SOLAR SAIL

Solar sails are a type of spaceship propulsion that uses the radiation pressure that sunlight exerts on large surfaces (also referred to as lightsails, light sails, and photon sails). Since the 1980s, a number of spaceflight missions to evaluate solar propulsion and navigation have been planned. The technology was initially applied during the 2010 launch of IKAROS, a spacecraft.

An effective comparison for solar sailing is a sailing boat; the force of the light acting on the big surface is comparable to the wind blowing a sail. Beam sailing is a technique that uses high-energy laser beams as an alternate light source to produce significantly stronger forces than would be feasible with sunlight. Compared to chemical rockets, solar sail vessels have the potential for low-cost operations, great speeds, and extended operational lifetimes. They can potentially be used again for payload delivery because they are low maintenance and don't require fuel. Solar sails make use of an occurrence that has a known, quantified impact on astrodynamics. All spacecraft, whether in interplanetary space or in orbit around a planet or tiny body, are impacted by solar pressure. Since solar pressure will displace a conventional spacecraft travelling to Mars by thousands of kilometres, this impact must be taken into consideration while calculating a spacecraft's trajectory, which has been done since the first interplanetary spacecraft were launched in the 1960s. A factor that needs to be considered in the design of spacecraft is how solar pressure influences a spacecraft's orientation.An 800 by 800 metre solar sail, for instance, experiences a total force of about 5 N (1.1 lbf) at Earth's distance from the Sun. This makes solar sails low-thrust propulsion systems, similar to spacecraft propelled by electric engines, but because they don't use propellant, the force is applied almost continuously, and the cumulative effect over time is significant enough to be taken into consideration as a possible method of propulsion for spacecraft.

Comet tails are known to point away from the Sun, and Johannes Kepler hypothesised that the Sun was to blame for this phenomenon. He stated in a 1610 letter to Galileo, "Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void." Though his articles on comet tails didn't appear for several years after he wrote those remarks, he may have had the comet tail phenomenon in mind.

James Clerk Maxwell's theory of electromagnetic fields and radiation, which demonstrates that light has momentum and may, therefore, impose pressure on things, was published between 1861 and 1864. The theoretical framework for sailing under little pressure is provided by Maxwell's equations. Interplanetary orbits, where height changes occur slowly, are the best for sailing operations. The sail force vector is oriented forward of the Sun line for outward bound trajectories, increasing orbital energy and angular momentum and causing the vessel to move away from the Sun. The sail force vector is positioned behind the Sun line for inward trajectories, which reduces orbital energy and angular momentum and causes the craft to move inward towards the Sun. It is important to note that there is no analogue to a sailboat tacking to windward—only the gravity of the Sun drives the craft towards the Sun. The force vector is turned away from the plane of the velocity vector to alter orbital inclination.

The biggest solar sail built to date is the IKAROS spacecraft developed by JAXA. IKAROS's sail's size is 196 square meters, about half the size of a basketball court. This was the first interplanetary solar sailing mission, travelling to Venus and onward on a trajectory to the far side of the Sun.Solar sail propulsion uses sunlight to propel vehicles through space, much the way wind pushes sailboats across water. The technology uses solar photons—sunlight—which are reflected off giant, mirror-like sails made of lightweight, reflective material 40 to 100 times thinner than a piece of writing paper. A novel method of spacecraft propulsion is solar sailing. Large reflective sails on a solar sail spacecraft collect the sun's light's velocity and utilise it to propel the craft forward. One example of this technology in use is the Light Sail 2 mission of the Planetary Society.

Light is made up of particles called photons. Photons don’t have any mass, but as they travel through space they do have momentum. When light hits a solar sail — which has a bright, mirror-like surface — the photons in that light bounce off the sail (i.e. they reflect off it, just like a mirror). As the photons hit the sail their momentum is transferred to it, giving it a small push. As they bounce off the sail, the photons give it another small push. Both pushes are very slight, but in the vacuum of space where there is nothing to slow down the sail, each push changes the sail’s speed. When a solar sail faces the Sun directly, photons push the spacecraft forward, away from the Sun. But a solar sail can move in other directions by tacking like a sailboat, changing the angle of the sail relative the Sun. It’s even possible to shift the spacecraft's orbit around the Sun, by angling the sail so that solar photons push against the direction it is traveling. Solar sails can also control their direction in other ways, such as changing their center of mass or using tip vanes.

Spacecraft gain most of their momentum when they are launched from Earth, and then most increase their speed or change course using chemical rockets that burn fuel that the spacecraft carries on board. But more rocket fuel means more weight, which limits how much can be carried. Most spacecraft reach their maximum speed and then coast through space or rely on gravity assists from other planets to reach their destinations. With solar sails, a spacecraft can continue accelerating as long as there is light pushing on it. Within a solar system, sunlight can continuously push on the sail, accelerating the spacecraft throughout its entire voyage. This means that solar sail-propelled spacecraft can reach speeds that would be practically impossible for chemical rockets to achieve.

