In great secrecy, the Pentagon is developing a nuclear-powered rocket for hauling giant weapons and other military payloads into space as part of the "star wars" program.
The goal is to build a special type of nuclear reactor that would power engines far more energetic than any rocket engines now in use, allowing very large and heavy payloads to be lofted high above the Earth.
The program was disclosed by the Federation of American Scientists, a private group based in Washington that has opposed the "star wars" anti-missile program and some uses of space reactors. The existence of the secret effort was confirmed by internal government documents obtained by the New York Times.
Henry Cooper, director of the Strategic Defense Initiative Organization, as the "star wars" effort is officially known, declined to comment on the program or on the federation's criticisms. "I won't be drawn into a discussion on that," Mr. Cooper said in a brief telephone interview.
The "star wars" program has long envisioned launching hundreds of large weapons and sensors too heavy for conventional rockets, including massive lasers, particle beams and homing rockets. The operation of some of the envisioned lasers would require tons of exotic fuel.
The secret rocket program was started in the 1980s as part of the Pentagon's SDI plan to send armaments into space for the destruction of enemy missiles. The nuclear-powered rocket is still in its early stages, the documents indicate.
It would be vastly different from the nuclear power packs used in NASA's deep space probes in that it would be a true reactor that splits atoms rather than a battery-like device that simply uses natural radioactive decay.
Engineers have fabricated and tested reactor fuels at Sandia National Laboratory in New Mexico and have drawn up plans for testing engines on the ground in Nevada and for flying a prototype rocket through the Earth's atmosphere.
About 1,500 people in the federal government have security clearances allowing them to know about the program, according to Steven Aftergood of the Federation of American Scientists. He said the federation's information on the effort came from federal and industry participants, who spoke on the condition of anonymity.
While currently run by the Defense Department, the effort is being quietly evaluated by the National Aeronautics and Space Administration, which is considering using nuclear reactors to power a manned mission to Mars.
The allure of nuclear rockets is that, in theory, they pack more punch than conventional ones, whose engines are energized by chemical reactions that often involve the explosive burning of oxygen and hydrogen.
By contrast, the reactor in a nuclear-powered rocket would heat liquid hydrogen to very high temperatures and blast it out of the engine's nozzle in a gaseous vortex, creating a stupendous thrust.
The standard measure of rocket performance is known as specific impulse, the length of time a pound of a given propellant will produce a pound of thrust. The higher the specific impulse, the more dazzling the engine. The space shuttle's main engines -- the most efficient rocket engines in the world, fueled by liquid hydrogen and liquid oxygen -- have a specific impulse of 455 seconds.
By comparison, the planned nuclear engines would have a specific impulse of more than 900 seconds.
Mr. Aftergood said industry studies indicated that a nuclear rocket could lift payloads of about 70 tons into a low orbit over the Earth. The Titan 4, the nation's largest rocket, can lift payloads of up to 20 tons.
But he said an accident during ascent could pose a severe radiation risk on Earth, and he argued that the program should be killed.
"It's wasting taxpayers' money on an idea that is stupid and possibly dangerous," Mr. Aftergood, a senior analyst with the federation, said in a telephone interview.
In the past, some federal experts have argued that the risks of nuclear rockets are minimal and that the benefits are potentially great.
For the secret program, whose code name is Timberwind, the Pentagon is also considering a flight test of a prototype rocket, largely over the open ocean near Antarctica.
The Federation of American Scientists opposes the use of nuclear power in Earth orbit because of the risk of a radiation release and would instead limit its use to deep-space missions of scientific exploration.
Mr. Aftergood said one danger posed by the program was that the rocket would put out a "cloud of radioactive material" from its exhaust into the Earth's atmosphere. Some experts have argued that any such releases would be harmless.