Astronauts aboard the International Space Station (ISS) are about to get a couple of new pets, though not the kind you would normally ask for. NASA is sending a batch of rats to the ISS for anywhere between 30 and 90 days (the details are still be worked out) to better understand how animals are affected by living in space, reports Elizabeth Howell at Fox News.

Nasa rats ISS

NASA: Rats will likely return in a SpaceX Dragon capsule

Rats have been sent to space dozens of times before, but typically just for a week or two, and scientists want to study some of the longer-term impacts on their physiology. Apparently rats are preferable to mice despite being larger (the extra food is a real cost when you have to put it into orbit) because their brain function is more similar to humans. And while it wasn’t said explicitly, scientists want to the effect that living in space has rat brains because poking around inside of an astronaut’s head is generally frowned upon.

The reason the exact duration of the experiment hasn’t been decided on is that the rats would likely come back on a SpaceX Dragon capsule, and cargo room is always tight. Besides, the capsules usually land in the ocean and drift for a bit before getting picked up and towed ashore, an arrangement that might not work for the rats.

NASA: Generations of fruit flies will also be studied aboard ISS

Fruit flies, another classic biology specimen, will be sent up along with the rats to give scientists a chance to see how life in space affects a species over many generations. If you got the chance to work with fruit flies in high school or college, you might remember that they have a generation time measured in days. Even though fruit flies don’t have much in common with humans, NASA is studying the effects of microgravity on a variety organisms (the Veg-01 experiment studying different types of vegetable life is already underway) both as basic research and part of the longer range goal of effecting space colonization. When the time eventually comes it won’t be enough to send humans up with food and supplies, they’ll have to bring a mini-ecosystem along with them, and it’s important to know how space flight and low gravity would affect different species.