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JUPITER MOON PROBE GOES NUCLEAR
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| Three companies will compete to design NASA’s Jupiter Icy Moons Orbiter, the world’s first space probe to be powered by a nuclear reactor.
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Under NASA’s Proj-ect Prometheus, an array of contractors, Energy Dept. labs, and NASA field centers are rekindling work on space nuclear power and on some propulsion technologies that have lain dormant since the 1960s. In the coming years, four teams of experts—one from government and three from industry—will pick and choose from this portfolio of “core technologies” to propose designs for the world’s first nuclear- reactor-powered space probe: JIMO, the Jupiter Icy Moons Orbiter.
The “in-house” government team will pursue its design almost as if it were one of the competitors. But its real job will be to give NASA managers the knowledge they will need to choose intelligently from among three JIMO proposals to be offered by Boeing, Lockheed Martin, and Northrop Grumman, says Ray Taylor, the JIMO program executive at NASA headquarters. The agency hopes to launch the orbiter as early as 2011 as the first of many nuclear-powered probes to be built with technologies reborn under Project Prometheus.
At the moment, all that exists of JIMO is a NASA drawing showing the craft’s essential parts. Using a nuclear reactor as a power source will enable the spacecraft to operate where the Sun is too dim to illuminate solar arrays. A power conversion system will turn a fraction of the reactor’s heat to electricity, and panel-like radiators will vent the excess heat to space.
Propelled by electrically powered ion thrusters, JIMO will head for Jupiter to orbit three of its planet-sized moons—Callisto, Gany-mede, and Europa. A suite of science instruments, installed as far as possible from the distorting radiation of the reactor, will measure and photograph the moons.
Scientists hope to prove that Jupiter’s “icy moons” are in fact dynamic worlds of subsurface oceans and tides, as suggested by NASA’s Galileo probe in the 1990s. This would make them worthy of further exploration, perhaps even for the existence of life. If NASA managers had it to do over again, they probably would have named the mission the “icy worlds” orbiter, Taylor says.
And so, NASA field centers, contractors, and Dept. of Energy labs have begun exploring the technical options for JIMO’s major elements: its reactor, power conversion system, and thrusters. “This is a fascinating system engineering assessment looking at different reactor types, different power conversion types, and different spacecraft configurations,” Taylor says.
The in-house government team, headquartered at NASA’s Jet Propulsion Laboratory, consists of experts from JPL, NASA’s Glenn, Ken-nedy, and Marshall centers, and two Dept. of Energy national laboratories (Oak Ridge in Tennessee and Los Alamos in New Mexico). By law, it is Energy Dept. experts who must build nuclear reactors and handle nuclear fuel. Taylor points out that the in-house team should not be seen as a competitor or emergency backup to the industry teams.
“This is not going to be an in-house build. This is going to be an industry build for sure. There’s no fuzz on that,” he says. The parallel-track strategy will create “a very experienced and independent project office that can evaluate the results from the industry teams,” he adds. To ensure that all options are explored fully and fairly, Taylor says the government and industry teams will not be allowed to share information. “We want to maintain a very clear firewall so as not to have the government team results flowing around” among the contractors, Taylor continues.
Reactor designs
In 1993 the U.S. abandoned work on its last space nuclear reactor, the SP-100, after engineers had successfully tested its ability to initiate a nuclear reaction. An earlier U.S. space reactor, called SNAP 10A, has been circling Earth ever since it malfunctioned on its 45th day of operation in 1965.
Energy Dept. experts at Los Alamos and Oak Ridge are leading the effort to “recapture” the lessons of these and other nuclear programs, Taylor says.
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| After launch, JIMO will unfold an array of heat-shedding radiator panels. |
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The JIMO reactor must be small enough to fit inside a launch vehicle yet powerful enough to generate 100,000 W of electricity for the satellite, once it is safely away from Earth and the nuclear reaction is turned on. The hotter the reactor, the more electricity it can generate per unit of mass. “You need higher temperatures in a space reactor to get the power-to-weight ratio high enough,” Taylor says. Thus a space reactor like JIMO must run hotter than an Earth-based nuclear reactor.
One major focus of the space reactor work is the selection of heat-resistant refractory metals to contain the reaction. Another is the assessment of cooling strategies for efficiently and reliably delivering heat to the JIMO power conversion system, which will generate the electricity. The cooling options are:
Liquid-metal-cooled reactor. This system would cool itself by circulating liquid lithium, which would transport heat to the power conversion system. The SP-100 was a liquid-metal-cooled reactor.
Gas-cooled reactor. Gaseous helium or xenon would transport heat to the power conversion system.
Heat-pipe reactor. Liquid sodium or lithium would be heated from liquid to gas to deliver heat to the power conversion system. The heat-pipe reactor would use an evaporator to create the gas and a condenser to convert the gas back to liquid, Taylor explains.
The Oak Ridge/Los Alamos experts are exploring the reactor options exclusively for the government’s JIMO team. The industry teams are studying the same three options. At their discretion, the three industry teams can tap experts from Sandia National Laboratories in New Mexico, Argonne National Lab in Illinois, and the Idaho Engineering and Environmental National Lab in Idaho Falls, says Taylor.
Heat to electricity
At best, JIMO’s power conversion system will convert only 6-20% of its reactor’s heat into electricity. The residual heat will be vented to space through an array of radiators, most likely deployed along the spacecraft’s center spar.
