EIGHT years from now a planetary probe could boost itself out of Earth’s orbit, fire up its fission reactors and head off on a nuclear-powered quest for alien life beneath the crust of one of Jupiter’s icy moons, Europa. This bold $3 billion mission will be part of NASA’s Project Prometheus, a huge effort announced in February that will develop a new generation of nuclear systems for missions to the outer planets. The culmination of Prometheus could even see the first astronauts set foot on Mars.
Nuclear propulsion offers obvious advantages over space travel’s workman-like chemical rockets. With a plentiful and controllable source of thrust, nuclear propulsion could transform space missions from one-way trips with preset goals into voyages of genuine exploration. Ground control could change the course of a mission on a whim and let craft take unscheduled detours to study planetary features they liked the look of.
But even as space scientists and mission planners are cheered by the prospect of the project, it is likely to spark a massive protest from those who worry about the safety of launching radioactive material into space. At least four nuclear-powered craft have crashed to Earth since the 1960s. And in the wake of the Columbia disaster, Congress may prove keener to bolster human space flight than embark on a new range of ambitious missions to the distant reaches of the Solar System.
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Ironically, in 1953 Werner von Braun, designer of Saturn V – the chemical rocket that finally put men on the Moon – predicted nuclear rockets within a decade. But high costs, and protests from those worried by the danger of an accident at launch have mostly kept space a nuclear-free zone.
And that’s not the best news from the standpoint of space exploration. Certainly it will be expensive to develop but nuclear propulsion and power generation has by far the greatest potential to get probes to their targets quickly and cheaply and to provide robotic explorers with a plentiful supply of electricity once they’re there. “It’s straightforward,” argues Raynor Taylor, the administrator in charge of the Jupiter Icy Moons Orbiter and formerly the administrator in charge of NASA’s Nuclear Systems Initiative. “To explore the Solar System the chemical and solar power sources we have are not adequate.”
Even though the smoke and fire of a launch looks dramatic and a chemical rocket can generate millions of newtons of thrust, the hydrogen and oxygen in a rocket booster contain relatively little energy for their weight. With uranium on the other hand, a fist-sized chunk would be enough to kick out up to 50 times as much energy as one of the shuttle’s external fuel tanks. And rather than releasing it all in one go, this source of energy can be switched on and off at will.
Nuclear energy also makes sense for powering instruments and communications, especially once you get beyond the orbit of Mars where the intensity of sunlight falls off dramatically. By the time you reach Saturn, solar energy is only about 1 per cent as strong as it is at Earth’s orbit.
So far a few dozen US and Soviet satellites and space probes, including Apollo, Pioneer, Galileo and Cassini, have used radioisotope thermoelectric generators, devices that generate electricity from the heat released by the radioactive decay of plutonium-238. NASA’s Galileo space probe, for example, uses two 55-kilogram RTGs each containing about 11 kilograms of plutonium. These generate a total of about 300 watts of electricity – enough to power a transmitter and a battery of sensors.
These RTGs are relatively simple. The plutonium heats one side of a thermocouple, a junction made from two semiconductors or metals, while the other side is kept cool. Link the two sides of the thermocouple into a circuit and the heat differential creates a current. With no moving parts there’s nothing to break, and in spacecraft such as Voyager 1 the RTGs are still running reliably after more than 25 years.
The downside is that RTGs aren’t efficient, converting as little as 6 per cent of the heat into electricity. Some probes must make do with the same amount of electricity it would take to power a single light bulb. So as part of the new project, NASA engineers want to create more efficient power sources that generate bags of electricity from less plutonium.
One solution is to replace the thermocouple with a Stirling engine which uses heat flow to expand a gas and drive a piston (żěè¶ĚĘÓƵ, 2 March 2002, p 36). The motion of the piston then drives a generator to create electricity. A Stirling-based converter of the same mass as a thermocouple-based device would operate at 23 per cent efficiency, meaning the same amount of energy from only a quarter of the plutonium. The trick will be making sure the moving parts don’t make the RTG less reliable.
