What force holds the satellite. Why don't satellites leave orbit? Earth's imperfect gravitational field

It may seem that satellites in Earth’s orbit are the simplest, most familiar and familiar thing in this world. After all, the Moon has been hanging in the sky for more than four billion years and there is nothing supernatural about its movements. But if we ourselves launch satellites into Earth orbit, they stay there for only a few or tens of years, and then they re-enter the atmosphere and either burn up or fall into the ocean and onto the ground.

Moreover, if you look at the natural satellites on other planets, they all last significantly longer than the man-made satellites that orbit the Earth. International space station(ISS), for example, orbits the Earth every 90 minutes, while our Moon needs about a month to do this. Even satellites that are close to their planets - like Jupiter's Io, whose tidal forces warm the world and tear it apart with volcanic catastrophes - remain stable in their orbits.

Io is expected to remain in Jupiter's orbit for the rest of its life solar system, but the ISS, if no measures are taken, will be in its orbit for less than 20 years. The same fate is true for almost all satellites present at low levels around earth's orbit: By the time the next century rolls around, almost all of the current satellites will have entered the Earth's atmosphere and burned up. The largest ones (like the ISS with its 431 tons of weight) will fall in the form of large debris onto land and into water.

Why is this happening? Why do these satellites not care about the laws of Einstein, Newton and Kepler and why do they not want to maintain a stable orbit all the time? It turns out there are a number of factors causing this orbital turmoil.

This is perhaps the most important effect and is also the reason why satellites in low Earth orbit are unstable. Other satellites - like geostationary satellites - also fall out of orbit, but not as quickly. We are accustomed to consider “space” everything that is above 100 kilometers: above the Karman line. But any definition of the boundary of space, where space begins and the atmosphere of the planet ends, will be far-fetched. In reality, atmospheric particles extend far and high, but their density is becoming less and less. Eventually the density drops - below microgram per cubic centimeter, then a nanogram, then a picogram - and then we can more and more confidently call it space. But atmospheric atoms can be present thousands of kilometers away, and when satellites collide with these atoms, they lose momentum and slow down. Therefore, satellites in low Earth orbit are unstable.

Solar wind particles

The Sun constantly emits a stream of high-energy particles, mostly protons, but there are also electrons and helium nuclei, which collide with everything they encounter. These collisions, in turn, change the momentum of the satellites they collide with and gradually slow them down. After enough time has passed, the orbits begin to become disrupted. And although this is not the main reason for the deorbiting of satellites in LEO, for satellites further away it is more important, as they get closer, and with it atmospheric resistance increases.

Earth's imperfect gravitational field

If the Earth did not have an atmosphere like Mercury or the Moon, would our satellites be able to remain in orbit forever? Not even if we removed the solar wind. This is because the Earth - like all planets - is not a point mass, but rather a structure with a variable gravitational field. This field and changes as the satellites orbit the planet result in tidal forces affecting them. And the closer the satellite is to Earth, the greater the impact of these forces.

Gravitational influence of the rest of the solar system

Obviously, the Earth is not a completely isolated system in which the only gravitational force that affects the satellites comes from the Earth itself. No, the Moon, the Sun and all the other planets, comets, asteroids and more contribute in the form of gravitational forces that push the orbits apart. Even if the Earth were a perfect point - say, collapsed into a non-rotating black hole - without an atmosphere, and the satellites were 100% protected from the solar wind, those satellites would gradually begin to spiral into the center of the Earth. They would remain in orbit longer than the Sun itself would exist, but this system would not be perfectly stable either; The satellites' orbits would eventually be disrupted.

Relativistic effects

Newton's laws - and Keplerian orbits - are not the only things that determine the motion of celestial bodies. The same force that causes Mercury's orbit to precess an extra 43" per century causes the orbits to be disrupted by gravitational waves. The speed of this disruption is incredibly low for weak gravitational fields (like those we find in the Solar System) and for large distances: it would take 10,150 years for the Earth to spiral down towards the Sun, and the degree of disruption to the orbits of near-Earth satellites is hundreds of thousands of times less than this . But this force is present and is an inevitable consequence of the general theory of relativity, effectively manifesting itself on the planet’s closer satellites.

All of this not only affects the satellites we create, but also the natural satellites we find orbiting other worlds. The closest moon to Mars, Phobos, for example, is doomed to be torn apart by tidal forces and spiral down into the Red Planet's atmosphere. Despite having an atmosphere that is only 1/140th the size of Earth's, Mars's atmosphere is large and diffuse, and, in addition, Mars has no protection from the solar wind (unlike Earth with its magnetic field). Therefore, after tens of millions of years, Phobos will be gone. It may seem that this will not happen soon, but this is less than 1% of the time that the Solar System has already existed.

