CAPE CANAVERAL, Fla. -- The year was 1959. Dwight Eisenhower was president; the nation's first commercial nuclear power plant was barely 2 years old; and the United States was still two years away from sending its first astronaut into space.
But in the Nevada desert a group of NASA scientists were test-firing a nuclear reactor called Kiwi that, they hoped, would some day power a manned spaceship to Mars.
"The people in the program all believed they could make it work," said Gary Bennett, now manager of nuclear-propulsion studies for NASA in Washington and a participant in later stages of the agency's reactor program. "I still believe it."
The reactor program folded in 1973, when money ran out, but NASA has never given up its dream of going to Mars. Now President Bush has set a goal of sending astronauts to the Red Planet by the year 2019, and his space agency wants to revive the effort to build a nuclear-powered rocket engine.
Such a rocket would be far different from the plutonium generator aboard the Ulysses solar that was launched last weekend by the shuttle Discovery. That device -- basically a battery filled with 24 pounds of radioactive material -- was designed to provide electricity for Ulysses' instruments and computers.
The engine for a Mars voyage would drive an entire spaceship, using a reactor similar to those that run commercial power plants or Navy ships and submarines.
NASA experts say such a rocket could be ready for a Mars flight by the year 2016, perhaps earlier. But building it could prove difficult.
One reason would be its estimated cost: $5 billion. Another would be the difficulty in finding a way to safely conduct ground tests producing radioactive exhaust. Another obstacle would be convincing the public that the rocket wouldn't endanger Earth once in orbit.
"Safety, reliability and environmental concerns are the biggest items we have to look at now," said Tom Miller, a nuclear-rocket project manager at NASA's Lewis Research Center in Cleveland. "They are all potential show-stoppers."
Critics warn that such a rocket would be much more worrisome than the plutonium that U.S. scientists have used to power not only Ulysses but seven other interplanetary probes and eight satellites since 1961.
"This would make the plutonium issue seem like child's play," said Henry Kendall, a physicist at the Massachusetts Institute of Technology and chairman of the Union of Concerned Scientists. "The needle just goes up to the end of the [danger] scale and stays there."
NASA is interested in building a nuclear rocket for the 49 million-mile voyage to Mars because such an engine would cut the yearlong trip in half. That would reduce astronauts' exposure to cosmic radiation, which scientists fear could penetrate a spaceship and endanger the crew.
"If cosmic radiation is a serious problem -- which some think it is -- we'll have to find a way to shorten the trip," said Jerry Grey, director of science and technology policy for the American Institute of Aeronautics and Astronautics in Washington.
Another benefit: A nuclear rocket would weigh much less than one carrying chemical fuel, such as liquid hydrogen. That would make it cheaper and easier to get the mission off the ground.
The heart of a nuclear rocket would be a reactor loaded with uranium-235, a substance that is not radioactive until it is activated in a process known as fission.
In one preliminary design known as a "nuclear thermal rocket," the reactor would heat gaseous hydrogen to superhot temperatures and shoot it through a rocket nozzle at very high speed to create thrust.
Such a rocket might be ready by 2016, NASA officials say.
Another design, known as a "nuclear electric rocket," would use the reactor to produce an electrical current to drive an engine and generate thrust.
That version could be ready sooner, perhaps by 2005, but it would shave little time off a Mars trip using ordinary non-nuclear fuel.
In either case the nuclear engine would be launched into orbit using a conventional rocket. There it would meet up with its spaceship and crew, about 800 miles above Earth. Only then would it be turned on and fired, reducing the risk of radioactive contamination.
"Until you run it hot, you don't have any danger," said Earl Van Landingham, deputy director of NASA's space technology office Washington. "That's the beauty of it."
A nuclear rocket would be fired twice: once to hurl the ship from Earth toward Mars and again to yank the ship out of its Martian orbit for the journey home.
Once the mission was over, the reactor would be left in a high, "graveyard orbit" above Earth.
That has opponents of nuclear power worried because from there a reactor could tumble back into Earth's atmosphere, break apart and, they fear, spread a cloud of radioactive dust.
Similar concerns apply to testing the engine on the ground.
The test-firing of the Kiwi reactor in 1959 was just one of 20 above-ground tests conducted by NASA at the Department of Energy's Nevada Test Site.