The Pioneer anomaly has a habit of doing the rounds of the “Einstein was wrong and scientists ignore this” crowd on the internet. The Pioneer 10 and 11 spacecraft were the first to venture past Jupiter and head out towards the edge of the solar system. However, careful calculations of their distance from the Earth suggests an unexplained acceleration towards the Sun over and above gravity. There are an array of possible explanations – maybe the heat distribution of the probes is asymmetrical, maybe getting hammered by radiation at Jupiter (which fried Pioneer 10 to the extent that it only just survived it) or interaction with micrometeorites has left something leaking or maybe the probes have washed through so much dark or ordinary matter they have just slowed down. The other explanation is that gravitational physics is slightly wrong at these distances – which is of course leapt on by the MOdified Newtonian Dynamics people who insist it is proof that extra additions to gravitational laws are required.
One way to test the latter theory is to look at the orbit of things at a similar distance to the probes when they were first exhibiting this effect. Fortunately, a small thing we know of is Pluto. Careful observations of Pluto’s orbit have been taken over some time and the minor planet doesn’t seem to be exhibiting any such behaviour. Now Gary Page and John Wallin from George Mason University in Virginia, USA and David Dixon from Jornada Observatory in New Mexico have pointed out that in their opinion the orbit of Pluto isn’t yet well constrained enough to make such a claim about the tiny acceleration affecting it. Although we have a rough idea of how Pluto orbits the Sun since its discovery in 1930, it is rather remote and difficult to track, so to get a reading as accurate as the one required is difficult to say the least. They note that this doesn’t mean the Pioneer effect is present, just that we don’t have accurate enough readings to say whether or not it is present.