Interstellar travel, a timeworn staple of science fiction, can already be science fact if one has cash to spare. For just $100 million or so, a customer could actually purchase a top-of-the-line commercial rocket and ride right out of the solar system. But patience would be key. If launched tomorrow toward the nearest port of call—Proxima b, a potentially habitable Earth-mass planet recently discovered in the triple star system of Alpha Centauri about four light-years away—that rocket would take 80,000 years to arrive.

Instead of spending $100 million on a slow boat to the stars, in April of last year the billionaire entrepreneur Yuri Milner announced he would use that same sum to forge a new path to Alpha Centauri within a human lifetime. Called Breakthrough Starshot, the initiative calls for largely abandoning rockets in favor of “light sails”—gossamer-thin reflective sheets that, once unfolded in space, could be propelled to very high speeds by laser beams. Starshot’s tentative plans involve using conventional rockets to place thousands of one-gram, four-meter-wide light sails in Earth orbit as early as the 2040s. Each sail would be embedded with a one-centimeter-wide chip containing cameras, sensors, thrusters and a battery. From Earth orbit, each featherweight spacecraft would be boosted toward Alpha Centauri at 20 percent light-speed by a minutes-long pulse from a ground-based, 100-gigawatt laser array. The interstellar crossing would take just a little over 20 years, so the probes could reach Alpha Centauri in the 2060s.

But such high speeds come at a high price. Even the most conservative cost estimates for Starshot far exceed Milner’s initial $100-million investment—the multi-decadal project could easily consume $10 billion, and perhaps much more, largely due to the enormous expense of building the ground-based laser array. Government assistance and international collaboration would likely be required. Moreover, the light sails that survive the 20-year voyage would pass through the Centauri system in a flash, moving so fast they would have only seconds to capture high-quality close-up images and other data from Proxima b and any neighboring planets that may be there. As they fall deeper into the dark between the stars, the light sails would attempt to transmit their precious findings back to Earth using laser beams no more powerful than the signal from a typical cell phone.

A Slower Sail to the Stars

Some critics say these problems make Starshot’s rush to Alpha Centauri look like a poor investment. “When we read about [Starshot], we found it wasteful to spend so much money on a flyby mission which is en route for decades, while the time for a few snapshots is only seconds,” says Michael Hippke, an independent researcher in Germany. Working with René Heller, an astrophysicist at the Max Planck Institute for Solar System Research in Göttingen, Hippke has developed an alternative mission profile that he says could offer greater scientific returns for a fraction of the cost. Rather than using multibillion-dollar laser arrays to boost small light sails to relativistic speeds for one-time flybys, Heller and Hippke propose using starlight alone to send larger sails on more leisurely journeys that would take them to all three stars in the Alpha Centauri system and leave them parked in orbits there. Their findings appear in the February 1 edition of The Astrophysical Journal Letters.

The crux of their proposal is not only to use sunlight to accelerate outbound light sails, but also rely on the light and gravitation of Alpha Centauri’s triple stars at journey’s end. Heller and Hippke have calculated that a mind-bogglingly low-density sail weighing approximately 100 grams but spread across 100,000 square meters (that’s about 15 football fields!) could do the trick. Such a sail’s construction appears possible, based on rapid progress in material science. Incrementally adjusting its angle as it approaches to soak up more radiation pressure from the stars, that sail could bleed off enough speed to be captured into orbit within the system.

To reach the potentially habitable planet Proxima b, these “photogravitational” assists counterintuitively require first sending the light sail swooping blisteringly close to the bright, sunlike stars Alpha Centauri A and B—even though they are nearly two trillion kilometers farther from us than Proxima b’s smaller, dimmer host star, Proxima Centauri. This is because the greater radiation pressure from Alpha Centauri A and B provides more deceleration, and thus a faster approach, for any light sail targeting the system. But the twin stars’ radiation pressure has its limits; if Heller’s and Hippke’s 100,000-square-mter light sail came in any faster than 4.6 percent light-speed, it would simply overshoot the system. All together, they envision their sail’s journey to Alpha Centauri A and B taking nearly a century, followed by another half-century voyage to the final destination—a stable orbit around Proxima.

