快猫短视频

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. How did tiny radioactive specks of decay products of uranium end up on
one of its instrument covers? 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. 鈥淎nything that is up in Earth鈥檚
orbit for more than a few weeks is bombarded with particles of space
诲别产谤颈蝉.鈥�

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鈥攂oth decay products of
uranium-238. 鈥淭hat is the thrill of science鈥攕eeing something that no one
has seen before,鈥� 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鈥攖he
highest known nuclear test, and higher than Mir鈥檚 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. 鈥淪upernovae are
isotope manufacturing machines,鈥� says Paul Murdin of Britain鈥檚 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, 鈥渁 supernova is the most intriguing鈥�.