The launch of the NASA mission on Saturday was called off less than two minutes before blast off when an alarm sounded. It was triggered by a malfunction in a radar tracking beacon.

All the systems have been tested

The Stardust mission is now due to begin its five billion kilometre (3.1 billion miles) journey on Sunday. It will start at Cape Canaveral in the United States and will end on salt flats of Utah in 2006 by which time a piece of a comet will be returned to Earth.

Prof Donald Browlee, Univ of Washington in Seatle, describes the mission

Not since 1972 have extra-terrestrial samples been returned to Earth. In that year Apollo 17 brought back moonrock. The Stardust mission will return the first samples from beyond the Moon.

After launch, the spacecraft will follow a long trajectory around the inner solar system until it encounters comet Wild-2 in January 2004.

Comet Wild-2 showing its dust cloud

Wild-2 is a special. Until 1974, it was confined to an orbit in the outer solar system. And because it had not been subjected to the heat of the Sun it had hardly changed since the solar system was young.

But that all changed after a close encounter with Jupiter - the planet's gravity changed the comet's orbit sending it much closer to the Sun. It is now a realistic prospect to try to reach Wild-2 with a spacecraft.

Stardust's fly-through of the comet will mean it can collect material that came from the same cloud of gas and dust out of which our Sun and planets formed.

Comet tails

The aerogel dust collector

A comet is a flying mountain of rock, ice and frozen gasses. If it nears the Sun, it begins to warm and the so-called nucleus - Wild-2's is estimated to be 9.5 km (6 miles) wide - becomes a bubbling mass of geysers and jets erupting from its surface.

The comet becomes enveloped in a cloud of dust and gas - some of which trails behind it, some of which is blown away by the pressure of sunlight. These are the comet's tails.

In January 2004, Stardust will plough through this gas and dust shroud flying within 100 miles of the nucleus.

Its delicate instruments and cameras will be protected behind an armoured shield. But a sample collection device will be deployed that will trap specks of dust.

Artist's impression of Stardust touchdown

The dust will be captured in a substance called aerogel. This is the lowest density material ever made and is designed to capture the dust without damaging it.

After the flyby, the aerogel with its trawl of perhaps 1,000 specs of dust will be enclosed in a capsule and ejected into an orbit that will bring it back to Earth.

The capsule should arrive on 15 January, 2006, parachuting down over the salt flats of Utah. It will then be taken to a sterile laboratory and opened.