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Mysterious Pioneer slowdown solved at last

Dr Amanda Doyle, Feature writer
Jul 25, 2012, 7:00 UTC

Sen—Scientists have finally unravelled the mystery behind the Pioneer Anomaly, a strange slowing down of two NASA spacecraft sent to explore the Solar System 40 years ago.

There had been speculation that a mysterious cosmic force affected the paths of the Pioneer 10 and 11 space probes. However, fresh analysis of the data shows that heat generated by electric currents on the craft themselves pushed back on them, causing their speed to decrease slightly.

The Pioneer 10 and 11 probes were launched in 1972 and 1973 respectively on a mission to explore the planets. A few years after they were launched, it was noticed that both craft were exhibiting a strange and unexpected deceleration as they neared Saturn. Not much thought was given to this at the time, as it was assumed that leftover propellant in the fuel lines was the cause of the deceleration.

However, in 1998 it was realised that this deceleration was still occurring at a rate of 0.9 nanometres per second squared. The effect was dubbed the Pioneer Anomaly and raised questions as to whether the cause was a new type of physics that could contradict Einstein’s general theory of relativity.

In 2004, Slava Turyshev and his colleagues started to gather old data from the Pioneer craft to try to analyse the strange effect further. They wanted to find out as much as possible about the anomaly before suggesting a deep space mission to NASA.

The data they needed was Doppler data, which is the pattern of data that the craft send back to Earth, and telemetry data. The problem was that all of this data was stored on punch cards and magnetic tapes, making it difficult to use. The punch card data had to be digitised, and a vintage tape machine has to be salvaged so that the tapes could be read. The total amount of data collected was 43 gigabytes, which is a substantial amount for a 1970s mission.

They discovered that this strange effect was unique to the Pioneer spacecraft, as they rely on spinning for stability rather than having thrusters aligned along three axes like the Voyager spacecraft.

They eventually concluded that it was not strange physics causing the deceleration, but heat from the electrical subsystems and the decay of plutonium which was pushing back on the craft.

"The effect is something like when you're driving a car and the photons from your headlights are pushing you backward," said Turyshev, lead author of the paper published on 12 June in Physical Review Letters. "It is very subtle."

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

NASA lost communication with Pioneer 10 in January 2003, and the last signal from Pioneer 11 reached Earth in 1995.