Researchers have proposed a new propulsion method that could make covering the vast distances required for interstellar missions feasible within a human lifetime.
The fundamental challenge in reaching a different star system lies in figuring out how to generate and transfer enough energy to a spacecraft both efficiently and affordably. The physical limitations of modern spacecraft pose significant challenges for reaching interstellar space in a human lifetime, especially with limited room onboard for carrying propellant or batteries. If we ever want to achieve the tremendous speeds necessary to cross interstellar distances in a human lifetime, we need to find outside-the-box solutions.
Enter relativistic electron beams made up of electrons moving close to the speed of light. “Beaming power to the ship has long been recognized as one way to get more energy […] than we can carry with us,” Jeff Greason, Chief Technologist of Electric Sky, Inc, and chairman of the Tau Zero Foundation, told Space.com. “Energy is power [multiplied by] time — so to get a given amount of energy from a beam, you either need very high power or you need to stay in the beam a long time.”
One such solution that was recently proposed uses electron beams accelerated to near the speed of light to propel spacecraft, something that could overcome the vast distances between Earth and the next closest star. “For interstellar flight, the primary challenge is that the distances are so great,” Greason explained. “Alpha Centauri is 4.3 light-years away; about 2,000 times further away from the sun than the Voyager 1 spacecraft has reached — the furthest spacecraft we’ve ever sent into deep space so far. No one is likely to fund a scientific mission that takes much longer than 30 years to return the data — that means we need to fly fast.”
A study by Greason and Gerrit Bruhaug, a physicist at Los Alamos National Laboratory, published in the journal Acta Astronautica, highlights that reaching practical interstellar speeds hinges on the ability to deliver sufficient amounts of kinetic energy to the spacecraft in an economic way.
“Interstellar flight requires us to collect and control vast amounts of energy to achieve speeds fast enough to be useful,” said Greason. “Chemical rockets that we use today, even with the extra speed boost from flying by planets, or from […] swinging by the sun for a boost, just don’t have the ability to scale to useful interstellar speeds.”
Most theoretical studies on “beam riders” for interstellar travel have focused on laser beams, which are composed of particles of light called photons. Notable examples include laser-powered interstellar ramjets and laser sails. Ramjets propel spacecraft by compressing hydrogen gas collected from the interstellar medium, with energy supplied by a laser beam transmitted from a distant source. In contrast, laser sails use the momentum of photons from the laser beam to push the spacecraft forward.
While both concepts appear to be ideal solutions, several limitations hinder their application. Interstellar ramjets face challenges such as the sparse density of the interstellar medium and immense energy and fusion requirements. Laser sails, though simpler in design, struggle with maintaining beam alignment and intensity over vast distances to ensure adequate power delivery.
Electrons, by contrast, are far easier to accelerate to near-light speeds and offer unique advantages, though they remain less explored due to their own set of limitations. “Since the electrons are all negatively charged, they repel each other which spreads the beam apart,” explained Greason.
But Greason and Bruhaug argue there are ways to counteract this.
At relativistic speeds — that is near the speed of light — time moves more slowly, which would mean the electron beam wouldn’t have enough time to spread out, keeping the beam focused.
The other advantage lies in the fact that space is not empty. “There’s a very thin spread of ionized gases called plasma that fills space, which has its own electrons and ions drifting around,” explained Greason. “When the electron beam passes through [this plasma], it repels the lighter electrons from this background gas but the ions, which are heavy, move more slowly and are left behind.”
As the electron beam passes through the plasma, it sees a magnetic field due to passing by the ions left behind from the space plasma; that magnetic field creates a force that pulls the electron beam together, effectively squeezing the beam and preventing it from spreading apart. “That’s called a ‘relativistic pinch,'” said Greason. “If this all works right, we can hold the beam together in space a very long distance — thousands of times the distance from Earth to the sun — and that would provide the power to accelerate a spacecraft.”
In their paper, the duo calculated that an electron beam traveling at these speeds could generate enough power to propel a 2,200 lb (1,000 kg) probe — about the same size as Voyager 1 — up to 10% of the speed of light. This would enable it to reach Alpha Centauri in just 40 years, a significant improvement over the current 70,000 years it would take.
Greason argues that examples of these pinched relativistic beams already exist in deep space, such as jets of charged particles released by black holes, indicating it is hypothetically possible. “But can we produce those kinds of conditions artificially?” he asked. “Will the sun’s own magnetic field break up the beam? How would we get the electron beam started? These are all questions that remain.”
In the paper, the team suggests placing a “beam-generating spacecraft” close to the sun, where the intense sunlight could provide the power needed for the beam. “While there is engineering work to do in making such a high-power beam, it’s not especially difficult compared to the other challenges,” commented Greason.
Projecting an electron beam out to a spacecraft is also only half the challenge — the power generated needs to be able to propel a spacecraft. “That means converting the energy of the beam into ejecting some kind of propellant or ‘reaction mass,'” said Greason. “This beam would be transmitting a lot of power, and that conversion would have to put very little waste heat into the spacecraft so it doesn’t melt!”
He says they have some ideas for how this could be accomplished, but they are all currently hypothetical and require more work to figure out. They also need to do more computer modeling studies to better understand the beam’s behavior and how it might be initiated, and then space-based experiments would provide concrete data to work from. “For example, a satellite far from Earth could transmit a beam to the Moon to experimentally confirm that the results match those predicted by the modeling,” said Greason.
While acquiring funding may be challenging, the scientists argue that compared to alternatives like laser-pushed sails, electron beams could achieve 10,000 times the range, thus requiring less power, and be capable of propelling heavier spacecraft. “The cost of making a big beam scales with the power, so the relativistic electron beam approach may be significantly more affordable,” said Greason. “The work being done on laser-pushed spacecraft for interstellar flight is looking at ships of only a few grams, and that’s very challenging to get scientific data back. If we can push larger spacecraft of tens of kilograms, we can include more power supply, instruments, and communications to send the data back to Earth.”
The ability to beam power over long distances has wide-ranging implications, from enabling faster travel within the solar system to transmitting power from the sun to other locations like the Moon.
Though it remains a distant goal, lowering the cost of interstellar transportation could one day allow humans to make voyages to other stars, pushing the boundaries of what was ever thought possible in space exploration.
