According to NASA scientists, the unexpected slowing of the Pioneer 10 and 11 spacecraft – the so-called ‘Pioneer Anomaly’ – turns out to be due to the effect of heat pushing back on the spacecraft.

They have found that the heat emanates from electrical current flowing through instruments and the thermoelectric power supply.

“The effect is something like when you are driving a car and the photons from your headlights are pushing you backward. It is very subtle,” said Dr Slava Turyshev of NASA’s Jet Propulsion Laboratory in Pasadena and lead author of a paper published in the June 12 issue of the journal Physical Review Letters (arXiv.org).

Launched in 1972 and 1973 respectively, Pioneer 10 and 11 are on an outward trajectory from our Sun. In the early 1980s, navigators saw a deceleration on the two spacecraft, in the direction back toward the Sun, as the spacecraft were approaching Saturn.

In 2004, Dr Turyshev decided to start gathering records stored all over the country and analyze the data to see if he could definitively figure out the source of the deceleration. In part, he and colleagues were contemplating a deep space physics mission to investigate the anomaly, and he wanted to be sure there was one before asking NASA for a spacecraft.

They went searching for Doppler data, the pattern of data communicated back to Earth from the spacecraft, and telemetry data, the housekeeping data sent back from the spacecraft. They collected more than 43 gigabytes of data, which may not seem like a lot now, but is quite a lot of data for the 1970s.

The scientists saw that what was happening to Pioneer wasn’t happening to other spacecraft, mostly because of the way the spacecraft were built. For example, the Voyager spacecraft are less sensitive to the effect seen on Pioneer, because its thrusters align it along three axes, whereas the Pioneer spacecraft rely on spinning to stay stable.

With all the data newly available, Dr Turyshev’s team was able to calculate the heat put out by the electrical subsystems and the decay of plutonium in the Pioneer power sources, which matched the anomalous acceleration seen on both Pioneers.

“The story is finding its conclusion because it turns out that standard physics prevail,” Dr Turyshev explained. “While of course it would’ve been exciting to discover a new kind of physics, we did solve a mystery.”

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Bibliographic information: Turyshev et al. 2012. Support for the Thermal Origin of the Pioneer Anomaly. Phys. Rev. Lett. 108, 241101; doi: 10.1103/PhysRevLett.108.241101