A month ago I mentioned an Economist article about how spacecraft are not behaving quite as expected. A physicist friend of mine e-mails:
The sling-shot anomaly you asked about is taken seriously enough to have been published in Phys Rev Letters last month. Of course it COULD indicate undreamed of physics. But it’s more likely to be a subtle systematic error.
The same first author (Anderson) is also on the 1998 paper on the Pioneer anomaly which seemed to indicate that Pioneer 10 and 11 were accelerating towards the sun a little more than they should. That’s been a thorn in the side of general relativity for some time now. That effect has now been PARTLY explained by a rather prosaic effect: the heat from the plutonium power source warms the spacecraft which radiate thermal radiation. But, because of their composition, it turns out that they radiate more in front of them than behind. So radiation pressure slows them down…. how annoying… but not exciting.
As a condition to posting his email, my friend asks me to point out that he is not an expert. Noted. He knows much more than I do, though.