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Cassini

Super Swooper: Cassini Wraps Up its Lowest Pass Through Titan Atmosphere: Posted on Jun 21, 2010 07:01:02 PM | Saturn News 4 Comments | Permalink |; Julie Webster,
Spacecraft Operations Team Manager, NASA's Cassini spacecraft On Sunday evening, my eyes were glued to eight windows on my computer screen, watching data pop up every few seconds. NASA's Cassini spacecraft was making its lowest swing through the atmosphere of Saturn's moon Titan and I was on the edge of my seat. Trina Ray, a Titan orbiter science team co-chair, was keeping me company. Five other members of my team were also at JPL. Between us, we were keeping an eye on about 2,000 data channels. One of the 34-meter antennas at the Deep Space Network's Goldstone complex, DSS-24, was pointed at Saturn and listening for the signal that was expected to be here in just a few minutes. The data would be arriving at my computer as quickly as they could be sent back to Earth, though there was an agonizing hour-and-18-minute delay because of the distance the data had to travel. (We call this flyby T70, but it is actually Cassini's 71st flyby of Titan.) It was a nervous time for me -- the previous night we had been at JPL to send some other real-time commands to the spacecraft when an alarm came in indicating that the magnetometer, the prime instrument taking data for the T70 flyby, needed a reset. Fortunately, the controller on duty immediately called the magnetometer instrument operations team lead in England. Within 90 minutes, the commands were on their way to do a computer reset and clear the alarm. At 2 a.m. Pacific time on Sunday, we got the email indicating all was well and the magnetometer was ready for the Titan closest approach. So here we were, past one hurdle, hoping nothing else would come up. We had run hundreds of simulations over the past three-and-a-half years, so I knew we had done everything we could think to do. We did more training for this event than anything else we had done since we dropped off the Huygens probe in January 2005 for a descent through the moon's hazy atmosphere. Right on time, at 7:26 p.m., the Deep Space Network locked on the spacecraft downlink, a good start. I was focused on the data for spacecraft pointing. As long as we stayed within an eighth of a degree of the expected pointing, everything would be fine. At 7:45 p.m., we got the data from closest approach, a mere 880 kilometers (547 miles) in altitude. Over the vocabox, a cross between a telephone and walkie-talkie, the attitude control team reported that the thrusters were firing about twice as much as we expected. The Titan atmosphere appeared to be a little thicker than we expected, even though we had fed about 40 previous low Titan flybys by Cassini and the descent data from Huygens into our modeling. But spacecraft control was right on the money, keeping the pointing within our predicted limits. Even with the extra thrusting, we stayed well within our safety margin. At 7:53 p.m., the spacecraft turned away to go to the next observation. I let out a sigh of relief, happy that everything during closest approach had gone just as we planned. Five attitude control guys crowded into my office with smiles on their faces. Trina and I were marveling at what a wonderful spacecraft we have to work with. Another first for the Cassini mission! Now, as Trina says, we have to finish the job by returning all the great science data. We have data playbacks today at two different Deep Space Network stations to make sure we have - as we say here - both belts and suspenders. Engineers will also go back to analyze the data with the scientists to see just how dense the Titan atmosphere turned out to be at our flyby altitude. But last night, at least, my team and I went home happy! Julie Webster, spacecraft operations team manager for NASA's Cassini spacecraft, oversees the engineering subsystems and overall spacecraft health and safety. She is a systems engineer at JPL.
Cassini to Swing Low Into Titan's Atmosphere: Posted on Jun 18, 2010 05:05:47 PM | Saturn News 3 Comments | Permalink |; Cesar Bertucci, Space Physicist, Instituto de Astronomía y Física del Espacio
This weekend, Cassini will embark on an exciting mission: trying to establish if Titan, Saturn's largest moon, possesses a magnetic field of its own. This is important for understanding the moon's interior and geochemical evolution. For Titan scientists, this is one of the most anticipated flybys of the whole mission. We want to get as close to the surface with our magnetometer as possible for a one-of-a-kind scan of the moon. Magnetometer team scientists (including me) have a reputation for pushing the lower limits. In a world of infinite possibilities, we would have liked many flybys at 800 kilometers. But we went back and forth a lot with the engineers, who have to ensure the safety of the spacecraft and fuel reserves. We agreed on one flyby at 880 kilometers (547 miles) and both sides were happy. Flying at this low altitude will mark the first time Cassini will be