New images of Comet Wirtanen's 1-km 'dirty snowball' nucleus have been obtained with the European Southern Observatory Very Large Telescope at Paranal (Chile). They show this object at a distance of approx. 435 million km from the Sun, about the same as when the Rosetta spacecraft of the European Space Agency (ESA) arrives in 2011.
The new observations indicate that the comet has a very low degree of activity at this point in its orbit - almost no material is seen around the nucleus. This means that there will not be so much dust near the nucleus as to make the planned landing dramatically difficult. As a result of these observations ESA will be able to refine plans for its Rosetta mission.
Rosetta will be launched next year and it will reach Comet Wirtanen in 2011. By that time the comet will be nearly as far from the Sun as Jupiter, charging headlong towards the inner Solar System at speeds of up to 135,000 km/h. To get there and to be able to match the comet's orbit, Rosetta will need to be accelerated by several planetary swing-bys, after which the spacecraft - following a series of difficult manoeuvres - will get close to the comet, enter into orbit around it and release a lander from a height of about 1 km.
The VLT observations were planned specifically to investigate the 'activity' of Wirtanen at about the same solar distance as at the time of the landing manoeuvres. Because of this timing requirement, they had to be carried out at a certain moment - unfortunately, when the comet was low in the twilight evening sky and descending rapidly towards the western horizon. However, even though the exposures therefore had to be quite short, the VLT with its superb light-gathering capability and opto-mechanical perfection was still able to produce excellent images of this rather faint, moving object (about 6 million times fainter than what can be perceived with the unaided eye).
"Rosetta is certainly a very challenging space mission. No one has ever tried to land on a comet before," says Gerhard Schwehm, Rosetta's Project Scientist. "We need to learn as much as possible about our target. The new VLT data will allow us to improve our models and make decisions once we get there."
"It is a pleasure to help our colleagues at ESA", says ESO astronomer and comet specialist Hermann Boehnhardt. "We will continue to keep an eye on this comet, in particular when Rosetta is approaching its target. We can then provide the spacecraft controllers and the astronomers with very useful, regular updates, e.g., about the 'cometary weather' at the time of arrival."
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