Today's astronauts don't have to wait for a slot aboard the space shuttle or the International Space Station (ISS) to experience orbital living conditions. They can dive to the ocean floor and enter a metal container that, like a spacecraft, both protects them from an inhospitable environment and doubles as a laboratory for undersea science.

"The living area here is actually smaller than that on the space station itself," said ISS astronaut Peggy Whitson of her aquatic habitat. "It's actually more equivalent to the space available on the space shuttle."

The recent mission, dubbed NEEMO 5, is the latest in a three-year partnership between NASA, the National Oceanic and Atmospheric Administration (NOAA) and the University of National Undersea Research Center at the University of North Carolina at Wilmington (UNCW) to conduct astronaut training on Aquarius.

The NOAA-owned lab houses six people comfortably, including operation support staff from UNCW, which runs the station. UNCW aquanauts James Talacek and Ryan Snow served as operational support for NEEMO 5.

Aquarius is more than just a tin can full of air on the bottom of the ocean. Inside, six people work, eat and sleep in an area nine feet (about 3 meters) wide, 45 feet long (about 14 meters) and packed full of equipment.

Whitson said the underwater cabin is about the size of the ISS Zvezda Service Module that serves as living quarters for the astronauts onboard. Whitson should know too since, unlike her NEEMO 5 colleagues, she spent about six months aboard the space station during Expedition 5 last year. Her experience as that mission's science officer led to her role as commander of NEEMO 5.

"We're very fortunate to have Peggy here," said Clayton Anderson of Whitson from aboard Aquarius. "She's provided a wealth of information on what it's like to fly on one of these space missions."

NASA officials said that is precisely the point of the NEEMO missions. For astronauts in training, Aquarius serves as a very wet analog to flying in space. Astronauts speak in NASA vernacular during communications. They eat space food and conduct "extravehicular activities" wearing wetsuits, masks and goggles instead of space suits. A veteran astronaut like Whitson typically accompanies an Aquarius in order to give other crewmembers a more personal perspective of the experience.

"This is not a simulation, where at the end of the day the astronauts get out of the simulator and go home for the night," said Bill Todd, NEEMO project manager at NASA's Johnson Space Center. "That's not the way is here."

Todd, who has spent the last 17 years working training astronauts in simulators, told SPACE.com that NEEMO missions take advantage of saturation diving in order to parallel the inaccessibility of space missions. Saturation diving is a technique that lengthens the amount of a time a person can work at ocean depths by saturating body tissues with nitrogen. The astronauts aboard Aquarius would have to go through a 16-hour decompression period if they wanted to resurface to prevent decompression sickness, so a quick exodus from the station is not possible, Todd said.

Like their space-bound colleagues, NEEMO 5 astronauts had extensive pre-"launch" training and adhered to a set schedule during their two-week mission that meticulously tracked the times of meals, extravehicular activities and even web chats with students. For an added touch of realism, NASA set up the Exploration Planning and Operations Control (ExPOC) room at Johnson Space Center that monitored NEEMO 5 alongside a Key Largo-based topside center.

But there are some differences between life under the ocean and life in orbit, just one of which is the view.

"When you go out on a 'space walk' down here, you see more small activity among the reefs which are beautiful," Whitson said. "But on a real space walk the view is more macroscopic, with the Earth below you and continuous stream of sunrises and sunsets."

But simply getting comfortable with living with others in an enclosed space surrounded by an inhospitable environment does not make a space mission. After all, astronauts in space are in space to do science, not just for the view.

During their two-week mission, NEEMO 5 astronauts conducted 12 different experiments ranging a nutritional assessment of space food to a study that will hopefully show whether Aquarius crewmembers experienced the same patterns of viral reactivation astronauts in space. The latter experiment, NEEMO scientists hope, could help determine whether a decrease in the human immune response exposes astronauts in space to unacceptable medical risks.

"As a scientist, I find this experience [aboard Aquarius] very valuable," Aquarius astronaut Emma Hwang told SPACE.com. "It really gives me an overall view of how experiments should be conducted in space."

The NEEMO 5 crew also tested a small, portable ultrasound instrument for use in medical diagnostics and wore watches that gauged sleep efficiency by measuring light and activity levels. During the team's extravehicular outings, they took samples and measurements of nearby coral reefs as part of an ongoing study by NOAA.

"It's really a symbiotic relationship we have with NOAA," said Todd.

But the NASA research is not the first conducted out of Aquarius. The undersea laboratory has been in operation almost continually for about 15 years, with a few years taken off for renovations or refurbishing. In all, there have been about 72 missions aboard the ocean floor habitat, staffed by about 400 aquanaut researchers conducting experiments.

"There is quite a demand for this facility," said Steven Miller, director of the National Undersea Research Center. "Aquarius is really on the frontier in terms of isolation and undersea science, which has matched up really well with NASA's goals."

Todd said that the training environment afforded by Aquarius could on day benefit astronauts and mission controllers tasked with a sending an expedition to Mars.

"When we first go to Mars, we're going to have to be able to get there and set up camp," he said. "We're going to have to be able to go out and get the lay of the land, take measurements and perform some rudimentary science. All of these things we are doing with these [NEEMO] crews."

Even ExPOC, NEEMO 5's miniature mission control, can help NASA researchers prepare for a mission to the Red Planet. Ground controllers, for instance, must learn the proper procedures needed to communicate, without being intrusive, with a space crew that has lived in close quarters for six months.

"Controllers must be ready to deal with the response time from Mars too, where it takes 40 minutes to get a com call," Todd said.

The NEEMO 5 mission is the last planned outing to Aquarius for NASA pending a review of the entire program by NASA officials to determine whether it should be continued. Todd said that, for his part, he hopes the NEEMO program goes on to train future astronauts on their way into space.

"It's really a fantastic tool and an incredible opportunity for these astronauts," he said. "For a lot of them, it's the first mission they've been on."