Comet Hartley 2: EPOXI probe flyby to yield rare close-up photos

Five years and 2.9 billion miles after its historic encounter with comet 9P/Tempel 1, a plucky NASA spacecraft is set to fly by a second comet, 103P/Hartley 2, Thursday morning.

The craft's second round of dancing with the comets is designed to give scientists an unprecedented look at an object thought to carry important clues about how planets in the solar system formed some 4.5 billion years ago.

Between the advanced instruments on the craft, formerly known as Deep Impact, and its relatively close 434-mile fly-by distance, the effort will provide "the most extensive observations of a comet to date," says Tim Larson, project manager for the mission -- renamed EPOXI -- at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

Only four other comets have received fleeting visitations from US spacecraft: Giacobini-Zinner, Borrelli, Wild 2, and Tempel 1.

Compared to the other four, Hartley 2 is the smallest, with a core of rocky rubble and ice only about three quarters of a mile across, versus an average of about four miles across for the others. Their more-famous sibling, Comet Halley, sports an even larger nucleus, some eight miles across.

The EPOXI team hopes to figure out what accounts for the wide range of shapes and surface features of comet nuclei when "there are no obvious differences in the processes that shape the comets," says Michael A'Hearn, a researcher from the University of Maryland at College Park and the lead scientist on the project.

Hartley 2 not only is a cometary munchkin, it's an unusually active one, says Dr. A'Hearn. For its size, it spews relatively large quantities of dust and gas as it approaches the sun.

This process sculpts the surface of the nucleus. A'Hearn and his colleagues say they hope to use images of this activity and its effects to help them distinguish between features on the nucleus that have been continuously reshaped versus those that still represent pristine patches of material from the early solar system.

A high-resolution spectrometer aboard EPOXI can then tease out information about the composition of these different kinds of materials.

For all the excitement the team says it feels as the encounter approaches, Hartley 2 was not its first choice as a target. That distinction fell to comet 85P/Boethin. Had EPOXI crossed Boethin's path, scientists would have had their data two years ago because it was closer than Hartley 2. During probe's cruise to meet it, Boethin vanished. Astronomers are still unclear about its fate. It could have become too faint to spot anymore. Or it could have disintegrated somewhere along its orbit, a rare occurrence for a comet, but one scientists have observed.

Hartley 2 swings around the sun every 6.46 years. It belongs to the Jupiter family of short-period comets, which orbit no farther from the sun than Jupiter's neighborhood.

When it meets up with EPOXI, the encounter will take place at a blazing 26,800 miles an hour. The craft itself is the size of a subcompact car, and weighs about 800 pounds less, says Amy Walsh, the craft's lead systems engineer at Boulder, Colorado-based Ball Aerospace and Technologies. And between the encounter with Tempel 1 and Thursday's event, "we've gone five and a half years on one tank of fuel," she quips.

During the final 18 hours of its approach, EPOXI will stop sending the stream of images it's been gathering as it closes in on Hartley 2; as the craft orients its high-resolution cameras in preparation for the fly-by, the antenna it uses for streaming science data will have its back turned to Earth.

Given the speeds involved, it may seem like a blink-and-you-miss-it encounter. But it's not quite that bad, Ms. Walsh says. The comet will be just over 400 miles away at closest approach, so from the spacecraft's standpoint, the pass will appear slower -- requiring that EPOXI turn itself at roughly a sixth of the speed a second-hand needs to travel around a clock face in order to keep its cameras trained on the comet.

The EPOXI mission is expected to return nearly 6,000 detailed images of the comet taken during the fly-by. The high-resolution camera that will serve as the fly-by's workhorse is said to be able to spot the difference between a pick-up truck and sedan at 400 miles.

Although the team says it won't have any data to share immediately after the encounter, it expects to begin releasing images from the fly-by perhaps as early as Friday.

In 2005, EPOXI, then Deep Impact, flew past Tempel 1 and sent an impactor hurting into the comet's nucleus. This provided scientists with their first look at the material beneath the heavily processed surface of the comet.