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.

Discovery landed with some 3,000 flies aboard 13 days later, a whole new generation that had hatched, developed, and matured in space. It is truly fascinating how 75 flies became 3,000 flies without ever landing on solid ground, and their bodies gave scientists something to investigate about what long-duration space travel might do to the rest of us.

Why NASA cares about a bug most people swat away


Fruit flies, also known as Drosophila melanogaster, aren't all that glamorous subjects of a scientific study, but they turned out to be capable of yielding significant results aboard the space mission STS-121. They breed quickly, are inexpensive to maintain, and have the same basic molecular wiring to their innate immune system as do we. NASA's Ames Research Center chose them for an experiment, dubbed FIT (Fungal Pathogenesis, Tumorigenesis, and Effects of Host Immunity in Space), due to that overlap. The principal investigator of the mission, Sharmila Bhattacharya, as per the NASA report, stated that it was eye-catching how the flies' behavior, development, and immune system were affected as soon as they landed on Earth.

The outcome wasn't very promising

This is what makes anyone who has ever dreamed of going to Mars a bit uneasy. The larvae that grew up completely in orbit returned smaller and with fewer of the specialized immune cells known as plasmatocytes, and those cells were significantly less capable of engulfing bacteria than were the Earth-raised immune cells. Marcu and colleagues in their peer-reviewed 2011 study titled “Innate Immune Responses of Drosophila melanogaster Are Altered by Spaceflight,” published in PLoS ONE, show exactly that. The researchers also discovered less activity in the genes involved in identifying a threat and responding. In other words, the flies' immune systems appeared to become underdeveloped and slower to respond to bacterial exposure in microgravity, measurably impaired, but not completely broken down.

A subsequent study only made matters worse. A 2014 study titled “Toll Mediated Infection Response Is Altered by Gravity and Spaceflight in Drosophila,” by Taylor et. al, also published in PLoS ONE., found that the flies' immune system responses to bacterial infection did not appear to be significantly affected by spaceflight; however, their response to fungal infection appeared to be affected, specifically by inhibiting the activation of the Toll pathway, a pathway in the immune system that is responsible for fighting fungal infections. This finding matters: it suggests that spaceflight may affect immune responses and infection susceptibility, at least for pathogens that depend on that pathway.

What this actually means

It is not a problem just with insects. Time in space has long been associated with impaired immune function among astronauts, and scientists have been unable to definitively explain the reason for this phenomenon, in part because each of us has a different immune system, as well as because of the considerable complexity of the subject. In order to get rid of that problem, flies became genetically identical and were disposed of for the purposes of science. When creatures that share about 75 percent of their disease-causing genes with humans return from space with a weakened immune system, it's a pretty good indicator to any space explorer thinking of spending multiple years on Mars without any medical help at hand.

The bigger picture

These findings do not mean that space travel is inherently dangerous in a sensational way. Rather, scientists have detected a real, replicated biological signal in one of the most trusted model organisms in genetics; one worth taking seriously as private companies and space agencies pursue longer human spaceflights. The fruit fly is a reminder that the hardest part of space travel may not be rocket science at all, it may be keeping a human body, including its immune system, functioning millions of miles from the nearest hospital, a problem science is still working out.