Engineers must balance their desire for a more efficient system—which could mean one with moving parts—against the need for reliability. “This spacecraft is going to deep space. It’s not going to be hanging out in Earth’s orbit. The technologies have got to be looked at in the context of that,” Taylor says.
NASA has suggested to the industry teams that they begin by studying three options:
A Brayton system. Here a gas would expand because of the heat from the nuclear reaction; the resulting motion of this medium would turn a turbine attached to an electrical generator.
A thermoelectric system. The reactor would warm pieces of metal to create temperature differentials across them, a condition that causes electrons to flow. This system would have no moving parts and would be similar to the static conversion system used in the radioisotope thermoelectric generators aboard the Cassini Saturn probe and other NASA spacecraft, says an agency official.
A Rankine system. This would be a space version of the Rankine thermal cycle used in Earth-based electric power plants. A Rankine system would convert a liquid into a gas, which would turn a turbine to generate electricity.
A Rankine system has the greatest potential for improvements in efficiency, but because it requires transporting a medium in two phases, it will take longer to develop, says Taylor. “That’s a longer range thing that would be attractive for a human mission beyond the time of JIMO,” he says.
The least efficient system would be the thermoelectric, and the most efficient would be the Rankine.
Electricity to thrust
Propelling the orbiter will be electric thrusters that push the craft forward by accelerating ions out of nozzles.
The basic concept of propelling a science probe this way was proven by NASA’s Deep Space 1 spacecraft, which tested an experimental ion engine. Called NSTAR, for NASA Solar Electric Propulsion Technology Application Readiness, this engine used a finely meshed metal grid to electrically accelerate ions out a cylindrical nozzle.
Until Deep Space 1, scientists were afraid that the exhaust from an ion engine would corrupt science readings or harm instruments in unexpected ways. JIMO has a more challenging mission than Deep Space 1. Its thrusters would have to be more powerful, and they would need to operate for up to 10 years.
NASA managers hope to prove this capability by ground testing three competing ion propulsion concepts. Two of the engines have begun nonnuclear-powered testing at NASA field centers. Development of a third engine, however, has been stalled by legal hurdles associated with procuring a Russian-built ion engine for testing in the U.S. The three thruster rivals are:
Nuclear electric xenon ion system (NEXIS). On December 12, 2003, engineers at JPL tested this upgraded version of NSTAR for the first time under high-power and high-thrust operating conditions.
The engine was installed inside the same vacuum chamber where a nonflying version of the NSTAR engine had set an endurance record by operating for 30,352 hr (3.5 years), according to a description of the test released by NASA. The NSTAR engine, which was identical to the one that flew on Deep Space 1, was removed from the chamber after an internal debate in which some engineers had advocated running the engine until it failed.
The NEXIS engine ran on 20 kW of electricity, or almost 10 times the power of the NSTAR engine. Engineers expect to achieve JIMO’s 10-year operating requirement by replacing key metal components of NSTAR with components made of advanced, carbon-based materials.
High-power electric propulsion (HiPEP). In November 2003, engineers at NASA Glenn ran this new engine for the first time. Unlike NEXIS, HiPEP used microwaves to ionize gas, and accelerated the ions through a rectangular rather than cylindrical nozzle. “The rectangular shape, a departure from the cylindrical ion thrusters used before, was designed to allow for an increase in engine power and performance by means of stretching the engine. The use of microwaves should provide much longer life and ion-production capability compared to current state-of-the-art technologies,” according to NASA.
By using microwaves instead of electricity to ionize the fuel, engineers have eliminated the need for a hollow cathode electron gun, says Joe Nainiger, the energetics project manager at Glenn, whose office managed the test. Engineers feared these cathodes could become clogged with debris, which would be a key factor limiting a thruster’s lifespan. The tests at Glenn and JPL are encouraging, “but there’s still a lot of work to be done,” Nainiger says.
Bismuth-fueled Hall-effect thruster. Scientists from Stanford and JPL hope to acquire a version of this thruster that was built by Russia’s Central Scientific Research Institute of Machine Building, known as TsNIIMash. They would test the thruster inside a vacuum tank at JPL and use the results to build their own version as a candidate for propelling JIMO. U.S. scientists have said they want to test whether an ion thruster fueled by the heavy metal bismuth would be able to operate over a multiyear mission.
Work on that engine remains stalled, however. NASA managers have not yet cleared the legal hurdles associated with acquiring the engine from Russia under the terms of the U.S.-Iran Non-Proliferation Act. The act restricts U.S. dealings with Russia out of concerns over the country’s close technical relationship with Iran.
U.S. work on that engine “is not able to proceed until this is resolved,” Taylor says.
No late surprises
NASA managers are taking steps now to uncover surprises that might otherwise show up late in the development of JIMO. Officials from NASA Kennedy, who will be in charge of preparing the spacecraft for launch and affixing it to the launch vehicle, are already involved in the project through the JPL office, Taylor says. In addition, NASA technology managers are working closely with scientists to ensure that the capabilities they are developing are in synch with the expectations of JIMO scientists. Taylor and other managers attended a briefing given by the JIMO Science Definition Team at the December 8, 2003, meeting of the American Geophysical Union in San Francisco.
Until that meeting, Taylor says, technologists did not know the types of orbits the scientists hope to achieve around the three moons, and how those expectations could affect the thrust required for JIMO. “We didn’t know—should it be in equatorial orbit, or something different? It turns out, for determining the presences of an ocean it needs to be in a high-inclination orbit around the moons,” Taylor says, highlighting the importance of the project’s team approach.
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| Aerospace America March 2004 |
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