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Cassini-Huygens

2007 Schools Wikipedia Selection. Related subjects: Space transport

                   Cassini-Huygens
   An artist's concept of Cassini
    Organization:            NASA/ESA/ ASI
    Mission type:     Fly-by, orbiter, and lander
      Flyby of:    Jupiter, Phoebe, Titan, Enceladus
    Satellite of:               Saturn
    Launch Date:           October 15, 1997
   Launch Vehicle: Titan IV-B/Centaur launch vehicle
      NSSDC ID:                1997-061A
      Webpage:           Cassini-Huygens Home

   Cassini-Huygens is a joint NASA/ESA/ ASI unmanned space mission
   intended to study Saturn and its moons. The spacecraft consists of two
   main elements: the NASA Cassini orbiter, named after the Italian-French
   astronomer Giovanni Domenico Cassini, and the ESA Huygens probe, named
   after the Dutch astronomer Christiaan Huygens. It was launched on
   October 15, 1997 and entered into orbit around Saturn on July 1, 2004.
   On December 25, 2004 the Huygens probe separated from the orbiter at
   approximately 02:00 UTC, with deployment confirmed by the Jet
   Propulsion Laboratory. The probe reached Saturn's moon Titan on January
   14, 2005, where it made an atmospheric descent to the surface and
   relayed scientific information. It is the first spacecraft to orbit
   Saturn and the fourth spacecraft to visit Saturn.

Overview

   Launch occurred at 4:43 a.m. EDT (8:43 UTC) on October 15, 1997 from
   Launch Complex 40 at Cape Canaveral Air Force Station, Florida.
   Enlarge
   Launch occurred at 4:43 a.m. EDT (8:43 UTC) on October 15, 1997 from
   Launch Complex 40 at Cape Canaveral Air Force Station, Florida.

   Cassini's principal objectives are to:
    1. Determine the three-dimensional structure and dynamic behaviour of
       the rings
    2. Determine the composition of the satellite surfaces and the
       geological history of each object
    3. Determine the nature and origin of the dark material on Iapetus's
       leading hemisphere
    4. Measure the three-dimensional structure and dynamic behaviour of
       the magnetosphere
    5. Study the dynamic behaviour of Saturn's atmosphere at cloud level
    6. Study the time variability of Titan's clouds and hazes
    7. Characterize Titan's surface on a regional scale

   The Cassini-Huygens spacecraft was launched on October 15, 1997 from
   Cape Canaveral Air Force Station's Launch Complex 40 using a U.S. Air
   Force Titan IVB/ Centaur launch vehicle. The launch vehicle was made up
   of a two-stage Titan IV booster rocket, two strap-on solid rocket
   motors, the Centaur upper stage, and a payload enclosure, or fairing.
   The complete Cassini flight system was composed of the launch vehicle
   and the spacecraft.

   The spacecraft is composed of the Cassini orbiter and the Huygens
   probe. The Cassini orbiter is planned to orbit Saturn and its moons for
   four years, and the plan sees the Huygens probe dive into the
   atmosphere of Titan and land on its surface. Cassini-Huygens is an
   international collaboration between three space agencies. Seventeen
   nations contributed to building the spacecraft. The Cassini orbiter was
   built and managed by NASA/ CalTech's Jet Propulsion Laboratory. The
   Huygens probe was built by the European Space Agency. The Italian Space
   Agency provided Cassini's high-gain communication antenna, and a
   revolutionary compact and light-weight multimode radar (synthetic
   aperture radar, radar altimeter, radiometer).

   The total cost of the Cassini-Huygens mission is about US$3.26 billion,
   including $1.4 billion for pre-launch development, $704 million for
   mission operations, $54 million for tracking and $422 million for the
   launch vehicle. The U.S. contributed $2.6 billion, the European Space
   Agency $500 million and the Italian Space Agency $160 million.

   Normally spoken, Cassini mission would end in 2008, but it is quite
   certain that further funding will be granted.

          A list of Cassini-Huygens abbreviations is available.

