Back in April 2009, I wrote a blog post on a fascinating riddle. Pioneer 10, one of the space probes that has left the solar system, is not where it should be. This has been a growing mystery as every possible thing that could cause the spacecraft to accelerate or decelerate was accounted for. At the time, there seemed to be no answer, but now it seems to be solved. Read the original blog post then come back here for the rest of the story.

RIDDLE SOLVED
It seems that the heat from the electronics was being reflected off the big antenna. Heat is infrared radiation of course, and when the radiation travels away from the spacecraft there is an opposite force created. The same happens when you shine a flashlight at something, the beam of light pushes back on your hand, but the force is so minuscule, it cannot be felt. Imagine shining a light day after day, and year after year, from a spacecraft in deep space. The force is extremely tiny. Not enough to feel, but year after year, decade after decade, it begins to add up, and Pioneer Ten started to slowly deviate from where it would be if no force was present.

How much force is the radiation heat putting on Pioneer 10? Something around 0.000000000874 meters/second/ each second

Technology Review at MIT has a good piece on this as well.