Suppose you were to read this:

The speed and direction of some spacecraft are being mysteriously altered as they pass near the Earth.

Would you assume it was from some nutty UFOlogy web site? Well you would be wrong. That’s scientists from JPL talking to Nature. The differences in trajectory are very small, but they seem to happen with every spacecraft that is monitored. Either there’s a problem with NASA’s model, or there’s something about physics that we don’t quite understand.