Archive for the ‘Saturn’ Category

A Different Slant:

Monday, July 9th, 2012

By Duane Roth

Cassini Has a Special View of Saturn These Days - How Did It Get There?

For the past 18 months, NASA’s Cassini spacecraft has been orbiting Saturn in practically the same plane as the one that slices through the planet’s equator. Beginning with the Titan flyby on May 22, navigators started to tilt Cassini’s orbit in order to obtain a different view of the Saturnian system. The measure of the spacecraft orbit’s tilt relative to Saturn’s equator is referred to as its inclination. The recent Titan flyby raised Cassini’s inclination to nearly 16 degrees. Seven more Titan flybys will ultimately raise Cassini’s inclination to nearly 62 degrees by April 2013. On Earth, an orbit with a 62-degree inclination would pass as far north as Alaska and, at its southernmost point, skirt the latitude containing the tip of the Antarctic Peninsula.

These graphics show the orbits NASA's Cassini spacecraft has made and will make around the Saturn system from September 2010 to April 2013.These graphics show the orbits NASA’s Cassini spacecraft has made and will make around the Saturn system from September 2010 to April 2013. As shown in gray, Cassini orbited within the plane of Saturn’s equator during the first 18 months of its current mission phase, known as the Solstice mission. Then, starting in May 2012, Cassini used the gravity of Saturn’s largest moon, Titan, to tilt its orbit as shown in the magenta loops, reaching a maximum tilt of about 62 degrees in April, 2013. Titan’s orbit is shown in red. The orbits of Saturn’s inner moons are shown in black. Image credit: NASA/JPL-Caltech

You may wonder why this change has been planned and how this feat is achieved. The “why” is to allow scientists to observe Saturn and the rings from different geometries in order to obtain a more comprehensive three-dimensional understanding of the Saturnian system. For instance, because Saturn’s rings lie within Saturn’s equatorial plane, they appear as a thin line when viewed by Cassini in a near-zero-degree orbit inclination. From higher inclinations, however, Cassini can view the broad expanse of the rings, making out details within individual ringlets and helping to unlock the secrets of ring origin and formation. Some of those images have already started to come in.

At higher inclinations, Cassini can also obtain excellent views of Saturn’s poles, and measure Saturn’s atmosphere at higher latitudes via occultation observations, where radio signals, sunlight or starlight received after passing through the atmosphere help to determine its composition and density.

The “how” is by using the gravity of Titan — Saturn’s largest moon by far — to change the spacecraft’s trajectory. Like the rings and Cassini’s previous orbit, Titan revolves around Saturn within a plane very close to Saturn’s equatorial plane. As Cassini flies past Titan, Titan’s gravity bends the spacecraft’s path by pulling it towards the moon’s center — similar to a ball bearing rolling on a smooth horizontal surface past a magnet. Near Titan, the motion is confined to a plane containing the spacecraft’s path and Titan’s center of mass. If this “local” plane coincides with Cassini’s orbital plane about Saturn, the trajectory’s inclination will remain unchanged. However, if this plane differs from Cassini’s orbital plane about Saturn, then the bending from Titan’s gravity will have a component out of Cassini’s orbital plane with Saturn, and this will change the tilt of the spacecraft’s orbit. Repeated Titan flybys will raise Cassini’s orbit inclination to nearly 62 degrees by April of next year and then lower it back to the Saturn equatorial plane in March 2015.

This view, from the imaging camera of NASA's Cassini spacecraft, shows the outer A ring and the F ring of SaturnNASA’s Cassini spacecraft has recently resumed the kind of orbits that allow for spectacular views of Saturn’s rings. This view, from Cassini’s imaging camera, shows the outer A ring and the F ring. The wide gap in the image is the Encke gap, where you see not only the embedded moon Pan but also several kinky, dusty ringlets. A wavy pattern on the inner edge of the Encke gap downstream from Pan and a spiral pattern moving inwards from that edge show Pan’s gravitational influence. The narrow gap close to the outer edge is the Keeler gap. Image credit: NASA/JPL-Caltech/SSI

