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Shoti Shoti Shipti (Solar)

Solar sails, powered by radiation and able to move in space and serve as a source of propulsion for spacecraft, are no longer science fiction. About solar sails and their corresponding space applications

Tal Inbar, the article was published in the August 2008 issue of Galileo magazine

Illustration of the Cosmos 1 spacecraft, the first spacecraft equipped with a solar sail
Illustration of the Cosmos 1 spacecraft, the first spacecraft equipped with a solar sail

In the short science fiction story "The Wind Blowing from the Sun" (written in 1963), Arthur C. Clarke Race of solar sails from the Earth to the Moon and back. In his fictional vision, Clark saw the creation of a new sport for billionaires - space flights that rely on the impulse of photons hitting thin giant sails. The force exerted by the photons is approximately 5,000 times greater than the thrust of the solar wind particles, which also hit the sail (however, there are designs for special solar sails that will also use the solar wind).

Today there are still no manned spaceflight competitions using photons or the solar wind as a motive force, but unmanned spacecraft using the idea have been built. One of them was even launched into space, however, due to the failure of the launch vehicle, it was unable to reach orbit and deploy its sails.

The dream of solar sails is indeed theoretical, but in scientific conferences and various studies it appears to be very promising - especially for unmanned research missions of the solar system, which can benefit from a large saving in the mass of the spacecraft, and hence also in the launch expenses. In this article we will briefly review the field of solar sails and their corresponding space applications. (On solar sails see also the article "Raising on the wings of the solar wind")

A solar sail - what is it?

A solar sail is a large surface with a high reflectivity of radiation (for example, of light in the visible range), capable of moving in space and serving as a source of propulsion for spacecraft. The original idea for a solar sail was put forward by Russian rocket scientist Friedrich Tsander back in 1920. Tsander, by the way, was the one who designed the Soviet Union's first liquid-fueled rocket.

The physical basis for solar sails is the fact that radiation that hits any body exerts a force on it. The type of radiation is not important, and it can be in the fields of visible light, infrared, microwaves, etc. Solar sails that will operate in space will be able to use radiation from the sun or alternatively a laser beam or radio waves that will be sent to them from the earth, the moon or satellites.

The force exerted by the sun's radiation, at the distance from it where the Earth is (an average distance of about 150 million km) is tiny - about nine million newtons per square meter of reflective material. Hence, a solar sail would have to be equipped with a sail with a huge surface area, and its mass would have to be as small as possible, in order to use this propulsion method.

The efficiency of a solar sail is a function of its ability to reflect radiation. As more radiation is reflected from it (ie less radiation is absorbed by it), the thrust provided by the sail will increase. If we want to reduce the area of ​​the sail, we can use an artificial radiation source, for example a laser. The use of an artificial radiation source will also make it possible to focus the light beam on the sail effectively - something that will reduce the area of ​​the sail. At the same time, it should be remembered that solar sails that will reflect sunlight will not depend on an external supply of radiation from an artificial source, thus simplifying the process in space.

What will the sun sails be made of?

The technical requirements for the sail are low mass, high ability to reflect radiation and the ability to store it in a folded state and deploy it in space. The material that is currently seen as the most suitable for solar sails that reflect sunlight is a thin aluminum mesh, with tiny holes in it with a diameter of half the wavelength that we wish to reflect.

Nanometric bumps will emit infrared radiation into space. This material exists today, but its mechanical strength is not sufficient for folding (for the launch phase) and retirement in space. The spaceship Cosmos 1, the first solar sail, made use of Mylar sheets (a plastic material) coated with aluminum (similar to the "escape blanket" known to travelers, soldiers and athletes). Japan successfully tested the retirement of a small solar sail made of Mylar for experimental purposes, in August 2004.

Advantages of solar sails

The spaceships we use are accelerated in the first stage of their journey into space by means of rockets, which give them sufficient speed to enter orbit around the Earth (about eight kilometers per second). However, if we want to fly somewhere else, outside of this coffee orbit, for example to the moon, Mars, Venus or the outer solar system, we must increase the speed of the spacecraft.

