Sailing across the cosmos is finally becoming a reality.
For centuries, it has been a dream: traveling through space powered only by the solar wind. It was first imagined in the 1600s by Johannes Kepler, the German astronomer. Centuries later, Arthur C. Clarke moved it into the realm of science fiction in “Sunjammer,” a 1964 short story. Carl Sagan, the cosmologist and co-founder of the Planetary Society, thought it could be more than a speculative fantasy, and began in the 1970s to promote building solar sails for space exploration.
On Tuesday, solar sailing could take its next major step to becoming a proven technique for navigating the voids between worlds.
Last month, the Planetary Society’s spacecraft, LightSail 2, traveled to space aboard a SpaceX Falcon Heavy rocket. After 10 years of planning and over 40,000 private donations worth $7 million, the cubesat has reached a high enough orbit around Earth and some technical issues have been worked out by engineers on Earth. With the press of a button on the ground in California on Tuesday morning, the mission’s controllers hope that the spacecraft’s sails will unfurl, opening the curtain on the next act of spaceflight.
What is solar sailing?
One limitation of space travel is that power sources eventually run out.
But the sun is a source of constant energy. It is always releasing photons into space. While these particles don’t have mass, they have momentum. Solar sailing relies on the ever so gentle nudge of photons to push a sail forward, moving whatever is behind the sail in another direction.
Sailing could be one of the most fuel efficient options for space travel. While the force exerted on a solar sail is about the same as you might feel from the weight of a piece of paper in the palm of your hand, the momentum is able to build, increasing the speed of the sail over time.
For example, NASA’s twin Voyager spacecraft, flying on pure momentum since they ran out of fuel, needed more than 40 years from launch to reach the boundary of our solar system. But if they had solar sails, the lengths of their trips could have been nearly cut in half.
How will LightSail 2 work?
LightSail 2 aims to become the first steerable solar sail ever launched into orbit around Earth. It is a loaf-sized cubesat carrying a boxing-ring-sized solar sail. Made from a thin mylar material, these sails are designed to blossom like a space-lotus and collect the sun’s energy.
The cubesat has a momentum wheel, which allows the Planetary Society’s engineering team on Earth to guide its mylar sails. That will keep the spacecraft at a 90-degree angle to the sun at all times, not unlike the way a sailboat needs to tack into the wind to move.
As LightSail orbits Earth, engineers on the ground will attempt to extend the farthest point in its orbit, called apogee. To do this, the sail must get enough of a push from the sun, and also rely on steering from the ground.
“If everything goes perfectly, we ought to be able to raise the apogee by about 1,640 feet per day,” says Dave Spencer, an aeronautics professor at Purdue University in West Lafayette, Ind. and LightSail’s mission manager.
What will happen during deployment?
On Tuesday at 11:40 a.m. in California, 2:40 p.m. Eastern time, engineers in California will send an order to orbit, and over about two minutes, LightSail 2 will unfurl its sails.
During the deployment, two wide-angle cameras on the cubesat will capture 32 images. “It will effectively give us a kind of movie of the sail deployment,” Dr. Spencer said.
The primary mission is to last around a month, and after that LightSail could orbit Earth for up to a year. Sometimes it will be visible from Earth with the naked eye, and the Planetary Society will provide updates on where it can be seen.
Eventually, Earth’s gravitational pull will drag the cubesat back toward the atmosphere, where it will burn up.
Has this been tried before?
LightSail 2 follows LightSail 1, launched in 2015 as a test. While accomplishing some of its goals, the test was hindered by a number of engineering snags.
The very first solar sail spacecraft, Ikaros, was launched in 2010 by the Japan Aerospace Exploration Agency. Although it was not steerable, it traveled past Venus. It entered orbit around the sun and was last heard from in 2015.
Early in the next decade, NASA plans to launch, NEA Scout or, Near Earth Asteroid Scout. This small cubesat will use a solar sail to visit a near-Earth asteroid to collect data and send it back to Earth.