Scientists ponder sluggish Pioneers
Is gravity wrong, or is it a leaky tank?
Astronomers tracking the paths of Pioneer 10 and 11 say that something seems to be slowing the progress of the two craft as they progress towards the outskirts of the solar system.
The Pioneer probes were launched 30 years ago to investigate Jupiter and Saturn, and are the oldest spacecraft launched by the US. Since their missions were completed, the two craft have been left to fly on, eventually to leave the solar system. But now, researchers at the Jet Propulsion Laboratory have reported that the probes are not on quite the path that they should be following.
John Anderson and Slava Turyshev of the Jet Propulsion Laboratory in Pasadena and Michael Martin Neito of the Los Alamos National Laboratory were tracking the Pioneers using NASA's Deep Space Network when they noticed that they were not where they were expected to be.
The drift from the expected trajectory is only small, but, according to Anderson, Turyshev and Neito, it is significant.
This part of the discovery, published in Physics World, may be clear enough, but the reasons for the drift have split the scientific community. Some scientists speculate that dark matter is responsible for slowing the flight of the probes, or suggest that models of gravity are wrong. Others (clearly lacking a sense of drama) suggest that the slow down could be due to leaks in the crafts' gas tanks providing a deceleration.
"Unless there is really good evidence to the contrary, we should stick to simple ideas like these and not go around blaming strange new types of particle or flaws in general relativity," Professor Martin Barstow, of Leicester University told The Observer.
Anderson is clearly not impressed with this sort of thinking: "It's hard to imagine such a leak happening on both probes at the same time in such a way as to produce an identical acceleration," he said.
Along with his fellow researchers, he is now calling for a follow-up mission to deep space that would determine the cause of the drift. He is confident that current sensor and communications technology would be able to uncover the root of the probes' strange behaviour. ®