More than 30 years ago, Apollo astronauts brought back rocks from the moon; since then, no mission has retrieved specimens from outer space. That's about to change. A small NASA spacecraft, launched today from Cape Canaveral, Florida, will collect material from the sun and bring it back to Earth. The Genesis probe will help reveal the composition of the primordial solar nebula, which gave birth to the sun and planets 4.6 billion years ago.
Genesis would fry if it landed on the sun, which has a surface temperature of some 6000 degrees C. Instead, the craft will fly 1.5 million kilometers toward the sun--a mere 1% of the distance between Earth and the sun. There it will deploy more than one square meter of collector foils made of silicon, diamond, sapphire, and aluminum, among other elements--foils chosen for their purity. The foils will intercept the solar wind, the continuous stream of electrons and atomic nuclei blown away from the hot surface and atmosphere of the sun. After 26 months, Genesis will bring some 20 micrograms--about the weight of a droplet of mist--of solar material back to Earth for analysis.
The Genesis material should help astronomers make sense of other solar system samples, says Peter Bochsler, a solar physicist at the University of Bern, Switzerland. Until now, knowledge about the composition of the solar nebula has largely been derived from studies of meteorites--a good approximation, since meteorites are primitive chunks of the solar nebula. But even meteorites differ in their chemical composition by a few percent. Particles emitted by the sun should give a better reading: "The sun contains 99.9% of the total mass of the solar system," says Bochsler, "and is probably representative of the primordial solar nebula."
One of the most challenging stages of the mission will take place when it's time to retrieve the samples. To avoid contamination, a mailbox-sized container housing the samples will descend through Earth's atmosphere by parachute and will be snatched in midair by two helicopters. NASA will have plenty of time to practice this daredevil operation; it's set for September 2004.