The scenario is now just a gleam in an engineer's eye: An ambitious mission to the outermost reaches of the solar system is ready to leave Earth orbit. After a flawless launch, a final rocket motor ignites. When it falls away spent after a few minutes, ground controllers check the heading of the craft, and with a punch of a button, activate a nuclear reactor the size of a small trash can.
The reactor represents NASA's technological declaration of independence from gravity as a tool for propelling interplanetary spacecraft. Whereas today, a trip to the outer solar system relies on five to 10 minutes of burning chemicals and months or years of coasting, nuclear propulsion holds the promise of faster, more direct, more experiment-packed missions to places where sunlight is too feeble to power spacecraft. Indeed, some say that manned missions to Mars and beyond are unthinkable without nuclear propulsion.
If the mission is hypothetical, the technology is not. Earlier this month, the National Aeronautics and Space Administration (NASA) announced that its budget proposal includes $125.5 million to explore the use of low-power reactors as part of the propulsion systems in new spacecraft. One prototype reactor already has been built by researchers at the Los Alamos National Laboratory in New Mexico.
The proposed nuclear program also aims to develop a new series of generators that converts decay heat from small amounts of plutonium into electricity. Similar devices have already flown on a range of space missions, such as the Voyager spacecraft, the Galileo mission to Jupiter, and the Cassini-Huygens mission, currently en route to Saturn.
From NASA's perspective, harnessing mini nuclear-power plants with electric propulsion units such as an ion-drive motor is the next logical step. Yet some worry about safety and suggest the program could also open the way for reactor-powered weapons in space.
Plans for nuclear rocket motors have been around since the mid-1950s. A series of them were built and successfully tested at the Nevada Test Site until the end of the Apollo program.
But these proved too expensive to use to send humans to Mars, one of the next-step ideas after Apollo, and public sentiment began to turn against nuclear energy. The program ended in the early 1970s.
Now, however, the political winds have shifted, if only slightly. The Bush administration has signaled that it is more open to nuclear power than were its predecessors. And the technology to take advantage of reactors in space has advanced.
For 40 years, science packages have ridden to distant planets on the brief punch from chemical rockets. It's like exploring the West with covered wagons, NASA officials say.
"You accelerate for 5, 10, or 15 minutes, then you stop and coast," says Ed Weiler, NASA's associate administrator for space science. "Occasionally, you're in the right spot for a little boost" from another planet's gravity, "but it's not always there. That's not the way to do exploration."
Instead, he says, it's time to begin running a railroad to the solar system, using spacecraft motors that constantly churn out small amounts of thrust, but build speed over time to make direct flights possible and shorten travel time. This would allow researchers to launch when they want to, not merely when the planets are in proper alignment.
The shift to nuclear propulsion is vital if solar-system research is to advance, argues Colleen Hartmann, NASA's director of solar-system exploration. "In terms of solar-system exploration as a whole, we're getting close to the limits of chemical propulsion. Now that we've visited every planet but Pluto, the next step is to orbit the planets and conduct full surveys."
Nuclear propulsion would give a particular boost to studying objects in the outer solar system, where sunlight is far too feeble to power a craft. Scientists would like to put an orbiter around Jupiter's moon Europa, as well as other Jovian moons. Neptune and its moon Triton present alluring targets for longterm studies, as does Uranus.
Nuclear propulsion also could allow scientists to conduct detailed studies of the Kuiper Belt - a chance they would jump at. Discovered in 1992, the belt lies beyond Neptune and is made up of at least 70,000 objects, many with diameters in excess of 100 kms (62 miles). The objects are thought to be the construction debris left over from planet formation during the solar system's first few million years. "To have any hope of achieving that vision, you really have to go nuclear," Dr. Hartmann says.
Nuclear propulsion is not a panacea, cautions Wesley Huntress, a former NASA official who heads the Geophysics Laboratory at the Carnegie Institution of Washington.
He notes that chemical propulsion and solar-electric systems are likely to remain the preferred approaches for conducting studies from Earth orbit or for visiting the inner planets. Still, he says, "I'm delighted to see this back on the table. It opens tremendous possibilities."
The notion of putting nuclear reactors on any type of spacecraft raises the hackles of some antinuclear activists. They point to mishaps in the 1970s and '80s, when two Soviet nuclear-powered reconnaissance satellites fell to Earth. One landed in the Indian Ocean. But Cosmos 954 crashed near Great Slave Lake in the Northwest Territories, spreading radioactive debris across nearly 50,000 square miles of the Canadian Arctic.
"When you look at the average failure rate for rockets, eventually, you are going to have a problem," says Bruce Gagnon, secretary and coordinator for the Global Network Against Weapons and Nuclear Power in Space, based in Gainesville, Fla.
He also is concerned that proposals for low-power nuclear reactors in space represent the leading edge of Pentagon efforts to move toward higher-power space reactors to energize space-based weapons.
He says he prefers to focus on solar-electric craft, to develop the technology to the point where it can be used in deep space. There's no rush, he suggests. The planets "aren't going anywhere," he points out.
Others counter that no developments in the wings are likely to allow solar energy to power a mission to the outer planets
And the planets are too going somewhere: Their positions relative to Earth are constantly changing, in some cases closing a window for exploration that won't reopen for another 248 years.
They add that if the military were interested in the types of reactors NASA has in mind, it would already have developed them. Moreover, they say, safety would be a paramount goal.
"Safety will be Job 1," says NASA's Weiler. New reactors and RTGs [radio isotope thermo-electric generators] must survive any bad-day scenario."
Is it possible to design a nuclear reactor for space exploration that isn't too hot to handle?
Seven years ago, David Poston decided to find out.
With a bit of discretionary money from his lab director's budget, the Los Alamos National Laboratory physicist and colleagues have pulled together a mini nuclear-power plant that they say could keep a craft humming long after it reached the outer solar system.
Although the reactor has not been fully activated, or taken "critical," it has passed several key tests during the past four years, highlighting its potential, says Dr. Poston, leader of the lab's space-fission power team.
Dubbed the SAFE-400 reactor, the project came together "pretty much in our spare time," Poston says.
The reactor's core, about the size of a small trash can, consists of uranium fuel surrounding a set of pipes that circulate gas through the core. Energy released in nuclear chain reactions would heat the gas, which can be fed into one of several types of systems to produce electricity.
That electricity would power a spacecraft's instruments as well as run advanced units such as ion-drive motors, which build momentum by constantly expelling electrically charged particles from its exhaust nozzle.
The core's small size contributes to what Poston calls its "passively safe" design. Standing on its own, the reactor can't go critical because neutrons from the decaying uranium are more likely to escape the core than they are to strike another nucleus and start a chain reaction. Indeed, the reactor requires special shields to keep the neutrons corralled for use.
The system could be engineered, he says, to move those shields into place only after command from the ground. Thus the likelihood of a launch accident activating the reactor would be vanishingly small.
Moreover, he adds, the inactive core emits so little radiation that a person would have to hug it for 84 days to receive a dose of radiation as large as a typical chest X-ray.
So far, Poston says, his team has tested the 400-kilowatt reactor at NASA's Marshall Space Flight Center and at CalTech's Jet Propulsion Laboratory. In lieu of chain reactions to generate heat, the team substituted electrical heating elements to achieve the operating temperatures the reactor is designed to reach. Tests included connecting the reactor to an ion-drive motor at JPL.