History

   Cassini-Huygens' origins date to 1982, when the European Science
   Foundation and the American National Academy of Sciences formed a
   working group to investigate future cooperative missions. Two European
   scientists suggested a paired Saturn Orbiter and Titan Probe as a
   possible joint mission. In 1983, NASA's Solar System Exploration
   Committee recommended the same Orbiter and Probe pair as a core NASA
   project. NASA and the European Space Agency (ESA) performed a joint
   study of the potential mission from 1984 to 1985. ESA continued with
   its own study in 1986, while American astronaut Sally Ride, in her
   influential 1987 report " NASA Leadership and America's Future in
   Space," also examined and approved of the Cassini mission.

   While Ride's report described the Saturn orbiter and probe as a NASA
   solo mission, in 1988 the Associate Administrator for Space Science and
   Applications of NASA Len Fisk returned to the idea of a joint NASA and
   ESA mission. He wrote to his counterpart at the ESA, Roger Bonnet,
   strongly suggesting that the ESA choose the Cassini mission from the
   three candidates at hand and promising that NASA would commit to the
   mission as soon as ESA did.

   At the time, NASA was becoming more sensitive to the strain that had
   developed between the American and European space programs as a result
   of European perceptions that NASA had not treated it like an equal
   during previous collaborations. NASA officials and advisors involved in
   promoting and planning Cassini-Huygens attempted to correct this trend
   by stressing their desire to evenly share any scientific and technology
   benefits resulting from the mission. In part, this newfound spirit of
   cooperation with Europe was driven by a sense of competition with the
   Soviet Union, which had begun to cooperate more closely with Europe as
   the ESA drew further away from NASA.

   The collaboration not only improved relations between the two space
   programs but also helped Cassini-Huygens survive congressional budget
   cuts in the US. Cassini-Huygens came under fire in both 1992 and 1994,
   but NASA successfully persuaded the U.S. Congress that it would be
   unwise to halt the project after the ESA had already poured funds into
   development because frustration on broken space exploration promises
   might spill over into other areas of foreign relations. The project
   proceeded politically smoothly after 1994, although, as noted below,
   citizens groups concerned about its potential environmental impact
   attempted to derail it through protests and lawsuits until and past its
   1997 launch.

Spacecraft design

   Cassini assembly
   Enlarge
   Cassini assembly

   The spacecraft was originally planned to be the second three-axis
   stabilized, RTG-powered Mariner Mark II, a class of spacecraft
   developed for missions beyond the orbit of Mars. Cassini was being
   developed together with the Comet Rendezvous Asteroid Flyby (CRAF)
   spacecraft, but various budget cuts and rescopings of the project
   forced NASA to terminate CRAF development in order to save Cassini. As
   a result, the Cassini spacecraft became a more specialized design,
   cancelling the implementation of the Mariner Mark II series.

   The Cassini spacecraft, including the orbiter and the Huygens probe, is
   the largest and most complex interplanetary spacecraft built to date.
   The orbiter alone has a mass of 2150 kilograms. When the 350-kilogram
   Huygens probe, launch vehicle adapter, and 3132 kilograms of
   propellants were loaded at launch, the spacecraft had a mass of about
   5600 kilograms. Only the two Phobos spacecraft sent to Mars by the
   Soviet Union were heavier. The Cassini spacecraft stood more than 6.8
   metres (22.3 feet) high, and was more than 4 metres (13.1 feet) wide.
   The complexity of the spacecraft is necessitated both by its
   trajectory, or flight path, to Saturn, and by the ambitious program of
   scientific observations to be undertaken once the spacecraft reaches
   its destination. It functions with 1,630 interconnected circuits,
   22,000 wire connections, and over 14 kilometers (8.7 miles) of cabling.

   Now that Cassini is at Saturn, it is between 8.2 and 10.2 astronomical
   units from Earth. Because of this, it takes between 68 to 84 minutes
   for signals to travel from Earth to the spacecraft, and vice versa. In
   practical terms, this means that ground controllers are not able to
   give " real-time" instructions to the spacecraft, either for day-to-day
   operations or in cases of unexpected in-flight events. Even if
   controllers respond immediately after becoming aware of a problem,
   nearly three hours will have passed before they receive a response from
   the spacecraft.

