lightsail 2's wing in the foreground, Earth and Australia in the background
LightSail 2 captured this image as it cruised over Australia.The Planetary Society

Long before anyone considered exploring space with thrusters, German astronomer Johannes Kepler—noticing that the sun appeared to blow back the tails of comets—dreamt about getting around on celestial winds. “Provide ships or sails adapted to the heavenly breezes,” he wrote to Galileo Galilei in 1608, “and there will be some who will brave even that void.”

Four centuries later, a crowd-funded spacecraft, the LightSail 2, has become the latest to do just that. While wind as we know it certainly doesn’t blow in space, the ship can directly harness something else: modest nudges from sunbeams that tweak its path as it orbits around the Earth. The Planetary Society, a private space exploration organization, spearheaded the development of the spacecraft to prove that the tiny force exerted when light hits the backside of its shiny sail suffices as a form of transportation—a technology that could someday open up new types of space missions and provide crucial insight into destructive solar storms.

After nearly half a year of experimentation, the LightSail 2 team has learned what it takes to pilot the ultimate solar-powered space vehicle. Although it orbits hundreds of miles above the International Space Station, the thinnest reaches of Earth’s atmosphere extend farther still, slowly dragging satellites down to their doom. While the craft hasn’t been able to beat such air resistance consistently enough to fly away from the Earth, improved maneuvering has noticeably slowed the vehicle’s sinking, and on occasion even reversed it.

“There have been a number of intervals where we’ve also been able to increase the energy of the orbits,” says Justin Mansell, a PhD candidate at Purdue University and co-author of a recently released paper describing the vehicle’s performance so far. “That’s justified our mission success.”

LightSail 2 is not humanity’s first success at sailing on sunbeams. That honor goes to IKAROS, a roughly 50-foot by 50-foot Japanese spacecraft weighing hundreds of pounds. In 2010, it caught a measurable push from the sunlight in interplanetary space on its way to Venus.

LightSail 1 eventually became the next solar sail to safely reach space in 2015. The test craft succeeded in its primary objective of spreading its sail, but with no way to control its orientation it soon floundered, fell back toward Earth, and burned up in the atmosphere.

The sequel, The Planetary Society hoped, would realize the hopes and dreams of the more than 50,000 donors who had contributed a collective $7 million dollars to fund the two prototypes. Hitching a ride with two dozen Department of Defense satellites on SpaceX’s Falcon Heavy last June, it entered an orbit more than 400 miles above the Earth—nearly twice as high as its predecessor. Between the thinner air and a proper control system, the team aimed to achieve true altitude-adjusting, solar sail-powered flight.

But learning to control the vehicle hasn’t been smooth sailing. Unfurled, the feather-weight sail (made from a material called Mylar) could cover a boxing ring, but all steering power originates from a loaf-of-bread-sized body stuck to the middle. At just 11 pounds, LightSail 2 boasts the best heft-to-size ratio and the quickest acceleration of any solar sail to date. But that superlative makes it cumbersome to control, and as any sailor knows, tight control is crucial to getting where you want to go. “There are aspects that are weirdly analogous to sailing a ship,” says Mansell, who became certified to sail terrestrial ships in 2018.

LightSail 2 sails fastest when moving away from the sun, as the star’s rays push the craft “downwind.” For that part of the orbit the team points it away from the sun for maximum light exposure (imagine the most perilous position to hold an umbrella in during a gust of wind). Later on in its hour-and-a-half long orbit, however, it slips into the Earth’s shadow—doldrums for a solar sailor. It then swings back around the other side, struggling “upwind” against the sunlight’s pressure. For this part of the orbit, the team aims to align the sail’s edge toward the sun to minimize pushback.

To steer, the craft has two systems. Its modest body holds a small “momentum wheel,” which, when spun, twists the craft in the opposite direction. LightSail 2 also features two electrified metal coils that generate magnetic fields. These coils can grab onto the Earth’s magnetic field, letting operators align the spacecraft. Figuring out how to steer most efficiently accounts for much of the solar sailing learning curve, Mansell says.

But after five months of practice and data collection, the solar mariners feel confident that the vessel’s sail is helping it stay aloft. During periods when the sailors take their hands off the controls, the vehicle falls an average of 113 feet closer to Earth due to drag from the thin air. While actively sailing, however, LightSail 2 tends to drop just 65 feet. On their best day of sailing the team managed to raise the orbit by 24 feet. With sparse satellite information from this band of space the team wasn’t originally sure what kind of performance to expect, Mansell says, so the spacecraft is also doubling as a probe of far-atmospheric thickness.

“Even though we’re sailing really well, there’s days when the atmosphere is a bit more dense than normal,” he says. “Other days when the atmosphere is a little bit thinner at that altitude, we have a better chance at raising the orbit.”

To raise the orbit regularly, team members estimate that LightSail 2 would need to start out at least 500 miles above the surface. In its current location about 400 miles in the air, the vehicle is fated to continue its downward spiral and eventually flame out in the atmosphere—perhaps this summer. Before then, however, the team has two more experiments to run. They first plan to test their ability to hold a steady course, keeping the craft pointed straight at the sun. And as LightSail 2 really starts to plummet, they’ll try controlling the speed of its fall by changing its orientation.

The Planetary Society has no current plans for a LightSail 3, Mansell says, but the future of solar sailing lies in deep space. Solar sails work best far from Earth, where no atmospheric traces can slow them down and where they aren’t constantly dipping into shadow. NASA plans to launch a light-driven craft as early as next year aboard the first test of its upcoming Space Launch System, and the LightSail 2 team has helped consult. The Near-Earth Asteroid Scout will, as its name suggests, fly on a solar sail to a nearby asteroid and scout it.

Later on, solar sails might also make new types of missions possible. Traditional spacecraft use thrusters for quick tweaks to their paths, but generally remain confined to the same elliptical orbits that planets follow, as Johannes Kepler once discovered. The sun’s gravity governs these celestial routes, but adding a constant push from sunlight enables so-called “non-Keplerian” orbits. One could, for example, indefinitely balance a solar sail spacecraft between the Earth and the sun at a point beyond the reach of a vehicle relying on thrusters. If equipped with sun monitoring instruments, such an outpost could provide warning for solar storms up to a dozen times earlier than current space-based solar observatories.

Mansell says he’s excited for the future of solar sailing, and grateful to have participated in the team helping it get off the ground. “It’s been an incredible opportunity to be part of a mission that means so much to so many people.”

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