Cassini: FAQ

Illustration of a spacecraft in space


  • What was Cassini-Huygens?
    Cassini was a robotic spacecraft that arrived at Saturn on July 1, 2004 Universal Time (June 30 in U.S. time zones), and orbited the planet, studying its famous rings and family of intriguing moons. Cassini plunged into Saturn’s atmosphere on Sept. 15, 2017, returning science data to the very end. The Huygens probe was attached to Cassini before landing on Saturn’s largest moon, Titan, in January 2005.The Cassini-Huygens mission was an international collaboration among three space agencies. The Cassini orbiter was built and 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 contributed to the mission, and more than 250 scientists worldwide are involved in studying the data coming in from the Saturn system.
  • Where can I find basic information about Cassini? How can I get updates as the mission progresses?You can find a lot of information on our website: For downloadable fact sheets and press kits, visit our resources section.
  • What were the fastest speeds reached by the Cassini spacecraft? The maximum speed clocked by Cassini was 98,346 miles per hour (44.0 kilometers per second) relative to the Sun on June 25, 1999. Relative to Saturn, the spacecraft reached 68,771 mph (30.7 kilometers per second) during the Saturn Orbit Insertion maneuver on July 1, 2004.
  • What did Cassini discover? The Cassini mission 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. The mission solved the mystery of the dual bright-dark surface of Iapetus, and much more.
  • What was the program to send people's names to Saturn with the Cassini spacecraft? Cassini carried a DVD with 616,400 digitized signatures of people from 81 countries. This was the first "Send your Name to" disk project.

