Our last day in flight promises to be the most challenging and the most rewarding for the project.  Our 112 days in orbit are focused entirely on the last four minutes, after the Centaur impacts our target crater and raises a plume of lunar material for the LCROSS Shepherding Spacecraft to observe for signs of water, but before the Shepherd also impacts the moon. 

From a Flight Team perspective, the LCROSS impact sequence is a dream occasion, and yet provides some cause for trepidation.  Many things can go wrong, and with so little time, there is only so much that can be done. 

During you day tomorrow, I thought it might be fun for you to know what the Flight Team will be doing in lead-up to the event.  To put it plainly, we won’t be idle!  Enjoy!

A Recent Development: TCM 9 put LCROSS On-Target

The latest data from our Navigation team indicates that TCM 9 has already put LCROSS on target to hit the designated impact area, without the need for executing TCM 10 on Thursday evening. Our predicted impact point is already within our target 3.5 km diameter circle, and our team will only make very small adjustments to improve our impact accuracy.  This alters our original plan for Thursday.

Instead of performing TCM 10, the team will plan and execute a very slow rotation by feeding the spacecraft new target attitudes each minute (see “Once More Around the Earth” for a description of our “quaternion creep” attitude change) to minimally disturb the current orbit while turning to an orientation that is optimal for Separation.  At Separation, LCROSS will use the velocity imparted by the springs between the LCROSS Shepherding Spacecraft and the Centaur (adding an estimated 15 cm/s to the Centaur) as a final means of nudging the Centaur toward the center of our target.  Analysis of the Centaur separation springs, along with actual tests of the system conducted to simulate very harsh conditions of space (far harsher than LCROSS has actually experienced) indicate the separation will impart a fairly precise change in velocity to the Centaur. 

This plan represents less risk (the slow attitude change will be simpler to plan, test and execute than a TCM), and introduces less uncertainty into the prediction of our impact point (firing thrusters for very short durations adds a lot of uncertainty, while the separation springs in the LCROSS-Centaur interface mechanism are very repeatable).  We’re fortunate to find ourselves in this situation, and we’ll take full advantage of it to ensure we impact on-target.

DOY 281 (October 8): TCM 10 (or not), Separation, Centaur Observation and Braking Burn

The last 24 hours of the mission, bridging DOY 281 and 281 (October 8 and 9), will be a flurry of activity.  Here is the sequence of events.  I’ve provided both UTC and Pacific Daylight Time references:

• 08:00 UTC/01:00 PDT: Final orbit determination delivery for Separation.  The Navigation team delivers its final orbit determination to the Maneuver Design Team.  This final trajectory estimate will be the basis for planning our slow rotation, Separation and Braking Burn.
• Maneuver planning and communications link analysis for slow rotation through Braking Burn. For eight hours, the Maneuver Design team will determine the optimal attitude for Separation (when we let go of the Centaur), then plan and double-check plans for the slow rotation, Separation and Braking Burn.  Braking Burn happens after Separation, so will have no influence on the path of the Centaur.  It accomplishes two things: first, it slows our Shepherding Spacecraft with respect to the Centaur, such that at the time of Centaur impact, the spacecraft will be 4 minutes behind it.  This allows LCROSS to observe the Centaur impact while not being too close (risking damage from debris) and not being too far away.  Second, the Braking Burn independently targets the Shepherding Spacecraft impact point, which will be a few kilometers away from the Centaur impact point.  During the same time period, the Communications Link Analyst will refine his estimate of our communications link margin through all phases of the slow rotation, Separation and Braking Burn.
• 13:00 UTC/06:00 PDT: Separation Activity Selection Review (ASR): Our team knows every last detail of what activities we’ll be running, but this meeting is our last chance to change any part of the command sequence, based on late-breaking data (e.g. changes on the spacecraft, etc).  The Maneuver Design Team and Communications Analysis Teams will present their results here, and will form the basis of command generation.
• 14:00 UTC/07:00 PDT: Command generation and checking for Separation through Braking Burn. Our Activity Planning and Sequencing Engineer will generate all of the command sequences for TCM 10, Separation, Braking Burn, a preliminary version of Impact, as well as for several contingency cases.  He will hand his products over to both Engineering Analysis, and to the Simulation Engineer, who provide different aspects of quality assurance checks.  Engineering Analysis performs a number of computer-based checking against LCROSS flight rules to make everything in the sequence is legal.  Both the Simulation Engineer and the Engineering Analyst will run the commands on our spacecraft simulator to confirm that they do what we want.  During this time, Shift B (Flight Directory Rusty Hunt’s shift) will hand off to Shift A (my shift).  Shift B will get some sleep, and return before Impact.  Shift A will oversee all of the events through Braking Burn.
• 19:00 UTC/12:00 PDT: Separation Command Approval Meeting (CAM): This is our final, team-level review of all plans, command products and quality assurance data before executing the slow rotation through Braking Burn.  We’ll make sure everything is correct, over a 90 minute review.  Then we’ll move to our seats in the Mission Operations Control Room (MOCR) to begin execution.
• 20:30 UTC/13:30 PDT: Command loads for Separation, Centaur Observation and Braking Burn, and slow rotation to Separation attitude. Once “on console”, Shift A’s first priority will be to load the commands for Separation, Centaur Observation and Braking Burn.  We then turn our attention to the slow rotation to the Separation attitude, by loading the burn commands to the spacecraft in an alternate memory bank.  The slow rotation command sequence will re-orient the spacecraft from our Cruise attitude to the Separation attitude.  We’ll confirm the loaded parameters, and then wait for the reorientation to start.
• ~00:00 UTC/17:00 PDT: Slow rotation to Separation attitude starts.  The maneuver is small, only 6 degrees or so, but will happen in chunks of less than 0.5 degrees each minute.  The onboard command sequence automatically switches over to our Separation, Centaur Observation and Braking Burn command sequence, just in case we lose communications with LCROSS.  In that off-nominal scenario, Separation would still happen without ground-based commanding by our team.
• 01:40 UTC/18:40 PDT: Separation onboard command sequence starts.  The pre-Separation command sequence starts running.  Ten minutes to Separation.
• 01:50 UTC/18:50 PDT: Separation.  Commands temporarily disable our ACS, then fire the relays that unlock the Centaur from our spacecraft.  Heavy springs push the Centaur and spacecraft apart at roughly 0.7 m/s, a firm but gentle shove.  The Centaur will accelerate approximately 15 cm/s, but with our optimal orientation, only 3.5 cm/s will be used for Centaur targeting.  After Separation, the ACS is re-activated with an entirely new set of parameters to handle the vastly different mass properties. With the Centaur separated, LCROSS will just have lost 2000 kg of mass.  The spacecraft motion (dynamics) will now behave very differently.  The Flight Team has only 10-15 seconds to confirm that Separation has occurred, and if not, only 50 seconds more to terminate the command sequence to progress any further.  We have practiced this critical timing many, many times.
• 01:51 UTC/18:51 PDT: Flip to point LCROSS instruments at Centaur.  Just 1 minute 6 seconds after Separation, the onboard command sequence initiates a 180 degree pitch flip to point spacecraft cameras at the departing Centaur.  This takes less than 3 minutes to perform.  The command sequence also powers up the Data Handling Unit (DHU), which powers the science instruments, in preparation for Centaur Observation.  Following the pitch flip, commands roll the spacecraft to optimize the pointing of our omni-directional antenna toward Earth for best downlink rate.  At the end of the pitch maneuver, the Flight Team will re-configure the LCROSS downlink data rate for 256 kbps, and will command the DHU to go to a high-rate camera sampling sequence.  Imagery of the departing Centaur, with the moon in the background, will begin flowing to Earth.
• 02:01 UTC/19:01 PDT: End of Centaur Observation.  Nineteen minutes after Separation, with the Centaur nearly 800 meters away, the Centaur Observation will terminate.  The Flight Team will reconfigure the communications downlink rate for Braking Burn (64 kbps).  The onboard command sequence automatically re-orients the spacecraft to the final burn attitude, and then squeezes down our attitude control deadband from 3.0 degrees to 0.1 degrees, in preparation for Braking Burn.
• 02:30 UTC/19:30 PDT: Braking Burn starts.  This burn is longer our last few TCM’s, just over four minutes.  This is because there’s not much time remaining in the mission to build up a 4-minute delay between the Centaur and the Sheperding Spacecraft.  At the end of the burn, the onboard command sequence will re-orient the spacecraft to our Cruise attitude. 
• ~03:00 UTC/20:00 PDT: Preliminary Impact command load.  As a precaution, the Flight Team will load a preliminary command sequence for Impact to the spacecraft.  If we lost communications with LCROSS sometime after this point, up until the final few minutes, this command sequence should be sufficient to point the LCROSS cameras at the Centaur impact point, run the instruments, and meet all mission objectives. However, before Impact, the team will re-estimate the orbit of the Centaur and Shepherding Spacecraft, and re-plan Impact with the best possible information.
• 03:30 UTC/20:30 PDT: Shift Handover.  Shift A (my shift) hands control over to Shift B.  Shift B will oversee the Impact event.  We’ll review the status of the spacecraft, in particular the dynamic behavior following Separation, and any last-minute items.
• 04:30 UTC/21:30 PDT: Final Orbit Determination Delivery. The Navigation team delivers its final estimate of the spacecraft and Centaur orbit. The spacecraft’s orbit can be measured directly, while, without a communications transponder aboard the Centaur, we have no direct measure of the Centaur’s orbit after Separation.  This final orbit determination will become the basis for Impact command sequences, in particular the spacecraft attitude sequence to maintain pointing on the Impact site, and the Impact timing.
• Final Impact Planning and Command Generation: The Mission Design team will re-plan Impact using the latest orbit data from the Navigation team.  The changes between preliminary and final Impact plan will be very subtle.  The plan involves literally hundreds of Shepherding Spacecraft orientation changes to keep the onboard science instruments pointing at the expected Centaur impact point as we approach the moon.  The new orbit estimate will change all of these orientations very slightly.  The Sequencing Engineer will re-implement the command sequences, then pass his results to the Engineering Analyst and Simulation Engineer for final checking. 
• 6:30 UTC/23:30 PDT: Disabling LCROSS Fault Management. Shift B will begin configuring LCROSS for the Impact.  One of the first steps is to nearly completely disable the LCROSS onboard fault management system.  Fault Management responds automatically to correct problems it detects onboard.  Sometimes these are benign responses, like switching from a primary sensor to a backup sensor. Other times, the responses can be all-encompassing.  It might seem strange to disable this function right before our most important phase of the mission.  However, the last thing the Flight Team wants is for a problem onboard the spacecraft to interrupt our Impact observations.  Some fault management responses are designed to throw LCROSS into a Survival State, turning off all power to the science payload, and disabling any onboard command sequences.  This could mean disaster for the Science Team, since there would not necessarily be sufficient time to recover and return to the pre-Impact configuration.  So, only minor fault management is enabled, but the more severe responses are disabled.  In preparation for Impact, aside from disabling fault management, Shift B will also coordinate with the Deep Space Network to transfer our downlink path from a 34 meter diameter antenna (DSS-24) to the Goldstone complex’s 70 meter dish (DSS-14).  The 70 meter antenna enables LCROSS to return science data at 1 megabit per second (1 Mbps).
• 8:30 UTC/01:30 PDT: Impact Command Approval Meeting (CAM).  Shift B will review the final Impact plan and the associated onboard command sequences and ground commanding products.  This is our last chance to get things right.  Since the team is focused on a very specific set of checks, and for lack of time, this CAM lasts only 30 minutes.  Then Shift B goes back to the MOCR to perform Impact.
• 9:00 UTC/02:00 PDT: Loading Impact command sequence to LCROSS. Shift B loads the final command products to the Shepherding Spacecraft, including a set of contingency command sequences to cover off-nominal scenarios.  In the event of a building fire or an earthquake, our team even has a command sequence that would allow Shift B to leave the building and have the entire Impact sequence and observation be automated.  The Deep Space Network has dedicated four antennas to this period of time, three from the Goldstone complex in California, and a fourth located at Madrid in Spain.  Shift B, with the help of DSN operators at JPL, will coordinate those antennas as LCROSS changes its communications configuration.  Hours earlie
• 10:00 UTC/03:00 PDT: Start of Impact onboard command sequence.  Its first commands will perform a reorientation of the Shepherding Spacecraft to point the science instruments towards the expected Centaur impact point on the moon.  The cameras and other instruments will not yet be on.  This reorientation will also point the –Z Medium Gain Antenna (MGA) towards the Earth, enabling the team to switch the LCROSS downlink path from the omni-directional antenna to this MGA, in preparation for high-rate science data transmission. 
• 10:10 UTC/03:10 PDT: Switch to –Z MGA. Shift B will command the switch from omnidirectional to the –Z MGA antenna.  This is a potentially critical step in achieving full-rate science data transmission after the Centaur impact.  However, since we did our combined Cold-Side Bakeout #3/MGA Test on September 24, we’re pretty confident this will work again.
• 10:15 UTC/3:15 PDT: Transitioning to Science Rate. The Flight Team will now command a transition from a standard downlink data rate of 64 kbps to our full science rate, 1 Mbps.  This is another very important step to achieving full science return.  However, we do have backup procedures that would allow us to transmit science data at a lower rate, 256 kbps, if the DSN 70-meter dish were to fail, or if the MGA was non-functional.
• 10:36 UTC/3:36 PDT: Payload powers on. The onboard Impact command sequence powers on and enables the DHU and science instruments.  At 10:41 UTC, the command sequence also starts DHU NVM sequence 1, a sequence of instrument commands that tests each instrument in the LCROSS  payload, save the Total Luminescence Photometer (TLP).  The MOCR at NASA Ames begins to receive data from the science instruments, and the Payload Team and Science Team begins analyzing the preliminary data to make sure everything is working.  This is still nearly one hour from Impact, but it’s the team’s last chance to find a problem in our suite of payload instruments that might otherwise foil our Impact observation.  The team continues checking the instruments, and via the Flight Controller and Flight Director, commanding small adjustments to exposure settings, for 35-40 minutes.
• 11:10 UTC/4:10 PDT: TLP Instrument powers on. The Total Luminescence Photometer (TLP) instrument powers on for the first time since before launch.  This instrument is very sensitive, and can only be powered on a limited number of times.  The Science Team has been very careful not to overuse the instrument in tests.  However, if the instrument powers on as expected, this is a major success on the road to the Impact event.  The TLP, which gathers light measurements at 1000 times per second, will “catch” the Impact flash as the Centaur hits the moon, and is hence a very important instrument for water detection.
• 11:30:20 UTC/4:30:20 PDT: Flash Mode begins. One minute prior to Centaur impact, the DHU will command NVM command sequence 2, which begins Flash Mode.  For the next 1 minute 3 seconds, Flash Mode will run the TLP and other instruments to capture the flash of light coming from the impact event. 
• 11:31:20 UTC/4:31:20 PDT: Centaur Impact. Centaur impacts the moon at Cabeus.  The energy of impact emits a brief, intense flash of light.  A plume of lunar debris will rise in a pattern similar in shape to an inverted conical lampshade.
• 11:31:23 UTC/4:31:23 PDT: Curtain Mode begins. The DHU will switch from Flash Mode to Curtain Mode, which is a sampling sequence optimized to observe the evolution of the debris plume as it rises from the lunar surface.  With this debris rising above the altitude of the Shepherding Spacecraft, our side-looking spectrometer will look towards the sun to measure light as it is transmitted through the debris.  The remainder of the payload will be pointed down towards the impact point. This mode lasts for 3 minutes.
• 11:34:23 UTC/4:34:23 PDT: Crater Mode begins. At this late stage, the DHU will now switch from Curtain Mode to Crater Mode, which is designed to capture data about the properties of the new crater generated by the Centaur impact.  The Shepherding Spacecraft now has less than one minute of time to capture and transmit data before it also hits the moon.  With the Centaur impact point now off to the side, LCROSS will continue to try and track that point until its own contact with the moon.
• 11:35:39 UTC/4:35:39 PDT: Shepherding Spacecraft impact. The Shepherding Spacecraft will also hit the moon at roughly this time.  The Flight Team will abruptly stop receiving telemetry a few seconds later, as the photons from LCROSS’s last transmission travel back to Earth to be received by the DSN 70 meter antenna.  The LCROSS flight mission will be over.

