Solar sails: How to fly in space without fuel

A spacecraft usually has to be accelerated by an engine. And an engine requires fuel. The farther and more complex the flight, the more fuel must be brought along—and the heavier the spacecraft itself becomes. But there is another method of propulsion that uses no fuel at all. You can use light.

This is the principle behind solar sails—giant, extremely lightweight reflective surfaces that receive a small but constant push from solar radiation. The idea sounds almost like science fiction. Yet it relies on standard physics: light is capable of transferring momentum to objects it hits. And if the surface reflects light, the effect becomes even stronger.

The authors of the study Solar Sailing – Mission Opportunities and Innovative Technology Demonstration view the solar sail precisely as a propulsion system that gains momentum from photons and therefore needs no propellant onboard.

Light can indeed push a spacecraft

Every single photon transfers a minuscule momentum to a surface. It is so small that we simply do not notice radiation pressure in everyday life. But in space, the situation changes.

If you create a reflective surface with an area of hundreds or thousands of square meters while keeping it extremely light, the impact of billions upon billions of photons begins to generate quite real thrust.

For this reason, a solar sail must possess two seemingly incompatible traits: it must be huge and nearly weightless. The ESA study considers designs with a mass of just tens of grams per square meter. The sail itself is an ultra-thin film with a reflective coating.

Reflection is fundamentally important here. If a photon is simply absorbed by the surface, it transfers its momentum. If the light is reflected back, the change in momentum is greater. For a nearly ideal reflector, the transferred momentum can approach double compared to absorption.

The result is a kind of space sailboat. However, while a conventional sail catches the wind, a solar sail catches the stream of light from the Sun.

If the thrust is so weak, what good is it?

At first glance, not much. The study looks at characteristic accelerations on the order of 0.1–1 mm/s². This is incomparable to the power of a rocket engine.

Yet a solar sail has a fundamental advantage: a rocket fires for minutes or hours until its fuel runs out. Light presses on a sail constantly. Second after second. Day after day. Month after month.

Therefore, a small acceleration steadily builds up. It is precisely the lack of need to expend fuel that makes solar sails viable for maneuvers that would require an immense fuel supply for a conventional spacecraft.

A sail can be controlled

A solar sail does not merely push a spacecraft directly away from the Sun. By changing the sail's angle relative to the sunbeams, you can alter the direction of the resulting force. In this way, the spacecraft can adjust its orbital momentum and gradually transition to different orbits—including moving not only outward from the Sun, but also spiraling inward toward the inner solar system.

Essentially, instead of firing an engine, the spacecraft changes the orientation of its gigantic mirror. This turns solar radiation pressure from a curious physical effect into a space propulsion system.

Why solar sails are needed in the first place

The primary reason is not just fuel economy. A solar sail potentially enables maneuvers that would be far too expensive or virtually impossible for a traditional spacecraft. The study offers a telling example—a spacecraft designed to study Earth's magnetotail.

This tail is continuously oriented relative to the Earth–Sun direction. A standard satellite on an elongated orbit stays in a favorable position for observation during only part of the year. To constantly rotate its orbit to follow the magnetotail, a speed change of roughly 3.2 km/s per year would be required—extremely costly in terms of fuel.

A solar sail can deliver a small, continuous force. The authors' calculations showed that a constant acceleration of about 0.14 mm/s² is enough for such an orbital change. Thus, the weakness of the solar sail unexpectedly becomes its main strength: where long-term, continuous force is required rather than a powerful short impulse, it proves particularly effective.

To Mercury on solar sails

The calculations for a flight to Mercury are even more compelling. The authors compared a traditional mission plan with a scenario where transport is provided by a solar sail.

In their calculated scenario, the flight to Mercury took about 2.4 years at a characteristic acceleration of roughly 0.3 mm/s². This required no gravity assists from other planets, eliminating any rigid dependence on favorable planetary alignments and launch windows.

The mass savings are especially striking. In the comparison provided by the authors, the solar sail mission had an estimated launch mass of 872 kg, whereas the alternative configuration using electric and chemical propulsion systems weighed 2272 kg.

The reason is simple: a conventional spacecraft must carry not only scientific instruments, but also engines, tanks, and hundreds of kilograms of propellant. Instead of fuel, a solar sail carries a large, thin reflective surface.

Reaching places where conventional spacecraft struggle

The study also considers an even more unusual possibility: approaching the Sun and then altering orbital inclination to observe its polar regions.

The proposed spacecraft could gradually descend to a distance of about 0.172 astronomical units from the Sun before transitioning into a solar polar orbit. For such a mission, the calculated square sail was truly gigantic—roughly 167 × 167 meters.

That is an area of nearly three football fields, and the entire structure must be an ultra-thin, extremely lightweight material that must first be folded for launch and then flawlessly deployed in space.

That is why the main challenge facing solar sails today is not the physics itself—which has long been understood—but the engineering.

The main challenge is building a giant sail that weighs almost nothing

The larger the sail's surface area and the smaller its mass, the more efficiently the spacecraft utilizes sunlight. But a giant film must be packed into the tight volume of a rocket, delivered into space, deployed, tensioned, and then precisely oriented.

In the demonstrator described by ESA, for example, the stowed structure was designed to occupy a volume of just about 60 × 60 × 80 cm, after which four long booms unfolded four sections to form a square sail measuring 20 × 20 meters. The larger the sail, the harder this task becomes. Engineers must simultaneously solve challenges related to structural strength, mass, control, thermal loads, and the durability of the ultra-thin film.

A spacecraft that never needs refueling

A solar sail will not replace rockets for launches from Earth. It cannot lift a payload off a planet's surface and provides no massive instantaneous thrust. Its advantage shines once in space. There, the nearly imperceptible pressure of light can be transformed into an engine capable of running for months and years without burning a drop of fuel.

This changes the fundamental logic of spaceflight. Instead of the pattern "take fuel—accelerate—deplete fuel," a new approach emerges: "deploy the sail—and let the Sun gradually alter the spacecraft's trajectory."

Consequently, the solar sail is valuable not as an exotic replacement for conventional engines, but as a tool for an entirely different class of missions: long-duration flights, continuous orbital adjustments, and reaching regions of space that are simply too expensive to access through traditional means.

As the authors of the study conclude, it is the ability to achieve an almost unlimited total velocity gain without consuming propellant that enables solar sails not only to enhance existing space missions, but to make fundamentally new ones possible.

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