When it comes to attracting interest in new mission plans to far-out places in the solar system, it is often the wildly futuristic concepts that get the attention.

Antimatter propulsion, solar and magnetic sails all make great stories, but such futuristic concepts don't do anything to get humans out to the moon, or Mars, or to various local comets or asteroids within the foreseeable future.

With these futuristic technologies barely out of their conceptual phases, practical use of such far-out concepts for human space transportation is decades away at best.

	Images
	This diagram shows the basic travel plans of the Bimodal Nuclear Thermal Rocket Crew Transfer Vehicle (CTV) on a trip to Mars. Click to enlarge.
	This diagram outlines the design of one nuclear thermal rocket engine. Click to enlarge.
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So when planners at NASA begin to examine space-travel goals beyond low Earth orbit, beyond 2005 when the International Space Station is scheduled to be complete, they are faced with making bigger, brawnier and incredibly more expensive versions of the chemical rockets in use today.

Either that, or consider a demonstrated technology that was abandoned almost 30 years ago: nuclear rocket engines.

"It's continually talked about. Whenever you start seriously contemplating human missions back to the moon and Mars in an economical way with reuse potential, nuclear always comes to the foreground," said Stanley Borowski, a nuclear and aerospace engineer at NASA's Glenn Research Center in Cleveland, Ohio.

In the past few months, several NASA notables, including associate administrators Joe Rothenberg and Gary Payton, have mentioned publicly that nuclear power in space transportation deserves a closer look. The comments indicate that if public relations efforts can gain acceptance for the possibility, future interplanetary missions may include nuclear-power options.

Meanwhile, engineers at NASA centers and various other research institutions, including Los Alamos National Laboratories in New Mexico, have been working quietly in the background to design several such missions.

One system that holds promise is a concept for a Bimodal Nuclear Thermal Rocket, a mission design that uses nuclear reactors to produce thrust and electricity for a human-crewed mission to Mars. It was developed during the past three years by Borowski and Leonard Dudzinski, also an aerospace engineer at the Glenn Research Center.

The detailed mission design would send two cargo vehicles to Mars in 2011, followed by a crew-carrier that would leave Earth in 2014. Each of the vehicles would be launched in two parts aboard chemical rockets made of modified space shuttle-style rocket boosters.

The two-part vehicles would be assembled in orbit before the nuclear reactors are turned on to propel the spacecraft to Mars. A block of three small nuclear rockets capable of producing 15,000 pounds of thrust each would drive each of the vehicles. The reactor cores would provide plenty of energy to get the cargo and crew to and from Mars quickly, to brake into planetary orbit, generate electrical power, and even to produce artificial gravity during transit (see video clips at top right).

For 25 years, nuclear has been a dirty word, even in space transportation. Despite the fact that nuclear propulsion has consistently come up as one of the most-promising propulsion concepts for human missions beyond Earth orbit, little more than study has been done since the Nuclear Engine for Rocket Vehicle Applications, or NERVA, program was killed in 1972.

Started in 1959, and conducted vigorously throughout the 1960s by NASA and the Atomic Energy Commission, the program built and tested 20 nuclear-reactor rocket engines at the federal government's Nevada Test Site. The total cost of the program was $1.4 billion, a figure that is equivalent to about $7 billion today.

The rockets, all of which were operated in the open air at the Nevada Test Site, ranged in output from 50,000 to 250,000 pounds of thrust. In comparison, the liquid-fueled rocket engines clustered at the rear end of the space shuttle produce about 400,000 pounds of thrust each, while the combined jet engines on a Boeing 747 yield about 220,000 pounds of thrust at full takeoff power.

Advocates of nuclear-powered rocketry blame a small but vocal and vehement faction of activists for creating a public climate that has prohibited space agencies from flying nuclear-reactor rockets.

Bruce Gagnon of the Global Network Against Weapons & Nuclear Power in Space, based in Gainesville, Florida is active in the movement to keep space free of nukes. He argues that nuclear power is dangerous to the health and safety of workers in the nuclear industry, and to people around the world.

Moreover, Gagnon warns, the so-called "peaceful" uses of nuclear power in space such as nuclear Mars rockets are merely a cover to develop power systems that can be used for space-based weapons. Once developed under the guise of space exploration, he said, nuclear reactors could be used to drive dangerous space-based laser weapons.

"These rockets are the foot in the door, the Trojan horse, if you will, for the militarization of space," Gagnon said.

But Borowski and other nuclear and aerospace engineers call themselves pragmatists. The fact is that nuclear power can get human-crewed missions to the moon, Mars and elsewhere in the solar system faster, safer and cheaper than any other alternatives, they say.

Nuclear-reactor rockets, like the ones that would be used in the Bimodal Nuclear Thermal Rocket, conduct nuclear fission reactions -- the same kind employed at nuclear power plants -- in which uranium atoms are split apart, releasing tremendous volumes of energy. In a nuclear thermal rocket, this energy is used to heat hydrogen propellant, which is stored aboard the rocket as liquid in supercooled fuel tanks.

The strength of nuclear propulsion is that it is more efficient than traditional chemically-propelled rockets. "It is the next step evolutionary step in chemical propulsion and it has twice the propellant mileage of the chemical rockets that we currently use," Borowski said.

All rockets require fuel. Chemical rocket engines burn it, heating up the fuel and accelerating the combustion byproducts out a rocket nozzle. Nuclear thermal engines employ a very compact mass of nuclear fuel to release tremendous amounts of energy. That energy is used to heat lightweight hydrogen gas, and shoot it through a nozzle to get thrust. The nuclear reaction heats the hydrogen to much higher velocities than chemical combustion can.