But Jupiter’s closest satellite is not Io: it is Metis, according to mythology the first wife of Zeus. Closer to Io are four small moons, of which Metis is the closest, at just 0.8 Jupiter radii from the planet's atmosphere. In the case of Jupiter, it is not atmospheric forces or the solar wind that are responsible for the disruption of orbits; With an orbital semi-axis of 128,000 kilometers, Metis experiences impressive tidal forces, which are responsible for this moon's spiral descent towards Jupiter.

As an example of what happens when powerful tidal forces dominate, comet Shoemaker-Levy 9 and its collision with Jupiter in 1994, after it was completely torn apart by tidal forces. This is the fate of all satellites who spiral towards their home world.

The combination of all these factors makes any satellite fundamentally unstable. Given sufficient time and the absence of other stabilizing effects, absolutely all orbits will be disrupted. After all, all orbits are unstable, but some are more unstable than others.

"Man must rise above the Earth - into the atmosphere and beyond - for only in this way will he fully understand the world in which he lives."

Socrates made this observation centuries before humans successfully launched an object into Earth orbit. But still ancient Greek philosopher, seemed to understand how valuable a view from space could be, although he had absolutely no idea how to achieve it.

This concept—of how to launch an object “into the atmosphere and beyond”—had to wait until Isaac Newton published his famous cannonball thought experiment in 1729. It looks something like this:

“Imagine that you placed a cannon on top of a mountain and fired it horizontally. The cannonball will travel parallel to the surface of the Earth for a while, but will eventually succumb to gravity and fall to the Earth. Now imagine that you keep adding gunpowder to a cannon. With additional explosions, the core will travel further and further until it falls. Add required quantity gunpowder and give the ball the correct acceleration, and it will fly constantly around the planet, always falling in the gravitational field, but never reaching the ground."

In October 1957, the Soviet Union finally confirmed Newton's hunch by launching Sputnik 1, the first artificial satellite to orbit the Earth. This initiated the space race and numerous launches of objects that were intended to fly around the Earth and other planets in the solar system. Since the launch of Sputnik, several countries, mostly the United States, Russia and China, have launched more than 3,000 satellites into space. Some of these man-made objects, like the ISS, are large. Others fit perfectly in a small chest. Thanks to satellites, we receive weather forecasts, watch TV, surf the Internet and make phone calls. Even those satellites whose operation we do not feel or see serve excellently for the benefit of the military.

Of course, launching and operating satellites has led to problems. Today, with more than 1,000 operational satellites in Earth orbit, our immediate space region has become busier than Big City during rush hour. Add to this inoperative equipment, abandoned satellites, pieces of hardware and fragments from explosions or collisions that fill the skies along with useful equipment. This orbital debris we are talking about has accumulated over many years and poses a serious threat to the satellites currently circling the Earth, as well as to future manned and unmanned launches.

In this article, we will climb into the guts of an ordinary satellite and look into its eyes to see views of our planet that Socrates and Newton could not even dream of. But first, let's take a closer look at how a satellite actually differs from other celestial objects.

To understand why satellites move this way, we must visit our friend Newton. He suggested that the force of gravity exists between any two objects in the Universe. If this force did not exist, satellites flying near the planet would continue to move at the same speed and in the same direction - in a straight line. This straight line is the inertial path of the satellite, which, however, is balanced by a strong gravitational attraction directed towards the center of the planet.

Sometimes a satellite's orbit appears as an ellipse, a flattened circle that revolves around two points known as foci. In this case, all the same laws of motion apply, except that the planets are located at one of the foci. As a result, the net force applied to the satellite does not travel uniformly along its entire path, and the satellite's speed is constantly changing. It moves fast when it is closest to the planet - at the perigee point (not to be confused with perihelion), and slower when it is farther from the planet - at the apogee point.

Satellites come in the most different forms and sizes and perform a wide variety of tasks.

• Weather satellites help meteorologists predict the weather or see what's happening in the weather. this moment. The Geostationary Operational Environmental Satellite (GOES) provides a good example. These satellites typically include cameras that show Earth's weather.
• Communications satellites allow telephone conversations to be relayed via satellite. Most important feature A communications satellite is a transponder - a radio that receives a conversation on one frequency, and then amplifies it and transmits it back to Earth on another frequency. A satellite typically contains hundreds or thousands of transponders. Communications satellites are typically geosynchronous (more on that later).
• Television satellites transmit television signals from one point to another (similar to communications satellites).
• Scientific satellites, like the Hubble Space Telescope once upon a time, carry out all types of scientific missions. They observe everything from sunspots to gamma rays.
• Navigation satellites help planes fly and ships sail. GPS NAVSTAR and GLONASS satellites are prominent representatives.
• Rescue satellites respond to distress signals.
• Earth-observing satellites are recording changes from temperatures to ice caps. The most famous are the Landsat series.

