A US space agency (Nasa) spacecraft has started collecting cosmic dust for the second time during its voyage to a distant comet.

The Stardust probe is on a mission to snatch particles from Comet Wild 2 and return them to Earth.

Stardust encounters Comet Wild 2 in early 2004 Tom Duxbury, Nasa

On its way, it is collecting specks of solid matter that were produced by the current generation of stars.

The galaxy is dotted with these interstellar dust grains, each smaller than one-hundredth the width of a human hair.

"If you look at the Milky Way on a dark night you may see a black band stretching along the centre," says Dr Don Brownlee, principal investigator of the Stardust mission.

"The band is interstellar dust blocking the light from distant stars. These are the particles that Stardust will be collecting."

Stardust collected some particles of interstellar dust for six weeks in 2000. The second phase continues until December.

Building blocks

The craft is using a device containing the world's lightest solid - aerogel - to capture the particles as it moves through the dust stream.

"Stardust's tennis-racket-shaped particle collector has shoulder and wrist joints that will point one side of the aerogel collector material into the dust stream to collect interstellar dust," says project manager Tom Duxbury of Nasa's Jet Propulsion Laboratory in Pasadena, California.

"When Stardust encounters Comet Wild 2 in early 2004, the reverse side of the collector will trap particles from the gas and dust escaping from the inside of the comet.

"When the dust samples return to Earth in 2006, we will extract and analyse the particles."

The dust samples are of interest because they could shed light on what the early galaxy was made of.

They could also give clues to how the Earth itself evolved.

Some scientists believe that the building blocks of life were carried to our planet via comets or interstellar dust.