Mystery Solved!

This is actually old news, but the Planetary Society's blog has a nice post with some new details.  The original mystery was that Pioneer 10 and Pioneer 11 were slowing down a tiny bit more than any known source could account for.  In a feat of data recovery and scientific analysis (paid for the by Planetary Society), Slava Turyshev and his colleagues figured out why: the nuclear power source (RGT) on board differentially radiated toward the direction of flight – acting like a teensy, tiny retro-rocket.  A very nice piece of scientific detective work, and an equally nice confirmation of radiation-induced acceleration...