NASA Practices Satellite Flyby of Asteroid

John Roach
for National Geographic News
November 1, 2002
Practice makes perfect, or so the thinking goes at mission control for the Stardust spacecraft. The satellite, launched in 1999 to collect samples from a comet, planned to run through a full dress rehearsal today in preparation for its one-shot encounter 14 months from now.

"As we know from vast experience, spacecraft do not necessarily respond exactly as predicted the first time we try things," said Thomas Duxbury, project manager for the mission at NASA's Jet Propulsion Laboratory in Pasadena, California.

Duxbury and his colleagues hope that any problems encountered during the flyby of the asteroid can be fixed in time for a perfect encounter with comet Wild 2 on January 2, 2004.

"It increases our probability of success," said Donald Brownlee, an astronomy professor at the University of Washington in Seattle and the mission's chief scientist. "Many of the mistakes made in space can be found in testing."

The asteroid, named Annefrank after the famed Holocaust victim, is a 2.5-mile-wide [four-kilometer-wide] rock in the inner asteroid belt, just beyond the orbit of Mars and on Stardust's planned route to comet Wild 2.

Stardust is expected to make its closest approach to Annefrank at 11:50 PM EST on Friday as it whips by at four miles [seven kilometers] per second. The satellite's main camera will take a few pictures, but Brownlee cautions that the pictures will not be very detailed.

Cautious Approach

At its closest point, Stardust will be nearly 2,000 miles [3,220 kilometers] from Annefrank. "We could have flown closer if science was our major goal, but this is the optimum distance for the full systems test," said Brownlee.

The scientists do not want to get too close to Annefrank in case it has an unknown moon or cloud of dust and debris. "That would represent a risk to the mission," said Duxbury.

The scientists want to detect any problems that could cause the comet particles to go awry. The spacecraft is designed so that almost any problems that might be detected could be fixed via remote communications.

Stardust was launched in February 1999 on a mission to collect particles from the coma—the gas and dust envelope—that surrounds comet Wild 2 and return them to Earth for scientific analysis.

Scientists believe comets are the oldest, most primitive bodies in the solar system, possibly composed of the basic building blocks of life. They contain the remains of materials used in the formation of stars and planets and likely hold clues about the formation of the solar system.

Capturing Dust Particles

Dust particles released from comets are thought to float around in the coma for a few hours before they are dispersed into space. Stardust will fly to within 75 miles [120 kilometers] of Wild 2's main body, close enough to fly through the coma.

"By gathering pristine dust within hours or less from when it was released from the comet Wild 2, we can perform detailed analyses of these particles on Earth to understand the chemical composition of Wild 2 and comets in general," said Duxbury.

This knowledge, he says, will give scientists a better understanding of the role comets played in the formation of our solar system and the Earth.

Stardust will capture particles with a blue, lightweight, high-tech glasslike substance known as aerogel. The particles are caught as they impact into the gel and slow down, forming an inverse carrot-shaped trail in the glass as they come to rest.

"Aerogel is a very exotic material with many remarkable characteristics," said Brownlee.

The scientists chose aerogel because it can capture the comet particles without damaging them, even though at impact they will be traveling at about six times the speed of a bullet fired from a shotgun.

The aerogel aboard the Stardust spacecraft is fitted into a collector shaped like a tennis racket. After the comet Wild 2 flyby, the collector will fold into a capsule that will be dropped on the desert floor of Utah on January 15, 2006.

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