For several years, the known laws of physics seemed inadequate to explain what should have been a simple physics textbook exercise: the trajectory of the Pioneer spacecraft through the outer Solar System. The Pioneer 10 and 11 probes were launched in the early seventies in preparation for the later Voyager Planetary Grand Tour. The mission was a success, and by the eighties, both Pioneer probes were minding their own business well beyond the orbits of Jupiter and Saturn. In the absence of significant gravitational forces other than from the Sun, the expected trajectory is easily calculated. However, telemetry data showed that both probes were systematically slowing down just a tiny bit more than could be expected from the gravitational pull of the Sun alone. Scientists were baffled and tried to come up with all sorts of hypotheses to explain what became known as “The Pioneer Anomaly”. In spite of the fact that the Voyager probes did not show this effect, some scientists even suggested that our theory of gravity needed revisions.

This week, there is finally an answer. After many years of painstakingly collecting all the necessary data, scientists have convincingly shown that the effect is due to thermal radiation. The effect is tiny, but over the long distances that the Pioneer probes have flown (check out their current position), it becomes noticeable. Too bad for sci-fi writers in need of new inspirational physics, but once more, ordinary physics saves the Universe!

The impressive efforts undertaken by the researchers are described in this story from sciencedaily, but you physics buffs probably want to jump straight to the paper in Physical Review Letters.