|Posted on Oct 08, 2009 02:51:54 AM | Paul.D Tompkins | 53 Comments ||
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!
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