A solar sail is a method of moving a spacecraft using the pressure of light and high-speed gases (also called the pressure of sunlight) emitted by a star. Let's consider its device in more detail.
The use of a sail involves low-cost space travel combined with an extended lifespan. Due to the lack of many moving parts, as well as the need to use a propellant, it is potentially possible to reuse such a ship for the delivery of useful cargo. Also sometimes used are the names light or photon sail.
History of the concept
Johannes Kepler once noticed that the tail of a comet is looking in the direction from the Sun, and suggested that it is the star that produces such an 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 too." Perhaps with these words he referred precisely to the phenomenon of the “tail of the comet,” although publications on this subject appeared several years later.
James C. Maxwell in the 60s of the XIX century published the theory of electromagnetic field and radiation, in which he showed that light has an impulse and thus can exert pressure on objects. Maxwell's equations provide a theoretical basis for movement using light pressure. Therefore, already in 1864 in the community of physicists and outside it was known that sunlight carries an impulse that puts pressure on objects.
First, Peter Lebedev experimentally demonstrated the pressure of light in 1899 , and then Ernest Nichols and Gordon Hull conducted a similar independent experiment in 1901 using a Nichols radiometer.
Albert Einstein presented a different formulation, recognizing the equivalence of mass and energy. Now we can write simply p = E / c as the relationship between momentum, energy and the speed of light.
Svante Arrhenius predicted in 1908 the possibility of pressure from solar radiation, transferring live spores to interstellar distances, and, as a consequence, the concept of panspermia. He was the first scientist to claim that light can move objects between stars.
Friedrich Zander published a paper incorporating a technical analysis of the solar sail. He wrote about "using huge and very thin sheets of mirrors" and "the pressure of sunlight to achieve cosmic speeds."
The first official projects to develop this technology began in 1976 at the Jet Propulsion Laboratory for the proposed rendezvous mission with Halley's comet.
The principle of the solar sail
Light has an effect on all the devices in the orbit of the planet or in interplanetary space. For example, an ordinary spaceship next to Mars will be displaced more than 1000 km in the direction from the Sun. These effects are taken into account when planning the trajectory of space travel since the very first interplanetary spacecraft of the 1960s. Radiation also affects the position of the device, and this factor should be taken into account in the design of the vessel. The force acting on the solar sail is 1 Newton or less.
Using this technology is convenient in interstellar orbits, where any action is performed at a slow pace. The force vector of the light sail is oriented along the solar line, which increases the energy of the orbit and the angular momentum, as a result of which the ship moves further from the Sun. To change the inclination of the orbit, the force vector is outside the plane of the velocity vector.
Position control
The spacecraft’s attitude control system (ACS) is required to achieve and change the desired position when traveling through the universe. The set position of the spacecraft changes very slowly, often less than one degree per day on interplanetary space. This process occurs much faster in the orbits of the planets. The control system of the apparatus using a solar sail must satisfy all the requirements for orientation.
Control is achieved by a relative shift between the center of pressure of the vessel and its center of mass. This can be achieved with the help of control vanes, the movement of individual sails, the movement of the control mass or a change in reflectivity.
A fixed position requires the ACS to maintain zero clean torque. The moment of force of the sail is not constant along the trajectory. Changes with the distance from the Sun and the angle that the sail shaft adjusts and deflects some elements of the supporting structure, which leads to changes in force and torque.
Limitations
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 strength of air resistance exceeds the strength of light pressure. That is, the influence of solar pressure is weakly perceptible, and it simply will not work. The turning speed of the sailing ship must be compatible with the orbit, which is usually a problem only for the configuration of the rotating disks.
Operating temperature depends on solar distance, angle, reflectivity, and front and rear emitters. Sail can only be used where the temperature is maintained within its material limits. As a rule, it can be used quite close to the sun, about 0.25 astronomical units, if the ship is carefully designed for these conditions.
Configuration
Eric Drexler made a prototype of a 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 construction has a high area per unit mass and, therefore, receives an acceleration of "fifty times higher" than those based on deployable plastic films. It consists of square sails with masts and paired lines on the dark side of the sail. Four intersecting masts and one perpendicular to the center to hold the wires.
Electronic design
Pekka Yanhunen invented the electric sail. Mechanically, it has little to do with the traditional light design. Sails are replaced by straightened conductive cables (wires) located 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 an electric field (like photons on a traditional solar sail). The ship can be controlled by adjusting the electrical 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 the Drexler solar sail is a thin aluminum film 0.1 microns thick. As expected, it has demonstrated sufficient strength and reliability for use in space, but not for folding, launch and deployment.
The most common material in modern designs is Kapton aluminum film 2 microns in size. It resists high temperatures near the Sun and is quite strong.
There were some theoretical assumptions about the application of molecular production methods to create an advanced, strong, ultralight sail based on fabric nanotube networks, where woven “gaps” are less than half the wavelength of light. Such material was created only in laboratory conditions, and means for manufacturing on an industrial scale are not yet available.
Light sail opens up enormous 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 the help of such a design of a spaceship becomes a familiar thing for mankind.