Enceladus Flyby Underway

Amanda HendrixAmanda Hendrix, Cassini Scientist on the Ultraviolet Imaging Spectrograph (bio)

Well our flyby sequence has officially started!! Last night we began our observations of Enceladus! We are very distant, but getting closer all the time, over the northern hemisphere.  The first observation was a long stare at Enceladus, which is still pretty far away and small, but this is a nice opportunity to do compositional measurements. As of 9 a.m. Pacific, radar observation of Enceladus began, which will give us an idea of the roughness of this side of Enceladus, at centimeter scales.  The closest approach is around 1 p.m. Pacific today.

The entire flyby sequence is on-board the spacecraft, and there’s really no opportunity to change it at this point. We’re in it for good. However, the sequence gets thoroughly tested prior to uplink, so we are confident that things will go smoothly. The next time we hear from Cassini will be tonight after the flyby at around 7 p.m. Pacific.  We are being fairly cautious, though: even though Cassini will come about 30 miles of the surface, while flying through the plume we will be 120 miles from the surface. So we’re “dipping our toes” in the plume a little more than we’ve done before!

Cheers from Houston,

Main Engine Cover Successfully Closed

Todd BarberTodd Barber, Cassini Lead Propulsion Engineer

Greetings again from Cassini engineering, as we are literally a day away from our date with destiny and a super-close Enceladus flyby. Another engineering event, critical for the health and safety of the spacecraft, has gone off without a hitch. Earlier this morning, around 4 am Pacific Daylight Time (PDT), we closed (or deployed, in our nomenclature) the main-engine cover. As we planned to head in closer to Saturn and Enceladus, we recognized the potential for a slight dust hazard, not uncommon during our multi-year orbital tour of the ringed planet. Even with minute dust grains, our delicate main-engine columbium coating could be damaged with hypervelocity impacts, so occasionally we have to deploy our “baby-carriage” cover to protect our twin main engines.

Our thermal control and devices team reports to me yet another successful deployment of the main-engine cover, an event that has taken place roughly three dozen times since launch over ten years ago. I can usually tell when this happens as a propulsion engineer because the closing of the cover helps to warm up the area around the engines, causing the temperature readings I monitor to increase. After the dust hazard is behind us, we’ll again open (or stow) the main-engine cover, although this will occur around 7 pm PDT on Wednesday. In other words, this event will occur after the thrilling E3 flyby, an important engineering event to be sure, but likely one that will be buried in the excitement of the prospect of new Enceladus science results mere hours away. 


John SpencerJohn Spencer, Cassini Scientist on the Composite Infrared Spectrometer (bio)

As I write this we are just minutes before closest approach.  The spacecraft is now completing the big turn to get its mass spectrometer and dust instruments facing forward, to best sample the plume as we fly through.  I have the simulator that we use for planning the observations open on my desktop, so I can ride along in my imagination as the flyby happens.  You can ride along too, using the cool CASSIE tool on the Cassini Web site http://saturn.jpl.nasa.gov/multimedia/CASSIE/ .  After closest approach, we’ll begin the temperature scans of the south pole.  Weeeeeee!

Cassies first screen

Enceladus Approach Maneuver Cancelled — We're Good to Go!

Todd BargerTodd Barber, Cassini Lead Propulsion Engineer (bio)

Monday greetings from the engineering side of the Cassini flight team!  I’m very happy to report via this blog that we just decided to cancel the final Enceladus approach Orbit Trim Maneuver (OTM), OTM-148, and its back-up maneuver labeled JTM-148.  Since these burns were scheduled after midnight local time in California last night and tonight, respectively, I don’t think many of us will mourn their cancellation.  These final targeting maneuvers for Wednesday’s thrilling close flyby of Saturn’s icy companion were deemed unnecessary, largely due to excellent performance at OTM-147 four days ago.  In fact, we were able to save a little bit of propellant for the mission overall by canceling these maneuvers!
We took an engineering image of Enceladus yesterday evening, a so-called “optical navigation” or “op-nav” image.  Rather than being used for science, we actually used this image to measure the position of Enceladus very accurately with respect to known background stars.  This helped us improve our knowledge of Enceladus’ location, and the result of this latest op-nav is that Cassini remains on target for its historic rendezvous with Enceladus and its icy south polar plumes in two short days.  We in engineering wish our science colleagues on the mission a very fruitful and eye-opening close encounter with one of Saturn’s most intriguing moons. 