Instruments

   Cassini's instrumentation consists of: a synthetic aperture RADAR
   mapper, a CCD imaging system, a visible/ infrared mapping spectrometer,
   a composite infrared spectrometer, a cosmic dust analyzer, a radio and
   plasma wave experiment, a plasma spectrometer, an ultraviolet imaging
   spectrograph, a magnetospheric imaging instrument, a magnetometer, an
   ion/neutral mass spectrometer. Telemetry from the communications
   antenna as well as other special transmitters (an S-band transmitter
   and a dual-frequency K[a]-band system) will also be used to make
   observations of the atmospheres of Titan and Saturn and to measure the
   gravity fields of the planet and its satellites.

   Cassini Plasma Spectrometer (CAPS)
          The Cassini Plasma Spectrometer (CAPS) is a direct sensing
          instrument that measures the energy and electrical charge of
          particles such as electrons and protons that the instrument
          encounters. CAPS will measure the molecules originating from
          Saturn's ionosphere and also determine the configuration of
          Saturn's magnetic field. CAPS will also investigate plasma in
          these areas as well as the solar wind within Saturn's
          magnetosphere.

   Cosmic Dust Analyzer (CDA)
          The Cosmic Dust Analyzer (CDA) is a direct sensing instrument
          that measures the size, speed, and direction of tiny dust grains
          near Saturn. Some of these particles are orbiting Saturn, while
          others may come from other solar systems. The Cosmic Dust
          Analyzer onboard the Cassini orbiter is ultimately designed to
          help discover more about these mysterious particles, and
          significantly add to the knowledge of the materials in other
          celestial bodies and potentially more about the origins of the
          universe.

   Composite Infrared Spectrometer (CIRS)
          The Composite Infrared Spectrometer (CIRS) is a remote sensing
          instrument that measures the infrared light coming from an
          object (such as an atmosphere or moon surface) to learn more
          about its temperature and what it's made of. Throughout the
          Cassini-Huygens mission, CIRS will measure infrared emissions
          from atmospheres, rings and surfaces in the vast Saturn system
          to determine their composition, temperatures and thermal
          properties. It will map the atmosphere of Saturn in three
          dimensions to determine temperature and pressure profiles with
          altitude, gas composition, and the distribution of aerosols and
          clouds. This instrument will also measure thermal
          characteristics and the composition of satellite surfaces and
          rings.

   Ion and Neutral Mass Spectrometer (INMS)
          The Ion and Neutral Mass Spectrometer (INMS) is a direct sensing
          instrument that analyzes charged particles (like protons and
          heavier ions) and neutral particles (like atoms) near Titan and
          Saturn to learn more about their atmospheres. INMS is intended
          also to measure the positive ion and neutral environments of
          Saturn's icy satellites and rings.

   Imaging Science Subsystem (ISS)
          The Imaging Science Subsystem (ISS) is a remote sensing
          instrument that captures images in visible light, and some in
          infrared and ultraviolet light. The ISS has a camera that can
          take a broad, wide-angle picture and a camera that can record
          small areas in fine detail. Scientists anticipate that Cassini
          will be able to use ISS to return hundreds of thousands of
          images of Saturn and its rings and moons. ISS includes two
          cameras; a Wide Angle Camera (WAC) and a Narrow Angle Camera
          (NAC). Each uses a sensitive charge-coupled device (CCD) as its
          detector. Each CCD consists of a 1,024 square array of pixels,
          12 μm on a side. The camera's system allows for many data
          collection modes, including on-chip data compression. Both
          cameras are fitted with spectral filters that rotate on a
          wheel—to view different bands within the electromagnetic
          spectrum ranging from 0.2 to 1.1 μm.

   Dual Technique Magnetometer (MAG)
          The Dual Technique Magnetometer (MAG) is a direct sensing
          instrument that measures the strength and direction of the
          magnetic field around Saturn. The magnetic fields are generated
          partly by the intensely hot molten core at Saturn's centre.
          Measuring the magnetic field is one of the ways to probe the
          core, even though it is far too hot and deep to actually visit.
          MAG's goals are to develop a three-dimensional model of Saturn's
          magnetosphere, as well as determine the magnetic state of Titan
          and its atmosphere, and the icy satellites and their role in the
          magnetosphere of Saturn.

