**2. Flying in space under a solar sail**

The principle of movement in space under a solar sail is based on the effect of light pressure. Friedrich Arturovich Tsander (or F.A. Zander) (1887–1933), a Soviet scientist and inventor, one of the pioneers of rocketry, determines the emergence and development of ideas for space navigation under a solar sail. F.A. Tsander was one of the creators of the first Soviet liquid-propellant rocket and the author of the first technical design for a solar-sail spacecraft. The development of his is an engineering project for a space flight with a low-thrust engine in the form of a metal mirror. The idea of such spacefaring was first expressed by him in 1910–1912 as having a scientific and engineering sense, whereas previously it appeared in only a few works of science fiction.

In 1920, F.A. Tsander and K.E. Tsiolkovsky (1857–1935) discussed the possibility that a very thin flat sheet, illuminated by sunlight, could reach high speeds in space. As for the question of whether it is possible to use the property of photons for cosmic motion, they answered in the affirmative. Tsander was the first who not only expressed the idea, substantiating its scientific reliability and technical feasibility of its implementation, but also embodied this in 1924 into a calculated engineering design of a spacecraft with a reflecting mirror.

In 1921, a report on this project was presented by Tsander at a conference of inventors, and in 1924, it was revised and published in the journal "Technics and Life" under the title "Flights to other planets." In the same article, Tsander expressed ideas about the benefits of using ramjet engines, and about the possibility of using and constructing a solar sail and transferring energy to a moving rocket.

He applied to the Committee on Inventions for a space plane that can use huge and very thin mirrors to travel in interplanetary space. The project was presented in the form of two manuscripts, which, however, remained unpublished at that time. They will see the light only in 1961. In the middle of the twentieth century, science fiction writers again returned to solar sails (e.g., in A. Clark's story "The Solar Wind"), and then engineers and scientists. Around the same time, the short term "solar sail" appeared and took root as a successful borrowing from foreign science fiction. The idea of using a solar sail as a low-thrust engine corresponds to the historically important ideas of F.A. Tsander on space flight under the influence of light pressure and makes a feasible contribution to the development of the scientist's scientific heritage. Spacecraft flights using the energy of light pressure are no longer fiction, but the reality of projects of the present time and the near future [1, 8–13].

In 1924, Tsander, based on the formulas and results of Lebedev's experiment, proposed the first engineering design of a space sailing ship with a mirror-screen sail made of the finest metal foil. However, the problem of space navigation has remained outside the field of vision of scientists for a long time and gains popularity only in the space age. Theoretical developments and design projects are resumed,

and variants of spacecraft with solar sails, different in design and shape, appear: with flat (solid, round or rectangular), such as parachutes or inflatable balloons, multi-bladed, that is, split like a helicopter propeller, honeycomb type, such as complex mirror systems and so on.

The main guarantee of the sail's efficiency is a high, close to unity, reflectivity of a light film mirror, which creates a high windage of the entire structure. At the level of modern technical capabilities, the value of windage, that is, the surface-to-mass ratio, is of the order of 1000 cm2 /g, which makes it possible to achieve an acceleration of the order of 0.1 cm/s<sup>2</sup> , which is only six times less than the acceleration in the Earth's orbit from the gravitational actions of the Sun.

Since the second half of the twentieth century, the rapid development of the sailing theme was accompanied by an intensive increase in the number of publications in Russia and abroad. Numerous articles began to be published in magazines, and monographs appeared, in whole or in part, related to solar sails. Bold projects of sailing flights to the Moon, Mars, or Halley's comet, projects of illuminating the Earth from space with the help of a mirror sail-illuminator on the Artificial Earth Satellite, and many other fascinating technical ideas were described. In the 90s. the domestic device "Znamya" has been successfully developed and implemented, but it was not yet an interplanetary flight, but a launch into a satellite orbit in the vicinity of the Earth.

The first attempt to implement the "Znamya" project and deploy a solar sail in space was successfully carried out in 1993 [2, 4, 13]. Such space sailing ships can be used for flights to large and small planets, to meet with asteroids or comets, and to form special orbits of motion in the vicinity of the Sun or the Earth. New technologies should bring visible results in the creation of space engines based on the direct use of an unlimited source of solar energy. Over the years, numerous variants of motion patterns and new possible forms of solar sails have appeared. The technology for large-scale solar reflector designs is in its infancy.

After constructing and orbiting such mirrors of certain proportions, we obtain a self-adjusting systemic orientation with respect to the Sun for coplanar or spatial trajectories. More sophisticated options and models make it possible to control the orbital and rotational movements of the spacecraft while in motion using a solar sail.

One of the most difficult problems of mankind at the turn of the twentieth/twenty-first centuries was the problem of providing energy. One of the most sensible ways to save energy resources in space is to develop renewable or "perpetual" energy sources. Such promising energy resources primarily include the energy of the sun's rays, both thermal for filling special devices and mechanical for the formation of additional pressure forces on the surface of spacecraft or special installations of "solar sails."

The latest version involves the use of the pressure of the sun's rays on all the bodies encountered in the luminous flux. This "eternal", and at the same time environmentally friendly type of energy resources is also beneficial because it does not require any expenses for its transportation to the place of consumption.

The forms of using the thermal energy of the Sun on Earth are widely known, but the problem of using a very small mechanical pressure of the light flux for the purposes of the so-called "space navigation" turns out to be much more complicated. Flying in space under a solar sail is just a real embodiment of the idea of full or partial replacement of the energy of jet engines with the "donated" energy of the sun's rays, the pressure of which on a mirror reflective sail is able to create, albeit a small, but quite tangible thrust force in space for a spacecraft... With the help of a solar sail, you can determine or change the direction of movement in orbit or perform complex gravity assist maneuvers around large planets.