Raw Images

  • Where can I find raw images from the Cassini mission? Unprocessed and uncalibrated (or “raw”) images from the cameras of Cassini’s Imaging Science Subsystem (ISS) are here:
  • Why don't I see stars in the images? The exposures needed to take images of Saturn and its moons were 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 moved during the exposure, stars also sometimes appear as parallel streaks.
  • Why do some images look sideways or upside down? 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 was true for Cassini - the images reflected the orientation of the photographer, in this case the spacecraft.
  • Why are parts of some images missing? It's hard getting data all the way from Saturn. Bad weather or antenna problems at one of the Deep Space Network stations caused data to be lost because of trouble locking the signal. This resulted in gaps in the images. To overcome these problems, data for important observations was played back again at a different time. For all of the other times there was just one chance to get the data.
  • Why were parts of the right side of some images missing?
    It takes a lot of data to create an image, much more than the old adage that a picture is worth a thousand words. For Cassini it was more like a million words – data words. Because of the limited space on Cassini's recorder and the time it took 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 were two kinds of compression the camera used – "lossless" and "lossy."
  • Why was the bottom of some images missing?
    It took a certain amount of time to read the data from the camera's sensor. It also took a different amount of time for the camera to send the data over wires to the spacecraft for recording. The camera had four different sized time windows in which it was allowed to read out an image. If the time window picked by the scientist who planned the image was too short, the image would be incomplete and cut off at the bottom. There can be two reasons for this. If the scientist had chosen to use compression, the scene might have contained more detail than expected and thus have had 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 was allowed to collect, the scientist may have the image cut off on purpose because the interesting thing is in the top half. An image also may have been cut off at the bottom due to what is called "data policing." Each instrument was given a certain amount of space on Cassini's recorder to store the data collected, and there was much demand among instruments to get this precious space. Once a science team designing an observation filled up the allotted space, the spacecraft stopped recording, thus resulting in a cutoff image.
  • Why were some images overexposed?
    Cassini's cameras had 63 different exposure settings, from 5 milliseconds to 20 minutes. Scientists planning an observation had to choose the exposure for each image taken. That could be tough when taking a picture of something you've never seen before. Thus, incomplete information on how bright something could be sometimes lead to underexposed or overexposed images. Images could be overexposed on purpose, too. If the scientist was looking for something dim next to something bright, the bright thing may be overexposed. Finally, Optical Navigation personnel used images to see where Cassini was located relative to Saturn and its moons. Often they overexposed images because they needed to see where these moons were in relation to the stars in the background sky.
  • Why are there images of different sizes?
    The Cassini cameras were 1-megapixel cameras. A normal image is 1024 x 1024 pixels. Using a technique called "summation" the Cassini cameras had the ability to combine pixels together to get smaller but less noisy images. This resulted in smaller images that took a lot less time to readout out and took up less data volume. Summation is very useful if a scientist needed to conserve both. In the 2 x 2 mode, the camera took a 2 x 2 pixel square and averaged those values into a single pixel. Images in that mode were 512 x 512 pixels. In the 4 x 4 mode, the camera took a 4 x 4 pixel square and made that a single pixel. Images in that mode were 256 x 256 in size.
  • What were those random streaks in some images?
    There are high-energy particles that fly though space called cosmic rays. When one of these particles hit the Cassini camera's sensor, it caused a bright spot. When one of the particles hit the camera's sensor edge-on, it could leave a trail across the image. Exposures shorter than a second did not have many of these spots or trails. However, long exposures, like those from a minute to 20 minutes contained many of these trails.
  • What were those dark doughnut shapes?
    Small doughnut-like dark spots in images were 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 appeared as the Cassini mission progressed.
  • What was that horizontal waviness in some pictures?
    There was a low level source of noise in the camera's signal as it came out of the sensor and got converted to numbers. This noise added and subtracted a small amount to the signal in a cycle. When the data was put into an image, it could appear as bright and dark bands in the image. The amount of noise was very small and not noticeable in most images. Images that had black sky or were very dark could show this noise. The camera recorded the baseline of the signal for each line so the noise can be removed in later processing. Both cameras were affected by the noise but the Narrow Angle Camera was worse.
  • Why were some images smeared?
    The Cassini spacecraft was moving very fast through the Saturnian system. When the cameras were taking pictures of objects very far away it didn't matter too much. However, if Cassini was taking images of a moon during a close flyby, the change in distance or position during the exposure could cause the image to be smeared – much like taking pictures of close-by things from a fast moving car. Also, many instruments could have been 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 have resulted in an image being taken while the spacecraft was turning from one position to another.
  • Why were some images fuzzy?
    The Cassini 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.
  • Why did the contrast look different between images?
    The camera measured light from an object at each point in an image and assigned it a number from zero to 4095 depending on its brightness. Sometimes the scientist couldn't afford to send this amount of data for each pixel because of the amount of storage it took. The camera had the ability to convert this range of values to those from zero to 255. The camera did this according to a preset table of values designed by the scientists. This table devoted many of the 256 levels for less bright things and less levels for brighter pixels. Part of calibrating an image on the ground was to reverse this table and get back pixels in the range of zero to 4095. Because they were looking at the raw data, images sent back in this mode would have dimmer things look brighter compared to the brighter parts of the image than in images not in this mode.
  • Why did some images look bizarre/psychedelic?
    As in the previous question, the other way the camera could send back less data (by sending pixels with values from zero to 255 instead of zero to 4095) was to send back only the lower binary digits of the number. This was 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, would appear almost white, while pixel values just over 255, like amounts just over a dollar with not many cents, would appear dark. The ideal use of this mode was for image scenes that were dark with almost all of the pixel values less than 255. If the scene was simple with gradual increases in brightness, then even if the original values went over 255 and went dark again, the scientists could figure out what the real value was. If the scene was very complicated or the original values were much brighter than 255, the image could have many bright and dark transitions with strange contours. In that case, the image would look very bizarre but not have much scientific value.
  • What were the ghostly lights?
    When the cameras took an image of something like a moon with a very bright Saturn just out of view, light shining from the planet could reflect off parts of the inside of the camera and onto the sensor. The inside of each camera was coated with a black non-reflective substance to minimize the scattered light. Still, some light did get in and the result could be rays or large fuzzy circles of light.
  • What does it mean when the caption says, "camera was pointing toward SKY?" What is SKY?
    When the Mission and Science Planning Teams built the computer commands that were sent to the spacecraft, one of the things they did was tell the spacecraft where to point. The computer on board the spacecraft had a catalog of pointing "targets," generally identified by a single word for easy reference. Some of the available options for pointing the Cassini spacecraft included: Saturn, rings, most major moons and "sky." For example, when the target was "Titan," during the observation the spacecraft targeted Titan and then tracked Titan as it moved relative to the background stars. The "sky" position was used to point the spacecraft at a fixed location in the sky and take a picture of whatever was there. It was typically used to take images of unrecognized moons (newly discovered ones, for example) and for optical navigation. Optical navigation images were 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 did not track the moon (which was orbiting around Saturn and therefore moving), but they stayed fixed on the background stars. After taking an optical navigation image, the navigation team compared the position of the moon relative to the stars in the background of the image and calculated its orbit accurately.
  • Why were there are no raw images between Aug. 19 and Aug. 29, 2007?
    On Aug. 19, 2007, Cassini entered "solar conjunction" – which meant 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 were taken. However, the position offered 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.
  • What was that small dark vertical band on the left part of some images?
    The sensor on the Narrow Angle Camera had a flaw where the first 12 or so pixels at the left of the image were 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.

