By Duncan Geere, Wired UK

A 30-year mystery as to why the Pioneer spacecraft have slowly been drifting off course is close to being explained — the latest analysis pins the blame on heat.

The Pioneer 10 and 11 spacecraft, launched in 1972 and 1973 respectively, were designed to fly past the asteroid belt and investigate the Jovian and Saturnian systems. Both are now located in the interstellar medium, and radio contact has been lost, but each is slowing gradually due to the influence of the Sun’s gravity.

In 1980, an algorithm to study gravitational effects in the outer solar system was concocted by astronomer John Anderson, but it didn’t quite work. There was a discrepancy — small but noticeable — between the readings predicted by the algorithm and those actually observed from the Pioneer spacecrafts’ radio signal. The phenomenon was named the Pioneer Anomaly.

Physicists struggled to explain the discrepancy, proposing complex theories that even included a suggestion that gravity might behave differently at large distances from the Earth. However, the most recent analysis suggests a rather more mundane explanation — heat from the plutonium inside the spacecrafts’ generators.

If heat is radiating evenly in all directions from the spacecraft, then there will be no effect on its course, but if there’s a difference of as little as five percent between the front and the back then that could explain the difference between the predicted course and the course that’s actually been observed. The smoking gun is that the level of deceleration seems to be decreasing at an exponential rate, which tallies with the radioactive decay of the plutonium-238 that powers the two spacecraft.

It’s not completely out of the question that there might be another culprit, but it’s looking increasingly unlikely. NASA is performing its own investigation to compare to the data, but if that comes out with heat as the cause too, then the 30-year mystery may finally be over.

Image: NASA

Source: Wired.co.uk

Via: Discovery News

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