This will be my last post until after Impact.  I hope you enjoy the show tomorrow – it should be very exciting.  Though we won’t have immediate feedback for water detection, I hope to report good news to you on Friday regarding the accuracy of our impact, and the collection of the science data.  Then, over the coming weeks after Impact, the Science Team will review their data and interpret the observations. I’m sure you’ll be hearing news one way or the other.

Thanks for reading!

Well, we all knew it was going to happen.  It was inevitable.  It was the whole design of the mission.  LCROSS was destined to end its wonderfully fantastic journey by intentionally crashing into a permanently shadowed crater at the south pole of the Moon.  We are the ones who devised this fate for LCROSS.  So why should we be surprised (and just a little bit sad) now that the time has finally come?

As a proud member of the LCROSS Science Team and as the Observation Campaign Coordinator, I would have to say that working on this mission has been one of the highlights of my career thus far.  The mission itself is truly amazing (We’re impacting the Moon!  We’re looking to see if there’s water ice at the poles!  We’re going to this utterly unexplored place in our Solar System, so close to home, and are so excited about what we’re yet to learn!).  LCROSS is so important to both science and exploration.  This mission is blazing a new path in how to build small, robust spacecraft both on schedule and on budget.  LCROSS uses eight (yes eight!) commercial off-the-shelf instruments for its payload – also a very novel way for NASA to get more bang for the buck as well as good science to boot.  The technical aspects of the LCROSS mission are astounding, but none of this would be possible without the dedication of the *people* working on this project.

The LCROSS Team is made up of an amazing cadre of individuals.  LCROSS has a relatively lean and nimble team.  There’s still a lot of work to be done to send a spacecraft to the Moon, and so that means that everyone has to pull together to make things happen.  If somebody is extra busy and needs help, you help them.  If there’s something that needs to be done and you’ve never done it before, you figure out how to do it.  If you are stuck and need some assistance, just ask your teammates and without hesitation people are willing to help.  We all naturally come together to get the job done.  There is a high level of trust and commitment on this team, starting with the top Project management and all the way through the people working the nitty-gritty technical aspects.  It is truly a glorious experience to work with a team such as this.  The best part is that everyone is working towards a common goal, and everyone is willing and able to contribute in whatever way is needed in order to achieve the objective.  It is amazing what a group of people can do when presented with a fascinating project and an exciting challenge.

And it’s not just the Project folks who have helped make this happen, but it’s all of the students and members of the general public who have so substantially contributed to the successes of LCROSS.  Student interns at NASA Ames have had the opportunity to work with real honest-to-goodness flight hardware.  Not everyone has the opportunity in college to hold an instrument that will be on the Moon within the next year!  Such opportunities are tremendously powerful for encouraging the students of today to continue the pursuit of careers in math, science, and engineering.  Amateur astronomers from around the world have been imaging LCROSS in the night sky during its trip to the Moon and are planning to collect observations of the impacts as well.  This is a great way to actively participate in a NASA mission.  We’ve also been having a great time keeping folks updated regarding LCROSS activities through our NASA website as well as the LCROSS Facebook and Twitter accounts.  Thousands of people are following LCROSS on these sites and we’re thrilled to be able to have a two-way dialog to discuss all things lunar!

So, although this mission was destined to end in a spectacular grande finale culminating with two lunar impacts, it is a bit sad to see this phase of the project come to a close.  However, next up is the exciting analysis of the data to try and learn all we can about these enigmatic regions on our very own Moon.  And here’s to hoping there are lots more missions coming up in the future, because we are fired up and ready to go!

The LCROSS Mission Design Team included the Centaur as a critical component of their mission from the very beginning.  It’s essentially a bonus impactor that stays with LCROSS after performing it’s normal function of delivering LRO and LCROSS on their way to the moon.  For the LRO-LCROSS launch and particularly for its contribution to the LCROSS mission, the Centaur was the focus of extra attention from both the LCROSS team and from the KSC/ULA-Atlas launch team.

Think of it as energy recycling.  The kinetic energy  [1/2 MV²] provided by the launch vehicle to get LCROSS and Centaur to lunar swing-by are later used to create the ejecta plume at the moon’s south pole.

Impacting a 2-ton Centaur upper stage enables the LCROSS mission to significantly increase the amount of lunar ejecta expelled from the target crater over what could have been achieved using only the spacecraft.

Centaur is a cryogenic (liquid hydrogen and liquid oxygen) powered rocket  upper stage. Its engine can be shut-down and restarted after one or more guided coast phases.  Centaur has a long history of successful  missions.  For LRO-LCROSS, instead of being parked in a safe orbit after sending LRO and LCROSS toward the moon, this is Centaur’s first role as a guided impactor for the science gathering  portion of a mission.

The Centaur upper stage used for the LCROSS mission is the same as Centaurs used to propel the Mars Reconnaissance Orbiter and Pluto-New Horizons on their way to Mars and Pluto, respectively.  The only significant modification to this Centaur is the addition of white thermal paint to help balance the temperature of the empty stage. 

Upper Photo: The LCROSS logo, painted on the Centaur.  Lower Photo: The Centaur upper stage is lifted for mating to the Atlas-V first stage (Tail Number 020) at the Vertical Integration Facility in Florida (April 30, 2009). The Centaur was painted white to help manage the thermal environment during the 100-plus day LCROSS mission.  Yellow lifting hardware is ground handling equipment and is removed after stacking on the first (booster) stage.  Courtesy of NASA Kennedy Space Center.

What is particularly unique about this Centaur was the way that the United Launch Alliance (ULA) Atlas Team designed the flight profile and maneuvers after LRO separation to minimize the amount of residual hydrogen, oxygen, hydrazine, and helium left on Centaur.  This special care to ensure that the Centaur is as empty is possible, makes the Centaur very clean. It reduces the hydrogen (H2), oxygen (O2), and hydroxyl (OH) species that could be confused with in-situ water ice ejecta.

Since Centaur would be attached to LCROSS until just before impact, all of the Centaur hardware and functional system were analyzed and scrutinized and if necessary tested to ensure that they would not adversely affect LCROSS after LCROSS took over control from Centaur.

The final function to liberate Centaur as a guided impactor will be the release of the clamp band holding the LCROSS/Centaur stack together. This separation command will be sent by LCROSS approximately 9 ½ hours before impact.  The clamp-band and the separation system that will push LCROSS and Centaur apart, underwent long-duration testing to give the Mission team confidence that it will function smoothly after 100-plus days in orbit.

Additional Centaur info and history, courtesy of NASA’s Glenn Research Center, can be found at: http://www.nasa.gov/centers/glenn/about/history/centaur.html

The anomaly robbed the LCROSS Flight Team of precious time to prepare for Impact.  But with a healthy spacecraft, and enough propellant to do the job, our team was all too happy to prepare for the future.   Ahead of us were five more Trajectory Correction Maneuvers (TCM 6 – 10) to precisely refine our crater targeting, then Separation, Centaur Observation, Braking Burn, and finally, Impact.  In the midst of our TCM series, the Science Team continued refining their selection our target – the specific crater, and the point within the crater - based on the latest data from other missions.  As a final confirmation of the payload instruments, the Science Team also wanted to look at Earth one last time before Impact.  On top of all that, we needed to practice those final two critical days as much as we could.

DOY 247-251 (September 4 – 8): Housekeeping

Out of Emergency Status, we resumed operations gradually, monitoring spacecraft health and performing typical housekeeping duties.  Of special note, as a result of the anomaly and our very small propellant margin, we decided to substitute Earth Look Cal 2, originally scheduled for DOY 250 (September 7), with a new, more propellant-efficient calibration maneuver that we termed “Earth Gaze”, on DOY 261 (September 18; see below).   

DOY 252 (September 9): TCM 6 Waived

We received more good news about our trajectory.  Despite the long train of thruster pulses that resulted from our anomaly, LCROSS’s orbit was still right on target.  The Mission Design Team and Navigation Team agreed that TCM 6 would be too small to be worthwhile.  This meant one less event to attend to in our busy schedule, and one less reason to take risks at this late, sensitive stage in the mission.

This decision worked in our favor for saving propellant too.  Each TCM typically requires the spacecraft to reorient to a specific direction so that LCROSS can add velocity in a particular direction via the 22 N Delta-V thrusters, and then back again to the Cruise attitude.  All of our remaining “burns” were expected to be small; so much so that the propellant devoted to the reorientation maneuvers was expected to be several times more than for the burn itself.  Waiving TCM 6 meant we could save more propellant, and reduce risk.

