Space
The featherlight devices are smaller than a dime and need no solar panels, propellers, or engines to move.
Tiny, sun-powered aircraft floating in Earth's upper atmosphere
With a blast of light, the wafer-thin metal “parachutes” levitate into the air. The curious inventions are each smaller than a dime and need no solar panels, propellers, or engines to move. Future swarms of the tiny flyers could explore our atmosphere at the edge of space or even beyond.
This sliver of air, called the mesosphere, is roughly 50 to 80 kilometers above sea level and bridges Earth’s atmosphere and space. Some studies hint that the layer is the “miner’s canary” for climate change, because cloud formation in this extremely cold region is highly sensitive to changes in carbon dioxide, temperature, and water vapor. Further study of the atmospheric layer could yield valuable insights, but the elevation is too high for balloons and aircraft to reach and too low for satellites—earning it the nickname the “ignorosphere.”
The featherlight devices, outlined in Nature, could in theory carry tiny monitors up to the mesosphere. One day, they may even be used to analyze the atmosphere on Mars at relatively low cost, powered only by the heat of the sun.
“If the full potential of this technology can be realized, swarms or arrays of such…flyers could be collecting high-resolution data on the temperature, pressure, chemical composition, and wind dynamics of the mesosphere within the next decade,” wrote Igor Bargatin at the University of Pennsylvania, who had previously championed a similar technology but was not involved in this work. “What began as a Victorian curiosity might soon become a key tool for probing the most elusive region of the atmosphere.”
“You don’t really believe it until you see it,” study author Ben Schafer at Harvard University told Nature.
A Playful Road
Most current space technologies use solar panels for power. But the panels are heavy and costly to shuttle into space. An alternative that started as a toy-like device invented more than 150 years ago would directly harvest the sun’s heat.
The toy itself seems simple. Picture a four-leaf pinwheel—like the ones that kids blow on to spin. Each leaf, called a “vane,” is painted black on one side and white on the other. These vanes are mounted on a spindle and encased in a low-pressure chamber similar to a light bulb.
If you’ve ever worn black or white clothing on a sunny summer day, you’ll know black absorbs light and heats up, while white reflects it and stays cooler. The contraption takes advantage of this effect. “When the vanes are exposed to bright light, they begin to spin, as if being pushed on the black side,” wrote Bargatin.
The phenomenon captivated brilliant scientists at the time, including James Maxwell and Albert Einstein. The movement couldn’t be due to solar radiation—which powers our solar panels—because the vanes would turn the other way. Photons, or light particles, “push” a surface harder when bouncing back, compared to being absorbed. In other words, the white side should contribute more energy to the spin.
Instead, the “toy” works as it does thanks to heat transfer and gas.
Gas to the Rescue
We’re surrounded by gas molecules—nitrogen, oxygen, and so on—that are constantly bouncing around. The higher their energy, the faster they move.
If the vanes are hotter than the surrounding air, nearby gas molecules gain speed as they randomly bump into the vanes' surfaces. Because the black side of the toy heats up more as it absorbs more energy, that side gives nearby gas more momentum than the white side, generating air flow. The effect, called photophoresis, is especially notable at low pressures. So, in a thin atmosphere, like high above Earth’s surface or on Mars, it could generate useful amounts of force. “When you’re at low pressures, things get a little bit wonky,” Schafer told Nature.
In 2021, Bargatin and team pioneered tiny flyable devices—each thinner than a sheet of cling wrap—based on the physics. These were far lighter than the original toy, but too delicate to carry cargo.
The new devices are sturdier. They have two layers of perforated aluminum oxide—each about 1,000 times thinner than a human hair—connected by a series of pillars. The top layer allows light to soak in. The bottom layer is coated with chromium, which absorbs sunlight.
This lower layer is like the black side of the toy: When gas bounces off the layer, which is hotter, it gains more energy than gas hitting the top side. Also, because the air above is colder and denser, it naturally sinks down and generates airflow through the holes in the layers.
“Overall, more porous structures can lift more mass at lower altitudes,” wrote the team.
In natural sunlight, the device produces an airflow that lifts it up. This is “similar to [the] downward jet of gas propelling a rocket upwards,” wrote Bargatin. Although scientists have previously made similar contraptions, they needed illumination far stronger than natural sunlight to work, making them less practical for space exploration.
Larger Payload
The team next used computer simulations to test how a palm-sized version of the new device would fly at low pressures like those that exist in the mesosphere.
This outermost region of the Earth’s atmosphere has often eluded scientific research because it’s hard to reach. Aircraft and balloons can’t fly that high. Ground-based radar and satellites offer some remote-sensing data but with low coverage.
Under pressure and temperature conditions that naturally occur in the mesosphere, the team’s simulations suggest a larger version of the ultralight devices could carry a 10-milligram payload—enough to support a small radio antenna, sensors, and other microelectronics to detect and communicate atmospheric changes.
And because they’re powered by the sun alone, the flyers could in theory stay aloft indefinitely during the summer months near the poles. They could even be powered at night by exploiting the infrared light Earth emits and, in this way, levitate for weeks to months.
If scaled up, the devices could, within a decade, begin studying high-altitude cloud and lightning events, tracking meteoric dust, and recording temperature fluctuations related to climate change. Away from Earth, swarms of the sun-powered devices could one day explore Mars, which has a thin atmosphere that roughly resembles the mesosphere.
“We did some modeling on how well these things will fly on Mars, and it turns out that they would have pretty comparable performance,” said Schafer. Because the devices are so lightweight they’d be easy to ship on a rocket. If loaded with sensors and communication devices, they could beam back data on water vapor, wind speed, and other conditions on the dusty planet.
Shelly Fan