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. According to Nasa's John
Anderson it is a real puzzle. He said: "We've been working on this
problem for several years, and we accounted for everything we could
think of." It is all to do with the motion of space probes in the outer
solar system. 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. By studying the doppler shift of the
radio signals from the craft scientists have been able to calculate how
fast it is travelling.
The puzzle is that Pioneer 10 seems to be slowing more quickly than it
should. Astronomers have detected an anomaly, as if Pioneer 10 were
getting an extra pull from the Sun that cannot be explained. 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. 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. Scientists are a long way from suggesting that Newton's
law of gravity should be reworked. But the same strange effect seen in
four space probes is making them think.