DOY 253 (September 10): Omni Pitch “Creep”

Throughout the mission, Omni Pitch maneuvers have been necessary to keep our primary antenna pointed towards Earth.  In all previous Omni Pitch renditions, the “slew” was performed in SIM DB2 (Stellar Inertial Mode, deadband 2), an Attitude Control System mode that enables relatively efficient attitude changes.  However, SIM DB2 significantly accelerates the spacecraft rotation to induce the change in orientation, and this costs propellant. 

In our bid to save propellant, we re-examined all of our previous maneuver approaches for opportunities to be more efficient, including for Omni Pitch maneuvers.  Someone on the Flight Team came up with the idea of providing LCROSS a long sequence of small attitude changes rather than one big one, with the goal of avoiding the accelerations of SIM DB2 (small attitude changes don’t justify large rotational accelerations, and therefore are cheaper propellant-wise).  Simulations of this new approach, what we informally called “Quaternion Creep”, indicated this would lead to a significant propellant savings, so we baselined this approach for our Omni Pitch maneuver on DOY 253. 

Normal Omni Pitch maneuvers used to take 40+ minutes to execute.  This new approach took 4 hours 20 minutes, but saved a lot of propellant.  Well worth the bargain.

DOY 257 (September 14): Guarding Against Single-Event Upsets

Cosmic rays are high-energy photons that are known to interfere with spacecraft computers by in effect reprogramming single bits of memory as they pass through.  When a bit in computer memory is reprogrammed by radiation, but not permanently damaged, it is called a Single-Event Upset (SEU).  Most spacecraft computers and onboard memory chips are “radiation-hardened” to prevent or otherwise sidestep the effects of cosmic rays. 

The spacecraft Data Handling Unit (DHU) is the independent computer that controls the operation of the science instrument payload.  Command sequences onboard the DHU control all of the sampling sequences and instrument settings to make sure LCROSS collects the best possible set of data at impact, and for other calibration events.  They are clearly very important to the success of LCROSS.

The DHU, and those instrument command sequences specifically, are somewhat vulnerable to SEU, and the Payload Team had not checked the contents of the DHU in a long time.  On this day, our team dumped (downloaded) the full contents of DHU non-volatile memory and confirmed that the sequences were still intact. 

DOY 257 – 259 (September 14 – 16): Full Rehearsal of TCM 10 through Impact

It was hard to find time in our busy operational schedule to practice all of our pre-Impact procedures.  In the months before launch, we spent all of our time readying for Transfer Phase and the first parts of Cruise Phase.  Impact seemed so far away.  Then Cruise Phase, with the anomaly, was far busier than expected.  With only weeks remaining, our perspective was entirely different.   

We took full advantage of two free days in our schedule to hold the “Last Two Days Rehearsal”, a full-team, high-fidelity rehearsal of the last two days of the mission, similar in style to our First Week Rehearsal (see the post on “First Week Rehearsal”).  For 42 hours, following the exact schedule we planned to follow in flight, and synchronized with the actual times of day when events would occur, the Flight Team practiced nearly every aspect of those operations.  We ran 24 hours a day, with three overlapping execution shifts (Shift B, then A, then B again), and two planning sessions.  In the rehearsal, the team successfully hit the target crater and collected all of the science data.  However, the there were some procedural and process shortcomings that made things just a bit rocky at times.  We learned a lot, and began working improvements in the days after the test. 

DOY 260 – 261 (September 17 – 18): Our Last Gaze at Earth

In substitution for Earth Look Cal 2, we conceived of a more propellant-efficient calibration event we coined, for lack of something more official, “Earth Gaze Cal”.  We loaded the commands for this event on DOY 260 (September 17), and executed the calibration the following day.  Rather than sweep back and forth over the Earth (see the full description of our first Earth Look in “During Our Second Trip Around Earth”), Earth Gaze looked straight at the Earth for an extended period, during which time we collected camera and spectrometer data.  This was the Science Team’s last chance to evaluate instrument performance and settings before Impact. 

We captured some more images.  Here are a few, along with a simulation to show the orientation of Earth during the event.

DOY 267 (September 24): Cold Side Bakeout #3 and a Test of the –Z MGA

Despite previous efforts to rid the Centaur outer skin of water on Cold Side Bakeout #1 and #2, our Navigation team continued to observe the accelerating effects of escaping water at the end of the second of those events.  With the amounts of water remaining, the Science Team was no longer concerned that this water could interfere with water measurements at Impact – there was just too little left.  However, Navigation was still concerned that remaining water might push our Centaur off course in the hours before Impact, after Separation when we no longer had any control over its orbit. 

Recall that we had planned to execute Cold Side Bakeout #3 on DOY 234 (August 22), but our plans were thwarted by the discovery of the anomaly.  Cold Side Bakeout #3 was unfinished business that had to be completed. 

In an unrelated thread, we also wanted to test the antenna we’d be using for Impact.  LCROSS has two Medium Gain Antennas (MGA’s), one on the +Z axis, the other on the –Z axis, used to downlink high-speed science data to Earth.  We had used the +Z MGA during Lunar Swingby, but had never tested the –Z MGA in flight.  We didn’t want to discover a problem with this antenna in the hours before impact, so we devised a test that would expose any issues immediately, and that would couple very nicely with Cold Side Bakeout #3. 

The combined Cold Side Bakeout #3 and –Z MGA Test took advantage of the fact that LCROSS was passing right through the ecliptic plane, the plane of the Earth’s orbit around the sun.  The LCROSS +X axis was perpendicular to the ecliptic plane at the time, and so by rotating about the +X (roll) axis, we could simultaneously face the “cold side” of the Centaur towards the sun, and the –Z MGA towards the Earth (required to test communications via this antenna).  It was a perfectly-timed opportunity.

On DOY 267 (September 24), we performed the maneuver.  Unlike previous versions of Cold Side Bakeout, we stopped the spacecraft twice, once at 135 degrees rotation, and again at 225 degrees.  The first position pointed the –Z MGA (which is canted by 45 degrees) straight at the Earth, and warmed one side of the cold skin of the Centaur.  After 20 minutes, LCROSS rotated another 90 degrees, moving the MGA off the Earth, and moving another part of the cold face of the Centaur into full sunlight.  We characterized one “slice” of the –Z MGA antenna gain pattern, confirmed that it was operational and mounted according to specification, and removed more water from our impactor.

One of the risks of Cold Side Bakeouts is that we might induce a thermal instability in our thrusters, as we had in Cold Side Bakeout #1, prompting our improvised fault management to fire the thrusters to keep them warm (see the post entitled “Our First Orbit Around the Earth” for details).  Happily, our thrusters remained thermally stable, and we avoided any additional propellant cost.

DOY 268 (September 25): TCM 7

LCROSS had not performed a Trajectory Correction Maneuver since TCM 5a, way back on July 21.  We’d been literally coasting along our orbit since then.  By this time, the Science Team had selected a satellite of the Cabeus crater, Cabeus A1, as our impact target.  Trajectory Correction Maneuver 7 (TCM 7) was performed on September 25 to target that location on Impact Day.  According to predictions on burn pointing errors, TCM 7 would guarantee a hit within 38 km of our intended target.  Future burns would reduce the error a lot further.  Still 14 days from Impact, TCM 7 nudged LCROSS by 32 cm/s (just over 1 foot/sec), but enough to make a big difference in impact position.

DOY 273 (September 30): TCM 8

Following TCM 7, the Science Team continued to receive data from other lunar missions.  Lunar Reconnaissance Orbiter (LRO), our partner at liftoff, devoted much of its operational time to scouring our top target regions, and data from Chandrayaan-1 and other missions continued to improve our knowledge of the mapping of possible water concentrations at the lunar south pole.  New information gathered after TCM 7 prompted the Science Team to redirect LCROSS to the main Cabeus crater.  Relatively close to the previous target, changing represented a small amount of extra propellant, but a potentially significant improvement in science data. 

TCM 8 was performed on September 30, and changed LCROSS’s velocity by 35 cm/s.  Not only did TCM 8 target the new impact position, but it also refined our impact timing.  Our narrow target time window will allow the Hubble Space Telescope to take images of our impact plume.

DOY 274 (October 1): Impact Contingency Rehearsal

Our Last Two Days Rehearsal on DOY 258-259 did not emphasize off-nominal events, but rather the full integration of the team in a single push.  On October 1, Shift B, who will be overseeing Impact, practiced two complete run-throughs of the final two hours of the mission.  I helped our Test Conductor in setting up the tests, and planning a series of anomalies that would really challenge Shift B.  During the simulations, we failed banks of temperature sensors and heater circuits, caused various instruments to malfunction and the DHU to crash.  We faulted the IRU, forcing a recovery in the midst of early pre-Impact science operations.  We failed the –Z MGA switch (even though this is far less likely having conducted our MGA test), and forced the team to perform the Impact sequence at a contingency low data rate.  Their team did very well, and succeeded in meeting mission objectives on both runs.  The members of Shift B were very happy they had that final opportunity to practice.

DOY 275 (October 2): Planning Rehearsal

The flight planning team has one of the biggest challenges in the final 24 hours before Impact.  Based on a final assessment of the LCROSS orbit, they have to plan the final series of maneuvers that culminate in the Impact event: TCM 10, Separation, Centaur Observation, Braking Burn, and Impact itself.  From the maneuver plan, the team must generate command sequences for each of these events, and then run them on our simulator to prove that they are flight-worthy.  Furthermore, they also need to generate and test command products to handle specific contingency events, like a failed Separation, or a late Braking Burn.  They even generate a preliminary Impact command sequence, just in case we lose contact with LCROSS for hours before the event.  Some of these products can be produced well in advance, but a great number of them depend on the final orbit assessment, and the specific orientations that are required for TCM 10, Separation and Braking Burn to ensure we accurately hit our target.

The Last Two Days Rehearsal proved that the planning team could generate all of its products, but could not quite perform its full list of quality assurance tasks in time.  Following the rehearsal, the team took some significant steps to streamline the planning, command generation and quality assurance process.  We found opportunities for working in parallel, for skipping unnecessary steps, for nailing down some variables in advance to reduce the amount of variance on Impact Day. 

To test these changes, we conducted a Planning Rehearsal for DOY 275 (October 2) to span the full 12 hours of the process, exactly as it will happen on the Impact Day.  I was really happy with the results.  The team actually finished most of its tasks early, including simulations of all command products, and their delivered products contained no errors.  The Command Approval Meeting, our last visual evaluation of commands, finished early.  I was very encouraged by these results, and the planning team is now fully confident that they could repeat this level of performance on Impact Day.

DOY 278: Final DHU NVM Sequence Loads and TCM 9

The remainder of this post has to do with current and future events.  I’m finally caught up!

This morning, as I write this post, Shift B is loading its final set of science payload command sequences to the spacecraft.  The Science and Payload Teams have pored over the data we collected from all previous science calibrations to derive a set of camera exposure/gain settings for Impact.  Over-exposure means washed-out images, which under-exposure means less contrast and detail.  Six of nine sequences will be changed to reflect the Science Team’s best knowledge of camera performance, and expected conditions on our final descent to the lunar surface.

Later this afternoon, Shift A will be performing TCM 9, our second-to-last, and possibly our last orbit adjustment.  After TCM 8, based on the latest lunar mapping data, the Science Team selected a different spot within the Cabeus crater.  This is our final target.  With only 7.6 seconds of firing, TCM 9 moves our impact point from our old point in Cabeus to this new location roughly 9 km away.  TCM 9 should put us within 1.75 km of our target.  If we can “nail” this maneuver by staying within that range, we’ll be able to skip TCM 10. 

TCM 9 is our last spacecraft activity, other than monitoring and housekeeping, until our final rapid-paced series of maneuvers in the final 24 hours.

As you can imagine, things are starting to get very busy!  I will submit one more post with a detailed description of the final 24 hours.  I also have a few guest authors who would like to share some things with all of you before Impact.  Stay tuned...this is going to be an incredible week.

Early in the second half of our second Earth orbit, while out of DSN contact, LCROSS experienced an anomaly.  An error detected with our Inertial Reference Unit (IRU) resulted in an automatic response that consumed a large amount of propellant in a short amount of time.  We spent the remainder of our second orbit recovering from that anomaly, and protecting against any future excessive propellant usage.  We emerged safely, but with so little propellant remaining that, since that event, we’ve had to step very carefully to avoid wasting any more of this precious resource.  This posting describes our discovery of the problem, and our recovery steps to protect LCROSS against a reoccurrence.

