Their simulations estimated that a 6-centimeter plate could carry 10 milligrams of cargo into the mesosphere under natural sunlight. Ten milligrams may not sound like a lot; a drop of water weighs five times more. But technical advances have reduced silicon chips to dust-sized sensors much smaller than that. These “smart dust” systems can integrate a power source, radio communication, and cube data collection sensor. only a millimeter in diameter. “Researchers can do a lot when you give them a cubic millimeter of silicon,” says Bargatin. “And a cubic millimeter of silicon weighs a few milligrams.”
In their vacuum chamber test, they found that by increasing the light intensity beyond the power of sunlight, that extra surge of energy carried the flyer higher. But after about 30 seconds, the disc began to curl under the effect of the photophoretic force, eventually collapsing. Ultrathin Mylar is inherently very fragile, says Bargatin. The shag of the carbon nanotubes makes the Mylar disc more rigid, but the force of high-speed molecular collisions eventually deforms the flyer. The team’s model can predict which disc sizes, air pressures and light intensities are causing it, and Bargatin says development of a lightweight frame is underway.
Bargatin plans one day researchers to release levitators loaded with sensors in the mesosphere and let them roam, like weather balloons or floating ocean sensors. “Another approach is to develop smart flyers that can control their destination,” he says. The same tilt that stabilizes the levitators could be used to steer them. And, he adds, hanging the sensor from the levitator like a parachuter hanging from a canopy would help keep the system straight into the wind.
Still, Marsh is not convinced that such a device can withstand mesospheric conditions. “Any instrument is going to have to operate in the extreme conditions of the mesosphere, where average winds can easily exceed 100 mph,” he writes. Winds in the upper mesosphere can be particularly shearing, temperatures can drop to 140 below zero, and space weather conditions radiate through the mesosphere and can damage communication systems.
Paul Newman, chief earth scientist at NASA’s Goddard Space Flight Center, agrees that accounting for mesospheric wind will be a big technical challenge, but he can’t help but rejoice in the possible applications. “I actually think that’s a really cool idea,” he says. One possibility would be to probe the water vapor in the mesosphere, where polar clouds form so high that the sun still shines on them at night. The mysterious clouds aren’t just beautiful, Newman says; their possible link to increasing greenhouse gases means they could become more common, but researchers cannot track the water content and temperature of the mesosphere as well as they would like. Mesospheric clouds are “another sign of climate change. And we need information to show it, ”Newman says. “That’s why it might be really cool to get data on atmospheric composition.”
Newman adds that the thinness and levitating ability of the plates could also be intriguing for research on Mars. The atmospheric pressure of the Martian atmosphere is similar to the Earth’s mesosphere, so perhaps light, self-contained levitators could collect measurements of temperature or composition. “You can take off once a day, climb up and down and land on your little Martian lander,” he imagines. “We don’t have this information on Mars. It would be just fantastic. (NASA plans to test a small helicopter called Ingenuity as part of his soon ashore Mission of the Perseverance rover, but this machine will be much bigger and is still at the stage of the test flight; he is not yet ready for science missions.)
Bargatin says they are currently exploring applications for Mars and the team also hopes to make their microflyers work at sea level on Earth. But whatever the eventual use, Azadi will always remember seeing Mylar’s creation float for the first time, exactly according to his theoretical predictions. “After that,” he said, “I called my girlfriend and said, ‘I think I’m going to graduate soon.’”
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