In January, astronomer Bill Gray got a big shock. He was checking the path of a near-Earth object when the computer sent a warning message. “IMPACT!” it said, in big red letters.

This object wasn’t an asteroid or a comet. It was a three-tonne rocket section, as big as a school bus. And it was on a collision course with the moon. It would hit the moon March 4.

Asteroid detective

The old rocket booster had been tumbling through space for years. But where was it from?