Engineering – Cassini

Xtreme Navigation,Not for the Faint of Heart

Shadan 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, flyby The 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!

Sascha 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.

Enceladus,Here We Come!

Amanda 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 animation Now, 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

Strapping in for the Ride

Geraint Jones
Scientist on the Cassini Plasma Spectrometer (bio)

Cassini’s next white-knuckle Enceladus flyby is quickly approaching, and the excitement is building for those of us working on the Cassini Plasma Spectrometer, CAPS. This instrument team is headed by Dave Young at the Southwest Research Institute, San Antonio, but the team members work in several countries. This includes the UK, where several of us on CAPS work at the Mullard Space Science Laboratory—part of University College London that’s located in the countryside to the south of the city.

CAPS is part of Cassini’s suite of magnetospheric and plasma science, or MAPS experiments. With these, the spacecraft can sense its immediate environment: the gases, plasma, electromagnetic fields, and dust that can tell us a huge amount about the Saturn system.

artist concept of Enceladus flyby The results from each of this year’s Enceladus flybys has a different flavor. The main reason for this is that not all instruments can be pointed in their respective “best” directions at once, and the flybys are so brief that there’s no time to turn Cassini, so every instrument gets a good look or samples the particles they want to… it’s a bit like deciding who gets the window and aisle seats! Luckily, because we’ve got several flybys, the teams can take it in turns to get the best data out of their respective experiments… this time, as it was back on March 12th, the MAPS instrument teams are in their favorite seats.

So what are the CAPS team going to be looking at during this daring dive? Enceladus is sitting in a flow of plasma—a mix of ions and electrons that’s trapped around Saturn by the planet’s magnetic field. We expect Cassini to briefly enter a wake where this magnetospheric plasma can’t penetrate, a “shadowed” region where CAPS should sense a big drop in the density of the plasma. Once past closest approach, only 25 kilometers, or 15.5 miles, from Enceladus’ surface, Cassini will spend a few minutes actually inside the plume of gas and ice particles being thrown out by the moon.

CAPS will be pointed in the right direction to scoop up this material at almost 18 kilometers. or 11 miles a second. as we whiz through the plume. Some of the gases released in the plume are electrically-charged by the time they reach Cassini; CAPS can measure the energies of these and how many are present. From this, we should be able to learn a lot about the plume: how dense it is, its composition, and how this plume affects the plasma flowing past it. This approach worked really well during the March encounter, so we’re hoping to repeat that success, but taking a different “cut” through the plume because Cassini will fly past Enceladus from a different direction.

We’re strapped in, noses against the window, and can’t wait for the ride!

Cassini Is 'GO' for Enceladus-5

Todd Barber
Cassini Lead Propulsion Engineer (bio)

Hello again from the realm of Cassini engineering, just 48 hours before another wonderful date with destiny! It is a distinct pleasure to kick off yet another Enceladus flyby blog, particularly one about a flyby so thrilling and daring. I’m happy to report the spacecraft is right on target for this historic encounter with Saturn’s icy and active companion. As an engineer, I think I’m most floored by the closest approach distance of 25 kilometers, roughly 16 miles. Cassini hasn’t been this close to any solid body since our ascent in a pre-dawn October sky in Florida in 1997! To further put this flyby in perspective, one can convert the minimum altitude into feet–yes, we’ll actually be that close! In this system of units, we will buzz this active water-geyser surface at only 82,000 feet, which puts Cassini closer to Enceladus (900 million miles from Earth, mind you) than an SR71 Blackbird can fly above the ground on the third rock! Wow!

flyby trajectories Given this truly up-close-and-personal flyby, the primary scientific focus will be “sniffing” the tenuous atmosphere of Enceladus, a rarefied collection of gas and dust spewed forth from a surprisingly active surface. As such, E5 will concentrate less on imaging results, but E6 will tip the scales towards imaging yet again, all before the end of the month. Truly, Cassini is embarking on a busy October for the ages!

From the spacecraft engineering and navigation teams, I’m happy to report our final E5 flyby approach maneuver, OTM-166, was executed successfully yesterday afternoon. It was a mere 15 millimeters per second (0.033 mph) speed change to our essentially on-target spacecraft. However, even traveling thousands and thousands of miles per hour, such corrections are necessary in the intricate billiard game that is celestial mechanics. We also monitored the spacecraft very carefully after OTM-166, looking for any rocket thruster leakage after their usage during the maneuver. Even though any such leakage would be exceedingly unlikely, it could have theoretically put Cassini on an Enceladus impact trajectory, and that might be, well, career-limiting for the scientists and engineers on the project. 🙂 I’m happy to report the maneuver went well, there is no sign of thruster leakage, and we are ready to hand over the spacecraft to our eager scientists. May you take a big whiff of whatever Enceladus has to offer, and may it offer the sweet scent of scientific promise and discovery!

