NASA comet crash mission enters risky home stretch

PASADENA, Calif., Monday (Reuters) A NASA spacecraft entered the risky home stretch of its 268 million-mile (431 million km) voyage to crash into a comet on Sunday, performing its final maneuvers in a mission to uncover the building blocks of life on Earth.

The Deep Impact spacecraft successfully deployed its coffee table-sized "impactor" into the path of comet Tempel 1 Saturday evening, and was on track for a 10:52 p.m. PDT (1:52 a.m. EDT, 0552 GMT on Monday) smashup, scientists at NASA's Jet Propulsion Laboratory said.

As the copper-fortified impactor entered the last two hours of its $333 million mission, it began taking four images per minute, officials said. The craft's auto-navigation system will use the data to modify its trajectory and determine the brightest place to target on the comet's surface.

Because communications between the spacecraft and Earth take more than seven minutes each way, controllers at the laboratory cannot correct the impactor's course. The probe must also dodge increasing amounts of debris as it gets closer to the comet.

Scientists and engineers in the mission's control room appeared focused but nervous as they awaited status reports from both the impactor and the fly-by spacecraft. The biggest remaining challenge, according to the mission's project manager, is determining the shape and behavior of the comet so that the impactor can strike it head on.

"It's presenting a very strange shape to us," said Rick Gremmier, Deep Impact project manager. "We're looking at the end of it, and the end is triangular, so it all depends on what the comet does. So that's the challenge."

The impactor is expected to create a spectacular collision that scientists hope digs deep into the comet's surface and unleashes a spray of below-surface material formed billions of years ago during the creation of the solar system.

Comets are made of gas, dust and ice from the solar system's farthest regions. They often show bursts of activity, during which their surfaces crack to create tails of dust. Scientists think comets may have been responsible for first bringing water to Earth by crashing into its surface.

The size of the crater on the comet, which is roughly half the size of Manhattan, could range from that of a large house to a football stadium. Bursts of debris from the cosmic collision could be visible to the naked eye in some areas of the world, scientists said.

Overnight on Saturday, the Jet Propulsion Laboratory team watched as Deep Impact, which will record the crash from a safe distance, oriented itself and slowed its speed in preparation for releasing the impactor onto its collision course.