Military satellites are also in orbit, but much of their operation remains secret. They can relay encrypted messages, monitor nuclear weapons, enemy movements, warn of missile launches, listen to ground radio, carry out radar surveys and mapping.

When were satellites invented?

Scientists didn't understand Clark - until October 4, 1957. Then the Soviet Union launched Sputnik 1, the first artificial satellite, into Earth orbit. Sputnik was 58 centimeters in diameter, weighed 83 kilograms and was shaped like a ball. Although this was a remarkable achievement, the contents of Sputnik were sparse by today's standards:

• thermometer
• battery
• radio transmitter
• nitrogen gas that was pressurized inside the satellite

On outside Sputnik's four whip antennas transmitted at shortwave frequencies above and below the current standard (27 MHz). Tracking stations on Earth picked up the radio signal and confirmed that the tiny satellite survived the launch and was successfully on a course around our planet. A month later, the Soviet Union launched Sputnik 2 into orbit. Inside the capsule was the dog Laika.

In December 1957, desperately trying to keep up with his opponents cold war, American scientists tried to put the satellite into orbit along with the planet Vanguard. Unfortunately, the rocket crashed and burned during takeoff. Shortly thereafter, on January 31, 1958, the United States repeated the Soviet success by adopting Wernher von Braun's plan to launch the Explorer 1 satellite with a U.S. rocket. Redstone. Explorer 1 carried instruments to detect cosmic rays and discovered in an experiment by James Van Allen of the University of Iowa that there were far fewer cosmic rays than expected. This led to the discovery of two toroidal zones (eventually named after Van Allen) filled with charged particles trapped in the Earth's magnetic field.

Encouraged by these successes, several companies began developing and launching satellites in the 1960s. One of them was Hughes Aircraft, along with star engineer Harold Rosen. Rosen led the team that implemented Clark's idea - a communications satellite placed in Earth's orbit in such a way that it could bounce radio waves from one place to another. In 1961, NASA awarded a contract to Hughes to build the Syncom (synchronous communications) series of satellites. In July 1963, Rosen and his colleagues saw Syncom-2 blast off into space and enter a rough geosynchronous orbit. President Kennedy used new system to speak with the Prime Minister of Nigeria in Africa. Soon Syncom-3 also took off, which could actually broadcast a television signal.

The era of satellites has begun.

What is the difference between a satellite and space debris?

Man-made objects like Sputnik and Explorer can also be classified as satellites because they, like moons, orbit a planet. Unfortunately, human activity has resulted in a huge amount of debris in Earth's orbit. All these pieces and debris behave like large rockets - rotating around the planet at high speed in a circular or elliptical path. In a strict interpretation of the definition, each such object can be defined as a satellite. But astronomers generally consider satellites to be those objects that perform a useful function. Scraps of metal and other junk fall into the category of orbital debris.

Orbital debris comes from many sources:

• A rocket explosion that produces the most junk.
• The astronaut relaxed his hand - if the astronaut repairs something in space and misses wrench, he is lost forever. The key goes into orbit and flies at a speed of about 10 km/s. If it hits a person or satellite, the results could be catastrophic. Large objects like the ISS are a big target for space debris.
• Discarded items. Parts of launch containers, camera lens caps, and so on.

NASA has launched a special satellite called LDEF to study the long-term effects of collisions with space debris. Over six years, the satellite's instruments recorded about 20,000 impacts, some caused by micrometeorites and others by orbital debris. NASA scientists continue to analyze LDEF data. But Japan already has a giant net for catching space debris.

What's inside a regular satellite?

All satellites have a power source (usually solar panels) and batteries. Solar panel arrays allow batteries to be charged. Newest satellites include fuel cells. Satellite energy is very expensive and extremely limited. Nuclear power cells are commonly used to send space probes to other planets.

All satellites have an on-board computer for control and monitoring various systems. Everyone has a radio and an antenna. At a minimum, most satellites have a radio transmitter and a radio receiver so the ground crew can query and monitor the satellite's status. Many satellites allow a lot of different things, from changing the orbit to reprogramming the computer system.

As you might expect, putting all these systems together - not an easy task. It takes years. It all starts with defining the mission goal. Determining its parameters allows engineers to assemble necessary tools and install them in in the right order. Once the specifications (and budget) are approved, satellite assembly begins. It takes place in a clean room, in a sterile environment, which allows you to maintain desired temperature and humidity and protect the satellite during development and assembly.

