As it moves further away from our solar system, pioneer 10 is slowing down. A mysterious force is reducing its speed by 6 mph per century. Not much, but baffling when you consider that effects like gravity and solar radiation decrease rapidly with distance. Other possibilities have also been ruled out.

Also at the Guardian.

Problem solved - In brief, the anomalous acceleration may arise from radiation of waste heat by the on-board spacecraft RTG power generator.