BY RIGHTS, the Pioneer 10 and 11 space probes should no longer be of interest to anyone. Launched in the 1970s, they are now far beyond the edge of our solar system, drifting silently out into the void. The last contact we had with Pioneer 10 was on January 10, 2003, when a weak signal made it back to Earth. It is now nearly 8 billion miles away, past the orbits of Neptune and Pluto, and we will not hear from it again because it no longer has any power left with which to send out a signal.
However, the Pioneer probes are of interest. That is because they are drifting off course in a very intriguing way. In every year of travel, the probes veer 8000 miles further away from their intended trajectory. It is not much when you consider that they cover 219 million miles a year; the drift is around 10 billion times weaker than the Earth’s pull on your feet. Nonetheless, it is there, and decades of analysis have failed to find a straightforward reason, such as a heat leak from the probes. So it is possible that these errant spaceprobes are telling us something extraordinary. Maybe Newton’s law of gravitation, the law that describes how their trajectories should be playing out, is in need of an overhaul. Or is there is a new and strange force waiting to be discovered?