About Saturn

  • Who discovered Saturn?
    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.
  • Where can I find Galileo's sketches of Saturn?
    You can see Galileo's sketches and read about the history of Saturn observation here:
  • What would happen if you tried to land on Saturn? 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.
  • How does Saturn's size compare with Jupiter? It depends on whether you're talking about volume or mass. Saturn has about 84% the diameter of Jupiter, but only about 30% 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.
  • What are Saturn's rings made of? How many rings does Saturn have? How big are the ring particles? 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 less than half an inches to 16 feet, 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.
  • How thick are the main 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.
  • What is the mean distance between the surface of Saturn and the first ring? What is the mean outer diameter of the Saturn ring system? 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.
  • Do any other planets have rings? 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.
  • Where do planetary rings come from? How did rings form around Saturn?
    There are currently several theories of how Saturn got its rings: 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). A medium-size moon might have strayed inside the Roche limit, and been pulled to pieces by tidal forces. 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.
  • What is the Roche limit? 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.
  • What is the cause of the "spokes" seen on the rings? 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 made many observations of ring spokes.
  • Why do Saturn's rings all lie in the same plane? Why are planetary rings always found in their equatorial planes and not sometimes crossing their poles? 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.
  • If Saturn's atmosphere is composed mainly of flammable gases, why don't the lightning storms ignite the planet? 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.
  • As particles in Saturn's rings form clumps and then disperse, do they disperse at the same size as they were before they clumped, or is it more like an explosion into different sized pieces? 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.

End of Mission

  • When did the mission end? After almost 20 years in space, the Cassini mission ended on Sept. 15, 2017.
  • Why was the end of mission called the Grand Finale? With input from more than 2,000 members of the public, Cassini team members chose the name for the final phase of the mission: the Cassini Grand Finale. Starting in late 2016, the Cassini spacecraft began a daring set of orbits that was, in some ways, like a whole new mission. The spacecraft repeatedly climbed high above Saturn's north pole, flying just outside its narrow F ring. Cassini probed the water-rich plume of the active geysers on the planet's intriguing moon Enceladus, and then hopped the rings and dove between the planet and innermost ring 22 times. Because the spacecraft was in close proximity to Saturn, the team was 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.
  • Why dispose of the spacecraft in Saturn’s atmosphere instead of other possible ends for the mission (for example, send Cassini somewhere else in the solar system, attempt a landing on one of the moons, crash into the rings and watch the impact with telescopes)? 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.
  • Did Cassini continue to transmit data as it entered Saturn’s atmosphere? The Cassini spacecraft continued to collect and transmit scientific data as it entered Saturn’s atmosphere.
  • Could microbes really have survived onboard Cassini for this long in space? Is this truly a concern that influenced the decision to deorbit into Saturn? Based on exposure experiments on the International 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 decided to dispose of the spacecraft in Saturn’s atmosphere to avoid the possibility that microbes from Cassini could potentially contaminate Saturn’s moons at some time in the future.
  • What challenges did the Grand Finale present?
    While the Grand Finale offered unprecedented opportunities for new discoveries during Cassini’s final orbits and a thrilling end to the Cassini mission, it was 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 was the effect of Saturn’s atmosphere. If it was denser than modeled, it could cause the spacecraft to lose its ability to maintain a fixed attitude. Though recoverable, this could have curtailed science observations while the spacecraft team reestablished attitude control. Another, more significant, concern was the potential impact with ring material. The Cassini spacecraft successfully made numerous passes through the G ring. 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 76,000 mph (34 kilometers per second) in uncharted regions; an impact with even a grain-of-sand-sized piece of ring material could seriously damage the spacecraft or an instrument.
  • When did the Grand Finale actually start? The first orbit in Cassini’s Grand Finale phase began when the spacecraft is at the farthest point from Saturn in its orbit (called apoapsis) on April 22, 2017. But the real excitement of the Grand Finale began on April 26, when Cassini made its first dive between Saturn’s rings and the planet itself.
  • Was it always planned that Cassini would end its mission by plunging into Saturn? The preferred end-of-mission plan for Cassini was 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.
  • Why was it safe to dispose of a spacecraft by burning it up in Saturn’s atmosphere? Disposing of Cassini in Saturn’s atmosphere was safe. The spacecraft entered Saturn’s atmosphere at high speed and burned up like a meteor. Any spacecraft material that survived atmospheric entry, potentially including its radioisotope fuel, sank deep into the planet where melted and became completely diluted as it mixed 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.
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