8 The Pioneer anomaly
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?