   Magnetospheric Imaging Instrument (MIMI)
          The Magnetospheric Imaging Instrument (MIMI) is both a direct
          and remote sensing instrument that produces images and other
          data about the particles trapped in Saturn's huge magnetic
          field, or magnetosphere. This information will be used to study
          the overall configuration and dynamics of the magnetosphere and
          its interactions with the solar wind, Saturn's atmosphere,
          Titan, rings, and icy satellites.

   Radio Detection and Ranging Instrument (RADAR)
          The Radio Detection and Ranging Instrument (RADAR) is a remote
          active and remote passive sensing instrument that will produce
          maps of Titan's surface and measures the height of surface
          objects (like mountains and canyons) by bouncing radio signals
          off of Titan's surface and timing their return. Radio waves can
          penetrate the thick veil of haze surrounding Titan. In addition
          to bouncing radio waves, the RADAR instrument will listen for
          radio waves that Saturn or its moons may be producing.

   Radio and Plasma Wave Science instrument (RPWS)
          The Radio and Plasma Wave Science instrument (RPWS) is a direct
          and remote sensing instrument that receives and measures the
          radio signals coming from Saturn, including the radio waves
          given off by the interaction of the solar wind with Saturn and
          Titan. The major functions of the RPWS are to measure the
          electric and magnetic wave fields in the interplanetary medium
          and planetary magnetospheres. The instrument will also determine
          the electron density and temperature near Titan and in some
          regions of Saturn's magnetosphere. RPWS studies the
          configuration of Saturn's magnetic field and its relationship to
          Saturn Kilometric Radiation (SKR), as well as monitoring and
          mapping Saturn's ionosphere, plasma, and lightning from Saturn's
          (and possibly Titan's) atmosphere.

   Radio Science Subsystem (RSS)
          The Radio Science Subsystem (RSS) is a remote sensing instrument
          that uses radio antennas on Earth to observe the way radio
          signals from the spacecraft change as they are sent through
          objects, such as Titan's atmosphere or Saturn's rings, or even
          behind the sun. The RSS also studies the compositions, pressures
          and temperatures of atmospheres and ionospheres, radial
          structure and particle size distribution within rings, body and
          system masses and gravitational waves. The instrument uses the
          spacecraft X-band communication link as well as S-band downlink
          and K[a]-band uplink and downlink.

   Ultraviolet Imaging Spectrograph (UVIS)
          The Ultraviolet Imaging Spectrograph (UVIS) is a remote sensing
          instrument that captures images of the ultraviolet light
          reflected off an object, such as the clouds of Saturn and/or its
          rings, to learn more about their structure and composition.
          Designed to measure ultraviolet light over wavelengths from 55.8
          to 190 nm, this instrument is also a valuable tool to help
          determine the composition, distribution, aerosol particle
          content and temperatures of their atmospheres. This sensitive
          instrument is different from other types of spectrometers
          because it can take both spectral and spatial readings. It is
          particularly adept at determining the composition of gases.
          Spatial observations take a wide-by-narrow view, only one pixel
          tall and 60 pixels across. The spectral dimension is 1,024
          pixels per spatial pixel. Additionally, it is capable of taking
          so many images that it can create movies to show the ways in
          which this material is moved around by other forces.

   Visible and Infrared Mapping Spectrometer (VIMS)
          The Visible and Infrared Mapping Spectrometer (VIMS) is a remote
          sensing instrument that is actually made up of two cameras in
          one: one is used to measure visible wavelengths, the other
          infrared. VIMS captures images using visible and infrared light
          to learn more about the composition of moon surfaces, the rings,
          and the atmospheres of Saturn and Titan. VIMS also observes the
          sunlight and starlight that passes through the rings to learn
          more about ring structure. VIMS is designed to measure reflected
          and emitted radiation from atmospheres, rings and surfaces over
          wavelengths from 350 to 5100 nm. It will also help determine the
          compositions, temperatures and structures of these objects. With
          VIMS, scientists also plan to perform long-term studies of cloud
          movement and morphology in the Saturn system, to determine the
          planet's weather patterns.

