We are all pretty sure we know what force is. Gravity, for instance, is a force and gravity needs no further explanation. Newon was the first scientist to state the law of gravity in mathematical terms and for many years we thought that this was the final answer to what the law of gravity really is. Then cam Einstein, who found that Newton’s law was only a good first approximation. Einstein’s relativity concepts gave us a more accurate computation, which has been proved many times to give the correct answers for orbital and space trajectories.
By this time, therefore, we should feel reasonably certain that at least we know the answer to how gravity affects massive bodies, so that we can predict any result. It seems, however, that nature still has a trick or two up its sleeve. Back in the early 1970s, two Pioneer spacecraft were sent to take a close look at Jupiter and Saturn. After that mission was completed, they then went on into the outer solar system. Soon after wards, it was noticed that their velocities deviated from those predicted by Einstein. They were slower than they should have been, by some 5000 kms. per year. It seemed as though an unknown force, emanating from the sun, slowed them down. To add to the mystery, another anomaly was discovered with several spacecraft when they were sent round the earth in what is called a slingshot maneuver, to pick up speed before going on to various space missions. This time, they picked up more speed than they should have, only four millimeters per second, but easily measured. Of course, in both cases, all sorts of precautions were taken to eliminate errors due to faulty instruments, inaccurate measurements or any other extraneous factors that would have allowed existing theories to remain intact. It seems that we might need another bored young patent clerk doodling on a piece of paper, another Einstein to astound us by finally explaining a very enduring mystery.