What force is holding the satellite of the earth. How do satellites work? Speed \u200b\u200band distance

To launch a satellite into a near-earth orbit, it is necessary to give it an initial velocity equal to the first cosmic velocity or slightly exceeding the latter. This does not happen immediately, but gradually. The multistage rocket carrying the satellite is gradually picking up speed. When the speed of its flight reaches the calculated value, the satellite is separated from the rocket and begins its free movement in orbit. The shape of the orbit: its size and eccentricity depends on the initial velocity given to it and its direction.

If there were no resistance of the environment and the disturbing gravitations of the Moon and the Sun, and the Earth would have 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 for various reasons.

The main force that changes the satellite's orbit is deceleration 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 its maximum distance at apogee. The density of the earth's atmosphere decreases rapidly with increasing altitude, 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 the circular one. The satellite moves around the Earth in a slowly coiling spiral and eventually ends its existence in the dense layers of the Earth's atmosphere, warming up and evaporating like a meteoroid. At large sizes, it can fly to the surface of the Earth.

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

Kepler's third law implies that

where C is a constant, M is the mass of the Earth, m is the mass of the satellite, P is the period of its revolution and a is the semi-major axis of the orbit. Neglect

by the mass of the satellite in comparison with the mass of the Earth, we obtain

For simplicity of calculations, we will take the satellite orbit as circular. Moving at a constant speed υ, the satellite travels in orbit the distance υ Р \u003d 2 πа for a full revolution, whence Р \u003d 2πa / υ. Substituting this value of P into formula (9.1) and performing transformations, we find

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

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

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

However, all these forces are outrageous. The main force that keeps the satellite in its orbit is the force of gravity. And here we meet with some peculiarities. We know that, as a result of axial rotation, the shape of the Earth differs 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 near the Earth reacts to the presence of "equatorial bulges" near the Earth. The plane of its orbit slowly but quite regularly rotates around the axis of rotation of the Earth. This phenomenon is clearly seen from observations carried out over one week. All these orbital changes are of great scientific interest, and therefore systematic observations are made of the movement of artificial satellites.

"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 put an object into Earth's orbit. And yet the ancient Greek philosopher seems to have realized how valuable a view from space can be, even though he did not know at all how to achieve it.

This notion of how to get an object "into and out of the atmosphere" had to wait until Isaac Newton published his famous thought experiment with a cannonball in 1729. It looks like this:

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

In October 1957, the Soviet Union finally confirmed Newton's guess 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 of 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 human-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 work we do not feel and do not see, perfectly serve the benefit of the military.

Of course, the launch and operation of satellites has led to problems. Today, with over 1,000 working satellites in Earth orbit, our closest space region is livelier than a major city at rush hour. Add to that non-working equipment, abandoned satellites, hardware pieces, and debris from explosions or collisions that fill the skies with useful equipment. This orbital debris that we have been accumulating over the years 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 crawl 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, in fact, the satellite differs from other celestial objects.

is any object that moves in a curve around a planet. The moon is a natural satellite of the Earth, and there are also many satellites made by human hands, artificial, so to speak, near the Earth. The path followed by the satellite is an orbit, sometimes taking the shape of a circle.

To understand why the 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, the satellites flying near the planet would continue their movement at the same speed and in one 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 to the center of the planet.

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

Satellites come in a wide variety of shapes and sizes and perform a wide variety of tasks.

• Meteorological satellites help meteorologists predict the weather or see what is happening to it at the moment. The Geostationary Operational Environmental Satellite (GOES) provides a good example. These satellites usually include cameras that show the Earth's weather.
• Communication satellites allow telephone conversations to be relayed via satellite. The most important feature of 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 a different frequency. A satellite usually contains hundreds or thousands of transponders. Communication satellites are usually geosynchronous (more on that later).
• Television satellites transmit television signals from one point to another (similar to communication satellites).
• Science satellites, like the Hubble Space Telescope once, carry out all kinds of scientific missions. They watch everything from sunspots to gamma rays.
• Navigation satellites help planes fly and ships sail. GPS NAVSTAR and GLONASS satellites are outstanding representatives.
• Rescue satellites respond to distress signals.
• Earth observation satellites observe changes - from temperature to ice caps. The most famous are the Landsat series.

Military satellites are also in orbit, but much of their work remains under wraps. They can relay encrypted messages, monitor nuclear weapons, enemy movements, warn of missile launches, listen to land-based radio, carry out radar surveys and mapping.

When were satellites invented?

Perhaps Newton launched satellites in his fantasies, but it took a long time before we actually accomplished this feat. One of the first visionaries was science fiction writer Arthur Clarke. In 1945, Clarke suggested that a satellite could be placed in orbit so that it would move in the same direction and at the same speed as the Earth. So-called geostationary satellites could be used for communication.

Scientists didn't understand Clark until October 4, 1957. Then the Soviet Union launched Sputnik 1, the first artificial satellite, into Earth's orbit. "Sputnik" was 58 centimeters in diameter, weighed 83 kilograms and was made in the form of a ball. While this was a remarkable achievement, Sputnik's content was meager by today's standards:

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

On the outside of Sputnik, four whip antennas were transmitting 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 successfully entered a course around our planet. A month later, the Soviet Union launched Sputnik 2 into orbit. Inside the capsule was Laika the dog.

In December 1957, desperate to keep up with their Cold War opponents, American scientists attempted to put a satellite into orbit alongside the planet Vanguard. Unfortunately, the rocket crashed and burned out during the takeoff stage. Shortly thereafter, on January 31, 1958, the United States repeated the success of the USSR by adopting Wernher von Braun's plan to launch the Explorer-1 satellite with the U.S. rocket. Redstone. Explorer-1 carried cosmic ray detection tools 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 (ultimately 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 of \u200b\u200ba communications satellite orbiting the Earth in such a way that it could reflect radio waves from one place to another. In 1961, NASA contracted Hughes to build the Syncom series of satellites (synchronous communications). In July 1963, Rosen and his colleagues saw Syncom-2 take off into space and enter a rough geosynchronous orbit. President Kennedy used the new system to speak to the Prime Minister of Nigeria in Africa. Soon, Syncom-3 took off, which could actually broadcast a television signal.

