For some years the Pioneer spacecraft have been demonstrating a phenomenon that has had astrophysicists tearing their hair out. They are drifting out into interstellar space; the only forces acting on them are gravity from the sun, drag against the fabulously thin interplanetary medium, incredibly weak photon pressure/solar wind from the sun, and a few other minor effects. All together, the result should be a slight deceleration (mostly due to the suns gravity). But careful measurements have shown that the spacecraft are actually decelerating more than physics says they should.
So, the Pioneer Anomaly has resulted in theory after theory, with physicists (and others) suggesting new phenomena that would generally overturn Relativity or introduce unseen words Out There, or something. But it appears that a careful examination of all the data has produced an explanation that explains everything and requires no new physics:
Pioneer Anomaly Solved!
The short form seems to be that heat being radiated from the spacecraft is producing the thrust needed for the deceleration. The level of thrust is incredibly low… keep in mind, this is produced by low-energy infra-red photons leaking away from the spacecraft. The dying RTGs radiate somewhat equally in all directions, but some of that heat is reflected off of the main antenna dish, producing that slight flicker of thrust.
Think of it as the thrust produced by a nearly-dead flashlight.