[FPSPACE] Pioneer spacecraft deceleration finally explained

[David R. Woods]

Folks may recall an anomaly that had developed with the two deep space 
Pioneer probes.  Each seemed to be decelerating faster than expected as 
they climbed farther and farther out of the Solar System.  The force 
necessary for the deceleration is comparable to that of your headlights 
on your car when the lights are turned on, emitting photons: weak but 
still enough to cause a noticeable result over time.  An IEEE Spectrum 
article describes the detective work that was done to finally find an 
explanation of this effect.

http://spectrum.ieee.org/aerospace/astrophysics/finding-the-source-of-the-pioneer-anomaly