Cassini

Cassini is a robotic spacecraft that arrived at Saturn on July 1, 2004 Universal Time (June 30 in U.S. time zones), and has orbited the planet ever since, studying its famous rings and family of intriguing moons. The Huygens probe was attached to Cassini before landing on Saturn’s largest moon, Titan, in January 2005.

The Cassini-Huygens mission is an international collaboration among three space agencies. The Cassini orbiter was built and is managed by NASA's Jet Propulsion Laboratory. The European Space Agency (ESA) built the Huygens probe. The Italian Space Agency provided Cassini's high-gain antenna. Seventeen countries had contributed to the mission at the time of launch, and more than 250 scientists worldwide are involved in studying the data coming in from the Saturn system.

You can find a lot of information on our Website: solarsystem.nasa.gov/missions/cassini. For downloadable fact sheets and press kits, visit our resources section. 

For weekly updates of significant events, visit our news section and follow along daily on social media at twitter.com/cassinisaturn and facebook.com/NASACassini.

The maximum speed clocked by Cassini was 44.0 kilometers per second (98,346 miles per hour) relative to the Sun on June 25, 1999.

Relative to Saturn, the spacecraft reached 30.7 kilometers per second (68,771 mph) during the Saturn Orbit Insertion maneuver on July 1, 2004.

With respect to Earth, the maximum speed reached by Cassini is 19.0 kilometers per second (42,561 mph) on Aug. 18, 1999, as the spacecraft flew past our home planet at an altitude of 1,171 kilometers (727 miles).

During the second Venus flyby, Cassini zoomed by the planet at 13.6 kilometers per second (30,523 mph) on June 24, 1999.

The spacecraft flew by Jupiter at a speed of 11.6 kilometers per second (25,951 mph) on Dec. 30, 2000.

The Cassini mission has yielded some of the richest results in the history of space exploration. For example, the Huygens probe made first landing on a moon in the outer solar system, Titan.

Cassini discovered active, icy plumes emanating from an underground ocean on the Saturnian moon Enceladus. The mission revealed Saturn’s rings to be active and dynamic -- a laboratory for how planets form. It found Titan to be an Earth-like world in some ways with rain, rivers, lakes and seas. Cassini’s instruments studies of the great northern storm of 2010-2011. The mission solved the mystery of the dual bright-dark surface of Iapetus, and much more.

See a list of some of the top discoveries of the mission overall, as well Enceladus findings and the best images so far.

To find out today's positions and speed, go to "Where is Cassini Now" and click the frame at the lower right. The spacecraft speed (relative to Saturn) is indicated in kilometers per hour next to Cassini's position.

Cassini carries a DVD with 616,400 digitized signatures of people from 81 countries. This was the first "Send your Name to" disk project. You can find the story of the disk at solarsystem.nasa.gov/news/102803/

The JPL Public Services web page has all the information you'll need to book a visit to the Jet Propulsion Laboratory www.jpl.nasa.gov/pso/index.cfm. 

“Rev” numbers assigned to each tour orbit, assuming each orbit begins at apoapsis (the most distant point from the planet). The partial orbit from SOI to the first apoapsis is Rev 0. The rev number is incremented at each succeeding apoapsis.

Add one to a Rev to get the total number of orbits Cassini has completed around Saturn. For example, Rev 251 is Cassini's 252nd trip around Saturn since arriving in 2004.

Unprocessed and uncalibrated (or “raw”) images from the cameras of Cassini’s Imaging Science Subsystem (ISS) are regularly published soon after they are received on Earth at: https://solarsystem.nasa.gov/missions/cassini/galleries/raw-images. The most recent images are displayed in reverse chronological order of when they were downlinked, with the most recently received images first. The images are also searchable by camera (wide angle or narrow angle), target, observation time and distance from target.

The fully validated and calibrated images are made available, with a time delay of between nine months and one year, at the Planetary Data System. In all cases, the images are available for free, but please refer to the official Image Use Policy.

The exposures needed to take images of Saturn and its moons are still fairly short compared with the exposure times it takes to see the stars, which are much dimmer. If you look really close you can sometimes see stars in images that are overexposed. In long exposures where the spacecraft has moved during the exposure, stars also sometimes appear as parallel streaks.

