While dozens of people have lived there over the years—including six right now—the International Space Station is unlike any other home. Its residents sleep zipped into bags tethered to the wall so they don’t float away. They pee into a plastic hose that suctions urine into a processor and then turns it into drinking water. Their showers require squeezing globs of water out of pouches.

But just like in homes found on Earth, the residents of the International Space Station share their space with thousands of invisible roommates: bacteria.

A team of microbiologists at the University of California recently sought to examine the population of microbes on the station. They wanted to see how the ecosystem orbiting 200 miles above Earth would resemble that of homes—and the people who inhabit them—below. They collected swab samples from more than a dozen surfaces on the station and compared them to microbes that were taken from surfaces in terrestrial households and different spots on the human body.

The researchers say the ISS turned out more species-rich than they expected. They identified 12,554 species of microbes, most of them harmless. They found that the composition of microbes of the space staton and a typical household was significantly different, but the microbes on the station more closely resembled home surfaces than human ones.

These results are both surprising and unsurprising. The researchers had expected to find differences in the microbial ecosystems between the space station and terrestrial homes. “Unlike the ISS, homes on Earth are exposed to a variety of sources of microbes, including the outdoor air, tracked-in soil, plants, pets, and human inhabitants,” the study authors write. On the ISS, you can’t exactly crack open a window to let some fresh air in.

These conditions would seem to suggest that the microbes living on the surfaces of the station would have more in common with the ones residing on the humans inside it. And yet, the researches found the microbes to be “more similar to the surfaces of human homes on Earth than it is to human bodies.” So the microbes on the ISS don’t match up with the ones found in houses or on bodies, but they’re still more like ones living on your kitchen counter than on your skin.

The findings were published Tuesday in the journal PeerJ. In May 2014, Koichi Wakata, an astronaut from Japan and the crew commander at the time, swabbed 15 surfaces around the station, including telephones, laptop keyboards, handrails, and air vents. In the station’s microgravity environment, microbe-carrying dust tends to accumulate in air filters rather than surfaces.

The samples were stored in a lab freezer, sent back to Earth on a SpaceX spacecraft, and then shipped to a lab, where the researchers extracted DNA to build a census of the microscopic inhabitants. The researchers compared them to three databases: the Human Microbiome Project, which characterizes human microbes; the Wildlife of Our Homes project, which tracks microbe samples taken from household surfaces like kitchen counters, toilet seats, pillowcases, and door handles; and Project MERCCURI, a microbial experiment on the ISS.

The researchers tried to target surfaces on the ISS that may be similar to those used in the Wildlife of our Homes project, but they hit a few snags. “The kitchen surfaces aboard the ISS are in the Russian module, which we did not have permission to access,” they write. And “swabbing the toilet seat was deemed inappropriate due to biosafety concerns.”

Studying the specific microbial environment on the ISS is crucial for future space missions beyond low-Earth orbit. When humans leave the comfort of Earth’s protective bubble for more dangerous, extreme worlds, they won’t be going alone—they’ll take with them thousands, perhaps millions, of these microorganisms, tiny beings just as unprepared as they are to face environments they didn’t evolve in.

The microbes of the ISS recently popped into headlines in November when the Russian news agency TASS reported that cosmonauts had scraped living bacteria off the outside of the station during a spacewalk. The article quoted a cosmonaut saying the bacteria “have come from outer space and settled along the external surface,” prompting some on the internet to think it was of extraterrestrial origin. He didn’t add much else, but as science journalists have since pointed out, it’s more probable that the bacteria was delivered there by terrestrial activities, like repairs and maintenance by astronauts and cosmonauts during spacewalks. The likeliest culprits behind mystery bacteria in low-Earth orbit are always going to be humans. Beyond that, however, who knows?