Last week, SpaceX launched CRS-14, a Dragon resupply capsule bound for the International Space Station carrying 5,800 pounds worth of tools, food, equipment and science experiments. Tucked somewhere among the hardware, reports Rae Paoletta at Inverse, was some frozen human and bull sperm. No, it’s not an April Fool’s prank—the crew will thaw the sperm to investigate how it acts in zero gravity conditions.

According to a NASA press release, the experiment is called Micro-11 and its aim is twofold. First, it’s designed to investigate how well those little swimmers do in microgravity in order to understand whether human reproduction can occur in outer space. Second, studying the movement of sperm without the interference of gravity could reveal processes that we just can’t see on Earth.

Previous studies of spermatozoa in space suggest that the lack of gravity might cause some problems. Most of us know the broad strokes of how mammalian reproduction works—sperm meets egg, sperm fuses with egg and reproduction is accomplished. But there’s actually a lot going on in between. First, the sperm cell has to be activated so its little tail starts wagging. Then it needs to build up steam and move faster while its head softens and becomes more fluid in preparation for fusion. But things don't go quite as planned when the sperm is in microgravity.

“Previous experiments with sea urchin and bull sperm suggest that activating movement happens more quickly in microgravity,” NASA writes in the release, “while the steps leading up to fusion happen more slowly, or not at all. Delays or problems at this stage could prevent fertilization from happening in space.”

In the latest experiment, the astronaut researchers will thaw the sperm from 12 humans and six bulls then use a chemical to activate half of the samples. They’ll then video the sperms’ movement using a microscopic camera before preserving the sperm for scientists on the ground to analyze. While human sperm is pretty diverse in its motion and appearance, bull sperm is much more uniform. The researchers will use the bull sperm as a quality control to make sure they are able to detect small changes in both types of sperm.

Right now, knowing whether the human reproductive system works in space isn’t a very pressing matter, especially since, at least officially, no humans have even had sex in space. But in future decades, as humans spend more time in space (and even perhaps travel to Mars), questions about reproduction will inevitably arise.

“One of the long-term interests of NASA is looking at multi-generational survival as we plan longer and longer-duration missions,” Joseph Tash, a researcher at the University of Kansas Medical Center who will examine the sperm when it returns says in a university press release. “As we plan to travel beyond the space station with thoughts of colonization on the moon and Mars and other heavenly bodies, the question of whether or not multi-generational survival can occur—not only in animals but in humans—is a very fundamental question that needs to be addressed.”

So far, mammals haven’t done so well breeding in space. As Maggie Koerth-Baker at FiveThirtyEight reports, in 1979 Russia attempted to breed rats in space. While two rats became pregnant, they both miscarried. Non-mammals, however, have fared better. “There have been several species that have gone through successful breeding in space, including frogs, salamanders, sea urchins, jellyfish, snails, medaka fish, nematode (roundworm, known as Caenorhabditis elegans), and other aquatic invertebrate animals,” Darryl Waller, public affairs officer at NASA’s Ames Research Center tells Paoletta. “The aquatic invertebrates; Amphipods, Gastropods (pond snails), Ostracods and Daphnia (water flea) produced their offspring or repeated their life-cycles under microgravity during four months in space.”

While this latest experiment is serious science, the quirkiness of sending sperm into space isn’t lost on Tash. That’s why he commissioned an old friend, The Simpsons' creator Matt Groening to create the mission patch, which shows Homer Simpson riding a bull sperm into space.