The Great Game

THIS is a tale of two spacecraft. Pioneer 10 was launched in 1972; Pioneer 11 a year later. By now both craft should be drifting off into deep space with no one watching. However, their trajectories have proved far too fascinating to ignore.

That's because something has been pulling - or pushing - on them, causing them to speed up. The resulting acceleration is tiny, less than a nanometre per second per second. That's equivalent to just one ten-billionth of the gravity at Earth's surface, but it is enough to have shifted Pioneer 10 some 400,000 kilometres off track. NASA lost touch with Pioneer 11 in 1995, but up to that point it was experiencing exactly the same deviation as its sister probe. So what is causing it?

Nobody knows. Some possible explanations have already been ruled out, including software errors, the solar wind or a fuel leak. If the cause is some gravitational effect, it is not one we know anything about. In fact, physicists are so completely at a loss that some have resorted to linking this mystery with other inexplicable phenomena.

Bruce Bassett of the University of Portsmouth, UK, has suggested that the Pioneer conundrum might have something to do with variations in alpha, the fine structure constant. Others have talked about it as arising from dark matter - but since we don't know what dark matter is, that doesn't help much either. "This is all so maddeningly intriguing," says Michael Martin Nieto of the Los Alamos National Laboratory. "We only have proposals, none of which has been demonstrated."

Nieto has called for a new analysis of the early trajectory data from the craft, which he says might yield fresh clues. But to get to the bottom of the problem what scientists really need is a mission designed specifically to test unusual gravitational effects in the outer reaches of the solar system. Such a probe would cost between $300 million and $500 million and could piggyback on a future mission to the outer reaches of the solar system.

"An explanation will be found eventually," Nieto says. "Of course I hope it is due to new physics - how stupendous that would be. But once a physicist starts working on the basis of hope he is heading for a fall." Disappointing as it may seem, Nieto thinks the explanation for the Pioneer anomaly will eventually be found in some mundane effect, such as an unnoticed source of heat on board the craft.