"For a given amount of propellant then, we can either carry a lot more payload, or we can - for the same amount of payload - travel faster to our destination," Borowski said. "Or we can just decide to travel at the same speed as the chemical with the same payload and just require a lot less mass and maybe a smaller vehicle."

Advocates of nuclear powered rocket engines point out that at the time of launch, there is almost no radiation released from the nuclear reactors. The nuclear-powered rockets aren't used to get off the ground, just to get to and from Mars, to generate power during the trip, and to brake into Mars and eventually Earth orbit on the return trip.

Plans call for the vehicles to be launched from Earth on traditional chemical rockets. The nuclear reactors would only be turned on, or "made critical" once the vehicles are parked safely in low Earth orbit, about 250 miles (419 kilometers) above the surface.

Each of the reactors would contain 77 pounds (35 kilograms) of enriched uranium, a concentrated form of the nuclear fuel that is found scattered in various amounts across the surface of the Earth. Thus, at the time of launch, the reactors in a new nuclear rocket are no more dangerous than large pile of dirt, Borowski said.

Astronauts are among the most enthusiastic boosters of such a nuclear-powered mission. The nuclear thermal rocket has the [extremely important] advantage of being able to dramatically reduce trip times to and from Mars. This reduces the amount of time that astronauts are exposed to the dangerous solar and cosmic radiation that permeates space.

Compared to the radiation released from a well-designed, adequately shielded nuclear rocket engine, the radiation environment of space is tremendously more dangerous.

"The risk is much greater from the normal radiation environment from space - by orders of magnitude," said Roger Crouch, a former space-shuttle astronaut and senior NASA scientist. "The issue with nuclear engines and nuclear power sources is people are afraid of them. You're dealing with an area where people have a fear, but their fear is not grounded on a realistic assessments of the risks involved."

According to Crouch, the most viable proposals to get a crew to and from Mars safely, efficiently and relatively quickly are nuclear-powered. Plenty of NASA astronauts would volunteer for a 2-year round trip Mars mission, Crouch said, but they are more hesitant to consider trip durations that stretch much beyond that, the durations that some chemically propelled missions require.

A fast mission using nuclear thermal rockets could get astronauts to Mars in as little as 4 months, Borowski said. It would allow them a one- to two-month stay on the planet and then bring them back to Earth on a return leg that would take about eight months, getting them home in just over a year.

One of the great added strengths of the Bimodal Nuclear Thermal Rocket is that it can be used to generate not only thrust, but all the power that a crew needs during interplanetary travel. Once the crew-transfer vehicle escapes from Earth orbit and reaches speed on its trip to Mars, the engines are brought down to an idle. Their heat is routed through a generator to produce power for crew survival, high data-rate communications, and even a refrigerator to keep the liquid hydrogen fuel from boiling off into space. Because liquid hydrogen boils at minus 423 degrees Fahrenheit (minus 217 degrees Celsius), the loss of hydrogen propellant is a serious problem which forces most mission designers to carry a great deal of extra propellant to make up for the loss.

With nuclear reactors, though, there is plenty of energy to run a refrigeration system to keep the hydrogen cold. This greatly reduces the total mass of the vehicle. Nuclear reactors even provide enough power to create artificial gravity, a feature that should protect the astronaut crew from the physiological ravages of living in low-gravity conditions for extended periods.

Astronauts who spend months aboard the Mir space station return to Earth crippled from muscle atrophy, bone loss and a host of other problems associated with life in microgravity. U.S. astronaut Jerry Linenger lived aboard Mir for five months in 1997. He said it took him only a month to become completely accustomed to living aboard the station, but more than two years to return to full health after he came back to Earth.

A Mars mission would hold dubious promise if it were to land six crippled astronauts on the Red Planet, Borowski points out. The solution is to create artificial gravity through the centrifugal force of a spinning Mars-bound vessel.

The crew-transfer vehicle that Borowski and Dudzinski propose would have thrusters that could bring the craft into a controlled end-over-end spin. With the crew habitat module at one end and the nuclear-reactor core stage at the other, the craft would swing around a center of mass located near the inside end of the engine stage. The astronauts would feel a "downward" force pressing them toward the outside end of the habitat module. That force depends on the distance from the center of rotation and the speed of the rotation, so the force of artificial gravity can be controlled by the speed of the rotation.

On the way toward Mars, the craft would tumble at about four rotations per minute to create a gravity similar to that the astronauts would experience on Mars, which is about one-third that of Earth's. On the way back from the planet, the speed of rotation could be increased to help the crew prepare for the higher gravity of Earth. At six rotations per minute, the crew would feel a force equivalent to about 80 percent of Earth's gravity.

A further advantage of the nuclear thermal rocket is its reuse potential. The core stages of the cargo craft would be used only once, and then jettisoned into deep space where they would be lost forever, Borowski said. The crew carrier, however, could be left in Earth orbit once the astronauts return, refueled with liquid hydrogen, and sent on a second trip to Mars or some other destination. After a few trips to Mars, the reactor core would be used up, and that stage could be disposed of in deep space, where it would have less than a one-percent chance in a million years of re-encountering Earth. "We view the probability of Earth re-encounter as non-existent," he said.

While a Mars mission could be completed with chemical propulsion, the size and cost are prohibitive, Borowski said, and the vast majority of the cargo would be propellant.

"It would take a lot to launch and assemble, and to my way of thinking it would be dead-ended, because you'd be throwing away the vast majority of all the pieces," he said.

"That is a prescription for a flags-and-footprints program that will quickly lead to termination rather than providing the technology and the in-space transportation that we need, that allows humans to expand into space economically, that has reuse capability and can ultimately lead to humans settling and colonizing the moon, Mars and planets beyond."