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NASA eyes nuclear rockets to reach deep space

By Peter N. SpottsStaff writer of The Christian Science Monitor / February 28, 2002

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.

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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."

A railroad through space

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.