Artificial satellites are usually made to order. Some companies have developed modular satellites, that is, structures whose assembly allows the installation additional elements according to specification. For example, Boeing 601 satellites had two basic modules- chassis for transporting the propulsion subsystem, electronics and batteries; and a set of honeycomb shelves for equipment storage. This modularity allows engineers to assemble satellites from blanks rather than from scratch.

How are satellites launched into orbit?

In most satellite launches, the rocket is launched straight up, this allows it to get through faster. thick layer atmosphere and minimize fuel consumption. After the rocket takes off, the rocket control mechanism uses the inertial guidance system to calculate necessary adjustments rocket nozzles to provide the desired tilt.

After the rocket enters the thin air, at an altitude of about 193 kilometers, the navigation system releases small rockets, which is enough to flip the rocket into a horizontal position. After this, the satellite is released. Small rockets are fired again and provide a difference in distance between the rocket and the satellite.

Orbital speed and altitude

The rocket must reach a speed of 40,320 kilometers per hour to completely escape Earth's gravity and fly into space. Space speed is much greater than what a satellite needs in orbit. They do not escape earth's gravity, but are in a state of balance. Orbital speed is the speed required to maintain a balance between the gravitational pull and the inertial motion of the satellite. This is approximately 27,359 kilometers per hour at an altitude of 242 kilometers. Without gravity, inertia would carry the satellite into space. Even with gravity, if a satellite moves too fast, it will be carried into space. If the satellite moves too slowly, gravity will pull it back toward Earth.

The orbital speed of a satellite depends on its altitude above the Earth. The closer to Earth, the faster the speed. At an altitude of 200 kilometers, the orbital speed is 27,400 kilometers per hour. To maintain an orbit at an altitude of 35,786 kilometers, the satellite must travel at a speed of 11,300 kilometers per hour. This orbital speed allows the satellite to make one flyby every 24 hours. Since the Earth also rotates 24 hours, the satellite at an altitude of 35,786 kilometers is in a fixed position relative to the Earth's surface. This position is called geostationary. Geostationary orbit is ideal for weather and communications satellites.

In general, the higher the orbit, the longer the satellite can remain there. At low altitude, the satellite is in the earth's atmosphere, which creates drag. At high altitude there is virtually no resistance, and the satellite, like the moon, can remain in orbit for centuries.

Types of satellites

Polar-orbiting satellites also pass the poles with each revolution, although their orbits are less elliptical. Polar orbits remain fixed in space while the Earth rotates. As a result, most of the Earth passes under the satellite in a polar orbit. Because polar orbits provide excellent coverage of the planet, they are used for mapping and photography. Forecasters also rely on a global network of polar satellites that circle our globe every 12 hours.

You can also classify satellites by their height above the earth's surface. Based on this scheme, there are three categories:

• Low Earth Orbit (LEO) - LEO satellites occupy a region of space from 180 to 2000 kilometers above the Earth. Satellites that orbit close to the Earth's surface are ideal for observation, military purposes and collecting weather information.
• Medium Earth Orbit (MEO) - These satellites fly from 2,000 to 36,000 km above the Earth. GPS navigation satellites work well at this altitude. Approximate orbital speed is 13,900 km/h.
• Geostationary (geosynchronous) orbit - geostationary satellites orbit the Earth at an altitude exceeding 36,000 km and at the same rotation speed as the planet. Therefore, satellites in this orbit are always positioned towards the same place on Earth. Many geostationary satellites fly along the equator, which has created many traffic jams in this region of space. Several hundred television, communications and weather satellites use geostationary orbit.

Finally, one can think of satellites in the sense of where they "search." Most of the objects sent into space over the past few decades are looking at Earth. These satellites have cameras and equipment that can see our world in different wavelengths of light, allowing us to enjoy spectacular views of our planet's ultraviolet and infrared tones. Fewer satellites are turning their gaze to space, where they observe stars, planets and galaxies, and scan for objects like asteroids and comets that could collide with Earth.

Known satellites

However, there are real heroes of orbit. Let's get to know them.

