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So far, no one has built a fusion reactor that generates more energy than it consumes, but researchers funded by NASA are developing small fusion rockets:

The large fusion reactors under development today, such as the International Thermonuclear Experimental Reactor (ITER), usually strive to generate hundreds of megawatts of power. In contrast, Paluszek and his colleagues at Princeton Satellite Systems are designing reactors meant to produce only a dozen megawatts or so. This humbler goal results in a smaller, lighter reactor that is easier to build and launch into space “for practical robotic and human missions,” Paluszek said.

In addition, these small fusion reactors are much cheaper than larger devices. Paluszek noted that, whereas modern fusion experiments might cost $20 billion, a prototype fusion rocket the researchers plan to develop should cost just $20 million. So far, they have received three NASA grants to fund the project, he said.

The aim for the fusion drives is to get about 1 kilowatt of power per 2.2 lbs. (1 kilogram) of mass. A 10-megawatt fusion rocket would therefore weigh about 11 tons (10 metric tons).

“It would probably be 1.5 meters [4.9 feet] in diameter and 4 to 8 meters [13 to 26 feet] long,” Paluszek said.

Nuclear fusion requires extremely high temperatures and pressures to force atoms to fuse, a process that converts some of the mass of the atoms into energy. The fusion reactors that Princeton Satellite Systems is developing uses low-frequency radio waves to heat a mix of deuterium and helium-3, and magnetic fields to confine the resulting plasma in a ring. (Deuterium is made of hydrogen atoms that each have an extra neutron; helium-3 is made of helium atoms, each of which is missing a neutron; and plasma is the state of matter found in stars, lightning bolts and neon lights.)

As this plasma rotates in a ring, some of it can spiral out and get directed from the fusion rocket’s nozzle for thrust. “We can get very high exhaust velocities of up to about 25,000 kilometers per second [55.9 million mph],” Paluszek said.

The large amounts of thrust this fusion rocket may deliver compared to its mass could enable very fast spacecraft. For instance, whereas round-trip crewed missions to Mars are estimated to take more than two years using current technology, the researchers estimated that six 5-megawatt fusion rockets could accomplish such missions in 310 days. This extra speed would reduce the risks of radiation that astronauts might experience from the sun or deep space, as well as dramatically cut the amount of food, water and other supplies they would need to bring with them.

In addition, the fusion reactors could also help generate ample electricity for scientific instruments and communications devices. For instance, whereas NASA’s New Horizons mission took more than nine years to get to Pluto and had little more than 200 watts of power to work with once it arrived, broadcasting about 1,000 bits of data back per second, a 1-megawatt fusion rocket could get a robotic mission to Pluto in four years, supply 2 million watts of power and broadcast more than 1 million bits of data back per second, Paluszek said. Such a mission could also carry a lander to Pluto and power it by beaming down energy, he added.

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Previous research suggested this kind of fusion rocket in the 1960s, but the designs proposed for them would not stably confine the plasmas, Paluszek said. About 10 years ago, reactor designer Sam Cohen figured out a magnetic-field design “that could make stable plasmas,” Paluszek explained.

One drawback of the kind of nuclear reactor that Princeton Satellite Systems is developing is that radio waves do not penetrate deeply into plasma. “We’re limited to something like 10 meters [33 feet] in diameter,” Paluszek said. To generate large amounts of power with this strategy, the researchers have to rely on multiple reactors.

Another pitfall is that, while this fusion reactor generates less deadly neutron radiation than most fusion reactors under development, it still does produce some neutrons, as well as X-rays. “Radiation shielding is key,” Paluszek said.

In addition, helium-3 is rare on Earth. Still, it is possible to generate helium-3 using nuclear reactors, Paluszek said.

Princeton Satellite Systems is not alone in pursuing small fusion reactors. For instance, Paluszek noted that Helion Energy in Redmond, Washington, also intends to fuse deuterium and helium-3, while Tri Alpha Energy in Foothill Ranch, California, aims to fuse boron and protons.

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