For some years now NASA scientists have noticed that two of our early space probes, Pioneer 10 and 11, are not quite where they should be. Launched in the early 1970s to study the outer planets, these two spacecraft are now billions of miles away from Earth on escape trajectories out of the solar system. As scientists have tracked them over the years they have noticed a tiny, unexplained error that has been accumulating in their predicted positions. This is called the Pioneer Anomaly. Efforts to explain the anomaly have been unsuccessful. Is it caused by a slight outgassing from the spacecraft materials, or perhaps infrared photons radiating from the onboard nuclear power supplies? If you eliminate all the possible onboard causes, a much more interesting possibility remains. Perhaps the theory of gravity needs to be adjusted a bit! Einstein's theory of general relativity has been astonishingly successful at explaining the motions of the planets and other heavenly bodies. It would be Big News if even a tiny adjustment needed to be made in theory. For that reason, most scientists have assumed some other explanation will turn up for the Pioneer Anomaly. After all, it has only been observed with those two spacecraft.
Until now. A new anomaly has now been observed in the motions of five other spacecraft. Each is going either slightly slower or faster than expected after a flyby of Earth. There's no obvious connection to the Pioneer Anomaly, but it seems unlikely that two independent anomalous effects would exist.
So is there something wrong with our theory of gravity? Probably not, but it's fun to wonder.