"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.
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