When We Left Earth: The NASA Missions (or NASA's Greatest Missions: When We Left Earth in the UK) is a 2008 Discovery Channel HD documentary miniseries consisting of six episodes documenting American human spaceflight from the first Mercury flights and the Gemini program, to the Apollo program and its Moon missions and landings, to the Space Shuttle missions and the construction of the International Space Station.

The miniseries was created in association with NASA to commemorate the agency's fiftieth anniversary in 2008. It first aired on June 8, and concluded on June 22. Each airing consisted of two hour-long episodes. The miniseries was then released on DVD on July 10, 2008, and released on Blu-ray disc on August 12.