Gravity assists are key to Cassini’s ever-changing orbital geometries. Onboard propellant alone would quickly become depleted attempting to accomplish these same changes. A gravity assist can be characterized by the amount of “delta-v,” or change in the velocity vector, it imparts to a spacecraft. Delta-v may of course also be imparted to the spacecraft via rocket engines and, either way, alters the spacecraft’s orbit. The eight Titan gravity assists responsible for raising Cassini’s inclination to 62 degrees will provide a delta-v of 15,000 mph (6.6 kilometers per second). For comparison, Cassini’s rocket engines had only enough propellant after initially achieving orbit around Saturn to deliver about 2,700 mph (1.2 kilometers per second) of delta-v. That’s 15,000 mph of capability spread over 11 months via gravity assists versus a modest 2,700 mph of capability spread over more than 13 years via rocket engines! Because delta-v is a vector, it may change both the speed and direction of Cassini at a point along its orbit, so the speed of Cassini is not changing by 15,000 mph, but mostly all of the directional changes sum to 15,000 mph. To give these values some context, Cassini’s speed typically varies between as low as 2,500 mph (1.1 kilometers per second) and as high as 79,000 mph (35 kilometers per second) relative to Saturn between apokrone and perikrone, the farthest and closest points from Saturn along its orbit. Gravity assists from the initial prime mission Titan flyby in 2004 to the final Solstice Mission Titan flyby in 2017 will provide nearly 200,000 mph (90 kilometers per second) of delta-v, leveraging the onboard propellant by a ratio of 75 to 1. The bulk of the Saturn tour trajectory is shaped by gravity assists, while the role of onboard propellant is to fine-tune the trajectory.

At the end of year 2015, Cassini will again begin climbing out of Saturn’s equatorial plane in preparation for its grand finale. After reaching an inclination of nearly 64 degrees, a Titan gravity assist in April 2017 will change Cassini’s perikrone so that Cassini will pass through the narrow 2,000-mile (3,000-kilometer) gap between Saturn’s atmosphere and innermost ring. Twenty-two spectacular orbits later, one final distant Titan gravity assist will alter Cassini’s course for a fiery entry into Saturn’s atmosphere to end the mission.

Cassini to Swing Low Into Titan’s Atmosphere

Thursday, June 17th, 2010
César Bertucci
César Bertucci

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.

Artist's concept of Cassini's Titan flyby
Cassini flies to within 880 kilometers (547 miles) of Titan’s surface during its 71st flyby of Titan, known as “T70,” the lowest in the entire mission. Image credit: NASA/JPL/Space Science Institute
› Full image and caption

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!

Smooth Sailing by Rhea and Helene

Monday, March 8th, 2010
Amanda Hendrix
Amanda Hendrix

Cassini’s closest-ever flyby of Saturn’s moon Rhea went quite smoothly and teams are busy checking out their data! These flybys never fail to amaze me. And the raw images — which give us an unprocessed first look — are really cool!

Raw image N00152150 gives us a view of part of the bright, fractured terrain we refer to as “wispy terrain” from about 14,000 kilometers (8,900 miles) away. We know that Rhea’s albedo overall is quite high. (When I say “albedo,” I basically mean “brightness” or “reflectivity.” Studying the albedo can tell a lot about surface composition, geologic processes, and interactions with external environment.) But this image demonstrates how bright these cracks are since they are so shiny that the surrounding terrain looks quite dark. There are also some interesting apparent albedo variations seen in this image, which are really intriguing.

The camera was pointing toward RHEA.
The camera was pointing toward RHEA at approximately 38,532 kilometers away, and the image was taken using the CL1 and CL2 filters. This image has not been validated or calibrated. Image credit: NASA/JPL/Space Science Institute.
› Full image and caption

The camera was pointing toward HELENE.
The camera was pointing toward HELENE, and the image was taken using the CL1 and CL2 filters. Image credit: NASA/JPL/Space Science Institute.
› Full image and caption

This raw image (N00152175) from Cassini’s narrow-angle camera image was taken about 40 minutes after closest approach. The image shows a region adjacent to the wispy terrain –craters, craters everywhere! And wow, are those crater rims bright compared to the surrounding terrain.

Cassini captured a full portrait of the serene moon with its wide-angle camera (raw image W00063107) on the outbound leg of the flyby, about 1.25 hours after closest approach. Keep in mind that the phase angle is quite low here (only about 2.5 degrees), meaning that the sun is almost directly behind Cassini and Rhea is nearly fully illuminated — so there are no shadows. Large-scale albedo variations are apparent across the surface.

The spacecraft also obtained a cool image of little Helene with raw image N00152211. We’re basically looking at the night side of the body — but it doesn’t appear very dark, because it’s illuminated by sunlight reflecting off Saturn. During the later image sequence of Helene, this small moon was transiting Saturn - so you can see Saturn in the background. Sometimes,pointing at these little guys can be very tricky, especially so close after a targeted flyby. It can be difficult (or impossible!) to get the positions of the spacecraft, the moon and the instruments all lined up — but boy are these close-up Helene images incredible! The detail on the surface is tremendous, and should go a long way to informing geologists about surface properties and processes.

