science – Page 3

Enceladus Play-by-Play Movie

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

This is a neat movie (click here to download or click here to watch it) put together by Brent Buffington, on the Cassini Navigation team. I love these kinds of movies, because you can really get a good idea of what the spacecraft is doing during a flyby. Everything goes pretty fast, so you may want to watch this a few times to really absorb what’s going on!

In the big left-hand window (you can see a still from the first frame of the movie below), you can see Cassini and a projection of the active (or “prime”) instrument’s field-of-view (FOV) (see key below). The right-hand windows show a view of what the boresights are seeing. Sometimes they’re staring, sometimes they’re scanning. Generally, since the optical remote sensing (ORS) instruments are pretty much bore-sighted (as shown in the upper right window), that means that when one of them is “prime,” the others are “riding along” and also getting data, resulting in a wonderful multi-wavelength suite of measurements.
view of Cassini video
So we start off with a long, distant stare, with the Visual and Infrared Mapping Spectrometer (VIMS) prime. Even though Enceladus is still far away and small, staring for such a long time gives all the optical remote sensing instruments a nice chance to build up signal and get a good spectrum, useful especially for investigating surface composition.

After VIMS, ISS (i.e., the camera) takes over for a stare at Enceladus (notice that Enceladus is still smaller than a Narrow Angle Camera–NAC, here). Also notice that the phase angle is pretty high, meaning that the body is only partially illuminated and will appear as a crescent (not unlike our own Moon last night!).

After the cameras, radar does a scan of Enceladus. To do this, we have to turn the spacecraft 90 degrees, since the radar is mounted on the spacecraft at a different orientation from the cameras.

After radar, the Composite Infrared Spectrometer (CIRS) is prime and does a scan and a stare to get temperature information, of the north polar region.

Then Ultraviolet Imaging Spectrograph (UVIS) does a slow scan of Enceladus to do surface composition (and potentially gas) measurements.

The Composite Infrared Spectrometer then does another scan (notice that we are getting pretty close to Enceladus and it’s looking bigger!).

For the final remote sensing observation before closest approach, the camera system does a three-panel mosaic of the cratered northern hemisphere. This will be our best look yet at this terrain.

After the cameras, the spacecraft does a big turn to put the in situ instruments’ sensors into the proper orientation for the closest approach (facing into the direction of motion, so they can “scoop up” particles around Enceladus). Remember that the closest approach (about 50 kilometers, or 31 miles) occurs near 20 degrees south latitude – not in the plume. About a minute after the closest-approach, we’ll be over the south pole and in the plume (at about 641 kilometers, or 398 miles). So we’re diving in the plume a little more than we’ve done before! The in situ instruments will get great measurements of particle size and composition, and gas composition, both in the plume and close to the surface of Enceladus.

Now, it isn’t obvious from this movie, but Enceladus goes into eclipse (i.e., it passes into the shadow behind Saturn), very close to the time that Cassini is making its closest approach, and remains in eclipse for a couple of hours. This is a prime opportunity for the Composite Infrared Spectrometer to do temperature mapping of the hot, active south pole region (which is now in view, after Cassini has swung through its closest approach), with no contribution or heating from solar illumination. The other remote sensing instruments will ride along, but without light from the sun, it isn’t clear exactly what they’ll measure – so that will be a surprise! While Enceladus is in eclipse is also a nice time to do radar measurements (radar doesn’t need the sun to illuminate the surface), which we do next, followed by another Composite Infrared Spectrometer scan, post-eclipse.

We complete the flyby with an Ultraviolet Imaging Spectrograph-prime stare at Enceladus (note that by this time, Enceladus is distant and small again), followed by a turn to Earth to downlink the data!

Key to instrument Field of Views:

Optical Remote Sensing, or ORS, instruments:
UVIS (Ultraviolet Imaging Spectrograph) – long narrow magenta field of view
VIMS (Visible-Infrared Mapping Spectrometer) – big red square
ISS (Imaging Subsystem, i.e., the camera) – white square (small=Narrow Angle Camera; large=Wide Angle Camera)
CIRS (Composite Infrared Spectrometer) – red circular field of view and two small red parallel narrow fields of views

HGA – high gain antenna, used for communication with Earth

Radar – green circle (centered on the high gain antenna)

Cheers,
Amanda, still in Houston

Trying to Be Patient

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

Yeah! We made it! I wasn’t too worried about the plume passage, as I wrote yesterday, but it was still wonderful to hear last night that Cassini had contacted Earth and was sending home its precious cargo of Enceladus data. Not only did we survive, which was never much in doubt, but the spacecraft was healthy and the data were looking good. This morning, the beautiful images of Enceladus posted on the Cassini raw image Web site provided further confirmation that things had gone well. And there was more welcome news from the folks at Goddard Spaceflight Center in Maryland, where our Composite Infrared Spectrometer (CIRS) instrument was built and is operated. The CIRS data have been collected from the Deep Space Network, compiled at JPL, transferred to Goddard, and everything looks as expected. The data are now going through the time-consuming calibration process, converting the raw bits into spectra that will reveal some of the secrets of the active south polar region. We should be able to transfer the calibrated data to Boulder and start work on it in an hour or two–I can’t wait!

