Retired astronauts Scott Kelly and Roy Bridges Jr. tell TMZ ... Tom's going to go through loads of training before blasting off to the ISS. Now, this might sound like a joke, but they agree the john will be a key part of his studies. They say it's no space cakewalk when nature calls in zero gravity.

As you know ... Tom is set to boldly go where no action star has ever gone before, going into orbit to film a new flick, and partnering with NASA, as well as Elon Musk's SpaceX.

Scott's commanded the ISS on 3 expeditions, and spent more time in space than any human being ... so he knows what he's talking about when he says Tom needs to move slowly aboard the ISS. It's common for most newbies to puke. Again, not pretty in zero-G.

On the positive side -- we think -- he adds, the only thing on Earth that compares to space travel is going over Niagra Falls in a barrel while you're on fire.

Roy, who piloted the Space Shuttle Challenger back in 1985, tells us Tom's body is in for a shock -- blood rushing to his head instead of his feet, due to the ol' zero-G.

Scott and Roy also told us what kind of training Tom can expect before getting on the ISS ... like how to move around safely, how to get in and out of the capsule, how to wear a spacesuit, how to make food AND get it in his mouth, plus how to sleep and clean himself -- think birdbath.

Oh, and we're told Tom's also gotta learn how to prepare for a space emergency, like space debris pummeling the ISS ... and how to survive an ocean splashdown when he comes back to Earth.

Sounds like TC's got his work cut out for him ... but this space mission is definitely possible!!!