When a photographer tilts his or her camera to best fit the scene, the resulting images may appear sideways or at an angle. The same is true for Cassini - the images reflect the orientation of the photographer, in this case the spacecraft. The images on this web page have not been processed in any way, so there is no guarantee that the images will consistently show North at the top of the frame.

Additionally, sometimes images are taken when another instrument or spacecraft subsystem controls the orientation. In this case, the scene may not be optimized for the imaging field of view, but represents a unique opportunity to capture an image.

It's hard getting data all the way from Saturn. Bad weather or antenna problems at one of the Deep Space Network stations can cause data to be lost because of trouble locking the signal. This results in gaps in the images. To overcome these problems, data for important observations is played back again at a different time. For all of the other times there is just one chance to get the data.

It takes a lot of data to represent an image, much more than the old adage that a picture is worth a thousand words. For Cassini it is more like a million words—data words. Because of the limited space on Cassini's recorder and the time it takes to transmit that data, cameras on board the spacecraft were built to have the ability to compress the data, that is, to have it take up less space on the recorder. There are two kinds of compression the camera can use—"lossless" and "lossy." With lossless compression the original data can be completely recovered once it reaches the ground—much like what happens with zip compression for personal computer files or the compression in GIF images. The lossy compression is more efficient and produces smaller files, but does not give the ability to get back all the original data—like the compression in JPEG images or in MP3 audio files.

In lossless compression mode, the camera guarantees the compression will result in data at most half the amount as the original image. If the scene has a lot of detail, it will not compress as a well as a less detailed image. In this case, when the image has too much detail to be compressed by half, the camera cuts data off the end of the second line until the compression is sufficient. This means that in images with a lot of detail, the right side of even lines can be cut off in some parts of the image or throughout the whole image.

It takes a certain amount of time to read the data from the camera's sensor. It also takes a different amount of time for the camera to send the data over wires to the spacecraft for recording. The camera has four different sized time windows in which it is allowed to read out an image. If the time window picked by the scientist who planned the image is too short, the image will be incomplete and cut off at the bottom. There can be two reasons for this. If the scientist has chosen to use compression, the scene might contain more detail than expected and thus have too much data to readout in the amount of time given. The other reason could be that, in order to conserve the limited amount of data the scientist is allowed to collect, the scientist may have the image cut off on purpose because the interesting thing is in the top half.

An image may also be cut off at the bottom due to what is called "data policing." Each instrument is given a certain amount of space on Cassini's recorder to store the data collected, and there is much demand among instruments to get this precious space. Once a science team designing an observation fills up the allotted space, the spacecraft stops recording, thus resulting in a cutoff image.

Cassini's cameras have 63 different exposure settings, from 5 milliseconds to 20 minutes. Scientists planning an observation must choose the exposure for each image taken. That can be tough if you're taking a picture of something you've never seen before. Thus, incomplete information on how bright something can be can lead to an underexposed or overexposed image.

Images can be overexposed on purpose too. If the scientist is looking for something dim next to something bright, the bright thing may be overexposed. Finally, Optical Navigation personnel use images to see where Cassini is relative to Saturn and its moons. Often they overexpose images because they need to see where these moons are in relation to the stars in the background sky.

The Cassini cameras are 1-megapixel cameras. A normal image is 1024 x 1024 pixels. Using a technique called "summation" the cameras have the ability to combine pixels together to get smaller but less noisy images. This results in smaller images that take a lot less time to readout out and take up less data volume. Summation is very useful if a scientist needs to conserve both. In the 2 x 2 mode, the camera takes a 2 x 2 pixel square and averages those values into a single pixel. Images in this mode will be 512 x 512 pixels. In the 4 x 4 mode, the camera takes a 4 x 4 pixel square and makes that a single pixel. Images in this mode are 256 x 256 in size.

There are high-energy particles that fly though space called cosmic rays. When one of these particles hit the camera's sensor, it causes a bright spot. When one of the particles hit the camera's sensor edge-on, it can leave a trail across the image. Exposures shorter than a second will not have many of these spots or trails. However, long exposures, like those from a minute to 20 minutes will contain many of these trails.

