The 'Pioneer anomaly' - the mystifying observation that NASA's
two Pioneer spacecraft have drifted far off their expected paths
- cannot be explained by tinkering with the law of gravity, a
new study concludes. The study's author suggests an unknown, but
conventional, force is instead acting on the spacecraft. But
others say even more radical changes to the laws of physics
could explain the phenomenon. Launched in the early 1970s,
NASA's Pioneer 10 and 11 spacecraft are drifting out of the
solar system in opposite directions, gradually slowing down as
the Sun's gravity pulls back on them. But they are slowing down
slightly more than expected and no one knows why. Some
physicists say the law of gravity itself needs revising, so that
gravity retains more strength in the outer solar system. But
there has been disagreement about whether such modifications
would accurately predict the orbits of the outer planets. Now,
Kjell Tangen, a physicist at the firm DNV in Hovik, Norway, says
tweaking the law of gravity in a