Close comet encounter produces surprises

Ann Arbor, Mich. -- The February passage of the spacecraft Ulysses through comet McNaugth's ion tail has given U.S. scientists vital clues about the solar system's evolution.

The spacecraft -- a joint venture of the National Aeronautics and Space Administration and the European Space Agency -- is on a mission to study the sun's polar region. In February it flew through the comet's tail about 160-million miles from the comet's core.

University of Michigan space scientists said their Solar Wind Ion Composition Spectrometer, or SWICS, instrument aboard the spacecraft not only detected unexpected ions, but also indicated that even at 160 million miles from the comet's nucleus, the tail slowed the solar wind to half its normal speed.

The solar wind would usually be about 435 miles per second at that distance from the sun, but inside the comet's ion tail, it was less than 249 miles per second.

"This was very surprising to me," said UM Professor Michael Combi. "Way past the orbit of Mars, the solar wind felt the disturbance of this little comet. It will be a serious challenge for us theoreticians and computer modelers to figure out the physics."

The study is reported in the Astrophysical Journal.

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