A U.S. spacecraft is nearly at the end of a five-year comet chase and will fly by the comet Wild-2 on Friday. The name of the spacecraft is Stardust, indicative of its goal to capture particles of matter formed in stars out of which our solar system, and we, formed.
The notion of stardust is romantic, the stuff of poetry and song.
But astronomers like Donald Brownlee of the University of Washington have a more pragmatic outlook. They see stardust as containing key information about the makeup of the early solar system. They want to study some of it under a microscope, so they designed a space mission to capture particles from a comet.
"Comets are containers that have preserved the fundamental building blocks that our solar system, our planet, and even ourselves were made of," said Mr. Brownlee. "The atoms inside our bodies were in stardust or interstellar grains before the sun formed, and we believe these are still preserved in comets."
To get some of these grains, the Stardust spacecraft, launched in early 1999, is finally catching up with Comet Wild-2 as the icy body speeds through the solar system on its six-year orbit around the sun. The approach will occur nearly 400 million kilometers from Earth and will bring the spacecraft as close as 300 kilometers to the comet.
As the craft moves through the cloud of gas and dust surrounding Wild-2, it will capture 1,000 or more of its particles in a wispy filter called an aerogel. Project manager Tom Duxbury says an antenna on the craft is to simultaneously transmit close-up pictures of the comet's nucleus and sensitive instruments will gather data about the object.
"During the flyby, the spacecraft is under its own control. It will turn itself to point our camera at the nucleus during the flyby. This will probably be the most tense period of the mission. But a few minutes past the flyby, the spacecraft will automatically turn itself back to Earth and start dumping [transmitting] data again," said Mr. Duxbury. "That's when our white knuckles turn back to normal color and we start breathing normally."
During the encounter, billions of high speed particles from the comet's surrounding dust cloud will pelt the U.S. spacecraft, which will be traveling at nearly 22,000 kilometers per hour. But Mr. Duxbury says Stardust is to deploy shields to protect it from the onslaught. "We have an extreme, severe environment where we're going through a storm of dust particles that are hitting us at over six times the speed of a bullet."
The risk comes if a bigger chunk of the comet smashes into the spacecraft - something larger than one centimeter in diameter. It could penetrate the shields, but the researchers rate the chance of this as one percent or less.
They hope the comet grains they collect will confirm long held suspicions - that the carbon material in comets played a major role in the origin of life. They point out that comets are the most carbon-rich bodies in the solar system, full of organic compounds that fall to Earth all the time.
But they add that it is necessary to journey to the comet to get them because it is difficult to distinguish the comet particles that blanket our planet from Earthly material. They would also not be in as pristine condition as dust in a comet.
Donald Brownlee says another goal of the mission is to help scientists learn more about comet structure.
"One of the interesting things we want ot know about comets is, how diverse are they? Are they all the same or are they made out of different materials? It's a fundamental question," he says.
The payoff for scientific research will begin in January 2006, when the Stardust spacecraft returns to Earth and releases a capsule containing its treasure - several grams of comet dust - by parachute onto the Utah desert. The U.S. space agency NASA will collect it by helicopter and distribute specimens to scientists around the world to begin to unlock secrets of the early solar system.
The Stardust mission is only one of several U.S. and European comet rendezvous flights either in progress or soon to be launched. The European Space Agency plans to send up its Rosetta craft in February to drop scientific instruments on a comet to study its composition. The most spectacular project will be NASA's Deep Impact mission. After its launch in one year, it is to crash into a comet to get data from below its surface.
NASA calls it a golden age of comet exploration.