1. Landsat satellites have been photographing the Earth since the early 1970s, and they hold the record for observing the Earth's surface. Landsat-1, known at one time as ERTS (Earth Resources Technology Satellite), was launched on July 23, 1972. It carried two main instruments: a camera and a multispectral scanner, built by the Hughes Aircraft Company and capable of recording data in green, red and two infrared spectra. The satellite produced such gorgeous images and was considered so successful that a whole series followed it. NASA launched the last Landsat-8 in February 2013. This vehicle carried two Earth-observing sensors, the Operational Land Imager and the Thermal Infrared Sensor, collecting multispectral images of coastal regions, polar ice, islands and continents.
2. Geostationary Operational Environmental Satellites (GOES) circle the Earth in geostationary orbit, each responsible for a fixed portion of the globe. This allows satellites to closely monitor the atmosphere and detect changes in weather conditions that can lead to tornadoes, hurricanes, floods and lightning storms. Satellites are also used to estimate precipitation and snow accumulation, measure the extent of snow cover, and track the movement of sea and lake ice. Since 1974, 15 GOES satellites have been launched into orbit, but only two satellites, GOES West and GOES East, monitor the weather at any one time.
3. Jason-1 and Jason-2 played a key role in the long-term analysis of Earth's oceans. NASA launched Jason-1 in December 2001 to replace the NASA/CNES Topex/Poseidon satellite, which had been operating above Earth since 1992. For nearly thirteen years, Jason-1 measured sea levels, wind speeds, and wave heights in more than 95% of Earth's ice-free oceans. NASA officially retired Jason-1 on July 3, 2013. Jason-2 entered orbit in 2008. It carried high-precision instruments that made it possible to measure the distance from the satellite to the ocean surface with an accuracy of several centimeters. These data, in addition to their value to oceanographers, provide extensive insight into the behavior of global climate patterns.

How much do satellites cost?

Building such a complex machine requires a ton of resources, so historically only government agencies and corporations with deep pockets could enter the satellite business. Most of the cost of a satellite lies in the equipment - transponders, computers and cameras. A typical weather satellite costs about $290 million. A spy satellite would cost $100 million more. Add to this the cost of maintaining and repairing satellites. Companies must pay for satellite bandwidth the same way phone owners pay for cellular communication. This sometimes costs more than $1.5 million a year.

Another important factor is the startup cost. Launching one satellite into space can cost from 10 to 400 million dollars, depending on the device. The Pegasus XL rocket can lift 443 kilograms into low Earth orbit for $13.5 million. Launching a heavy satellite will require more lift. The Ariane 5G rocket can launch an 18,000-kilogram satellite into low orbit for $165 million.

Despite the costs and risks associated with building, launching and operating satellites, some companies have managed to build entire businesses around it. For example, Boeing. The company delivered about 10 satellites into space in 2012 and received orders for more than seven years, generating nearly $32 billion in revenue.

The future of satellites

Another solution is to reduce the size and complexity of satellites. Since 1999, scientists at Caltech and Stanford University have been working on a new type of CubeSat satellite, which is based on building blocks with a border of 10 centimeters. Each cube contains ready-made components and can be combined with other cubes to increase efficiency and reduce stress. By standardizing design and reducing the cost of building each satellite from scratch, a single CubeSat can cost as little as $100,000.

In April 2013, NASA decided to test this simple principle with three CubeSats powered by commercial smartphones. The goal was to put microsatellites into orbit at a short time and take some pictures with our phones. The agency now plans to deploy an extensive network of such satellites.

Whether large or small, future satellites must be able to communicate effectively with ground stations. Historically, NASA relied on radio frequency communications, but RF reached its limit as demand for more power emerged. To overcome this obstacle, NASA scientists are developing a two-way communication system using lasers instead of radio waves. On October 18, 2013, scientists first fired a laser beam to transmit data from the Moon to Earth (at a distance of 384,633 kilometers) and achieved a record transmission speed of 622 megabits per second.

To launch a satellite into low-Earth orbit, it is necessary to give it an initial speed equal to the first cosmic speed or slightly higher than the last. This does not happen immediately, but gradually. A multi-stage rocket carrying a satellite smoothly picks up speed. When its flight speed reaches the calculated value, the satellite separates from the rocket and begins its free movement in orbit. The shape of the orbit depends on the initial speed given to it and its direction: its dimensions and eccentricity.

If there were no resistance from the environment and the disturbing attractions of the Moon and the Sun, and the Earth had a spherical shape, then the satellite’s orbit would not undergo any changes, and the satellite itself would move along it forever. However, in reality, the orbit of each satellite changes due to various reasons.

