The International Space Station (ISS) has seen many interesting experiments. From growing lettuce in space to studying twins (one in space and one on Earth) to compare the different developments their bodies went through.

One study carried out by NASA doesn't involve human subjects, but animal ones: the researchers sent mice up to the ISS to see how organisms react to microgravity.

"Since the environment of space alters multiple, interacting biological systems — including bones, muscles, the heart, blood flow, and the immune system – sometimes it is better to study everything at once in the entire organism," NASA states on their "rodent research" site. "This can be achieved by working with research model organisms, such as mice and other rodents."

While it might not seem like it at first glance, humans and mice actually have quite a lot in common, which makes the little rodents perfect guinea pigs, so to speak. Another plus is mice's faster development, so effects of microgravity can be studied on a shorter timescale.

All of this is why NASA has run several different experiments with mice in space. We have footage of the most recent set of space mice:

The start of it all

NASA first launched its rodent mission in 2014. Its primary interest was to see whether mice could actually survive the journey into space on an uncrewed vehicle and life on the ISS — and that the astronauts onboard the space station could work with the little rodents. Rodent Research-1 lasted 37 days and laid the groundwork for what was to come. Before this, rodents flew aboard 27 space shuttle missions between 1983 and 2011, but those only ranged from four to 18 days in duration.

Developing treatments for muscle- and bone-diseases

The second rodent mission, launched in 2015, focused on the effect space travel had on mice's muscles, bones and neurological systems. Once results could be transferred to human health research, they could "help scientists discover ways that therapies could act on muscle- and bone-related diseases, which could facilitate the development of new treatments," NASA says.

A year later, the space administration partnered with a US pharmaceutical company to look into developing new treatments for skeletal muscle wasting and weakness. And in 2017, Rodent Research-4 focused on how bones grow and heal in microgravity and on Earth. The results will be useful for astronauts as well as for people with injuries down on Earth.

Effects of a Mars mission

Several other rodent missions have examined effects of microgravity on blood vessels in the brain and eyes and how the body readapts to Earth's conditions after an extended stay in space.

The mice on NASA's most recent space mission are part of a study that's supposed to generate insights on how humans could survive voyages to Mars and beyond. They live in a habitat box that has been significantly improved from earlier versions. Scientists can now observe the mice easier, and cameras were installed in positions that made it less likely for them to be obstructed by fluids or small pieces of dirt.

Initial observations have shown scientists that the mice were nonplussed about their floating state at first. But within days, they returned to normal routines like grooming — and then began to take advantage of microgravity by running laps along their enclosure's walls. While many factors still need to be examined, this activity level bodes well for humans that will go on long space journeys in the future.