Month: October 2008

Amanda Hendrix
Cassini scientist on the Ultraviolet Imaging Spectrograph (bio)

Well, here we go again! Close on the heels of the first two exciting and successful targeted Enceladus flybys of the Cassini Equinox Mission, we have another Enceladus encounter this week!

Tomorrow is the third of three Enceladus flybys in a series … first the August flyby (referred to as E4 since it was the 4th targeted flyby of the entire mission), then on October 9 (E5) and on Friday, Oct. 31, we’ll do another flyby (E6). These three flybys comprise kind of a set, since they all have similar geometries: the spacecraft approaches Enceladus on an inclined trajectory over the northern hemisphere, closest approach is at a low latitude (near the equator), and then we pass through the plume over the southern hemisphere.  Shortly after closest approach, Enceladus enters eclipse behind Saturn and is in darkness for a couple of hours.

Having three flybys with similar geometry is really nice, because it allows us to perform different science experiments on each flyby, since we can’t do everything all in one flyby. It also lets us look for any temporal changes that could be happening at Enceladus, since it is such a dynamic body. E4 was geared toward remote sensing (especially imaging and CIRS) near closest approach, though the fields-and-particles instruments got good data too. E5 was designed for the fields-and-particles instruments, so that they could directly sense the plume as we flew through it. E6 is again designed primarily for remote sensing-and it has a bit more distant closest-approach altitude (200 kilometers or 120 miles), compared to E4 and E5, which approached Enceladus within 50 kilometers (30 miles) and 25 kilometers (16 miles), respectively.

I’ll tell you about the science activities we’ll be doing during this flyby, and you can follow along by watching the accompanying movie (made as usual with grace and skill by Cassini navigator Brent Buffington). Click here for the movie. (At left, a still from the movie.)

Those of you who have been following along with the blog might be familiar with this flyby geometry, and you may also be familiar with this type of movie. For those of you who may be new to this, I’ll introduce the picture: the three panels show what the Cassini spacecraft is doing at each moment during the flyby. The left panel shows the spacecraft relative to the target body Enceladus, and shows which instrument is “prime” by highlighting the field-of-view of that instrument. The lower right shows the field-of-view of the prime instrument, and the upper right shows the fields-of-view of the remote sensing instruments (the cameras, UVIS, VIMS and CIRS), which are all co-aligned.

Here’s the key:
UVIS = magenta long skinny rectangles
CIRS = red circle and red small parallel rectangles WAC = large white box NAC = smaller white box VIMS = red box

The movie starts about 8.5 hours before closest-approach, with a UVIS observation of Enceladus and its environment, performing measurements of neutral gases near the moon. That lasts for about 3.5 hours, and then VIMS is prime and stares at Enceladus to get compositional information as Enceladus gets closer and closer. CIRS then takes over (at 4 hours before closest-approach) and does a series of stares and scans with its FP1 (circular) and FP3 (small rectangular) slits, to get surface temperature measurements. Then VIMS performs a half-hour stare (closer to Enceladus this time). ISS takes over at 40 minutes before closest approach, and the spacecraft executes a large turn to put the cameras in position so that they can see the south polar region just as soon as Cassini gets to that location in the trajectory. Paul Helfenstein on the imaging team designed the “skeet shoot” sequence to image the south pole at the highest resolution possible (8.4 meters per pixel for this flyby)! The skeet shoot sequence starts just about two minutes after closest approach. (Closest approach is at 27 degrees south latitude, 97 degrees west longitude.) The imaging team will be able to study the south polar region to look for evidence of varying levels of geyser activity, by combining images from E6 and E4. The skeet shoot is followed by an eight-panel mosaic. Then ISS hands off to UVIS, to execute its closest, highest-resolution-ever scan of Enceladus to image the south pole and get compositional information on the tiger stripe region, as well as on the environment close to Enceladus (notice how the UVIS slit is long and how it extends past the limb of Enceladus). At 50 minutes after closest-approach, CIRS takes over, just as Enceladus is entering eclipse. Without solar illumination, it’s a perfect opportunity for CIRS to measure the thermal situation at Enceladus’ south pole, to determine what kind of heat is coming from the interior. CIRS executes a series of stares and scans with its different fields-of-view, to make these measurements. By the time CIRS is finished and Enceladus is out of eclipse, it’s about 4 hours after closest approach and Enceladus is getting farther away (it’s now smaller than a NAC). VIMS does another stare, this time of the southern hemisphere, and finally UVIS does a measurement analogous to the first one of the sequence, now of the southern hemisphere.

It should be really great — we’ll keep you posted on how things go!

Amanda.

Todd Barber
Cassini Lead Propulsion Engineer (bio)

Early Halloween greetings from sunny Pasadena, California! 

That was a quick few weeks to catch our collective breaths since Cassini’s daring and successful plunge near Enceladus a few weeks ago.  Scientists are still grinning profusely from the treasure trove of data from that stunning close approach, largely interpreting in situ measurements of Enceladus’ perplexing plumes.  However, as I mentioned last time, this rendezvous with Saturn’s icy satellite is mostly about imaging.  Even though the closest approach distance is about eight times higher than our last flyby, imaging is not performed right at closest approach, anyway, so the pictures promise to be spectacular, as always.  I can’t wait to see the fruits of our labors!

As always, I’m privileged to provide a maneuver status report a few days before the flyby.  This morning, Cassini dutifully fired its small Reaction Control System (RCS) thrusters for about 191 seconds to nail flybys of not only Enceladus on Halloween but Titan three days later as well.  Talk about a scientific doubleheader!  The burn this morning went quite well, changing the speed of the spacecraft by about 0.23 meters per second (0.51 mph).  We will execute a routine Reaction Wheel Assembly (RWA) bias tomorrow, but then it will be time for engineering to hand off the spacecraft to science.