August 21/22 (DOY 234): Anomaly Detection, Spacecraft Safing, and Entry to Emergency Status

Shift A came into the Mission Operations Control Room (MOCR) on Saturday with a big agenda on LCROSS – to execute a third “Cold Side Bakeout” to rid the Centaur of more ice trapped in its skin, and to perform an Omni Pitch maneuver to flip the spacecraft to re-point the primary omni-directional antenna at the Earth, all under a tight schedule with little margin for error.  The shift started at 2:25 AM Pacific time, but despite that early hour, we were excited to have such a challenging pass ahead of us, and eager to get started.

We acquired spacecraft telemetry at 3:25 AM.  In the MOCR, each Flight Team operator sits behind a set of computer monitors that display fields of telemetry data – numbers, status indicators, etc. that indicate the health of LCROSS.  Many of the telemetry data fields show up in stoplight colors – green, yellow, or red, to indicate whether the value is “nominal” (green), or approaching an emergency state (yellow), or is in an emergency state (red).  Typically, telemetry comes up entirely green. 

• The Inertial Reference Unit (IRU), our onboard gyro, and primary means of measuring rotation rates around each axis for attitude control, had faulted;
• The Star Tracker (STA) had replaced the IRU in providing rotation rate estimates to the attitude controller;
• Spacecraft rotation rates were periodically exceeding yellow high alarm limits (rotating too fast);
• LCROSS was firing thrusters almost continuously,
• The propellant tank pressure sensor (our primary means of determining how much fuel we have left), indicated we had consumed a LOT of propellant.

In short, we were in serious trouble, and needed to make corrections immediately.

First, we needed to save our remaining propellant by reducing our thruster firings.  LCROSS has an Attitude Control System (ACS) to control its orientation automatically, by firing thrusters (LCROSS has no reaction wheels).  The ACS controls each of the axes (roll, pitch and yaw; see the diagram in “First Orbit  Around the Earth”) to within an acceptable error bound, defined by a control “deadband” from a fixed orientation in space.  The ACS has several modes, all with different characteristics.  The mode we were in, “Stellar Inertial Mode, deadband 2” (or “SIM DB2” for short), controlled each axis to a +/- 0.5 degree deadband.  Through the anomaly, LCROSS was keeping to within its assigned deadband.  Under attitude control with a deadband, thrusters typically fire most often when the spacecraft rotates to the very edge of the deadband, causing the spacecraft to stop its rotation and preventing it from exceeding the deadband limit.  So, in an effort to slow down the firings (and our propellant consumption), we switched to a wider deadband, SIM DB3, which controls to a 10 degree range.  Unfortunately, this didn’t reduce our thruster firing rate very much.  We needed to do something else.

The anomaly had caused LCROSS to automatically switch from using the IRU to the Star Tracker (STA) for measuring spacecraft rotation rates. However, when we studied the output of the IRU, it was producing good rotation rate data (or just “rate data” for short).  So, we ran through the IRU recovery procedure to re-engage the IRU with attitude control, and sure enough, that finally returned the thruster firing profile to normal and bought ourselves time to think.

One thing was immediately clear – we had suffered a significant “anomaly”, or bad problem and our plans for a Cold Side Bakeout and Omni Pitch maneuver were dashed.  We’d be spending the rest of the shift, at least, and possibly a lot longer, figuring out what had happened and how to prevent it from ever happening again.

With the spacecraft stable, but still in jeopardy, the next order of business was to extend our DSN coverage.  The Mission Operations Manager, with full agreement from the rest of the Flight Team, declared a “Spacecraft Emergency” with the Deep Space Network.  Under DSN guidelines, we could only call this if we thought the spacecraft was in immediate jeopardy of partial or total mission failure.  Under an emergency declaration, all missions using the Deep Space Network volunteer their normally-scheduled antenna time in a community effort to help the ailing mission.  Their help provided LCROSS with enough antenna time to work out its problems.

As a step to protecting LCROSS, the Flight Team had to figure out what caused the anomaly.  Since the spacecraft was healthy on our last contact, the anomaly had occurred while LCROSS was in a normally-scheduled 66 hour “out-of-view” period with the Deep Space Network (DSN).  The Flight Team could not collect telemetry during that time.  However, just for this purpose, LCROSS constantly records a part of its telemetry onboard, to enable our team to diagnose problems that happen while out of contact.  We set immediately to “downlinking” and analyzing our virtual recorder telemetry data to gather clues.  

Using this data, and over an hour or two of analysis, our team gradually pieced together the story.  As with most anomalies, this one stemmed from multiple vulnerabilities whose combined harmful effects had not been anticipated.

In summary, a spurious, short-lived error on the IRU was interpreted as a more serious fault by the spacecraft fault management system, resulting in a switch to a backup rate sensor (STA). That rate signal was noisy (including random variations over the “true” rate signal), but was misinterpreted as real, “clean” rate data, causing over-control by the attitude control system and resulting in a great deal of propellant consumption.  LCROSS detected the associated tank pressure drop, but with no fault management option available for disabling thruster control (thrusters are required to keep the LCROSS solar array pointed to the sun), and no ability to determine the specific nature of the pressure loss, the spacecraft fault management system performed steps to stop a leaking thruster (another potential reason for a pressure drop), and to power-up its transmitter to “phone home” to warn the operations team that there was a problem.  However, this call could not be detected over the Southern hemisphere, since there were no DSN assets that could “see” LCROSS in that location, so the call was missed.

The LCROSS team knew that it had a serious problem on its hands.  Did we have enough propellant to complete our mission?  Was the IRU truly healthy, or would it fault again and trigger another loss of fuel?  Thanks to the DSN Emergency Status agreement, LCROSS was able to get continuous ground antenna coverage, whenever the spacecraft was in view of one of the three DSN antennas.  However, given our position in our Cruise Phase orbit, only the Canberra antenna complex could actually “see” LCROSS at that time, and only for 15 hours per day. 

• To increase the “persistency” of the IRU fault check by flight software.  By requiring an IRU fault to persist for 5 consecutive seconds rather than 1 second before tripping to STA, we hoped to avoid switching to the STA for an inconsequential IRU fault, yet remain protected against a serious IRU fault.
• To augment the IRU fault response by automatically recovering the IRU (as we had done manually) first, before switching to STA for rate information.  Even if an IRU fault lasted for 5 seconds, this second-tier change would try to re-instate the IRU first, and fail over to the STA only if this had failed.

We designed, implemented, tested and loaded these changes to the spacecraft in our remaining time.  By 1:50 PM Pacific Daylight Time, with these temporary fixes in place, we began configuring LCROSS for our forced DSN outage.  Just 10 ½ hours after discovering our problem, we had to release our hold on LCROSS again and anxiously look to the next time when we would reacquire communications with the spacecraft.

August 23 – September 3 (DOY 235 - 246): Our Recovery

• Improve our fault protection against an IRU failure for the long term.  We had short-term fixes in place from the first day, but we ultimately needed to implement a more complete solution.  These would take longer to design and test.
• Improve fault protection against further excessive propellant loss.  This was a core issue.  The loss of the IRU was one potential vulnerability, but there might be others we didn’t yet know about.  LCROSS could not tolerate another similar loss of propellant.
• Determine whether the IRU was really showing signs of failure, or whether it was actually fine.
• Assess our propellant margin.  Could we complete all of our mission objectives? Would we have to give up some of our planned activities to save propellant?
• Develop a plan for LCROSS health monitoring for the remainder of the mission.  With so much at stake, we felt we needed a way to regularly monitor LCROSS after we emerged from Emergency Mode.  However, with a team as small as ours, watching the spacecraft 24 hours a day for the rest of the mission would exhaust us.  Besides, we needed to devote a lot of time in the coming weeks to prepare for impact.
• Continue performing the nominal events that could not be put off until our anomaly resolution was complete.

Propellant Usage Monitor and Free-Drift Mode

Our first priority was to develop fault protection against excessive propellant consumption.  The sure way to save propellant is to not fire thrusters.  But firing thrusters is the only way to maintain attitude control, and that is critical for generating solar energy and to keep the spacecraft thermally stable.  It seemed we were in a bind.  But then we recalled a strategy the team had originally designed to fight large Centaur gas leak torques (see the post entitled “Real-Life Operations: Day 3” for a description of the Centaur leak issue). 

The attitude control system has a special safe mode called “Sun Point Mode” (SPM) that points the solar array at the sun using special Coarse Sun Sensors, and without the use of the Star Tracker.  SPM spins LCROSS very slowly, end over end, but keeps the solar array pointed at the sun at all times.  SPM still requires thruster control to maintain stability. However, through simulations, our engineering team discovered that by modifying SPM to spin faster, LCROSS would have enough angular momentum to keep it stable for a very long time.

We still needed a means to detect excessive propellant usage.  We deemed propellant tank pressure measurements a little too coarse for such an important job, and yet there was no other single, direct indicator of propellant usage in the system.  However, one of our team members came up with a great idea. LCROSS generates and stores special telemetry “packets” each time a thruster is fired.  These packets accumulate over time in spacecraft memory, and are downloaded to Earth for analysis.  His idea was to monitor the accumulation of thruster telemetry packets as an indirect indicator of propellant usage.  It was unconventional, but the idea worked.

Efficient Control under Star Tracker Rates

IRU Healthy

Enough Propellant for Full Mission Success

There is no way to directly measure the amount of propellant left in the LCROSS tank.  One can estimate the remaining propellant load by measuring pressure, temperature and tank volume (and compute the result using gas law equations), and alternatively one can estimate the propellant consumed through the mission by measuring the accumulated on-times for all the thrusters, along with predictions of how much propellant each firing consumes. Neither method is perfect. 

The Mission Goes On

During our anomaly recovery, we had to support a number of activities that couldn’t wait until later.  The day after our discovery of the anomaly (DOY 235, August 23), orbit geometry dictated we needed to rotate our spacecraft to re-point our omnidirectional antenna toward the Earth again, or suffer very poor communications for many days to come.  We performed another of these 10 days later on DOY 246 (September 3).

In a bit of good news, TCM 5c, originally scheduled for DOY 236 (August 24), was determined to be unnecessary.  As with so many TCM’s in our original plan, the Mission & Maneuver Design and Navigation teams planned TCM 5a so accurately that neither of the follow-up burns (TCM 5b and TCM 5c) needed to be performed.

However, not all of our news that day was good.  Also on DOY 236, we thought we had also experienced signs of a failing Star Tracker.  Out of concern for its survival, we shut down the STA for two days, and spent time away from our main anomaly recovery developing protections for the STA.  As it turns out, the unusual STA signature was related to an onboard clock calibration we had just performed.  The STA was just fine, but you can imagine, in the midst of the tension of the anomaly recovery, this extra scare didn’t help matters!  As in so many cases with space flight, even seemingly benign changes can lead to unexpected results.

Negotiations with Deep Space Network and other Missions

To address the need to monitor LCROSS more often after our exit from Emergency Status, the LCROSS project negotiated a schedule with the DSN mission community that shortened all out-of-contact periods to 9 hours duration or less.  This came at great impact to other missions, but was invaluable to LCROSS in achieving its Emergency Status exit criteria. 

DOY 247 (September 4): Emergency Over

Twelve days after the anomaly discovery, having augmented LCROSS fault protection and our contact plan with the DSN and other DSN-supported missions, we requested that DSN remove LCROSS from Emergency Status. 

One of the gratifying aspects of this effort was how the LCROSS team responded to make sure LCROSS stayed safe.  Members of the Flight Team, LCROSS project and NASA Ames worked tirelessly.  Northrop Grumman mobilized to provide a number of valuable improvements to LCROSS.  The DSN mission community generously volunteered their antenna time to support our bid to restore LCROSS to nominal status.  NASA management, from Headquarters to NASA Ames, provided constant support and valuable assistance. 

With our first orbit around the Earth behind us, the Flight Team team looked optimistically ahead to a number of upcoming events, including two science instrument calibrations, and two Trajectory Correction Maneuvers, TCM 5b and 5c.  Thankfully for us, the schedule of planned events for the next 34 days to be a little more relaxed than for the previous 42 days.  The key distinction being the planned events.

