Rosetta was originally intended to rendezvous with the periodic comet 46P/Wirtanen, but, after the launch was delayed, the target was changed to another regular visitor to the inner Solar System, 67P/Churyumov-Gerasimenko.
67P/Churyumov-Gerasimenko was discovered in 1969, when several astronomers from Kiev visited the Alma-Ata Astrophysical Institute to conduct a survey of comets. On 20 September, Klim Churyumov was examining photographs of comet 32P/Comas Solá taken by Svetlana Gerasimenko when he found a comet-like object near the edge of the plate.
He assumed that the faint object was the expected periodic comet, but upon returning to Kiev, he studied the plates very carefully and eventually realised that a new comet had been found, less than two degrees from comet Comas Solá.
The comet has a particularly unusual history. Up to 1840 its perihelion distance was 4.0 AU (four Sun-Earth distances or about 600 million km) and the comet was completely unobservable from Earth. That year, a fairly close encounter with Jupiter caused the orbit to move inwards to a perihelion distance of 3.0 AU (450 million km). Over the next century, the perihelion gradually decreased further to 2.77 AU. Then, in 1959, a further Jupiter encounter reduced the perihelion to just 1.29 AU. It currently completes one orbit of the Sun every 6.57 years.
The comet has now been observed from Earth on six approaches to the Sun - 1969 (discovery), 1976, 1982, 1989, 1996 and 2002. It is unusually active for a short period object and has a coma (a diffuse cloud of dust and gas surrounding the solid nucleus) and often a tail at perihelion. During the 2002/2003 apparition, the tail was up to 10 arcminutes long as seen from Earth, with a bright central condensation in a faint extended coma. Even 7 months after perihelion the tail continued to be very well developed, although it subsequently faded rapidly.
The comet typically reaches a magnitude around 12, although this is because the comet has outburst at perihelion at three of its last four returns in 1982/83, 1996/97 and 2002/03. Despite being a relatively active object, even at the peak of outburst the dust production rate is some 40 times lower than for 1P/Halley. Nevertheless, 67P/Churyumov-Gerasimenko is classed as a dusty comet.
The peak dust production rate in 2002/03 was estimated at approximately 60kg per second, although values as high as 220kg per second were reported in 1982/83. The gas to dust emission ratio is approximately 2.
Sixty-one images of comet 67P/Churyumov-Gerasimenko were taken with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope on 11-12 March 2003. The HST's sharp vision enabled astronomers to isolate the comet's nucleus from the coma. The images showed that the nucleus measures five by three kilometres and has an ellipsoidal (rugby ball) shape. It rotates once in approximately 12 hours.
"Although 67P/C-G is roughly three times larger than the original Rosetta target, its elongated shape should make landing on its nucleus feasible, now that measures are in place to adapt the lander package to the new configuration before next year's launch," said Dr. Philippe Lamy of the Laboratoire d'Astronomie Spatiale in France.