What Is Solar Sailing, and How Does It Impact the Environment?

An illustration of a solar sail above the Earth.
An illustration of a solar sail above the Earth.

Mark Garlick/Science Photo Library/Getty Images

Solar sailing is done in space, not at sea. It involves using solar radiation rather than rocket fuel or nuclear energy to propel spacecraft. Its energy source is nearly unlimited (at least for the next few billion years), its benefits can be substantial, and it demonstrates the innovative use of solar energy to propel modern civilization.

How Solar Sailing Works

A solar sail works the same way that photovoltaic (PV) cells work in a solar panel—by converting light into another form of energy. Photons (light particles) don't have mass, but anyone who knows Einstein's most famous equation knows that mass is merely a form of energy.

Photons are packets of energy moving by definition at the speed of light, and because they are moving, they have momentum proportional to the energy they carry. When that energy hits a solar PV cell, the photons disturb the cell's electrons, creating a current, measured in volts (thus the term photovoltaic). When a photon's energy hits a reflective object like a solar sail, however, some of that energy is transferred to the object as kinetic energy, just as happens when a moving billiard ball hits a stationary one. Solar sailing may be the only form of propulsion whose source is massless.

Just as a solar panel produces more electricity the stronger the sunlight hitting it, so too a solar sail moves faster. In outer space, unprotected by Earth's atmosphere, a solar sail is bombarded with portions of the electromagnetic spectrum with more energy (such as gamma rays) than are objects on the surface of Earth, which is protected by Earth's atmosphere from such high-energy waves of solar radiation. And since outer space is a vacuum, there is no opposition to the billions of photons striking a solar sail and moving it forward. As long as the solar sail remains close enough to the Sun, it can use the Sun's energy to sail through space.

A solar sail operates just like the sails on a sailboat. By changing the angle of the sail relative to the Sun, a spacecraft can sail with the light behind them or tack against the direction of light. The speed of a spacecraft depends on the relationship between the size of the sail, the distance from the light source, and the mass of the craft. Acceleration can also be enhanced by the use of Earth-based lasers, which carry higher levels of energy than ordinary light. Because the bombardment of the Sun's photons never ends and there is no resistance, the acceleration of the satellite increases over time, making solar sailing an effective means of propulsion over long distances.

Environmental Benefits of Solar Sailing

Getting a solar sail into space still takes rocket fuel, since the force of gravity in Earth's lower atmosphere is stronger than the energy that a solar sail can capture. For example, the rocket that launched LightSail 2 into space on June 25, 2019—SpaceX's Falcon Heavy rocket—used kerosene and liquid oxygen as rocket fuel. Kerosene is the same fossil fuel used in jet fuel, with roughly the same carbon dioxide emissions as home heating oil and slightly more than gasoline.

While the infrequency of rocket launches makes their greenhouse gases negligible, the other chemicals that rocket fuel releases into the upper layers of Earth's atmosphere can cause damage to the all-important ozone layer. Replacing rocket fuel in outer orbits with solar sails reduces the cost and atmospheric damage caused by burning fossil fuels for propulsion. Rocket fuel is also expensive and finite, limiting the speed and distance that spacecraft can travel.

Solar sailing is impractical in low-Earth orbits (LEOs), due to environmental forces like drag and magnetic forces. And while interplanetary travel beyond Mars becomes more difficult, due to the decreasing energy in sunlight in the outer solar system, spacecraft solar sailing can help reduce costs and limit damage to Earth's atmosphere.

Solar sails can also be paired with solar PV panels, which convert sunlight into electricity just as they do on Earth, allowing the satellite's electronic functions to continue working without other external fuel sources. This has the added benefit of allowing satellites to remain in a stationary position over the poles of the Earth, thus increasing the ability to constantly monitor by satellite the effects of climate change on the polar regions. (A “stationary satellite” normally stays in the same place relative to the Earth by moving at the same speed as the Earth's spin—an impossibility at the poles.)

Illustration of a future solar sailing spacecraft studying the exoplanets in the Centauri system
Weak sunlight means solar sailing still faces challenges when attempting to explore deep space.

Photon Illustration/Stocktrek Images/Getty Images

A Timeline of Solar Sailing
1610 Astronomer Johannes Kepler suggests to his friend Galileo Galilei that some day ships could sail by catching solar wind.
1873  Physicist James Clerk Maxwell demonstrates that light exerts pressure on objects when it reflects off of them.
1960  Echo 1 (a metallic balloon satellite) records pressure from sunlight.
1974  NASA angles the solar arrays of Mariner 10 to work as solar sails on its way to Mercury.
1975  NASA creates a prototype of a solar sail spacecraft to visit Haley's Comet.
1992  India launches INSAT-2A, a satellite with a solar sail meant to balance the pressure on its solar PV array.
1993  The Russian Space Agency launches Znamya 2 with a reflector that unfurls like a solar sail, though this is not its function.
2004  Japan successfully deploys a non-functioning solar sail from a spacecraft.
2005  The Planetary Society's Cosmos 1 mission, containing a functional solar sail, is destroyed on launch.
2010  Japan's IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) satellite successfully deploys a solar sail as its main propulsion.
2019 The Planetary Society, whose CEO is famed science educator Bill Nye, launches the LightSail 2 satellite in June 2019. LightSail 2 is named one of TIME magazine's 100 Best Inventions of 2019.
2019 NASA selects the Solar Cruiser as a solar sail mission for deep space research.
2021 NASA continues development of the NEA Scout, a solar sail spacecraft meant to explore near-Earth asteroids (NEA). Planned launch is November 2021, delayed from May 2020.

Key Takeaway

Solar sailing still requires fossil fuels to launch spacecraft into orbit or beyond, but it nonetheless has its environmental benefits, and—perhaps more importantly—demonstrates the potential of solar energy to solve Earth's most pressing environmental problems.

View Article Sources
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