Monthly Archives: October 2008

More Fun Awaits!

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Amanda HendrixAmanda 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.

animation previewI’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.

 

 

Here We Come Again,Enceladus

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Todd BarberTodd 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!

The Extraordinary Becomes Routine

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John SpencerJohn 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.

flyby trajectories

 

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!

 

Catching Our Breaths: Getting Ready For More

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

Hi everyone, Just got back from the DPS science meeting in Ithaca, NY (where lots of great Enceladus results from previous flybys were presented!).  It was a busy week but I got a chance to have a quick look at the new UVIS data from last week’s flyby. It looks good!!

Looks like we got some really good-quality data, and we’ll be able to say something about surface composition at the south pole, and potentially about variations in surface composition, as well as about the environment around Enceladus. I need to dig much deeper into the data though, before I say anything more!

Wow. I have to say, even though it’s pretty hectic to analyze new data while going to meetings and giving talks and writing papers (all at once)– and all the while preparing for the next flyby (Oct.31!) –it is so useful to have multiple flybys of Enceladus.  Enceladus is such a crazy and dynamic object, that it is incredibly vital to get multiple observations and flybys to be able to really find out what’s going on. Hearing some of the DPS talks on Enceladus made me feel like we’re just starting to get enough data to begin looking at trends and really get to know Enceladus. So it’s really getting exciting. So even though this flyby went very smoothly, we can’t get all the data we need to understand Enceladus on just one flyby! This is partly because Cassini is a complicated spacecraft and not all instruments can get data optimally at once, but also because Enceladus is so interesting and puzzling that many flybys are required to start to understand it.

Anyway, that’s all for now. Thanks for your support and interest!

Amanda.

Waiting,Patiently . . .

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Sascha KempfSascha Kempf
Cassini Scientist on Cosmic Dust Analyzer

 

Right now we are excited that we collected many mass spectra of fresh dust emerging from Enceladus.  Mass spectra provide information about the composition of the particles.  We obtained about 1,500 spectra during the flyby.

 

We just started the data reduction process which may take some time. Like many of you, we are all eager to know what this data are telling us; however our data require careful analysis and interpretation. 

 

Our mass spectrometer is not like instruments you find in university labs. In space we cannot just catch the particle and prepare it carefully for examination– the particles hit the detector with a speed of about 18 kilometers per second (40,000 miles per hour). Instead, we analyze the composition of the plasma created by the striking particles. The plasma constituents are separated in a strong electric field and accelerated toward a multiplier at the instrument centre. From the ions’ time of flight we get the ion masses and finally the composition of the particle–in theory.  In reality things are much more difficult because within the plasma, the ionized atoms and molecules react with each other and form new compounds. Thus, to interpret an impact ionization mass spectrum correctly we have to understand the chemistry going on in the impact plasma.  In other words, things aren’t always clear cut: a Carbon Monoxide (CO) reading does not necessarily imply that the grain really contains CO. 

 

We are working hard in Heidelberg and as soon as we are certain of our results, we will share it with all of you.  This could take several weeks.  So we wait patiently for the news to unfold on this tiny moon.

 

–Sascha from Heidelberg, Germany

 

Xtreme Navigation,Not for the Faint of Heart

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Shadan ArdalanShadan Ardalan, Cassini Navigator (bio)

Good morning from sunny Southern California.  I have literally been up all night waiting to hear back from the Cassini spacecraft after its closest ever flyby of Enceladus.  It’s nights like this that I feel like a kid again waiting up for Santa on Christmas Eve.

As you know from all the other blog posts, last night we skimmed above the surface of Enceladus at an altitude of 82,000 feet (nearly 16 miles) while traveling at about 40,000 miles per hour…..and as if that wasn’t cool enough, we gave Cassini a “cosmic car wash” by flying the spacecraft through the plumes of the geysers on Enceladus.

Enceladus, from Oct. 9, 2008, flybyThe challenge of navigating a spacecraft with the precision required for such a feat is two-fold.  Not only do my navigation teammates need to determine the orbit of the Cassini spacecraft (i.e., where it is and where it’s going), but there are other members of the navigation team trying to better figure out where Enceladus is (it’s one thing to know where you are….it’s something entirely different to know where your target is).  The experience from the other two Enceladus close encounters earlier this past March and August has improved our knowledge of this moon to such a degree that we feel confident that we can hit this tighter bullseye nearly a billion miles away from Earth.

