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. (New Scientist)