Closest Earth-Size Planet May Get Robot Visitors—Here’s How



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An illustration shows what a visitor might see if they could stand on the surface of the Earth-size planet Proxima b.

Photo illustration by ESO, M. Kornmesser

Just a few months ago, astronomers revealed that there is an Earth-size world orbiting Proxima Centauri, the next star over. Now, a team of astrophysicists thinks there’s a way to pay that planet a nice long visit sometime in the not-so-distant future.

The method involves something similar to the Breakthrough Starshot Initiative, a plan unveiled last year that would send a fleet of small spacecraft toward the three stars in the Alpha Centauri system, which includes Proxima. Propelled by a giant laser, those tiny spacecraft would zip through the system in a matter of moments, furiously snapping photos, gathering data, and somehow relaying that information back to Earth.

But what if instead, a small, lightweight spacecraft could slam on the brakes, pull into orbit around Proxima, and stay for a while?

“The time for a few snapshots is only seconds, and if you miss the target, it’s all for nothing,” says astrophysicist Michael Hippke. “In contrast, if you manage to go in orbit around your target, you can stay there as long as your probe functions.”

Major Laser

Astro-entrepreneurs developed the Breakthrough Starshot Initiative even before the announcement of the Earth-size exoplanet around Proxima. That’s because Alpha Centauri’s trio of stars is the closest system to ours. Now, that alien world—dubbed Proxima b—is especially exciting because it is a scant 4.24 light-years away, making it the best chance we have to robotically visit an exoplanet during a human lifetime.

But Breakthrough Starshot doesn’t leave a lot of time for exploration. As envisioned, the nanosized StarChip spacecraft would employ highly reflective sails to harness the power of a still-to-be-built enormous laser. Momentum from the laser’s photons would be transferred to the sails and ultimately rocket the StarChips through space at roughly 20 percent the speed of light.

Even at that pace, it would take up to 20 years to reach the Centauri trio—and without a way to slow down, the nanocraft would hurtle through the system in a matter of minutes.

Now, Hippke and his colleague René Heller think they have come up with a way to use starlight to not only slow down those nanocraft, but also to help them slip into orbit around Proxima. The team’s idea would alter the Starshot plan in some significant ways, and though it has already drawn some skepticism, it’s an intriguing concept.

“The energy required to send an interstellar probe to Proxima b would essentially be the energy required to put the sail into orbit around the Earth using a conventional rocket,” says Heller, of Germany’s Max Planck Institute for Solar System Research. “This would reduce the technological and energy demands for an interstellar mission substantially at the price of being five times more slow.”


Using the distant stars’ light as a brake is a concept born from the idea that starlight can propel spacecraft through the void, which is what solar sails are made to do. These big, extremely shiny, ultrathin sheets capture the energy from the sun’s photons in the same way that a sea-faring ship catches the wind.

The thinking goes that if photons can be used to push a spacecraft from behind, they can also be used to slow down an incoming probe—just as wind can both accelerate and decelerate a sailboat.

In a study published by the Astrophysical Journal Letters, Heller and Hippke lay out how this could work. They start by considering a spacecraft weighing about as much as an average bar of soap, festooned with sails measuring more than 1 million square feet, or about the area of 14 soccer fields.

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Best seen in southern skies, the bright blue star Alpha Centauri is really a system of three stars just over four light-years from Earth.

Photograph by ESO

Propelled by the sun’s light striking those sails, the craft could be set on a course for the Centauri system. Then, by reorienting its sail, the craft could be effectively halted by the incoming photons from Alpha Centauri.

Once there, it could either remain in orbit around the brighter of Alpha Centauri’s two stars or, after a series of maneuvers involving both stars, it could use the pair’s gravity to slingshot toward Proxima, where it could eventually pull into orbit.

While in residence around Proxima Centauri, the craft could gather data and photos at its leisure and send them back to Earth.

“This is our interesting target, because we know it has a planet in the habitable zone,” Hippke says.

Slow and Steady

It’s a compelling idea, but it would take a bit longer than a single lifetime. Pushed by sunlight, the Centauri-bound spacecraft would leave the solar system traveling at just 4.6 percent the speed of light.

At that rate, it would take about 95 years to snuggle up to the Centauri system. After being slowed by the starlight from Alpha Centauri, the craft would take another 46 years to reach Proxima.

That means any science data would be delivered to future generations.

“Our main constraint in defining the Starshot concept was to visit Alpha Centauri within our lifetime,” says the Harvard-Smithsonian Center for Astrophysics’ Avi Loeb, chair of the Breakthrough Starshot Initiative’s advisory committee.

“Our envisioned laser array can push the sail with an energy flux that is a million times larger than the local solar flux.”

A spacecraft traveling that quickly can’t be stopped on the other side by the force of starlight alone—but it would get there much quicker. Loeb is also concerned that Hippke and Heller’s large, ultralight sails would rely on materials that have not quite been developed yet.

Still, plausible advances in materials science could help produce the gossamer sails that would make the spacecraft lighter and faster.

“If you can mass-produce large quantities of graphene, and coat them with a metamaterial, you’re almost there,” counters Hippke. “To this you add some sensors, a communications laser, and a few other things—like in a smartphone—and there you go!”

And the science returns of a mission that might overstay its welcome at the neighbors’ could be seriously worth the wait.

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