Optical Navigation Images

 Todd Barber, Cassini Lead Propulsion Engineer

Todd Barber Ok, folks, some readers have expressed additional interest in the Optical Navigation (op-nav) images of Enceladus we took on Sunday night.  They were posted within our raw image gallery , but I thought it would be a good idea to have them linked here.  If nothing else, these images remind us of the scientific thrills to come in a few short days!
Enceladus The Enceladus_no_zoom image (at left) shows the entire frame, with exposure times  optimized for Enceladus.  In this view, it is difficult to discern background stars. 

One can zoom in on this image of Enceladus, and that’s seen in the image Enceladus_zoom (the next image down).  These images, though intriguing and titillating, aren’t too useful for measuring the relative position of Enceladus.



< Enceladus_zoom

The next three images have a longer exposure time, in order to capture background stars.  Enceladus_stars_bg_noID may not win any photo contests, but all the important stuff is there — a brightly lit crescent Enceladus, background stars, and even some typically seen cosmic ray hits.  The stars seen in this image may seem very nondescript to you and me, but our op-nav experts know exactly which stars are which, thanks to excellent astrometry from orbiting telescopes like Hipparchos. 


< Enceladus-stars_bg_noID

The picture labeled Enceladus_stars_bg_withID identifies a few background stars seen in the prior image.
Enceladus < Enceladus-stars_bg_withID
Finally, Enceladus_stars_nobg_no/WithID just represents some quick-and-dirty image processing to remove the
artifact of horizontal banding seen in earlier images.
Enceladus < Enceladus_stars_nobg_no/WithID
The bottom line is our op-nav team helped nail Enceladus’ location within a few kilometers (or miles) with a
60-millisecond peek at Saturn’s orbiting ice ball. Not bad!


The Gory Details

John SpencerJohn Spencer, Cassini Scientist on the Composite Infrared Spectrometer (bio)

For the nerds out there (you know who you are), here’s a somewhat technical presentation that I gave a couple of weeks ago to the rest of the Satellites Orbiter Science Team (SOST) — the group that plans the details of all the non-Titan satellite encounters.  SOST holds a “preview” telecon before each major satellite encounter, where each team reminds the rest of us what their instrument will be up to, so this presentation summarizes what the Composite Infrared Spectrometer (CIRS) will be doing during tomorrow’s Enceladus flyby.

CIRS_061EN_FP3HOTSPT001 is the observation that I’ll be pouncing on once the data are calibrated and available on Thursday.  Occupying the time between 15 and 63 minutes after closest approach, it will give us by far our most detailed look so far at the heat from the south polar fractures.  As you see on slides 6 and 7, that one observation has a lot of different things crammed into it.

Here’s a secret decoder ring to translate some of the Cassini-speak in the presentation:

CIRS: The Composite Infrared Spectrometer.  CIRS has three different focal planes sensitive to different wavelengths of infrared radiation.  Yes, there is no focal plane 2- it was eliminated early in the design phase as a cost-cutting measure.

FP1:  CIRS focal plane 1, which measures long-wavelength heat radiation (wavelengths longer than 16 microns).  These are the wavelengths where most of Enceladus’ heat is radiated, so we hope to get improved measurements of Cassini’s total internal heat flow from this part of CIRS.  FP1 has rather coarse spatial resolution, represented by the red circles on the observation preview diagrams.

FP3:  CIRS focal plane 3, measuring shorter-wavelength radiation (wavelengths between 10 and 16 microns).  This detector can see much finer details than FP1, and is good for measuring the higher temperatures along the tiger stripes.  The FP3 field of view is  represented by the pink rectangle, which contains a row of ten pixels, each making independent measurements.

FP4: CIRS focal plane 4, measuring radiation at wavelengths shorter than 10 microns.  Enceladus doesn’t put out much radiation at these short wavelengths, unless we find some *really* hot spots along the tiger stripes, but FP4 will be recording data anyway, just in case.

