In a space reactor, the fuel elements would be surrounded by neutron-reflecting materials. Without the reflectors in place, there aren’t enough neutrons bouncing around to cause a chain reaction. So engineers would devise a system to move the reflectors into place to start the reactor, then back them out to stop it. With the reflectors in safe mode during launch, the uranium fuel would be no more than “marginally radioactive,” Wahlquist says. If the rocket exploded on the launch pad or suffered some other catastrophic failure on its way to orbit, “the rocket fuel would be more toxic than the uranium,” he says.
JIMO and its reactor would be launched on a conventional rocket to an altitude of just over 600 miles; only then would the reactor be turned on. At that altitude, say NASA officials, if something went wrong after controllers start the reactor, it wouldn’t pose a threat to people on the ground. SNAP 10A, a reactor-powered spacecraft launched by the U.S. military in 1965, has been circling Earth ever since malfunctioning on its 43rd day of operation. About 1,000 years from now, its orbit will have decayed to the point where the spacecraft will reenter the atmosphere. By then, its radiation will have dissipated, and “we think it will be [just] a hunk of metal,” Newhouse says.
DOE expects to build the JIMO reactor at one of its facilities, most likely the National Environmental and Engineering Laboratory in Idaho or the Argonne National Laboratory in Illinois. Wahlquist says it won’t be a simple matter of resuming work on SP-100; other designs will also be considered. For example, SP-100 used liquid lithium metal for its coolant, but Prometheus may use a light gas like helium, or vapor transported through heat pipes. Whatever the choice, the reactor will have to be as light as possible, a requirement for any hardware that is space-bound.
Once the JIMO reactor is turned on, the heat it produces will be converted to electricity to drive a new type of thruster that propels the craft with a glowing stream of ions. “This is not a nuclear rocket,” Newhouse says, still chafing from an article in the Los Angeles Times last year that failed to distinguish Prometheus’ nuclear electric engines from more advanced—and controversial—nuclear thermal rockets, which would circulate hydrogen through a reactor and spew the exhaust out a nozzle. JIMO’s reactor is only a power source, not part of the engine itself.
Stanley Borowski, a 15-year veteran of the agency’s Glenn Research Center in Ohio and unofficial keeper of the nuclear flame at NASA, has another nit to pick. The word “is pronounced ‘nu-clee-ar,’ ” he says. “It’s not ‘nook-u-ler,’ which still a lot of people say.” After SP-100 was scrapped and all talk of nuclear-powered Mars missions ended in the early 1990s, Borowski, a nuclear engineer with a Ph.D., retreated into the bowels of the Glenn center, where he continued working on low-level internal studies. Now his field is hot again. The budget plan that NASA sent last summer to the White House Office of Management and Budget, where O’Keefe used to work, included a stepped-up nuclear program. Having seen official excitement rise before, only to fade away quickly, some nuclear proponents were skeptical that the new plan would go anywhere. “Quite frankly, I didn’t think we had a ghost of a chance,” Newhouse says, “but it was approved.” Prometheus was born.
Engineers Dave Manzella and Rob Jankovsky bend down to look through a porthole at the base of a white schoolbus-size vacuum chamber at the Glenn center. Inside the tank, a circular rocket engine about the size of a large pizza gives off a steady, pale blue glow, like a TV in a darkened room. The only sound is the hum of the chamber itself. No need to hide in a blockhouse from the thundering rocket blast. In fact, the thrust from this engine is imperceptible to all but the sensitive disk it’s mounted on.
It will be just such an engine, powered by a nuclear reactor, that pushes JIMO toward Jupiter and lets it maneuver through the Jovian system with a new kind of nimbleness. The thrust produced by ion propulsion—the slow, steady expulsion of ions to accelerate a spacecraft—ranges from a fraction of an ounce to three pounds, minuscule by the standards of most liquid-fuel rockets. Cassini’s twin thrusters, for example, produce 100 pounds each. But, explains Jankovsky, chief of Glenn’s onboard propulsion branch, the ion engine would be so fuel-efficient that it could run continuously, building up speed and eventually outpacing chemical rockets.
When the Galileo probe approached Jupiter in 1995, it fired its liquid-fuel main engine for 49 minutes to slow down so Jupiter’s gravity could pull it into orbit. After that big burst, Galileo had only enough fuel left for minor tweaks to its trajectory. Most of the subsequent course correction came from carefully timed gravity-assisted swing-bys of the Jovian moons. “If you look at a chart of the Galileo orbits, it looks like a line drawing of a flower where each orbit represents a petal of the flower,” explains Ron Greeley, a planetary geologist at Arizona State University.
Mission designers measure such maneuvers in terms of delta-V, or change in velocity—basically, how much energy is needed to change a spacecraft’s speed and direction. Cassini carries enough fuel to provide a total of 6,500 feet per second of delta-V over the lifetime of the mission. JIMO may have 30 or 40 times that. “With JIMO, we’ll orbit Callisto, then slip over and orbit Ganymede, and finally over to orbit Europa,” says Greeley. Such dramatic, energy-demanding orbit shifts were well beyond the capability of earlier planetary spacecraft, because the trajectories would have required many times more liquid fuel than they could affordably carry.
The efficiency of a rocket is generally given in terms of specific impulse, measured in seconds. A typical planetary spacecraft thruster might have a specific impulse of 300 seconds. “People who build chemical rockets would kill for a couple extra seconds of specific impulse,” says Jankovsky. With JIMO’s ion drive, NASA engineers hope to achieve 4,000 seconds.