Tuesday, September 28, 1999 Published at 21:23 GMT 22:23 UK
Old spacecraft makes surprise discovery
Pioneer 10: Still doing the business
By BBC News Online Science Editor Dr David Whitehouse
Scientists have discovered a new object orbiting the Sun after a spaceprobe was mysteriously knocked off course.
Researchers have yet to identify the object, but they are confident it exists because of the way it appears to have deflected the tiny Pioneer 10 craft, which is hurtling out towards the stars.
If the observations are confirmed by other astronomers, it will be only the second time in history that a Solar System object has been discovered by its gravitational effect alone.
The first was the planet Neptune which was discovered in 1846. Its position was predicted because of its gravitational tug on the planet Uranus, which appeared to be behaving oddly following its discovery 59 years earlier.
The new body, found by a team at Queen Mary and Westfield College in London, UK, and the Jet Propulsion Laboratory in California, is probably a so-called Kuiper Belt object.
The astronomers looked at Pioneer 10's tracking data obtained with the Nasa Deep Space Network, an array of large radio telescopes designed to communicate with far-off space probes.
On 8 December, 1992, when Pioneer was 8.4 billion km (5.2 billion miles) away, they saw that it had been deflected from its course for about 25 days.
The scientists have been looking for such an effect for years and are currently analysing the data using several different methods to confirm their findings.
"We are quite excited that we have found one of these events. It is a very neat signal," Dr Giacomo Giampieri of Queen Mary and Westfield College told BBC News Online.
If confirmed, it would be one of over 100 known icy and rocky objects that circle the Sun at vast distances, mostly beyond the most distant planet Pluto.
They are small in planetary terms, just a few hundred kilometres at most, but astronomers believe there are millions of them swarming around the Sun in a vast belt. The first one was detected in 1992.
The Pioneer 10 spacecraft was launched in March 1972 and has lived up to its name.
It was the first spacecraft to travel through the asteroid belt, the ring of rocky debris that orbits the Sun between Mars and Jupiter. Before it successfully traversed this region of space, scientists did not know if a spacecraft could get through unharmed.
It was also the first craft to reach a gas-giant planet, Jupiter, after which it left the solar system becoming the first craft to effectively leave the Sun's planetary system, even though it has not yet passed into interstellar space.
It is currently 11 billion km (6.8 billion miles) away and still transmitting even though Nasa ceased monitoring its signals in 1997 after it had spent 25 years in space.
Earlier this year, scientists were puzzled by what was described as a mysterious force acting on the probe. It led to speculation that there was something wrong in our understanding of the force of gravity.
Eventually the effect was tracked down to the probe itself, which was unexpectedly pushing itself in one particular direction.
Pioneer 10 will reach the stars of the constellation of Taurus in about two million years.