Reaching for the stars: Using light to sail through space

The word “liftoff” will come up a lot between now and the end of September. NASA’s Nancy Grace Roman Space Telescope is slated to launch Aug. 30, followed by a Roscosmos Progress resupply mission to the International Space Station (ISS), targeted for Sept. 9. Then, NASA’s SpaceX Crew-13 mission, which rotate the ISS crew, is scheduled to launch no earlier than Sept. 12.

At the School of Engineering and Applied Science, mechanical engineer Igor Bargatin's lab has spent the last decade building toward its own launches. A few weeks ago, the team proposed a new light sail prototype, an ultrathin, hexagon-patterned film that reflects more than half the laser light hitting it while absorbing almost none. Their findings are published in Nature Communications.

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It’s the latest and most concrete step in an idea he’s chased for years: using light alone to hurl objects through space, at scales from a few square inches to interstellar distances.

No fuel needed

It started with nanocardboard, a corrugated, nanometer-thin material that Bargatin’s group showed could levitate on the heat of a light beam. Those results were published in Nature Communications in 2018. Years later, in a study published in Science Advances, they pushed the trick further, edging toward real payloads with a more stable version of their prototype— sturdier discs with carbon nanotubes on their underside.

Both studies targeted the mesosphere, the band of atmosphere between about 30 and 50 miles up, too thin for planes and too low for satellites. “It’s a region of the atmosphere that is almost empty,” Bargatin says—and, until recently, entirely unused.

Bargatin calls it the only regime where people can fly on sunlight alone, with no propellant and no moving parts.

A data center in orbit

Jump to low Earth orbit, and the same penchant for lightweight, self-orienting structures reappears—this time aimed at putting data centers into orbit.

In February, Bargatin’s group presented at a conference a proposal for stringing computing nodes along a tether—solar panels, chips, and radiators threaded like beads—holding its orientation toward the sun using gravity and solar pressure alone, no thrusters required. “This is the first design that prioritizes passive orientation at this scale,” Bargatin says. Individual satellites, he argues, will never beat Earth-based data centers once launch costs are counted—so the game is shaving weight, node by node.

Designing, then building, a sail for another star

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The farthest reach of the lab’s work is Project Starshot, the Breakthrough Initiatives-backed plan to send a chip-sized probe to Alpha Centauri—a star system containing the closest star to our sun, about 4.2 light-years away—at a fifth of light speed. In 2022, Bargatin and colleagues laid out the theoretical requirements for a sail that could survive a laser blast without melting or tearing. “Reaching another star within our lifetimes is going to require relativistic speed,” he says.

That theory becomes real this summer. Bargatin is candid about the mission’s scientific payoff being modest: Telescopes will likely see more than the probe’s centimeter camera ever could by the time it arrives. The real value, he believes, is that the same sails, tuned to a more modest 1% of light speed, could intercept interstellar visitors like Oumuamua—the first confirmed object to visit our solar system from another—within hours of a sighting. That’s something no chemical rocket could do.

“Thinking in space,” Bargatin says, “means starting from first principles instead of optimizing what already exists. I enjoy thinking about something that would be a first of its kind.”

Igor Bargatin is an associate professor in the Department of Mechanical Engineering and Applied Mechanics at Penn's School of Engineering and Applied Science.