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Between 50 and 100 kilometers (30–60 miles) above the Earth's surface lies a largely unstudied stretch of the atmosphere, called the mesosphere. It's too high for airplanes and weather balloons, too low for satellites, and nearly impossible to monitor with existing technology. But understanding this layer of the atmosphere could improve the accuracy of weather forecasts and climate models.

A new study in Nature by researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), University of Chicago, and others introduces a novel way to reach this unexplored near-space zone: lightweight flying structures that can float using nothing but sunlight.

"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, lead author of the paper and a former Harvard graduate student in the research groups of Joost Vlassak, the Abbott and James Lawrence Professor of Materials Engineering at SEAS, and David Keith, now a professor at the University of Chicago.

How photophoresis works

Photophoresis occurs when gas molecules bounce more forcefully off the warm side of an object than the cool side, creating continuous momentum and lift. This effect only happens in extreme low-pressure environments, which are exactly the conditions found in the mesosphere.

The researchers built thin, centimeter-scale membranes from ceramic alumina, with a layer of chromium on the bottom to absorb sunlight. When light hits this structure, the heat difference between the top and bottom surfaces initiates a photophoretic lifting force, which exceeds the structure's weight.

"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 said. "However, we are able to make our structures so lightweight that the photophoretic force is bigger than their weight, so they fly.

The concept originated more than a decade ago when Keith hypothesized different uses of photophoresis particles, including their potential to reduce climate warming. A collaboration began with then-graduate student Schafer, and Vlassak, an expert in nanofabrication and experimental mechanics, in order to help move the concepts from theory to reality.

The collaboration became feasible through recent advances in nanofabrication technology, which allow researchers to build low-mass, nanoscale devices with greater precision.

"We developed a nanofabrication process that can be scaled to tens of centimeters," Vlassak said. "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."

Testing devices in the lab

Using these fabrication methods, the research team created centimeter-scale structures and directly measured the photophoresis forces acting on them inside a low-pressure chamber Schafer and former Harvard postdoctoral fellow Jong-hyoung Kim built in Vlassak's lab.

They compared those results to predictions of how such structures would behave in the upper atmosphere. Device design and fabrication were led by Kim, who is now a professor at Pukyong National University in South Korea.

"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.

A key experiment detailed in the paper shows a 1-centimeter-wide structure levitating at an air pressure of 26.7 Pascals when exposed to light at just 55% the intensity of sunlight. This pressure condition models what's found 60 kilometers above Earth's surface.

"This is the first time anyone has shown that you can build larger photophoresis structures and actually make them fly in the atmosphere," said Keith. "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."

Possible applications: Sensing, communication, Martian exploration

The team envisions a range of possible applications for their new device, especially in climate science. If equipped with lightweight sensors, this device could collect key data like wind speed, pressure, and temperature from a region of the atmosphere that has long remained a blind spot. This data is critical for calibrating the climate models that build the foundation of weather forecasting and climate change projections.

Other potential applications include telecommunications for defense and emergency response scenarios. Using a fleet of these devices could enable a floating array of antennas with data transmission capabilities comparable to low orbit satellites like Starlink, but with lower latency due to their closer proximity to the ground.

Since Earth's upper atmosphere shares key characteristics with the thin atmosphere of Mars, the device could facilitate new modes of planetary exploration and communication in that environment as well.

The team's next step is to integrate onboard communications payloads that would allow the device to transmit real-time data during flight.

"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."

Research described in the paper formed the building blocks of a Harvard spinoff company, Rarefied Technologies, that Schafer and co-founder Angela Feldhaus launched in 2024.