As the imaging team is taking a closer look at images such as these, other instrument teams — including those for the radar instrument, composite infrared spectrometer, visual and infrared mapping spectrometer and the ultraviolet imaging spectrograph (the instrument I work on) — are also busy processing their data. At a science meeting Friday, we talked about a few of the preliminary results. Some of the magnetospheric and plasma science instruments teams reported that they’re seeing some really interesting and surprising results! So stay tuned to hear more about those!

Of course, after one successful flyby, we get right to work on another. Coming up next: Dione on April 7!

Road-Tripping to Rhea with Cassini

Monday, March 1st, 2010
Amanda Hendrix
Amanda Hendrix

Here in Cassini-land, we are really excited about Tuesday’s Rhea flyby! This will be the mission’s second targeted flyby of the moon in the mission, so it’s sometimes referred to as R-2 or Rhea-2.

The spacecraft will fly by Rhea at an altitude of about 100 kilometers (60 miles), the closest encounter yet with Saturn’s second largest moon. (Our first targeted flyby of Rhea in 2005 was at an altitude of 500 kilometers, or 300 miles, so this is way closer.)

We’ve been focusing a lot on the moon Enceladus because it is sort of the darling of the Saturn system — but Rhea is a good example of why the other moons are interesting too. We know a decent amount about this moon, but we still have more questions, especially about the debris that could make up a ring around the moon and the composition of its surface.

The first targeted flyby in 2005 was focused on a radio science experiment doing gravity measurements to understand Rhea’s interior structure. We also got some nice remote-sensing data from the cameras and spectrometers (see for example PIA07764) as well as radar measurements for surface and subsurface composition. We also did a much more distant flyby (5,000 kilometers or 3,000 miles) of Rhea in August 2007; that flyby was dedicated to remote sensing of the moon, including imaging (such as PIA08402). So we have a pretty good understanding of Rhea as being pretty heavily cratered with no super obvious signs of activity. It has this “wispy terrain” (see PIA08120), which is a lot like the type of feature seen on another Saturnian moon, Dione, and is basically a large series of fractures that are relatively bright compared to the surrounding regions.

Rhea's Wisps in Color
Bright, wispy markings stretch across a region of darker terrain on Saturn’s moon Rhea. In this extreme false-color view, the roughly north-south fractures occur within strips of material (which appear greenish here) that are a different color from the surrounding cratered landscape. Image credit: NASA/JPL/Space Science Institute.
› Full image and caption

Rhea-2 Flyby Animation
On Tuesday, March 2, 2010, NASA’s Cassini spacecraft will make its closest encounter yet with Saturn’s second largest moon. Image credit: NASA/JPL
› Full animation and caption

One of the most interesting results to come out of the 2005 and 2007 flybys came from the fields and particles instruments: the mysterious signature of electron depletion around Rhea, suggestive of a debris ring. (Basically, solid material appears to be absorbing electrons in the vicinity of Rhea.) So Rhea could be a moon with its own ring! The ring has not been seen by any of the remote sensing instruments on Cassini, however. It can be difficult to get the viewing geometry just right in order to see this type of thing — recall that the Cassini cameras didn’t definitely see Enceladus’ plume until after being in orbit for more than one year!

Tuesday’s flyby should give us some clues about the suspected debris disk around the moon, but the slam-dunk experiment to “see” Rhea’s debris disk is what we call a stellar occultation through the ring plane - looking to see if debris particles or clumps block out light from stars. Unfortunately we won’t get to do such an occultation on this flyby. This is a tricky experiment to do because you have to get the timing and the geometry just right, but we’re hoping to do it at some point later in the mission.

Anyway, on to Tuesday’s flyby! To get a sense of what we’re going to do, check out the movie made by Cassini navigator Brent Buffington that shows each of the activities performed during the flyby.

We will approach Rhea on the night side, so the moon will be dark. This is an especially good opportunity for the radar instrument to make measurements. (The cameras and imaging spectrometers typically prefer to observe the dayside, not the nightside.) Radar will do synthetic aperture radar imaging scans similar to those at Titan and will also do measurements to understand the surface composition. Previous measurements had suggested an asymmetry in brightness (which could be due to compositional differences) between the leading and trailing hemispheres of the moon, so this flyby will help with investigating that.

