Solar Sails to New Heights

The space industry has long been interested in the opportunities offered by solar sails. Developing the analogy between space and maritime exploration, early space scientists and fiction writers have thought of concepts that could harness “the wind from the sun” as Arthur C. Clarke calls it in his short story “Sunjammer” from 1964. But three centuries prior, Johannes Kepler had already theorized that sails and ships could be “adapted to the heavenly breezes."

Solar sails work by using solar photons (sunlight) reflected off supersized sails made of very light material up to 100 times thinner than paper. When bouncing off the sail surface, photons transmit energy to the solar sails. Given the almost omnipresence of photons and the fact they constantly hit the sail, a constant energy transfer takes place, producing a continuous acceleration of the spacecraft.

The potential acceleration offered via solar sails can surpass that of traditional chemical propulsion: the constant push provided by sunlight will allow spacecrafts to build up speeds of up to 50,000 kilometers per second, compared to orbital speeds of 8 kilometers per second observed for the Space Shuttle. And in cases of absence of sunlight, there are known solutions that already exist via the use of lasers that replace solar photons and provide the necessary propulsion to the tails. While agility is altered, solar sails will maintain maneuverability of their spacecraft, by allowing for the tilting of the sail.

Space agencies have long explored ways to apprehend the potential of solar sails due to the potentialities identified for space exploration and the space industry overall. We have seen that solar sails pave the way for faster travel, and combining the use of solar sails with powerful lasers or microwave emitters could generate a propulsion equal to a tenth of the speed of light. This would allow us to reach Proxima Centauri, the closest star to our solar system, in about 20 years. But other advantages exist such as economies in the lift-off mass that would allow for lighter spacecraft, in turn able to carry larger payloads for lower access to space costs. This has applications for deep space exploration missions, paving the way for missions that would otherwise be impossible to conduct.

For example, it has become imaginable to design missions that constantly orbit over the pole of the sun to perform scientific research, at costs that could be divided by 10 compared to current deep space missions for a development time divided by a factor of five, according to Gama Space’s CEO. There are also industrial applications, with a growing interest for a permanent human presence in space and on the Moon in particular, and space cargo could benefit from lightweight propulsion alternatives that save available mass for cargo. This would help optimize economic and business models behind future space cargo and logistics markets. It comes at no surprise then that French shipping and logistics leader CMA-CGM is partnering with Gama Space on their technology demonstration.

The Japanese Aerospace Exploration Agency (JAXA) was the first to launch an interplanetary solar sail spacecraft named IKAROS with its 196 square meter sail that would set it on Venus’ course. NASA followed on JAXA’s heels, developing the NanoSail-D2 demonstration thought as an option for passive satellite de-orbiting that was deployed in 2011, the same year the Solar Sail Demonstrator ended up being scrapped for launch, yet providing valuable technological insights for NASA’s engineers.

This institutional push in solar sails is now taken over by new players who, for some of them, see perspectives in the commercialization of these new means of space mobility, including in the markets precedingly mentioned. In 2022, Lightsail 2 succumbed to gravitational drag and burned upon re-entry following a three-year journey orbiting the Earth thanks to a 32 square meter sail deployed a 720 kilometer altitude that allowed the spacecraft to validate its ability to change orbits. Lightsail 2 itself was the second demonstrator of the Planetary Society, a nonprofit that has been developing such demonstrators since the mid 2000s.

More recently, the launch of Gama Space’s “Gama Alpha” mission on January 3 alongside 113 other nanosatellites onboard the SpaceX Transporter-6 mission made the headlines and brought solar sails under the spotlight. The French startup has confirmed nominal early operations of its mission and is now expected to deploy a 73 square meter solar sail in spring 2023. The mission is the first of three already planned by the French startup in the coming years.

While many incentives justify the interest borne by solar sails for both exploration and the commercial space economy of tomorrow, solar sail technologies still must navigate a mare incognita of technological and market uncertainties. Technological demonstrators will still take years before presenting the market with mature technologies and the fragility of solar sails, a couple microns thick, might pose resilience problems for the future missions they will equip.

The commercial success of solar sails will also remain pegged on specific use cases in exploration and, eventually, long-distance logistics. Truth is indeed that solar sails will require compromises even when technologically mature. Below a 1500 kilometer altitude, solar sails tend to behave like giant parachutes, leading to early re-entries of satellites into the atmosphere, creating an initial limitation in Low-Earth Orbit (LEO) use cases. In addition, the very nature of solar sails, while still maneuverable, remain poorly agile which also limits ability to move a spacecraft in orbit contrary to what alternative thrust options will offer for smallsats and constellations alike – a capacity particularly useful for Earth observation, for example.

While their technical constraints will definitely limit applicable use cases of solar sails, they should most certainly be considered for nothing more but nothing less than what they pretend to be: a complementary propulsion method for specific applications that will require cost-efficient, long-distance space travel. VS Mathieu Luinaud is a manager in strategy consulting within the PwC Space Practice and a senior lecturer in Economics at Sciences Po Paris. His experience covers space domains from upstream to downstream. He has worked extensively with satellite manufacturers and satellite operators on business model development and market assessments. He is a member of the International Institute of Space Law (IISL) and currently serves as deputy-mayor for Paris 15th district.