We Nailed It!

John Spencer

Cassini Scientist on the Composite Infrared Spectrometer (bio)

Click here to browse Cassini raw images site for Enceladus images

All sorts of emotions over the last couple of days. Yesterday morning began with the great (but not surprising) news that our trusty spacecraft had successfully negotiated its latest and deepest- yet passage through the Enceladus plume, successfully executed its observations, and was starting to ship its cargo of data home. There wouldn’t be any calibrated data to look at for hours, so I focused for a while on something much more down-to-Earth: my wife Jane and I put in an hour harvesting produce at a local vegetable garden. Searching through the luxuriant, dripping-wet foliage for green beans and tomatoes, Jane remarked, “Isn’t it amazing what can happen on a planet that has water on it”? That of course, is part of the reason why we’re so excited to be exploring Enceladus–the geysers breaking through that intensely cold surface harbor heat, lots of it, maybe enough to melt the ice below the surface and, just maybe, enough to give Enceladus its own chance for life. Maybe our new Composite Infrared Spectrometer (CIRS) observations of Enceladus’ internal heat, now on their way home, could give us new clues about whether liquid water really does lurk beneath the surface.

close-up view of Enceladus

Image left: This image shows our initial discovery, back in 2005, that the south polar tiger stripe fractures were warm. Larger view

The rest of the day was an exercise in patience as we waited for the CIRS data to be calibrated at CIRS’s home at NASA’s Goddard Spaceflight Center in Greenbelt, Md., a complicated and time-consuming process. By late afternoon, right before I had to leave for the evening, we got a nibble–a short sequence of data from the few minutes right after closest approach. The processed data ended, tantalizingly, just before our planned stare at the active fracture Damascus Sulcus, which we hoped, if targeting was perfect, would give us perhaps our best-yet determination of the temperatures of the tiger stripe fractures. But something bothered me–CIRS was operating in a mode that I didn’t expect. Had something gone wrong with the instrument commanding? It was too late in the day to check with the folks in Maryland. That worry preyed on my mind all evening, so this morning I pounced on the data as soon as I could, to run some more checks. To my great relief, everything was fine–I had forgotten that we had planned to use that unfamiliar instrument mode for this unusual close-up observation. Still, the rest of our data were still not calibrated, and I had to wait a bit longer.

In the meantime, there were the close-up ISS camera images to look at. Like the other bloggers here, I was blown away by both the quality of the images, which were taken under very difficult
circumstances, and by the bizarre landscape that they revealed. Utterly stunning. Hats off to the imaging team, particularly (as Bonnie and Carolyn also mentioned) to Paul Helfenstein, who sweated for months on the details of planning that sequence.

Then, finally, it was our turn–the Goddard team completed the CIRS calibration this morning, and I downloaded the data. More nervousness, until the plots started coming up on the screen and showed a beautiful spike in the signal strength, right when we expected to be staring at Damascus. It was obvious that we were pointing right at the warm fracture, just as planned. We nailed it! Not that CIRS gets credit for this bit of precision targeting–the camera team was driving and we were along for the ride. Credit goes once more to Paul Helfenstein and rest of the ISS team, and also the navigation team who put the spacecraft exactly where it was supposed to be. Now we have to delve deeper to find out what that beautiful observation of Damascus is telling us.

Cassini Calls Home!

Todd Barber

Cassini Lead Propulsion Engineer (bio)

Hello from the “flip” side of Enceladus! I’m happy to report via this blog that the Deep Space Network in Canberra, Australia, locked onto Cassini’s radio signal around 9:03 p.m. PDT on Monday, Aug. 11.

We’ve executed yet another successful flyby of Saturn’s ice-geyser moon! Playback data has just begun streaming to our breathless scientists and engineers at JPL, and it will continue throughout the evening and into tomorrow morning. With the rest of you, I’m pumped up for the raw (unprocessed) images that will be posted Tuesday morning, Pacific Time. For the latest raw images, check here:

http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-list1.html

Cassini engineering may pay my bills, but Cassini science sparks my passions.

