Scientists have detected a mysterious gravitational-like force which is pulling on distant spacecraft. They became aware of the force after analyzing the trajectories of four deep-space probes.
"It is almost as if the probes are not behaving according to the known law of gravity," said Dr John Anderson, of the American space agency's (Nasa) Jet Propulsion Laboratory, the scientist heading the study into this anomaly. "We've been working on this problem for several years, and we have accounted for everything we could think of."
Pioneer 10, one of the probes being affected by the mystery force, was launched towards the outer planets in 1972. It is now far beyond Jupiter but still in radio contact with Earth. By studying the Doppler shift of the radio signals from the craft, scientists have been able to calculate, very precisely, how fast it is travelling.
The puzzle is that Pioneer 10 is slowing more quickly than it should.
At first, scientists speculated that the slowing might be due to the gravitational pull of some nearby, unseen object. However, when the trajectory of Pioneer 10's sister spacecraft, Pioneer 11, launched in 1973, was analyzed, it was found that it too was being subjected to the same mysterious pull. Since Pioneer 11 is on the opposite side of the Solar System from Pioneer 10, the effect cannot be the gravitational force of some unseen body.
"Our analysis strongly suggests that it is difficult to understand how any of these mechanisms can explain the magnitude of the observed behaviour of the Pioneer anomaly," Anderson's team stated.
The mystery deepened when it was revealed that the same unexplained force has been affecting on the Galileo spacecraft on its journey to Jupiter, and the Ulysses spaceprobe that is orbiting the Sun.
Several scientists have noted that the strength of the effect seems to be related to two of the Universe's physical constants: the speed of light and the speed of the expansion of the Universe. If this were true it would suggest a fundamental flaw in our understanding of gravity.

Related link: BBC Report