A solar sail is a way to propel a spacecraft using the pressure of light and high-velocity gases (also called solar light pressure) emitted by a star. Let's take a closer look at its device.
Using a sail means low-cost space travel combined with extended lifespan. Due to the lack of many moving parts, as well as the need to use propellant, such a ship is potentially reusable for the delivery of payloads. The names light or photon sail are also sometimes used.
Concept story
Johannes Kepler once noticed that the tail of a comet looks away from the Sun, and suggested that it was the star that produced this effect. In a letter to Galileo in 1610, he wrote: "Provide the ship with a sail adapted to the solar breeze, and there will be those who dare to explore this void." Perhaps, with these words, he referred precisely to the phenomenon of the "comet tail", although publications on this topic appeared several years later.
James K. Maxwell in the 60s of the XIX century published the theory of the electromagnetic field andradiation, in which he showed that light has momentum and thus can exert pressure on objects. Maxwell's equations provide the theoretical basis for light pressure locomotion. Therefore, as early as 1864, it was known within and outside the physics community that sunlight carries an impulse that exerts pressure on objects.
First, Pyotr Lebedev demonstrated experimentally the pressure of light in 1899, and then Ernest Nichols and Gordon Hull conducted a similar independent experiment in 1901 using the Nichols radiometer.
Albert Einstein introduced a different formulation, recognizing the equivalence of mass and energy. Now we can simply write p=E/c as the ratio between momentum, energy and speed of light.
Svante Arrhenius predicted in 1908 the possibility of pressure from solar radiation carrying living spores over interstellar distances, and, as a result, the concept of panspermia. He was the first scientist to claim that light could move objects between stars.
Friedrich Zander published a paper including a technical analysis of the solar sail. He wrote about "the use of huge and very thin sheets of mirrors" and "the pressure of sunlight to achieve cosmic speeds."
The first formal projects to develop this technology began in 1976 at the Jet Propulsion Laboratory for a proposed rendezvous mission with Halley's Comet.
How a solar sail works
Light affects all vehicles in the orbit of the planet or ininterplanetary space. For example, a conventional spacecraft bound for Mars would be more than 1,000 km away from the Sun. These effects have been factored into space travel trajectory planning since the very first interplanetary spacecraft in the 1960s. Radiation also affects the position of the apparatus, and this factor must be taken into account in the design of the vessel. The force on the solar sail is 1 newton or less.
The use of this technology is convenient in interstellar orbits, where any action is carried out at a low pace. The light sail's force vector is oriented along the sun's line, which increases the orbit's energy and angular momentum, causing the ship to move farther away from the sun. To change the inclination of the orbit, the force vector is out of the plane of the velocity vector.
Position control
A spacecraft's Attitude Control System (ACS) is needed to reach and change the desired position while traveling through the universe. The set position of the apparatus changes very slowly, often less than one degree per day in interplanetary space. This process occurs much faster in the orbits of the planets. The control system for a vehicle using a solar sail must meet all orientation requirements.
Control is achieved by a relative shift between the vessel's center of pressure and its center of mass. This can be achieved with control vanes, moving individual sails, moving a control mass, or changing the reflectiveabilities.
Standing position requires ACS to maintain net torque at zero. The moment of force of the sail is not constant along the trajectory. Changes with distance from the sun and angle, which corrects the shaft of the sail and deflects some elements of the supporting structure, resulting in changes in force and torque.
Restrictions
The solar sail will not be able to work at an altitude lower than 800 km from the Earth, since up to this distance the air resistance force exceeds the light pressure force. That is, the influence of solar pressure is weakly noticeable, and it simply will not work. The rate of turn of the sailing vessel must be compatible with the orbit, which is usually only a problem for spinning disc configurations.
Operating temperature depends on solar distance, angle, reflectivity, and front and rear radiators. The sail can only be used where the temperature is kept within its material limits. It can generally be used fairly close to the sun, around 0.25 astronomical units, if the ship is carefully designed for those conditions.
Configuration
Eric Drexler made a prototype solar sail from a special material. It is a frame with a panel of thin aluminum film with a thickness of 30 to 100 nanometers. The sail rotates and must be constantly under pressure. This type of structure has a high area per unit mass and thereforeacceleration "fifty times faster" than those based on deployable plastic films. It is a square sail with masts and twin lines on the dark side of the sail. Four intersecting masts and one perpendicular to the center to hold the wires.
Electronic design
Pekka Janhunen invented the electric sail. Mechanically, it has little in common with traditional light design. The sails are replaced by straightened conductive cables (wires) arranged radially around the ship. They create an electric field. It extends several tens of meters into the plasma of the surrounding solar wind. Solar electrons are reflected by the electric field (like photons on a traditional solar sail). The ship can be steered by regulating the electric charge of the wires. The electric sail has 50-100 straightened wires, about 20 km long.
What is it made of?
The material developed for Drexler's solar sail is a thin aluminum film 0.1 micrometer thick. As expected, it has demonstrated sufficient strength and reliability for use in space, but not for folding, launching and deployment.
The most common material in modern designs is aluminum film "Kapton" 2 microns in size. It resists high temperatures near the Sun and is strong enough.
There were some theoreticalspeculation about applying molecular manufacturing techniques to create an advanced, strong, ultra-light sail based on nanotube fabric grids where the woven "gaps" are less than half the wavelength of light. Such a material was created only in the laboratory, and the means for manufacturing on an industrial scale are not yet available.
The light sail opens up great prospects for interstellar travel. Of course, there are still many questions and problems that will have to be faced before traveling through the universe with such a spacecraft design becomes a common thing for mankind.
