The American scientists who discovered the radioactivity say it is the first evidence that space around the Earth is contaminated with uranium. The scientists, from California Polytechnic State University in Saint Luis Obispo, have three possible scenarios for its source.

It could have come from nuclear weapons tested in space in the 1960s, or from uranium-powered satellites that have burnt up on re-entry into the atmosphere.

Alternatively, an exploding supernova could have blasted the uranium into our Solar System many thousands of years ago. More data is needed to establish the true origin, the scientists say.

Space debris expert John Zarnecki of the Open University in Milton Keynes says that all the explanations are plausible. "Anything that is up in Earth's orbit for more than a few weeks is bombarded with particles of space debris."

The Californian researchers, led by Roger Grismore, came across the uranium almost by accident. In June 1991, a small mitten-shaped space blanket made in California was placed over a glass instrument on the outside of Mir.

The blanket, which consists of ten thin layers of aluminium and polyester, protected the instrument from solar radiation and showers of tiny meteorites. The blanket was removed in August 1995, returned to Earth and kept in a clean room for 16 months before Grismore and his team looked at it.

They used two spectrometers to analyse gamma radiation given off by the blanket. This revealed that the gamma rays had energies characteristic of two radioactive isotopes, lead-214 and bismuth-214-both decay products of uranium-238.

"That is the thrill of science -- seeing something that no one has seen before," says Grismore. To check that the blanket had not been contaminated in storage, the researchers also analysed a similar blanket that had stayed back on Earth. It emitted less than a tenth as much radiation.

Among the possible culprits, Grismore lists Starfish Prime, a US nuclear bomb test carried out on 9 July 1962 at an altitude of 399 kilometres -- the highest known nuclear test, and higher than Mir's average orbit of 320 kilometres. China and the Soviet Union may also have experimented with atomic bombs at high altitudes.

Another possible source is one of the hundreds of satellites launched into space over the past 40 years, some of them powered by mini-reactors and some with depleted uranium for ballast. Two uranium-fuelled Cosmos satellites from the Soviet Union burnt up re-entering the atmosphere around 20 years ago.

The wild card is the idea of contamination from deep space. When supernovae explode they may spew out heavy elements such as uranium. "Supernovae are isotope manufacturing machines," says Paul Murdin of Britain's Particle Physics and Astronomy Research Council. He agrees that the supernova which 340,000 years ago spawned the neutron star Geminga, roughly 400 light years from Earth, could be a source of the uranium.

Grismore believes high-altitude nuclear tests are the most probable source of the radioactive specks. But, he adds, "a supernova is the most intriguing".

source Journal of Environmental Radioactivity (vol 53, p 231)

This article appeared in the February 3 issue of New Scientist New Scientist. Copyright 2001 - All rights reserved. The material on this page is provided by New Scientist and may not be published, broadcast, rewritten or redistributed without written authorization from New Scientist.

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