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Cassini-Huygens is a joint NASA/ESA unmanned space mission intended to study Saturn and its moons. The spacecraft consists of two main elements: the Cassini orbiter and the Huygens probe. It was launched on October 15, 1997 and is estimated to enter Saturn's orbit on July 1, 2004.
This is an artists concept of Cassini during the Saturn Orbit Insertion (SOI) maneuver, just after the main engine has begun firing.
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Cassini's principal objectives are to:
The Cassini-Huygens spacecraft was launched on October 15, 1997 from Kennedy Space Center 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 will orbit Saturn and its moons for four years, and the Huygens probe will 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'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.
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. However, various budget cuts and rescopings of the project have forced a more special design, postponing indefinitely any implementation of the Mariner Mark II series.
The Cassini spacecraft, including the orbiter and the Huygens probe, is one of the largest, heaviest, and most complex interplanetary spacecraft built to date. The orbiter alone weighs 2,150 kilograms (4,750 pounds). When the 350-kilogram Huygens probe, launch vehicle adapter, and 3,132 kilograms (6,905 pounds) of propellants were loaded, the spacecraft weighed about 5,600 kilograms (12,346 pounds) at launch. Only the two Phobos spacecraft sent to Mars by the former Soviet Union were heavier. The Cassini spacecraft stood more than 6.8 meters (22.3 feet) high and was more than 4 meters (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 interconnect circuits, 22,000 wire connections, and over 14 kilometers (8.7 miles) of cabling.
When Cassini is at Saturn it will be between 8.2 and 10.2 astronomical units from Earth. Because of this, it will take 68 to 84 minutes for signals to travel from Earth to the spacecraft, or vice versa. In practical terms this means that ground controllers will not be able to give "real-time" instructions to the spacecraft either for day-to-day operations or in cases of unexpected in-flight events. By the time the controllers become aware of a problem and respond, nearly three hours will have passed.
Cassini's instrumentation consists of: a 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 Ka-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.
Because of Saturn's distance from the Sun, solar arrays were not feasible power sources for the spacecraft. To generate enough power, such arrays would have been too large and heavy. Thus, the Cassini orbiter gets its power from three radioisotope thermoelectric generators or RTGs, which use heat from the natural decay of plutonium (in the form of plutonium dioxide) to generate direct current electricity. These RTGs are of the same design as those flying on the Galileo and Ulysses spacecraft and are designed to have a long operational lifetime. At the end of the 11-year Cassini mission, they will still be capable of producing at least 628 watts of power.
Cassini's use of plutonium -- 32.8 kg, at the time the most ever launched into space -- attracted significant protest from environmental groups, physicists, and some former NASA staff. NASA made several statements about the safety of the mission, all of which intended to mean the mission was acceptably safe: the chances of radioactive release during the first 3 1/2 minutes after launch were 1 in 1,400; the chances of a release later in the rocket's climb into orbit were 1 in 476; the chances of the craft falling to earth later into the mission were less than 1 in a million; a worst-case scenario would mean 120 humans could die from Cassini-caused cancer over 50 years. These figures were derided as wild guesses by commentators that included noted physics professor Michio Kaku, who suggested 200,000 would die if Cassini landed in a city.
To gain momentum for the voyage to Saturn, Cassini's trajectory included several gravitational slingshot maneuvers: two passes of Venus, one past the Earth, then one past Jupiter. The Earth fly-by, which occurred successfully on August 18, 1999, was the final point at which Cassini posed any danger to humans. Had it suffered a malfunction that caused it to impact, the plutonium contents of the RTGs would have been dispersed into Earth's atmosphere. A small number of activists continued to protest after the maneuver. Counterdemonstrators from the National Space Society carried signs reading CASSINI IS GO.
The Huygens probe descends through Titan's murky, brownish-orange atmosphere of nitrogen and carbon-based molecules, beaming its findings to the distant Cassini orbiter. (Artist's impression)
The Huygens probe, supplied by the European Space Agency (ESA) and named after the Dutch 17th century astronomer Christiaan Huygens, will scrutinize the clouds, atmosphere, and surface of Saturn's moon Titan. It is designed to enter and brake in Titan's atmosphere and parachute a fully instrumented robotic laboratory down to the surface. The Huygens probe system consists of the probe itself, which will descend to Titan, and the probe support equipment (PSE), which will remain 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 will be transmitted or "downlinked" to the ground.
The probe will remain dormant throughout the 6.7-year interplanetary cruise, except for bi-annual health checks. These checkouts follow preprogrammed descent scenario sequences as closely as possible, and the results are relayed to Earth for examination by system and payload experts.
Prior to the probe's separation from the orbiter, a final health check will be performed. The "coast" timer will be loaded with the precise time necessary to turn on the probe systems (15 minutes before the encounter with Titan's atmosphere), and then the probe will separate from the orbiter and coast to Titan for 22 days with no systems active except for its wake-up timer.
The main mission phase will be the parachute descent through Titan's atmosphere. The batteries and all other resources are sized for a Huygens mission duration of 153 minutes, corresponding to a maximum descent time of 2.5 hours plus at least 3 additional minutes (and possibly a half hour or more) on Titan's surface. The probe's radio link will be activated early in the descent phase, and the orbiter will "listen" to the probe for the next 3 hours, which includes the descent plus 30 minutes after impact. Not long after the end of this three-hour communication window, Cassini's high-gain antenna (HGA) will be turned away from Titan and toward Earth.
A chronology of the mission can be found under Cassini-Huygens timeline. Following is discussion of the more notable events and discoveries.
Jupiter flyby picture
Cassini made its closest approach to Jupiter on December 30, 2000, and performed many scientific measurements. About 26 thousand images were taken of Jupiter during the course of the months-long flyby. The most detailed global color portrait of Jupiter ever was produced (see image at right), in which the smallest visible features are approximately 60 km (37 miles) across.
A major finding of the Jupiter flyby, announced[1] 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 particle 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.
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 space craft, as those signals traveled close to the Sun. 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.
A new, high-resolution picture of Saturn taken by Cassini on February 9, 2004 was publicly released a few weeks later. Mission scientists were puzzled by the fact that no "spokes" in Saturn's ring were visible. These dark structures in the "B" section of the ring had been discovered in pictures taken by the Voyager probe in 1981. (See JPL Press Release Image)
The chart above was computed by JPL's HORIZONS System, where shows the spacecraft's speed related to the Sun in date/time range from 1997-Oct-15 16:00:01 UT (1997-Oct-15 16:01:04.184 TT) to 2008-Aug-08 16:00:00 UT (2008-Aug-08 16:01:04.184 TT) in SCET. The chart was generated by Microsoft Excel.
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