Scientists have caught hold of the tail of a comet and have a collection of tiny "snowflakes" which should prove it.

The balloon's payload included a digital video camera

The particles of dust are the debris left in the wake of comet Temple Tuttle. They entered the Earth's atmosphere last November during the Leonid meteor shower and were scooped up by a Nasa balloon hovering in the stratosphere.

Details of the catch have been revealed at a conference in San Francsico. NASA astrophysicist Dr David Noever says that the size, shape and chemistry of at least one of the tiny fragments point to an extra-terrestrial origin.

If he and his team are correct, they will have succeeded in grabbing material from the very distant edge of our solar system without leaving the Earth.

Rooftop of the world

On 18 November, during the peak of the 1998 Leonid meteor shower, scientists at the Nasa/Marshall Flight Centre in Alabama launched their 10-metre helium-filled weather balloon.

A crater left in the aerogel by a dust particle

It climbed rapidly above the thick blanket of fog covering Huntsville that night and carried on up to a height of 20 km. At this breathtaking altitude, above 98% of the Earth's atmosphere, it bumped along on the rooftop of the world for a couple of hours, observing and sampling the meteor storm from a privileged vantage point.

The balloon's payload included a digital video camera which Webcast live images of the bright fireballs. It also carried a soft aerogel micrometeorite catcher, similar to the cosmic dust collector on board the Stardust spacecraft, on its way to encounter comet Wild 2 early next century.

"At 20 km up the balloon had expanded to over 30 metres across and after a couple of hours it eventually burst and a parachute took over returning the precious payload back to Earth," explains Noever.

Men in Black

It landed 240 km (150 miles) away in the garden of a puzzled Georgian resident called Homer Yarnshop. "It was just like the movie Men in Black," laughs Noever. "Five government men turn up in dark suits and knock on his door one night asking if they could retrieve this alien looking package marked 'flight sensitive Nasa hardware'".

A fireball from the outer reaches of our Solar System

Back in the lab, the team used a scanning electron microscope to scrutinise the surface of the retrieved Xerogel collecting plates. "We didn't expect to find anything interesting", admits Noever. "The 1998 mission was really just an engineering test for this year's Leonid shower.

"Before the flight, I regarded the probability of capturing a piece of micrometeorite dust at less than 10%." Against all the odds, however, Noever and his colleagues discovered eight tiny craters in the gel plates.

Noever says one fragment looks extra-terrestrial in origin. The particle in question is an irregular-shaped speck, best described as a miniature iceberg with rough edges about 50 microns across. A translucent rim to the particle and an opaque core imply that it has come from space.

Fluffy particle

"It's easy to attribute it to meteoric dust," Noever suggests. "But it's harder to claim that it's definitely a Leonid particle. We're fishing for a very rare catch, but our best evidence that it's cometary is the video film of the fireballs."

A Leonid photographed over Tokyo

The upper atmosphere was certainly being bombarded by cometary dust at the time the balloon was flying last November, increasing the chances of a catch.

"Our Holy Grail is to determine the density and composition of cometary material," says Noever.

"We need to test the theory that life was brought to Earth on seed-like particles. But if comet dust is dense and compact then there's less chance of life coming to Earth in this way."

Their dream was to find a more "fluffy" or porous particles and Noever says that their candidate particle is exactly that - fluffy.