Remarkable new images of Comet Wirtanen
1 July 1999
In 2011, after an eight-year trek through the inner Solar System, the Rosetta spacecraft will rendezvous with periodic comet Wirtanen before releasing a lander onto its tiny nucleus.In order to ensure the success of this ambitious mission, a joint team from ESA's Space Science Department and the European Southern Observatory (ESO)has agreed to make a series of ground-based observations of the comet. The first fruits of this collaboration are a series of remarkable images of thecomet's nucleus which have been released today by ESO to coincide with theESA Rosetta press conference in London.The new images were taken on 17 May with one of the most advanced telescopes in the world - the second 8.2 metre Very Large Telescope Unit (KUEYEN) - and the VLT Test Camera. They show the comet near its aphelion (furthest point from the Sun), 745 million km from the Sun. At this distance, the comet is over 50 million times fainter than the faintest star visible to the naked eye. This is equivalent to seeing a black golf ball in space over 20 000 km away!
Analysis of the images shows that the nucleus appears 'sharp' with no indication of a surrounding coma. This is good news, since it means that there is unlikely to be any damage from comet dust when the Rosetta spacecraft nears the nucleus. Assuming that the comet's surface is blacker than coal (like most comets), the diameter of the nucleus seems to be about 1 km.
Another important result for the scientists planning the Rosetta observations and landing was that, over the three hours of VLT observations, the comet's brightness remained constant. Since such a small object is unlikely to be spherical in shape, it seems that the nucleus must rotate very slowly.
Further observations with ESO's ground-based telescopes will be made before and after launch so that scientists can have as much information as possible about Rosetta's elusive target before its arrival 12 years from now.