Small doughnut-like dark spots in images are actually out of focus dust specks on the filter wheels, lenses or other parts of the optics of the cameras. Because there is no way to clean the cameras in space, more of these spots have appeared as the Cassini mission progresses.

There is a low level source of noise in the camera's signal as it comes out of the sensor and gets converted to numbers. This noise adds and subtracts a small amount to the signal in a cycle. When the data is put into an image, one can see it as bright and dark bands in the image. The amount of noise is very small and is not noticeable in most images. Images that are of black sky or very dark can show this noise. The camera records the baseline of the signal for each line so this noise can be removed in later processing. Both cameras are affected by this noise but the Narrow Angle Camera is worse.

The Cassini spacecraft is moving very fast through the Saturnian system. When the cameras are taking pictures of objects very far away this doesn't matter too much. However, if Cassini is taking images of a moon during a close flyby, the change in distance or position during the exposure can cause the image to be smeared—much like taking pictures of close-by things from a fast moving car.

Also, many instruments may be taking data at the same time but the spacecraft pointing for the observation would have been generated by a single instrument team. In this case, a mistake or miscommunication could result in an image being taken while the spacecraft was turning from one position to another.

The Narrow Angle Camera was plagued with a contamination problem. This caused images taken between May 2001 and early 2002 to look hazy. After a year of heating treatments, the haze problem was resolved. For more information see this article. 

The camera measures light from an object at each point in an image and assigns it a number from zero to 4095 depending on its brightness. Sometimes the scientist can't afford to send this amount of data for each pixel because of the amount of storage it takes. The camera has the ability to convert this range of values to those from zero to 255. The camera does this according to a preset table of values designed by the scientists. This table devotes many of the 256 levels for less bright things and less levels for brighter pixels. Part of calibrating an image on the ground is to reverse this table and get back pixels in the range of zero to 4095. Because you're looking at the raw data, images sent back in this mode will have dimmer things look brighter compared to the brighter parts of the image than in images not in this mode.

As in the previous question, the other way the camera can send back less data (by sending pixels with values from zero to 255 instead of zero to 4095) is to send back only the lower binary digits of the number. This is like having a list of amounts of money and only recording the amount of cents for each one and assigning the brightness in an image to the amount of leftover cents. Pixels with brightness values just under 255, like amounts just under a dollar, will appear almost white, while pixel values just over 255, like amounts just over a dollar with not many cents, will appear dark. The ideal use of this mode is for image scenes that are dark with almost all of the pixel values less than 255. If the scene is simple with gradual increases in brightness, then even if the original values get over 255 and go dark again, the scientists can figure out what the real value was. If the scene is very complicated or the original values are much brighter than 255, the image can have many bright and dark transitions with strange contours. In this case, the image will look very bizarre but not have much scientific value.

When the cameras take an image of something like a moon with a very bright Saturn just out of view, light shining from the planet can reflect off parts of the inside of the camera and onto the sensor. The inside of each camera is coated with a black non-reflective substance to minimize this scattered light. Still, some light does get in and the result can be rays or large fuzzy circles of light.

When the Mission and Science Planning Teams build the computer commands that are sent to the spacecraft, one of the things they need to do is tell the spacecraft where to point. The computer on board the spacecraft has a catalog of pointing "targets," generally identified by a single word for easy reference. Some of the available options for pointing the spacecraft include: Saturn, rings, most major moons and "sky." For example, when the target is "Titan," during the observation the spacecraft targets Titan and then tracks Titan as it moves relative to the background stars.

The "sky" position is used to point the spacecraft at a fixed location in the sky and take a picture of whatever is there. It is typically used to take images of unrecognized moons (newly discovered ones, for example) and for optical navigation.

Optical navigation images are used as a way for the navigation team to fine-tune their understanding of the exact orbit of moons. While capturing an optical navigation image, cameras on the spacecraft do not track the moon (which is orbiting around Saturn and therefore moving), but they stay fixed on the background stars. After taking an optical navigation image, the navigation team compares the position of the moon relative to the stars in the background of the image and calculates its orbit accurately.