Rev. 11, 32, 61:  Cassini orbit numbers.  Rev. 61 is the encounter coming up tomorrow.

C/A-02:30, etc.:  Times measured relative to closest approach time, in hours and minutes.  Closest approach is at 19:06:12 Universal Time on March 12th.  Other times are in Universal Time.

CIRS_061EN_FP1INMAP001, etc.:  The official names used to identify each discrete Cassini observation.

Bolometric Albedo:  The fraction of the sunlight hitting Enceladus (at all wavelengths), that is reflected back into space (in all directions).  The remaining sunlight is absorbed by the surface and heats it, so understanding Enceladus’ bolometric albedo is important for understanding out how much heat escapes from the interior.  Enceladus’ bolometric albedo is about 0.8, higher than for any other known planetary body, due to its coating of plume fallout, which is almost pure, white, water ice.

Thermal Inertia:  A measure of how well a planetary surface can store heat.  A high thermal inertia surface will not cool down much at night, while low thermal inertias mean rapid nighttime cooling.  Again, it is important to understand Enceladus’ thermal inertia to separate out the warmth of absorbed sunlight from the warmth of its internal heat.

MAPS: Magnetospheric and Plasma Science, the group of instruments on Cassini that directly measure gases, plasmas, dust, and magnetic fields.  Tomorrow’s flyby is a very important one for the MAPS instruments, as they directly sample the material in the plume as we fly through.  MAPS will be in charge of the spacecraft for the critical few minutes around closest approach.

Plume source VI: The source of one of the jets identified from analysis of the Cassini plume images, by Joe Spitale and Carolyn Porco in a recent “Nature” paper.

Hot spot C: A discrete region of high surface temperatures seen by the CIRS instrument during the July 2005 flyby, listed in a 2006 “Science” paper by myself and the rest of the CIRS team.  Plume source VI seems to correspond to hot spot C, so that’s a region we’ll  
be focusing on.

Here’s the full presentation.

Wish us luck!

Holding our Breaths

John Spencer

John Spencer, Cassini Scientist on the Composite Infrared Spectrometer (bio)

Two days before the close encounter with Enceladus, there’s not much for us on the science team to do regarding the upcoming flyby except to hold our breaths and cross our fingers.  We made our plans months  ago, haggling over the details of where the spacecraft would point, and when, and for how long, and how much precious onboard storage space we could use.  As I type, the spacecraft is arcing high above Saturn’s north pole, scanning the heat radiation from Saturn’s rings. 

It is already beginning its million-kilometer (600,000 mile) plunge towards the equatorial plane, where it will brush past Enceladus at a range of just 50 kilometers (30 miles) and a speed of 14.4 kilometers per second (32,000 miles per hour).  I’m glad someone else is driving…

So I’ve been spending the morning drafting plans for our Composite Infrared Spectrometer (CIRS) observations on the next Enceladus flyby, which is on August 11th this year.  We are blessed with four Enceladus flybys in 2008, each with rather similar geometry (approach over the north pole, depart looking back at the south pole), but we’ll be doing something different on each of them.   
On this Wednesday’s flyby, Enceladus plunges into Saturn’s shadow right after closest approach, preventing high-resolution imaging of the south pole in sunlight.  This is perversely good news for us, because our infrared detector can see in the dark, so we can do our thermal scans of the heat from the active south pole without competition from the Cassini cameras.  On the August 11th flyby, however, Enceladus will be in sunlight as we look back at the south pole, so it will be the camera team’s chance for super-high-resolution imaging of the strange south polar landscape.  The Composite Infrared Spectrometer will take over later, when Enceladus again goes into Saturn’s shadow, so those are the observations I’ve been sketching out today.  I just e-mailed my suggestions to my colleagues John Pearl and Marcia Segura at Goddard Spaceflight Center, where John will review their scientific value and Marcia will begin the process of translating them into detailed designs.
I’m also trying to get some non-Enceladus related work out of the way before I get my hands on the new Enceladus south polar thermal scans sometime on Thursday.  Once we have those data, I don’t expect to get 
much other work done for a few days.