Oumuamua

For eleven weeks in 2017 the first known object from another star crossed the solar system, and in 2018 a team measured a push on its way out that the Sun's pull does not explain.

Oumuamua and the 2018 Measurement of a Push Nobody Saw

For eleven weeks in 2017 the world watched a fragment of another star system pass through. In 2018 a team reported it had accelerated on the way out. Nothing visible was pushing it.