Cassini–Huygens is a joint NASA/ESA/ASI robotic spacecraft mission currently studying the planet 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, mathematician and physicist 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; it reached Saturn's moon Titan on January 14, 2005 where it made an atmospheric descent to the surface and relayed scientific information. On April 18, 2008, NASA announced a two year extension of the mission. Cassini is the first spacecraft to orbit Saturn and the fourth to visit it.
Hundreds of scientists and engineers from 16 European countries and 33 states of the United States make up the team responsible for designing, building, flying and collecting data from the Cassini orbiter and Huygens probe. The mission is managed by NASA’s Jet Propulsion Laboratory, where the orbiter was designed and assembled. Development of the Huygens Titan probe was managed by the European Space Technology and Research Center, whose prime contractor for the probe is Alcatel in France. Equipment and instruments for the probe were supplied from many countries, including the United States. The Italian Space Agency (ASI) provided Cassini's high-gain communication antenna, and a revolutionary compact and light-weight multimode radar (synthetic aperture radar, radar altimeter, radiometer).
The Cassini–Huygens spacecraft was launched on October 15, 1997 from Cape Canaveral Air Force Station's Launch Complex 40 using a US 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 total cost of the 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 US contributed $2.6 billion, ESA $500 million and ASI $160 million.
The nominal end of the mission is in 2008 but an extension to the mission until 2010 was approved. It is possible that funding will be granted for additional extensions.
A list of Cassini–Huygens abbreviations is available.
Cassini–Huygens's 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 advisers 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 United States. Cassini–Huygens came under fire politically 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.
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, canceling the implementation of the Mariner Mark II series.
The spacecraft, including the orbiter and the probe, is the largest and most complex interplanetary spacecraft built to date. The orbiter has a mass of 2,150 kg (4,739 lbs), the probe 350 kg (770 lbs). With the launch vehicle adapter and 3,132 kg (6,900 lbs) of propellants at launch, the spacecraft had a mass of about 5,600 kg (12,345 lbs). Only the two Phobos spacecraft sent to Mars by the Soviet Union were heavier. The Cassini spacecraft is more than high and more than wide. The complexity of the spacecraft is necessitated both by its trajectory (flight path) to Saturn, and by the ambitious program of scientific observations once the spacecraft reaches its destination. It functions with 1,630 interconnected electronic components, 22,000 wire connections, and over of cabling.
Now that Cassini is orbiting 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. Thus, ground controllers cannot give "real-time" instructions to the spacecraft, either for day-to-day operations or in cases of unexpected events. Even if they respond immediately after becoming aware of a problem, nearly three hours will have passed before the satellite receives a response.
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. Instead, the Cassini orbiter is powered by three radioisotope thermoelectric generators (RTGs), which use heat from the natural decay of plutonium (in the form of plutonium dioxide) to generate direct current electricity. The RTGs have the same design as those 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 able to produce at least 628 watts of power. One of Cassini's spare RTGs was used to power the New Horizons mission to Pluto and the Kuiper Belt.
The use of 32.8 kg (72 lbs) of plutonium—the most launched into space until then—attracted significant protest from environmental groups, physicists, and some former NASA staff. NASA made several statements intended to mean that the mission was acceptably safe: the chances of radioactive release during the first 3½ 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 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 the theoretical physicist Professor Michio Kaku, who suggested that 200,000 humans would die if the plutonium canisters survived reentry and crashed in a heavily populated area, though his estimates were based on atmospheric dispersal of the plutonium over a metropolis. Cassini's launch trajectory did not bring it within suitable vicinity of any large metropolis and the design of the RTGs would mean that they would be very unlikely to fracture even in the case of a catastrophic mission abort.
