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

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."

Boethin's disappearance did not come as a total shock. Whether because of the gravitational pull of a nearby planet, the pressure of pent-up gas from sublimating ice or some other mechanism, comet breakups are rather common. One study had estimated that short-period comets such as Boethin each have a better than 3 percent chance of falling apart in any given century. "Comets really don't have much strength," Meech says. "We don't know specifically the mechanism that will cause them to break apart, but we know they're pretty weak."

Still, Deep Impact's mission planners had high hopes of recovering the comet by the fall of 2007, the cutoff for making course-correction maneuvers to set up an encounter the following year. Meech recalls Deep Impact principal investigator Michael A'Hearn of the University of Maryland, College Park, reassuring a review committee assessing the extended mission that Meech could recover Boethin in time. "I thought, 'Please, Mike!'" she recalls. "Things can happen to comets. Oh God, the pressure!"

Meech's concerns aside, the odds favored Boethin's having survived its two orbits around the sun since 1986. And its absence from the astronomical record in 1997 made perfect sense, given its position behind the sun at the time. "I was pretty confident that we would find it," A'Hearn says. "I convinced myself that there were very good reasons for why we didn't recover it" by the time Deep Impact's mission extension came up for discussion, he adds.

In July 2007 NASA announced that it had approved the EPOXI extended mission for Deep Impact, with Boethin as the prime target. Meanwhile, if still extant, Comet Boethin was drawing nearer to the sun, which would expose it more easily to viewing by terrestrial telescopes. Years of intermittent searching gave way to weeks of more intensive hunting with Subaru and VLT. But the comet refused to show.

In August the cold trail suddenly grew warm. The Canada–France–Hawaii Telescope detected a faint blob along Boethin's orbital path. Meech and her colleagues scrambled to confirm the sighting by commandeering whatever telescopes they could. The directors of several large observatories awarded discretionary time on their instruments. Independent astronomers who had earned telescope time on the strength of their own proposals put their projects on hold to help in the search. "They gave up significant amounts of time for the project," Meech says. "Just gave it to us because they thought it was neat to help find a wayward comet for a space mission."

The clock was ticking—mission planners had precious few months to design and execute a course correction if the spacecraft was to rendezvous with Boethin in 2008. "People were calling me every day and saying, 'Have you got it? Have you got it? Have you got it?'" Meech says. By October 18, 2007, the results from the search were in—no other telescope could confirm the tentative sighting. The cometlike blob was a fluke.

The next day EPOXI mission managers made an official decision. Boethin was out and the backup target, Comet Hartley 2, was in. Hartley 2 was a promising scientific destination in its own right, but the spacecraft would not reach it until two years after the originally planned encounter with Boethin, and even longer after the budget termination of the Deep Impact project. "It was going to be four years, and it was going to be very hard to fit that in the budget limits for the extended mission," A'Hearn says.

Despite the added time and expense, NASA ultimately extended the mission, and in 2010 the spacecraft completed a successful flyby of Comet Hartley 2 at a cost of about $40 million—more than originally estimated but still a fraction of the cost of a standalone mission. "It turned out to be a blessing because that new target was absolutely fascinating and has changed a lot of our view of how comets work," Meech says.

In the meantime, though, the specter of Comet Boethin haunted Meech and her colleagues once more. In 2008, as Boethin would have been near perihelion (its closest approach to the sun), the astronomical record-keepers at the Minor Planet Center received notice that the wayward comet had been sighted. But the report, submitted by an inexperienced observer, soon proved unfounded—much to Meech's relief. "Once we declared it lost and redirected the spacecraft, we really want to be right," she says. "I was keeping my fingers crossed that it really was dead and nobody found it."

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