Scientists in Houston, Texas, are "excited and awed" after opening the Stardust capsule which returned from space with samples of cometary dust.
Scientists have opened the capsule at Nasa in Houston, Texas
The lead scientist on the US space agency mission said there were "more than a million" specks of material in the capsule's sample-return canister.
The Stardust probe released the capsule as it flew back to Earth after a 4.6-billion-km (2.8-billion-mile) trip.
The disc-shaped vessel landed in the Utah desert on Sunday.
The comet particles and interstellar dust it holds are the first ever returned to Earth; and will shed light on the origins of the Solar System, scientists say.
The canister, which was packed away inside a protective shell, was opened after being moved to Nasa's Johnson Space Center (JSC).
Stardust returned to Earth after a seven-year mission to collect particles from Comet Wild-2 and samples of interstellar dust streaming into our Solar System from other parts of the galaxy.
The comet dates from the formation of the Solar System 4.6 billion years ago.
"It exceeds all expectations," said Donald Brownlee, a University of Washington astronomy professor who is lead scientist for Stardust.
The dust particles are clearly visible in the aerogel
"It's a huge success. We can see lots of impacts. There are big ones, there are small ones."
Professor Brownlee calculated there might be more than a million microscopic specks of dust embedded in Stardust's aerogel collector.
Aerogel, a remarkable material that is as much as 99.9% empty space, greatly reduced the stress of impact on the particles, he said.
The carrot-shaped tracks of much larger particles are visible in the aerogel from several feet away, Professor Brownlee added, and in some of the tracks, the black comet dust is visible at the end of the track.
One track, he said, "is almost large enough to put your little finger into it."
Professor Brownlee said the mission already has its first scientific result: the first cometary particle the team looked at turned out to be a mineral grain.
"There's been a lot of discussion over whether comets contain minerals or glass or water," he told journalists in a news conference at the JSC.
The aerogel is contained within collector trays
Stardust co-investigator Mike Zolensky added that some of the impacts could have been from grains containing ice, and that some of the water from this ice could still be retained by the aerogel.
"We have locations where particles went in and exploded. You can see little grains curling off in all directions from the cavity. That could be a particle that was an icy, dirty grain where the water evaporated," said Dr Zolensky.
"Aerogel combines with water and if you get rid of that and dry it out, not all of the water goes away. Some of it stays behind, bonded to the aerogel. It's possible that some of the cometary water is locked up in the interiors of the cavities around these big impacts."
About 150 scientists around the world will get a chance to carry out a preliminary analysis of the contents, including researchers at the UK's Open University (OU).
"A very important part of the study of cometary grains is the study of organics. We know comets contain abundant organics and abundant water. We're not sure what kinds of organics are in there," said Mike Zolensky.
Stardust took remarkable pictures of Comet Wild 2
"But we think that most of the Earth's water and organics - most of the molecules in our bodies - came from comets."
Members of the public are being asked to sift through millions of pictures of the gel to locate the precise positions of the tiny interstellar dust grains.
The project, known as Stardust@home, has been set up by researchers at the University of California, Berkeley. Volunteers will be able to access the images via a web-based "virtual microscope".