Dec29 Written by:admin
Saturday, December 29, 2007
A NASA comet probe heading back to Earth for a gravity boost will be redirected after the disappearance of its intended target.
Deep Impact, which won funding for an extended mission after its successful close-up studies of Comet Tempel 1 in 2005, was expected to use its Dec. 31 flyby of Earth for a slingshot maneuver to rendezvous with a comet known as 85P/Boethin in 2008.
Scientists spent the summer and early fall searching for the target so Deep Impact could be dispatched on the right path. A hunt by the most sophisticated telescopes on the planet, however, turned up nothing.
"We couldn't find it," University of Maryland astronomer Michael A'Hearn, who heads the Deep Impact science team, told Discovery News.
The comet had only been seen twice before, but was expected to reappear last summer. A'Hearn believes the comet escaped detection because its orbit differs from predictions. But it could have fragmented into smaller pieces or even dissipated completely during its last pass around the sun.
"Comet Linear did that in 2000, but it's quite rare. If that is the right explanation, that's really fascinating," A'Hearn said.
Scientists had a backup target, Comet Hartley 2, but reaching it would take Deep Impact an extra two years and cost another $8 million.
"That's why we were trying so hard to find Comet Boethin," A'Hearn said.
With budgets tight, it was far from assured that NASA would pick up the extra expense, but last week the U.S. space agency agreed to do so.
Deep Impact has another mission before it reaches its new target: Beginning in January, it will use the larger of its two telescopes to hunt for planets circling nearby stars. The study is funded for six months, but scientists are considering asking for an extension because Deep Impact's comet rendezvous, originally planned for late 2008, is now delayed until October 2010.
Although A'Hearn and his colleagues will have to wait longer, Comet Hartley 2 may be an even better target than Comet Boethin. While both are about the same size -- roughly a half-mile in diameter -- Hartley 2 may be more active, A'Hearn said.
The original Deep Impact mission blasted a hole into its target so instruments could study its underlying structure and composition. With no more probes to release, the follow-up study of Hartley 2 will be restricted to measurements of surface features and naturally occurring phenomena, such as ice jets.
While only a handful of comets have been studied in detail, scientists have been surprised by their physical and chemical differences.
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