The main force that changes the satellite's orbit is braking, which occurs due to the resistance of the rarefied medium through which the satellite flies. Let's see how it affects his movement. Since the satellite's orbit is usually elliptical, its distance from the Earth changes periodically. It decreases towards perigee and reaches maximum distance at apogee. The density of the Earth's atmosphere decreases rapidly as altitude increases, and therefore the satellite encounters the greatest resistance near perigee. Having spent part of the kinetic energy to overcome this, albeit small, resistance, the satellite can no longer rise to its previous height, and its apogee gradually decreases. A decrease in perigee also occurs, but much more slowly than a decrease in apogee. Thus, the size of the orbit and its eccentricity gradually decrease: the elliptical orbit approaches a circular one. The satellite moves around the Earth in a slowly winding spiral and eventually ends its existence in the dense layers of the Earth's atmosphere, heating up and evaporating like a meteoric body. At large sizes it can reach the surface of the Earth.

It is interesting to note that braking a satellite does not reduce its speed, but, on the contrary, increases it. Let's do some simple calculations.

From Kepler's third law it follows that

where C is a constant, M is the mass of the Earth, m is the mass of the satellite, P is its period of revolution and a is the semimajor axis of the orbit. Neglected

By the mass of the satellite compared to the mass of the Earth, we obtain

For simplicity of calculations, let us assume the satellite’s orbit to be circular. Moving at a constant speed υ, the satellite travels a distance υ Р = 2 πа in its orbit for a full revolution, from where Р = 2πa/υ. Substituting this value P into formula (9.1) and performing transformations, we find

So, as the size of the orbit a decreases, the speed of the satellite v increases: the kinetic energy of the satellite increases due to the rapid decrease in potential energy.

The second force that changes the shape of the satellite’s orbit is the pressure of solar radiation, that is, light and corpuscular flows (solar wind). This force has practically no effect on small satellites, but for satellites such as Pageos it is very significant. At launch, Pageos had a circular orbit, but two years later it became a very elongated elliptical.

The movement of the satellite is also affected by the Earth's magnetic field, since the satellite can acquire some electric charge and when it moves in a magnetic field, changes in the trajectory should occur.

However, all these forces are disturbing. The main force that holds the satellite in its orbit is the force of gravity. And here we encounter some peculiarities. We know that as a result of axial rotation, the Earth's shape is different from a spherical one and that the Earth's gravity is not directed exactly towards the center of the Earth. This does not affect very distant objects, but a satellite located close to the Earth reacts to the presence of “equatorial bulges” near the Earth. The plane of its orbit slowly but quite regularly rotates around the Earth's axis of rotation. This phenomenon is clearly visible from observations carried out over one week. All these orbital changes are of great scientific interest, and therefore systematic observations are carried out on the movement of artificial satellites.

As you know, geostationary satellites hang motionless above the earth over the same point. Why don't they fall? At that height there is no force of gravity?

Answer

A geostationary artificial Earth satellite is a device that moves around the planet in the eastern direction (in the same direction as the Earth itself rotates), in a circular equatorial orbit with a period of revolution equal to the period of the Earth’s own rotation.

Thus, if we look from the Earth at a geostationary satellite, we will see it hanging motionless in the same place. Because of this immobility and the high altitude of about 36,000 km, from which almost half of the Earth's surface is visible, relay satellites for television, radio and communications are placed in geostationary orbit.

From the fact that a geostationary satellite constantly hangs over the same point on the Earth’s surface, some draw the incorrect conclusion that the geostationary satellite is not affected by the force of gravity towards the Earth, that the force of gravity disappears at a certain distance from the Earth, i.e. they refute the very Newton. Of course this is not true. The launch of satellites into geostationary orbit is calculated precisely according to Newton’s law of universal gravitation.

Geostationary satellites, like all other satellites, actually fall to the Earth, but do not reach its surface. They are acted upon by a force of attraction to the Earth (gravitational force), directed towards its center, and in the opposite direction, a centrifugal force (force of inertia) repelling the Earth acts on the satellite, which balance each other - the satellite does not fly away from the Earth and does not fall on it exactly just like a bucket spun on a rope remains in its orbit.

If the satellite did not move at all, then it would fall to the Earth under the influence of gravity towards it, but satellites move, including geostationary (geostationary - with an angular velocity equal to the angular velocity of the Earth’s rotation, i.e. one revolution per day, and satellites in lower orbits have a higher angular velocity, i.e. they manage to make several revolutions around the Earth per day). The linear speed imparted to the satellite parallel to the Earth's surface during direct insertion into orbit is relatively large (in low Earth orbit - 8 kilometers per second, in geostationary orbit - 3 kilometers per second). If there were no Earth, then the satellite would fly at such a speed in a straight line, but the presence of the Earth forces the satellite to fall on it under the influence of gravity, bending the trajectory towards the Earth, but the surface of the Earth is not flat, it is curved. As far as the satellite approaches the Earth's surface, the Earth's surface moves away from under the satellite and, thus, the satellite is constantly at the same height, moving along a closed trajectory. The satellite falls all the time, but cannot fall.

