The Pioneer Anomaly
The Pioneer missions yielded the most precise information we have about
navigation in deep space. However, analysis of their radio tracking
data indicates a small unexplained acceleration towards the Sun! The
magnitude of this acceleration is roughly 10-9 m/s2.
It is called the "Pioneer anomaly".
This anomaly has also been seen in the Ulysses spacecraft, and possibly
also in the Galileo spacecraft, though the data is much more noisy,
since these were Jupiter probes, hence much closer to the Sun, where
there is a lot more pressure from solar radiation. The Voyagers are not
set up to provide good data on this issue, since they are able to
rotate themselves and this messes things up. The Viking mission to Mars
did not detect the Pioneer anomaly - and it would have, had an
acceleration of this magnitude been present, because its radio tracking
was incredibly accurate - good to about 12 meters.
Many physicists and astronomers have tried to explain the Pioneer
anomaly using conventional physics, but so far nobody seems to have
succeeded. Radiation pressure from the Sun, the solar wind, the
back-reaction from the radio emissions: all these point the wrong way!
Other explanations also seem to fail, like gravity from the Kuiper
belt, small amounts of gas venting from the spacecraft, and thermal
radiation from the crafts.
As conventional explanations seemed to fail, people started trying to
explain the anomaly using new physics - for example, modified theories
of gravity, or dark matter. But it's hard to get these explanations to
work, either. For example, explaining the Pioneer anomaly by the
gravitational attraction of dark matter would require more than 0.0003
solar masses of dark matter within 50 AU units of the Sun. But this is
in conflict with our highly successful calculations of planetary orbits
- even a millionth of a solar mass of dark matter in this region would
be enough to throw those off! [text from John Baez]