Remember NASA’s "Twins Study"? That was the one where an astronaut in space and his identical twin brother on Earth were monitored, to see how being in space affects the human body.

While the results from that are still ongoing, we’re about to see the sequel of sorts. As 20 identical twin mice have just been sent to the International Space Station (ISS), while their siblings remain on Earth.

The mice were launched into space in a SpaceX Dragon capsule, which lifted off on Friday, June 29 in a rather stunning launch. The capsule arrived earlier today, with the mice among the various other experiments and cargo that are also on board.

The experiment is being led by Fred Turek and Martha Vitaterna from Northwestern University in Illinois, the primary goal of which is to see how being in space affects the body’s microbiome – or gut bacteria. And that could have important implications for future long-duration missions.

“Because a trip to Mars and back is expected to take several years, we need to determine how the gut’s microbiota might be altered in zero gravity over long timescales,” said Turek in a statement.

The original Twins Study involved astronaut Scott Kelly in orbit on the ISS from March 2016 to March 2017, with his twin brother Mark on Earth. One criticism, however, was that Mark was not required to mimic exactly the conditions or diet of Scott in space.

This mice study will attempt to advance on that somewhat. The mice on Earth will live inside a NASA simulator that replicates the conditions on the ISS, except that they’ll be subject to Earth’s gravity. This means the exact effect of weightlessness on biology can be studied more closely.

“With the twins we only had two subjects and we could not prescribe that they eat the same, or live similar lifestyles while Scott was in space,” said Vitaterna. “With this new project, we will be able to control for diet and also further examine the liver, spleen, and fat to learn the ways in which individual components affect one another.”

The team also want to study how being in space affects circadian rhythms. These internal clocks tend to regulate most of our functions, so working out if space travel changes them is pretty vital to long-term missions.

It’s also worth noting that the mice actually comprise two different strains, something that’s never been done before on the ISS. Half of the mice on the ISS will also spend 30 days in orbit before returning to Earth, while the others will spend 90 days up there.

Good luck, micetronauts. We’re all counting on you.