WCYDWT: Pioneer 10 & the Mysterious Force

From the Daily Telegraph

Research to be published shortly in The Physical Review, a leading physics journal, will show that the speed of the two probes is being changed by about 6 mph per century – a barely-perceptible effect about 10 billion times weaker than gravity.

Scientists initially suspected that gas escaping from tiny rocket motors aboard the probes, or heat leaking from their nuclear power plants might be responsible. Both have now been ruled out. The team says no current theories explain why the force stays constant: all the most plausible forces, from gravity to the effect of solar radiation, decrease rapidly with distance.

There’s lots more in the article. What can we do with this mysterious space probe force?