This schematic animation shows how a giant, low-density light sail traveling at nearly 5 percent the speed of light could decelerate and enter orbit in the Alpha Centauri star system.

“You would need to travel about seven times as long as Starshot’s 20-year mission, but in return you would get years or decades of close-up exploration instead of only seconds,” Heller says. Comparing the ratio of exploration time with travel time for both cases, Heller adds, “Starshot could use one hundred-millionth of the mission’s lifetime for in situ science whereas we could use of the order of one hundredth—or about a million times more.” Plus, by using sunlight to launch the sail, the new proposal obviates the need for a multibillion-dollar gigawatt-scale laser array.

Even so, their proposed 150-year voyage could not start tomorrow. Heller’s and Hippke’s proposal utilizes a rare configuration of Alpha Centauri’s stars that only occurs once every 80 years, when their orbits all align in a plane that intersects the trajectory of any incoming probe from our own solar system. Alpha Centauri’s 80-year triple-alignment next occurs in 2035, far too soon for any conceivable light sail from Earth to be anywhere close to the system; instead, Heller and Hippke suggest it might be more realistic to target the subsequent alignment, in 2115.

As far-off as that is, the timing could be much worse, Heller says: Sending their sail directly to Proxima Centauri would demand much slower interstellar speeds due to the smaller star’s weaker radiation pressure and braking ability, raising the total travel time to nearly a millennium.

Patience, Please

For Hippke, a multigenerational mission ending in orbit around Alpha Centauri would be worth the wait, even if he would never see its returns. “Our children and grandchildren will receive the amazing photos from these space probes. Imagine alien rivers, volcanoes and perhaps exotic life!” Opting for a mission with a century timescale also opens possibilities for exploring other nearby bright stars, Hippke says. The massive star Sirius, for example, is just over twice as far away as Alpha Centauri—but because it shines some 25 times brighter than our sun, it offers a stronger radiation-pressure braking effect, allowing light sails to approach at much higher speeds. If nothing else, the possibility of sending light sails to orbit many nearby stars suggests a natural next-generation, longer-term follow-up to Starshot’s more urgent mission goals.

Despite these perks, Avi Loeb, an astronomer at Harvard University and chair of Breakthrough Starshot’s Scientific Advisory Committee, remains unconvinced this alternative proposal offers realistic advantages over Starshot’s plan to use gigawatt-class lasers to boost smaller sails to the stars. “In using starlight to reach relativistic speeds, one must use an extremely thin sail,” Loeb says, noting that the weaker push of sunlight calls for a correspondingly lower-density light sail. Hippke and Heller say their sail could in theory be constructed from low-weight, high-strength material such as graphene, but Loeb questions whether making and using a few-atoms-thick 100,000-square-meter sheet of graphene for an interstellar probe would actually be any easier than building a massive laser array. “Such a surface is orders of magnitude thinner than the wavelength of light that it aims to reflect, and so its reflectivity would be low,” Loeb says. “It does not appear feasible to reduce the weight by so many orders of magnitude and yet maintain the rigidity and reflectivity of the sail’s material.” In other words, a 100,000-square-meter graphene sail could prove far too flimsy to actually fly. Additionally, Starshot plans to fly thousands of sails, not just one—even if each successful interstellar crossing yielded only a few seconds of close-up observations, they could quickly accrue across multiple successive flybys.

Perhaps the biggest issue, Loeb says, is whether ambitious plans for multigenerational projects can really survive their inescapable encounters with human mortality. “If one ignores the duration of the journey, one can always use conventional rockets and reach Alpha Centauri in 80,000 years for a very modest cost,” he says. “But the people who work on Starshot are more ambitious. We want to get there within our lifetime.”