Stardust: a comet’s tale

A comet's tale The Stardust collector tray

Comet dust collected by NASA's Stardust mission is providing scientists with their closest ever look at the fundamental building blocks of planets and stars. 'Study of the dust that arrived back on Earth in January is already providing unexpected results. Comets form in the coldest parts of the Solar System,' explains Monica Grady of the Open University. 'It was therefore a great surprise to find material from the hottest places in the Solar System in the comet dust samples.' High temperature minerals containing aluminium, calcium, and titanium were found in the comet dust, substances normally formed close to stars. 'These grains could have formed near the Sun and then blown out to colder regions to mix with the dust forming the comet,' says Monica, 'but as these are such preliminary findings we cannot yet be sure about the process that led to them being part of the comet'.

The Stardust mission took five years from its launch in 1999, to reach comet Wild 2 (pronounced vilt 2). In a very risky manoeuvre, the spacecraft flew within 149 miles of the comet's nucleus to collect the dust freshly released from the surface. The mission was completed successfully when a canister containing the dust parachuted back to Earth providing scientists with the first ever samples of pristine' dust for analysis. The dust from this comet has not been processed in the same way as the dust that eventually formed the Earth,' explains Monica. Because the dust has been collected directly from the comet it resembles far more closely the dust particles that were the starting point for all the planets and objects in our Solar System'. Stardust also captured dust from in between planets and in between stars, all of which will be studied in the UK by the Open University's Planetary and Space Sciences Research Institute, a partner in the Stardust mission.

Key to the capture and safe return of the dust is a solid called aerogel, the world's lightest solid' according to the Guinness Book of Records. Aerogel is 99.8% air, 1000 times less dense than glass and 39 times more insulating than the best fibreglass insulation. These properties enabled the solid, also known as 'frozen smoke', to capture dust travelling at great speed without it becoming damaged. As the particles hit the aerogel, they travel through it forming trails and finally slowing down to remain trapped in the solid. The trails in the pale blue aerogel are used to pin point dust samples that are much smaller than a grain of sand. In addition to trapping the dust the mission also recorded the noises made as these particles impacted with the aerogel. The sounds are rather like dried peas hitting a dinner plate in bursts', says Monica.