NASA's Deep Impact mislays comet

Boethin goes awol

NASA has cancelled a scheduled liaison between its Deep Impact spacecraft and Comet 85P/Boethin because the latter has disappeared without trace, New Scientist reports.

Deep Impact completed its principal mission back in 2005, when it fired a 360kg probe into Comet Tempel-1 in an attempt to deduce the body's composition.

It then set off for a late 2008 hook-up with Boethin, a body which has been spied only twice, "first when it was discovered during a close approach to the Sun in 1975, and again during a second close passage in 1986".

It was supposed to put in an appearance in 1997 on another approach to the Sun, but may have escaped detection from Earth due to it being behind the star's glare. In October, astonomers deployed various telescopes including the Very Large Telescope array in Chile, and that at Hawaii's Subaru observatory to track it down, without sucess.

Quite what has happened to Boethin remains a mystery. While it may have broken up during its 1997 approach, Michael A'Hearn of the University of Maryland in Baltimore, chief scientist for the Deep Impact mission, said such a heat-provoked disintegration was unlikely since it "never comes closer to the Sun than just beyond Earth's orbit".

He told New Scientist: "Disappearing in the sense of breaking up and dissipating is actually very rare. If it disappeared, then that is fascinating in itself - only one other comet has done that in recent memory."

The previous self-destructing comet in question was Linear-S4, which fell apart and disappeared back in 1990.

A'Hearn says it's more likely that Boethin "broke into a few large chunks that are still intact but have drifted too far from the original comet's orbit to have been spotted in searches to date". More plausible still is that since Boethin has only been seen twice, scientists did not correctly predict its trajectory and the telescopes have been looking in the wrong place.

Whatever the case, NASA will now send Deep Impact off for a meeting with Comet Hartley 2, around the same size as Boethin at 1.6km, although more active. The planned encounter will take place in 2010. ®