The Huygens probe

   The Huygens probe, supplied by the European Space Agency (ESA) and
   named after the Dutch 17th century astronomer Christiaan Huygens,
   scrutinized the clouds, atmosphere, and surface of Saturn's moon Titan
   in its descent on 15 January 2005. It was designed to enter and brake
   in Titan's atmosphere and parachute a fully instrumented robotic
   laboratory down to the surface. The Huygens probe system consisted of
   the probe itself, which descended to Titan, and the probe support
   equipment (PSE) on Cassini, which remained attached to the orbiting
   spacecraft. The PSE includes the electronics necessary to track the
   probe, to recover the data gathered during its descent, and to process
   and deliver the data to the orbiter, from which it was transmitted or
   "downlinked" to Earth. The Probe Data Relay Subsystem (PDRS) for the
   radio link between Huygens and Cassini to transmit the scientific data
   collected during the descent on Titan, and the Command Data Management
   Subsystem (CDMS), that will automatically manage the entire mission of
   the probe that cannot be telecommanded because of the distance from
   Earth, were provided by the Italian Space Agency ( ASI).

Important events and discoveries

Venus and cruise to Jupiter

   Cassini performed two Gravity-assisted flyby of Venus on April 26, 1998
   and June 24, 1999.
   Picture of Moon during flyby
   Enlarge
   Picture of Moon during flyby

   On August 18, 1999 at 03:28 UTC Cassini did a gravity-assisted flyby of
   Earth. An hour 20 minutes before closest approach, Cassini made the
   closest approach to the Moon at 377,000 km, and took a series of
   calibration images.

   On January 23, 2000 Cassini performed a flyby of Asteroid 2685 Masursky
   around 10:00 UTC. Cassini took images 5 to 7 hours before at 1.6
   million km distance and estimated a diameter of 15 to 20 km.

Jupiter flyby

   Jupiter flyby picture
   Enlarge
   Jupiter flyby picture

   Cassini made its closest approach to Jupiter on December 30, 2000, and
   performed many scientific measurements. About 26,000 images were taken
   of Jupiter during the course of the months-long flyby. The most
   detailed global colour portrait of Jupiter ever was produced (see image
   at right), in which the smallest visible features are approximately 60
   km (40 miles) across.

   A major finding of the Jupiter flyby, announced on March 6, 2003, was
   of the nature of Jupiter's atmospheric circulation. Dark "belts"
   alternate with light "zones" in the atmosphere. Scientists had long
   considered the zones, with their pale clouds, to be areas of upwelling
   air, partly because many clouds on Earth form where air is rising.
   Analysis of Cassini imagery, however, told a new story. Individual
   storm cells of upwelling bright-white clouds, too small to see from
   Earth, pop up almost without exception in the dark belts. According to
   Anthony Del Genio of NASA's Goddard Institute for Space Studies, "We
   have a clear picture emerging that the belts must be the areas of
   net-rising atmospheric motion on Jupiter, with the implication that the
   net motion in the zones has to be sinking."

   Other atmospheric observations made included a swirling dark oval of
   high-atmosphere haze, about the size of the Great Red Spot, near
   Jupiter's north pole. Infrared imagery revealed aspects of circulation
   near the poles, with bands of globe-encircling winds, with adjacent
   bands moving in opposite directions.

   The same announcement also discussed the nature of Jupiter's rings.
   Light scattering by particles in the rings revealed the particles were
   irregularly shaped (as opposed to being spherical) and likely originate
   as ejecta from micrometeorite impacts on Jupiter's moons, probably
   Metis and Adrastea.

Test of Einstein's theory of general relativity

   On October 10, 2003, the Cassini science team announced the results of
   a test of Einstein's theory of general relativity, using radio signals
   from the Cassini probe. The researchers observed a frequency shift in
   the radio waves to and from the spacecraft, as those signals traveled
   close to the Sun. According to the theory of general relativity, a
   massive object like the Sun causes space-time to curve, and a beam of
   radio waves (or light) that passes by the Sun has to travel further
   because of the curvature. The extra distance that the radio waves
   travel from Cassini past the Sun to the Earth delays their arrival; the
   amount of the delay provides a sensitive test of the predictions of
   Einstein's theory. Although deviations from general relativity are
   expected in some cosmological models, none were found in this
   experiment. Past tests were in agreement with the theoretical
   predictions with an accuracy of one part in one thousand. The Cassini
   experiment improved this to about 20 parts in a million, with the data
   still supporting Einstein's theory.
   Spokes imaged by Cassini in 2005.
   Enlarge
   Spokes imaged by Cassini in 2005.