August 1 (DOY 213): A Look at Planet Earth, and a “Discovery” of Water

Before the mission began, our team had identified key geometric opportunities for performing our Earth and moon-looking science instrument calibrations.  One of the important factors in determining the best time is spacecraft-to-target distance.  The closer the Earth and moon are to LCROSS, the better resolution we can attain, and the stronger the signals will be (hence why Lunar Swingby was so important).  The below plot shows the distance between LCROSS and the Earth (light blue) and LCROSS and the moon (dark blue), over the entire mission.  The labels indicate when specific LCROSS events happened (or will happen), including launch, Lunar Swingby, Earth Look Cal 1, Earth Look Cal 2, Moon Look Cal, and Impact.  As you can see, the Earth Look events coincide with low points on the curve, while Moon Look timing was influenced by other factors.

Another factor is “phase” with respect to sun illumination on the target.  Both the Earth and the moon go through cycles depending on the relative positions of the sun, the Earth or moon, and the observer, in this case LCROSS.  So at various times, the Earth will look like a crescent, at other times will be “full” (like a full moon) and still other times will be “new”, with its face completely un-illuminated.  For science measurements, our Science Team tended to prefer “fuller” views of the Earth and moon. 

As planned prior to launch, Earth Look started by rotating the payload boresight (pointed along the LCROSS –X axis) to point at the center of the Earth, then repeatedly “swept” the boresight in alternating East-West and North-South passes across the Earth’s disk.  The objective was to collect spectra of the known Earth signal (for spectral calibration) and also re-affirm the alignment of the instruments.  Similar to the limb crossings in Lunar Swingby (see “Lunar Swingby: Development of a Procedure”) by sweeping the cameras across the disk, the Science Team could compare the timing of the rising and falling signal with the well-known motion of the spacecraft relative to the position of Earth.  Consistent mismatches in timing would indicate a misalignment. 

We collected some cool photos of the event, included here.  Note that our instruments are optimized for lunar impact and finding water, so you can’t make out the landforms in the visible images, and the IR images detect thermal differences in the upper atmosphere.  It was really exciting to be looking back at our home planet from so far away.  In contrast to images taken from the Space Shuttle, International Space Station, or even Apollo, our images really show how insignificant the Earth is in comparison to the immensity of space.

LCROSS, via its spectrometers, also “discovered” water on the Earth – an event that didn’t exactly make headlines, but made the Science Team quite happy.  Detailed analysis of the spectra and imagery indicated that all of our instruments were all performing very well.

 

In an unrelated piece of good news, the Mission & Maneuver Design team determined that TCM 5a had been so accurate that TCM 5b would be unnecessary.  Originally, TCM 5b would have incorporated another Omni Pitch maneuver, but now with TCM 5b unnecessary, we’d perform a standalone Omni Pitch instead.

August 6 – 7 (DOY 218, 219): Re-Pointing our Antenna – Omni Pitch 3

Nearing the “top” of our orbit (see the flight path diagram), it was time again to perform an Omni Pitch maneuver to re-point the primary omni antenna towards the Earth.  These times are tricky because at the time of the maneuver, neither the initial nor the final orientation provide very good angles to the omni antenna.  You can plan to perform the maneuver early (favoring the initial attitude, but making the final attitude margins really bad), or perform the maneuver late (favoring the final attitude).  Omni Pitch 3 favored the final attitude.  We lost telemetry data sporadically before it began, indicating very poor link margins (downlink rate at our lowest rate of 2 kbits per second or kbps), but then had sufficient margin to boost our downlink data rate to 32 kbps for post-maneuver virtual recorder playbacks.  Throughout the mission, the Flight Team constantly has to evaluate LCROSS link margins to make sure we can command the spacecraft and monitor maneuvers via telemetry.

August 8 – 13 (DOY 220 – 225): Good and Boring

The mission operations philosophy states that “a boring spacecraft is a good spacecraft.”  This is certainly true because “exciting” usually means risky, dangerous and proximity to failure.  As a Flight Team member, you really don’t want to be responsible for a mission disaster!  Despite the boredom that sets in on those uneventful days, slow and steady is a good thing.  Thankfully for us, LCROSS was behaving very well, and occasionally we found ourselves in long stretches of time between planned events.  During those times, the Flight Team simply monitored spacecraft health, and hoped for boredom.

Cruise Phase health monitoring passes typically include the following steps:

August 14 (DOY 227): Opportunistic Science and Design on the Fly

During the mission, the Science team and Engineering team occasionally requested activities that were not on the original schedule.  The Science team wished to find an opportunity to image a point source with a strong infra-red (IR) signal in order to work on a solution to an Mid-IR 2 camera focusing issue. The considered Venus and Jupiter, and though both were very bright in the visible spectrum, neither provided a very good IR signature.  Working with the Mission & Maneuver Design team, the Science team discovered a golden observation opportunity – on August 16, just as LCROSS would be passing through orbital plane of the moon, the Earth and moon would appear only a few degrees apart from each other (see the plot of the angle between the Earth and the moon, as viewed by LCROSS).  The moon has known IR reflective properties, and because the moon would be on the opposite side of the Earth from LCROSS, it would approximate a point source (look very small).  About the only problem with the proposed timing is that LCROSS would be very far from Earth (see the “Special Earth/Moon Cal” in the above plot of Earth and moon distance.  However, our team considered the opportunity, and decided to go for it.

With a simple mission like LCROSS, a Flight Team is wise to pre-plan as much as possible before launch.  As the saying goes, “the devil is in the details”, and that certainly holds true when piecing together command sequences and operational procedures.  We worked very hard before launch, without the pressure of a flying spacecraft to care for, to pre-plan, construct and test commands and procedures for each event in the mission plan. 

This new activity, dubbed “Special Earth/Moon Look” didn’t match any of our pre-planned maneuvers, so we had to design it on the fly.  We designed the overall approach in a meeting between the Science team, Mission & Maneuver Design Lead, Sequencing Lead, Systems Engineers, Flight Directors and Flight Controllers.  To the credit of the team, especially the Activity Planning & Sequencing Lead (John Bresina), we were able to create command a sequence, adapted from our Star Field Calibration sequence, that could accomplish the goals of the activity.  One major difference was that we’d be looking at the Earth and moon (two targets) instead of a star field (one target), so it was like merging two Star Field Calibrations together.  There were a lot of other changes at the detailed level to make this work.  The Engineering Analyst had to extend his automated checking tools to work on this new command sequence.  The Simulation Engineer also simulated the commands on the LCROSS simulator to make sure things were going to work.  Within a few days we had a set of command products ready to go.

A day before the event, to save time on the scheduled observation DSN pass, we loaded the new command sequences to LCROSS and hoped we’d gotten everything right.

August 16 (DOY 229): Family Portrait: Observing the Earth and Moon

Our Special Earth/Moon Look Cal went off pretty much just as it should have.  We collected images of both the moon and the Earth on all of the cameras, and met the full objectives set out by the Science team.  There were some minor issues with payload heaters, and a small command sequence timing error, but we were satisfied with the results.

The photo here shows a family portrait taken by the LCROSS visible camera.  At the time of the calibration, the LCROSS was approximately 520,000 km from Earth and 881,000 km from the moon.

August 19 (DOY 231): Re-Scaling our Faulty Coarse Sun Sensor

Recall from my previous post (“Our First Orbit Around the Earth”), one of our Coarse Sun Sensors (CSS 1) had been producing low signal outputs since Day 1 of the mission.  Now “South” of the moon’s orbit plane, on the second half of our second orbit around the Earth, we took the opportunity of an open DSN contact period to finally load parameters to the spacecraft that would effectively re-scale the output of CSS 1 to have it match the output levels of the other CSS’s.  This closed out another of our original spacecraft anomalies, and left LCROSS even safer.

August 20 - 21 (DOY 232 - 233): Planning for Cold Side Bakeout 3, Omni Pitch 4 and TCM 5c

We had a challenging few days ahead of us.  The Navigation team had identified another good opportunity to perform a third Cold Side Bakeout (unfortunately on a Saturday).  Then on Monday and Tuesday, we’d have a back-to-back execution of TCM 5c and another Omni Pitch maneuver.  We generated, tested and reviewed commanding products for all three events on Thursday and Friday.  Excited to observe our next Cold Side Bakeout, I elected to take the weekend DSN pass so that I could oversee the maneuver.

August 22 (DOY 234): Innocence Lost

I came onto shift on Saturday morning, very excited for a difficult, busy shift in overseeing Cold Side Bakeout.  This third repetition was planned to look a lot like the first one, which ended up being pretty exciting and somewhat complicated.  After the standard coordination with our Canberra antenna, we prepared for Acquisition of Signal or AOS.

We acquired telemetry, and ran through our standard set of health checks, but it was immediately clear that LCROSS was in trouble.  Inertial Reference Unit (IRU) faulted, Kalman Filter faulted.  Attitude control system using the Star Tracker for body rate information.  Propellant tank low pressure fault.  Thrusters firing nearly continuously.    

It’s hard for me to believe, but LCROSS is now only 18 days from impact.  After a long time away from the LCROSS Flight Director’s Blog, I wanted to catch you all up in the amazing journey LCROSS and the Flight Team have been through in the time since I last wrote. 

On my last post (from July 22), LCROSS had not yet completed its first orbit about the Earth.  We are now on our third and final orbit.  Since then, we have observed the Earth and moon from various distant vantage points in the orbit to calibrate our science instruments.  We’ve baked a lot of ice off of the surface of the Centaur upper stage.  We orbited the Earth freely, with rarely a problem at all, and those only minor ones.  Then in late August, nearly tragically, LCROSS encountered a major problem, which resulted in the loss of most of its propellant reserves, and could very easily have resulted in the end of our mission.  You’ve read the headlines, but I’m happy to report that we’ve climbed our way out of that very precarious position, and are now poised to meet our full set of mission objectives, with less than three weeks remaining until impact.  The Science Team recently selected the impact target – Cabeus A1 – and the LCROSS team is now making final refinements in the orbit to make sure we hit that spot.  This is an exciting time.

In this post, I’d like to provide a summary of the events following Lunar Swingby on our first orbit around the Earth.  The next post will cover our second orbit and the lead-up to our “anomaly”.  Then in following posts I’ll detail the anomaly, and provide some running commentary in the lead-up to impact. 

By Deep Space Network convention, our mission uses a “day-of-year” reference for dates, rather than standard calendar months/days.  So, January 1 is DOY 1, and December 31 is either DOY 365 or 366 (depending on whether it’s a leap year or not).  I’ll provide both for you here, so that you can interpret the orbit diagram I’ve included.  Each of the below events are shown along the flight path LCROSS took in its first trip around the Earth.  Each major “tick” mark is a 24-hour day.  Transfer Phase events are shown for reference.

June 23-25 (DOY 174 - 176): Taming our Thruster Thermal Control Problem

The Flight Team was working at full capacity during the first five days of the mission (Transfer Phase), with 24-hour operations, and major events happening daily.  It was an impossible pace to maintain indefinitely with such a small team.  But before we could ease off to our 4-out-of-72-hours contact schedule planned for Cruise Phase, we had to protect against some of the problems we encountered early in the mission, and build confidence that LCROSS would remain safe long-term. This took some extra work.

Just after Lunar Swingby, five days into the mission, we were still fighting our newly-discovered thruster thermal problem (see “Real-Life Operations: Day 3” post).  In a temporary move to keep them above the freezing point of our hydrazine propellant, our team watched thruster valve temperatures for T1 and T7 (both 5 Newton attitude control thrusters – see diagram), and fired specific sets of thrusters via ground commanding if we saw any of them get too cold.  This was not an effective long-term solution – we commanded the sequence from the ground many times per shift (very crew intensive), and this was also a waste of propellant.  But by the end of Day 6, we had loaded our first automated solution (no human intervention required) that performed the same sequence of thruster firings if either of the cold thruster valves fell below 7 degrees Celsius.  Though we refined the solution over the coming weeks, establishing this “safety net” was a major milestone in freeing up the Flight Team from having to staff 24 hours a day to keep LCROSS safe.

In a parallel effort, our team worked to find an orientation for LCROSS that would keep our cold thrusters warm using the sun, without firing thrusters at all – intended as a primary means of thermal control.  By June 25, we settled on a “yaw” bias of -20 degrees, tipping the “top” of LCROSS towards the sun slightly to warm the cold thrusters (T1 and T7 – see the figure).  We’re still using the same sun-relative orientation for LCROSS today, and plan to use it for the rest of the mission.