Cosmos 1

The spaceship Cosmos 1 was the world's first solar sail, and was first in another aspect - the budget for its development, construction and launch was collected by a public organization (The Planetary Society) from donations. The spacecraft was designed and built by the Russian Babakin Design Office, and was launched into space using a satellite launcher converted from a ballistic missile, from a Russian submarine. Unfortunately, the launch, which took place in June 2005, failed, due to a problem in the second stage of the rocket, and the spacecraft did not reach space. These days the Cosmos 2 spacecraft is being built, intended for launch in a few years.

This operation is carried out by rocket engines that burn fuel (and oxygen) - a mass that can add up to hundreds of kilograms, coming at the expense of the spacecraft's payload. Few spacecraft have so far used an ion engine, which uses xenon ions accelerated by a powerful magnetic field and emitted from the spacecraft. Such an engine is more economical in terms of propellant, but produces very little thrust (over time). A significant part of the launcher's power is invested, therefore, in accelerating the mass into space within a short period of time - the spacecraft's fuel.

Of course, in addition to the fuel we must take into account the mass of the tanks for its storage, of its transmission system and of the engines themselves. Using a solar sail will make it possible to reduce the mass of the spacecraft and channel most of the energy to accelerate the payload, rather than a large mass of the engine systems.

The biggest advantage is that you can accelerate a solar sail for a very long time - as long as radiation hits the sail, the spacecraft will continue to accelerate. Basically, using powerful lasers and proper planning of the sail, it is possible to accelerate solar sails to extremely high speeds, up to tens of percent of the speed of light - a speed that no conventional propulsion - chemical or nuclear - used by us today can achieve.

Disadvantages of solar sails

In addition to the technical difficulties involved in building a sail that is sufficiently reflective, lightweight and can be folded and retracted, the most prominent obstacle is the ability of the solar sail to move in the same direction as the radiation. The spacecraft's maneuverability - including braking upon reaching its destination - is limited, and at this stage of planning the solar sails, several conventional maneuvering engines will also be installed in them.

The enormous size of the sail will be exposed to the danger of being hit by bodies, which are rare in space, but still exist. A collision with a small meteoroid will not cause much damage: such a body will create a small tear in the sail and pass through it. But a collision with a larger body, such as an asteroid, would be devastating. The strength of the sails that are currently being designed will not allow survival in the event of collisions, so the path of the sail will have to be planned taking into account small celestial bodies (comets and asteroids) within the solar system. Of course, passing through planetary rings (such as Saturn's ring system) will not be possible either.

Another problem is the intensity of the sun's illumination when the sail moves away from it. As we know, the intensity of illumination is inversely proportional to the square of the distance from the light source. A possible solution to the dramatic decrease in radiation power could be an improvement in the return ability of the material from which the sail will be built on the one hand, and the use of artificial light sources such as lasers on the other hand. Satellites equipped with lasers to propel solar sails do not currently exist, but in the future, as solar sails become common, such satellites may be launched to various places in the solar system.

Possible applications

The use of masers (Microwave Amplification by Stimulated Emission of Radiation) could - in principle - enable the acceleration of solar sails to enormous speeds, and allow for the first time to build an interstellar spaceship and reach neighboring stars (suns) in research spaceships. Solar sails will also be able to propel spacecraft within our solar system, especially small and light spacecraft.

There is a plan to launch a nanosatellite (NanoSail-D; see "My little satellite") to be equipped with a small solar sail, to examine the feasibility of using solar propulsion for nanosatellites. There are proposals - which are seen today as customers from the world of science fiction - to use solar sails to deflect asteroids that endanger the Earth; At this point, the large mass of asteroids, combined with the low thrust of solar sails, make the solution impractical. But in the future, especially with the retirement of an array of lasers and masers in space, it will be possible to think of a system to protect the Earth from asteroids, which will make use of this technology.

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