Pioneer Spacecraft Warmth Takes Heat off Relativity

Pioneer Spacecraft Warmth Takes Heat off Relativity: Pioneer 10 and 11 are slowing down due to a small-but-ever-present thermal recoil. Both spacecraft give off heat from their electronics and from the radioactive decay of their plutonium fuel, and that’s enough to impart the measured deceleration. The researchers liken it to photons from a car’s headlights pushing gently back on the vehicle. The analysis is in the journal Physical Review Letters.