Dipping Our Toes

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

Today’s Enceladus flyby is a bit more adventurous than most satellite flybys by Cassini.
We are dipping into the jet of water vapor and ice spewing from Enceladus’ south pole, because by doing so we can take full advantage of the amazing opportunity to study
fresh samples from inside this strange world. The mass spectrometer, dust, and plasma
instruments will be running flat-out, gathering priceless information on the composition
of the gases and the ice particles for the sixty seconds or so that Cassini will be in
the dense part of the plume.

graphic showing Enceladus flyby Plume particles are wonderful things to study, but it’s possible to have too much of a
good thing – at the speed that Cassini is going, particles as small as a millimeter in
size could cause serious damage to the spacecraft if we ran into one. So the decision to
enter the plume was not taken lightly. The project convened an “Enceladus plume working group”, led by fellow-blogger Amanda Hendrix, which held a series of meetings to determine whether there was any hazard to the spacecraft from flying as close as we plan to go. Though we fly 50 kilometers (30 miles) above Enceladus’ surface, this happens near the equator and away from the plumes–the closest approach to the source of the plumes is more like 200 kilometers (120 miles)–see the graphic on the left by David Seal (E3 is the official name of today’s Enceladus flyby).

We know from the Cassini images, and from observations from the dust detector during the July 2005 flyby, that there are many ice particles a few microns (a few 1000ths of a
millimeter) in diameter in the parts of the plume we’ll be traversing, and these provide
juicy samples without posing any hazard. But was there any way that much bigger
particles could be lofted into Cassini’s path? We reviewed the observational evidence,
and theories about how the plumes might work (for instance, figuring out how much gas it would take to accelerate an ice grain of a dangerous size to a speed high enough to reach the spacecraft) and we decided that the danger was very small. Still, it will be good to hear from the spacecraft when it turns its antenna back to Earth and sends home word that the flyby was safely negotiated. That will be at 7:05 p.m. Pacific time this evening, not that I’m counting…

Future flyby plans could probe even deeper into the plume, coming as close as 100 kilometers (62 miles) from the source on November 2, 2009 (though this depends on official approval of the proposed extended mission). But we’ll review the results from this flyby before committing to anything that close. If Cassini reports “come on in, the water’s fine!”, as we expect, we’ll dip our toes in a little further next time.

Enceladus – Then and Now

Linda Spilker, Cassini Deputy Project Scientist

As the hours tick down and Cassini gathers data during our closest flyby yet of Enceladus I am thinking about the two Voyager flybys of the Saturn system that took place over 25 years ago. How in the world did we miss the Enceladus plumes back then???

In a nostalgic mood, I am looking over some of my old Voyager timelines today. (Yes, I actually kept all of my old timelines from each Voyager flyby)! I see that we took pictures of Enceladus during the first Voyager flyby in 1980, discovering a tiny, sparsely cratered world at the heart of the E ring. I remember wondering how such a tiny moon could create such a huge, tenuous ring. That mystery was one of the puzzles left for Cassini to solve. Remember that Cassini is in orbit around Saturn so we do multiple flybys, but the Voyagers only flew by Saturn, and each only had one encounter with Enceladus in their itinerary.

< This view of Enceladus was taken by Voyager 2 in 1981.

My timelines show that we planned to take more pictures of Enceladus during the Voyager flyby in 1981. Alas, the Voyager 2 scan platform containing the cameras and spectrometers stuck just as we flew close to Saturn, and the observation of our outbound pictures and spectra of Enceladus (and everything else!) were never made. I remember feeling sad about how much unique data we wouldn’t get on that fateful day and the days that followed. What discoveries remained for some other lucky scientists to make, I wondered? Little did I know that I would be one of those lucky scientists! Good thing Cassini is there to keep an eye on Enceladus!

Enceladus Flyby Underway

Amanda 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,
Amanda

The Gory Details

John 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!
–John

Holding our Breaths

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.