To gain momentum, Cassini's trajectory included several gravitational slingshot maneuvers: two passes of Venus, one of Earth, then one of Jupiter. The Earth fly-by was the final point when Cassini posed any danger to humans, and occurred successfully on August 18, 1999. Had it suffered a malfunction that caused it to impact, NASA's final environmental impact study estimated that in the worst case a significant fraction of the plutonium inside the RTGs would have dispersed into Earth's atmosphere, but the chances of that were nearly ten million to one. This worst case involved an acute angle of entry in which Cassini would gradually burn up and be vaporized in the upper atmosphere, a highly unlikely scenario. A small number of activists continued to protest after the maneuver.
The primary mission for Cassini ended on 2008-07-06, with a two-year mission extension already approved, and a second one possible. NASA is targeting decommissioning Cassini in 2012. Unlike the Galileo spacecraft, which was plunged into Jupiter to disintegrate in a fiery atmospheric entry, a similar approach for Cassini may impact a large object within the rings and make it uncontrollable. Instead, NASA is considering a high-altitude parking orbit and an impact on a smaller moon where RTG contamination will not be a problem. Specifically, scientists do not want to contaminate Enceladus or Titan with the radioactive waste since those satellites may have organic materials.
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 January 15, 2005. It was designed to enter and brake in Titan's atmosphere and parachute a fully instrumented robotic laboratory down to the surface.
The probe system consisted of the probe itself which descended to Titan, and the probe support equipment (PSE) which remained attached to the orbiting spacecraft. The PSE includes electronics that tracks the probe, recovers the data gathered during its descent, and processes and delivers the data to the orbiter that transmits it to Earth. The data was transmitted by a radio link between Huygens and Cassini provided by Probe Data Relay Subsystem (PDRS). As the probe's mission cannot be telecommanded from Earth because of the great distance, it is automatically managed by the Command Data Management Subsystem (CDMS). The PDRS and CDMS were provided by the Italian Space Agency (ASI).
On August 18, 1999 at 03:28 UTC Cassini did a gravity-assist flyby of Earth. An hour and 20 minutes before closest approach, Cassini made the closest approach to the Moon at 377,000 km, and took a series of calibration images.
The New Horizons mission to Pluto captured more recent images of Jupiter, with a closest approach on February 28, 2007.
A major finding of the flyby, announced on March 6, 2003, was of Jupiter's atmospheric circulation. Dark "belts" alternate with light "zones" in the atmosphere, and 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. But analysis of Cassini imagery showed that 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, "the belts must be the areas of net-rising atmospheric motion on Jupiter, [so] the net motion in the zones has to be sinking."
Other atmospheric observations included a swirling dark oval of high atmospheric-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 showed the particles were irregularly shaped (rather than spherical) and likely originate as ejecta from micrometeorite impacts on Jupiter's moons, probably Metis and Adrastea.
Using images taken by Cassini, three new moons of Saturn were discovered in 2004. They are very small and were given the provisional names S/2004 S 1, S/2004 S 2 and S/2004 S 5 before being named Methone, Pallene and Polydeuces at the beginning of 2005.
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 known moon inside Saturn's ring system is Pan.
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.
The Saturn Orbital Insertion (SOI) maneuver performed by Cassini was complex, requiring the craft to orient its High-Gain Antenna away from Earth and along its flight path, to shield its instruments from particles in Saturn's rings. Once the craft crossed the ring plane, it had to rotate again to point its engine 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.
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. It 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.
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, which gives 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 contain liquid water.
On March 12, 2008, Cassini made a close fly-by of Enceladus, getting within 50 km of the moon's surface.. The spacecraft passed through the plumes extending from its southern geysers, detecting water, carbon dioxide and various hydrocarbons with its mass spectrometer, while also mapping surface features that are at much higher temperature than their surroundings with the infrared spectrometer. Cassini was unable to collect data with its cosmic dust analyzer due to an unknown software malfunction. The probe will pass near Enceladus again later in 2008, and could make another attempt to collect the data.
Radar images obtained on July 21, 2006 appear to show lakes of liquid hydrocarbon (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.
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 a very small proportion of its orbital energy to the insignificant mass, in this case, the probe. However, due to the spacecraft's small mass, this energy transfer gives it a relatively large energy increase in proportion to its orbital energy, speeding its travel through space.
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.