Destination: Missing--Comet Once Targeted by NASA Mission Vanished

Little-known Comet 85P/Boethin, chosen as the destination for an extended mission of NASA's Deep Impact spacecraft, disappeared sometime after 1986

By John Matson

COMET CRUISER: The Deep Impact spacecraft has now visited two comets, but a different onetime destination has now vanished without a trace. Image: NASA/JPL-Caltech

In 2005, after NASA's Deep Impact spacecraft had completed its objective of slamming an impactor probe into the nucleus of Comet Tempel 1, mission scientists began plotting their next move.

The spacecraft that had released the probe and documented its cometary collision was intact, with fuel to spare, leaving it well equipped to rendezvous with another comet in the inner solar system. Comets preserve some of the primordial materials from the early solar system, and the rare close look offers planetary scientists a glimpse of conditions that prevailed billions of years ago. The prospect of visiting another comet without having to build and launch a new spacecraft?getting two missions for little more than the price of one?seemed too good to pass up. The only question: Where to?

The best option, it seemed, was a comet called 85P/Boethin. The little-known object, named for its discoverer, Leo Boethin, a priest in the Philippines, would be drawing close to Earth in 2008. Boethin's timely orbit would provide an opportunity for NASA to visit another comet without having to fund the Deep Impact mission for more than a few additional years.

The only catch: Comet Boethin had not been sighted in almost 20 years. In fact, it had been seen on only two occasions?at its 1975 discovery and on its subsequent orbit around the sun, in 1986. Boethin's roughly 11-year elliptical orbit placed it in the class of so-called short-period comets. But when it was due to make another pass through the inner solar system in 1997, Boethin was inconveniently positioned on the opposite side of the sun from Earth, precluding astronomers from getting another look at it.

In the years leading up to 2008, when Boethin was to return once more, astronomers designing the extended Deep Impact mission, called EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation), moved to locate the comet again. But despite a few promising leads, the trail went cold, and mission scientists opted to send Deep Impact to a backup target instead.

The mission ultimately succeeded, but Comet Boethin was never found. And after an exhaustive search, which engaged some of the largest telescopes on Earth and in orbit, the researchers have declared the comet gone for good. It most likely disintegrated sometime after the last known sighting in 1986, but astronomers may never know exactly when, where or how Boethin met its demise. Its unexpected disappearance delayed the payoff of EPOXI mission by two years and added millions to the price tag.

In a forthcoming issue of the journal Icarus, astronomer Karen Meech of the University of Hawaii at Manoa's Institute for Astronomy and her colleagues describe the laborious, ultimately futile search for Boethin. From 2005 to 2007 the hunt engaged several of the world's most powerful observatories, including in Chile the 8.2-meter Very Large Telescopes (VLT), the 8.1-meter Gemini South Telescope, the twin 6.5-meter Magellan telescopes and two four-meter telescopes at the Cerro Tololo Inter-American Observatory; in Hawaii, they employed the 8.3-meter Subaru Telescope and the 3.6-meter Canada?France?Hawaii Telescope. They even turned to space, harnessing the observing power of the orbiting Spitzer Space Telescope. Later the astronomers looked in vain for a debris trail in the data collected in 2010 by NASA's Wide-Field Infrared Survey Explorer satellite.

"I was almost embarrassed to say how much telescope time we had because the eight- and 10-meter-class telescopes are very precious resources," Meech says. "If you're really lucky, you might get a night of eight-meter time."

Meech even considered marshaling robotic planetary spacecraft for the campaign. "I was actually looking at the facilities in orbit around Mars," she says. "Could we turn around Mars Odyssey or something else to look for it?" She soon learned that such maneuvers were possible in principle but that Mars orbiter cameras were ill equipped to detect such a faint object.

The fact that none of the telescopes spotted the comet can only mean one thing: "It had to have broken up," Meech says. "Because of the interest in this comet, everyone was looking for it."

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