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Professor of Samara University Department of Space Engineering Olga Starinova and Director of the Center for Theoretical Physics of the City University of New York Roman Kezerashvili told Indicator.ru how the general theory of relativity can be proved with the help of a space sailboat, how it can help in the exploration of Mars and why the calculation of its trajectory resembles a chess game.
A solar sail is a device that uses the pressure of sunlight (or laser) on a mirror surface to propel a spacecraft. Although the idea of using this kind of device still causes a lot of controversy and skepticism, scientific research in this area is gaining momentum around the world.
The convincing evidence that paved the way for the future of interplanetary space sailing ships was the successful launch of the Japanese IKAROS apparatus on May 21, 2010. In June 2019, LightSail 2 Cubesat flew into space with a 32 m² solar sail. It was the first satellite to successfully change the trajectory of its movement with the help of a sail. At present, enthusiasts supported by Yuri Milner and Stephen Hawking are developing the Breakthrough Starshot project, a solar-sail probe that is planned to be accelerated with lasers to one fifth of the speed of light, which is supposed to allow it to reach Alpha Centauri in 20 years.
One of the leading centres for the development of the idea of moving spaceships with the help of a solar sail is Samara National Research University. Research in this area has been conducted at the university since the beginning of the 1990s, now it is headed by Professor of Space Engineering Department Olga Starinova. Since 2016, the project received additional recharge thanks to cooperation with the theoretical physicist, Member of the International Academy of Astronautics, Director of the Center for Theoretical Physics of the City University of New York Roman Kezerashvili. In June 2019, he officially joined Samara University research group, which is working on an innovative sail design. Recent works of the scientists have been published in Acta Astronautica and Advances in Space Research journals. We talked to the researchers about what the solar sailing ship of Samara University is going to be and what tasks it can help to solve.
- How did you come up with the idea of the device, where the main engine is sunlight? And how does a solar sailboat work?
Olga Starinova: The first spacecraft project using a solar sail was patented in 1924 by Friedrich Zander, shortly after Peter Lebedev experimentally confirmed Maxwell’s theoretical prediction of light pressure that is exerted on solid bodies. This is the concept of a solar sailing ship – a special type of a spacecraft, where the driving force is solar pressure, but not jet engines. The principle of movement is similar to ordinary ships, but there the wind blows the sails, and in our case, the sunlight exerts the pressure on the sail. The solar sail unfolds and focuses on the Sun. The particles of sunlight – photons – possess an impulse and transmit it to any illuminated surface, creating pressure, which carries the spacecraft. When the ship moves away from the Sun, the pressure decreases, but theoretically by that time the device can already pick up the initial speed, which allows it to move to the target.
However, of course, it is not so simple. The light pressure is very small. The surface of the sail should be huge and at the same time as light as possible, so that the sailboat receives sufficiently large acceleration. For this, a solar sail is made of the thinnest metallized mirror film, the thickness of the surface is calculated so that it is minimal and at the same time does not become transparent. In addition, it is necessary to be able to deploy this fragile canvas in space, to equip it with mechanisms that protect against collapse and to calculate the dynamics of flight.
– Are solar sailboats interesting from a research point of view?
Roman Kezerashvili: I, as a theoretical physicist, am fascinated by the idea that space sailing ships can provide convincing evidence of fundamental laws that are almost impossible to obtain under the Earth's conditions. In particular, this refers to the general theory of relativity. Due to the space sail, we can get close to the Sun, see, how its gigantic mass bends space and time, and then record this fact. When the space sailboat approaches the Sun, it will begin to revolve around it, and then the Sun will carry space itself along with the sailboat into this rotation. The trajectory of motion in a static space and in a rotating one is different. Deviations associated with the “frame-dragging” effect affect the sail’s rotation period and can be tracked and measured. We can calculate the classical trajectory, see the real one, and then compare them. And that is not all. Since large masses have the property to bend not only space, but also to affect time, we will find another effect: the time near the Sun will slow down for a sailboat.
– Are there any other possibilities to fix such effects?
R. К.: Yes, this type of phenomenon can be observed another way, for example, at the perihelion of Mercury, but for this you need to wait for a hundred years. Why do we observe the perihelion shift only on Mercury? Because it is the closest to the Sun. The Earth also has such a shift, but in this case, to measure it, you will have to wait for about a thousand years. And the experiment with a space sailing ship can be completed in one year: the sailing boat will need only 71 days to make one revolution, and with each new revolution the effect of space curvature will increase.
О. S.: Of course, the ability to confirm the theory of relativity is only one of the possible applications of space sailing ships. There are others, no less interesting. For example, today the solar sail is practically the only device that can move away from the Sun by its thrust. To get out of its scope it is able to develop sufficient speed. The duration of flights to other stars is gigantic, but we can, for example, launch the device into the Oort cloud or into the focus of the gravitational lens of the Sun. For comparison: the most high-speed and "remote" space probe Voyager-1 will take 300 years to reach the cloud. The solar sail that we are now designing is able to reach it in 20-30 years. The figures are different. This means that we can receive unique research information in our lifetime or the next generation will receive it. We will be able to get convincing evidence of the existence of the cloud itself, supposedly generating all comets, and to study the substance that is left from the formation of the solar system about 4.6 billion years ago.
