There are no electrical outlets on the moon. No power cables either, no transmission towers, no grid, no generating plants.

So when astronauts arrive sometime after 2020 for long-term occupation, they'll have to bring their power system with them.

During the three-day stays of the Apollo era, batteries and fuel cells did the job. But NASA is planning much longer and more ambitious missions this time around, with crews of at least four living and working on the moon for weeks or months at a stretch. They'll need electricity for everything from shavers and computers to heaters and lights for the long, frigid lunar nights.

Engineers at NASA's

in Cleveland are leading work on a possible solution: an ultra-compact nuclear power plant that can run for eight years or longer without maintenance, and can make more than enough electricity to meet the daily needs of the average American house.

The whole rig could fold up to fit in a tractor-trailer with room to spare. The heart of the system is a reactor no bigger than an office trash bin.

"Unfortunately, we can't send up a big extension cord from Earth," said Lee Mason, a Glenn mechanical engineer and the principal investigator for the fission surface power project. "Our job is to bring up the [electrical] outlet for the crew, and all the things that have to go behind that."

Project members want to prevent a repeat of the Apollo 13 crisis in 1970, when the command module, its fuel cells crippled by an explosion, almost ran out of power to keep the crew alive as they struggled to return to Earth.

"We don't ever want to put astronauts in that situation again," said Don Palac, the fission power project's manager. "We want them to have an excess of power – what we call a power-rich environment. We think we owe that to future space explorers."

The lunar outpost's design is still evolving, and NASA's overall human space exploration program is in flux as the White House and Congress wrestle with the cost of building new spacecraft to send astronauts to the moon and Mars.

Space agency planners won't decide for several years yet what kind of facilities astronauts will occupy while on the moon's surface.

A pair of internal reviews next year of NASA's lunar exploration strategy, along with input from the space agency's international partners and data from a NASA satellite looking for evidence of frozen water on the moon, will help determine the outpost's scope, said Pat George, the Glenn-based manager of NASA's lunar surface power systems.

That decision, in turn, will dictate how much electricity lunar explorers need, and how it will be generated.

One scenario is to send crews to multiple landing sites, using each landing craft as a temporary base and relying on rovers to explore nearby surroundings.

Another concept involves picking a single location of major scientific interest – possibly the

, where permanently shadowed impact craters deeper than the Grand Canyon may hold frozen water – and setting up a large, semi-permanent habitat.

Visiting astronaut crews would swap in and out, as they do now on the International Space Station. Initially, they would build living and working quarters. Later they would conduct science experiments, and mine the surface for rocket fuel ingredients, oxygen and raw building materials.

The larger, busier and more permanent the outpost, the greater its power demands. NASA figures it will need at least a continuous 35 kilowatts to operate equipment, keep batteries charged and maintain a shirtsleeve environment in a place where there's no air, and temperatures swing from daytime highs hotter than boiling water to nighttime lows of minus 300 degrees.

Solar power seems an obvious choice. It's a well-understood technology in use on many spacecraft. Sunlight collection doesn't pose a problem; lunar days are nearly a month long, and there are no clouds to block the sun's rays.

But nights on the moon are long, too – about 350 hours long. Some areas near the poles never get direct sunlight at all. During lengthy dark periods, an outpost would have to get by on battery power. Batteries are heavy, and every pound on a cargo rocket to the moon costs big money. They'd also have to be able to reliably charge and discharge many hundreds of times.

So NASA is studying options in addition to solar, and has turned to the Glenn center because of its expertise in designing space power systems.

About 10 Glenn engineers, working with colleagues at the U.S. Department of Energy, NASA's Marshall Space Flight Center in Huntsville, Ala., and industry partners, have a preliminary design of a lunar nuclear power system and are already testing some of its components.

The nuclear micro-plant won't come cheap. Team members estimate it will cost NASA $1.5 billion to fully develop and manufacture the first production model if the space agency chooses to go with nuclear power on the moon.

That's about 1 percent of the $119 billion price tag through 2020 for NASA's overall Constellation program to design and build new rockets, spacecraft and hardware for exploring the moon. "In that context, this [fission power] system is pretty small to provide an important commodity," Mason said.

Here's the Glenn-led team's basic plan: The small reactor would be buried about 6 feet deep in the lunar soil and shielded with a plug made of boron carbide, a material that blocks radiation as effectively as lead but is much lighter.

