Researchers have tested a new approach to monitoring a difficult-to-reach area of Earth’s atmosphere using lightweight flying structures.

In a process known as photophoresis, the novel structure would use nothing but sunlight to fly as it navigates its way through the mesosphere, a layer of the Earth’s atmosphere between 50 and 85km above the surface.

Credit: Angela Cini / iStock / Getty Images Plus.

The team says the device could collect key data like windspeed, pressure and temperature and help calibrate any blind spots in current climate models and weather forecasting. 

“Previously, nothing could sustainably fly up there,” says lead author Ben Schafer, from the University of Chicago in the US. “It’s a bit like the ‘Wild West’ in terms of applied physics.”

The mesosphere remains largely unstudied as it is too low for satellites and too high for airplanes.

“This is the first time anyone has shown that you can build larger photophoretic structures and actually make them fly in the atmosphere,” adds David Keith, a professor at University of Chicago.

Photophoresis happens when light heats up molecules on one side of an object more than another side. Gas molecules on the warm side bounce more forcefully than on the cooler side, which creates a push and lifts the object up. This effect can only be seen in low pressure settings, making the mesosphere a perfect environment to apply it.

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

“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.”

However, the team made use of recent nanofabrication technology advances to build a structure so lightweight that the photophoretic force was bigger than the weight of the structure itself, allowing for flight.

“We developed a nanofabrication process that can be scaled to tens of centimetres,” says Joost Vlassak, professor of material engineering at the University of Chicago. “These devices are quite resilient and have unusual mechanical behaviour for sandwich structures.”

The researchers built the structure with a layer of chromium on the bottom which can absorb sunlight. When light from the sun hits this structure, the heat difference between the top and bottom of the object creates a photophoretic force which lifts the object up.

An illustration of the devices’ use cases. Credit: Ben Schafer and Jong-Hyoung Kim.

“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,” says Schafer.

The researchers tested the centimetre-tall structures in a low-pressure chamber they built in a lab.

In one of the experiments, the team levitated a 1cm-wide structure at an air pressure of 26.7 Pascals by exposing it to light at about 55% of the Sun’s intensity. The conditions in this experiment model the environment 60 kilometres above the surface in the mesosphere.

While the team hopes this lightweight structure will be instrumental in climate science, the device could also be used for defence and emergency telecommunication.

“It opens up an entirely new class of device: one that’s passive, sunlight-powered, and uniquely suited to explore our upper atmosphere,” says David Keith.

“Later they might fly on Mars or other planets.”

Mars has a thin atmosphere not dissimilar to the Earth’s upper atmosphere so it’s possible that this device could be used to explore and communicate on the nearby red planet too.

The team’s next step is to integrate the onboard communications which would allow the device to transmit real-time data while it’s still up in the atmosphere.

“I think what makes this research fun is that the technology could be used to explore an entirely unexplored region of the atmosphere,” says Schafer.

The results from the air pressure experiments have been published in Nature.