Cassini–Huygens ( kə-SEE-nee HOY-gənz), commonly called Cassini, was a joint space-research mission by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, including its rings and natural satellites. The Flagship-class robotic spacecraft comprised both NASA's Cassini space probe and ESA's Huygens lander, which landed on Saturn's largest moon, Titan. Cassini was the fourth space probe to visit Saturn and the first to enter its orbit, where it stayed from 2004 to 2017. The two craft took their names from the astronomers Giovanni Cassini and Christiaan Huygens.
Launched aboard a Titan IV-B with a Centaur D-1T upper stage on October 15, 1997, Cassini was active in space for nearly 20 years, spending almost 7 years in transit and 13 years orbiting Saturn, studying the planet and its system after entering orbit on July 1, 2004.
The voyage to Saturn included flybys of Venus (April 1998 and July 1999), Earth (August 1999), the asteroid 2685 Masursky, and Jupiter (December 2000). The mission ended on September 15, 2017, when Cassini's trajectory took it into Saturn's upper atmosphere and it burned up in order to prevent any risk of contaminating Saturn's moons, which might have offered habitable environments to stowaway terrestrial microbes on the spacecraft. The mission was successful beyond expectations – NASA's Planetary Science Division Director, Jim Green, described Cassini–Huygens as a "mission of firsts" that revolutionized human understanding of the Saturn system, including its moons and rings, and our understanding of where life might be found in the Solar System.
Scientists and individuals from 27 countries made up the joint team responsible for designing, building, flying and collecting data from the Cassini orbiter and the Huygens probe.
Cassini's planners originally scheduled a mission of four years, from June 2004 to May 2008. The mission was extended for another two years until September 2010, branded the Cassini Equinox Mission. The mission was extended a second and final time with the Cassini Solstice Mission, lasting another seven years until September 15, 2017, on which date Cassini was de-orbited to burn up in Saturn's upper atmosphere. The Huygens module traveled with Cassini until its separation from the probe on December 25, 2004; Huygens landed by parachute on Titan on January 14, 2005. The separation was facilitated by the SED (Spin/Eject device), which provided a relative separation speed of 0.35 metres per second (1.1 ft/s) and a spin rate of 7.5 rpm. It returned data to Earth for around 90 minutes, using the orbiter as a relay. This was the first landing ever accomplished in the outer Solar System and the first landing on a moon other than Earth's Moon.
At the end of its mission, the Cassini spacecraft executed its "Grand Finale": a number of risky passes through the gaps between Saturn and its inner rings. This phase aimed to maximize Cassini's scientific outcome before the spacecraft was intentionally destroyed to prevent potential contamination of Saturn's moons if Cassini were to unintentionally crash into them when maneuvering the probe was no longer possible due to power loss or other communication issues at the end of its operational lifespan. Cassini's atmospheric entry on Saturn ended the mission, but analysis of the returned data will continue for many years.
NASA's Jet Propulsion Laboratory, which developed the orbiter, managed the mission. The European Space Research and Technology Centre developed Huygens. The centre's prime contractor, Aérospatiale of France (which became part of Thales Alenia Space in 2005), assembled the probe with equipment and instruments supplied by many European countries (including Huygens's batteries and two scientific instruments from the United States). The Italian Space Agency (ASI) provided the Cassini orbiter's high-gain radio antenna, with the incorporation of a low-gain antenna (to ensure telecommunications with the Earth for the entire duration of the mission), a compact and lightweight radar, which also used the high-gain antenna and served as a synthetic-aperture radar, a radar altimeter, a radiometer, the radio science subsystem (RSS), and the visible-channel portion VIMS-V of VIMS spectrometer.
The 90s were difficult years for NASA and JPL: with the Cold War and the Space Race between the US and the USSR finished, NASA saw dwindling budgets and introduced the so-called "faster, better, cheaper" approach, that encouraged smaller, cheaper missions built with the help of third-party contractors, efficiently "commercializing" the research lab and forcing it to work with industry. Caltech leadership even feared that JPL could be closed.
JPL had little experience in small missions at the time: its "flagship" missions, like Voyager, Cassini, and Galileo, employed hundreds of people for decades. Cassini "directly supported maybe 500 work-years, about 10 percent of total lab staff [and] provided close to 20 percent of the lab budget". In order to comply with budget restrictions and save the mission, Cassini was downsized; to save $250 million, the scan platform had to be removed from the plans.
NASA provided the VIMS infrared counterpart, as well as the Main Electronic Assembly, which included electronic sub-assemblies provided by CNES of France. On April 16, 2008, NASA announced a two-year extension of the funding for ground operations of this mission, at which point it was renamed the Cassini Equinox Mission.
It was extended again in February 2010 as the Cassini Solstice Mission.
The mission consisted of two main elements: the ASI/NASA Cassini orbiter, named for the Italian astronomer Giovanni Domenico Cassini, the discoverer of Saturn's ring divisions and four of its satellites; and the ESA-developed Huygens probe, named for the Dutch astronomer, mathematician and physicist Christiaan Huygens, discoverer of Titan.
The mission was commonly called Saturn Orbiter Titan Probe (SOTP) during gestation, both as a Mariner Mark II mission and generically.
Cassini–Huygens was a Flagship-class mission to the outer planets. The other planetary flagships include Galileo, Voyager, and Viking.
Cassini had several objectives, including:
Determining the three-dimensional structure and dynamic behavior of the rings of Saturn.
Determining the composition of the satellite surfaces and the geological history of each object.
Determining the nature and origin of the dark material on Iapetus's leading hemisphere.