Thankfully, in the midst of this work, the Navigation and Mission & Maneuver Design teams informed us that we could skip our first post-Swingby Trajectory Correction Maneuver, TCM 4a.  It was always optional, but it was great to hear that Lunar Swingby was so accurate that performing this burn was unnecessary.

June 26 – July 14 (DOY 177 – 195): Gaining Confidence

Since our mission wasn’t certain how quickly we could transition to our Cruise Phase schedule, we grabbed as much leftover time on the Deep Space Network (DSN) as we could in the early days of the mission to allow a “gentle” transition to hands-off operations.  This wasn’t easy on the Flight Team – odd hours, numerous contacts, and lots of planning to do on the side.  We were anxious to scale back, but only when we could prove LCROSS would remain safe during long periods out of contact. 

In our new -20 degree yaw-biased sun attitude, LCROSS never triggered the automatic thruster firings to keep our two cold thrusters warm.  This was great news.  With automatic thruster thermal control showing promise, we moved to retire some of our other concerns with LCROSS.  Recall that the Centaur upper stage was leaking some of its residual propellant, and causing LCROSS to “fight” the resulting torque with thruster firings each day (again see “Real-Life Operations: Day 3”).  Well, our team observed a gradual but steady decline in the daily thruster firings to counteract the Centaur leak.  We were using only 0.24 kg/day at this point, easily sustainable for the remainder of the mission with our substantial propellant margin (warning: foreshadowing).  Good news for propellant usage, but the reduced thruster firing frequency also caused our cold thrusters to get colder!  Space operations are never simple.

We also got to skip Trajectory Correction Maneuver (TCM) 4b, scheduled for June 30 (DOY 181), our second skipped maneuver of the mission. Our orbit remained right on track stemming entirely from the accurate targeting of Lunar Swingby.

As we gained confidence, we eased off of our DSN contact schedule, de-staffing some contact periods, and releasing others.  This finally gave our team some much-needed down-time.  We also started figuring out our operational “rhythm” – what shift schedules worked and didn’t work, how to communicate information over the team without regular shift handovers, etc.  Simulations only go so far, and there’s nothing like flight experience to understand how things really work.  We were learning every day.

July 15 (DOY 196): Thawing the Centaur

As I describe in “Introduction to Cruise Phase”, we had anticipated that water ice might accumulate on the exterior of the Centaur prior to launch and remain there throughout the mission.  It was essential that we remove as much of that ice as possible prior to Impact, to avoid having it interfere with both our trajectory and water measurements at Impact.  Mid-July provided a great opportunity for our first attempt.

The plan for Cold Side Bakeout (CSB for short) was pretty simple.  From our sun-pointed (but yaw-biased) orientation, we would command the spacecraft to first remove the yaw bias, then to slowly rotate 180 degrees about the pitch axis (the long axis of the Centaur – see the figure) to point the solar array directly away from the sun, and the “cold side” of the Centaur directly towards the sun.  We would remain in that orientation for one full hour to warm the Centaur surface and to bake off as much water as possible.  At the end, we’d rotate back to our yaw-biased Cruise attitude and continue the mission. 

The risky part of CSB is that we’d be rotating the spacecraft in exactly the wrong direction for the Power and Thermal design, with the solar array dark and cold, and normally hot electronics equipment panels straight into sunlight.  Analysis indicated we could only tolerate this thermally for two hours.  We had to be on the alert to terminate the maneuver if anything heated or cooled more quickly than expected. 

The really interesting part is that our Navigation team selected July 15 for its particular geometry (with LCROSS passing through the ecliptic plane – the plane of Earth’s orbit about the sun) that would make it possible to actually measure the tiny change in LCROSS speed resulting from water escaping from the Centaur surface!  We were very curious to see what the measurements would tell us.

In practice, Cold Side Bakeout was a very interesting event.  Halfway around to the bakeout attitude, our Star Tracker (STA) suddenly dropped into Standby Mode just as it had before TCM 3 when we inadvertently pointed it at the Earth (read “Multi-Tasking: Day 4”).  We’re uncertain what caused the STA to trip to standby mode (it was not pointed at the Earth, moon or sun), but an enticing theory is that escaping water vapor from the Centaur scattered incoming sunlight and confused the sensor (though it may also have been sunlight reflected off the inside of the conical shroud protecting the STA).  Magically, as we pointed the Centaur cold side to the sun, the Navigation team noticed a Doppler shift in our ranging signal, indicating a change in our spacecraft velocity.  The Engineering team also noticed a change in the attitude control behavior, presumably due to a torque induced by escaping water vapor.   On its final assessment the following day, the Navigation had some startling news.  The impulse induced by the escaping water was actually 3.5 cm/second, nearly 3 times more than expected, and the effect did not dissipate over the hour-long bakeout.  This meant one thing – the Centaur was carrying lots of water, and we’d need to perform yet another Cold Side Bakeout in the future.

In a great act of forethought, one of our Flight Controllers, Matt D’Ortenzio, predicted that exposing all of our back-side thrusters to the sun during the CSB might actually prevent the thruster heaters from activating themselves later.  In our original implementation of the automated thruster thermal protection monitors, we only protected the valve temperatures for the two coldest of eight (T1 and T7) attitude control thrusters (all others were warming via heaters).  On the prediction that we might have trouble with others, we loaded similar monitors for the remaining six.  This proved to be a good call.

July 16-17 (DOY 197 – 198): Omni Pitch Flip and Thruster Thermal Flare-Up

On this day we made our second “Omni Pitch Flip” of the entire spacecraft (about the Pitch axis – see figure) to reorient the primary omnidirectional antenna toward the Earth again (see “Introduction to Cruise Phase”). 

Before we executed the Pitch Flip, however, we found that our automated thruster warming sequences had fired 70 times while we were out of contact with LCROSS.  The thruster 2 heater, which had up to this point properly activated to control the T2 valve temperature, had stopped activating.  The thruster now depended on our thruster firing “safety net” to avoid freezing.  Matt’s prediction of CSB had come true.  We were worried that perhaps we had induced a thermal instability – that the thruster would never settle back into its nice heater-based thermal control cycle.  It took another day before the T2 temperature equilibrated, allowing its heater to warm the valve without using additional propellant.  In the final assessment, we used on the order of 2.1 kg of propellant to fight the thermal issue after Cold Side Bakeout.  CSB had its cost.

July 18 – 20 (DOY 199 – 201): Steady as She Goes

LCROSS stayed healthy as we periodically monitored telemetry, downloaded and reviewed telemetry data stored onboard while out of contact, and performed a clock calibration.  Our thrusters continued behaving well, and we were burning less and less propellant in fighting the Centaur leak.  Things were looking great!

July 21 (DOY 202): Trajectory Correction Maneuver (TCM) 5a

As I described in “Welcome to Cruise Phase”, the LCROSS mission plan contained two major “delta-v” maneuvers, or changes in velocity, that had to be performed.  The remaining planned TCMs were “non-deterministic”, to be performed only as needed to clean up the errors stemming from previous maneuvers (e.g. minor mis-pointing, thruster performance modeling inaccuracies) and due to slight mis-prediction of effects on-orbit (e.g. solar radiation effects, effects of imbalance between attitude control thrusters).  We had been able to skip TCM 4a and 4b because of how well things had been planned and predicted by our Navigation and Maneuver Design teams. 

TCM 5a, however, was our second “deterministic” burn, meaning it had to be done.  TCM 5a, which used the larger 22 Newton thrusters, increased our velocity by 21.1 m/s, and was the biggest “delta-v” maneuver of the mission.  It went off just as planned, and put us on a collision course with the moon.

July 22 – 29 (DOY 203 – 210): Health Monitoring and Maintenance

Most of our time in this week was spent monitoring LCROSS health, reviewing both real-time stored telemetry data.  Because of onboard clock drift effects, we also re-calibrated the clock again to have it better match ground time.

Aside from monitoring, we loaded a new command sequence to the Data Handling Unit to control science instrument sampling for our upcoming Earth Look Calibration, our first payload calibration since Lunar Swingby.  We also downloaded the contents of all 10 of the DHU command sequences, to make sure none had been affected by radiation effects.  The Payload team determined everything was fine.

On Day 1 of the mission, we noticed that one of our Coarse Sun Sensors (CSS 1) was outputting abnormally low readings.  LCROSS uses the CSS’s to automatically point the spacecraft solar array toward the sun in Sun Point Mode (SPM), our “safe mode” for more severe fault management responses (and also the mode LCROSS “woke up” in on Day 1).  If you have a significant problem on the spacecraft, the first thing you want to do is switch to a safe power and thermal configuration.  The low output readings on CSS 1 meant degraded sun-pointing performance, potentially a risk if we ever had a serious problem.

Fortunately, LCROSS has two sets of CSS’s, called Primary and Redundant.  Upon entry to SPM, LCROSS defaults to the Primary set.  If the Primary set is malfunctioning, LCROSS will switch to the Redundant set automatically.  Unfortunately, CSS 1 was in the Primary set.  So, way back on Day 2, to avoid using CSS 1 by default, we designated the Redundant set as the default.  However, if the Redundant set ever malfunctioned, we wanted LCROSS to switch to the Primary set.  On July 23, we loaded a small flight software patch to allow LCROSS to switch automatically to the Primary set.  Meanwhile, we worked on an upload to actually re-scale the CSS 1 output to make it fall in line with the other CSS’s.  This is an example of the active maintenance that happened in various ways throughout the mission.

July 30 (DOY 211): Cold Side Bakeout 2 – Still More Ice on the Centaur

We executed a second Cold Side Bakeout maneuver at the end of our first orbit around the Earth, again near our passage through the ecliptic plane.  This time we rotated to the bakeout orientation more quickly (with high propellant expense), and faced the cold side of the Centaur to the sun for longer.  The good news is that Navigation saw a reduction of the delta-v effects due to escaping water – about 1/3 of what we saw for CSB 1.  However, there was still a substantial disturbance to our orbit, so we began looking for a third CSB opportunity in a future orbit.

All of the above happened in our first orbit around the Earth.  At this point, we were well on our way in the mission, having mastered our initial problems with LCROSS, having performed the largest TCM, and having removed a significant quantity of water from the Centaur.  However, we hadn’t operated the science payload instruments since Lunar Swingby.  This would be our first priority on the second revolution about the Earth.  In, my next post, I’ll pick up where this leaves off.

The most frequent questions I get about LCROSS go something like, “You’ve been flying for 'x' days…aren’t you at the moon already?”, “Why does it take so long to reach the moon?”, or “What are you doing with all the extra time between now and impact?” Admittedly, the answers are hard to communicate, because the LCROSS mission profile isn’t as straightforward as one might guess. 

Between Transfer Phase and Impact Phase is the longest phase of the LCROSS mission – Cruise Phase.  Transfer and Impact Phases are the most intense and busy, and therefore get the most attention.  Cruise Phase is when the Flight Team performs long-term preparations for Impact in every respect – we calibrate our science instruments, we better characterize our spacecraft, we finalize our lunar impact target and refine our impact trajectory.  Many of us on the Flight Team envisioned Cruise to be a lot less busy than Transfer, but our team has been steadily busy since Lunar Swingby.

Over the next two posts, I’ll introduce you to Cruise Phase, and get you all up to speed on what’s happened since Lunar Swingby so far.  Maybe in doing so, I’ll answer those questions that you’ve wanted to ask about Cruise Phase that seems, at first glance, to be nothing more than a long delay between the really cool parts of the LCROSS mission.

The Real Reason for Cruise Phase: Impact Energy

Cruise Phase is first and foremost about trajectory and impact energy, so I’ll start there.  So, why does it take so long to get to the moon, and why are we still orbiting the Earth?  Actually, it doesn’t take very long to get to the moon.  LCROSS passed by the moon on Day 5 of our mission, way back on the morning of June 22.  We could have impacted the moon back then, but our objective was to impart as much energy into our impact as possible, and it just wasn’t possible to do that on our first lunar encounter.  I’ll explain.

