Fusion-powered space travel has long held the promise of rapid trips across the solar system: Mars in weeks, Saturn in months, Pluto in years.
For decades, such possibilities have remained theoretical, like something plucked out of a science fiction novel. But several companies are now working to build practical nuclear fusion propulsion engines, with significant milestones being hit.
Pulsar Fusion, a U.K.-based startup, hopes to launch a demonstration mission to space in 2027, while Princeton University and Helicity Space in the U.S. are continuing their own work on fusion drives.
Richard Dinan, the CEO of Pulsar Fusion, displays the Sunbird spacecraft at the company’s facility in Bletchley, England.
(Image credit: Pulsar Fusion)
If any of these efforts prove successful, missions across the solar system for robots and humans could be unlocked like never before, turning us into a true spacefaring species.
“If we continue on the current trajectory, everything we know about space travel is going to change within a decade,” Stephane Lintner, CEO and co-founder of Helicity Space, told Live Science.
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But is it too good to be true? Can the dream of nuclear fusion propulsion ever be fully realized, or will it remain a sketchbook fantasy? After decades of dreaming, we might be on the cusp of finding out.
Nuclear fusion is a process whereby two atoms combine, or fuse, to form a heavier one, releasing excess energy in the form of light and heat. The best-known type of fusion is the process that occurs inside the sun at up to 27 million degrees Fahrenheit (15 million degrees Celsius), where atoms of hydrogen fuse to form helium.
If this process could be replicated on Earth, it would provide us with huge amounts of energy. “The holy grail is free energy for everyone,” Lintner said. Entire cities and countries would be transformed, bringing about a new age of clean and abundant power.
Many experiments and tests have been performed, and large fusion experiments, like the International Thermonuclear Experimental Reactor (ITER) in France, are under construction. However, commercially viable fusion reactors have yet to be built. One of the main problems is keeping the turbulent, superhot plasma required for fusion confined and at the right temperature. The record is currently 22 minutes.
If you take the same concept and put it in space, you suddenly have a means to power a spacecraft. What’s more, the same problems with sustaining the plasma don’t arise. Instead, the plasma can be fired out of the spacecraft, providing a steady thrust that can be used to accelerate a spacecraft to enormous speeds in the frictionless vacuum of space.
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“Fusion propulsion in some ways is harder, and in some ways is easier, than terrestrial energy production,” Bhuvana Srinivasan, a professor of aeronautics and astronautics at the University of Washington, told Live Science.
A fusion-powered spacecraft could be accelerated to hundreds — or even thousands — of miles per second through this method, multiple times faster than any spacecraft in history. In the most optimistic scenario, such a spacecraft could reach significant fractions of light speed, making interstellar travel a possibility.
There are tremendous challenges, of course. A space-based fusion reactor must be small enough to fit inside a spacecraft on a rocket. That’s no easy feat, considering fusion reactors like ITER are the size of houses. You also need a way to create the immense heat required for fusion to occur, a source of fuel and propellant, and powerful magnets to keep the plasma from burning through the engine’s walls.
Fusion propulsion in some ways is harder, and in some ways is easier, than terrestrial energy production.
Bhuvana Srinivasan, professor of aeronautics and astronautics at the University of Washington
Producing meaningful thrust requires staggering numbers of fusion reactions every second. A quintillion reactions would provide about 10 newtons of thrust — equivalent to the weight of a 1-liter bottle of water in your hand. But a fusion-powered engine would provide this thrust for months, rather than minutes for typical chemical propulsion, allowing large speeds to be reached eventually.
At first glance, this might sound nearly impossible, but some recent tests have shown it could be plausible
In March, Pulsar Fusion demonstrated “first plasma” inside a nuclear fusion engine for the first time. In the test, which took place at the company’s facility in Bletchley, one of Pulsar’s Sunbird engines briefly transformed krypton gas into a plasma, demonstrating how the company might confine plasma within its exhaust system.
The demonstration showed “the plasma will sit in the system where you want it to sit,” confined by an electromagnetic field, Richard Dinan, the company’s CEO and founder, told Live Science. “The difficult work now is to be able to heat the plasma to temperatures nearer to fusion.”
A display model of a Pulsar Sunbird engine, which briefly transformed krypton gas into a plasma in March.
(Image credit: Pulsar Fusion.)
Dinan founded Pulsar Fusion in 2011 with the goal of building a fusion-powered propulsion system. The company’s plan is to fuse helium-3 and deuterium, a lighter form of helium and a heavier form of hydrogen, and use the energy produced to heat helium-4 and expel it as propellant to produce thrust. The amount of thrust that can be produced depends on “how good you are at diverting those particles out the back of the spacecraft,” Dinan said. “And right now, nobody knows how efficient we can be.”
Just a few hundred grams of deuterium and helium-3 fuel would be needed to sustain the fusion process on a trip to Mars, Dinan said, but he claims their rocket would need 10 to 20 metric tons (11 to 22 U.S. tons) of deuterium propellant to provide the thrust to reach the speeds they’re hoping for.
If they can make it work, their plans are ambitious. The company imagines fleets of its Sunbird vehicles traversing the solar system, taking people and cargo to and from destinations at speeds of up to 329,000 mph (529,000 km/h) — about 10 times faster than the Voyager 1 spacecraft currently traveling outside the solar system.
