The name of the light sail is famous these days and scientists want to use this technology to carry out space travel.

Image Source

A light sail, also known as solar sail and photonic sails, is a kind of propulsion that the spacecraft uses to propel light from the sun to the large mirrors of the spacecraft. Consider an yacht. This boat provides its propulsion by wind, which means that due to high-speed airflow to the sail, a propulsion force is created and the boat moves. The light sail also works exactly the same way, with the difference that the light sparkled on mirrors like boats plays the same role as the airflow, and makes the spacecraft move.

Instead of sunlight, high-energy laser beams can also be used to produce a far more powerful force. These beams are more energy and more powerful than the sun's rays, which makes the spacecraft move faster.

Spacecraft equipped with optical sailing can help reduce the cost of missions and be used in longer periods. Such spacecraft have only a few moving parts and do not perform a refueling process, so they can be used to move shipments between space and land for a long time.

The light sailors use a phenomenon that has an effect fixed and measured on the spacecraft to move. Solar pressure affects all spacecraft, regardless of whether they are in the interstellar space or circle around a massive mass in a given orbit. When a conventional spacecraft moves toward Mars, thousands of kilometers deviate from its original path due to the presence of solar pressure, so all of these effects must be measured and calculated during planning, so that there is no problem. In fact, scientists have been doing this for spacecraft since the 60's to this day. It's interesting to know that solar pressure also affects the orientation of a spacecraft, and this should also be considered when designing the spacecraft.

Image Source

That makes the spacecraft based on the sail optical launch power is very weak and hence spacecraft electric motor are similar, but since this spacecraft based on the sail-optic any thrust of their own, all in Solar power is available and this force is also constantly introduced into sails.

How does a light sail work?

Many people think that sailboats are driven by solar winds, just as they are for yachts. We know that in the oceans and seas, wind is the source of sailing boats; but this does not apply to solar sails, and in fact there is no wind. The pressure that the solar radiation enters on the solar sail is due to the reflection of light and a small fraction of the radiation absorbed.

Image Source

According to Einstein's equivalence relation, the momentum or flux of a photon is calculated as follows: p = E / c

We all know that a photon has no weight; how can it then move an optical sail forward? How is it possible to transfer an impassable thing to a blooming thing?

An interesting question is that the problem of transferring momentum by a photon is not something that can be easily observed. This issue is also related to the specificity of light. We have already known that there is a wave of particle behavior; in other words, to accept this explanation, we have accepted that the light-producing particles do not weighed. To know exactly how this might be, let's first look at the light as a wave and ignore the particle behavior of the photon.

Image Source

We divide it by the amount of energy transmitted by the wave, electromagnetic spectrum. A more energetic wave has a higher frequency; therefore, by dividing the frequencies, it is possible to separate the energy levels. When these waves hit the surface and absorbed by it, they will carry energy to that surface. That is why we are burned when exposed to sunlight. The ultraviolet light of the sun that we do not observe, carries a lot of energy, and this energy directly penetrates our skin. Our skin can not tolerate this amount of energy well, and so we face a burn, as if we directly touched a very hot thing.

But what if the surface can not absorb light? In other words, what will happen for a surface like a mirror that can reflect visible light?

If the wave fully hits the surface and reflects it, then there is no energy transfer to the surface. The fact is that none of the known materials is fully reflective; therefore, the light shined into the mirror, when it reflects, loses some of its initial energy and transmits it to the mirror. In many cases, this lost energy is negligible. If you're thinking about making an optical sail, this small amount of energy that remains in the mirror can move the sail a little forward.

Let's go back and this time, look at the light as a particle, and leave the wave of vision behind. Particle light still carries energy. The light in the particle is a bit more weird because its mode is such that it has the ability to carry a momentum even without a slight weight. Here, we see the momentum-energy transition; even without it's weight, this happens for all the photons of light. By shining the light into a mirror, a piece of particles hits and reflects the surface of the mirror (like the collision of the ball with the surface and returning it), and transfers part of its energy to the surface it encounters. This energy transfer gives a certain amount of momentum, and if this level floats completely in space (such as light sails), it moves slowly and speeds up.

With these interpretations, it would seem that if we consider light in the wave mode and in particle mode, we will have the same result, which is energy transfer and motion creation. In some special situations, it is better to consider math only in a wave or particle mode separately, and never see it as a wave-particle because in this case it becomes clear that light can be a factor Propulsion turns into an optical sailor.

Image Source

The term for which this energy transition is a little energy is "radiation pressure." If this pressure enters the sail that moves in the solar system, we call it "solar pressure." In other parts of the world, there are a lot of radiation; in other words, light sources are very large in the universe. With this in mind, it can be said that the optical sailor has the ability to move at any point in the known world.

Unfortunately, our sun is not a powerful source of light and can not create huge radiation pressures. If it was strong enough, we did not need to build large optical sails. The current light sails that are under construction are a few hundred square meters in size so that they can fly swiftly.

Are light sails Future Space Trips?

Scientists plan to use optical sails for many of the space missions that are leading. Such spaces do not need to be fueled and can move freely with light; it can also be sent to any point in the Solar System or beyond. According to NASA researchers working on interstellar probe design, a spacecraft equipped with a 400-yard yacht can move about 2.1 billion kilometers in a year and can within a decade or so. Most of it escapes from the sun's gravitational range.

Image Source

Bruce Campbell, a NASA aerospace engineer, says:

At present, we do not know any chemical fuel that could bring such a speed to a ship. This is while the optical sail has the ability to achieve high speeds.

As it has been said, the radiation pressure moves slightly into the sail; however, this pressure is permanent and continuous, and over time, it accelerates the movement of the ship and increases its velocity. Unfortunately, there is still a long way to go, so that light sailors come out of the conceptual design and get the color of reality. Until 2010, we did not see the introduction of a true version of the optical sails, until the same year, the Japanese explorer Ikaros with a sail area of

14 square meters was introduced. The probe was the first human structure accelerated by photons in space.

Five months later, NASA sent the NanoSail-D probe to earth orbit to display technology. The probe lifted its sails three months later and rolled around the Earth for eight months until it was fired in the atmosphere. Other technologies are now in the development so that they can turn the optical sails into commercialization. There are many people and companies that intend to invest in this field.

For example, look at the Sunjammer mission for $ 27 million. L'Garde, a Californian company, has been responsible for building sailboats and has developed a yacht with a total area of

38 square meters. The sail is made of an advanced material called Capton, which is only 5 microns thick. NASA officials say the entire collection weighs 32 kilograms and is similar in size to the dishwasher.

Image Source

Scientists plan to send a Sunjammer probe that is equipped with solar observation equipment to a point at a distance of 3 million kilometers from Earth. This is twice the L1 point spacing. The L1 point is said to be a region around the Earth that has been gravitationally stable and has so far been a host of weather satellites. It was supposed to launch the solar sail in 2015, but unfortunately, the scientists canceled the mission. Of course, if everything goes according to plan, we will see the launch of two sailboats next year.

What is your opinion about the future of this technology? 

Can it be an alternative to chemical fuels?

References:

• https://en.wikipedia.org/wiki/Solar_sail
• https://planetary.org/explore/projects/lightsail-solar-sailing/ 
• https://space.com/26011-solar-sail-tech-space-exploration.html
• https://space.com/22799-interstellar-solar-sails-light-propulsion-infographic.html
• https://zoomit.ir/2017/10/10/223209/what-is-solar-sail/