Deep Impact at ESO
... Comet 9P/Tempel 1
Comet 9P/Tempel 1 belongs to the class of short period comets. Its orbital period - i.e. the time ittakes to accomplish a full revolution around the Sun - is 5.52 years. This is almost exactly one half the orbital period of Jupiter, meaning that Comet 9P/Tempel 1 makes 2 revolutions around the Sun when Jupiter does one.
Its mean distance to the Sun is 3.12 the distance from the Earth to the Sun (3.12 AU, or 466.7 million km) but as it follows quite an excentric orbit (its excentricity is 0.5176), its distance to the Sun varies between 1.506 AU and 4.735 AU It does thus not cross the path of the Earth - located at 1 AU - but does cross the orbit of Mars and of the main asteroid belt. In fact, in 1885, the comet came at less than 0.033 AU (4.94 million km) from the asteroid Pallas. In 2011, it will be 0.041 AU away from the asteroid (1) Ceres.
Comet 9P/Tempel 1 will reach its perihelion - the closest point to the Sun - on July 5, i.e. one day after the impact. As most short-period comets, its inclination on the ecliptic - the orbital plane of the Earth - is relatively low: 10.5 degrees.
Comet 9P/Tempel 1 was discovered in 1867 in Marseille (France) by Wilhelm Tempel. It was the ninth periodic comet to be recognised as such and the first periodic comet found by W. Tempel, hence its name. W. Tempel also 10P/Tempel 2 in 1873, 11P/Tempel-Swift-LINEAR in 1869 and 55P/Tempel-Tuttle (the parent comet of the Leonid Meteor Shower) in 1866, as well as 9 other non-periodic comets and 5 asteroids: (64) Angelina, (65) Cybele, (74) Galathea, (81) Terpsichore, and (97) Clotho.
In its recent history, Comet Tempel 1 made several close approaches to Jupiter, which modified its orbital elements. In 1881, an approach to 0.56 AU pushed the comet's perihelion to 2.1 AU while later approaches, in 1941 and 1953, brought it back to about 1.5 AU. Since 1967, it has been observed at each of the eight perihelion passages.