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New Microflyers Could Soar in the Atmosphere’s Most Mysterious Layer

The mesosphere is too dense for satellites to orbit there, but too thin for planes and balloons to fly

When the two microflyers twirled around eachother midair, the researchers dubbed the maneuver "The Tango." (Courtesy of Mohsen Azadi / Science Advances)
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Researchers have created tiny disks out of mylar and carbon nanotubes that can levitate in conditions that mimic the mesosphere, reports Inside Science’s Katharine Gammon.

The microflyers, described in a study published in Science Advances on February 12, might be able to stay aloft in the mesosphere because they don’t use conventional methods to generate lift. They rely entirely on heat generated by light, a clever choice of materials and Newton’s Third Law.

The mesosphere is so little-studied that it’s jokingly called the “ignore-sphere,” says University of Pennsylvania mechanical engineer Igor Bargatin to Inside Science. The layer is between 31 and 53 miles high, and it’s a tough place to take direct measurements because the air pressure is in an anti-Goldilocks Zone: there isn’t enough air pressure for planes to generate lift or balloons to float, but there is enough air density that if a satellite tried to orbit at a mesospheric altitude, it would burn up. Rockets fly through the mesosphere on their way to outer space, but they’re only passing through.

“What we’re looking at is a new mechanism of levitation that has been used in the past for really small particles, ones that you can’t see,” says Bargatin to Krista Charles at New Scientist. “But what we’re doing is making it work for structures that are big enough that you can hold them in your hands and therefore, at least potentially, will have real-life applications.”

The team of engineers created six-millimeter-wide disks with two layers. The top layer of each disk is made of mylar, the same shiny, lightweight material used to make party balloons. The bottom layer is made of a carpet of carbon nanotubes, each of them only a few atoms wide.

The disks heat up when they are exposed to bright light. Then, when individual air molecules bounce off of a disk, the disk transfers some of its warmth to the molecule, which makes the molecule move a little bit faster. By Newton’s Third Law, which states that every action has an equal and opposite reaction, the air molecule pushes back on the disk, too.

How does that make the disk levitate? Mylar is not very good at transferring energy, but the mat of carbon nanotubes is very good at it. This imbalance in force causes air molecules to bounce off the bottom of the disk faster than molecules ricochet off of the top. The molecules below the microflyer push harder on the bottom of the disk, lifting it up like a weightlifter doing an overhead press.

The research team tested their design by putting two of the disks in a vacuum chamber that had bright LEDs at the bottom, and was filled with air at the same pressure as the mesosphere.

“We didn't know what we were expecting to see, but we hoped to see something,” says University of Pennsylvania mechanical engineer Mohsen Azadi to Max G. Levy at Wired. “When the two samples lifted, there was this gasp between all four of us.”

The experimental disks flew in the same way that their computer model had predicted, which meant that the team could use the model to predict how different-sized disks might perform. For example, they estimate that a disk that’s just over two inches wide could carry ten milligrams worth of payload, which is enough to carry small sensing equipment, according to the paper.

Earth’s mesosphere comes with challenges that couldn’t be replicated in a vacuum chamber: 100 mile per hour winds, temperatures around minus-140 degrees, and space weather caused by sunspots could all impact a microflyer’s performance. The disks are also so flimsy that the force of molecules bouncing off of them was enough to make it crinkle, so the team is developing a lightweight frame, reports Wired.

“In general, it’s unclear how many practical aspects of this technology would work, such as delivering the microflyers to this part of the atmosphere, which would presumably need to be dropped from rockets,” says University of Bristol atmospheric physicist Karen Aplin to New Scientist. “At the moment, this technology looks like a bit of a solution searching for a problem.”

If the technology improves enough to handle the challenges of the mesosphere, then the microflyers are “a really cool idea,” says NASA Goddard Space Flight Center chief scientist of Earth Science, Paul Newman, to Wired. Newman adds that the flyers could be used to study mesospheric clouds, which are associated with climate change, or could even have applications on Mars, which has a mesosphere-like atmospheric pressure.

“I should say that every time a new flight mechanism is implemented or discovered,” says Bargatin to New Scientist, “people find new applications that are hard to think about from the beginning.”

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