Galileo to ACS3: How NASA’s solar sails could change future space travel

On September 5, NASA shared the first image captured of its solar sail prototype spacecraft flying in space.

While solar sail technology is still in the early stages of development, it could revolutionize spaceflight. The solar sail could powerfully enhance our ability to reach distant regions of space, even allowing us to send a probe to another star.

The idea’s inception dates all the way back to the time of Galileo. Its long history shows that worthwhile ideas might be momentarily forgotten, but they never truly go away.

ACS3: Taking solar sail technology to the next level

On April 23, NASA’s Advanced Composite Solar Sail System, or ACS3, launched to orbit aboard a Rocket Lab Electron rocket. A few months later, on August 29, NASA confirmed that the ACS3 solar sail was successfully unfurled. The space agency shared an image of the fully unfurled solar sail in space.

In a blog post , NASA explained that “the five sets of markings on the booms close to the spacecraft are reference markers to indicate full extension of the sail. The booms are mounted at right angles, and the solar panel is rectangular, but appear distorted because of the wide-angle camera field of view.”

NASA also explained that the spacecraft was slowly tumbling, as expected, due to attitude control being disengaged while the sail was unfurling. It will soon be re-engaged, and then engineers will initiate maneuvers to raise and lower the spacecraft’s orbit.

The LightSail 2 legacy

The ACS3 mission builds on the work of the Planetary Society’s 2019 LightSail 2 orbital mission. LightSail 2’s goal was to show that light sail technology worked.

The mission’s mylar solar sail was able to successfully raise the orbit of a spacecraft using only sunlight. The engineers behind the mission expected their spacecraft to stay in orbit for a few months before being brought down by atmospheric drag. It lasted for more than a year, exceeding all expectations.

NASA aims to go a step beyond LightSail 2 by testing composite materials – a combination of materials with different properties – in a new lightweight boom that will allow faster travel.

On its website , NASA explains how its ACS3 technology works: “Just as a sailboat is powered by wind in a sail, solar sails employ the pressure of sunlight for propulsion, eliminating the need for conventional rocket propellant.”

The ACS3 solar sail measures roughly 30 feet (9 meters) on each side, which, according to NASA, makes it approximately the size of a small apartment. The spacecraft is light enough that the propulsive force of photons, or light beams, from the sun propels it forward in microgravity. Essentially, it allows for fuel-free transportation of very light CubeSat spacecraft.

NASA has committed to developing the technology. In recent years, the space agency has announced two other solar sail missions. One, the Near Earth Asteroid (NEA) Scout, was launched aboard Artemis I. Unfortunately, NASA lost contact with the spacecraft after launch. The other, the Solar Cruiser, was canceled due to budgetary constraints.

The history of the solar sail

The Planetary Society was the first organization to successfully test a solar sail in space. However, the non-governmental organization drew inspiration from famous science popularizer Carl Sagan. He, in turn, cited Planetary Society founder Louis Friedman, as well as important historical astronomers.

In a 2021 interview with IE , Planetary Society CEO Bill Nye (of “Science Guy” fame) explained the history of the solar sail.

“The concept of solar sailing dates back to 1607, when a bright comet appeared in the night sky, which both frightened and fascinated people,” Nye explained.

“Johannes Kepler, after whom Kepler’s Laws of Planetary Motion are named, observed the tails of what came to be called Comet Halley, and concluded that there must be something about sunlight that creates a comet’s dazzling tails.”

“The notion of sunbeams interacting with a celestial object inspired Kepler to imagine ships sailing through space on starlight, just as ships sail before the wind on Earth,” Nye added.

“In 1608, he wrote a letter to his friend Galileo Galilei: ‘Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void.”

In the 1970s, Planetary Society founder Louis Friedman aimed to make that prediction a reality. He led a NASA effort to send a solar sail to Comet Halley itself. While that mission never materialized, the Planetary Society took the idea up once again in 2019, flying the first solar sail spacecraft – LightSail 2 – in Earth’s orbit.

Reaching distant stars

Other companies are attempting to take the technology even further. The Breakthrough Starshot initiative, for example, aims to send a light sail mission to our nearest star system, Alpha Centauri, within 20 years. It was co-founded by Mark Zuckerberg, Yuri Milner, and Stephen Hawking with an early backing of $100 million.

The key difference with LightSail 2 and ACS3 is that the Breaktrhough Starshot spacecraft wouldn’t be powered by sunlight alone. Instead, it would train one million lasers onto the lightsail, allowing it to travel at up to a fifth of the speed of light.

Unfortunately, the Breakthrough Starshot mission seems to have been hit but funding problems. One of the scientists working on the project, Martijn de Sterke, told IE last year that his team had stopped working on the project due to a “lack of funding.”

While de Sterke believes reaching Alpha Centauri within our lifetime is still possible, he emphasized it would “require a lot of money.”

Either way, the Breaktrhough Starshot project shows the incredible potential of light sails. That potential explains why NASA aims to progress the technology with ACS3.

In the short term, the space agency says solar sails could eventually be used for space weather early warning satellites, near-Earth asteroid reconnaissance missions, or communications relays for crewed exploration missions. Much further in the future, they may one day help to finally make us an interstellar civilization.