So, all artificial Earth satellites fall to Earth, but along a closed trajectory. Satellites are in a state of weightlessness, like all falling bodies (if an elevator in a skyscraper breaks down and begins to fall freely, then the people inside will also be in a state of weightlessness). The astronauts inside the ISS are in weightlessness not because the force of gravity to the Earth does not act in orbit (it is almost the same there as on the surface of the Earth), but because the ISS freely falls to the Earth - along a closed circular trajectory.

What is geostationary orbit? This is a circular field, which is located above the Earth’s equator, along which an artificial satellite rotates with the angular velocity of the planet’s rotation around its axis. It does not change its direction horizontal system coordinates, but hangs motionless in the sky. Geostationary Earth orbit (GEO) is a type of geosynchronous field and is used to place communications, television broadcasting and other satellites.

The idea of ​​using artificial devices

The very concept of geostationary orbit was initiated by the Russian inventor K. E. Tsiolkovsky. In his works, he proposed populating space with the help of orbital stations. Foreign scientists also described the work of cosmic fields, for example, G. Oberth. The man who developed the concept of using orbit for communication is Arthur C. Clarke. In 1945, he published an article in Wireless World magazine, where he described the advantages of the geostationary field. For his active work in this field, in honor of the scientist, the orbit received its second name - the “Clark Belt”. Many theorists have thought about the problem of implementing high-quality communication. Thus, Herman Potochnik in 1928 expressed the idea of ​​how geostationary satellites could be used.

Characteristics of the “Clark Belt”

For an orbit to be called geostationary, it must meet a number of parameters:

1. Geosynchrony. This characteristic includes a field that has a period corresponding to the rotation period of the Earth. A geosynchronous satellite completes its orbit around the planet in a sidereal day, which is 23 hours, 56 minutes and 4 seconds. The same time is needed for the Earth to complete one revolution in a fixed space.

2. To maintain a satellite at a certain point, the geostationary orbit must be circular, with zero inclination. An elliptical field will result in a displacement either east or west, as the craft moves differently at certain points in its orbit.

3. The “hovering point” of the space mechanism must be at the equator.

4. The location of satellites in geostationary orbit should be such that the small number of frequencies intended for communication does not lead to frequency aliasing different devices during reception and transmission, as well as to avoid their collision.

5. Sufficient amount of fuel to maintain a constant position of the space mechanism.

The geostationary orbit of the satellite is unique in that only by combining its parameters can the device remain stationary. Another feature is the ability to see the Earth at an angle of seventeen degrees from satellites located in the space field. Each device captures approximately one-third of the orbital surface, so three mechanisms are capable of covering almost the entire planet.

Artificial satellites

Satellites have been launched by many countries and companies. The world's first artificial device was created in the USSR and flew into space on October 4, 1957. It was named Sputnik 1. In 1958, the United States launched a second spacecraft, Explorer 1. The first satellite, which was launched by NASA in 1964, was named Syncom-3. Artificial devices are mostly non-returnable, but there are those that are partially or completely returned. They are used to conduct scientific research and solve various problems. So, there are military, research, navigation satellites and others. Devices created by university employees or radio amateurs are also launched.

"Standing point"

Geostationary satellites are located at an altitude of 35,786 kilometers above sea level. This altitude provides an orbital period that corresponds to the Earth's rotation period relative to the stars. The artificial vehicle is motionless, therefore its location in geostationary orbit is called the “standing point”. Hovering ensures constant long-term communication, once oriented the antenna will always be pointed at the desired satellite.

Movement

Satellites can be transferred from low-altitude orbit to geostationary orbit using geotransfer fields. The latter are an elliptical path with a point at a low altitude and a peak at an altitude that is close to the geostationary circle. A satellite that has become unsuitable for further work is sent to a disposal orbit located 200-300 kilometers above GEO.

Geostationary orbit altitude

A satellite in a given field keeps a certain distance from the Earth, neither approaching nor moving away. It is always located above some point on the equator. Based on these features, it follows that the forces of gravity and centrifugal force balance each other. The altitude of the geostationary orbit is calculated using methods based on classical mechanics. In this case, the correspondence of gravitational and centrifugal forces is taken into account. The value of the first quantity is determined using Newton's law of universal gravitation. The centrifugal force indicator is calculated by multiplying the mass of the satellite by the centripetal acceleration. The result of the equality of gravitational and inertial mass is the conclusion that the orbital altitude does not depend on the mass of the satellite. Therefore, the geostationary orbit is determined only by the altitude at which the centrifugal force is equal in magnitude and opposite in direction gravitational force, created by the gravity of the Earth at a given height.