Solar sailing spacecraft are also advantageous because they can be placed in orbits that would otherwise be unstable by using the sail acceleration as a balancing force.? As an example, this could enable solar monitoring missions to sit between the Earth and Sun at a closer distance than otherwise possible to provide more warning of solar storms. Current solar sails are made of lightweight materials such as Mylar or polyimide coated with a metallic reflective coating. LightSail 2 uses 4 triangular Mylar sails that are just 4.5 microns (1/5000th of an inch) thick. They unfold using 4 cobalt alloy booms that unwind like tape measures. The sails have a combined area of 32 square meters (344 square feet), about the size of a boxing ring.

There is theoretically no minimum size for a solar sail, but for the same mass spacecraft, bigger sails will capture more sunlight and accelerate the spacecraft more quickly. A NASA team in the 1970s, headed by Planetary Society co-founder Louis Friedman, proposed a solar sail with a surface area of 600,000 square meters (6.5 million square feet), that would be used to send a spacecraft to rendezvous with Halley’s comet. This is equivalent to a square of 800 meters (half-mile) by 800 meters – the size of 10 square blocks in New York City! Of course, the practicality of building and deploying such an enormous sail is questionable. But if such a sail could be successfully developed, amazing destinations could be reached. A solar sail’s speed depends on its size and its mass. A bigger sail captures more sunlight, gaining more momentum and accelerating more quickly for the same mass. For a given sail size, a lower mass spacecraft will have a higher acceleration.? The acceleration also depends on its distance from a light source and strength of the light source. As a solar sail spacecraft gets farther away from the Sun, the amount of sunlight available to it decreases, meaning that it accelerates less quickly. Theoretically, powerful lasers could be aimed at a distant solar sail, providing some extra acceleration as the spacecraft gets further from the Sun. To give a specific example of solar sail speed, Light Sail 2’s 32-square-meter sails accelerate it at just 0.058 mm/s2. In one month of constant sunlight, the spacecraft’s speed would increase by a total of 549 kilometres per hour, roughly the speed of a jet airliner at cruising speed. Larger sails, or small sails accelerated by lasers, could theoretically go much faster. In 2016, the group Breakthrough Initiatives announced a plan to send a fleet of tiny, laser-powered solar sails to our nearest star, Alpha Centauri. The spacecraft would be targeted in space by Earth-based lasers, and accelerate to 20 percent the speed of light.

Building a solar sail, especially a very large one, is a feat that still needs development. And that research and development can be expensive. But once solar sails are better tested and understood, they could be a relatively inexpensive means of propulsion. Sunlight is free and unlimited, meaning that a solar sail-propelled spacecraft could travel greater distances without having to increase the amount of fuel it carries on board, and could potentially use smaller, cheaper launch rockets for the same trip. So to travel at great speeds to distant destinations, or to provide long term stability in an otherwise unstable orbit, solar sailing could be an affordable option. The Planetary Society’s LightSail spacecraft were funded entirely by Society members, private donors, and backers of a crowdfunding campaign through Kickstarter. Together, these tens of thousands of people pooled their resources to build and launch solar sailing spacecraft. This is a historic demonstration of what is possible when people unite in support of space exploration.

The most exciting thing about solar sails is that they could open up new avenues for space science and exploration. A solar sail-propelled spacecraft could reach distant planets and star systems much more quickly than a rocket-propelled spacecraft because of the continual acceleration that solar sailing provides. The technology for interplanetary or interstellar solar sailing is still far from being developed, however. In the near-term, solar sailing can also be used effectively for other classes of missions including solar monitoring, multi-object flybys, and “pole-sitting” spacecraft for continuous observations of Earth’s or another object’s polar regions. Solar sails can also provide propulsion for CubeSats—small, inexpensive satellites that are increasingly being used by emerging space faring nations, small companies, and even school groups—allowing them to manner in space without relying on rocket fuel. The Planetary Society’s LightSail mission is demonstrating the potential use of solar sails for CubeSats.

Since the failed Cosmos 1 mission, solar sails have been successfully built and launched by the Japanese Aerospace Exploration Agency (JAXA) with their IKAROS spacecraft that first demonstrated controlled solar sailing, by NASA with their NanoSail-D spacecraft, and by The Planetary Society with our LightSail 1 spacecraft.

Many more solar sailing missions are in development, including The Planetary Society’s LightSail 2 mission and NASA’s NEA Scout mission to a near-Earth asteroid.?NASA’s Advanced Composite Solar Sail System, or ACS3, launching in 2022?will test out a larger?sail in Earth orbit than previous missions. NASA’s?Solar Cruiser mission launching in 2025 will test an even?larger solar sail the size of over?six tennis courts.