Solar energy generation has grown far cheaper and more efficient in recent years, but no matter how much technology advances, fundamental limitations will always remain: solar panels can only generate power during the daytime, clouds often get in the way and much of the sunlight is absorbed by the atmosphere during its journey to the ground. What if instead we could collect solar power up in space and beam it down to the surface?
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Sunlight is on average more than ten times as intense at the top of the atmosphere as it is down at the surface of the Earth. And up at a sufficiently high orbit sunlight would be available on a continuous basis, to capture all the sunlight available, able to be beamed to receiving stations across the planet, wherever it is needed.
The basic concept has been around for a long time, but has been given fresh urgency by the need for new sources of clean and secure energy to aid Europe’s transition to a Net Zero carbon world by .
Decades of research has led to a diversity of concepts using different forms of power generation, conversion and transmission principles. The so-called reference design transforms solar power into electricity via photovoltaic cells in geostationary orbit around Earth. The power is then transmitted wirelessly in the form of microwaves at 2.45 GHz to dedicated receiver stations on Earth, called ‘rectennas’, which convert the energy back into electricity and feed it into the local grid.
Space-Based Solar Power, SBSP, is based on existing technological principles and known physics, with no new breakthroughs required. Today’s telecom satellites transmitting TV signals and communication links from orbit are basically power-beaming satellites – except at a far smaller scale of size and power.
The biggest challenge is that – in order to generate optimal, economically-viable levels of solar power – the required structures need to be very large, both on Earth and in space. A single solar power satellite at geostationary orbit might extend more than a kilometre across, with the receiver station on the ground needing a footprint more than ten times larger.
It took dozens of launches to construct the International Space Station in low-Earth orbit, and would likely require an order of magnitude more launches to assemble a solar power satellite that weighs in at many thousands of tonnes. In the past, due to the high costs of launch, solar power satellites were not deemed to be economically competitive with terrestrial solutions.
But worldwide launch costs continue to trend downwards, making such construction economically feasible, and the end-result would be a continuously available source of clean energy. A single solar power satellite of the planned scale would generate around 2 gigawatts of power, equivalent to a conventional nuclear power station, able to power more than one million homes. It would take more than six million solar panels on Earth’s surface to generate the same amount.
Humans have been finding creative ways to harness the Sun's heat and light for thousands of years. But the practice of converting the Sun’s energy into electricity — what we now call solar power — is less than 200 years old. Yet in that short time, solar power has revealed the Sun’s limitless potential to power an increasingly technological society. Since the s, NASA has harnessed the energy of the Sun to power spacecraft and drive scientific discovery across our solar system. Today, NASA continues to advance solar panel technology and test new innovations.
Video credit: NASA's Goddard Space Flight Center/Lacey Young
In , French scientist Alexandre Edmond Becquerel (who was 19 at the time) was working in his father’s laboratory, experimenting with two metal sheets placed in an electricity-conducting liquid. As he shined light on the device, he detected a weak electric current — what we now know to be a flow of electrons through the material. This phenomenon was the first demonstration that light could generate electricity, known today as the photovoltaic effect.
By selenium had been incorporated in the world’s first solar array, which was installed on a New York City rooftop. Scientists continued to develop and experiment with selenium and other photovoltaic materials for the next 70 years, but practical applications were limited by their low efficiency – only about 1% of light energy could be converted to electricity.
That's when scientists at Bell Labs used an abundant material called silicon to create the first solar cell that achieved 6% efficiency. Solar panels today use this same basic design, with adjustments that have allowed industrial and commercial solar panels to achieve between 15% and 23% efficiency.
Silicon is an abundant material used in many technological applications because it is a very good “semiconductor,” or material whose ability to carry electric current can be easily manipulated by adding energy. In typical solar cells, silicon is layered in three thin sheets. A middle layer is made of pure silicon. The outer two silicon layers are injected with other elements (typically phosphorous on one side, and boron on the other) that differ in their capacity to “donate” or “accept” electrons. As light strikes the pure silicon layer, it energizes the silicon’s electrons, which then begin to move within the material. Those electrons are attracted to the silicon layer designed to “accept” electrons, leading to a buildup of negative and positive charges in the outer layers. These two sides are then connected with wires to form a circuit that facilitates the flow of electrons from one side to the other, generating usable power.
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Silicon-based solar cells power many of NASA’s spacecraft, including the James Webb Space Telescope. Learn more about why this abundant material is used in solar panels in this excerpt from NASA’s Elements of Webb video series.
The Soviet Union kicked off the space race with the launch of Sputnik on Oct. 4, , quickly followed by the United States’ Explorer 1 on Jan. 31, . But as both satellites ran exclusively on battery power, they were dead within a few weeks. In March , the United States launched the first solar-powered spacecraft, Vanguard 1 (pictured at right), which transmitted data for the next six years.
