On September 21, 2003, controllers drove Galileo into Jupiter at nearly 108,000 miles per hour. The spacecraft had found evidence of a saltwater ocean beneath Europa’s ice, and with its propellant running low, NASA could not guarantee that the unsterilized orbiter would never crash into the potentially habitable moon carrying microbes from Earth.

On September 21, 2003, controllers drove Galileo into Jupiter at nearly 108,000 miles per hour. The spacecraft had found evidence of a saltwater ocean beneath Europa’s ice, and with its propellant running low, NASA could not guarantee that the unsterilized orbiter would never crash into the potentially habitable moon carrying microbes from Earth.

Galileo’s final plunge was a planetary-protection decision: NASA used the orbiter’s remaining control to keep an unsterilised spacecraft away from Europa.