Astrobiology Top 10: Stardust
|Stardust Aerogel dust collector. Click image for larger view
As they clustered around the Stardust sample return capsule, Donald Brownlee, Stardust principal investigator from the University of Washington in Seattle, warned his team they might not be able to see any comet dust. The tiny particles may have made such small tracks in the aerogel collector that they would not be visible to the naked eye.
Wearing white bunny suits in a clean room at Johnson Space Center, the team anxiously examined the collector tray… and then broke into delighted celebration. Small black holes dotted the wispy aerogel tiles, and some were as large as half a centimeter wide.
The holes are carrot-shaped, with a large entry hole that tapers to a point. The first photograph of a cometary particle shows it residing in the very tip of the tunnel it drilled, like the dot of an exclamation point. The particle is only 11 microns across, and appears to be a transparent mineral grain.
"Scientifically, that’s great, because there’s been lots of discussion of whether comets contain minerals or glass," says Brownlee.
Michael Zolensky, Stardust curator and co-investigator from the Johnson Space Center in Houston, Texas, was among those excited to see evidence of the tiny comet grains. During Stardust’s long seven-year journey to the comet Wild 2, he had given in to some pessimistic speculation.
"Maybe (the collection tray) wouldn’t open properly. Maybe the particles would just smash all the aerogel out of the tray, and we’d come up with nothing at all. Or maybe (the aerogel) would even be covered with gunk from outgassing from the spacecraft," says Zolensky. "We were really worried about that, and got more and more worried as time went by. And so when we opened the tray just two days ago in the lab, we were relieved to find that everything went exactly right."
|Stardust aerogel samplers post-recovery
The scientists estimate they have up to a million comet particles, with a dozen or so that are the thickness of a human hair, and maybe even one that is larger than a millimeter. But it will take some time to know exactly what the aerogel collector tray holds.
"In the coming days we’ll be documenting the positions and character of all the tracks, looking at some of them in more detail," says Brownlee. Afterwards, they will remove some of the aerogel tiles from the collector and pull out the particles so they can be studied with different instruments.
The Johnson Space Center will be sending out Stardust samples to about 150 scientists worldwide by next week. The first official science findings will be revealed at the Lunar and Planetary Science Conference being held in Houston in March.
The opposite side of the collector tray holds samples of interstellar dust. Any tracks made by these miniscule particles are not readily apparent, so it will take some time to scan the aerogel tiles and find them. The scientists estimate they have collected up to two hundred interstellar dust grains, each no larger than a micron in size. In a project called Stardust@home, volunteers can help locate these particles by using their home computers to examine images of the aerogel. More than 65,000 people have volunteered so far.
Stardust’s sample return capsule returned to Earth on Sunday, January 15. The spacecraft had flown halfway to Jupiter to collect samples of interstellar dust and particles from the comet Wild 2. The comet dates from the formation of the solar system 4.6 billion years ago.
"We think that much of the Earth’s water and organics – the molecules in our bodies – perhaps came from comets," says Zolensky. "So we’ll learn a lot about the history of organics in the solar system. Basically it’s like looking at our great great grandparents."
NASA launched another exploration mission on Thursday: New Horizons is now on its way to the planet Pluto. Brownlee notes that both Wild 2 and Pluto were born in the Kuiper Belt, a region of ice and dust that encircles the outer region of the solar system. Wild 2 was only recently kicked out of that region and now travels between the orbits of Mars and Jupiter.
Planetary processes will have altered Pluto over time, since planets even as cold as Pluto can experience interior heating. But because Wild 2 is a small comet that hasn’t seen much solar heating, it’s remained relatively unchanged since the birth of the solar system.
"Pluto is a fascinating place, but it’s a planetary-sized object and so it’s modified the materials that it came from," says Brownlee. "Wild 2 is four-and-a-half kilometers in diameter, and it has a large amount of volatiles in it. The vapor that we saw coming out of the comet when we flew by was generated when the ice got heated up to 150 Kelvin. That ice had never been hotter than that temperature, or it would have been lost. So there is a good link to the Pluto probe, in that we’re sampling the materials that Pluto was made of."