Comet dust delivered to Earth
Cosmic grains should tell tales of the early Solar System.
The first ever sample of a comet parachuted safely down to the Utah desert early on Sunday 15 January. The Stardust mission's capsule left a bright streak of light in the night sky as it tore through the atmosphere at more than 46,000 kilometres per hour, before popping a series of parachutes and driving into the desert sand at 2:10 local time.
The milligram or so of dust inside is the first geological sample returned from space since the Apollo flights of the early 1970s, says Phil Bland, a planetary scientist from Imperial College London, UK, who will be one of the first to get his hands on the grains. "It's so exciting," he says. "I was three years old when the last Apollo samples came back, and there have been no rocks brought back from space since then."
Stardust's 4.6-billion-kilometre round trip to the comet Wild 2 took a total of seven years. Its close encounter in January 2004 gave us our best picture of a comet yet, with a surface pockmarked by craters and a surprisingly rigid core.
Skimming just 240 kilometres from the comet's surface, the craft detected simple organic molecules in the particles drifting from Wild 2. To get a closer look, it swept a soft, lightweight material called aerogel through the halo of dust and gas surrounding the comet's tail, and bagged a sample.
The few thousand specks of dust collected are thought to date back 4.6 billion years, to a time when the Solar System was first forming. Scientists hope that the samples will give them clues about the chemical make-up of the primordial rubble that spawned the planets.
Identifying minerals in the grains should reveal which elements were available as building blocks for our Solar System, and what sorts of stars created them. And if researchers find minerals that have been altered by water in the past, it might help to determine whether comets were instrumental in delivering much of the water in Earth's oceans, says Bland.
Dividing the spoils
Stardust's racquet-sized collector will be shipped to NASA's Johnson Space Centre in Houston, Texas. There it will be divided into minute crumbs and sent to about 180 researchers around the world for analysis.
"We've never had a comet in the lab before," says Bland, adding that the firsts tests should reveal whether rocks that have previously been identified as meteorites could in fact have come from disintegrating comets.
"To see the capsule safely back on its home planet is a thrilling accomplishment," says Don Brownlee of the University of Washington, Seattle, who leads the Stardust science team.
The engineers who built Stardust, from aerospace giant Lockheed Martin of Bethesda, Maryland, also heaved a sigh of relief. In September 2004, their Genesis spacecraft smashed into the Utah desert after switches designed to detect its re-entry into the Earth's atmosphere failed to deploy its parachutes. Investigations revealed that the switches had been installed upside down.
Having delivered its sample capsule, the Stardust mother ship has been put into orbit around the Sun. Although no extended mission is planned, NASA officials say they are open to any suggestions that could send the craft on another adventure, such as exploring asteroids.
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