A few engineers are still here, working away this evening, making sure the spacecraft came through its scientific marathon unscathed. I can’t speak for all these engineers, but I can tell you the propulsion subsystem is healthy and is ready for yet another Reaction Wheel Assembly (RWA) bias after midnight PDT. Thankfully, this activity is fairly commonplace, so long before then I plan to be celebrating another engineering success with some Olympics viewing, perhaps an appropriate beverage, and an indefatigable smile. It’s been a long day but oh so tremendous! Now let the flood of Enceladus science data commence!

Todd

Today,Monday August 11th,Is the Day!

John Spencer

Cassini Scientist on the Composite Infrared Spectrometer (bio)

Our next Enceladus encounter is very soon, at 21:06 Universal Time or 14:06pm Pacific time. This promises to be a spectacular encounter, giving our first high-resolution sunlit view of the south polar region since the discovery of activity there three years ago (the most recent encounter, in March 2008, observed the south pole only in the darkness of Saturn’s shadow). Our instrument, the Composite Infrared Spectrometer (CIRS), will be mapping the heat radiation from the warm tiger stripes as we did in last March’s flyby, but on that flyby our best views were from a range of 14,000 km (8,500 miles), allowing us to see details of the heat radiation on scales as small as 4 km (2.5 miles). This time we’ll start our observations from a range as close as 900 km (560 miles), showing us Enceladus in fabulous close-up, with CIRS mapping details as small as 270 meters (0.17 miles). As we scan the south pole we’re screaming away from Enceladus at nearly 18 kilometers/second (40,000 mph) so we have to work fast to make the most of this high-resolution opportunity.

Image left: Heat radiation from tiger stripes. Full caption

I’m most excited about the observation we’ll be attempting at about 21:11 UT, when we will try to put the CIRS short-wavelength detector right along one of the most active tiger stripes, called Damascus Sulcus, from a distance of only 4,500 km (2,800 miles). On our last flyby we saw temperatures as high as at least 180 Kelvin (-135 Fahrenheit) on this part of Damascus, from 15,000 km (9,000 mile) range, and from three times closer we might see even higher temperatures because the warm material, which we think occupies a strip just tens or hundreds of meters wide along the fractures, will fill more of our detector and give us a more accurate reading. However, this is a challenging observation because our detector consists of a linear array of ten pixels, which will be aligned parallel to the fracture. Pointing may not be perfect this close to the moon, so we may get all ten detectors, or none of them, on Damascus. You might ask why we didn’t align the detector across the fracture rather than parallel to it, to be sure that at least some of our pixels would fall on the warm material. But we needed to align the spacecraft in the direction that allows it to rotate as fast as possible to keep up with Enceladus as we zoom past, and we didn’t have the luxury of also optimizing the orientation of the detector.

The animation to the right depicts Cassini flying close to Enceladus.

So tomorrow morning, when we hope the data will be processed and ready for us to look at, we’ll be both nervous and excited to see if this particular gamble paid off. But even if we are unlucky in this case, we’ll get lots of other spectacular data. Here’s a blow-by-blow account of the planned CIRS observations of the south pole, with times given in Universal Time (UT):
21:07-21:11 UT Very high resolution scan of the tiger stripes, riding along with the ISS camera “skeet shoot” observation that Amanda described in her previous blog entry, and ending with that view of Damascus Sulcus that I discussed above.

21:11-21:34 UT Ride along with the ISS camera mosaic of the south pole- more very valuable high-resolution views of selected locations on and near the tiger stripes.

21:36-21:50 UT A long-wavelength map of the entire southern hemisphere, which will allow us to measure total heat flow from the active south polar region. Enceladus enters Saturn’s shadow during this period, at 21:41.

21:52-22:09 UT A single scan of the south pole using our short-wavelength detectors, which will pick up the small part of the tiger stripes, along Alexandria Sulcus, that we missed in our March 2008 scan.

22:10-22:56 UT A global 7-16 micron map of the southern hemisphere with our short wavelength detectors, including the south pole, to look for changes in thermal emission since previous observations.

22:57-23:37 UT A stare at Damascus Sulcus with each of our two short-wavelength detectors, to give other instruments a chance to watch Enceladus in eclipse.

23:37-23:55 UT A maneuver to change the spacecraft orientation.

23:55-00:24 UT Watch the warming of Enceladus as it emerges from Saturn’s shadow at 00:07 UT, with the our long-wavelength detector.

