Stars and planets: September

In January 2000, observers saw a brilliant fireball lighting up the sky above the town of Whitehorse in western Canada. It was also picked out by a US Department of Defense satellite. Together, these observations proved that a 150-ton asteroid had plunged to a fiery demise in Earth's atmosphere.

A week later a local pilot, Jim Brooks, spotted some strange black rocks lying on the icy surface of the frozen Tagish Lake. Suspecting that they were meteorites from the disintegrated asteroid, he collected them carefully and sent them to Nasa's Johnson Space Center in Houston.

"It was a very exciting time for me," recalls Nasa's Mike Zolensky. "The last time we had the fall of a meteorite such as this was 30 years ago, so it happens about one time in a career."

The first fragments alerted Zolensky to the fact that the Tagish Lake meteorite was no ordinary rock from space. Nasa scrambled a team to the site, and collected hundreds more fragments. "This particular meteorite is really special," Zolensky continues. "It has a very high water content, and the highest carbon content of any meteorite yet recovered."

The Tagish Lake stones are fuelling a suspicion that's growing in the minds of many scientists: that the carbon compounds that make up life on Earth – including our own bodies – didn't originate on our planet. Instead, these organic compounds have come to Earth from space.

It's not just big exploding chunks of rock that are seeding the world with organic material. At the University of Washington in Seattle, Don Brownlee has spent years collecting space material that's almost impossible to see.

"As well as the rare meteorites," he explains, "we have a lot more smaller particles, less than a millimetre across, which float down through the atmosphere." To catch space-dust that isn't mixed in with everyday materials, Brownlee has enlisted the help of two spy planes now owned by Nasa. They fly at 60,000ft, and sticky plates catch dust drifting down from space.

"Put them in an electron microscope," enthuses Brownlee, "and you see this wonderful array of minerals, glass and carbon – the building blocks of planets, the building blocks of life."

Most of these dust particles have been shed by comets, and Brownlee now has a space mission to return more pristine specimens. The Stardust space probe is now en route to comet Wild-2. As it flies past in January 2004, Stardust will catch dust particles from the comet, and return them to Earth.

In January 2006, almost six years to the day after the Tagish Lake asteroid hit Earth, Stardust will return to our planet. It will make a softer landing, as it parachutes down to the Utah desert carrying its precious cargo of comet dust.

What's Up

These September evenings are rather a poor time for planet aficionados. Venus sets less than an hour after the Sun this month, although it's so bright that you can see the Evening Star well down into the twilight glow. Then it's a question of waiting till after 11pm for Saturn to rise, with Jupiter following in the early hours.

But, if you live in a dark location, it's a good month to catch up with the Milky Way, our own galaxy of stars. This gently glowing band arching almost overhead is in fact the light from billions of distant stars in the galaxy where we reside. The Galaxy is flat and circular in shape. Since we live within it, we see this disc edge-on and it looks like a band of light running right round the sky.

If we could see the Milky Way from outside, it would look even more spectacular. The stars and gas in the flat disc are bunched up to make a set of spiral-shaped "arms" curving outwards from the centre. We live in a small spiral arm about two-thirds of the way out from the centre of the Galaxy. As well as stars, spiral arms contain dark clouds of interstellar dust that block out our view of the more distant universe. In time, this dust will form into new solar systems and – like Don Brownlee's microscopic particles (see main story) – seed planets with the materials of life.

You don't need a telescope to see cosmic dust in action. Locate the Summer Triangle high in the south – the bright stars Vega, Deneb and Altair. On a really clear black night, you'll see the Milky Way running from past Deneb and Altair to the horizon. And there's something odd in the region between these two bright stars: the glowing band appears double, split by the dark Cygnus Rift.

The Cygnus Rift is a long trail of dense dark clouds, silhouetted against the distant star clouds of the Milky Way. It runs along the inner edge of the spiral arm that we inhabit. Astronomers have discovered that the dust is veiling many brilliant stars and glowing gas clouds called nebulae. If it were not for the dust, September skies would be a far more exciting sight.

HEATHER COUPER AND NIGEL HENBEST

Nigel Henbest is the scriptwriter of 'The Day the Earth Was Born', which will be shown on Channel 4 on 16 and 23 Sept