Researchers say Pioneer 10, which 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 effect shows no sign of getting weaker as the spacecraft travels deeper into space, and scientists are considering the possibility that the probe has revealed a new force of nature.
Dr Philip Laing, a member of the research team tracking the craft, said: "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," said Dr Laing, of the Aerospace Corporation of California.
Pioneer 10 was launched by Nasa on March 2 1972, and with Pioneer 11, its twin, revolutionised astronomy 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 27,000mph towards stars that they will encounter several million years from now. Scientists are continuing to monitor signals from Pioneer 10, which is more than seven billion miles from Earth.