below the moon's ionosphere, a shell of electrons and other charged particles that make up the upper part of the atmosphere. As a result, the spacecraft will find itself in a region almost entirely shielded from Saturn's magnetic field and will be able to detect any magnetic signature originating from within Titan. Titan orbits within the confines of the magnetic bubble around Saturn and is permanently exposed to the planet's magnetic disturbances. Previous measurements by NASA's Voyager spacecraft and Cassini at altitudes above 950 kilometers (590 miles) have shown that Titan does not possess an appreciable magnetic field capable of counterbalancing Saturn's. However, this does not imply that Titan's field is zero. We'd like to know what the internal field might be, no matter how small. The internal structure of Titan can be probed remotely from its gravitational field or its magnetic properties. Planets with a magnetic field -- like Titan's parent Saturn or our Earth -- are believed to generate their global-scale magnetic fields from a mechanism called a dynamo. Dynamo magnetic fields are generated from currents in a molten core where charge-conducting materials such as metals are flowing around each other and also undergoing other stresses because of the planet's rotation. We might not find a magnetic field at all. A positive detection of an internal magnetic field from Titan could imply one of the following: a) Titan's interior still bears enough energy to sustain a dynamo. b) Titan's interior is "cold" (and therefore has no dynamo), but its crust is magnetized in a similar way as Mars' crust. If this is the case, we should find out how this magnetization took place. c) Something under the surface of Titan got charged temporarily by Saturn's magnetic field before this Cassini flyby. While I said earlier that the ionosphere shields the Titan atmosphere from Saturn's magnetic bubble, the ionosphere is only an active shield when the moon is exposed to sunlight. During part of its orbit around the planet, Titan is in the dark and magnetic field lines from Saturn can reach the Titan surface. A temporary magnetic field can be created if there is a conducting layer, like an ocean, on or below the moon's crust. Once Cassini leaves Titan, the spacecraft will perform a series of rolls to fine-calibrate its magnetometer in order to assess T70 measurements with the highest precision. We're looking forward to poring through the data coming down, especially after all the negotiations we had to make for them! César Bertucci, a space physicist working at the Instituto de Astronomía y Física del Espacio in Buenos Aires, Argentina, is a Titan expert on the Cassini magnetometer team. He is also a specialist in the solar wind interaction with weakly magnetized bodies such as Mars, Venus and comets.
Cassini's Swoop over Enceladus: First Morsels of Science Coming Back Now: Posted on Nov 05, 2009 11:31:45 AM | Kimberly Orr 8 Comments | Permalink |; Bonnie J. Buratti, Cassini scientist on the Visual and Infrared Mapping Spectrometer Team
Phew! We made it through the deepest swoop yet down into the plume of Enceladus, the encounter we call "E7" because it's the seventh targeted flyby of Enceladus. But now we have our work cut out for the next few weeks as we pore over the data, painstakingly analyzing every signal to understand the composition of the plume and its structure. So far, we know the Visual and Infrared Mapping Spectrometer (VIMS) was able to get images and data in a variety of wavelengths of light and saw that the plume extends out to at least 1,000 kilometers (600 miles). We also have striking images of the moon crowned by its glorious plume, which Cassini captured right before its plunge. The images illustrate well that the spectacular plume spewing from the south polar region is composed of many much smaller jets. The images and VIMS data both show that as the moon becomes less and less illuminated by the sun (similar to when our moon approaches the phase known as "new moon"), the plume gets much brighter. These data will be valuable for understanding the detailed structure of the plume and where it connects to the surface. We have also learned that the density of the plume appears to be less than half of that predicted. Still, the heart of the plume measured on this flyby was about three times denser than the sparser parts of the plume we flew through previously. There is more good news. We will be able to do the Enceladus flyby on April 28, 2010, on the spacecraft's reaction wheels. This means we will be able to perform the Radio Science Subsystem experiment with Cassini's main antenna to understand the interior of Enceladus under the hot south polar region. During this experiment, antennas from the Deep Space Network (DSN) on Earth will be tracking the spacecraft to see how much Enceladus tugs on it. By measuring this tug, scientists will be able to answer such questions as: How much is the shape of the moon deformed by tidal forces from Saturn? Is there an unusually dense mass under the south pole? (The higher the mass, the larger the tug?) We know that heating by tidal forces is what drives the plumes, but we're not sure exactly how. In addition to a possible liquid subsurface ocean, Enceladus may be harboring a dense mass underneath its surface that helped to start and maintain the moon's current activity. Just wanted to share our excitement about the reams of data we're combing through. Now, back to work!
