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The Planetary Society’s LightSail 2 is a winner of the 2024 Gizmodo Science Fair for testing the feasibility of using photons from the Sun as a form of propulsion for satellites.
The question
Can solar sailing effectively propel and maneuver small spacecraft, and can it be done in a low-cost, eco-friendly way?
The results
The Planetary Society’s LightSail 2 launched in June 2019, unfurling its 344-square-foot (32-square-meter) solar sail a month later. Solar sails catch photons from the Sun the way a traditional sail would catch wind, creating small bursts of momentum that propel the spacecraft. If such a spacecraft can overcome Earth’s atmospheric drag, it has the potential to reach very high altitudes.
© Vicky Leta/Gizmodo LightSail 2 was a crowdfunded project, with The Planetary Society raising $1.24 million in donations from space enthusiasts to help make it happen. The spacecraft carried a disk with names, selfies, and other messages from its fans for the duration of its flight.
The spacecraft itself was the size of a shoebox, with its sails made from mylar sailcloth tucked neatly together. When unfurled, the sails were about the size of a boxing ring.
Just two weeks after entering solar sailing mode, LightSail 2 gained 2 miles (3.2 kilometers) of altitude, making this experiment a success. “We were able to demonstrate sustained increase in our orbit altitude. We raised it about 800 meters, I believe, over that month-long timeframe,” said David Spencer, LightSail project manager and mission system manager for NASA’s Mars Sample Return Campaign.
Throughout its mission, LightSail 2 enjoyed literal highs and lows. The orbiter would sometimes lose a few meters of altitude per day, while other times gaining a few meters. It smashed into atmospheric particles as it traveled at speeds reaching 20,000 miles per hour (32,000 km/hr), causing the spacecraft to slow down. After a couple of years sailing around Earth, the experimental device began to experience a steep drop in altitude, mainly due to atmospheric drag.
“It took some of the more subtle things to be able to actually control the spacecraft,” Spencer said. “We figured out that our attitude control system had some bias to it, and we had to model that over time and update the flight software level to change that.”
“That allowed us to really do our best solar sailing of the mission two years into the mission, which is interesting because our orbit had come down quite a bit,” he added.
In November 2022, LightSail 2 reentered and burned up in Earth’s atmosphere, ending its demonstration in dramatic fashion. In total, it completed 18,000 orbits and traveled 5 million miles (8 million kilometers).
Why they did it
The idea of solar sailing was first imagined more than 400 years ago, when German astronomer Johannes Kepler observed a comet and wondered what had caused it to develop a tail of material that trailed behind it across the sky. It was sunlight itself that was pushing on the comet’s dust particles, causing them to scatter. Kepler believed that a space sail could one day capture the Sun’s rays the same way a boat sail uses the wind.
“Light is pushing on all of us right now, it’s just this little teeny tiny bit of momentum that’s conveyed by the photons of light,” said Bruce Betts, chief scientist and LightSail program manager for The Planetary Society. “If you’re in the vacuum of space and you deploy a really large, shiny sail, and you have a low-mass spacecraft, then you can harness that push and use it to sail on the radiation pressure from sunlight.”
One of the founders of The Planetary Society, Louis Friedman, developed a personal interest in solar sailing. The organization began developing the technology in the early 2000s with the help of the Russian space agency, leading them to build the first solar sail, Cosmos 1. However, the solar sail was lost in a Russian rocket failure in 2005. “The spacecraft never made it to space. We never knew what happened with that original solar sail,” said Jennifer Vaughn, chief operating officer of The Planetary Society. “From that concept, and from the momentum that had already been built, we said, let’s try again, let’s build something on our own—and that was the real birth of the idea of what became LightSail.”
LightSail 1 launched in May 2015 and was placed in low Earth orbit, but it encountered several problems. A software glitch caused the initial solar sail deployment attempt to fail. Although LightSail 1 eventually managed to unfurl its sails, atmospheric drag had already taken its toll, preventing the spacecraft from sailing across orbit as intended. The mission ended with the spacecraft’s re-entry on June 15, 2015.
The team wanted to give it another go, this time pairing cubesat technology (which was still emerging at the time) with solar sails. “The opportunity for us was, we thought, can we partner cubesat technologies and light sail technology as a propulsion method?” Vaughn said. “So that was the new big idea around LightSail 2: Can we build something that we can actually control in space?”
Thus, LightSail 2 was born. The spacecraft succeeded in using sunlight alone to change its orbit, without the need for an engine or thruster. Unlike chemical rockets that produce small bursts of momentum, solar sails provide continuous, slight thrust that can build up to higher speeds over time. What’s more, conventional spacecraft must carry propellants to orbit (which means more weight and cost), while sunlight is unlimited and free.
“Solar sails are one of the few technologies that can allow a spacecraft to get to the outer reaches of our solar system, or even go interstellar, on a timeline that is consistent with the human lifespan,” Spencer said.
Why they’re a winner
Solar sailing is based on a simple idea, harnessing the packets of energy produced by light and using them to propel spacecraft forward. Its execution, however, is a lot more complicated, which makes LightSail 2’s achievements so impressive.
The momentum that the spacecraft does get from the Sun is very small, so it is extremely challenging to scale up, and atmospheric drag is always working against that progress. “I think it was more challenging that I expected, personally,” Spencer said. “We had to overcome some of these challenges to be able to see that sustained orbit raise over time.”
The team was working with a “shoestring budget,” as they described it. “It was our biggest project ever, but it was small in terms of resources available to it,” Betts said. “It was great once we got it up there and it kept surviving.”
What’s next
LightSail 2 may have burnt up in Earth’s atmosphere, but its legacy has inspired several other missions, including NASA’s NEA Scout mission to a near-Earth asteroid, NASA’s Advanced Composite Solar Sail System to test out sail boom material in Earth orbit, and NASA’s Solar Cruiser (scheduled for a 2025 launch).
As it stands, however, LightSail 2 will not have a successor of its own. The team behind the mission is hanging up its solar sail, with no plans for a LightSail 3 mission. But other exciting missions are likely ahead, as The Planetary Society puts out a call for proposals every other year, searching for new innovative ideas to put to the test.
The team
Bruce Betts, LightSail program manager; David Spencer, LightSail 2 project manager; Jennifer Vaughn, chief operating officer of The Planetary Society.
Click here to see all of the winners of the 2024 Gizmodo Science Fair.
