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During the last decade or so, the Pioneer Anomaly has become one of the great unsolved puzzles in astrophysics.
The problem is this. The Pioneer 10 and 11 spacecraft were launched towards Jupiter and Saturn in the early 1970s. After their respective flybys, they continued on escape trajectories out of the Solar System, both decelerating under the force of the Sun's gravity. But careful measuremenrs show that the spacecraft are slowing faster than they ought to, as if being pulled by an extra unseen force towards the Sun.
This deceleration is tiny: just (8.74±1.33)×10^−10 ms^−2. The big question is where does it come from.
Spacecraft engineers' first thought was that heat emitted by the spacecraft could cause exactly this kind of deceleration. But when they examined the way heat was produced on the craft, by on board plutonium, and how this must have been emitted, they were unable to make the numbers add up. At most, thermal effects could account for only 67 per cent of the deceleration, they said.
That led to a host of other ideas some of which I've covered in this blog. For example, last year we looked at work ruling out the possibility that gravity could be stronger at these distances, since we ought to be able to see a similar effect on the orbit of other distant objects such as Pluto.
Now Frederico Francisco at the Instituto de Plasmas e Fusao Nuclear in Lisbon Portugal, and a few pals, say they've worked out where the thermal calculations went wrong.

To read the rest of the article, click here.
Category: Science