How much gravity do animals need to stay healthy? Will lunar or Martian gravity be enough? Do animals change because of low gravity or because of some other aspect of space, like radiation? Scientists will use the space station's centrifuge to find out.
A spinning centrifuge creates artificial gravity. The centrifuge is shaped like a tuna can eight feet (2.5m) wide. It rotates habitats so the animals inside are pushed toward the outside hull.
Gravity levels change with the speed of rotation or the distance of the habitats from the center of the centrifuge. Asteroid, lunar, Martian, and up to twice Earth's gravity can be simulated.
The biggest difference between Earth and space aquariums is in the way the fish behave. With no gravity, most fish get confused and swim in circles (the fish version of space sickness).
Some spin several times a second and bump themselves senseless against the glass of the tank. Like astronauts, most adjust after a few days. But some fish never get "sick" in the first place. Why not?
Japanese scientist Kenichi Ijiri did some experiments to find out. First, he tested lots of fish to find some that didn't get sick. He bred these fish and found their ability to not get sick was passed down to their offspring.
What was different between this family of fish and families that swam in circles in space?
Besides up and down, fish use plants and light to define where they are. Dr. Ijiri guessed that fish with poor eyesight would be confused and swim in circles, but fish with good eyes would quickly choose an up and down based on light as the "sky."
Results of experiments support this theory. So astronauts who don't get sick may have inherited eyes especially sensitive to light and movement.
Scientists think that after many generations, fish living in space will be different from fish on Earth. On Earth, fish die outside of water because gravity makes their gills collapse. In space, their gills might work in air. Can you imagine a fish aquarium without water? It might happen on a space station.