In July 1962 the US exploded a 1.4 megaton nuclear device at 400km. The Mir space station is determined to go out with a bang. Just a month before it is due to crash into the Pacific Ocean, it has thrown up one last puzzle. 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.
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.