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Using his experience with comets and NASA’s , Dr. Harold Weaver, of The ����ֱ�� University Applied Physics Laboratory, played a key role in preparing the European Space Agency’s Rosetta comet-study mission for its new target.

Weaver was part of a team that used Hubble to measure the size, shape, and rotational period of comet 67P/Churyumov-Gerasimenko (67P/C-G), on which will attempt an unprecedented landing and in-depth study. Hubble’s observations revealed that the comet’s nucleus is approximately a three-by-two mile, football-shaped object on which it is possible to land a spacecraft. Mission scientists were concerned that the nucleus was much larger and that its higher gravity would make a soft landing more difficult.

“Although 67P/C-G is roughly three times larger than the original Rosetta target, its highly elongated shape should make landing on its nucleus feasible, especially if the lander package can be strengthened a bit between now and next year’s launch,” says Dr. Philippe Lamy, of the Laboratoire d’Astronomie Spatiale, France. Lamy is presenting the Hubble results on comet 67P/C-G today at the annual meeting of the Division of Planetary Sciences of the American Astronomical Society in Monterey, Calif.

“Previous ground-based observations indicated the comet’s nucleus could be nearly 6 kilometers [3.6 miles] across, or about twice as large as what was considered safe for landing on its surface,” says Weaver. “But, fortunately, we realized that the Hubble Space Telescope could measure both the size and shape of the nucleus, which would allow a more accurate assessment of the risks of retargeting Rosetta to this new comet.”

Mission scientists began considering the backup target when Rosetta’s launch date was postponed. The delay put the original target comet, 46P/Wirtanen, out of easy reach. But scientists did not have enough information on the new target, 67P/C-G, and sought data from the largest telescopes. Using a technique developed over the past decade by Weaver, Lamy and Imre Toth (Konkoly Observatory, Hungary), the Hubble team snapped 61 images of comet 67P over a 21-hour period on March 11-12, 2003.

Hubble’s Wide Field Planetary Camera 2 isolated the comet’s nucleus from its coma, the diffuse cloud of dust and gas surrounding the nucleus, quickly providing the missing figures. The telescope showed that the nucleus is ellipsoidal and measured its rotation rate of approximately 12 hours.

Rosetta’s launch is planned for February 2004, with the comet rendezvous scheduled about 10 years later.

For the electronic images, a movie and additional information on the project, visit: .