NASA is already betting on an improved nuclear RTG to run the Mars Science Laboratory, scheduled for launch in 2009. This robotic rover, powered by an RTG named HOMER, will wander across the Martian surface drilling and digging for samples and analysing their chemistry. A similar RTG could allow a probe to explore deep inside the icy moons of Jupiter. With enough power, the probe could melt its way through the surface ice of Callisto or Europa to the liquid water scientists think lies beneath and hunt for life with a battery of sensitive detectors.
Nuclear energy isn’t just about powerful heaters or smart rovers. It’s also about taking the energy created by nuclear fission to heat propellants and create thrust – an approach called nuclear thermal propulsion.
In the 1950s the US government began building the Nuclear Engine for Rocket Vehicle Application, a powerful rocket designed to launch huge payloads from Earth by pumping cold hydrogen through a nuclear reactor. This would heat the gas which would expand and explode out of a nozzle to create thrust. The engines were test fired several times in Nevada, but the programme was cancelled in 1972 after it had gobbled up $7 billion.
By the 1980s Ronald Reagan’s “Star Wars” project refocused interest in nuclear propulsion, this time to power fast rockets designed to shoot down incoming enemy missiles. This time they didn’t even get off the drawing board before political interest waned, and funding with it.
NASA has now thrown its weight behind a different technology, called nuclear electric propulsion. The idea is to convert the heat generated by fission into electricity for driving the engine. While this approach can’t create the gut-wrenching thrust that nuclear thermal designs achieve and certainly couldn’t launch a rocket from Earth, nuclear electric propulsion is great for accelerating craft in the weightlessness of space.
Dave Poston’s team at Los Alamos National Laboratory in New Mexico think they’ve come up with a reactor design that could be launched safely and work reliably in a nuclear electric engine. Their Safe Affordable Fission Engine (SAFE), is the size and shape of a garbage can – about 50 centimetres tall and 30 centimetres across – and weighs about 1200 kilograms. According to their calculations, it would use about 100 kilograms of uranium-235 to produce 100 kilowatts of electric power – hundreds of times as much as most space probes have to get by on.
SAFE builds on generations of reactor designs aimed at creating controllable and safe fission in a compact, lightweight unit. It uses a hexagonal array of approximately 200 cylindrical rods packed with uranium oxide, surrounded by 6 larger cylinders containing a neutron reflector on one side and a neutron absorber on the other. These drums act as control rods: you switch the reactor on by rotating the rods to reflect neutrons back into the uranium core, triggering fission. To switch the reactor off, the drums are simply rotated so that the neutrons strike an absorber and the rate of fission drops. Pass gas or liquid through the core and the heat it collects can be used to power a Stirling engine and generate kilowatts of electricity (see Diagram).
How do you use this electricity to propel a rocket? We already have an example in Deep Space I, the solar electric rocket NASA launched in 1998 to study the asteroid Braille and the comet Borrelly. Deep Space 1 uses solar panels to generate about 2.5 kilowatts of electricity, which power an ion drive.
The propellant for Deep Space 1’s ion drive uses the gas xenon. A high voltage strips an electron off the xenon atoms, giving them a positive charge. A pair of grid electrodes then expels them out of the nozzle at more than 30 kilometres a second to provide thrust.
Ion drives provide a weak but steady thrust. They would be hopeless for launching a rocket into orbit but once in space they can go on accelerating for months on end. They are also efficient, converting electrical to kinetic energy with about 99 per cent efficiency. And because of the distances involved in deep space exploration, electric propulsion systems can accelerate craft to great speeds and still have enough fuel to change course or decelerate at the other end.
That means shorter mission times since the probes don’t need to adopt the complicated trajectories necessary to get gravity assisted slingshots from the Sun or Jupiter. And if the Los Alamos reactor were hooked up to an ion drive, it could provide 50 times the power of the solar cells on Deep Space 1. The result would be greater speed, faster data collection and transmission back to Earth. Also, an ion drive could halve the duration of trips to Mars.
But proponents of nuclear thermal propulsion believe NASA’s decision to back nuclear electric propulsion is flawed. Although not quite as efficient, nuclear thermal engines have a much higher thrust to weight ratio. So they could be powerful enough to actually land on a moon or small planet and take off again. “I feel that nuclear electric propulsion has serious limitations for planetary exploration,” says George Maise, a physicist who worked on nuclear rockets at Brookhaven National Laboratory in Upton, New York. “I’m puzzled by NASA’s decision.” Maise and his colleagues have developed an idea for a hydrogen-breathing thermal nuclear rocket that could explore the atmosphere of Jupiter and other planets (żěè¶ĚĘÓƵ, 2 December 2000, p 32).
