In 2006, NASA launched 75 fruit flies on STS-121 for 13 days; Discovery returned with about 3,000, including a new generation that developed in space

In 2006, NASA placed 75 normal fruit flies aboard the space shuttle Discovery and launched them into space as part of the STS-121 mission, a routine, crewed shuttle flight to the International Space Station, not a headline-grabbing astronaut or lunar mission. It was about something much more sedate, and much stranger, than the immune system itself: what happens to an immune system in zero gravity? According to a 2013 NASA article, scientists made a direct comparison between two groups of flies: one group was placed in five different containers on Discovery, and the other was placed in five similar containers on Earth.