NASA says it's preparing a new class of astronauts to be sent to the International Space Station soon, only they're the four-legged variety -- rats, in this case.

Although rodents have gone on short forays into space before on NASA space shuttle missions, the space agency says it want to puts rats on the space station for months at a time in the interest of science.

The rodents would live aboard the station from 30 to 90 days, ferried into orbit and then brought pack to Earth on supply missions carried out by spacecraft like the Dragon capsule from commercial launch firm SpaceX.

Rats, although larger than mice and needing more food than their smaller cousins, are better candidates for studies on the effects of long-term stays in space because their neurocognitive functions are more similar to those of humans, NASA scientists say.

Of particular interest is the effect of microgravity on living organism subjected to long stays in space, they say.

That's the benefit of having the rats remain in space for months at a time, says NASA's lead scientist for the ISS, Julie Robinson, although it will require some new animal husbandry efforts on the part of station crew.

"This will allow animals to be studied for longer period of time on space station missions," she says.

Several experiments into the impacts of microgravity on living organisms are already underway.

One is the space agency's Veg-01 or "Veggie" test of the possibility of growing food plants like lettuce and other crops with an eye to future space colonization.

The Veggie experiment has been underway aboard the space station since arriving on a Dragon capsule in April.

NASA is also considering putting fruit flies on the station, chosen because large numbers of the genes linked to human diseases are also present in the flies, and their short lifespans would allow researchers to track the effects of microgravity over many generations of the insects.

"The space station is the perfect laboratory for these long-term types of study experiments," says Marshall Porterfield, head of the space life and physical sciences division at NASA's Washington, D.C., headquarters.

NASA says it has not determined exactly when the rats will go into space, noting it will depend on the timing of flights by the Dragon capsule, on which cargo space is always limited.

When the rats arrive at the space station they will join some 200 other experiments currently underway during the orbiting lab's Expedition 40 mission.

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