Fans of extreme sports would especially appreciate what we accomplished.  I’ve bungee-cord dropped before and tonight’s flyby kinda reminded me of that.  This flyby 82,000 feet above the surface and through the plume of water ice is like bungee-cord jumping off a bridge and dipping your head into the river below before getting sprung back up.

As I’m anxiously waiting to receive a signal from the spacecraft, I’ve been spending my time surfing the web to see what the world has to say about us. One of the coolest aspects of working on such a historic mission as Cassini is reading stories on our work in the media.

As I’ve been typing this blog entry (and doing a little more net-surfing), I’ve been monitoring the real-time doppler signal, waiting for a call back home just to tell me everything is all-right.  AND THERE IT IS – YIPPEE!!!!!! and WHEEW!!!!! (wiping sweat off of brow).  This signal tells me in real-time, that Cassini successfully flew by Enceladus.  My friends will process this signal later this morning to tell us how well we hit our target.

And like that kid who just heard Santa land on his rooftop, I best be off to bed because I know when I wake up later this morning, I will have great gifts waiting for me in the form of spectacular images—I can hardly wait!!!!

I have pasted one raw image from the flyby here.To view all the latest images from before closest approach and as the spacecraft sped away from Enceladus, go to:

http://saturn.jpl.nasa.gov/multimedia/images/raw/index.cfm
and click on browse latest images.

–Shadan

 

We Sniffed the Plume!

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Sascha KempfSascha Kempf
Cassini Scientist on Cosmic Dust Analyzer

 

Everything went great for the Cosmic Dust Analyzer (CDA) during this flyby. We got good data during the entire flyby—before, during and after closest approach. We recorded mass spectra even in the deep plume with no data gaps as far as I can see.

The High Rate Data rate count profile shows pronounced peaks at the time we traversed the jets. This data is key for pinning down the structure of the dust jets.

None of it would have been possible without an excellent team overseeing the instrument and the team at JPL for flying us through. Now we are looking ahead to an exciting period of evaluation.

–Sascha, who is heading off to pick up the kids from school.

Where No Hoover Has Gone Before

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Sascha KempfSascha Kempf
Cassini Scientist on Cosmic Dust Analyzer

 

Cassini’s closest flyby so far of Enceladus is rapidly approaching, and we here at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, can’t bear the suspense any longer. Our team has built and operates the Cosmic Dust Analyzer, CDA.

CDA is part of Cassini’s suite of magnetospheric  and plasma science, or MAPS instruments, and is capable of determining the mass, speed, composition, and electrostatic charge of typically micron-sized grains, so called dust, striking the detector with speeds of a few tens of kilometes per second (how many miles is that?). Dust particles can tell us a lot about the Saturnian system, because each grain is a messenger and a participant in the physical processes responsible for forming Saturn’s magnificent rings, structuring the planet’s magnetosphere, and reshaping the surfaces of its moons. Like photons, dust particles carry precious information about their site of origin and about their history.

So what does the CDA team want to achieve during this flyby? Enceladus is the major dust producer of the E ring. Most of the particles are emerging from the plumes in the moon’s unusually warm south pole region. Those particles tell us a lot about the conditions inside the surface fractures, where the particles condense from the water vapor ascending through the cracks. There is, however, a second, although less effective way that Enceladus produces fresh dust: fast impacts by either interplanetary meteoroids or by ring particles onto the moon’s surface
produce many new particles—the surface ejecta. We believe that the composition of these ejecta differs from that of the plume particles, and this is one of the main goals of this flyby. If this turns out be true then we can follow the evolution of plume particles through the E ring until their end of life. This knowledge would also help us to understand whether the plumes’ exhaust is deposited on the surface of Enceladus and if so, how much We need just the right geometry to accomplish this goal since we can observe the surface ejecta only when the spacecraft is very close to the Enceladus surface but outside the south pole region.  That’s because the Enceladus plumes are such a strong dust source.

