Here’s why Pioneer slowed down…

Physics World has a solution for the Pioneer Anomaly – the strange slowdown experienced by both Pioneer space probes as they passed beyond the farthest reach of the solar wind. It’s not as exciting as aliens with tractor beams or magnetic walls built to keep humans from exploring the galaxy, but it is an explanation for what happened at the edge of the solar system:

Physicists have known for more than a decade that the Pioneer 10 and 11 probes are following trajectories that cannot be explained by conventional physics. Known as the “Pioneer anomaly”, both craft seem to be experiencing an extra acceleration towards the Sun as they exit the solar system….

In 2011 a team led by Slava Turyshev of the Jet Propulsion Laboratory in California – and including Viktor Toth, Jordan Ellis and Craig Markwardt – showed that the magnitude of the acceleration is decreasing exponentially with time. Given that for both craft electricity is supplied by a radioisotope thermoelectric generator (RTGs) powered by the heat given off by the radioactive decay of plutonium – an energy source that decays exponentially with time – Turyshev and others suggested that the extra acceleration could be caused by thermal radiation being emitted from the craft in a preferred direction.

To to see if thermal emissions really are driving the anomaly, Turyshev, Toth and Ellis joined forces with three other researchers – Gary Kinsella, Siu-Chun Lee and Shing Lok – to create a detailed computer simulation of the thermal properties of the spacecraft and the directions in which key components emit thermal radiation.

The simulation reveals that the two main sources of thermal emissions on the spacecraft are the RTG itself and the scientific instruments that it powers. These instruments, which are mostly mounted on the back of the spacecraft, face away from the Sun and, according to the simulations, their thermal emissions have a relatively high efficiency of accelerating the spacecraft towards the Sun.

In other words, the instruments up front are acting like miniature thrusters, because they’re hot.

Go figure.