In 1998, scientists sent 4 pregnant swordtail fish and 200 babies into space; near the end of the mission, the fish suddenly began swimming in continuous loops

In January 1998, four pregnant swordtail fish and 200 of their young left for space on the space shuttle Endeavor. They traveled inside a sealed aquatic module built to house them for the mission. According to NASA records, Endeavor launched from Kennedy Space Center, Fla., on Jan. 22, 1998, to test the effects of space travel on various forms of life. The fish swam as they always had for most of the journey. Then, near the end of the voyage, their motions took a turn that researchers were not prepared for.

Four pregnant fish became unlikely space travelers

The fish were swordtails, or Xiphophorus helleri, a small freshwater species commonly found in home aquariums. The fish flew inside the CEBAS Minimodule, a sealed aquatic ecosystem reportedly designed by German and American researchers to test if a self-contained tank could sustain animals during spaceflight. The module reportedly contained four adult pregnant females and 200 juvenile fish less than a week old, along with water snails and aquatic plants used to help recycle oxygen within the closed system.


The fish swam normally for most of the mission

Cameras captured everything the tank had to offer at all times, recording what was happening throughout the day. In the first several days, the adult swordtails swam close to the tank walls, a behavior known as thigmotaxis, and the juveniles swam in an equally routine fashion. The footage reportedly showed no abnormal swimming patterns during this time, suggesting the fish had quickly adapted to weightlessness.

On day nine, the fish suddenly began looping

That changed on the mission's ninth day. It started when video footage revealed that the fish were performing a rapid, curving body motion known as a C-start, an escape reaction normally used by fish when they are surprised or suddenly grabbed. But instead of stopping after one C-start, the fish kept right on going, as if they were in an endless loop of surprise. Researchers noted that the looping behavior was most likely caused by a mechanical disturbance aboard the shuttle, and once the movement started, the fish had no gravity-based cue to tell them to stop. On Earth, gravity helps reset an animal's inner ear after rapid movement, but that doesn't happen in space, so the fish's escape reaction kept going and going.

Landing brought its own strange moment

The video footage taken of the fish just after they landed also showed that the animals had no problem making the transition. They were seen with their heads up and rapidly beating their fins to move up toward the top of the tank. The same research also shows that during the first hour after landing, the fish had some difficulty swimming and moved awkwardly. This seems to have improved after four days, as they then swam normally just like they did before they took off.

A small tank that helped explain how balance works

A makeshift study with a couple of aquarium fish may seem like a minor observation from a 1990s shuttle mission, but it contributed to research on one of the body's major balancing systems: the vestibular system. In space, astronauts often experience some form of balance issue during flight or after returning to Earth. By observing the effect of zero gravity on a simple animal, scientists saw an early, straightforward example of how balance systems operated. The mission was also an early test of the idea of a self-sustaining aquatic ecosystem. The systems that could keep fish, plants, and other water-dwelling creatures alive in space and on long flights between destinations would be incredibly useful to NASA, given its plans to travel to places like the Moon and Mars. The 1998 fish study remains a reference point for understanding how living organisms respond to microgravity, work that continues to inform NASA's planning for long-duration missions.