Riding the beam to Mars and the stars: Laser space propulsion

Somehow, lasers and space travel just seem to go together like peanut butter and jelly. No doubt that’s the result of decades of science fiction films in which various types of lasers, phasers, “death rays,” and other such weapons are used to zap monsters, alien invaders, and even entire planets such as the hapless Alderaan in the original Star Wars. 

But the steady advance of laser technology in sophistication, precision, and power since Theodore Maiman created the first operational laser in 1960 has inspired ideas for more constructive real-world uses, including communications, medical imaging, and playing with cats. Not to mention a bevy of concepts for using the laser not as a space weapon, but for space propulsion—not just to the planets of the Solar System, but perhaps even other star systems.

The notion of using light to drive spacecraft with a solar sail or light sail—using the radiation pressure from photons hitting a large reflective surface in a vacuum to generate momentum—goes back as far as Johannes Kepler in 1610. It’s an elegant and romantic concept that’s been successfully tested in a number of actual space missions, but as technical analyses demonstrated in the late 1950s, one problem is the classic inverse square law: The farther you get from the Sun, the less light hits your sail. “Since the solar light flux drops off as the square of the distance to the Sun, the propulsion capability drops rapidly if one attempts to ‘sail’ very far out in the Solar System,” wrote a Hughes Research Laboratory physicist and engineer in a 1962 article.

But in the same paper, the engineer, a visionary with the apt name of Robert L. Forward, also offered a way around the inverse-square-law problem. “There is a way to extend the idea of solar driven sails to the problem of interstellar travel at large distances from the Sun,” he wrote. “That is to use very large lasers in orbits close to the Sun. They would convert the random solar energy into intense, coherent, very narrow light beams that can apply radiation pressure at distances of light years.” The catch, he continued, was that “since the laser would have to be over 10 kilometers in diameter, this particular method does not look feasible for interstellar travel.”

The laser, it seemed, would have some considerable growing-up to do before such grand visions could be realized. Forward, also a noted science-fiction writer, continued to develop the idea in his Rocheworld series of novels, but realized that was about as far as it could go. “Most people concluded that Forward’s idea would work, but we’d have to wait until the 23rd Century to build a laser like this,” observes Andrew Higgins, a professor of mechanical engineering at McGill University.

Some people aren’t waiting that long. Lasers have come a long way in terms of attainable wavelengths and power since the 1960s, and there’s more than one possible way to drive a spacecraft with a laser beam than by pushing a sail.

One possibility involves using the laser not so much as a source of light but of heat. This is the principle behind laser thermal propulsion, the brainchild of physicist Arthur Kantrowitz. In laser thermal propulsion, the fuel or reaction mass isn’t burned as in a chemical rocket, but rather heated to extreme temperatures and pressures with laser energy and then directed out of a nozzle for thrust. Kantrowitz, a multidisciplinary type who has worked in both fluid and gas dynamics and in designing high-power lasers, first published on the idea of laser propulsion in 1972. He didn’t propose anything quite as ambitious as Forward’s starships, only a means to more efficiently launch larger satellite payloads to orbit.

Things looked rosy for laser propulsion for a while during the 1980s, thanks to US President Ronald Reagan’s Strategic Defense Initiative (SDI) or Star Wars program. Vast amounts of funding were flowing for researchers to develop massively powerful space-based lasers to knock out Soviet intercontinental ballistic missiles. And, if SDI had actually succeeded in building megawatt or even gigawatt lasers, they could also have been used for laser propulsion. But when the Cold War ended and SDI funding dried up, mega lasers were suddenly out of vogue.

Yet the general idea of laser space-propulsion (also known as directed energy or DE) didn’t die. Maybe it was a little ahead of the times at first, but as the 21st Century settled in, ongoing advancements in lasers, microelectronics, and semiconductors made it more practical. In 2016, laser propulsion burst back into the public consciousness when the Breakthrough Initiatives (BI) program—founded by late physicist Stephen Hawking and a group of billionaires including Meta’s Mark Zuckerberg and Yuri Milner—became aware of work by astrophysicist Philip Lubin at University of California, Santa Barbara.