On Aug. 19, 2007, Cassini entered "solar conjunction"—which means the Sun was between Earth and the spacecraft. During solar conjunctions, sending images to Earth is very difficult, if not impossible due to interference from the Sun, thus no images are taken. However, the position offers a great opportunity to study the Sun, and the Cassini radio science group conducted a study to characterize the Sun's corona.

Once the spacecraft exited "solar conjunction" it resumed taking images, starting on Aug. 29 with the moon Tethys.

The sensor on the Narrow Angle Camera has a flaw where the first 12 or so pixels at the left of the image are darker than the rest. This flaw was found before Cassini was launched but it was determined that it would cost too much to fix it.

Saturn is easily seen with the naked eye, so people must have viewed it since prehistoric times. Galileo was first to see its rings, but his telescope wasn't powerful enough to show them clearly, and he initially thought he was observing a triple planet. Dutch astronomer Christiaan Huygens was first to identify a ring around Saturn.

You can see Galileo's sketches and read about the history of Saturn observation here: http://galileo.rice.edu/sci/observations/saturn.html

For an overview of what the Cassini spacecraft has discovered, visit our Saturn page.

For general information about Saturn, visit NASA's Solar System Exploration Website.

For more technical information, visit these National Space Science Data Center pages:

• Saturn Fact Sheet
• Saturn's Moons Fact Sheet
• Saturn's Rings Fact Sheet

Saturn has no solid surface, so you'd sink down into it. The deeper you sank, the more heat and pressure you'd experience. Eventually, not even a spaceship built like the strongest submarine would be able to withstand the pressure, and you'd be crushed and roasted.

It depends on whether you're talking about volume or mass. Saturn has about 84 percent the diameter of Jupiter, but only about 30 percent as much mass. Such a large volume with so little mass makes Saturn the least dense of all the planets, and the only one that would float if it were possible to place it into a tub of water.

Saturn's rings are an optical illusion on a cosmic scale. Far from solid, they're actually a blizzard of water-ice particles mixed with dust and rock fragments. Most are from around 1 cm to 5 meters, but they range from the size of smoke particles to boulders as big as a house. A few may reach kilometer-size. Each ring particle orbits Saturn independently, a moon unto itself. There are thousands of rings.

Based on stellar occultation measurements of ring edges, it is looking like the main rings (A, B and C) are on the order of 33 feet (10 meters) thick, or less. One exception are the bending waves, created by Mimas, whose orbit is slightly inclined with respect to the rings. Bending waves are vertical waves, and their height might reach .6 miles (1 kilometer) or so above and below the rings.

The mean distance from Saturn to the outer edge of the F ring is 2.33 Saturn radii, or 87,160 miles (140,270 km). The mean distance between the surface of Saturn and the first ring (we use the inner B ring) is 1.52 Saturn radii or about 57,000 miles (92,000 km).

To make your own model of Saturn and its rings, cut a 1.5-inch Styrofoam ball in half, and glue each half to opposite sides of a CD. That gives you pretty accurate proportions. Two differences: Saturn's rings don't actually touch the planet, and the CD is too thick. To get that proportion right, you'd need a single sheet of paper about 2 miles (3 km) in diameter and a much larger Styrofoam ball.

Yes, Jupiter, Uranus and Neptune all have rings. None of those rings are as spectacular as Saturn's, but it appears to be a common phenomenon for large planets.

That's one of the things we're hoping Cassini will help to answer. There are currently three theories of how Saturn got its rings:

1. The rings may be remnants of the material that formed Saturn's moons, but which were prevented from coalescing into a moon because they were inside the Roche limit (see below).

2. A medium-size moon might have strayed inside the Roche limit, and been pulled to pieces by tidal forces.

3. A moon might have been shattered by meteor impacts, and its debris might have moved to within the Roche limit, where it was unable to reunite into a large body.

The closer you are to a planet, the stronger is its gravitational pull on you. For a large moon, this means that the side closest to the planet is being pulled substantially more forcefully than the side facing away from the planet. Within a certain distance from the planet, that difference can be enough to pull the moon apart. The Roche limit is the minimum distance that a moon (or other large object) can be from a planet without being torn to bits. (For smaller objects, the difference in gravitational pull from one side to the other isn't enough to pull it apart.) If the planet and the orbiting body have the same density, that distance is about 2.5 times the radius of the planet.

