During the 1980s, NASA researchers noticed the Pioneer 11 spacecraft was slowing more quickly than expected as it neared the edge of the solar system. A similar effect occurred with the Pioneer 10 spacecraft, which was sent in the opposite direction. Finally, in 1998, John Anderson, then at the Jet Propulsion Laboratory in Pasadena, Calif., and colleagues made their findings public.
Since then, other NASA and European Space Agency probes have also exhibited unexplained changes in speed.
Anderson, now with the Global Aerospace Corp., says although it's possible an overlooked effect from ordinary physics might account for the anomalies, something more exotic could also be involved.
For example, the spacecraft trajectories could be influenced by the presence of dark matter in the solar system, says Michael Nieto of the Los Alamos National Laboratory. Or maybe the laws of gravity need reworking.
"We're just throwing it out as a possibility that the anomalies might have a single cause," said Anderson. "We thought it was really time to get the community thinking about it."