NASA probe bears down on comet target

PASADENA, Calif. — A NASA space probe was bearing down on its comet target Sunday in a suicide mission scientists hoped would provide new insight into the origins of the solar system.

The 820-pound copper probe was on course to intercept Tempel 1, a pickle-shaped comet half the size of Manhattan, and smash a hole in it so scientists could get their first peek at the heart of one of the icy celestial bodies.

The "impactor" probe, which separated from the Deep Impact spacecraft just after midnight MDT Saturday, appeared in photos taken hours later from the mothership as a bright, distinct dot as it made its 500,000-mile dive toward the sunlit section of Tempel 1.

The mothership, meanwhile, fired its thrusters to slightly change course and stake out a front-row seat 5,000 miles from the high-speed collision, expected to occur at 11:52 p.m. MDT Sunday.

"We anticipate a little bit of a bumpy ride," said systems engineer Jennifer Rocca, back at NASA's Jet Propulsion Laboratory in Pasadena some 83 million miles from the comet.

Comets are leftovers building blocks of the solar system, which formed when a giant cloud of gas and dust collapsed to create the sun and planets. Because comets were born in the system's outer fringes, their cores still possess some of the primordial ingredients, and studying them could yield clues to how the solar system formed 4.5 billion years ago.

The probe was to designed to shoot close-up pictures at a relative speed of 23,000 mph until hitting the comet. After that, the mothership was to take over recording the scene through its high-resolution telescope.

Among the challenges workers in mission control at the Jet Propulsion Laboratory were facing was the probe's switch to autopilot two hours before the encounter, relying on computer software and thrusters to steer itself into the path of the comet.

About 15 minutes after the probe's impact, the mothership was to make its closest flyby of the comet nucleus, approaching within 310 miles. Scientists expected it would be bombarded with flying debris and will stop taking pictures, turning on its dust shields for protection.

NASA's brigade of space-based observatories, including the Hubble Space Telescope, Chandra X-ray Observatory and Spitzer Space Telescope, also were pointing toward the comet to record the impact.