Oops

The Minor Planet Center keeps track of asteroids and comets, especially those that might collide with Earth. Recently, the MPC found an object that would soon pass within 5,000 miles of Earth-- less than the diameter of the planet. In astronomical terms, that's dangerously close. The MPC alerted astronomers worldwide to watch this thing.

Well, one Russian astronomer noticed something familiar about the object's path. The MPC's potentially dangerous "asteroid" turned out to be the Rosetta space probe, launched by the ESA in 2004. Rosetta is on its way to investigate a comet, and is swinging by Earth to get a gravity assist from the planet to pick up more speed-- a common technique used in deep space missions.

The MPC quickly issued another alert, saying, in effect, "Never mind." The system worked, however. The actual situation was soon determined. This time, it turned out to be good news.