WASHINGTON – When scientists sent a spacecraft to get a close look at a comet, they thought they knew what they’d see. They were wrong.
The Stardust spacecraft sent back pictures of broad mesas, craters, pinnacles and canyons with flat floors on the surface of comet Wild 2.
“It’s completely unexpected. We were expecting the surface to look more like it was covered with pulverized charcoal,” said Donald Brownlee, a University of Washington astronomy professor and Stardust’s principal investigator.
A description of Wild – pronounced “vilt” – is reported in today’s issue of the journal Science.
Launched by NASA in 1999, Stardust collected samples of material coming off the comet during a flyby in January. The spacecraft is bringing the dust back to Earth and landing is expected in 2006.
In addition to Brownlee’s report, other papers on the comet describe jets visible on the comet’s surface that spew material into space at supersonic speeds.
Brownlee and colleagues report seeing two kinds of craters, probably created when other space bodies slammed into the comet nucleus. One type has a rounded central pit and a rough surrounding terrain, presumably because material was ejected during the impact. The other type has a flat floor and nearly vertical cliffs.
The makeup of the nucleus, Brownlee said, could be one reason there is so little debris and dust. He said the nucleus is like a clump of hard dirt that can absorb impact but is brittle so it loses some material when hit. Another reason is that there’s almost no gravity to keep dust at the surface.
Researchers think the comet may be several billion years old. They hope to learn something about the early universe by studying the material the spacecraft brings back.
Collecting that material exposed the spacecraft to two swarms of microscopic comet dust as it approached within 150 miles of Wild 2.