At those speeds — which aren’t reached instantly, as the spacecraft need months to accelerate and decelerate — travel time to Mars could be halved from the nine months it takes with conventional chemical propulsion. Weight could be freed up for equipment because fusion is more efficient and needs less fuel. Quicker missions to farther destinations, such as to Saturn’s moon Titan or the metal-rich asteroid Psyche, would also be in reach.
“I think we will see fusion as a propulsion system pretty soon,” Dinan said. “The demand is certainly there.”
Pulsar Fusion’s method of propulsion is known as dual direct fusion, where the plasma from the process is directly used to provide thrust. Samuel Cohen, a physics professor at Princeton University, is working on a similar concept, the Direct Fusion Drive, as part of a project called Starfire.
Cohen and his team have been investigating this concept for more than 20 years. Their idea is to fuse deuterium and helium-3, which is rare on Earth but abundant on the moon. His team has built a prototype fusion thruster at Princeton with plasma that reaches 18 million F (10 million C).
“It’s a start, but you’ve got to get to a billion degrees” Celsius for deuterium and helium-3 fusion to occur, Cohen told Live Science. There are a few promising approaches to reaching those temperatures, including squishing the plasma or heating it up with radio or neutron beams.
Using their current plasma, Cohen claims the team has demonstrated a small amount of thrust — a few milligrams, or a few hundred-thousandths of a newton. Now, they hope to get more funding to take their concept further. “We need a few million dollars to start things off,” Cohen said.
With enough support, Cohen said, they could have a working fusion propulsion system in 10 to 20 years. Meanwhile, Pulsar Fusion, which has received support from the European Space Agency and the U.K. Atomic Energy Authority, hopes to perform a demonstration of its thrusters in orbit by 2027.
Helicity Space is targeting a launch of its fusion propulsion system in the 2030s. The company, which raised $5 million in 2023, is developing the Helicity Drive, which relies on pulses of plasma, rather than continuous fusion, to provide thrust.
An artist’s illustration of Helicity Space’s Helicity Drive.
(Image credit: NASA/Ryan Weed)
“If you get a reaction every few seconds, that’s an amazing science experiment on Earth, but you can’t do anything with it,” Lintner said. “In space, you have the most advanced electric propulsion drive ever built. You change the game for propulsion.”
Lintner said the company is aiming “for fusion temperatures very, very soon,” and is then “planning on flying a first prototype within three years.” By the 2030s, Lintner hopes to reach net gain in its fusion engines — producing more energy than is put in — to have a true fusion drive ready for use in space.
But not everyone is convinced that fusion drives will propel us through the solar system. John Slough, who is working on his own fusion rocket design called the Fusion Driven Rocket (FDR), said he is wary of “false promises” in the field. He noted that much of the underlying physics around fusion propulsion remains extremely difficult.
“It still relies on scientific discovery for us to push it,” Srinivasan said. “We need to figure out, how do we keep this fuel stable? Have we sorted a lot of the engineering and scientific challenges? It’s not going to happen next year.”
Remaining challenges include not only keeping the plasma stable, but also developing walls that can sufficiently contain the plasma, and shrinking the fusion system to fit inside a spacecraft, she added.
Other forms of nuclear propulsion might be more promising. NASA is developing SR-1 Freedom, a spacecraft powered by nuclear fission, not fusion. The goal for the spacecraft, announced in March, is to launch to Mars by December 2028, where three small robotic helicopters would be sent to explore the surface. The fission reactor would split uranium-235 to generate thrust.
The planned flight trajectory of the SR-1 Freedom craft.
(Image credit: NASA)
“SR-1 Freedom will pave the way for space nuclear hardware,” Steven Sinacore, the Space Reactor-1 Freedom Program Director at NASA, told Live Science in response to questions via email. “To sustain a presence on the Moon, send crews to Mars, and explore the outer solar system, the nation needs power that works everywhere, independent of the sun, and fission is that power.”
However, fission-based propulsion, while extremely useful, cannot match the speeds promised by fusion propulsion. That’s because fusion systems magnetically confine the plasma, meaning much higher temperatures can be reached. Fission temperatures, by contrast, are limited by the materials of the reactor where the fission reactions take place.
If any of the proposed fusion concepts were to become a reality, however, the possibilities would be enormous. Last year, Elena Ancona, a flight dynamics engineer at the Polytechnic University of Bari in Italy, and her colleagues described how a direct fusion drive could enable a mission to Sedna, a remote icy dwarf planet far beyond Neptune. For that mission, time is of the essence.
An artist’s illustration of Sedna. Named after the Inuit goddess of the sea, this dwarf planet has one of the most distant orbits in the solar system, stretching far beyond Pluto.
(Image credit: CoreyFord via Getty Images)
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The dwarf planet will reach its closest point to the sun in 2075, and it will not be that close again for another 11,000 years. “If we want to make this mission, it’s either now or in 11,000 years,” Ancona said. With a traditional chemical spacecraft, a mission would need to be launched 30 years in advance. But with fusion propulsion, it could get to Sedna in just 10 years.
Many more destinations would be within reach if fusion propulsion were to become a reality. Perhaps even interstellar travel would be possible much further down the line. “People have known that fusion is the answer to deep-space travel since the 1960s,” Lintner said. “It’s only in the last decade that, commercially, people started to spend a lot of time on cracking the fusion code.”
But there’s a long way to go. “It’s not for the faint of heart,” Lintner said. But if fusion propulsion is ever going to happen, the next few years look as good a bet as any.
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