Taking the Plunge: Cassini Soars by Enceladus: Posted on Nov 05, 2009 11:33:44 AM | Kimberly Orr 9 Comments | Permalink |; Bonnie J. Buratti, Cassini scientist on the Visual and Infrared Mapping Spectrometer Team
After so many close flybys of Enceladus, we're starting to feel as if this little moon of Saturn is an old friend. But during the encounter planned for Nov. 2, 2009, we are going to get up-close and personal. Cassini is going to take its deepest dive yet into the plumes spewing out from the moon's south pole to try to learn more about their composition and density. The spacecraft is going to approach within about 100 kilometers (62 miles) of the surface. We've been closer before (25 kilometers or 15 miles), but we've never plunged quite so deeply into the heart of the plume. To get a better sense of our flyby, watch the animation created by my colleague Brent Buffington. This is the seventh targeted flyby of Enceladus, so we sometimes refer to it as "E7." The video starts out with our approach to Enceladus, rotating through the various instruments scanning Enceladus for data. Then at around 7:40 a.m. UTC (Coordinated Universal Time), we do our long-anticipated flyby through the plumes. The passage will be quick: traveling at about 8 kilometers per second (about 5 miles per second) - fast enough to go from Los Angeles to New York in less than 9 minutes - we'll spend only about a minute in the plume. Then, we zoom away from the plumes and Cassini turns on an infrared instrument (red rays in the animation) to take the temperature of the south-pole fissures known as "tiger stripes" where the plumes originate. A few minutes later, Cassini uses an ultraviolet instrument (purple rays in the animation) to measure the plumes against the background of the peach-colored Saturn. The infrared instrument then gets another turn to examine Enceladus. For more details, see the mission description . The focus of this flyby is to analyze the particles in the plume with instruments that can detect the size, mass, charge, speed and composition. Instead of using its eyes (the cameras), Cassini is going to use its senses of taste and smell. But we're going to get some pretty good pictures too, including some impressive shots of the plumes from far away. So far, we have detected water vapor, sodium and organic chemicals such as carbon dioxide in the plumes that spew out from the tiger stripes, but we need more detail. Are there just simple organic molecules, or something more complex? This is the first time we've found activity on a moon this small (the width of Arizona, 500 kilometers or 310 miles in diameter). We really want to understand what's driving the plumes, especially whether there is liquid water underneath the surface. If we can put the pieces together - a liquid ocean under the surface, heat driving the geysers and the organic molecules that are the building blocks of life - Enceladus might turn out to have the conditions that led to the origin of life on an earlier version of Earth. So if this is all so interesting, why did we wait so long to travel into the plumes? One reason is the plunge is tricky. We wanted to make sure we could do it. We worried that plume particles might damage the spacecraft. We did extensive studies to determine that it was safe at these distances. We also wanted to have the right trajectory so we didn't use an excessive amount of rocket fuel. We are going very fast through this sparse plume; so to play it safe, we're using Cassini's thrusters to keep it stable through this flyby. We'll be monitoring the thrusters closely because we don't want to have to use them on another flyby through the plumes planned for April 28, 2010. In the future flyby, we plan on tracking the spacecraft very closely with the radio instruments on Cassini and on Earth so we can measure how the spacecraft wobbles as it passes near Enceladus. These measurements should tell us more about the interior of the moon, including whether it really does have a liquid subsurface ocean. With the thrusters on, we won't be able tell if the motion of the spacecraft comes from the gravity of Enceladus or the thrusters. We'd like to know whether we can rely on other kinds of attitude control equipment. We're all eager to pore over the results of this flyby. Stay tuned. In the meantime, feast your eyes on this map of the surface of Enceladus that the Cassini imaging team has updated and released today. The tiger stripes are located in the lower middle left and lower middle right of the image.
Greetings to All You Fellow Cassini Travelers!: Posted on Nov 01, 2008 08:17:26 PM | Saturn News 33 Comments | Permalink |; Carolyn Porco
Cassini Imaging Team Leader

