Sunlight-powered flyers could unlock secrets of Earth's atmosphere

Flyers may soon help unlock one of the most mysterious regions of Earth’s atmosphere. Between 30 and 60 miles above our heads lies the mesosphere – a stretch of sky too high for planes and weather balloons, too low for satellites, and nearly impossible to reach with current tools.

Yet inside this no-man’s land sits valuable data that could sharpen weather forecasts and strengthen climate models.

Now, a group of scientists from the Harvard John A. Paulson School of Engineering and Applied Sciences and the University of Chicago may have found a way in.

Their new study introduces an ultra-light flying device that can lift itself into this thin, upper atmosphere using only sunlight.

Lightweight flyers lifted by sunlight

The secret behind these floating flyers is a little-known physics trick called photophoresis.

“We are studying this strange physics mechanism called photophoresis and its ability to levitate very lightweight objects when you shine light on them,” said Ben Schafer, the lead author of the study.

Photophoresis happens when gas molecules bounce harder off the warm side of an object than the cool side. That creates a tiny push. In most environments, the force is too weak to notice. But in very low-pressure conditions – like those in the mesosphere – that small push can turn into a steady lift.

The research team built ultra-thin structures from ceramic alumina and coated one side with a thin chromium layer to absorb sunlight. When exposed to light, the temperature difference between the two sides generates a lift strong enough to counter gravity.

“This phenomenon is usually so weak relative to the size and weight of the object it’s acting on that we usually don’t notice it,” Schafer explained. “However, we are able to make our structures so lightweight that the photophoretic force is bigger than their weight, so they fly.”

Breakthroughs in technology

The idea for these light-powered devices started over a decade ago. David Keith, now at the University of Chicago, had been exploring ways photophoretic particles might be used in climate engineering.

That sparked a collaboration with Schafer and Joost Vlassak, a materials engineering expert with deep knowledge in nanofabrication.

Recent breakthroughs in nanotech made it all possible. Scientists can now build featherweight structures with precise control over material thickness and shape.

“We developed a nanofabrication process that can be scaled to tens of centimeters,” said Vlassak. “These devices are quite resilient and have unusual mechanical behavior for sandwich structures. We are currently working on methods to incorporate functional payloads into the devices.”

Sunlight-powered flyers in the atmosphere

To see if the theory would hold up, the team designed and built their devices in the lab. They placed them inside a low-pressure chamber, designed by Schafer and Harvard postdoctoral fellow Jong-hyoung Kim, to simulate conditions found in the upper atmosphere.

“This paper is both theoretical and experimental in the sense that we reimagined how this force is calculated on real devices and then validated those forces by applying measurements to real-world conditions,” Schafer said.

In one key experiment, a structure just one centimeter wide was able to float at a pressure of 26.7 Pascals when exposed to light at 55% the intensity of sunlight. That’s about what you’d experience 60 kilometers above Earth.

“This is the first time anyone has shown that you can build larger photophoretic structures and actually make them fly in the atmosphere,” Keith said. “It opens up an entirely new class of device: one that’s passive, sunlight-powered, and uniquely suited to explore our upper atmosphere. Later they might fly on Mars or other planets.”

New ways to study the mesosphere

These devices could open up entirely new ways to monitor and study the mesosphere. With small sensors attached, they could collect data like wind speed, temperature, and pressure – information that’s hard to come by today, but critical for accurate forecasting and climate predictions.

Fleets of flyers in the mesosphere could also be used for emergency communication networks or defense, acting like near-Earth satellites but without the high cost or lag time.

And since the atmosphere on Mars shares similar characteristics with our mesosphere, these devices could even help in future planetary missions. The team is now working on building communication payloads that will let the devices send back data in real time.

“I think what makes this research fun is that the technology could be used to explore an entirely unexplored region of the atmosphere. Previously, nothing could sustainably fly up there,” Schafer said. “It’s a bit like the Wild West in terms of applied physics.”

The full study was published in the journal Nature.

Image Credit: Ben Schafer and Jong-hyoung Kim

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