Pioneer 10, a space probe launched in 1972 that took the first close-up pictures of Jupiter before leaving our solar system in 1983, is being pulled back to the sun by an unknown force. The nature of this force baffles scientists and could cause them to rewrite the laws of physics.

Dr. Philip Laing, of the Aerospace Corporation of California, a member of the team that is tracking the craft, says, ?We have examined every mechanism and theory we can think of and so far nothing works. If the effect is real, it will have a big impact on cosmology and spacecraft navigation.?

Pioneer 10 was launched by NASA on March 2, 1972, and with its twin probe, Pioneer 11, provided us with detailed images of Jupiter and Saturn. In June 1983, Pioneer 10 passed Pluto, the most distant planet in our solar system. Both probes are now travelling at 27,000 mph towards stars they will reach several million years from now. Scientists are continuing to monitor signals from Pioneer 10, even thought it is more than seven billion miles from Earth.

The speed of the two probes is being changed by about 6 mph per century, which is a barely-perceptible effect about 10 billion times weaker than the force of gravity. At first scientists suspected that gas escaping from rocket motors aboard the probes or heat leaking from their nuclear power plants might be responsible, but these have now been ruled out. Scientists have also eliminated the possibility that the two probes are being affected by the gravitational pull of unknown planets beyond the solar system. None of our current knowledge of physics can explain why the force stays constant, since all other forces of nature, from gravity to the effect of solar radiation, decrease rapidly with distance.

It has now been discovered that the same force is affecting Galileo and Ulysses, two other space probes that are still in the solar system. Data from these two probes suggests the force is of the same strength as that pulling on the Pioneers.

Dr. Duncan Steel, of Salford University, believes that even a weak force such as this could have huge effects on a cosmic scale. He says, ?It might alter the number of comets that come towards us over millions of years, which would have consequences for life on Earth.?

In case Pioneer 10 comes in contact with alien life, it carries a gold-plated aluminum plaque with the figures of a man and woman, along with a map showing its origin. NASA calls it ?the cosmic equivalent of a message in a bottle.?

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