Lubin, whose interests range from cosmology and early universe studies to infrared astronomy and laser physics, had been revisiting Forward’s laser-driven light sail dreams in the context of advances like inexpensive fiber-optic lasers that could be combined in phased arrays, removing the need for a single, massively powerful laser. Working on a grant from the National Aeronautics and Space Administration (NASA), Lubin proposed using lasers to propel wafer-scale miniaturized space probes—about the size of a postage stamp and weighing no more than a few grams—to interstellar destinations. “We propose a system that will allow us to take the step to interstellar exploration using directed-energy propulsion combined with miniature probes, including some where we would put an entire spacecraft on a wafer to achieve relativistic flight and allow us to reach nearby stars in a human lifetime,” he wrote in a paper boldly entitled “A Roadmap to Interstellar Flight” published in the Journal of the British Interplanetary Society.

The Solar Cruiser will demonstrate solar sail propulsion by flying sunward of L1 and maintaining its position along the Sun-Earth line using only reflected light for propulsion. Photo credit: NASA.

That was just what the BI program wanted to do. Late in 2015, Lubin met with Pete Worden, a former NASA Ames Research Center director now working with the BI program. Lubin told Worden about the program, which he’d never heard of. “He asked me to send him a study paper, which we did. He showed it to Yuri, and then Yuri called for a meeting with me.” Excited by the ideas described in Lubin’s “roadmap” paper, Milner and Hawking announced the Breakthrough Starshot project on 12 April 2016, which aimed to launch humanity’s first dedicated interstellar mission. With the announcement four months later of the discovery of Proxima Centauri b, a possibly habitable planet orbiting the nearest star to the solar system, Starshot even had a destination.

The idea was to aim a powerful 10-km-square ground-based laser array at light sails in high Earth orbit to propel tiny “StarChip” spacecraft to relativistic speeds and into interstellar space. As a proof-of-concept, Starshot attracted a lot of press attention but faced daunting challenges.

“It’s gone into a kind of a pause mode, ” Lubin explains of the project today. After further detailed study, a decision was made that Starshot was feasible, but still too expensive to be practical. “So, they said, let’s wait a while and see how technology progresses. That’s a very reasonable decision to make…It requires a lot of technology development.”

But while interstellar voyages are off the table for the moment, laser propulsion is not. As Lubin’s roadmap explains, the technology is scalable, making a smaller laser array just right for trips in the local space neighborhood. If it can send a postage-stamp-sized craft to Proxima Centauri at 1/20th of light speed, it can also send tons of payload to closer destinations at slower but still quite respectable speeds. That’s why the current objectives for laser propulsion projects have shifted from the stars to our own Solar System. “What we’re working on now is trying to find ways to think of more near-term applications,” Higgins says.

A 2018 NASA solicitation for a revolutionary propulsion system that would make possible a 45-day trip to Mars spurred a flurry of activity. Lubin’s UC Santa Barbara group realized that a different approach would be required for a “Mars-in-a-month” sort of mission than for Starshot. “While the basic infrastructure to power the mission is basically the same as it would be for interstellar flight relativistic [near light speed] missions, it is applied in a very different way,” he says. “NASA asked us to look at how we could apply the same technology to high mass [large payload] missions in our solar system.”

This composite photograph depicts in the laboratory an operating laser-thermal thruster model.Photo credit: E. Duplay for the Interstellar Flight Experimental Research Group.

Navigating within the solar system to the planets requires considerably more finesse and maneuverability than simply barreling across the galaxy with a light sail to another star system. Relativistic speeds are mostly a drawback in the Solar System. Lubin explains, “If you’re going to Mars or Jupiter, you don’t want to go near the speed of light.” That would cost too much energy to travel a fairly short (astronomically speaking) distance, including the problem of slowing down at your destination. Instead, says Lubin, “you normally want high-mass missions,” moving large amounts of equipment, people, and supplies on a planetary scale in a reasonably short time.

Working with colleagues at MIT, Lubin and his team designed a mission using a different take on laser propulsion called laser electric propulsion (LEP). The laser beam is directed at photovoltaic (PV) panels aboard the spacecraft to generate electrical power for ion engines, which accelerate ions through an electric field to create thrust. It’s a highly efficient way to create power without carrying the massive amounts of fuel required by chemical rockets. Less fuel onboard, of course, means a lighter and thus faster spacecraft.

Higgins was spending a sabbatical from McGill University as a fly on the wall with the Lubin group as they worked on the project. Some limitations with the laser electric/photovoltaic idea led him to reconsider the earlier laser-thermal concept. “There’s a limit to how much laser flux you can put onto a photovoltaic,” he explains. “Like all semiconductor devices, they don’t respond well to heat. It’s going to overheat, and you just can’t operate it efficiently anymore. And so, a laser is a bit better than sunlight. I was trying to convince Philip that we need to go back to the old Arthur Kantrowitz idea of just using that laser to heat propellant and expand it out a nozzle.”