The mysterious dark "spokes" that radiate across the B-ring are thought to consist of tiny charged particles that have become trapped within the lines of Saturn's magnetic field. They can develop quite rapidly and then slowly fade. Voyager spotted one that grew over 3,700 miles (6,000 km) in just five minutes. Cassini has made many observations of ring spokes.

Saturn, its rings, and most of its moons all probably formed from a spinning disk of gas and dust within the original solar nebula. As it collapsed toward its center, it would have formed a central sphere (which became Saturn), leaving a disk of material orbiting its equator. That material probably coalesced into many or most of Saturn's moons. The ring particles may be material that was left over from this process, or they may be the remnants of a moon that was shattered by collision or by tidal forces (see Roche limit). In either case, the ring particles would have kept the angular momentum of the original disk, and continued orbiting Saturn in its equatorial plane.

If Saturn captures particles coming in from other directions (along the polar plane, for example), they will tend to be pulled toward the equatorial plane, too. Saturn's rapid rotation creates a centrifugal effect that produces a bulge around its equator. With more mass around its equator than at its poles, Saturn's gravity is stronger around its middle, so incoming particles would tend to be drawn there. Once in the area, they'd be likely to collide with some of the many other particles orbiting in that plane, which would rob them of their initial momentum and encourage them to join the throng moving along the equatorial plane.

Although composed mainly of hydrogen, helium and methane, Saturn's atmosphere lacks enough oxygen to sustain a fire. Only in the presence of a rich supply of oxygen, such as that found on Earth, do gases like hydrogen and methane ignite when hit by lightning.

The authors of the papers on clumping and dispersal in Saturn's rings do not specifically address fragment sizes during dispersal. It is probably safe, though, to use terrestrial analogs to answer the question.

First, the dispersal process is not explosive. A dispersal would be explosive if there was a hyper-velocity collision. Such an event is more likely due to an external meteoroid/asteroid collision with the clump in the rings rather than collisions of particles or clumps internal in the rings, which are moving at similar speeds around Saturn.

When it occurs, dispersal is most likely a gentle separation of the constituent fragments that had gently combined earlier. The separation is gentle because it is induced by the force of gravity, with some external source of gravity (another clump, a moon of Saturn, or even a planetary or solar tide) overcoming the strength of the gravity holding the clump together.

Earth's ocean tides are an example of a fragment (water) gravitationally bound to another fragment (Earth) which is being pulled away by the gravity of another body (the Moon).

Think about what happens when you wash a clod of dirt in water. The components in the clod, mineral fragments, roots, rocks, etc. separate out without changing their sizes or shapes. A snowball or crust of snow shatters into pieces the size of the snow that was originally collected. Unless chemical reactions occur (doubtful) or physical changes occur due to significant compression (possible in the cores of larger clumps), the unconsolidated components of a clump will generally disperse back into pieces of similar size that formed the clump.

After almost 20 years in space, the Cassini mission will end on September 15, 2017 at about 5 a.m. PDT (8 a.m. EDT). Read More ›

With input from more than 2,000 members of the public, team members on NASA's Cassini mission to Saturn have chosen a name for the final phase of the mission: the Cassini Grand Finale.

Starting in late 2016, the Cassini spacecraft will begin a daring set of orbits that is, in some ways, like a whole new mission. The spacecraft will repeatedly climb high above Saturn's north pole, flying just outside its narrow F ring. Cassini will probe the water-rich plume of the active geysers on the planet's intriguing moon Enceladus, and then will hop the rings and dive between the planet and innermost ring 22 times.

Because the spacecraft will be in close proximity to Saturn, the team had been calling this phase "the proximal orbits," but they felt the public could help decide on a more exciting moniker. The Cassini mission invited the public to vote on a list of alternative names provided by team members or to suggest ideas of their own.

"We chose a name for this mission phase that would reflect the exciting journey ahead while acknowledging that it's a big finish for what has been a truly great show," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory in Pasadena, California.