Spoke phenomenon verified

   In images captured 5 September 2005, Cassini finally detected spokes in
   Saturn's rings, hitherto seen only by famed visual observer Stephen
   James O'Meara in 1977 and later confirmed by the Voyager spacecraft in
   the early 1980s. The exact cause of the spokes is not yet understood;
   some models predicted spokes would not be visible again until 2007.

New moons of Saturn

   Discovery photograph of moon Daphnis
   Enlarge
   Discovery photograph of moon Daphnis

   Using images taken by Cassini, two new moons of Saturn were discovered
   in June, 2004. They are both very small and were given the provisional
   names S/2004 S 1 and S/2004 S 2 before being named Methone and Pallene
   at the end of 2004.

   On May 1, 2005, a new moon was discovered by Cassini in the Keeler gap.
   It was given the designation S/2005 S 1 before being named Daphnis. The
   only other moon inside Saturn's ring system is Pan.
   Image of Phoebe
   Enlarge
   Image of Phoebe

Phoebe flyby

   On June 11, 2004, Cassini flew by the moon Phoebe. This was the first
   opportunity for close-up studies of this moon since the Voyager 2
   flyby. It also was Cassini's only possible flyby for Phoebe due to the
   mechanics of the available orbits around Saturn.

   First close up images were received on June 12, 2004, and mission
   scientists immediately realized that the surface of Phoebe looks
   different from asteroids visited by spacecraft. Parts of the heavily
   cratered surfaces look very bright in those pictures, and it is
   currently believed that a large amount of water ice exists under its
   immediate surface.

Saturn rotation

   In an announcement on June 28, 2004 Cassini scientists described the
   measurement of the rotational period of Saturn. Since there are no
   fixed features on the surface that can be used to obtain this period,
   the repetition of radio emissions was used. These new data agree with
   the latest values measured from Earth, and constitute a puzzle to the
   scientists. It turns out that the radio rotational period has changed
   since it was first measured in 1980 by Voyager, and that it is now 6
   minutes longer. This doesn't indicate a change in the overall spin of
   the planet, but is thought to be due to movement of the source of the
   radio emissions to a different latitude, at which the rotation rate is
   different.

Orbiting Saturn

   On July 1, 2004, the spacecraft flew through the gap between the F and
   G rings and achieved orbit, after a seven year voyage. It is the first
   spacecraft to ever orbit Saturn.

   The Saturn Orbital Insertion (SOI) maneuver performed by Cassini was
   notably complex, requiring the craft to orient its High-Gain Antenna
   away from Earth and along its flight path, in order to shield its
   instruments from particles in Saturn's rings. Once the craft crossed
   the ring plane, it then had to rotate again so that its engine was
   pointed along its flight path, and then the engine fired to decelerate
   the craft and allow Saturn to capture it. Cassini was captured by
   Saturn's gravity at around 8:54 p.m. Pacific Daylight Time on June 30,
   2004. During the maneuver Cassini passed within 20,000 km (13,000
   miles) of Saturn's cloud tops.

Titan flybys

   Titan's surface
   Enlarge
   Titan's surface

   Cassini had its first distant flyby of Saturn's largest moon, Titan, on
   July 2, 2004, only a day after orbit insertion, when it approached to
   within 339,000 kilometers (211,000 miles) of Titan and provided the
   best look at the moon's surface to date. Images taken through special
   filters (able to see through the moon's global haze) showed south polar
   clouds thought to be composed of methane and surface features with
   widely differing brightness. On October 27, 2004 the spacecraft
   executed the first of the 45 planned close flybys of Titan when it flew
   a mere 1,200 kilometers above the moon. Almost four gigabits of data
   were collected and transmitted to Earth, including the first radar
   images of the moon's haze-enshrouded surface. Radar imagery observed no
   conclusive evidence of lakes of liquid hydrocarbons, though it did not
   dismiss the possibility such lakes could exist. It also revealed the
   surface of Titan (at least the area covered by radar) to be relatively
   level, with topography reaching no more than about 50 meters in
   altitude. The flyby provided a remarkable increase in imaging
   resolution over previous coverage. Images with up to 100 times higher
   resolution were taken and are typical of resolutions planned for
   subsequent Titan flybys.