I'm very happy to report that we've just put one more major milestone in this remarkable adventure successfully behind us.

Another bold dip over the south pole of Enceladus and another skillful setup for imaging the moon 'on the fly' have brought us another bounty of positively glorious views of one of the most fabulous places in the solar system.

On this run, we have captured, by design, jet source regions we didn't catch the first time: sources VI (see http://www.nasa.gov/mission_pages/cassini/multimedia/pia11134.html )
and VII ( see http://www.nasa.gov/mission_pages/cassini/multimedia/pia11127.html ) on and near the Baghdad tiger stripe, and we repeated our imaging of II and III on Damascus. In all, we've now seen at very high resolution (tens of meters per pixel) sources I, II, III, V, VI, and VII. (See http://photojournal.jpl.nasa.gov/catalog/?IDNumber=pia08385
for a map of these locales). And of course, as before, we note that the region of the active tiger stripes is finely-fractured throughout and littered with icy blocks.

Our next flyby of Enceladus, as you may know, is not for another year. The sun will be disappearing from the south pole throughout that time, so that by next year we will have a far dimmer view of a shrinking portion of the south polar terrain. So, take your fill of this fabulous place now, because it will be a very, very long time before you see it like this again.

Here are three more images we posted from yesterday's flyby:
http://www.nasa.gov/mission_pages/cassini/multimedia/pia11125.html
http://www.nasa.gov/mission_pages/cassini/multimedia/pia11124.html
http://www.nasa.gov/mission_pages/cassini/multimedia/pia11108.html
Cassini Begins To Transmit Science ‘Treats’ Home: Posted on Nov 01, 2008 12:32:24 AM | Saturn News 5 Comments | Permalink |; Amanda Hendrix
Cassini scientist on the Ultraviolet Imaging Spectrograph (bio)
Just in time for Halloween, Cassini does not disappoint and successfully begins to transmit a bag of “science” goodies home. And just like when I was a kid looking through my bag of candy after a long night of trick or treating . . . tonight will be the same as scientists around the country begin to pour over their treats, or in this case data captured on this, our last flyby this year of Saturn’s icy moon, Enceladus. The excitement and joy is relived all over again each time we fly by. What treats and treasures await? I can’t wait to find out!

The downlink will run through the evening. Raw images may pop up on the Web site early tomorrow: http://saturn.jpl.nasa.gov/multimedia/images/raw/index.cfm Just click on the latest images icon . . . or you might select Enceladus from the drop down menu.
--Amanda
More Fun Awaits!: Posted on Oct 30, 2008 05:10:28 PM | Saturn News 4 Comments | Permalink |; 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.

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