9P/Tempel, also known as Tempel 1 or 9P/Tempel 1, is a periodic comet. It was discovered on April 3, 1867 by Ernst Wilhelm Leberecht Tempel, an astronomer working in Marseille. At the time of discovery, it approached perihelion once every 5.68 years (designations 9P/1867 G1 and 1867 II). It was subsequently observed in 1873 (9P/1873 G1, 1873 I, 1873a) and in 1879 (1879 III, 1879b). However, Tempel 1's orbit occasionally brings it sufficiently close to Jupiter to be altered and its orbital period consequently change. This event occurred in 1881 (closest approach to Jupiter of 0.55 AU), lengthening the orbital period to 6.5 years. Perihelion also changed, increasing by 50 million kilometres, rendering the comet far less visible from Earth. Photographic attempts during 1898 and 1905 failed to recover the comet, and astronomers surmised that it had disintegrated.
Tempel 1 was re-discovered 13 orbits later, in the 1960s (as 9P/1967 L1, 1966 VII) after American astronomer Brian G. Marsden performed precise calculations of the comet's orbit, taking into account Jupiter's perturbations. Marsden found that further close approaches to Jupiter in 1941 (0.41 AU) and 1953 (0.77 AU) had decreased both the perihelion distance and the orbital period to values smaller than when the comet was initially discovered (5.84 and 5.55 years, respectively). These approaches moved Tempel 1 into its present libration around the 1:2 resonance with Jupiter. Despite an unfavorable 1967 return, Elizabeth Roemer of the Catalina Observatory took several photographs. Initial inspection revealed nothing, but in late 1968 she found a June 8, 1967 exposure (Tempel 1 had passed perihelion in January) that held the image of an 18th magnitude diffuse object very close to where Marsden had predicted the comet to be. Unfortunatly, a single image does not allow an orbit computation, so the next return had to be awaited.
Roemer and L. M. Vaughn recovered the comet on January 11, 1972 from Steward Observatory (9P/1972 A1, 1972 V, 1972a). The comet became widely observed, reached a maximum brightness of magnitude 11 during May, and was last seen on July 10. Since that time the comet has been seen at every apparition, in 1978 (1978 II, 1977i), 1983 (1983 XI, 1982j), 1989 (1989 I, 1987e1), 1994 (1994 XIUX, 1993c), 2000 and 2005. Its current orbital period is 5.51 years.
Tempel 1 is not a bright comet; its maximum magnitude so far has been 11, far below naked eye visibility. Its size is believed to be 14 by 4 kilometers (8.7 by 2.5 miles), based on measurements taken by the Hubble Space Telescope in visible light and the Spitzer Space Telescope in infrared light. Combining these observations also revealed a low albedo of only 4%. A two-day rotation rate was also determined.
Deep Impact space mission
On July 4, 2005 at 05:52 UTC (01:52 EDT), Tempel 1 was impacted by the NASA Deep Impact probe, one day before perihelion. Earth-bound and space telescopes observed brightening of several magnitude after the impact while Deep Impact's observation section recorded a bright spray from the impact site.
The exact details of the crater that was formed are not yet known. It is believed that the crater could be up to 200 meters in diameter and 30-50 meters deep, but the crater could be much smaller depending on the composition and structure of Tempel 1. The purpose of the Deep Impact mission is to study the interior composition of a cometary nucleus.