July 4, 2005

NASA comet crash mission enters risky home stretch

By Nichola Groom and Nigel Hunt

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

The Deep Impact spacecraft successfully deployed its coffeetable-sized "impactor" into the path of comet Tempel 1 Saturdayevening, 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 JetPropulsion Laboratory said.

As the copper-fortified impactor entered the last two hoursof its $333 million mission, it began taking four images perminute, officials said. The craft's auto-navigation system willuse the data to modify its trajectory and determine thebrightest place to target on the comet's surface.

Because communications between the spacecraft and Earthtake more than seven minutes each way, controllers at thelaboratory cannot correct the impactor's course. The probe mustalso dodge increasing amounts of debris as it gets closer tothe comet.

Scientists and engineers in the mission's control roomappeared focused but nervous as they awaited status reportsfrom both the impactor and the fly-by spacecraft.

The biggest remaining challenge, according to the mission'sproject manager, is determining the shape and behavior of thecomet so that the impactor can strike it head on.

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


The impactor is expected to create a spectacular collisionthat scientists hope digs deep into the comet's surface andunleashes a spray of below-surface material formed billions ofyears ago during the creation of the solar system.

Comets are made of gas, dust and ice from the solarsystem'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 firstbringing water to Earth by crashing into its surface.

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

Overnight on Saturday, the Jet Propulsion Laboratory teamwatched as Deep Impact, which will record the crash from a safedistance, oriented itself and slowed its speed in preparationfor releasing the impactor onto its collision course.

After placing itself in the proper orbit, the fly-by craftturned back and snapped a black-and-white image of the impactorhurtling away at 23,000 mph (37,100 kph) -- the speed it wouldtake to fly from New York to Los Angeles in about six minutes.

The impactor was expected to make three course correctionswith its thrusters during its last 90 minutes of flight.

Scientists said the impactor could continue sending imagesuntil 2 seconds before impact, although chances are also goodthat dust hitting the spacecraft could disable it.

Tempel 1 will be about 83 million miles away from Earth atthe time of the crash.

(Additional reporting by Gina Keating)