Does living in space make your gut bacteria bug out? Twenty mice headed to the International Space Station are about to find out. Early Friday morning, SpaceX plans to launch its 15th resupply mission from Cape Canaveral, sending its Dragon spacecraft packed with nearly 6,000 pounds of cargo into orbit. Most of that will be a scientific payload—instruments and equipment to help the six crew members do some science on board.

One of those experiments will include pricking, weighing, and collecting poop from the new mouse arrivals. Oh yeah, and videotaping them while they sleep.

NASA

The rodents are part of a study to determine how living on a sterile ship in the dark vacuum of space alters circadian rhythms—and potentially disrupts a healthy microbiome. It’s a murine mirror of astronaut Scott Kelly’s twin experiment. Because before NASA can start sending humans to Mars, they want to know exactly how the long journey will mess with their bodies, including all their resident bacteria.

Ten of the mice will spend a record 90 days in space (that’s about nine years in Homo sapiens time). “We’re trying to understand the cascade of events that happens when microgravity interrupts sleep and wake cycles,” says Martha Vitaterna, a co-principal investigator on the study and the deputy director of Northwestern University’s Center for Sleep and Circadian Biology. Microbes make all kinds of essential molecules humans and mice can’t produce themselves; Vitaterna and her colleagues are hoping to learn if shifts to those chemical signals impair their host’s ability to weather things like sleep loss, which can throw metabolism and immune systems out of whack.

Her team is sending two different strains of mice into space, one that they expect to snooze just fine at zero-G and one that was a light sleeper in simulation experiments. Thanks to their genes, mice of the poor-sleeping strain don’t make melatonin.