The upcoming encounter is just right for our needs. Around the closest approach, just 25 kilometers (16 miles) from Enceladus’ surface we hope to scoop particles that come directly from the moon’s surface. Later on, when Cassini traverses the dust plumes, we hope to collect genuine plume particles. This approach worked quite well during the last encounter in August.

We can hardly wait!

Today's the Day!

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Todd BarberTodd 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!

Enceladus,Here We Come!

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

Hi everybody! I’m very excited about today’s Enceladus flyby, which will take us deeper into the plume than we’ve ever been before! Here’s the scoop on the science activities that will take place during the encounter, accompanied by a neat movie provided by Cassini navigator Brent Buffington. Click here to see the movie (30Mb).

We’ve posted these types of movies for previous flybys, but in case you haven’t seen one before, here’s the setup: the left-hand panel shows the spacecraft and its relationship to Enceladus and will indicate the view of the “prime” instrument by showing its viewing frustum in the color corresponding to that instrument. The upper right panel shows the fields-of-view of the remote sensing instruments (i.e. the cameras and the imaging spectrometers), and the lower right panel shows the “active” field-of-view, since at any time, one instrument is “prime” (though other instruments may be simultaneously taking data, while “riding along”). (By the way, you can go to the end of this post for a key to help watch the animation.)

view from Cassini flyby animationNow, this flyby has a similar trajectory to the previous two flybys in March and August: a fast, inclined path coming in over the northern hemisphere and leaving over the southern hemisphere, with closest approach at a low latitude. Recall that summer in the southern hemisphere is winding down, meaning that the active “tiger stripe” region is illuminated by the sun less and less every day. CIRS and RADAR can see in the dark and don’t care too much about solar illumination, but ISS, UVIS and VIMS usually measure solar light reflected from the surface—so opportunities to measure the wild south pole using these methods are dwindling!

OK, so the movie starts while we’re inbound to Enceladus, about eight hours before closest approach, when RADAR makes measurements to get at centimeter-scale roughness and to investigate the energy balance in the upper layers of the surface – so you can see the green circle doing repeated scans over Enceladus. The scans are accomplished by slewing the entire spacecraft.

Then the spacecraft turns 90 degrees so that the remote sensing instruments can point at Enceladus. First CIRS is prime and does a series of stares and scans. CIRS measures the temperature of the surface. Fellow blogger John Spencer will probably tell you later more about CIRS measurements, which are super important and interesting at Enceladus.

After CIRS, we’re about two hours from closest approach, and UVIS is prime, starting from several radii away from the body and slowing scanning onto Enceladus, to map out any neutral gases, such as oxygen or hydrogen, that are present in the vicinity.

When UVIS is finished with the slow scan, the spacecraft executes a big turn to put the fields-and-particles instruments (especially CDA and INMS) into position to “scoop up” dust particles and gas species during closest approach and while in the plume. Such a close approach and relatively deep plume passage are going to provide really interesting and key results on plume composition and also the composition of material sputtered from the surface away from the plume. (Note that during the closest approach period, the remote sensing instrument boresights are actually on the planet – see the upper right panel – though they’re not taking data.)

Closest approach goes by quickly (we whiz by at nearly 40,000 mph!), and about 15 minutes after closest approach, the spacecraft turns so that the remote sensing instruments can check out the south pole. ISS is prime first, for about 30 minutes, to image the active tiger stripes while Enceladus is in sunlight. Then CIRS takes over, and at around 19:52, Enceladus will enter eclipse, and will be in eclipse for about 2.5 hours, so CIRS will be able to map south polar surface temperatures without the influence of solar input.

Finally the Enceladus sequence ends with a distant view of the body with UVIS as prime instrument!  That’s the instrument I work on.

I’ll report again on progress, from the DPS science meeting in Ithaca, NY.

It’s going to be great! Thanks for coming along with us as we fly by this crazy moon.

Amanda

Here’s the key to help watch the animation:
UVIS = magenta long skinny rectangles
CIRS = red circle and red small parallel rectangles
WAC = large white box
NAC = smaller white box
VIMS = red box
RADAR/HGA = green circle

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