Spacecraft mysteriously speed up when they fly past Earth

When the Galileo spacecraft flew past Earth in 1990 to slingshot toward Jupiter, it exited the encounter moving 3.9 millimeters per second faster than physics predicted. While that seems like a rounding error, in the precise world of orbital mechanics, it is a glaring anomaly. Since then, multiple probes have experienced similar 'glitches,' with the NEAR spacecraft speeding up by 13 millimeters per second during its 1998 flyby.