The next generation of American spacecraft designed to carry people into low-Earth orbit will be required to function as a lifeboat for the International Space Station for up to seven months. This service has not been provided by an American spacecraft since an Apollo command module remained docked to Skylab for about three months from 1973 to '74.

Like a lifeboat on a cruise ship, the spacecraft is not expected to be called into service to quickly evacuate people but it has to be ready for that job just in case.

Right now, the lifeboat function on the space station is served by requiring a pair of Russian Soyuz spacecraft to be docked at all times. Each Soyuz holds three people. So with two docked, there can be six people working on the station at any one time. The crew drops to three when one Soyuz leaves and before another arrives during a procedure called an indirect handover.

There are fundamentally two capabilities a spacecraft must perform to be called a lifeboat, said NASA engineers who are working with companies developing spacecraft in the agency's Commercial Crew Program (CCP).

First, the spacecraft needs to provide a shelter for astronauts in case of a problem on the station. Second, the ship has to be able to quickly get all its systems operating and detach from the station for a potential return to Earth.

"You've got to make sure it provides the same capability on day 210 as it does on day 1," said Justin Kerr, manager of CCP's Spacecraft Office.

Two things make it tough for spacecraft designers when it comes to the lifeboat feature: power and protection from things outside the spacecraft like micrometeoroids. The vast amount of electricity generated by the space station's acre of solar arrays is reserved for the station's systems and science experiments.

The amount of power dedicated for a docked crew spacecraft is similar to the amount of electricity a refrigerator uses.

"There's very little power available for these spacecraft so what we're really driving the partners to do is develop this quiescent mode that draws very little power," Kerr said.

Ideally, designers want to have the spacecraft powered off when it is attached to the station. That might not be possible, though, because air doesn't automatically circulate in microgravity the way it does on Earth. So a spacecraft, even with its hatch open inside of the station, can develop dead spots, or sections of the cabin without air for breathing, unless there is something to move the air around.

"You don't want someone to go into the spacecraft and immediately pass out because there's no breathable air in that one area," said Scott Thurston, deputy manager of CCP's Spacecraft Office.

Designers also have the unique challenge to build a spacecraft strong enough to withstand impacts from micrometeoroids, but cannot carry a lot of armor because it would be too heavy to launch. Although numerous impacts are not expected, designers are still expected to show their craft can survive an occasional hit.

"It's something you have to design for, the magic bb scenario," Thurston said.

The situations when the craft will be needed are not only hypothetical. There have been occasions on the International Space Station when the crew members took refuge in the Soyuz because space debris was passing near the station.

CCP gave aerospace companies a list of requirements their spacecraft need to meet during NASA's certification process for use as in-orbit lifeboats, Thurston said.

Boeing, Sierra Nevada Corporation and SpaceX are working in partnership with NASA on spacecraft designs that meet these criteria under their Commercial Crew Integrated Capability agreements.

Thurston said each company is coming up with its own novel solutions for the best way to meet the needs of a spacecraft that docks with the station and then stays in orbit for seven months.

"There's no rock left unturned," Thurston said. "Some have started out with very extravagant environmental control and life support systems and as they're doing their studies, they're slowly figuring out exactly what they need and what they don't need."

With a new American spacecraft also offering another four to seven seats, the station can host more astronauts than its current complement of six. That means more science on the station since more people would be available for research duties.

"You never kept more on station than you could get off the station and back home," Thurston said. "It's why we staff that station the way we do. Now, you expand the crew capacity and then the crew and that really expands the amount of science you can do."