Concepts were evaluated for parking Cassini in an orbit around Saturn that would have been stable for a long time, along with a variety of other mission scenarios. However, the Grand Finale of close dives past the outer and inner edges of the rings, and ultra-close brushes with the planet and its small, inner moons, offered such enormous scientific value that this scenario was chosen for the mission’s conclusion.

The Cassini spacecraft will continue to collect and transmit scientific data as it enters Saturn’s atmosphere. However, we do not expect to get much data back after atmospheric entry. Cassini’s entry will be very rapid and atmospheric drag will quickly cause the spacecraft to tumble, severing the connection.

Based upon exposure experiments on the Space Station, it is known that some microbes and microbial spores from Earth are able to survive many years in the space environment– even with no air or water, and minimal protection from radiation.  Therefore, NASA has chosen to dispose of the spacecraft in Saturn’s atmosphere in order to avoid the possibility that viable microbes from Cassini could potentially contaminate Saturn’s moons at some time in the future.

While the Grand Finale offers unprecedented opportunities for new discoveries during Cassini’s final orbits and a thrilling end to the Cassini mission, it is not without risk.

The Cassini spacecraft and instruments were designed to observe Saturn, its rings and satellites from a relatively benign vantage point away from Saturn’s rings and atmosphere. A primary concern is the effect of Saturn’s atmosphere. If it is denser than modeled, it can cause the spacecraft to lose its ability to maintain a fixed attitude. Though recoverable, this could curtail science observations while the spacecraft team reestablishes attitude control.

Another, more significant, concern is the potential impact with ring material. The Cassini spacecraft has successfully made numerous passes through the G ring, which is similar to the D ring region the spacecraft will pass through during its final orbits. While the engineers and scientists believe that the flight path during the Grand Finale will pose no greater threat, the spacecraft will be traveling at over 34 kilometers per second (76,000 mph) in uncharted regions; an impact with even a grain-of-sand-sized piece of ring material could seriously damage the spacecraft or an instrument.

The Grand Finale will allow unprecedented measurements of Saturn’s atmospheric composition and interior structure. Cassini’s dramatic end-of-mission plan enables three new scientific campaigns:

• Cassini will be able to measure Saturn’s gravity and magnetic fields to study its interior structure, reveal its internal rotation rate and provide a basic understanding of how giant planets form and how they work.
• It will evaluate the total main-ring mass to estimate ring system age and longevity.
• Finally, it will directly sample Saturn’s ring particles, innermost radiation belts and upper atmosphere in a region never previously explored.

These orbits will also provide some of the mission’s highest resolution views of the innermost rings and planet. After twenty orbits passing within 620 miles (1000 kilometers) of the narrow F ring, the spacecraft will dive through the 1240-mile- (2000-kilometer-) wide gap between the D ring and the planet 22 times before finally entering Saturn’s atmosphere, where it will burn up.

Technically, the first orbit in Cassini’s Grand Finale phase begins when the spacecraft is at the farthest point from Saturn in its orbit (called apoapse) on April 22, 2017. But the real excitement of the Grand Finale begins on April 26, when Cassini makes its first dive between Saturn’s rings and the planet itself, which is why the mission is celebrating this milestone. 

The preferred end-of-mission plan for Cassini has always been to safely dispose of the spacecraft in the upper atmosphere of Saturn. The exact “when” and “how” of the mission’s conclusion has evolved over the years as the scientifically productive mission has been granted three extensions by NASA. The current “Grand Finale” scenario – to send the spacecraft on a series of orbits between the planet and its rings -- has been part of the mission plan since 2010 and was developed in detail over the past four years.

Disposing of Cassini in Saturn’s atmosphere is safe. The spacecraft will enter Saturn’s atmosphere at high speed and will burn up like a meteor. Any spacecraft material that survives atmospheric entry, potentially including its radioisotope fuel, will sink deep into the planet where it will melt and become completely diluted as it mixes with the hot, high-pressure atmosphere of the giant planet.

Saturn’s atmosphere does not have conditions that would be favorable to life as we know it, according to evaluations by the Committee on Space Research of the International Council for Science.