On July 23rd, a tiny spacecraft in orbit around Earth unfurled a thin sheet of mylar that’s the area of a boxing ring. The result is a reflective sail that rides not on wind, but on light from the Sun to propel the vehicle through space.
The spacecraft that’s now sporting a shiny new accessory is called LightSail 2, which is operated by The Planetary Society, a nonprofit organization that advocates for space exploration. Its goal is to demonstrate a way to move a vehicle through space without relying on chemical propellants, which can be heavy, toxic, and expensive to use. This solar technology could provide a cost-effective alternative to propelling vehicles around Earth and through the Solar System. “You never run out of fuel,” Bill Nye, the CEO of The Planetary Society, told The Verge prior to LightSail 2’s launch. “That’s ideal for certain missions.”
Instead of relying on traditional fuels, LightSail 2 will rely on particles of light, known as photons, to maneuver through space. Photons may not have any mass, but they have momentum. Whenever photons collide with a reflective surface, they bounce off, imparting a tiny push on whatever they hit.
Over the course of the next month, LightSail 2 will perform something of a dance in orbit, twisting its sail back and forth to ride on sunlight. As it approaches the Sun, the spacecraft will keep its sail edge on toward the light. Then, once it’s directly in front of the Sun, it will twist and face the sail toward the Sun. “It’ll work very much like a sailboat, where you push, twist, and tack into the ‘wind,’” Nye said. “And then you twist and take advantage of sailing ‘downwind.’” If all goes well, the sunlight will push on LightSail 2, and the spacecraft’s orbit will rise slightly as it whips around the Earth.
This concept of sailing on sunbeams was a dream of famous astrophysicist Carl Sagan, who co-founded The Planetary Society. Sagan even presented a model of a solar sail on the Tonight Show Starring Johnny Carson in 1976. “It can go out from the Sun. It can tack inwards to the Sun,” Sagan told Carson at the time. “And because it has a constant acceleration, it can get you around the inner part of the Solar System a lot faster and a lot more conveniently than the usual sorts of rocket propulsion.”
For the last 10 years, The Planetary Society has worked hard to turn the dream of its former boss into a reality. It developed a CubeSat — a type of standardized small satellite about the size of a shoebox — that can deploy its own sail in space. Their design extends four cobalt-alloy beams from the spacecraft, pulling a mylar sail into its square configuration. It’s a delicate process; the mylar is thinner than the width of a human hair.
In 2015, The Planetary Society launched its first solar sail spacecraft, called LightSail 1, aboard the United Launch Alliance’s Atlas V rocket. After a few snafus, the sail deployed successfully, proving the technique could work. However, LightSail 1 was in too low of an orbit to sail properly. The atmosphere was just thick enough to overcome any pushing from the Sun’s light.
With lessons learned from that mission, The Planetary Society created LightSail 2 and launched it into space on June 25th aboard SpaceX’s Falcon Heavy rocket. Soon after the launch, it was deployed into an orbit just over 720 kilometers above Earth. At this altitude, there is less resistance from Earth’s atmosphere, which means, this time, LightSail 2 should be able to harness the momentum from the Sun to raise its orbit by about 15 kilometers.
Eventually, this orbit raising will lead to the spacecraft’s demise. LightSail 2 will only be able to raise its orbit on one side of the Earth, and each time the orbit gets higher on that side, the orbit gets lower on the other. In about a year, LightSail 2 will pass close enough to Earth that it will be dragged into the planet’s atmosphere, burning up during its descent.
However, its fiery demise won’t be a tragedy since LightSail 2 is only intended to prove that this type of in-space propulsion can work. If it manages to do that, The Planetary Society plans to share its solar sail technique with other spacecraft developers that are looking for innovative ways to maneuver vehicles through space. A solar sail could be used for something as simple as maintaining a spacecraft’s position in orbit, or it could be amplified for something as complex as sending cargo to Mars.
It all depends on how this mission goes. In the meantime, it’s possible that you could catch a glimpse of LightSail 2 as it steadily climbs to its higher orbit. Its large sail is so reflective that it may be visible to the naked eye if you happen to be looking up at just the right time. The Planetary Society will provide information about where LightSail 2 is in the sky so that stargazers can potentially spot the satellite as it rides on sunbeams.