Splitting uranium atoms in the reactor's core would release energy in the form of heat, as much as 1,200 degrees. A couple of gallons of liquid metal – probably sodium or a sodium/potassium mix – piped around the core would transfer that heat energy to four Stirling engines mounted on a metal truss above the reactor.

The Stirling engines

, driving an alternator that makes electricity. Together, the engines should put out 40 kilowatts.

Waste heat would be absorbed by coolant and pumped through a series of radiator panels that unfurl from the truss like giant bat wings. The fully extended panels would be 100 feet long, but when folded they're less than 5.

The whole system would weigh 11,000 pounds, about as much as a fully armored Humvee. NASA's Altair lunar lander conceivably could haul two at a time to the surface.

"The idea is to deliver [the nuclear power plant] in this nice, compact, stowed configuration, and once you install the reactor in the hole, you can deploy the radiators," said Mason. "You can do it remotely from Earth, or from the habitat on the moon. It's meant to be a very simple setup."

The 40 kilowatts of electricity is a fraction of the yield of a typical commercial nuclear plant on Earth. The tiny lunar reactor is capable of more; during its eight-year lifespan it will use up less than 1 percent of its fuel. But the designers have sacrificed greater power output for simplicity and safety.

"The DOE guys call this a dog of a reactor," Palac said. "It's sort of motoring along at idle, practically, and you don't have to watch it."

To further ensure the lunar fission plant's reliability, the Glenn and Marshall centers and DOE are testing parts of it under the harsh operating conditions it would encounter on the moon.

In one Glenn lab, pressurized 440-degree water shoots through nine titanium pipes like those that would shed heat from the power plant's radiators. The heat pipes are as long and thin as pool cues. The 24-hour-a-day durability test has been underway for three years, and will last at least another eight.

Nearby, in a

the size of an airplane cabin, Glenn engineers recently set up one of the radiator panels, then pumped out the chamber's air and dialed down the thermostat to minus 150 degrees. Despite the brutal, airless cold, the radiator panel and its plumbing worked properly, getting rid of the required amount of heat.

In two years, Glenn engineers will run a much larger-scale test. They'll assemble a working portion of the power plant inside a vacuum chamber. The heat will come from an electric-powered dummy reactor, not the real thing. But the rest of the rig – the liquid metal coolant, two Stirling engines and six radiator panels – is supposed to function like it would on the moon, cranking out about 10 kilowatts. The experiment will judge how well the components work together while simulated conditions in the chamber switch from lunar day to night.

Future testing of the nuclear reactor would not take place at the Glenn center; it would be done at one of the DOE's labs.

The proposal for a nuclear-powered lunar outpost isn't NASA's first foray into using fission reactors in space. During the 1960s, the space agency worked with the Atomic Energy Commission and a company called Atomics International to develop compact reactors that could power satellites.

The program was called

, or SNAP. On April 3, 1965, a rocket carrying a small reactor, SNAP 10A, lifted off from California's Vandenberg Air Force Base. For 43 day as it circled the globe, the reactor steadily churned out more than 600 watts of power. An electrical system failure prompted the reactor's shutdown. It remains in orbit, the only such nuclear reactor flight test the government has conducted.

NASA acknowledges the political sensitivities and safety issues of launching a nuclear-powered device like the proposed lunar power plant. According to declassified documents, the president would have to approve such flights.

Protestors unsuccessfully tried to stop NASA's 1997 launch of Cassini, a Saturn probe carrying 72 pounds of radioactive plutonium fuel. (The plutonium generates a tiny amount of electricity from the heat of nuclear decay, not fission.)

"We're not naive to think that there won't be people who are concerned" about the possibility of sending a fission reactor to the moon, said Mason. The proposed design locks down the reactor's controls during flight, preventing a nuclear chain reaction from starting unintentionally, he said. "There is no launch accident scenario that would cause this reactor to go critical and start producing radiation."

The radiation level of the reactor's individual uranium dioxide fuel pins is low enough that they can be safely hand-held, Palac said. If the rocket carrying the reactor exploded, its nuclear fuel would be dispersed, he said, and the remnants, although they would require cleanup, would not exceed normal background radiation levels.

Palac said he hopes people will react to the project like his wife's aunt, a member of several environmental groups. He'd been worried about her response to his research, and described it obliquely as developing fission surface power for the moon..

"She said, 'Oh, you mean nuclear,'" Palac recalled. "She said, 'You know, nuclear power has tremendous potential to be a solution to our global energy crisis, and how wonderful you're taking it to outer space, where I'm sure it's even more useful.' I was blown away by that."