Cassini

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Cassini

(käs-sē`nē), name of a family of Italian-French astronomers, four generations of whom were directors of the Paris Observatory. Gian Domenico Cassini, 1625–1712, was born in Italy and distinguished himself while at Bologna by his studies of the sun and planets, particularly Jupiter; he determined rotational periods for Jupiter, Mars, and Venus. He was called to Paris in 1669 to supervise the building of the Royal Observatory and remained there to direct it. While at Paris he discovered four of Saturn's satellites, studied the division in the planet's ring system that now bears his name, and began the mapping of the meridian passing through Paris in order to verify the Cartesian hypothesis of the elongation of the earth. His son Jacques Cassini, 1677–1756, took over the observatory after 1700 and continued the mapping of the Paris meridian, adding to it a measurement of the perpendicular to the arc in 1733–34. The triumph of the opposing Newtonian hypothesis of the flattening of the earth caused him to retire in 1740, and he was replaced by his son, Cesar-François Cassini de Thury, 1714–84, who continued his father's geodesic work and planned the first modern map of France. On his death, his son Jean-Dominique Cassini, 1748–1845, undertook the reorganization and restoration of the observatory. He completed his father's map of France and participated in the geodesic operations joining the Paris and Greenwich meridians. He lost his post in 1793 because of his monarchial views and was briefly imprisoned by the revolutionary government in 1794. He abandoned scientific work in 1800, becoming president of the General Council of Oise. He was decorated by Napoleon I and Louis XVIII and retired in 1818.

Cassini

(ka-see -nee) (Cassini/Huygens) A joint NASA/ESA mission to the Saturnian system, launched by a Titan IVB/Centaur rocket on Oct. 15 1997. It arrived at Saturn in 2004 and, after passing through a gap in the planet's complex ring system, entered orbit on June 30 of that year. The US Cassini probe carried 12 experiments, and on a 4-year tour orbiting Saturn was set to study the planet itself – atmosphere, rings, and magnetosphere – and some of its moons – Titan and the icy satellites, such as Tethys, Iapetus, Dione, and Phoebe.

Cassini used the gravitational effects of both Venus and the Earth to catapult itself across interplanetary space and reduce its journey time to Saturn to a mere seven years. It completed a flyby of the asteroid (2685) Masursky in January 2000 and swung by Jupiter in December 2000, picking up even more momentum in the process. During its Jupiter flyby, it teamed up briefly with NASA's Galileo probe in a joint investigation of the Jovian system. Cassini was scheduled to make 23 flybys of Saturn.

ESA's Huygens probe, carried aboard the Cassini spacecraft, was detached Dec. 24 2004 and landed on Titan Jan. 14 2005, after plunging into its dense atmosphere. During its parachute-controlled descent to the surface, which lasted about 2½ hours, and after touchdown, Huygens sent back a stream of data on the physics and chemistry of the satellite, including both pictures and sound. It transmitted from Titan's surface for about 70 minutes before its batteries ran out, and despite a computer malfunction aboard Cassini, which lost half the expected number of photographs, Huygens sent back via its mother ship about 350 pictures and a wealth of sensor readings. Ground-based radio telescopes also picked up some of Huygens' transmissions and rescued some science data that might have otherwise been lost. Preliminary analysis of the Huygens data appeared to confirm the view that Titan resembles the early Earth. Its atmosphere consists mostly of nitrogen, but concentrations of simple hydrocarbons such as methane exist near the surface. Evidence was found of liquid methane falling as ‘rain’ and penetrating just below the surfaces. At the Huygens landing site, the surface seemed to consist of a thin crust overlying a sandlike hydrocarbon soil; chunks of dirty water ice lay strewn nearby. High ridges were apparently cut by ‘river channels' draining down to lower terrain or even to a ‘lake’ of liquid hydrocarbons.

Cassini

Giovanni Domenico. 1625--1712, French astronomer, born in Italy. He discovered (1675) Cassini's division, the gap that divides Saturn's rings into two parts, and four of Saturn's moons