Well, there's no need to rewrite physics, and it's got nothing to do with aliens. The unexpected slowing of NASA's Pioneer 10 and 11 spacecraft - the so-called Pioneer Anomaly - has been explained.
It turns out to be due to the tiny effect of heat, emanating from electrical current flowing through instruments and the thermoelectric power supply, pushing back on the spacecraft.
"The effect is something like when you're driving a car and the photons from your headlights are pushing you backward," says Slava Turyshevof NASA's Jet Propulsion Laboratory. "It is very subtle."
Launched in 1972 and 1973 respectively, Pioneer 10 and 11 are on an outward trajectory from our sun. In the early 1980s, as they approached Saturn, navigators saw a deceleration on the two spacecraft.
At first, they dismissed it as the effect of dribbles of leftover propellant still in the fuel lines after controllers had cut off the propellant. But by 1998, this theory started wearing a bit thin, and scientists realized there was an actual deceleration of about 300 inches per day squared.
They suggested that this could represent some new type of physics that contradicted Einstein's general theory of relativity.
Turyshev started investigating further - partly, he says, because he was angling for a deep space physics mission to investigate the anomaly - and thought he'd better be sure there was one before asking NASA for a spacecraft.
He gathered Doppler data stored all over the country and cleaned it up.
The team then saw that what was happening to Pioneer wasn't happening to other spacecraft, mostly because of the way they were built. For example, the Voyager spacecraft are less sensitive to the effect seen on Pioneer, because its thrusters align it along three axes, whereas the Pioneer spacecraft rely on spinning to stay stable.
With all the data newly available, Turyshev and his colleagues were able to calculate the heat put out by the electrical subsystems and the decay of plutonium in the Pioneer power sources - which matched the anomalous acceleration seen on both Pioneers.
"The story is finding its conclusion because it turns out that standard physics prevail," says Turyshev. "While of course it would've been exciting to discover a new kind of physics, we did solve a mystery."