A $125-million initiative for developing the technology, which has frustrated scientists and engineers since the 1950s, was quietly inserted into the proposed fiscal 2003 NASA budget, which was unveiled this week. The space agency's plan calls for a $1-billion program over five years, a project that would include significant roles for the Jet Propulsion Laboratory in Pasadena, Glenn Research Center in Cleveland and Marshall Space Flight Center in Huntsville, Ala.

Such spaceships would have small nuclear reactors, which would give the engines greater thrust and virtually unlimited fuel supply to travel to the farthest reaches of the solar system.

Using nuclear technology would in theory slash a trip to Mars and back by more than half from about two years to less than a year, for instance, and alleviate lingering concerns with the health effects of long-duration space travel, NASA officials said. Astronauts who occupied the former Soviet operated Mir space station for months at a time suffered from muscle atrophy, bone loss and other crippling effects of prolonged exposure to micro gravity.

With nuclear power, "missions will be able to speed through the outer reaches of the solar system, at speeds as much as two times faster than is possible even with the most sophisticated space probes available today," NASA officials said in its budget proposal.

The technology will "allow NASA to consider more ambitious possibilities involving missions that could travel from one interesting planet, moon or comet to another for a close-up, in-depth study."

The proposed initiative was lauded by astronauts and astronomers but slammed by antinuclear activists.

"We welcome the proposal to develop nuclear power and propulsion technology to make the entire solar system more accessible with much shorter flight-times and more powerful investigations of the planets," said Wesley T. Huntress Jr., president of the Planetary Society. "These developments will revolutionize space exploration in the same way that the Navy was revolutionized by nuclear power."

But Bruce Gagnon, secretary for the Global Network Against Weapons & Nuclear Power in Space, said that in addition to potential health concerns from radiation exposure, the NASA initiative represents the Bush administration's covert move to develop power systems for space-based weapons such as lasers on satellites.

"It's our position that just like missile defense is a Trojan horse for the Pentagon's real agenda for domination of space, NASA's nuclear rocket is a Trojan horse for militarization of space," Gagnon said.

Donald Savage, a NASA spokesman, dismissed the argument.

"This is a solar system exploration program," he said. "This is a scientific program and that's what we are involved in."

Still, sensitive to the criticism and safety concerns, NASA officials stressed that the nuclear propulsion systems envisioned by the agency would be used only for traveling through space outside Earth's orbit. They would not be used for launching rockets into space.

In one possible scenario, the spacecraft would be assembled in space and the reactors would be turned on only after leaving Earth's orbit. The reactor and other components would be ferried into space by conventional rockets that use chemicals such as hydrogen and oxygen for combustion.

Under the so-called Nuclear Systems Initiative, NASA proposes funding three programs including developing nuclear power systems to power on-board equipment such as sensors to survey planets and instruments to control the craft and communicate with Earth.

Another major element would be to develop a nuclear propulsion system that would use a small nuclear reactor to generate electricity and propel ionized or plasma gas out of the rocket nozzle to provide thrust. The reactor would produce enough heat to generate electricity and ionized gas.

One advantage of a nuclear propulsion system is that it can generate significantly more energy for almost unlimited duration compared with a conventional chemical combustion engine. A soft-drink can full of uranium, for instance, yields 50 times the energy contained in the Space Shuttle's massive external tank, according to NASA.

The reactor would be co-developed with the Department of Energy, which has been looking for ways to maintain its know-how in nuclear power development.

More immediately, NASA envisions manufacturing new radioisotope thermoelectric generators that would provide power for long-duration exploration of Mars.

The system would allow the Mars Smart Lander, scheduled for launch in 2009, to scout sites for future missions.

It would be able to operate on the surface for years instead of months. The currently available solar cell system would become obscured by dust from the planet's atmosphere.

NASA officials said a small, rudimentary nuclear electric propulsion system has been used on exploratory spacecraft such as Deep Space One to help propel it in conjunction with solar panels. But such systems would not be sufficient for the kind of missions envisioned for the future.

But so far, developing nuclear powered rockets has been a frustrating endeavor and has remained in the realm of artist's conceptual drawings.

It has not been for lack of trying, however. Even American rocket pioneer Robert Goddard concluded as far back as in 1907 that nuclear propulsion would be essential to interplanetary exploration.

Emboldened by how nuclear subs revolutionized naval warfare, aerospace engineers began pursuing nuclear propulsion for rockets in earnest in the 1950s. And in 1959, NASA, with the then-Atomic Energy Commission, began a program to develop a nuclear rocket. Known as the Nuclear Engine for Rocket Vehicle Applications, the program eventually built and tested 20 nuclear-reactor rocket engines. The engines never produced the kind of thrust they had hoped for, however.