I can’t think of a better way to ring in Halloween—exploring a ghostly white, mysterious, active moon of Saturn nearly one billion miles from planet Earth.  May Enceladus show us no tricks and only provide us scientific treats!

John Spencer
Cassini scientist on the Composite Infrared Spectrometer

Another Enceladus encounter- the fourth (and last) for this year!

It’s become tempting to think of these barnstorming flybys as routine, and to forget how extraordinary they are.  Here we are, on one planet in this amazing solar system, flinging this wonderful machine with exquisite precision between the moons of another planet so far away.

This flyby (as show in the illustration), on orbit 91, doesn’t come quite so close to Enceladus as the last three, so we won’t be penetrating the plume so deeply as we’ve been used to recently.
That’s OK, because our focus this time is on remote sensing-we’ll be getting images and spectra of the active south polar region comparable in detail to the gorgeous data we obtained from our August 11th, flyby.  Originally the October 31st flyby had been planned for a 2,000 kilometer (1,245-mile) altitude, but we moved it down to its present 200 kilometer (120-mile) altitude so we could get a closer look at the surface.  We can’t image the surface from 200 kilometers away- we just can’t rotate the spacecraft fast enough to keep track of Enceladus from that range.  Instead, our goal is to lock onto Enceladus as soon as we can after the flyby, when we’ll have another good view of the south polar region.  The closer we fly to Enceladus, the more quickly the direction to the moon stabilizes as we recede, and the sooner we can track it.  Imagine trying to read a billboard from a speeding car-it’s a lot easier to read (if you’re not driving!) when it’s receding in your rear view mirror than when you’re passing it.

Image above: This diagram shows the trajectory of Cassini’s closest Enceladus flybys from the Prime and Equinox missions, relative to the south polar plume, which is shown in false color based on an image taken by the Cassini cameras in April 2007.  Flybys on orbits 3 and 121 do not come quite close enough to Enceladus to appear on this graphic.

Credit: NASA/JPL/SwRI/SSI

Our own instrument, the Composite Infrared Spectrometer (CIRS) will once again be mapping the heat radiation from the warm tiger stripe fractures, trying to understand how hot the fractures are, and exactly where the heat is coming from.  We only had time to map a small fraction of the active fractures from close range in August, so this time we’ll be looking at different regions, in cooperation with the imaging camera and the ultraviolet spectrometer which will be observing simultaneously for much of the time.

We’ll be following up on some of our August 11th discoveries.  Back then, we got a beautiful spectrum of one of the most active regions of the fracture Damascus Sulcus, allowing us to make some precise temperature measurements.  We’re really interested in the temperatures, because the warmer the fractures are on the surface, the more likely it is that liquid water lurks somewhere below.  The highest temperatures we saw, 167 Kelvin or -159 Fahrenheit, were a bit lower than we had estimated from our less precise data from the March 12th flyby (at least 180 Kelvin or -136 Fahrenheit), though both numbers are dramatically warmer than we would expect at this time of year if internal heat wasn’t leaking out of Enceladus (roughly 60 Kelvin or -352 Fahrenheit).  Whether the Damascus fracture has really cooled since March, or whether our March measurement was an overestimate, or some other factor explains the discrepancy, is one of the things we’re still puzzling over, and maybe this week’s data can help us sort that out.

Another thing we’ll be doing is measuring the total amount of heat energy coming out of the south pole.  Knowing the total horsepower of Enceladus’ heat engine is crucial for understanding what’s driving the activity there.  We’re pretty sure the deformation of Enceladus by Saturn’s tides is the ultimate power source, but so far none of the theorists have been able to explain in detail how that source can continuously supply the roughly 6 Gigawatts of power that CIRS is seeing.  So we’ll be trying to refine that power estimate on this flyby.  Whether the new observations will make life easier or more difficult for the theorists’ remains to be seen!

Todd Barber
Cassini Lead Propulsion Engineer (bio)

Hello again from Pasadena, California!  As I type these words, our spacecraft is dutifully executing a dynamic encounter with Saturn’s perplexing satellite, Enceladus.  Some of you have expressed an interest in the engineering particulars of our daringly close flyby, with respect to its effects on the spacecraft.  I’m happy to say that even though scientists should gather plenty of material for chemical and physical analysis, the icy plumes of Enceladus are rather tenuous, even with our very close flyby distance of only 25 kilometers (16 miles).  One reason this is true is our dive into the plumes occurs at a much higher altitude than the closest approach, as you can see in the image below.  As such, we don’t expect Cassini to slow down or change course due to impacts.  However, gaseous plume material may place small torques on the spacecraft which will cause our thruster deadbands to be exceeded, a common occurrence while in thruster control.  In that case, our small hydrazine thrusters (0.2 pounds or 0.9 Newtons) will fire to correct the pointing of the spacecraft.  In fact, our attitude control engineers may be able to estimate the density of the Enceladus plume by studying these rocket firings!  

As far as the potential for ice damage to Cassini, fortunately the ejecta from Enceladus consist of very tiny particles only.  This, coupled with our micrometeoroid shielding and relatively high altitude while within the plume, will help keep Cassini safe and sound.  It’s truly the best of both worlds–we’ll be close enough to really ingest the smorgasbord offered up by Enceladus, but we’ll still be comfortably far from the surface and well away from affecting the spacecraft adversely.

A new Cassini video update was recently released, further explaining our busy October plans for twin Enceladus flybys.  You may view the video here

Unfortunately, I won’t be here to report our first signal tomorrow after the flyby, as I am leaving for a leaf-peeping vacation to New England this evening.  Rest assured I’ll be thinking of our intrepid robotic friend and the flood of science data to come.  Go, Cassini, go!