Huygens encounter with Titan

   Cassini released the Huygens probe on 25 December 2004, by means of a
   spring. It entered the atmosphere of Titan on January 14, 2005. For
   more information on the landing, see Huygens probe.

Enceladus flybys

   During the first two close flybys of the moon Enceladus in 2005,
   Cassini discovered a "deflection" in the local magnetic field that is
   characteristic for the existence of a thin but significant atmosphere.
   Other measurements obtained at that time point to ionized water vapor
   as being its main constituent. Cassini also observed water ice geysers
   erupting from the south pole of Enceladus giving more credibility to
   the idea that Enceladus is supplying the particles of Saturn's E ring.
   Mission scientists hypothesize that there may be pockets of liquid
   water near the surface of the moon that fuel the eruptions, making
   Enceladus one of the few bodies in our solar system to have liquid
   water present.

Radio occultations of Saturn's rings

   In May 2005, Cassini began a series of occultation experiments,
   designed to measure the size-distribution of particles in Saturn's
   rings, and to measure the atmosphere of Saturn itself. For over 4
   months, Cassini will complete orbits specifically designed for this
   purpose. During these occultation experiments, Cassini will fly behind
   the ring plane of Saturn, as seen from Earth, and transmit radio waves
   through the particles. The radio signals are received on Earth, where
   the frequency, phase, and power of the signal is analyzed to help
   determine the structure of the rings.

Lakes of Titan

   Lakes of Titan
   Enlarge
   Lakes of Titan

   Radar images obtained on July 21, 2006 appear to show lakes of liquid
   hydrocarbons (such as methane and ethane) in Titan's northern
   latitudes. This is the first discovery of currently-existing lakes
   anywhere besides Earth. The lakes range in size from about a kilometer
   to one which is one hundred kilometers across.

Saturn hurricane

   In November 2006, scientists discovered a storm at the south pole of
   Saturn with a distinct eyewall. This characteristic of a hurricane on
   Earth had never been seen on another planet before. Unlike a hurricane,
   the storm appears to be stationary at the pole. The storm is 5,000
   miles across, 45 miles high and packing winds blowing 350 miles per
   hour.

Trajectory

   The image above displays the initial gravity-assist trajectory of
   Cassini/Huygens. This is the process whereby an insignificant mass
   approaches a significant mass 'from behind' and 'steals' some of its
   orbital energy. The significant mass, usually a planet, loses virtually
   zero of its orbital energy yet the insignificant mass (in this case,
   the probe) gains a very large proportion of its orbital energy.

   Cassini/Huygens performed two gravity assists at Venus, one at Earth
   and one at Jupiter.

   The above simplified diagram shows, in two dimensions, the orbital
   motion of Cassini/Huygens on and after arrival at Saturn.
   Cassini's speed relative to the Sun. The various gravitational
   slingshots form visible peaks on the left, while the periodic variation
   on the right is caused by the spacecraft's orbit around Saturn. The
   data came from the JPL Horizons Ephemeris System in 2005. The speed
   above is instantaneous distance in kilometers per second. The date/time
   is UTC in Spacecraft Event Time, which is from 1997-Oct-16 00:00:01 to
   2008-Jul-06 23:59:59 UTC, two leap seconds during this period. Note
   also that the minimum velocity achieved during Saturnian orbit is more
   or less equal to Saturn's own orbital velocity, which is the ~5km/s
   velocity which Cassini matched to enter orbit.
   Enlarge
   Cassini's speed relative to the Sun. The various gravitational
   slingshots form visible peaks on the left, while the periodic variation
   on the right is caused by the spacecraft's orbit around Saturn. The
   data came from the JPL Horizons Ephemeris System in 2005. The speed
   above is instantaneous distance in kilometers per second. The date/time
   is UTC in Spacecraft Event Time, which is from 1997-Oct-16 00:00:01 to
   2008-Jul-06 23:59:59 UTC, two leap seconds during this period. Note
   also that the minimum velocity achieved during Saturnian orbit is more
   or less equal to Saturn's own orbital velocity, which is the ~5km/s
   velocity which Cassini matched to enter orbit.
   Retrieved from " http://en.wikipedia.org/wiki/Cassini-Huygens"
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