— How will a solar sailboat develop such high speed?
R. К.: Firstly, this new space sailing ship will be round, secondly, it will be an inflatable structure, and thirdly, it will receive jet acceleration without jet engines. Our device is a torus, in other words, a “bagel”, in which the middle part is covered with a plate. The plate is a solar sail, it will open due to the fact that the gas will come into the torus at a certain moment and will inflate the whole structure. The surface of our solar sail will be covered with a special substance. The trick is that when the sailboat approaches the Sun, due to heating, this substance will begin to turn from a solid state into a gaseous one and evaporate, which will provide the structure with acceleration according to the principle of a jet engine. Sublimation (evaporation of a substance without a phase transition into a liquid) will quickly reach the speeds of about 300-400 km per second, and then the Sun will continue to push the sail forward with the pressure of sunlight.
This method demonstrates how the thing that prevents us can turn into our ally. The usual view is the necessity to cope with high temperature near the Sun, as it can melt the sail! I, on the contrary, suggested using this. It is the high temperature that triggers sublimation, it gives the structure great acceleration, the sailboat is rapidly moving away from the Sun, losing energy, and, therefore, is cooling.
– That means that you need to constantly provide the sailboat with a balance of temperature conditions near the Sun, right?
О. S.: Yes. We are in the position of a tightrope walker – but in our case we deal with numbers to calculate this balance accurately. We must calculate the trajectories that would allow us to get as close to the Sun as possible, and also “fit” into the restrictions on the surface temperature of the sail. This becomes especially significant when we talk about distant missions, to the same Alpha Centauri. We need to pump the sail with solar energy, without burning it. In addition to the trajectory and distance, we also calculate the sail itself, analyze what tension arises in it, what speed of such an apparatus will be, and at what angle it should approach the Sun.
R. К.: This interesting step was suggested by Olga. The idea is that by adjusting the position of the sail and the angle relative to the Sun, you can change its temperature. For example, in one position the sail will warm up to 1200 ° C, and if you change the angle, the temperature will drop to 800 ° C. And this means that we can come much closer to the Sun and the apparatus will not melt.
О. S.: Yes, by slightly deviating the sail from the Sun, we will lower the temperature, but, of course, the flight direction will also change. Ballistics – the calculation of the necessary maneuvers – resembles a complex chess game, where you count all the moves available to you in advance.
R. К.: The difficulty is that for long-haul flights, when the object develops high speeds, calculations should be carried out, relying not on Newton’s laws, but on the general theory of relativity, which takes into account the curved geometry of space-time (i.e. gravity) and relativity, taking into account gravity. For example, you are going to fly to the gravitational focus of the Sun, having adjusted the apparatus, but it will not get there if you do not take into account the effects of the general theory of relativity. In this case, the deviation can be about 6 million km, moreover, there will be an error in time – the device will arrive later.
— It turns out, that solar sailing ships are designed primarily for long-distance missions, right?
О. S.: No, not necessarily. At the Department of Space Engineering, we have already calculated interplanetary flights to all the planets of the solar system. Our studies have revealed that all the planets of the solar system can be reached by the usual traditional sail – this is a polyamide film with aluminum coating. It is quite possible to send a small spaceship weighing 100-200 kg to Jupiter, Saturn or to Mars with a research mission. It can exist in orbit almost indefinitely, transmitting scientific data. In a solar sailing ship, the resource is limited only by surface degradation and the resource of electronics.
— What future do you see for solar sailing ships? Can they become space cruisers with passengers on board?
О. S.: If they do, then far not soon. To take people on board, a very large apparatus with a giant sail will be required, while real devices weigh less than 100 kg – this includes the device itself, the sail and scientific equipment. Now research is going in exactly the opposite direction – the size of the apparatus does not increase, but decreases; nanosatellites with a solar sail are being designed. Such devices require less cost, they are easier to put into orbit, and for research purposes they are simply irreplaceable.
The applied problems that can be solved with the help of space sailing ships in the very near future are also interesting. For example, the world space community is now actively developing manned flight projects to Mars. In addition to the tasks associated with the delivery of the mission, it is obvious that it is necessary to think over the technologies that ensure a person's stay on the Red Planet. Mars is not the warmest place, the average temperature on its surface is −50 ° С. With the help of space sailboats it is possible to maintain favourable temperature for people at the Martian base. Due to its physical features, the sail is also a mirror and can reflect sunlight to a given surface area. The reflected light will heat the solar collectors or illuminate the solar panels even at night, ensuring the proper temperature and increasing the generation of electricity at the base. Due to the sail, such a device can be in non-Keplerian orbit for an infinitely long time. For the first time we formulated this idea at a congress in Mexico, then at the Department of Space Engineering we developed an algorithm and a programme and calculated the launch trajectories of such an apparatus. Therefore, speaking about the future of space sailing ships, we can say: despite the fact that we are only taking the first steps in this area, we are now on the verge of new revolutionary technologies and scientific discoveries.
Source: indicator.ru
