Strange Grip on Space Probes?

A mysterious force may be tugging at distant spacecraft. For 15 years, Pioneer 10 and 11 have both been slowing down ever so slightly--a change in speed that cannot be explained by any known mechanism, scientists say. The finding, to be published in Physical Review Letters, might imply that current theories of gravity need a slight tweaking.

Pioneer 10 and 11, the first space probes to study Jupiter and Saturn, are now leaving the solar system in opposite directions. Since 1980, John Anderson of NASA's Jet Propulsion Laboratory in Pasadena, California, and his colleagues have used Pioneer radio data to precisely determine any changes in the crafts' speed. Relative to the sun, both Pioneers appear to be slowing down more than expected. But not by much: The unexplained change in speed is about 10 billion times smaller than the change of speed of an apple falling on Earth. The European space probe Ulysses, which studies the sun from a very large distance, is showing a similar effect.

The team claims to have ruled out all known possible causes. Most explanations, such as gravity exerted by comets in the Kuiper Belt, gravity from the Milky Way, and minute gas leaks in the spacecraft, appear much too small to account for the slowing. And software errors are unlikely, Anderson says, because an independent team arrived at the same conclusion using different algorithms. "We can't think of anything else," he says, "although we may still discover a systematic error. The most likely solution is that we're overlooking something."

Other scientists agree. "The chances of this result signifying a fundamental problem in physics are extremely small," especially because the effect does not show up in the motions of planets, says Irwin Shapiro, a theorist from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Meanwhile, Anderson hopes that future spacecraft to the outer solar system, such as the proposed Pluto Express, might confirm the finding. His team would also like to see other groups track comets and Kuiper Belt objects and look for a similar effect.