Did the mile-wide comet explode due to volatile gases inside it? Did something crash into it and cause it to vaporize? Are little bits of it still on the comet's orbital path, too small to be detected from Earth? Or is it something else?
Astronomers don't know. “It's an intriguing mystery,” said University of Hawai'i astronomer Karen Meech, in a telephone conversation.
(Photo: Artist's conception of Deep Impact spacecraft flying by Earth. Credit: NASA, University of Maryland.)
"We were confident we could find the comet, and we were astonished when it wasn't there," said Meech, of the university's Institute for Astronomy and co-investigator for the mission of the Deep Impact spacecraft. That's the vehicle that in July 2005 crashed a big chunk of metal into Comet Tempel 1 to find out what it was made of.
The Deep Impact team had hoped to redirect their spacecraft to check out Boethin when it arrives near Earth next year. But after a search by an international team of observatories, it seems to be gone.
Deep Impact, whose mission is named EPOXI*, will be redirected to a backup comet, Hartley 2. It should pass by that comet in October 2010.
Boethin was scheduled to be visible within the inner solar system in fall 2008. While Boethin is normally visible from Earth only during the six months when it is closest to the sun, astronomers went looking for its infrared radiation signature when it was still out of sight. They'd expected it could be detected by October 2007.
When they couldn't find it, they enlisted 10 of the world's biggest telescopes, as well as NASA's Spitzer Space Telescope. Among the Earth-bound scopes were two on Mauna Kea, the 8-meter Subaru Telescope and the 3.6-meter Canada-France-Hawai'i telescope
“Their combined international observations constituted the most intense search for an astronomical object every undertaken,” the astronomy institute said in a press release.
Michael A'Hearn, principal investigator for the Deep Impact and EPOXI missions, said researchers feel there may be remnants of the comet, even if it exploded. They say blasts big enough to destroy a comet are rare.
"The last time we saw comet Boethin, we estimated it was at least a mile across. If we can't see it, it must have broken into pieces no more than a quarter-mile across," he said. Astronomers around the world will be watching late next year to see if pieces are visible, and whether remnants form a visible tail.
Meech said the situation is compounded by the fact that astronomers were not confident about their estimate of Boethin's location. It normally takes three comet passes to get an accurate comet orbital assessment. Boethin was only first spotted in 1975, and was viewed again in 1986. But its 1997 pass was on the other side of the sun from Earth, so only two passes have been measured.
Astronomers calculated the possible errors in their positions, and searched all the likely locations for the comet, with no luck.
“We searched that whole area very deeply,” Meech said.
Meanwhile, the Deep Impact/EPOXI spacecraft will have a longer voyage before its next cometary contact. Getting to Comet Hartley 2 will take two years more than a visit to Boethin would have, but Meech said the comet study effort is yielding lots of new information.
"Up to now we have seen only four comets close up, and each was different. Since comets tell us about the chemistry and physical conditions of the early solar system, the more we learn about comets, which delivered some of Earth's early water and organic compounds, the more we may learn about how life began on Earth," Meech said.
© 2007 Jan W. TenBruggencate .
* EPOXI is a blending of the acronyms for two space missions. During the next six months or so, Deep Impact's spacecraft will use one of its two telescopes to search for Earth-sized plans around five stars, a project called Extrasolar Planet Observations and Characterization (EPOCh). The second mission, to fly by and use telescopes and other gear to inspect Comet Harley 2, is the Deep Impact eXtended Investigation (DIXI).