Comet-Hunting Spacecraft Nears Icy Target
Hubble Space Telescope observations of Comet 103P/Hartley 2, taken on Sept. 25, 2010, are helping in the planning for a Nov. 4 flyby of the comet by the Deep Impact eXtended Investigation (DIXI) on NASA's EPOXI spacecraft.
Credit: NASA, ESA, H. Weaver (The Johns Hopkins University/Applied Physics Lab)

A comet-chasing spacecraft is closing in on its icy quarry, gearing up for a close flyby on Nov. 4.

On that date, NASA's Deep Impact spacecraft will pass a mere 435 miles (702 kilometers) from Comet Hartley 2. The probe has already trained its instruments on the ice ball, preparing for what mission planners describe as the best-ever extended look at a comet. [Photo of Hartley 2.]

"There are billions of comets in the solar system, but this will be only the fifth time a spacecraft has flown close enough to one to snap pictures of its nucleus," mission science team member Lori Feaga, of the University of Maryland, said in a statement. "This one should put on quite a show."

Solar system leftovers

Comet Hartley 2 is small — only about 0.9 miles (1.5 km) in diameter — and completes one orbit around the sun every 6.5 years or so. Cometary orbits tend to be big and looping; they travel far from the sun and then swing in much closer.

At the time of Deep Impact's encounter, Hartley 2 will be nearing the sun and warming up after its cold, deep-space sojourn. The ices in its nucleus will be vaporizing furiously, spitting dust and spouting gaseous jets, researchers said.

The aim of the mission is to gather details about what comprises Hartley 2's nucleus and compare it with other comets, scientists said. Since comets spend much of their time far from the sun, the cold temperatures preserve their composition — and that composition tells a story.

"Comets are leftovers from the 'construction' of our solar system," said mission science team member Sebastien Besse, also of the University of Maryland. "When the planets formed out of the stuff in the solar nebula spinning around the sun, comets weren't drawn in."

Researchers can study these pristine specimens of the primal solar system to learn something about how it formed, and how it birthed a life-bearing planet like Earth, researchers said.

A close flyby

During the one-day close encounter, Deep Impact will swoop down into the comet's bright coma (the sparkling cloud of debris, illuminated by the sun,that shrouds the nucleus). The spacecraft's cameras will take high-resolution pictures throughout the flyby, scientists said.

"We hope to see features of the comet's scarred face: craters, fractures, vents," Besse said. "We may even be able to tell which features are spewing jets."

The spacecraft's instruments — two telescopes with digital color cameras and an infrared spectrometer — are already trained on their speeding target, according to researchers. Deep Impact took its first photo of the comet Sept. 5, when the probe was 37 million miles (60 million km) away from Hartley 2.

"We're still pretty far out, so we don't yet see a nucleus," Besse said. "But our daily observations with the spectrometer and cameras are already helping us identify the species and amounts of gases in the coma and learn how they evolve over time as we approach."

New life for the mothership

The Deep Impact spacecraft was originally built as the mothership for NASA's Deep Impact mission, which intentionally crashed a probe into Comet Tempel 1 in 2005 to study the object's composition.

Now, the Deep Impact probe is being put to other uses — it is tracking and studying various celestial objects under the umbrella of NASA's broad EPOXI mission. The name is derived from the mission's dual science investigations — the Extrasolar Planet Observation and Characterization (EPOCh) and Deep Impact Extended Investigations (DIXI).

Among other objectives, the EPOXI mission hopes to understand more about how comets form and evolve, NASA officials have said.