NASA Probe on Course for Friday Comet Encounter

John Roach
for National Geographic News

December 30, 2003

On Friday, a NASA spacecraft will make a pass on the wild side.

Flying at five times the speed of a rifle bullet, the Stardust spacecraft will pass through a cloud of dust particles swirling off comet Wild 2, which is zipping through space some 242 million miles (389 million kilometers) away on the far side of the sun.

Using a tennis-racket-shaped collector filled with gaseous jelly, the spacecraft will snare hundreds of comet particles and fly them back to Earth. If all goes according to plan, the collector will parachute down to the Utah desert in January 2006.

Scientists believe comets are the remnants of exploded stars, leftover planetary building blocks that may hold secrets to the origins of life. Stardust will allow scientists for the first time to study unaltered bits of this primordial matter.

"One of the reasons we want to go to a comet is there is so much we don't know about comets," said Thomas Duxbury, program manager for the Stardust mission at NASA's Jet Propulsion Laboratory in Pasadena, California.

The 770-pound (350-kilogram) spacecraft will zoom to within 186 miles (300 kilometers) of the comet's icy nucleus. In addition to the dust collector, a camera onboard the spacecraft will snap close-up photos of the chunk of gravel and ice and transmit them to Earth.

Second "Biggie"

Getting Stardust to Wild 2 (pronounced "vilt two") and back to Earth is an epic seven-year journey. Friday's flyby of Wild 2 is the second of what Duxbury refers to as the "three biggies" of the adventure.

The first was the spacecraft's launch from Cape Canaveral, Florida, on February 7, 1999. The third will be the return of the capsule containing the comet dust to the Utah desert in 2006.

"We are at the second of the three, and I kind of laugh at how I'm really feeling," said Duxbury. "We've been working hard for the last month. The team's tired. Even though we're quite excited—underneath the surface is excitement—we are getting worn out."

Duxbury and his team have sacrificed their Christmas and New Year's holidays to do everything they can to ensure the Wild 2 flyby goes according to plan.

Using a steady stream of images of the comet taken by the spacecraft's camera, engineers are guiding Stardust in for the encounter.

"Finding the comet with a camera and figuring out how to get close to it is a challenge, but it is going really well," said Donald Brownlee, the mission's principal investigator and a professor of astronomy at the University of Washington in Seattle.

At 18 hours before the flyby, engineers will make their last adjustment. If the mission scientists see something in the images relayed by the spacecraft after that time period—like a new jet spewing from the surface or the comet broken into pieces—a maneuver stored onboard will steer the spacecraft out of harm's way, said Duxbury.

Otherwise, traveling at four miles (six kilometers) per second Stardust will pass by the sunlit side of the comet nucleus and capture freshly boiled off particles in the coma. A shield enveloping the spacecraft should protect it from the particle bombardment.

The shield is built to withstand impacts from marble-like particles up to a centimeter in diameter. "Anything larger would penetrate the meteor bumper shield, and we don't know what could happen. There is a risk if we get hit by something that big it could hurt a vital component," said Brownlee.

Duxbury said he is confident the decision to flyby at a distance of 300 kilometers (186 miles) from the comet nucleus is a sufficient balance between risk and science: close enough to collect particles, but distant enough to avoid catastrophic impacts.

"We expect to survive, but no one is going to guarantee it," he said.

Immediately after the flyby, a dust counter built by University of Chicago researchers will relay to Earth the number and size of the particles collected. Meanwhile, a German built instrument will begin analysis of the composition of the comet dust.

Lab Analysis

When the capsule containing the particles finally lands in the Utah desert over two years from now, it will be sent to NASA's Johnson Space Center in Houston, Texas. Samples will then be parceled out to scientists around the world.

Although tons of comet particles bombard Earth every year, they are difficult to find among earthly materials; and when found, their cosmic origins are uncertain. As well, such particles cannot give the same kind of information Wild 2 may yield.

Only in 1974, when Wild 2's orbit was altered by Jupiter's gravitational pull (bringing it in from beyond Uranus to just outside Mars), did it start orbiting close enough to the sun for primordial material to boil off its surface.

There hasn't been enough time for heat to destroy the comet's characteristics that have been kept frozen in deep space for billions of years, making it a prime target for Stardust.

"This is the initial pre-solar interstellar dust that formed a disc that ended up forming our entire planetary system," said Brownlee. "Actually, this is a universal process in the sense that the same kinds of grains initiated planet formation around other stars, so there are several links with astrobiology."

Understanding the role comets played in the origin of life here on Earth may help astronomers understand how life could form on planets orbiting distant stars, according to Brownlee.