As a researcher at a Canadian institution, Higgins wasn’t eligible to respond officially to the NASA solicitation, but when he returned to Montreal, he got his research group together. Emmanuel Duplay, one of his students at the time, recalls, “We used [the NASA solicitation] as a baseline to hit the targets that were set by NASA and write a paper that would use this as a starting point and show that it could be done.”

Duplay became the lead designer of the study, which he, Higgins, and the rest of the team conducted over summer 2020. Observing the NASA mission’s specifications of getting one ton of payload to Mars in 45 days (or less), the group designed a mission using a ground-based 10-m-square phased laser array. The laser propulsion system, with spacecraft payload attached, would be launched into medium Earth orbit with a conventional chemical rocket, such as a SpaceX Falcon 9. The laser array is fired up with 100 MW of power, beamed to orbit using adaptive optics to correct for atmospheric perturbations, and focused onto the propellant heating chamber of the spacecraft by a large inflatable parabolic reflector. After about an hour of continuous heating, the hydrogen fuel reaches about 10,000 degrees K, enough to provide thrust of about 14 km-per-second out of the engine nozzle. The spacecraft payload is then released on a direct trajectory to Mars, while the rest of the laser propulsion apparatus executes another maneuver to keep it in Earth orbit for reuse.

Illustration of a laser-thermal propulsion spacecraft in orbit, awaiting its 45-day transit to Mars. Photo credit: E. Duplay for the Interstellar Flight Experimental Research Group.

“It’s like a drag racer,” says Higgins of the group’s design. “You just go full out with thrust, and you accelerate quickly while you’re near the Earth. We do all the thrusting in about one hour, between the Earth and the Moon, while we’re still in the near field of this relatively small laser. And then that gives us the delta-V we need to go on to Mars in 45 days.”

The major hitch in the scheme is slowing down once the craft reaches Mars to enter orbit or land, since there would be no corresponding laser array to act as a brake. The McGill University group modeled several aerobraking scenarios in which the thin Martian atmosphere could be used for deceleration, though at rather high Gs, especially if astronauts are onboard. But it’s not an insurmountable challenge, and the laser-thermal mission architecture also remains well-suited for rapid flyby missions to the outer planets, comets, and asteroids of our Solar System, or intercepting interstellar objects passing through our Solar System, such as the recently discovered Oumuamua.

The laser thermal and laser electric approaches each have pluses and minuses, as do other variations on these themes, but they’re not mutually exclusive. Higgins notes that “there is no reason the laser-thermal approach cannot be combined with laser-electric propulsion or other techniques,” using laser thermal for the initial kick of delta-V and then switching to laser electric later in the mission.

The key development that has transformed the dreams of laser-propelled spaceflight from science fiction to something approaching reality is the advent of inexpensive, very small wavelength (1 µm) fiber-optic lasers that can be combined in massively parallel phased arrays with extremely long focal lengths. “They could reach a hundred times further into space than previously considered,” Higgins explains.

For now, it seems that not just the general public, but also most deep-pocketed billionaires and government agencies continue to think of lasers as devices for rock concerts, amusing cats, or blowing up planets. Ideas such as Breakthrough Starshot, or McGill University’s 45-days-to-Mars plan, attract more media attention than dollars. “Certainly, nobody in industry is looking at this, because it really only makes sense for these rapid transit missions,” Duplay observes. “If you don’t need to get there quickly, there’s no real reason to use this other than maybe fuel efficiency. Chemical rockets still work pretty well. So, at least until there’s a reason to get to Mars as fast as possible, I don’t expect there to be a strong industry interest in this kind of technology.” Lubin stresses that “it’s an incremental program with long-range aspirations,” with the proof-of-concept arrays he’s building in the lab eventually leading to more ambitious projects later on. “We are very hard-nosed, very practical looking at this. Not from just the purely dreaming point of view.”

Laser propulsion may have started with the dreams of imaginative scientists like Forward, but it’s steadily moving toward practical reality. “A decade ago, what we now propose would have been pure fantasy,” Lubin writes. “It is no longer fantasy. Recent dramatic and poorly appreciated technological advancements in directed energy have made what we propose possible, though difficult…. This is not a trivial project, but it is one where the rewards are truly profound in their consequences for all humanity.”  

It’s certainly a far more inspiring application for lasers than teasing the cat.

Mark Wolverton is a freelance science writer and author based in Philadelphia.