The Stardust mission to bring back samples of comet and interstellar dust was more successful than they had hoped, scientists said on Thursday.
The 45kg sample container from the seven-year mission, which landed on the salt flats of Utah on Sunday, captured thousands of particles, perhaps even millions, that originated at the edge of the solar system or from distant stars, they said.
While they had expected mostly microscopic samples, the researchers said, a surprising number of the particles were large enough to be seen with the naked eye.
"It exceeded all of our grandest expectations," Donald Brownlee of the University of Washington said.
The cargo in Stardust's sample container, which was opened on Tuesday, "was an ancient cosmic treasure from the very edge of the solar system," Brownlee said. Scientists believe these particles are the pristine remains of the material that formed the planets and other bodies some 4.6 billion years ago.
The spacecraft flew with a 36cm-wide collector that resembled a tennis racket and was filled with aerogel, a low-density silicon material also called "glass smoke" because it is composed of 99.8 percent air. The aerogel gently slowed and trapped particles without significantly damaging them. When the particles hit the aerogel, they left tracks in the material.
Before Stardust returned from its 4.6-billion kilometer trek around the inner solar system, which included a close encounter with comet Wild 2 near Jupiter on Jan. 2, 2004, Brownlee cautioned fellow researchers not to be disappointed if they did not see evidence of the particles with the naked eye, he said.
"The prediction was that we would get a dozen particles larger than human hair size and one particle a little bit larger than a millimeter," Brownlee said.
"We were totally overwhelmed by the ability to actually see this stuff so straightforwardly," he said, "Just looking at it, you can see hundreds and hundreds of tracks."
Brownlee said some of the tracks in the aerogel might have been left by ice particles that later evaporated. If such empty cavities are found, he said, they still may contain a residue of the ice and its constituents.
"If a sample hit the aerogel, it's still there in some form," he said.