On our initial outbound trajectory from the Earth in the days after launch, we could have hit the moon’s equatorial region very directly, but the only way we could have impacted the lunar south pole is at a very low, grazing angle.  A low impact angle results in relatively little energy release.  Our goal is to hit our south pole target as steeply as possible, transferring all of our kinetic energy into the impact site, thereby raising the greatest amount of lunar material from the surface for analysis.  The only way to do that is to approach the moon roughly perpendicularly to its orbit plane, to smack the moon from “below”, at the south pole.  At Lunar Swingby, we used the moon like a slingshot to throw LCROSS from its Trans-Lunar Orbit, roughly in the same plane as the moon’s orbit about the Earth, into a steeply-inclined Cruise Phase orbit that would allow us to do just that.  So, we got to the moon in five days, but we never entered an orbit around the moon.  Instead, we kept on orbiting the Earth, but in a much different orbit than we started in, thanks to the help of lunar gravity as the moon flew by us.  That’s right…you read this right: when you look at the animations of the LCROSS trajectory, the moon had the larger component of velocity (as compared to LCROSS) relative to the Earth, and actually flew by LCROSS rather than the other way around.  LCROSS was at the right place at the right time to be thrown into its Cruise orbit!

This figure illustrates the Transfer Phase and Cruise Phase segments of the orbit, as seen for July 22, 2009.  The moon's gravitational influence threw LCROSS into the Cruise Phase trajectory where it will remain, orbiting the Earth, until October 9.  The moon has already made a full revolution around the Earth, and LCROSS is (at the time of posting) far below the moon's orbital plane, not quite 3/4 of the way around its first orbit around the Earth.  At the time of Impact, LCROSS will rise and intersect the moon's orbit just at the time the moon crosses the same point.  Courtesy of NASA Ames Research Center.

Now that we’re in the Cruise Phase orbit, we’ll orbit the Earth three times before we impact.  As of today’s post, we haven’t even made it around the Earth once yet, though the moon has made one orbit around the Earth since Lunar Swingby.  Among other things, Cruise Phase is a waiting period for when the orbits of the moon and LCROSS cross paths again on Impact day on October 9.  Next time, the moon will be squarely in our sights!

Trajectory Correction Maneuvers: Fine-Tuning Impact

At special positions in the orbit during Cruise, LCROSS will perform additional TCM’s (Trajectory Correction Maneuvers) to converge on the impact target parameters – position, timing, and impact angle.  Originally we had 10 Cruise Phase TCM’s planned: TCM 4a, 4b, 5a, 5b, 5c, 6, 7, 8, 9, 10.  Some of these “maneuvers” are mandatory, others are optional.  The mandatory ones cannot be skipped if we want to hit the moon.  In the LCROSS mission, these are TCM 1 (executed during Transfer Phase) and TCM 5a.  The others are optional, and are only performed if errors remain from previous burns, or stemming from other influences (e.g. solar radiation pressure, disturbances caused by our own attitude control thruster firings, etc).  It’ll be rare that we skip a burn opportunity, but it has happened already.  We skipped the TCM 4a and 4b opportunities because we were so close to our target trajectory after Lunar Swingby that performing the burns wasn’t worth the extra risk and effort involved in doing such maneuvers. 

These burns vary dramatically in magnitude.  Earlier burns change the orbit more significantly.  By TCM 10, we’ll really be polishing things.  To give you a feel for the difference, TCM 5a was designed to alter the LCROSS velocity by 21.1 meters/second (47 miles per hour).  By comparison, our final burn will be changing the velocity by less than 10 cm/s (0.22 miles per hour), perhaps even less.  However, when you think about it, this kind of refinement is necessary.  Imagine the effect of 1 cm/s of velocity error over 10 hours of drift time prior to Impact.  The distance adds up fast: 0.01 m/s x 3600 s/hr x 10 hr = 360 meters of position error!

The fewer burns we have remaining in the mission, the more “locked in” we’ll be to a very specific impact target position and timing.  We started the mission with quite a bit of flexibility in both, but that will soon change.  In fact, the LCROSS science team has to make its final impact target selection by 60 days prior to Impact.  Any later, and the propellant cost to change to a different target becomes prohibitive. 

Science Instrument Calibrations

Cruise Phase provides some nice opportunities to continue calibrating our science instruments before Impact.  Our most valuable calibration opportunity was Lunar Swingby, executed on Day 5 as the close of Transfer Phase.  The close range to the moon meant that the moon filled a high percentage of the fields of view of our instruments, thereby improving our resolution, etc.  As I mentioned, I’ll have the Science/Payload team provide a guest posting sometime soon to discuss those results so far. 

Cruise Phase never gets another opportunity like Lunar Swingby.  We only get that close to the moon once more - on our final approach before Impact.  It’s far too late to be worrying about calibrations by then.  However, there are times on each orbit when LCROSS gets closest to the moon and Earth (making them look bigger in the instruments), and these are valuable opportunities for follow-on calibrations. 

Our mission has three nominal calibrations planned: two “Earth Look” calibrations, and one “Moon Look” calibration.  As the names indicate, we’ll train the instruments on the Earth and moon, and perform a series of sweeps back and forth across their distant shapes to improve our knowledge of instrument pointing accuracy, radiometric response and spectral response to known inputs.  The Science team’s lessons from Lunar Swingby will guide the specific measurements and focus of these activities.

Omni Pitch Maneuvers: Staying in Contact

The Cruise Phase orbit takes LCROSS high above and far below the moon’s orbital plane on each revolution.  Our standard spacecraft attitude for Cruise places our long axis (the one common to LCROSS and the Centaur) perpendicular to the ecliptic plane – the plane of the Earth’s orbit around the sun.  There’s nothing particularly special about this orientation, but it makes things a lot easier to think about operationally, and that’s worth a lot.  The catch is that our primary low-gain (omni-directional) antenna points in one direction off one side of the spacecraft, and it’s not always pointed toward the Earth.  It enables communications over about a full hemisphere of angle, but as we orbit the Earth without changing orientation, the Earth will move into and out of the antenna pattern.  When Earth is out of the pattern, we can’t communicate with LCROSS (note that we have a secondary omni antenna on the opposite side of the spacecraft, but it’s less effective, so we try to stay on the primary whenever possible). 

This figure depicts the LCROSS shepherding spacecraft and the Centaur, shortly after separation.  The spacecraft depiction shows the location of the primary omni-directional antenna and the solar array, relative to the standard body reference frame axes.  Omni Pitch Maneuvers rotate the spacecraft about the Pitch axis to re-orient the primary omni antenna.  The Cold Side Bakeout maneuvers rotate the spacecraft 180 degrees about the Roll axis to orient the cold side of the spacecraft (and Centaur) towards the sun.  Artwork courtesy of Northrop Grumman.

The remedy is to flip the spacecraft 180 degrees, keeping our solar array pointed at the sun, but turning the omni antenna pattern to the opposite direction.  At the time of the flip, neither the starting nor the ending orientation is very good for communications.  However, if left in the starting orientation, the Earth would move out of the antenna pattern.  By flipping the antenna (and the spacecraft) over, the Earth will tend to move deeper into the pattern over time.  These flips are rotations about the spacecraft “pitch” axis, and so are officially called “omni pitch maneuvers”.  During Cruise Phase, we have to perform these roughly every two weeks, though the timing depends on the specific orbit geometry.

Cold Side Bakeout: Getting the Water (and Other Stuff) Out of the Centaur

The Centaur upper stage (our impactor) is covered with material that is naturally absorbent and was expected to absorb some water from the atmosphere when sitting on the pad in the humid conditions in Florida.  In fact, if you watch the LCROSS launch video closely (the Centaur-mounted camera view), you can see ice accumulated in ridges on the vehicle surface, frozen by the extremely cold temperatures caused by the cryogenic contents (liquid oxygen and hydrogen) of the Centaur propellant tanks. 

In space, this accumulated water might melt or sublimate (a straight conversion from ice to vapor) if exposed to the extreme heat of the sun.  This is what we expected the water to do on the side of Centaur aligned with our spacecraft’s solar array, since that side is almost always pointed toward the sun.  After several days, we expected all of the water on the sun side of the Centaur to be removed naturally. 

Water trapped within the very cold foam on the opposite side of the Centaur, however, might remain for a lot longer, perhaps through the end of the mission.  In fact, Lunar Swingby spectrometer measurements show a significant water signal (along with other exotic hydrocarbons) – given the dry mid-latitude targets we were measuring, the water must have come from the spacecraft, and the strongest possibility is that it was from ice debris floating around the spacecraft.  There are two problems with water trapped on the Centaur.  The more obvious of these, but actually the less problematic, is that LCROSS is trying to find water on the moon.  Bringing water to the moon from the Earth will only confuse the issue!  Our Science team is actually not so worried about this, because we’re talking about only 10 kg of water or so, and they wouldn’t expect to “see” such a small amount relative to the hundreds of tons of material LCROSS is expected to loft and the percentages (albeit small) of that material expected to be water.  Still, it’s a concern.

Interestingly, the larger problem is that water escaping from the Centaur is predicted to cause a disturbance to its trajectory (via conservation of momentum).  The worry is that in the hours before Impact, as the Centaur is descending alone to the lunar surface, its shadowed side will be exposed to sunlight, causing a release of water and a notable trajectory disturbance.  A significant release of water could turn a perfectly-aimed impactor into a poorly-targeted dud.  We’re talking potentially 100’s of meters error or more!  It’s hard to believe that water escaping the surface could cause that much harm, but welcome to space physics.

To prevent this from happening, we’ve scheduled a Cold Side Bakeout maneuver that rotates the spacecraft about its long axis to point the solar array directly away from the sun, and bakes the water right out of the Centaur in full sunlight.  Over enough exposure time, the water should vent out of the Centaur.  The challenge is that LCROSS is designed to spend all but short periods of time pointing its solar array toward the sun.  Thermal and Power subsystem flight rules prevent long dwells in this opposite orientation.

Other Engineering Stuff

Aside from giving us time to conduct all of the above activities, Cruise Phase is a chance to get more familiar with our spacecraft so that we can totally predict how it will behave at Impact.  Recall that we came out of Transfer Phase with a few “anomalies”, or unexpected problems.  The Flight Team needs time to sort through all of the issues, starting with the most severe ones, and gradually moving down to the more benign ones.

Our goal is to figure out why each of the problems is occurring, and then to come up with ways to work around the problems, or to show that their behaviors are completely benign and not worth worrying about.  Each must be considered in terms of risks against achieving our mission objectives.  Our team can only bring an anomaly investigation to closure by proving to our mission stakeholders (NASA Ames, LPRP and NASA Headquarters management) that our mitigating strategies maintain our risk of failure to acceptable levels.

Flight Team Planning, Training and Rest

As Flight Team Lead, I’m responsible for creating and modifying the short and long-term plans for the mission, including Cruise Phase.  Before launch, most of our time was spent preparing for the first week of flight.  Once Lunar Swingby came and went, I had to drop into overdrive to get our Cruise Phase planning in order.  This meant (and continues to mean) nailing down times for TCM’s and other maneuvers (courtesy of our MMDS team – see the post on “LCROSS Flight Team Breakdown”), nailing down DSN contact times (via our CLASS team at Ames and JPL), putting together staffing schedules, etc. 

Then there’s planning for Impact.  Again, our team spent the 2-3 months prior to launch working out final details on Transfer Phase operations.  We last practiced Separation and Impact in February.  We have things worked out pretty well, but now we need to practice some more so that we nail our Impact in October.  As Flight Team Lead, I am responsible for setting down the rehearsal schedule, working with the CLASS team to schedule DSN readiness tests, and refining our procedures as needed.

When we’re not planning for our next activity, coordinating on anomaly resolution, working out schedules, or training for Impact, the Flight Team has a little extra time to rest than it did before launch, and during Transfer Phase.  We’re certainly not avoiding work – it’s what you as a taxpayer are paying us to do.  But the Flight Team has been working full steam ahead for six months or more, and now that the schedule is a little more relaxed, we can afford to take some hours and days off here and there.  It’s not much, but it sure helps!

This is it for now.  In the next post, I’ll present a brief diary to catch everyone up on all the activities we’ve performed since Lunar Swingby! 

All of you keeping up with LCROSS have undoubtedly already heard the good news – that our team at NASA Ames successfully performed our capstone event on Day 5 of the mission – the Lunar Swingby.  In execution, it worked just as we had hoped.  But getting it to work wasn’t easy.  It took us many months of development, testing and updates to get the procedures right, and throughout our months of training, our team rehearsed this event far more than any other. 

Lunar Swingby (LSB for short) is one of two close lunar encounters during the mission – in many ways it was a dress rehearsal for impact.  It’s an intricate coordination between orbit dynamics, attitude control, communications, thermal, power and of course, the science payload.  In this post, I’d like to walk you through the evolution of the procedures and command sequences that we ultimately flew on Day 5.

