A SpaceX Dragon capsule has returned from the International Space Station (ISS) with an unusual crew – one composed of mice rather than humans.

Dragon splashed down in the Pacific Ocean at 10.14am EST (2.14pm GMT) on Sunday, September 17, having launched to the station on August 14. Dragon is currently the only vehicle that can return cargo from the ISS to Earth.

The spacecraft is often used to transport experiments and other equipment back to our planet. This time around it carried mice from NASA’s Rodent Research-9 study, which has been looking at how microgravity affects mice. Often mice on the ISS are euthanized after experiments, so it’s nice that these ones managed to come home.

The purpose of these mouse experiments is to prepare for human missions to Mars. By studying how microgravity affects blood vessels in the brain and eyes, we can find out how the vision of astronauts might be affected on long-duration missions to Mars. These could have travel times of about eight months. Rodent Research-9 also looked at the effect of weightlessness on tissue degeneration in hip and knee joints.

"The changes we see during this rodent research experiment will allow us to better prepare our astronauts for long-term exposure to low-gravity environments,” Kevin Sato, project scientist for the space biology project at NASA’s Ames Research Center in California, said in a statement.

It wasn’t just cute little mousetronauts that Dragon returned from the ISS. On board was also a lung tissue experiment, used to test strategies for growing new lung tissue. And there was also an experiment to grow an important protein implicated in Parkinson’s disease.

In total, about 1,700 kilograms (3,750 pounds) of cargo was returned to Earth from the ISS. It took Dragon about 5.5 hours to get back to Earth after leaving the ISS. It was retrieved by a recovery ship just off the coast of California.

This was the 12th Dragon cargo mission to the ISS by SpaceX. The company is going to be in the news quite a bit in the coming months, with a planned launch of its new heavy-lift rocket – the Falcon Heavy – scheduled for later this year, and an inaugural launch of its manned Crew Dragon spacecraft expected in summer 2018.