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In the early 1970s, NASA sent two spacecraft on a roller coaster ride towards the outer Solar System. Pioneer 10 and 11 travelled past Jupiter (and Saturn in Pioneer 11’s case) and are now heading out into interstellar space.

But in 2002, physicists at NASA’s Jet Propulsion Laboratory in Pasadena, noticed a puzzling phenomenon. The spacecraft are slowing down. Nobody knows why but NASA analysed 11 years of tracking data for Pioneer 10 and 3 years for Pioneer 11 to prove it.

This deceleration, the Pioneer anomaly, has become one of the biggest problems in astrophysics. One idea is that gravity is different at theses distances (Pioneers 10 and 11 are now at 30 and 70 AU). That would be the most exciting conclusion.

But before astrophysicists can accept this, other more mundane explanations have to be ruled out. Chief among these is the possibility that the deceleration is caused by heat from the spacecraft’s radioactive batteries, which may radiate more in one direction than another.

Back in March, physicists from Europe claimed that a new computer model of heat emission from the spacecraft had finally nailed the problem. This proved that heat was to blame, they said.

NASA, which has its own team looking at this, has kept quiet about this result and today we can see why. Slava Turyshev at JPL and a few pals say they’ve trawled through JPL’s records looking for more data. And they’ve found it.

These guys say they’ve been able to double the datasets for both spacecraft. That increases the tracking data for Pioneer 10 to 23 years and Pioneer 11 to 11 years. That’s a jump from 20,055 to 41,054 data points for Pioneer 10 and from 10,616 to 81,537 for Pioneer 11.

So what does it show? Firstly, the new data confirms that anomalous deceleration exists.

But it also throws up something interesting. Turyshev and co say there appears to be an exponential drop in the size of the anomalous deceleration over time. It’s not easy to see in the data for sure, but there are certainly signs it is there.

That’s an important clue. Pioneer 10 and 11 are powered by the radioactive decay of plutonium-238, which of course decays exponentially.

NASA is currently performing its own computer simulation of the way that heat is emitted by the spacecraft to see whether it can explain the new dataset.

All the clues point to the notion that heat can explain the Pioneer anomaly. As Turyshev and co put it: “The most likely cause of the Pioneer anomaly is the anisotropic emission of on-board heat.”

So it looks as if NASA is set to agree with the European conclusion and that astronomers will soon be able to put this great mystery to rest once and for all.

Ref: arxiv.org/abs/1107.2886: Support For Temporally Varying Behavior Of The Pioneer Anomaly From The Extended Pioneer 10 and 11 Doppler Data Sets

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