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Sailing by Sunlight: Solar Sail-Propelled Spacecraft Launches Tonight

Weather permitting, a SpaceX Falcon Heavy rocket will blast off into the dark Florida sky late tonight (live stream here). In each of its 27 first-stage engines, liquid propellant RP-1 will mix with liquid oxygen, igniting chemical reactions that will thrust the 3-million-pound system into the night sky.

After its first-stage engines are spent and the boosters fall away, a second-stage engine will kick in, powered by the same reaction. This engine will provide the final pushes required to deliver the 24 satellites on board to their appropriate orbits.


Artist's concept of LightSail 2 above Earth. Image credit: Josh Spradling / The Planetary Society.

Folded up neatly within one of these satellites is a small, crowdfunded package that will be set free a week after launch. The spacecraft inside that package—LightSail 2—will be powered by a completely different, much gentler propulsion system than the one that carried it to space. If all goes as planned, LightSail 2 will be the first satellite to go around the Earth in a controlled orbit propelled solely by sunlight. (In case you’re wondering, LightSail 1 was a proof-of-concept spacecraft successfully deployed for testing in 2015.)

LightSail 2 weighs just over 10 pounds and, one unfolded, will be about the size of a boxing ring. It consists of a 32-square-meter sail composed of extremely thin mylar and supported by four rigid, metallic booms. The spacecraft also has electronic systems for communication, orientation, and navigation.

A solar-powered sail has no resemblance to the solar-powered systems on Earth that convert energy from the sun into electricity. Instead, solar sails harness the momentum of photons emitted by the sun.

Shiny objects, like mirrors and pieces of mylar, reflect incoming photons. When this happens, some of the photon’s momentum is transferred to the object. In other words, the photon gives the object a tiny shove forward before bouncing backwards. One shove might not do much, but continuous small shoves can add up to a force strong enough to propel the object forward. That’s the idea behind solar sails—large, lightweight structures that harness the nonstop flow of photons traveling out from the sun for propulsion.

By carefully orienting its sail with respect to the sun, LightSail 2 should be able to use this momentum to reach a higher orbital altitude than where it starts. Simulations suggest that it should be able to increase its orbit altitude by about 0.5 km per day.

Bill Nye holding the CubeSat for the LightSail 1 project. Credit: The Planetary Society







Solar sailing isn’t a new idea, but because of its nature, there are a limited number of possible applications. LightSail 2 is designed to test how well solar sails can propel CubeSats, mini-research satellites with a standard size and weight. CubeSats have become increasingly popular ways for scientists, students, and companies to explore space. More than 1,000 have been launched since 2003 and at least that many are in development now. Solar sailing is potentially an inexpensive, small, lightweight propulsion option that could expand the research capacity of CubeSats.

A lot of people are invested in this project, and not just for professional reasons. LightSail 2 was entirely funded by individuals, in part through a wildly successful Kickstarter campaign. That’s really uncommon for such a big research effort! The project is housed within The Planetary Society, a nonprofit foundation established in 1980 to “empower the world’s citizens to advance space science and exploration.” You might be familiar with the CEO: Bill Nye the Science Guy.

The Planetary Society has been promoting and developing solar sail technology since its early days but has no plan for a LightSail 3. That’s because the technology is now mature enough for space agencies, companies, and other researchers to put it to good use, and because the society’s membership has lots of other ideas worth exploring, according to COO Jennifer Vaughn. The society plans to hold a wide-open call for science and technology project proposals next year. “[W]e're hoping to really democratize this process of getting the best ideas into the hands of the people that could potentially make these dreams into reality,” Vaughn said at a press conference on Thursday.

In the meantime, all eyes will be on tonight’s launch. “It's really exciting to be flying this thing at last. It's almost 2020 and we've been talking about for, well, for 40 years. It's very, very cool,” says Nye.

If things go as planned, the launch will be live streamed on NASA TV and on NASA Live YouTube channel at 11:30 PM EDT tonight.

Kendra Redmond 

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