Ready for test
Joe Naininger, energetics project manager at NASA’s Glenn Research Center in Cleveland, says nuclear electric rockets are closer to flying. “There’s been a lot of technical work on nuclear electric propulsion and it’s easier to resurrect that technology,” he says.
The trip to Jupiter’s icy moons will be a showcase for the advantages of nuclear power per se. Using nuclear propulsion, the Jupiter Icy Moons Orbiter (JIMO) will have 100 times as much propulsion power as a chemical rocket – enough to go into orbit around Jupiter’s moon Europa, and drop in on two of its other moons – Ganymede and Callisto – after that. Probes that use chemical rockets are so underpowered that they have to either fly by their targets or stay put once they are in orbit.
When it reaches orbit, JIMO will be able to look for oceans of liquid water under the ice and detect organic compounds in the ice. Instruments would include radar and lasers to map surface features and ice thicknesses. Here again, nuclear power would be important, since it would allow the instruments enough power to make detailed measurements and transmit the data back to Earth at a high rate.
Nonetheless, NASA’s nuclear future is far from guaranteed. Although intense opposition to the plutonium powered RTG in NASA’s Cassini probe didn’t stop the launch in 1997, protests did seem to break NASA’s will to pursue further nuclear-powered missions. Now the opposition is mobilising once again.
SAFE has already been tested successfully with an ion drive at NASA’s Jet Propulsion Lab in Pasadena (using electric heaters to reach operating temperatures rather than uranium), but Poston himself admits that the biggest hurdle remaining is safety. “It’s more of a perceived problem than a technical one,” he says. Even so, he claims the design goes to great lengths to make sure the reactor would be safe if a launch should fail.
A standard rocket will blast the reactor into orbit. Should it crash on the way, the reactor is designed to stay in one piece to prevent the uranium being scattered. SAFE won’t be turned on until it’s travelling away from the Earth, and since the reactor uses uranium-235, it will be just mildly radioactive. Dangerous radionuclides will only build up once the reactor is running. For extra safety, a team at Marshall Space Flight Center in Huntsville, Alabama, have devised a system that would carry the fuel rods into orbit inside a shielded canister, and inject them into the reactor only when the craft had reached a safe orbit.
These safety measures don’t reassure everyone. Bruce K. Gagnon, coordinator of the Global Network Against Weapons and Nuclear Power in Space, helped lead the worldwide protests against the Cassini mission, and says people are ready to mount the same kind of protest against the renewed nuclear programs.
There are two major objections to nuclear power in space, he says. One is safety. Despite NASA’s assurances, Gagnon worries that a launch accident could spread radioactive material over the surface of the Earth, and possibly even over thickly populated areas. At least four spacecraft carrying nuclear material have already fallen to Earth: a US communications satellite in 1964, two Soviet Cosmos satellites in the 1970s and 80s, and a Russian Mars probe in 1996. These failures spread radioactive debris into the environment.
But Gagnon is also concerned that NASA’s efforts will give the military a chance to introduce powerful weapons into space through the back door. He worries that the same space-ready reactors that would supply energy for instruments and propulsions systems might soon power space-weapon systems such as lasers and electromagnetic cannons (żěè¶ĚĘÓƵ, 3 June 2001, p 26).
On top of everything else, there could be one more spanner in the works for nuclear propulsion. Congress could still refuse to cough up for a major programme of new planetary exploration, arguing that NASA should focus on investigating the Columbia shuttle disaster. Project Prometheus and JIMO are likely to cost many times that of conventional missions and the agency’s 2004 budget proposal reached Congress just before Columbia was destroyed. In February NASA administrator Sean O’Keefe faced critical questioning from congressmen who said NASA needs to concentrate on the safety of manned missions rather than spend more money on ambitious planetary exploration.
O’Keefe shows no signs of backing down. NASA can and should run both manned and unmanned missions, he says, including nuclear-powered probes. “The technology is mature enough that we can get on with it, and finally make the technology breakthrough that is long overdue.”