A six-member team headed by astronomer John Anderson from the jet propulsion laboratory in Pasadena, California, believes the unknown force has nothing to do with the accepted laws of physics.

Anderson told the press recently that he first noticed the discrepancies in 1980 when studying the path of the US space probes Pioneer 10 and Pioneer 11.

Launched in 1972 and 1973 to explore the outer planets Jupiter and Saturn, the two spacecraft are now on the edge of the solar system -- a distance 68 times farther from the sun that the earth is.

The deviations documented by Anderson and his team are relatively minor, however. If the braking force was applied to a car travelling at 500 kilometres per hour, it would take 500 years for it to come to a halt. What baffles scientists is that the intensity of the force was the same for both Pioneer craft.

Inaddition to the Pioneer probes, two space missions that started out later sent radio signals back to earth that have helped scientists in their investigation of the phenomenon. They were the Jupiter probe Galileo, launched in 1989, and Ulysses, which blasted off for the moon a year later. The trajectories of the four spacecraft revealed evidence of a weak force that influenced their speeds and directions. The force was discovered after the effects of all known influences on spacecraft were added up by the scientists, who found that there was something unexplained left over. The investigations by Anderson and his team are based mainly on radio data exchanged between the spacecraft and mission control on earth. The calculations were performed in two different ways, using two different kinds of radio data. They took into consideration factors such as the gravitational pull of comets and asteroids, possible errors in software and hardware used in computer analysis and the effects of gravity exerted by the sun,its planets and our solar system's galaxy, the milky way.

Other factors considered were possible leaks from the crafts' fuel tanks or manoeuvring rockets, heat relinquished by onboard batteries, and influences exerted by so-called dark matter and theoretical particles, which make up the mass of the universe.