An Introduction to Lunar Swingby

So, what was LSB designed to do?  There were two primary goals – one, to throw LCROSS into its phasing orbit, called the LGALRO, or Lunar Gravity Assist Lunar Return Orbit.  As that name suggests, our mission uses the gravitational well of the moon to drastically change the Transfer Phase orbit without any propellant cost.  The post-LSB orbit is designed to hit the moon with one additional Trajectory Correction Maneuver (ideally), enabling the steep impact angle at the lunar south pole that otherwise would have been impossible to achieve using the outbound Transfer Phase (trans-lunar) orbit we were provided by the Atlas/Centaur. 

The second goal of LSB was to calibrate the science instruments, taking advantage of the relatively close range of LCROSS to the moon.  The calibration was divided into two phases: one to measure lunar surface targets, and the other to sweep the payload over the “limb” of the moon (the “limb” is the edge of the apparent disk of the moon).  Looking at lunar surface targets with known mineralogy and optical properties (albedo) allows our science team to calibrate the cameras and spectrometers.  Having “ground-truth” means they can compare the actual instrument outputs against expected outputs, knowing how the instruments should respond to the known surface properties. On the other hand, sweeping the payload over the lunar limb allows the science team to calibrate the payload alignment.  The LSB incorporated “slews”, or changes of orientation, that alternately pointed the instrument boresights off the moon, then on the moon, back and forth, such that the boresight would inscribe a path perpendicular to the edge of the lunar disk.  Since we know the spacecraft orientation accurately, and we know the precise positions of the moon and the spacecraft, we can predict, for a given alternating sequence of slews, the precise timing of when the boresight should cross onto and off of the lunar disk.  A consistent bias between the predicted and measured timing indicates an alignment offset. 

LSB was designed to measure three surface targets for 5 minutes each (three craters: Mendeleev, Goddard C, and Giordano Bruno), and two limb targets, one in a “left-right” slewing orientation, and a second using an “up-down” orientation to detect misalignments in both possible directions.  Each of these limb sequences was also 5 minutes long.  Sounds pretty straightforward, right?  Well, the details make things tricky.

For reference in the next few sections, check out the link to an animation of the "as-executed" Lunar Swingby. The animation is sped up so that you can see the various sub-events (slews from Cruise attitude to initial sampling attitude, surface samples, limb slews, etc). Vectors coming out of the spacecraft model indicate the direction to the Sun (yellow) and Earth (Blue), and the body-fixed vectors representing the LCROSS body coordinate frame (Red).  The projected square represents the field of view of the LCROSS visible camera, and the narrow cone projection represents the 1.0 degree field of view of the nadir-pointing Near-Infrared Spectrometer, the most important instrument for LSB science return.  The little bullseyes are our surface targets.  See if you can identify the "setup" and "cleanup" attitude changes from those between targets.

LSB Constraints

To get into LSB constraints, let’s continue with the discussion of pointing the spacecraft to the targets.  The requirement was to point to within 1 degree of the intended target, and to slew no faster than 0.15 deg/s during measurements to avoid “smearing” our images (since we don’t have a super-fast effective “shutter speed”).  For LSB, we designated an attitude control mode that would maintain our orientation to within 0.5 degrees of a given target orientation.  The trick is that our spacecraft cannot autonomously identify the targets in an image, nor can it track them automatically once we point it in the right initial direction.  So, we had to tell the spacecraft where to point each few seconds, relying on knowing the precise position and orientation of the moon, as well as the position of the spacecraft (via orbit predictions) to predict the right set of orientations that would point the science instruments to the targets.  This means the whole sequence of orientation targets had to be planned in advance, but no earlier than several hours prior to the LSB event, since with TCM 3 happening the day before LSB, we couldn’t possibly know our orbit accurately enough to meet the pointing accuracy requirements.

To make matters slightly more complicated, instrument pointing was not our only concern with spacecraft attitude.  LSB and Impact both call on our highest available communications downlink rate – 1 megabit per second (Mbps).  This rate is only possible via one of two Medium Gain Antennas (MGA’s), and typically would require the use of a DSN 70 meter antenna.  For lower rates, our Omnidirectional antennas provide adequate link margin in almost any orientation.  The MGA’s, however, need to be pointed to within 20 degrees of the Earth to enable a strong signal to the ground, with more precise pointing providing better results.  With two attitude constraints, one for the instruments and the other for communications, our attitude was fully constrained.  Any other orientation requirement would have to be balanced against sacrificing one of the first two.  So, for example, we’d have to deviate from our normal “happy” orientation pointing the solar array at the sun during all of these observations.

This brings me to thermal and power constraints.  Both the Power and Thermal subsystems were designed to take only short deviations from the nominal spacecraft sun-pointed orientation.  Prior to flight, our Power subsystem was predicted to allow two hours off-sun.  Our Thermal subsystem’s constraints were a little more complicated, but it was predicted to enable up to one hour in some orientations, and two hours in other orientations.  We’d have to follow the one-hour limit since we couldn’t rely on satisfying the two-hour orientation constraint.

What about the science instruments?  The Data Handling Unit (DHU) is a separate computer that controls the science payload.  The DHU includes 10 buffers in non-volatile memory to store instrument command sequences.  The Science team defined different sequences for different purposes.  Originally, the Science team defined three sequences for the nominal LSB – one for surface target sampling, and another two for the limb measurements.  Later, as I’ll describe below, they had to define an additional three to cover contingency cases.  Another job of the operations team was to perform power-up and power-down of the DHU and operational heaters.  Thermal constraints dictate that payload operational heaters must be enabled at least 4 hours prior to using the science instruments to warm them to their operational temperature range.  Furthermore, the DHU, running with the full set of instruments, cannot run for more than 60 minutes without risking overheating.  Finally, the spacecraft command sequences, had to coordinate the execution of the LSB science command sequences with the commands to control the spacecraft attitude so that everything happened at the right times.

The following is a depiction of our surface and limb targets for LSB.

Wringing Out the Nominal Procedure

Last September, we thought we had a pretty good handle on flying Lunar Swingby (recall that our original launch date was October 28, 2008, so we had to be ready).   Due to the precise timing required to get this right, all of the LSB commands were originally packaged into an onboard command sequence, including several hundred attitude commands and other commands to power the DHU on and off, to start DHU command sequences, and to configure our communications system.  We had a nominal procedure that worked well in simulation, and both shifts had practiced it enough to gain confidence that we could do this in flight.  We had already tackled many of the issues, but there were problems. 

For one, we were exceeding our one-hour thermal time constraint.  The science calibrations were predicted to require a total of 45 minutes (five targets, five minutes each, plus four five-minute slews between targets), leaving only 15 minutes for other preparation and cleanup steps.  The initial reorientation from our Cruise attitude (which started the thermal clock ticking), the communications configuration steps, and the return to Cruise attitude (ending the thermal clock) required another 25-35 minutes, so we exceeded the limit by 10-20 minutes.  Ultimately, this forced our lead Thermal Engineer to revisit the thermal subsystem analysis, and to prove that the Thermal system could actually handle up to two hours off-sun in almost any orientation. 

Another LSB problem was with attitude control fault management.  As I mentioned earlier, to achieve the accuracy needed for science calibrations, LSB needed to employ an attitude control mode that enabled ]0.5 degree pointing precision.  Early tests of our command sequences worked just fine.  However, this mode was not originally designed with LSB slews in mind.  One day in the middle of an LSB simulation, our simulated spacecraft suddenly reset and dropped into Survival State, terminating LSB.  As we later discovered, we had exceeded the maximum attitude rate limit for the ACS mode we were using (the limits were tailored for near-stationary operation), causing a significant spacecraft fault management response.  Later versions of our procedures added steps to temporarily adjust the rate limits for that control mode to allow the spacecraft to slew a little faster without tripping fault management.

While testing LSB, the Flight Team also was temporarily stymied by a bug on the spacecraft that seemed to be activated when flowing high-rate data from the science payload (as happens during LSB).  In some runs, we found that the bug prevented telemetry and science data flow, and even prevented most commanding - a really bad problem to have in the middle of such an important event.  Over many trials, we found the only effective Flight Team response was to reboot our spacecraft processor – which would also put the spacecraft into Survival State and totally remove hope of successfully collecting our calibration data.  This behavior had been observed in earlier, lower-level tests of the payload and spacecraft, but had never reared up to affect operations.  Our operational discovery placed LSB in serious jeopardy.  After a lot of very careful debugging, our Northrop Grumman partners ultimately discovered the cause of the problem – an FPGA (Field Progammable Gate Array) that encoded the communications protocol had a programming bug in it, sourcing from a faulty specification of the protocol.  Once the problem was isolated, NG wrote software workarounds that enabled us to completely avoid the problem.

Contingency Responses

Once our original launch date had slipped briefly to December 2008, and ultimately into 2009, the Flight Team began focusing on “risk buy-down”.  With every aspect of our mission design, we asked ourselves, “with this new-found time, how could we make our mission less risky?”  With all nominal procedures worked out, we began to work on possible contingencies – things that might go wrong and cause LCROSS to have a bad day, either through loss of important data, or worse, a loss of spacecraft.

What were the big ones for Lunar Swingby?  LSB payload calibration data is really important to the Science Team.  I mentioned the high downlink rates required that we use our Medium Gain Antenna, and a DSN 70 meter ground antenna.  What if one of those two elements failed?  Could we still get enough data back to make our Science Team happy?

In November and December 2008, we worked on two low-rate contingency options, one assuming an MGA failure that utilized the Omnidirectional to 70 m antenna, the other assuming a DSN 70 meter antenna failure, utilizing the MGA and a 34 m antenna.  We spent hours working through how to address these failures, and it became very clear that the structure of the nominal procedures and command sequences was not well-suited to the contingencies.  Our challenge: at the time the failure is detected, the contingency response was to reconfigure the spacecraft for a lower downlink rate (256 kbps), then to run a specially-tailored low-rate DHU instrument command sequence that would approximate the higher-rate data return, with minimal sacrifice.  The trick was that things had to be done pretty quickly (within 10-15 minutes in some cases) and, depending on when the failure happened, it required different responses.  For example, if the DHU and instruments already running, the response was very different than if they were still several minutes from powering up.  Doing the wrong thing could get the team tied in knots and make a bad problem worse.  The nominal commanding structure made it very difficult to do this in a timely way. 

We went back to the drawing board to re-structure our commands and procedures to suit both nominal and contingent execution.  Ultimately, the changes we came up with were significant, but simple, and the result was far cleaner.  However, in practice, things weren’t yet up to snuff.  January Operational Readiness Tests (ORT’s) proved our initial contingency solutions were technically correct, but too slow – they required lots of human detective work and decision making under time pressure.  The right decision made too late was not much of a help.  In February, after multiple full-team tests, and hours more re-work, we finally arrived at a solution that addressed both the MGA and DSN 70 meter failures under a common approach that streamlined the decision process dramatically.  As a side bonus, the same approach provided a contingency response option for similar failures for the Impact procedure.  We finally had this one nailed down.

Aside from the low-rate contingency responses, we developed a few other major contingencies, including a general abort response that would enable our team to return to nominal Cruise state in the event of an unspecified problem aboard the spacecraft, and another response that would, in the event of a fire in our building or an evacuation, would enable the Flight Team to leave the control room and still collect LSB data successfully.

Details, Details: Deep Space Network Scheduling 

As we waited for launch, now moved to April, May and then to June, our team worked with JPL to finalize our Deep Space Network schedule.  Before launch, every mission that uses the DSN has to negotiate antenna time to support early-mission communications between the mission operations team and the spacecraft.  Typically, negotiations happen between the new mission team and teams representing other spacecraft that are already flying.  Newly-launched spacecraft tend to get high priority, since every team needs to have a lot of dedicated time at the beginning to get into orbit, for commissioning and to work out problems.  Our launch was a little different because there were two new spacecraft (LRO and LCROSS), both going to the moon, and both relying on DSN for critical periods.  As we worked through the details, we discovered that LRO and LCROSS were in conflict for two big LCROSS events: TCM 1 and, you guessed it, Lunar Swingby. 

Lunar Swingby coincided with LRO’s Lunar Orbit Insertion burn (LOI), which would place LRO into orbit about the moon.  Even though LSB was quite important to LCROSS, LOI was mission-critical for LRO.  Missing LOI would result in a total mission failure for LRO, so LCROSS had to take back seat for DSN scheduling in this case.  To ensure LOI success, LRO requested dual