The Pioneer Anomaly may not be anomalous

The Pioneer Anomaly sounds like some kind of thriller novel, but is actually a slight accelleration on the Pioneer probes, these are space probes launched in the 1970s and now the most distant man made objects. As they headed away from the solar system their path was carefully tracked, and a strange anomoly was discovered, they were decellerating slightly faster than they should be according to einsteinian gravity.

(c) NASA

' alt='Pioneer 10 and Pioneer 11' >This effect was tiny, about 1 part in 10^-10 ms^-2 , but it was measurable, and it got lots of theoratitions very interested. This is because studies of galaxies indicate that gravity may be stronger at longer distances, so lots of theorists were creating new theories of gravity to fit the data.

In 2008 a third of the anomoly was explained by Frederico Francisco and collegues, the spacecraft generates power from heat produced by radioactive decay, this heat is lost by thermal radiation, which is a form of light. Light carries momentum, so can apply a force to the spacecraft. But they hadn't managed to explain the whole anomaly.

Now they think they have - a source of heat - the main electronics compartment is immediately behind the parabolic dish the spacecraft uses to communicate with earth. So thermal radiation from the electronics will reflect off the dish giving the spacecraft a slight extra decelleration, explaining very neatly the anomaly.

So unfortunately for the theorists and their chances of a Nobel prize, Einstein's equations still hold.