At closest approach, the fields and particles instruments will take data that will help us understand the environment of Rhea — its interaction with Saturn’s magnetosphere, its debris disk, and its ejecta cloud density. Ejecta clouds are dust or material that is being ejected or sputtered or otherwise lost from Rhea and its environment and contributing to populations of neutral particles and plasma in the Saturn system. This material may also be contaminating Saturn’s rings.

Outbound, the remote sensing instruments will take over. They will make measurements — in wavelengths as short as the ultraviolet all the way to the far infrared — of Rhea’s surface terrains and composition, as well as its surface temperature. The cameras have seen some “bluish spots” that could be related to the debris ring material - so those regions will be investigated more during this encounter, as will the fractured “wispy” terrain. The visual and infrared mapping spectrometer and the ultraviolet imaging spectrograph will do imaging spectroscopy to search for and map out water ice grain sizes, carbon dioxide, ammonia and fine-grained iron particles, among other materials. The composite infrared spectrometer will map temperatures across portions of Rhea’s sunlit disk at high resolution. Ninety minutes after closest approach, Rhea will enter Saturn’s shadow, giving the composite infrared mapping spectrometer a good opportunity to measure the cooling of the surface, which will provide information about the texture of the uppermost surface layers.

But wait - there’s more! Not only do you get a Rhea flyby, but we’re going to throw in a close approach to the small moon Helene! Helene is one of the “co-orbitals” of Dione. That means it orbits Saturn at the same radial distance as Dione, but it happens to be 60 degrees ahead of Dione. Helene is only about 30 or 35 km across (19 or 22 miles) and it’s not spherical (see PIA10544). Cassini will approach Helene within about 1,825 kilometers (1,130 miles) — by FAR the closest we’ve ever gotten to Helene — allowing the cameras and imaging spectrometers to obtain information about individual regions across the surface.

So this promises to be an exciting period. Please stay tuned to see the great results!

Cassini’s Swoop over Enceladus: First Morsels of Science Coming Back Now

Thursday, November 5th, 2009
Bonnie J. Buratti
Bonnie J. Buratti

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).

Raw image of Cassini's Nov. 02, 2009 flyby of Enceladus
Cassini captured this raw image on its Nov. 02, 2009, flyby of Enceladus. The camera was pointing toward Enceladus from approximately 10,000 kilometers (6,000 miles) away. Image credit: NASA/JPL/Space Science Institute.
› Full image and caption

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

Friday, October 30th, 2009
Bonnie J. Buratti
Bonnie J. Buratti

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.

Mars Chart
This animation shows Cassini’s approach to Enceladus, rotating through the various instruments scanning Enceladus for data. Image credit: NASA/JPL.
› View animation

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.

Exciting Times for Cassini

Wednesday, November 5th, 2008
by Amanda Hendrix

It’s an exciting time in Cassini-land these days! We are well into the Equinox Mission, an extension to Cassini’s mission that includes seven flybys of the Saturnian moon Enceladus, discovered in July 2005 to be geologically active. Prior to the prime mission, we knew that Enceladus was interesting and unique, and thus planned and executed three targeted flybys for the prime mission. With the tremendous discovery of water plumes at the south pole of this small icy moon (which happened on the second targeted flyby), we planned a more in-depth investigation for the Equinox Mission. And we are well into it! Our first Enceladus flyby of the Equinox Mission was in August, and we had two in October.

My job on Cassini is two-fold: I am on the science planning team, helping to plan out the science activities that occur during each icy moon encounter, and I am on the team for the ultraviolet imaging spectrograph instrument, studying ultraviolet data of the surfaces of these icy moons. So it’s really fantastic to be involved in planning each encounter, and then analyzing data to understand the moons.

