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Deep Impact and the Case of the Missing Comet

Deep Impact spacecraft.
Rendering of the Deep Impact spacecraft by Pat Rawlings.

Millions of armchair astronomers were thrilled in 2005 when the Deep Impact spacecraft placed a large metal impactor in the path of Comet Tempel 1. NASA has recently announced that the spacecraft will be redirected to study a second comet, Hartley 2. The new mission is named EPOXI.

The decision was made after an international consortium of astronomers led by IfA astronomer Karen Meech, a co-investigator on the mission, announced that the first-choice target, called Comet 85P/Boethin, has apparently disappeared.

"We were confident we could find the comet, and we were astonished when it wasn?t there," said Meech.

Comet Boethin had been selected as a target because its orbit takes it to a region of the solar system that the Deep Impact spacecraft could have been directed to in 2008. Boethin has an 11.8-year orbit, but can be seen from Earth only during the six months when it is closest to the Sun.

Comet orbits are well known only if they have been observed on three revolutions around the Sun, and Comet Boethin, discovered in 1975, had only been seen one other time, in 1986. In 1997, Boethin's approach to the Sun was not visible from Earth. To navigate the spacecraft to the comet, its position had to be measured again before the end of 2007, when the spacecraft will swing by Earth and alter its course to encounter the comet's orbit.

The last time Comet Boethin was visible from Earth was in 1986, but Meech and her colleagues expected to observe it no later than October of this year. They used ten of the world's largest telescopes, plus NASA's orbiting Spitzer Space Telescope, which searched for infrared radiation from the comet. Their combined international observations constituted the most intense search for an astronomical object ever undertaken.

The recovery of Comet Boethin was particularly challenging because it is not going to develop its tail and get bright until fall 2008, so astronomers were going to have to pinpoint the position of a small very dark object moving through a region of the sky heavily carpeted by stars. It was the cosmic equivalent of finding a needle in a haystack, but the EPOXI team thought they could do it. The only way to find the missing comet was to look for a faint spot that moved against the background of stars and galaxies.

So what happened to Comet Boethin? It may have broken up into pieces that are too small to see from Earth, possibly because of a catastrophic "explosion" caused by volatile gases inside it. Cometary outbursts are very frequent, but something large enough to completely destroy a comet, or as large as the recent Comet Holmes outburst, is 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,” said Michael A'Hearn, the principal investigator for the Deep Impact and EPOXI missions.

Will Comet Boethin ever be seen again? Although it is too late for this space mission, astronomers, both professional and amateur, will search the skies for Comet Boethin during the fall of 2008, when a head and tail may develop.

“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,” said Meech.

Comet Hartley 2, which has an orbital period of 6.4 years, was first discovered in March 1986, and has been viewed on four trips around the Sun, so its orbit is well known. However, it will take two years longer to reach Hartley 2 than it would have taken to reach Boethin.

The Deep Impact spacecraft, launched in January 2005, encountered Comet Tempel 1 in July 2005. The comet's collision with the 370-kilogram (815-pound) impactor created a crater and ejected dust and ice that were studied by instruments on Deep Impact, as well as numerous ground- and space-based observatories, to learn more about the composition and structure of the comet. Deep Impact's encounter with Hartley 2 is a flyby mission only.