To that end, NASA has a specially designed training room at the Johnson Space Center in Houston where astronauts can carefully hone their "technique" before departing on a trip to orbit. Astronaut Michael Massimino put the space potty trainer in the spotlight in a behind-the-scenes video with the six-man crew of NASA's current shuttle mission – going on now – on the shuttle Atlantis.

On the shuttle, urine is handled differently than solid waste, so it doesn't go through the 4-inch opening. Instead, a long hose with suction power attaches beside the seat, and each astronaut attaches her or her own funnel for urination to this hose.

Funnels are different for men and women.

Women need to place the top of their funnels directly against their bodies, so the sides of the female funnels need to be vented so that air can flow in when the suction is turned on, Weinstein explained. And women can choose among three funnels with differently shaped tops — there are two funnels with oval-shaped tops and one with a circle-shaped top.

Male funnels are simpler. They only come in one shape — the top is circular — and they do not have venting.

"For men, we do not want them ... docking to the funnel," said Weinstein, so male funnels do not need venting at the top to let in air.

Lastly, for paper waste, a separate suction hose on the side of the toilet can be fitted with a larger cup and lined with a plastic bag.

Straps on the foot rest can help to hold an astronaut in place, and there are two thigh restraints on the sides of the toilet that can be swung over the top of the legs to help a person stay down on the toilet. But not everyone uses them.

Before the most recent Atlantis mission, astronauts gathered in the room and compared their techniques for staying in position when they are weightless.

"I stick my hands on the roof," said astronaut Piers Sellers, pressing his hands palm-up over his head.

The thigh restraints are helpful as handles for getting in and out of the toilet, said astronaut Steve Bowen, but he also uses the low roof over the toilet to stay in position.

Massimino followed Atlantis' astronauts as they trained for a 12-day mission to the International Space Station.