Close-up view of Enceladus, taken on the Oct. 31 flyby. Image credit: NASA/JPL

In order to learn as much as we can about crazy Enceladus (it’s so small and icy — yet it’s got these geysers!), we want to let all of the instruments make measurements, and it isn’t possible to simultaneously get measurements from all instruments. (That’s just the way the spacecraft is built.) We know that the cameras will tell us a lot about the current and historical geology of the surface, the ultraviolet and infrared imagers will tell us about the surface composition, and the long-wavelength infrared instrument will reveal surface temperatures. These four “remote-sensing” instruments can take data simultaneously. But if we want to get the best data from the “in situ” instruments (like the ion and neutral mass spectrometer and cosmic dust analyzer), we need to orient the spacecraft such that it’s nearly impossible to get remote sensing data. So we divide up the flybys and allow many instruments the opportunity to get data. The period around the closest approach during the August flyby (called “E4″) was allocated to the remote-sensing instruments — and this resulted in the highest-resolution images of the active “tiger stripes” ever! (See one of these images here: The closest approach of the next Enceladus flyby - called “E5,” on October 9 — took the spacecraft deeper into the south polar plume than ever before. Here the priority was given to the in situ instruments, which obtained great, high-signal data of the plume, telling us about the composition of both the gaseous and particle components. And the October 31 flyby - called “E6″ — was again dedicated to remote-sensing, for a last look at the south pole before it heads mostly into seasonal darkness.

It’s so fortunate that Cassini has multiple opportunities to execute close encounters of an object as dynamic as Enceladus. The Voyager spacecraft had just one shot as they flew through the Saturn system, but Cassini, as an orbiter, gives us the chance to analyze our data, figure out what we’ve learned, and make thoughtful decisions on what experiments we need to make to follow up on those discoveries.

Things aren’t completely within our control, however! For instance, southern summer in the Saturn system is coming to a close, limiting the amount of sunlight illuminating the fascinating south polar region of Enceladus. But there’s plenty of important science to do in the dark with the in situ instruments, as well as the composite infrared spectrometer and radar, which is great. Who knows — we’ll see what the equinox season (and hopefully the following solstice) has in store for us! We may get some surprises!

Here We Are … at Saturn - by Bob Pappalardo

Monday, July 14th, 2008

Cassini arrived at Saturn in 2004. › Full caption

Here we are, four years after the Cassini spacecraft entered orbit around Saturn. We’re about to begin the extended mission, termed the Cassini Equinox Mission. Cassini has been a scientifically remarkable mission and a fantastic return on the investment. If you are reading this blog, then you might already know about Cassini’s discoveries at Enceladus, Titan, the other icy moons, the rings, the magnetosphere and Saturn itself. But if you’re new to following this mission, you can catch up on those discoveries by reading about them here: This great science is accomplished by an international team of scientists and engineers. I am thrilled to be able to carry the scientific reins for Cassini as its incoming project scientist. The project scientist is essentially the mission’s chief scientist, who watches out for the overall scientific integrity of the mission.

Unprocessed image
of Saturn’s moon Enceladus, taken during a
close flyby in March 2008. › Read more

My own background is in the geology of icy moons of the outer solar system. Though the planets have always enthralled me, I trace this specific icy interest back to a course I took as an undergraduate at Cornell University in about 1984, taught by Carl Sagan and his post-doctoral research associate Reid Thompson, entitled “Ices and Oceans in the Outer Solar System.” The course included discussion of Jupiter’s moon Europa, which it was thought might have a globe-girdling ocean beneath its icy surface — an idea that would be further tested by the Galileo spacecraft when it arrived at Jupiter a decade later. We also learned about Saturn’s haze-shrouded moon Titan, which might just have seas of organic rain and liquids on its surface — but we wouldn’t know for certain until the Cassini spacecraft arrived at Saturn two decades later. Who could possibly wait so long? And who would have thought that once we all did, both of these seemingly far-fetched ideas would turn out to be correct? (If only Carl and Reid could be here today to know it.)

Artist concept of Europa Orbiter concept mission.
Artist concept of Europa Orbiter concept mission.
› Larger image

Two years ago I came to JPL with the goal of getting the next flagship mission to the outer solar system off the ground. It takes a great deal of time and energy to make such a mission a reality. They are relatively expensive and take a long time from concept to completion. But just as others before me — such as Galileo Project Scientist Torrence Johnson and Cassini Project Scientist Dennis Matson — have worked to send those missions into space, I would help create the next mission, potentially to orbit Jupiter’s moon Europa. Currently I serve as JPL’s study scientist for the Europa Orbiter mission concept (described at This mission concept is in friendly competition with a mission that would orbit Titan. I hope that somehow, in time, we can make both of these spectacular mission concepts come to fruition.

Entering into the wonderland that is Cassini, my eyes are wide open to the science and engineering behind the curtain, while wary of its history and complexity. My operating philosophy is to always be true to the science. With good planning and good fortune, Cassini will keep going down the road for many years to come, following up on its prime mission discoveries and in making new ones that we can’t dream of yet.

Stay tuned for more to come. It’ll be a great ride!