[meteorite-list] Stardust Spacecraft Returns Seven Interstellar Dust Particles

[Ron Baalke]

http://news.sciencemag.org/space/2014/03/spacecraft-returns-seven-particles-birth-solar-system
 
Spacecraft Returns Seven Particles From Birth of the Solar System
Richard Kerr
Science Magazine
21 March 2014 

THE WOODLANDS, TEXAS - After a massive, years-long search, researchers 
have recovered seven interstellar dust particles returned to Earth by 
the Stardust spacecraft. The whole sample, reported here this week at 
the Lunar and Planetary Science Conference, weighs just a few trillionths 
of a gram, but it's the first time scientists have laid their hands on 
primordial material unaltered by the violent birth of the solar system.

The Stardust spacecraft, launched in 1999, has already accomplished its 
prime objective: collecting dust particles in the tail of comet Wild 2 
and returning them in a reentry capsule ejected as Stardust passed by 
Earth in 2006. NASA went to all that trouble because comets were supposed 
to be the repository of the primordial ice and rock - the product of eons 
of star birth and death - that went into building the solar system. But 
it turned out that the minerals in the comet dust that Stardust managed 
to collect weren't that pristine: They had been heated, melted, and totally 
transformed somewhere near the nascent sun and then carried outward to 
be incorporated into comets in the ultradeep freeze far beyond the outermost 
planets.

That left the hunt for pristine star stuff to Stardust's other mission, 
collecting dust streaming in from interstellar space. For a total of 200 
days in 2000 and 2002, Stardust stuck out its tennis racket-like dust 
collector panel to catch interstellar dust as the solar system and the 
spacecraft with it plowed through the galaxy. The panel's centimeter-thick 
blocks of aerogel - "frozen smoke" made of airy silica that is 99.8% empty 
space - were intended to slow and retain dust particles without vaporizing 
them. That would be no mean feat, as interstellar particles are a thousandth 
the mass of comet dust and can be traveling well in excess of 15,000 kilometers 
per hour.

Once the sample panel was back on Earth, the problem quickly became finding 
any collected particles embedded in the aerogel. Out of desperation, Stardust 
team members called on 30,714 members of the general public. "We really 
did not know how else to find" the embedded particles, says Stardust team 
member Andrew Westphal of the University of California, Berkeley. The 
"dusters" of the Stardust at home project - who as a group were listed as 
authors of the talk - volunteered to examine microscopic images taken 
down through the aerogel. They used the world's best pattern-recognition 
system - the human eye and brain - to pick out the telltale tracks left 
by speeding particles.

One hundred million searches later, Stardust team members had seven "probable" 
dust impacts on the collector. Two particles weighing in at about 3 trillionths 
of a gram each - 100 billion of them would equal a grain of sugar - drove 
into the aerogel at less than 18,000 kilometers per hour and lodged there. 
A third came in so fast that it didn't even leave a chemical residue, 
only a track. Four more particles serendipitously blasted into thin aluminum 
foil around the edges of the aerogel, leaving measurable material in their 
craters.

"It's an enormous achievement that the Stardust team got this far," says 
cosmochemist Scott Messenger of NASA's Johnson Space Center in Houston, 
Texas, who is not on the team. He agrees with Westphal that these "are 
the most challenging extraterrestrial samples ever." To be sure that the 
grains are truly interstellar, researchers must next transfer the infinitesimal 
specks of dust from inside the aerogel into instruments for further analysis, 
in particular of their isotopes. Developing the techniques "will be fundamentally 
boring but necessary," Westphal says. "It would be very easy to lose them."