From the formula for calculating centripetal acceleration, you can find the angular velocity. The radius of the geostationary orbit is also determined by this formula or by dividing the geocentric gravitational constant by the angular velocity squared. It is 42,164 kilometers long. Taking into account the equatorial radius of the Earth, we obtain a height equal to 35,786 kilometers.

Calculations can be carried out in another way, based on the statement that the orbital altitude, which is the distance from the center of the Earth, with the angular velocity of the satellite coinciding with the rotational motion of the planet, gives rise to a linear velocity that is equal to the first cosmic velocity at a given altitude.

Speed ​​in geostationary orbit. Length

This indicator is calculated by multiplying the angular velocity by the field radius. The value of the speed in orbit is 3.07 kilometers per second, which is much less than the first cosmic speed on the near-Earth path. To reduce the rate, it is necessary to increase the orbital radius by more than six times. The length is calculated by multiplying the number Pi and the radius, multiplied by two. It is 264924 kilometers. The indicator is taken into account when calculating the “standing points” of satellites.

Influence of forces

The parameters of the orbit along which the artificial mechanism rotates can change under the influence of gravitational lunar-solar disturbances, inhomogeneity of the Earth's field, and ellipticity of the equator. The transformation of the field is expressed in such phenomena as:

1. The displacement of the satellite from its position along the orbit towards points of stable equilibrium, which are called potential holes in the geostationary orbit.
2. The angle of inclination of the field to the equator grows at a certain speed and reaches 15 degrees once every 26 years and 5 months.

To keep the satellite at the desired “standing point,” it is equipped with a propulsion system, which is turned on several times every 10-15 days. Thus, to compensate for the increase in orbital inclination, a “north-south” correction is used, and to compensate for the drift along the field, a “west-east” correction is used. To regulate the satellite's path throughout its entire lifespan, a large supply of fuel on board is required.

Propulsion systems

The choice of device is determined by individual technical features satellite For example, a chemical rocket engine has a displacement fuel supply and operates on long-stored high-boiling components (dianitrogen tetroxide, unsymmetrical dimethylhydrazine). Plasma devices have significantly less thrust, but due to long work, which is measured in tens of minutes for a single movement, can significantly reduce the amount of fuel consumed on board. This type of propulsion system is used to maneuver the satellite into another orbital position. The main limiting factor in the service life of the device is the fuel supply in geostationary orbit.

Disadvantages of an artificial field

A significant flaw in interaction with geostationary satellites there are large delays in signal propagation. Thus, at the speed of light of 300 thousand kilometers per second and an orbital altitude of 35,786 kilometers, the movement of the Earth-satellite beam takes about 0.12 seconds, and the Earth-satellite-Earth beam takes 0.24 seconds. Taking into account the signal delay in the equipment and cable systems transmissions of terrestrial services, the total delay of the signal “source - satellite - receiver” reaches approximately 2-4 seconds. This indicator significantly complicates the use of devices in orbit for telephony and makes it impossible to use satellite communications in real-time systems.

Another disadvantage is the invisibility of the geostationary orbit from high latitudes, which interferes with communications and television broadcasts in the Arctic and Antarctic regions. In situations where the sun and the transmitting satellite are in line with the receiving antenna, there is a decrease, and sometimes complete absence of signal. In geostationary orbits, due to the immobility of the satellite, this phenomenon manifests itself especially clearly.

Doppler effect

This phenomenon consists of a change in the frequencies of electromagnetic vibrations with the mutual movement of the transmitter and receiver. The phenomenon is expressed by a change in distance over time, as well as the movement of artificial vehicles in orbit. The effect manifests itself as low stability of the satellite's carrier frequency, which is added to the hardware instability of the frequency of the onboard repeater and earth station, which complicates the reception of signals. The Doppler effect contributes to a change in the frequency of modulating vibrations, which cannot be controlled. In the case when communication satellites and direct television broadcasting are used in orbit, this phenomenon is practically eliminated, that is, there are no changes in the signal level at the receiving point.

Attitude to geostationary fields in the world

The birth of space orbit has created many questions and international legal problems. A number of committees, in particular the United Nations, are involved in their resolution. Some countries located on the equator made claims to the extension of their sovereignty to the part of the space field located above their territory. The states stated that the geostationary orbit is a physical factor that is associated with the existence of the planet and depends on the Earth's gravitational field, so the field segments are an extension of the territory of their countries. But such claims were rejected, since there is a principle of non-appropriation in the world outer space. All problems related to the operation of orbits and satellites are resolved at the global level.