Sails have been in use to propel ships across lakes and oceans for thousands of years. They’re simple tools that use the wind to move vessels through the water — and in some cases, across the land. Now, NASA is using the basic features of the simple boat sail to harness the power of solar winds, allowing space vessels to sail through the stars. Many of the reasons why wind sails have persisted in use — even beyond the point at which we had access to motorized propulsion — also give the solar sail the potential to be vital to the future of human space exploration.
Conventional means of getting a ship to slip the bonds of Earth’s gravity and propel it through space require fuel. Lots of it. But it’s limited, heavy, and expensive. When sending a ship to Mars, it needs enough fuel to get there, perform maneuvers along the way, and make it back to Earth — if it’s a roundtrip flight. Fuel adds weight to a ship — at least as it pertains to the gravitational pull of the planets and other space bodies it interacts with. The heavier the ship gets, the more fuel is necessary to allow it to blast off from the surface of our planet.
Traditional fuel isn’t cheap. According to NASA, when the average Space Shuttle launched, it carried more than 835,000 gallons of fuel. The total weight of the orbiter and external tanks exceeded 1.6 million pounds, and the cost of that rocket fuel was a staggering $1,380,000.
Here’s how solar sails work
Solar sails don’t have the limitations set by traditional rocket engines because they utilize sunlight — also known as nature’s inexhaustible resource. NASA’s 2005 description says the technology requires a highly reflective sail made with material 40 to 100 times thinner than your average sheet of paper. Solar sails are deployed only after a ship is floating in space — at which point the sun does most of the work.
As the sun sends photons of energy outward into space, a solar sail is positioned to intercept. At the point of interception, each photon transfers its momentum to the sail and the craft to which the sail is attached, propelling the craft through space. Again, the whole process is similar to how wind pushes a sailboat across a body of water.
Solar sailing attempt with Sunjammer
In 2011, a NASA team comprised of manufacturer L’Garde Inc. and the National Oceanic and Atmospheric Administration started working on the Solar Sail Demonstrator project. The goal was to prove that solar sails were feasible and show their overall value to the space program. The project was nicknamed "Sunjammer" to honor Arthur C. Clarke, who wrote a story by the same name, marking literature’s proper mention of the expression "solar sailing."
A test flight was scheduled for 2015, where they planned to deploy and operate a sail some 13,000 square feet in size. It would have been about one-quarter the size of a football field and seven times larger than any other sail previously tested. Before deployment, it would sit inside a disposable support module the size of a dishwasher and only weighed 110 pounds. However, the project ended before launch in 2014, but not before the team obtained valuable data that has since been used for upcoming NASA solar sail projects.
Solar sailing from science fiction to science fact
In 2019, a small form factor satellite of 10-centimeter cubes called a CubeSat successfully deployed a prototype solar sail in space. The "LightSail 2," created by the Planetary Society, was about the size of a loaf of bread and had a solar sail of approximately 433 square feet.
NASA is currently working on new deployable structures and materials technologies with their Advanced Composite Solar Sail System. Later in 2022, they plan to launch the Near-Earth Asteroid Scout (NEA Scout) with the Orion capsule aboard the Space Launch System rocket. NEA Scout will be NASA’s first foray into deep space using solar sails.
Solar sails have appeared in science fiction tales from Alien Covenant (2017) back to Russell Saunders’ "Clipper Ships of Space," a short story released in 1951. If you believe Harvard professor Avi Loeb, aliens have been using solar sails for a while, too. It’s only a matter of time before we fulfill Johannes Kepler’s original dream of sailing through space, as written in a 1608 letter to Galileo Galilei: "Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void."