Some missions, such as NASA’s Parker Solar Probe, require specialized solar panels that can operate in extreme environments. Flying on an elliptical orbit into the Sun’s hot outer atmosphere, Parker Solar Probe uses solar panels angled away and partially shaded from the Sun. It also uses a special cooling system to ensure the system isn’t overwhelmed by heat and was designed to be extra robust to deal with the intense ultraviolet rays it receives when close to the Sun, which can degrade materials rapidly. The spacecraft’s elliptical orbit also takes it far from the Sun, even beyond Venus. Engineers designed the solar array to compensate for how the light changes at different distances to the Sun, which alters the color and intensity of the sunlight it receives.
At Jupiter, which receives 25 times less light than Earth, the Juno spacecraft (pictured at right) needs three 30-foot-long panels to generate 500 watts of energy — about how much a typical refrigerator uses. Its orbit around Jupiter also helps keep the solar panels almost constantly exposed to sunlight to maximize power generation.
Solar power becomes less viable for missions that venture even farther, where there’s not even enough light to charge a battery. Deep space missions like NASA’s Voyager 1 and 2 rely instead on energy from the radioactive decay of plutonium-238 to keep them running well into interstellar space.
NASA scientists and other researchers around the world are working to improve the efficiency and durability of solar panels. In addition to using silicon, scientists have discovered that adding a layer of minerals known as perovskites can dramatically improve panel efficiency. Perovskites help capture bluer visible wavelengths, complimenting silicon’s redder wavelength coverage and allowing a solar cell to capture more light. In , several independent research teams created small perovskite-silicon solar cells that exceeded 30% efficiency, and the best experimental cells today are approaching 50% efficiency.
Featured VideoNASA is also developing technology for flexible and rollable solar panels that can improve their use in constrained spaces. Using different materials for the base layer of a solar panel can make a panel lighter and more flexible — essential attributes for space missions that need to be packed into a small space in a rocket. The first two sets of solar arrays used by NASA’s Hubble Space Telescope in the s and s were designed with solar cells mounted to a flexible blanket-like material so they could be rolled up and stowed to fit inside the space shuttle cargo bay for launch.
In , NASA and its partners started working on the next iteration of flexible solar panels called roll-out solar arrays (ROSAs). These arrays, which unfurl like a roll of paper towels, are even lighter and more affordable than previous arrays. They have been used on NASA’s DART (Double Asteroid Redirection Test) mission, on commercial geostationary satellites, and on the International Space Station to augment its traditional solar array. NASA plans to include ROSAs on Gateway, an orbiting outpost crucial to NASA’s Artemis campaign.
NASA is also involved with envisioning the next generation of solar power usage in space. To advance the Artemis campaign, NASA tasked three companies with developing and building prototypes of vertical deployable solar array systems to power human and robotic exploration of the Moon. Most space solar array structures are designed to be used horizontally, but on the Moon, vertically oriented structures atop tall masts will be needed to maximize sunlight collection at the lunar poles, where the Sun stays close to the horizon. Scientists are also investigating the feasibility of space-based solar power, which would collect sunlight from space and beam the energy back to Earth, potentially serving remote locations across the planet to supplement power transmission infrastructure on the ground.
Along with working to improve the efficiency of solar panels, NASA is also looking beyond photovoltaics to an old technology: sails. Humans have crossed open waters by sail for thousands of years. And now, NASA is working on a system to traverse space using solar sails. Unlike photovoltaics, which work by capturing the energy of light, solar sails use the pressure of light. When a photon, or individual particle of light, bounces off a reflective solar sail, it imparts a small push. With enough photons, these tiny nudges can move an entire spacecraft, much like how traditional sails harness the multitude of tiny air molecules that make up the wind. In the future, solar sails could replace heavy propulsion systems and enable longer-duration and lower-cost missions.
In , the Advanced Composite Solar Sail System, a microwave-sized spacecraft, launched to test a new composite boom — a sail’s framework — made from materials that are stiffer and lighter than previous boom designs. The spacecraft has a solar sail measuring about 860 square feet — about the size of six parking spots. The seven-meter-long boom that holds out the solar sail can collapse into a bundle that would fit in your hand, which allowed it to fit compactly inside the spacecraft. The mission demonstrated the boom’s deployment and is now testing the sail’s performance using a series of maneuvers to adjust the spacecraft orbit using the sail angle. The technology could eventually allow for future sails up to half the size of a soccer field, enabling travel to the Moon, Mars, and beyond.
Explore NASA's Sun-related stories and download high-resolution images of the solar system, agency missions, and more.
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.
On Oct. 2, , the Moon will pass in front of the Sun, casting its shadow across parts of Earth.
Our closest star is so much more than meets the eye.
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