EPOXI) passed by comet Hartley 2 at 7:01 am PDT, the fifth time in history that a spacecraft has been close enough to photograph the heart of a comet. The probe flew through the comet’s diffuse corona at about 27,500 miles per hour and came within 435 miles of its icy, dirty core.

Observations leading up to the flyby showed that Hartley 2 is small but active. It’s only about 1.36 miles across, a shrimp compared to other comets that have been visited by spacecraft. But it spews several times more gas and dust than other comets. In this image from the moment of closest approach, the comet looks like a bowling pin or a peanut, with at least two jets streaming off toward the sun.

“It’s hyperactive, small and feisty,” said Don Yeomans, a senior research scientist at NASA’s Jet Propulsion Lab.

Deep Impact locked its instruments and began taking pictures of the comet 18 hours before nearest approach, when it was 496,000 miles away. By the time of closest approach, it was snapping photos once every four seconds.

But astronomers had to wait almost an hour after closest approach to get the good shots. The close-ups were stored on the spacecraft because Deep Impact couldn’t point its antenna toward Earth and its cameras toward the comet at the same time.

Hartley 2’s jets of material act as little rocket thrusters, making the comet’s orbit hard to pin down.

“It acts like a knuckleball,” said project manager Tim Larson. “You can’t predict where it’s going.”

So instead of trying to fly the spacecraft directly, engineers switched Deep Impact to autopilot, or AutoNav, about 50 minutes before closest approach. In AutoNav mode, the spacecraft aims for the brightest nearby object, excluding the sun.

Models predicted that without AutoNav, there would be just a 0.1 percent chance of actually imaging the nucleus, according to EPOXI flight director Rich Rieber.

“AutoNav’s prediction for the orbit of the comet was dead on,” Rieber said. “Those guys did magic.”

Deep Impact is the first spacecraft ever to visit two comets. Its primary mission was to make a crater on the surface of the comet Tempel 1 in July 2005, which showed scientists what that cosmic dirtball was made of. Deep Impact then spent a year observing extrasolar planets before firing its thrusters and heading toward Hartley 2.

“This is the third bonus mission we’ve gotten out of this,” said Rick Grammier, director for solar system exploration at JPL, on NASA TV this morning.

The spacecraft still goes by the name Deep Impact, but its mission was renamed EPOXI, a mishmash of acronyms for an extrasolar planet observation mission (EPOCh) and the Deep Impact Extended Investigation (DIXI).

Hartley 2 was discovered by Australian astrophotographer Malcolm Hartley in 1986, when it showed up as a smudge on a photographic plate. Hartley was at JPL mission headquarters when the images arrived.

“It’s absolutely awesome,” he said. “I’m overwhelmed by everything that’s happened in the past few weeks.”

Hartley 2 wasn’t actually NASA’s first choice for the next comet encounter. The A-list comet, called 85P/Boethin, disappeared without a trace. Scientists think it may have broken up into untraceable fragments.

NASA will continue to download new data from the close encounter until Nov. 6, and Deep Impact will keep taking images for the next 3 weeks as it leaves the comet.

“Despite the smiles and tears that you see, our job is far from over,” Rieber said. “After the next 3 weeks, the future life of Deep Impact is uncertain, but we have many ideas about how to use our great spacecraft.”

Image: 1. NASA 2. The first image of Hartley 2. AAO/SidingSpring

See Also:

• Aging Spacecraft Set On New Comet Hunt
• Close Encounter Coming With Ghostly Green Comet
• Reader Photos: Ghostly Green Comet Approaches Earth
• Comet’s 10 Million-Mile Tail Lights Up in Infrared
• Video: Cold, Little Comet Is No Match for Big, Hot Sun

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Authors: Lisa Grossman

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