In January the Stardust spacecraft cruised by Earth and tossed down a 95-pound canister packed with comet particles and interstellar dust, souvenirs scooped up during its seven-year journey past comet Wild 2. The bits are probably more than 4 billion years old, dating to an era when comets spread the chemistry of life among the planets. So when NASA popped the lid, did scientists worry about uncorking a deadly alien virus?
"Your backyard collected more comet dust over the last seven years than we did," says Donald Brownlee of the University of Washington at Seattle, principal investigator on the Stardust mission. Tons of comet detritus rains down on Earth every year. Stardust's cache was sanitized by intense heat as comet particles collided at 14,000 miles per hour with foamy aerogel in the probe's dust collector.
NASA's lack of concern is a far cry from the anxiety that surrounded the moon rocks retrieved by Apollo astronauts between 1969 and 1972. Upon arrival, the rocks were studied in glove-box vacuum chambers and elaborately quarantined. Departing visitors had to strip and bathe in sterilizing ultraviolet light. Part of the difference is that at the time of Apollo, scientists did not appreciate how much alien matter was already underfoot. More than 30 lunar meteorites have been found here on Earth. Meteorite impacts have also flung bits of Mars into space; Mike Zolensky, a space geologist with the Stardust mission, estimates that roughly one Mars rock a day hits Earth.
Still, there is no substitute for retrieving extraterrestrial matter at its source (see list below) to conduct a hands-on study of Earth's chemical and physical origins. Stardust's sample may be enough to reconstruct how material shuttled from planet to planet and even from star to star as Earth took shape 4.6 billion years ago.
SAMPLE-RETURN SPACE MISSIONS
Apollo(1969–72, United States) Six missions brought back 842 pounds of moon rocks.
Luna (1970–76, Soviet Union) Three robotic probes scooped up 11.5 ounces of lunar soil.
Genesis (2005, U.S.) The probe collected about a millionth of an ounce of solar wind atoms.
Stardust (2006, U.S.) The probe swept up one-thousandth of an ounce of comet particles.
Hayabusa (2010, Japan) It will drop off an unknown amount of asteroid dust when it returns.