- It seems that space probes travelling
through the solar system are not behaving according to the known law of
gravity, puzzling scientists. Our science editor Dr David Whitehouse reports.
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- According to Nasa's John Anderson it
is a real puzzle.
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- He said: "We've been working on
this problem for several years, and we accounted for everything we could
think of."
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- It is all to do with the motion of space
probes in the outer solar system.
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- In 1972, Nasa launched Pioneer 10 to
Jupiter. Since then scientists have maintained regular radio contact with
it as it passed Jupiter and sped into deep space.
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- By studying the doppler shift of the
radio signals from the craft scientists have been able to calculate how
fast it is travelling.
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- The puzzle is that Pioneer 10 seems to
be slowing more quickly than it should.
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- Astronomers have detected an anomaly,
as if Pioneer 10 were getting an extra pull from the Sun that cannot be
explained.
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- No-one is sure what is to blame. A fuel
leak, friction from gas in space, thermal radiation from the spacecraft's
batteries and gravity from an unknown asteroid have all been ruled out.
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- Pioneer 11, launched in 1973, is also
slowing at about the same rate. The Ulysses probe, launched in 1990, is
also affected. Signals from Galileo, now orbiting Jupiter, may also show
the same effect.
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- Scientists are a long way from suggesting
that Newton's law of gravity should be reworked.
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- But the same strange effect seen in four
space probes is making them think.
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