Andy Calloway loses a friend today.

Calloway is the missions operation manager—the "MOM"—for Messenger, an uncrewed spacecraft that has, for four years, orbited Mercury, 48 million miles away. He has essentially been the project hub. Each day he and his team interface with the engineers, scientists, management and, of course, the craft itself. He even takes a peek at how it's doing on weekends from his laptop at home.

Today, Messenger will make one last orbit around the planet, disappear behind the far side, and crash into its surface.

Messenger—an amalgam-acronym for Mercury Surface, Space Environment, Geo-chemistry and Ranging—has endured the most hostile space weather in the solar system. And, despite solar flares and coronal mass ejections, Calloway says, the craft has surpassed every challenge and met every request.