From: "Al G"
Date: Mon, 20 Dec 2004

Q What is the pioneer anomaly, and does it substantiate the theory of dark matter?

A An excellent question about a real mystery! The Pioneer anomaly is an
unexplained additional acceleration that the space probes Pioneer 10 and
Pioneer 11 seem to have experienced as they flew out of our solar system.

Pioneer 10 was launched in 1972 and flew by Jupiter before it swung out of
the solar system. Pioneer 11 was launched in 1973 and visited both
Jupiter and Saturn before heading out of the solar system (in roughly the
opposite direction from Pioneer 10). Scientists were able to monitor the
motion of both probes to very great precision, and used this data to study
various astronomical forces (the gravity of the sun and planets, the solar
wind, etc.) as they pushed and pulled on the spacecraft.

Around 1980, when the probes reached roughly the orbit of Uranus, the
scientists found that both craft were slowing down a little bit faster
than they could account for. It seemed like there was a tiny extra force
on the probes directed toward the Sun. This force continued until good
contact was lost with the probes in the mid-1990s. After years of
extremely careful study, scientists have still been unable to explain the
origin of this acceleration.

Some people have suggested that this anomaly might be caused by a real
force from some kind of strange gravitational effect - maybe this is an
indication that we don't understand gravity as well as we think we do!
Some physicists claim that this could be connected to new explanations for
dark matter or dark energy, which would be a very exciting possibility.

Unfortunately, no one is quite certain if the anomaly is even real. Other
spacecraft (Galileo, Ulysses, and most recently Cassini) have not
definitively seen this effect - though there are some hints, they can't
say for certain. This effect has also not been seen in the laboratory or
in the motions of the planets themselves. It's still possible that the
acceleration is caused by, say, fuel leaking from the spacecraft or some
kind of weird error in the system. Even if it is real, it may not be
connected with gravity or dark matter at all. At this point, nobody
knows!

Thanks for the question!

Jeff Filippini
Berkeley Cosmology Group