Thursday, September 10, 1998 Published at 15:06 GMT 16:06 UK
Spacecrafts pulled by mystery force
Artist's impression of Pioneer 10 at Jupiter
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.
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.