00:25-00:42 UT ISS camera and VIMS compositional maps of the now-sunlit south pole.

00:42-01:04 UT Complete the CIRS long-wavelength observation of the post-eclipse warming.
It’s going to be one heck of a ride- I’ll report back once we’ve had a look at our data.

Flyby Underway!

Todd Barber,

Cassini Lead Propulsion Engineer (bio)

Best wishes from Pasadena, California on this fine day! As I write
the highly anticipated Enceladus flyby is underway.

I was in the office this weekend trying to get caught up a bit (a
weekend tradition), allowing me to hit the ground running on another
busy work week. This week holds exciting promise and scientific
anticipation, though, in addition to my typical engineering duties.
Over the weekend, we did two Reaction Wheel Assembly (RWA) biases to
optimize the rotational speeds of these objects. Through angular
momentum changes, these wheels can turn and point the spacecraft,
allowing incredible precision. One of my tasks is to calculate how
much propellant these biases use, so that’s one thing I’m working on
today.

arist concept of Enceladus flyby Speaking of propellant usage, there have been some questions about how
much propellant we will use in the maneuvers around Enceladus and for
the flyby itself. As I mentioned in my prior blog entry, we were
actually able to cancel our approach maneuver, OTM-163. In addition,
the next propulsive maneuver (OTM-164) is nearly two weeks off,
setting up Cassini for solar conjunction and yet another Enceladus
encounter (E5) in October. OTM-164 will be rather large, however,
using roughly 10 kg of bipropellant, about 6 kg of nitrogen tetroxide
(oxidizer) and 4 kg of monomethylhydrazine (fuel). For the E4
encounter itself, I don’t have a propellant-usage prediction handy,
but I can tell you it will be at most a few tens of grams of hydrazine
(the monopropellant fuel), for yet another RWA bias.

The bottom line is that we have plenty of propellant for our two-year
extended mission and beyond. Thanks for that great question, and
we’ll see all of you on the other side of Enceladus!

Todd

Cassini Engineering Is Ready for Enceladus!

Todd Barber

Cassini Lead Propulsion Engineer (bio)

Greetings from the engineering side of the Cassini mission to Saturn on this auspicious date, 8/8/08! As most folks on the third rock from the Sun turn their attention toward China and the Summer Olympics, Cassini scientists and engineers at JPL are turning their gaze skyward for some Winter Olympics of our own at Enceladus. As Amanda so eloquently posted in her blog entry, this is truly an exciting time for the mission. We are a mere three days away from a daring pass near Enceldaus’ tumultuous south pole, peppered with fractures called “tiger stripes” and active ice volcanism! Though we’ve been to Enceladus a few times before, every encounter is a new adventure, allowing each science instrument its time to shine.

E3 is March flyby and E4 is August flyby.

Image left: E3 is March flyby and E4 is August flyby.

Before any science results may be achieved, though, we have to make sure the spacecraft is on course for a truly close approach to Enceladus. Next Monday, Cassini will zip over this icy orb within 50 kilometers (30 miles), a close shave indeed considering Enceladus is roughly 1.5 billion kilometers (900 million miles) from Earth. From the spacecraft engineering and navigation teams, I’m happy to report our final E4 flyby approach maneuver, OTM-163, was canceled yesterday afternoon. These cancellation-decision meetings are always very interesting, representing a wonderful example of the collaborative efforts required of our scientists and engineers. It turns out we could have performed OTM-163 today (i.e., it was large enough to execute), and we know where the spacecraft is very precisely. The “tall tent pole” in the process turned out to be our uncertainties in the position of Enceladus itself, rendering the tiny maneuver superfluous! Naturally, our understanding of Enceladus’ position will improve after Monday’s close flyby, but I thought it was quite interesting that we know where our (relatively) small spacecraft is better than we know the position of this moon of Saturn. Among countless scientific reasons, this is another incentive for close flybys of moons like icy Enceladus.

Perhaps I could have predicted this maneuver would get canceled, given its cushy positioning on a regular working Friday, during prime shift. Why aren’t the off-shift, holiday, and weekend maneuvers afforded the same cancellation probability, statistically? I’m sure this perception of mine is incorrect, but no matter, because the important point is we’re on course for Enceladus. Our engineering and navigation teams are happy and excited, and scientists are about to enjoy another thrilling encounter with this most perplexing of Saturn’s moons.

Todd