July 3, 2005

NASA comet-chasing spacecraft streaks toward impact

By Gina Keating

PASADENA, Calif. (Reuters) - The Deep Impact spacecraft hassuccessfully deployed its coffee table-sized "impactor" intothe path of a comet in the final stage of a mission to tracelife on Earth to its celestial origins, NASA scientists said onSunday.

The impactor was on track to collide with the Tempel 1comet at 10:52 p.m. PDT on Sunday (1:52 a.m. EDT, 0552 GMT onMonday), as Deep Impact's fly-by spacecraft, watching from asafe distance, captures images and data with its onboardinstruments, scientists at NASA's Jet Propulsion Laboratory(JPL) said.

"The spacecraft are perfectly healthy and where they aresupposed to be," Andy Dantzler, head of NASA's solar systemprogram, told a news conference. "The stage is set and we'relooking forward to seeing what happens in the next 12 hours."

The impactor is fortified with copper to create aspectacular collision that scientists hope digs deep into thecomet's surface and unleashes a spray of below-surface debriswhose composition may reveal more about how life came to Earth.

The size of the resulting crater could range from a largehouse to a football stadium, and be from two to 14 storiesdeep. Bursts of debris from the cosmic collision could bevisible to the naked eye in some areas of the world, scientistssaid.

Overnight on Saturday, the JPL team watched as Deep Impactoriented itself and slowed its speed in preparation forreleasing the impactor onto its collision course.


Deep Impact turned on the impactor's batteries and releasedit through a series of pyrotechnic explosions that separatedand pushed the smaller craft away.

A 14-minute rocket burn took the fly-by craft out of thepath of Tempel 1's nucleus and large debris that could disableits instruments or antennae.

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 travel from New York to Los Angeles in about sixminutes.

The impactor has begun communicating with JPL and willrelay its data and images from its final plunge to the comet'ssurface through the fly-by craft, scientists said.

The last two hours of the $333 million mission are the mostrisky, as the impactor switches to auto-navigation system andaims itself at the brightest part of the comet.

Because communications between the spacecraft and Earthtake more than seven minutes each way, controllers at JPLcannot correct the impactor's course, scientists said.

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, which JPL scientists have described as "a jetblack, pickle-shaped, icy dirt ball the size of WashingtonD.C.," will be about 83 million miles away from Earth at thetime of the crash.

The aim of the mission, the first to come in direct contactwith a comet's nucleus, is to photograph pristine materialformed billions of years ago during the creation of the solarsystem.

Comets are made of gas, dust and ice from the solarsystem's farthest regions. Scientists have long held the theorythat comets first brought water to Earth by crashing into itssurface.