TORONTO – 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.

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.

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