The Pioneer Anomaly is a long-standing mystery where the solar-system-escaping Pioneer 10 and 11 spacecraft have been experiencing a tiny, unexplained sunward acceleration over the course of their journey. That is, the spacecraft have been coasting for 40 years, and should only be experiencing gravitational slowdown from the sun (plus other known small accelerations, like radiation pressure).

However, they have been showing a slight, unaccountable deceleration. And by slight, I mean slight: 10^-9 m/s², or one hundred millionth of a g. Perhaps the most impressive thing in the entire business is that such a small acceleration can be accurately measured! (Although it does add up to a shortage in distance travelled of about 500 km per year, but that is for spacecraft 15 billion km away!)

A bevy of theories have been offered to try to explain the existence of this anomaly, the most creative of which is probably Modified Newtonian Dynamics, or MOND, which adds a fudge factor to Newton’s law of gravity that would account for this (and also get rid of the need for Dark Matter).

However, several Portuguese researchers now claim to have explained the anomaly in a much simpler way: By doing a better job of accounting for all of the heat radiating off of the spacecraft. We’ll have to wait for JPL to confirm the analysis, but it sure seems more likely than needing to modify the laws of physics – go Occam’s razor!

Here’s the original research article. From Slashdot.