Sunlight-powered flying structures could transform space exploration

High above the clouds, but far below the satellites, there exists a slice of Earth’s atmosphere that has remained frustratingly hard to explore. Known as the mesosphere, this region sits between 30 to 60 miles above the ground. It’s too high for balloons and airplanes, and too low for satellites to orbit. Yet this layer holds valuable data that could improve our weather forecasts and deepen our understanding of climate change.

Now, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), along with the University of Chicago and others, have found a way to reach it. Their new study, published in Nature, showcases ultra-light flying structures that float by harnessing sunlight itself—a phenomenon known as photophoresis.

The lead author, Ben Schafer, began exploring this concept as a graduate student in the labs of professors Joost Vlassak and David Keith. Together, their team designed and tested tiny structures that, when hit by sunlight, could lift off and hover in the mesosphere, with no engines, propellers, or fuel.

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

Photophoresis is a lesser-known force that pushes objects when light heats one side more than the other. In extremely thin air—like that found in the mesosphere—this heat difference causes gas molecules to bounce unevenly off a surface. The warmer side gets more force, creating a small push that lifts the object upward.

It’s a gentle force, almost always too weak to notice. But when the object is light enough and the pressure is low enough, photophoresis becomes powerful.

“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.”

The team built their devices from ultrathin ceramic alumina, a strong and lightweight material. They coated the bottom with chromium to absorb sunlight. The design also included perforations and a layered structure, allowing for better heat flow and structural strength.

The idea to use photophoresis for flight dates back over a decade, when Keith first proposed it as a way to cool the planet. But the practical engineering needed to make such flyers real has only recently become possible, thanks to breakthroughs in nanofabrication.

“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.”

To see if these tiny flyers could actually work in Earth-like conditions, the team built a special low-pressure chamber in Vlassak’s lab. There, they simulated the thin atmosphere found around 60 kilometers above Earth’s surface.

In one key experiment, a device just one centimeter wide levitated when exposed to light equal to 55% of normal sunlight. This occurred at an air pressure of 26.7 Pascals—close to what’s found in the mid-mesosphere.

“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.

Design and fabrication of the floating membranes were led by Jong-hyoung Kim, a former Harvard postdoc who is now a professor at Pukyong National University in South Korea. Their approach blends careful modeling with hands-on experimentation, a rare combination in this field.

Keith added, “This is the first time anyone has shown that you can build larger photophoretic structures and actually make them fly in the atmosphere. 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.”

The possibilities for these sunlight flyers reach far beyond academic curiosity.

First, they could revolutionize how we study Earth’s climate. By attaching sensors to the structures, scientists could measure pressure, temperature, and wind speeds in a region that is usually a blind spot. This data could sharpen the accuracy of climate models and help predict weather patterns more reliably.

These devices could also change communication systems. A group of them could form a floating array of antennas, similar to what satellites like Starlink offer—except closer to Earth, with lower data delays and potentially cheaper deployment.

The flyers even hold promise for exploring other planets. Mars, for example, has a thin atmosphere similar to Earth’s mesosphere. That makes it a natural target for these sun-powered flyers. Unlike traditional Mars rovers, these devices wouldn’t need motors or wheels. They could glide silently across the Martian sky, collecting data or even relaying signals.

“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 next steps include adding communication tools to the flyers, so they can send data back to Earth during flight. That would make them more useful for real-time sensing and monitoring.

To bring this technology into the real world, Schafer co-founded a startup called Rarefied Technologies in 2024, along with Angela Feldhaus. The Harvard Office of Technology Development helped license the invention and offered support for launching the business.

The company’s goal is to turn these floating flyers into a practical tool for science, communication, and exploration.

While these flyers may seem small, their design is built on years of advanced scientific work. The structures use a technique called thermal transpiration, where air flows from cold to warm through tiny holes, adding thrust in thin atmospheres.

The research team also developed a model to predict the best design for different altitudes. This includes the ideal number of holes, their size, and how the membranes are spaced. Using this model, they created devices with customized layouts that balance strength with performance.

In tests, they measured how different gases—some with heavier molecules—affect lift. They found that photophoretic forces remain strong even when using gases with high molecular weight, opening more doors for future use on various planets and altitudes.

Other floating materials have been studied before—such as mylar disks or nanocardboard—but none matched the power-to-weight ratio seen in these new alumina sandwich structures. Their performance, measured by how much weight is lifted per watt of light, puts them at the top of current photophoretic flyers.

While the current payload capacity is small—just 10 milligrams in a 3-centimeter device—the approach can scale. Meter-wide flyers may one day lift heavier tools into the mesosphere and beyond.

By tapping into this newly accessible region of the sky, these featherweight flyers may soon carry weather sensors, emergency communication gear, or even tiny Mars-bound probes. And they’ll do it all with nothing but sunlight.

Note: The article above provided above by The Brighter Side of News.

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