Light-powered spacecraft bound for space's no-man's land

It's a bird, it's a plane... no, it's a tiny, light-powered, levitating aircraft designed to fly in the "ignoro-sphere."

We have been putting both craft and humans into space since the 1960s, but never has a spacecraft been able to sustain flight in a specific zone of Earth's atmosphere called the mesosphere. Sandwiched between the airspace occupied by planes and the upper atmosphere occupied by satellites, the mesosphere is essentially a no man's land in the liminal region between Earth and space. Until now.

A team of engineers from the University of Pennsylvania have created a tiny spacecraft which they say can levitate in the mesosphere, using only sunlight.

Why it matters — Built using low-cost, easily accessible materials like mylar (a plastic used in some balloons), this near-spacecraft could finally offer scientists an opportunity to explore Earth's mesosphere. Doing so would give access to crucial measurements of carbon dioxide, which scientists need to monitor climate change accurately.

The research was published Friday in the journal Science Advances.

Here's the background — Earth's atmosphere is made up of four primary layers:

• Thermosphere — the highest layer, up to 375 miles above Earth and home to orbiting observatories, like Hubble (370 miles) and the International Space Station (250 miles).
• Stratosphere — the ozone layer lives here.
• Troposphere — the layer of the atmosphere, up to 10 miles above the ground, which planes fly through.
• Mesosphere — the atmosphere's middle child, the mesosphere lies between the troposphere and stratosphere, at about 31-53 miles above the ground.

In the video describing the research below, Igor Bargatin, lead author on the study and associate professor of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania, jokingly refers to the mesosphere as the "ignoro-sphere."

And for good reason.

Here's why: Because of the pressure that exists at this height above Earth — roughly 10-30 Pascals — this part of the atmosphere exerts too much drag on satellites, which can cause them to overheat, and not enough lift for airplanes.

Rockets, for their part, occasionally pass through the mesosphere on their way out of Earth's atmosphere, but they never stay long.

So the research team decided to try build a craft which could somehow go beyond the needs of planes and satellites by using a different, more natural fuel source: the Sun.

Using a process called photophoresis, the same technique responsible for powering light sails, the research team set out to design a miniature spacecraft capable of maintaining flight in Earth's mesosphere.

"It's an exciting idea, that you can just shine light on something and make it float and overcome gravity," Bargatin says in the video.

What they did — The researchers designed a craft shaped like a disk, six millimeters in diameter, with a smooth mylar top and a scratchy bottom made from carbon nanotubes.

The idea, explain the researchers, is that air molecules at mesosphere-like pressure react differently to these two surface textures. Air molecules will bounce off the scratchy bottom of the disk with a higher velocity than they will the smoother top side of the disk, creating a net lift force that balances, or levitates, the disk in mid-air.

The researchers did their best to recreate the atmospheric pressure and sunlight conditions in their Pennsylvania lab, creating an optical light trap that they could shape and change to control the movement of the disks.

Why they discovered — Unlike the team's previous research, which demonstrated how temperature differences on either side of the disks were also required for levitation, the new design shows the same levitating effect is possible with the same temperature on both sides.

Not only were their tiny disks able to sustain levitated "flight," but the researchers show in the study they can carry significant payloads, including scientific instruments many times the disks' own weight. For crafts like those in this study, a payload could weigh up to 10 milligrams, which the authors write is enough for the disks to carry up a small dust or carbon dioxide sensor into the mesosphere.

What's next — While these initial results are promising, the researchers write there's still much work to be done before these tiny crafts can begin calling the mesosphere home. For one, this study took place in a controlled lab setting, so it remains to be seen whether these light-powered spacecraft can actually make it in the real world.

The researchers also need to experiment with ways to scale their technology up, so a craft can hold more than 10 milligrams of cargo at a time. It would also be good to be able to steer and control the craft once they're in this forgotten layer of the atmosphere, too.

This article was updated on February 15th to correct the definitions of troposphere and thermosphere. Inverse regrets the error.

Abstract: We report light-driven levitation of macroscopic polymer films with nanostructured surface as candidates for long-duration near-space flight. We levitated centimeter-scale disks made of commercial 0.5-micron-thick mylar film coated with carbon nanotubes on one side. When illuminated with light intensity comparable to natural sunlight, the polymer disk heats up and interacts with incident gas molecules differently on the top and bottom sides, producing a net recoil force. We observed the levitation of 6-mm-diameter disks in a vacuum chamber at pressures between 10 and 30 Pa. Moreover, we controlled the flight of the disks using a shaped light field that optically trapped the levitating disks. Our experimentally validated theoretical model predicts that the lift forces can be many times the weight of the films, allowing payloads of up to 10 milligrams for sunlight-powered low-cost microflyers at altitudes of 50 to 100 km.