The era of satellites has begun.

What's the difference between satellite and space debris?

Technically, a satellite is any object that orbits a planet or lesser celestial body. Astronomers classify moons as natural satellites, and over the years they have compiled a list of hundreds of such objects orbiting the planets and dwarf planets of our solar system. For example, 67 moons of Jupiter were counted. And still.

Man-made objects such as Sputnik and Explorer can also be classified as satellites because, like moons, they revolve around the 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 - they revolve around the planet at high speed in a circular or elliptical path. In the strict interpretation of the definition, each such object can be defined as a satellite. But astronomers tend to think of objects as satellites that serve a useful function. Debris and other debris fall into the orbital debris category.

Orbital debris comes from many sources:

• The rocket explosion that produces the most junk.
• The astronaut relaxed his hand - if the astronaut repairs something in space and loses the 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 a satellite, the results can be disastrous. 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 have recorded about 20,000 collisions, some of which were caused by micrometeorites and others by orbital debris. NASA scientists continue to analyze LDEF data. But in Japan there is already a giant net for trapping space debris.

What's inside an ordinary satellite?

Satellites come in many shapes and sizes and perform many different functions, but all are basically the same. They all have a metal or composite frame and body, which English-speaking engineers call a bus, and Russians call a space platform. The space platform puts everything together and provides enough measures for the tools to survive launch.

All satellites have a power source (usually solar panels) and batteries. Arrays of solar cells allow you to charge batteries. Newer satellites include fuel cells. The energy of the satellites is very expensive and extremely limited. Nuclear batteries are commonly used to send space probes to other planets.

All satellites have an on-board computer to control and monitor various systems. They all have a radio and 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 orbits to reprogramming a computer system.

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

Artificial satellites are usually custom made. Some companies have developed modular satellites, that is, structures that can be assembled to fit additional elements to specification. For example, Boeing 601 satellites had two base modules - a chassis for transporting the propulsion subsystem, electronics and batteries; and a set of honeycomb shelves for storing equipment. This modularity allows engineers to assemble satellites from a blank, not from scratch.

How are satellites launched into orbit?

Today, all satellites are launched into orbit on a rocket. Many transport them in the cargo department.

Most satellite launches launch a rocket straight upward, allowing it to travel faster through the thick atmosphere and minimize fuel consumption. After the rocket takes off, the rocket's control mechanism uses an inertial guidance system to calculate the necessary adjustments to the rocket nozzle to achieve the desired tilt.

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

Orbital speed and altitude

The rocket must pick up a speed of 40 320 kilometers per hour to completely escape from Earth's gravity and fly into space. The space velocity is much greater than the satellite needs in orbit. They do not avoid Earth's gravity, but are in a state of balance. Orbital speed is the speed required to maintain a balance between gravitational attraction 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 the satellite moves too fast, it will be blown into space. If the satellite moves too slowly, gravity will pull it back towards Earth.

The orbital speed of a satellite depends on its height 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 its orbit at an altitude of 35,786 kilometers, the satellite must rotate at a speed of 11,300 kilometers per hour. This orbital speed allows the satellite to fly once every 24 hours. Since the earth also rotates 24 hours, the satellite at 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 communication satellites.

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

Types of satellites

On earth, all satellites look similar - shiny boxes or cylinders adorned with solar panel wings. But in space, these clumsy machines behave very differently depending on flight path, altitude and orientation. As a result, satellites are difficult to classify. One approach is to determine the orbit of the spacecraft relative to the planet (usually the Earth). Recall that there are two main orbits: circular and elliptical. Some satellites start out in an ellipse and then enter a circular orbit. Others follow an elliptical path known as the Lightning orbit. These objects, as a rule, circle from north to south through the poles of the Earth and complete a full circle in 12 hours.

Polar-orbiting satellites also pass through the poles with each revolution, although their orbits are less elliptical. The polar orbits remain fixed in space while the Earth rotates. As a result, most of the Earth passes under a satellite in 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 orbit our balloon in 12 hours.

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

• Low Earth Orbit (LEO) - LEO satellites cover an area of \u200b\u200bspace from 180 to 2000 kilometers above the Earth. Satellites that move close to the Earth's surface are ideal for observing, military, and weather information gathering.
• 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. The approximate orbital speed is 13,900 km / h.
• Geostationary (geosynchronous) orbit - geostationary satellites orbit around the Earth at an altitude exceeding 36,000 km and at the same rotational speed as the planet. Therefore, satellites in this orbit are always positioned to the same place on Earth. Many geostationary satellites fly around the equator, which has created a lot of "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 where they "search". Most objects sent into space over the past few decades look at the Earth. These satellites have cameras and equipment that can see our world in different wavelengths of light, allowing you to enjoy the spectacular ultraviolet and infrared colors of our planet. Fewer satellites turn their gaze to space, where they observe stars, planets and galaxies, as well as scan objects like asteroids and comets that could collide with the Earth.

Notable satellites

Until recently, satellites remained exotic and top-secret devices that were used mainly for military purposes for navigation and espionage. Now they have become an integral part of our daily life. Thanks to them, we find out the weather forecast (although forecasters, oh, how often they are mistaken). We watch TV and work with the Internet also thanks to satellites. GPS in our cars and smartphones allows you to get to the right place. Is it worth talking about the invaluable contribution of the Hubble telescope and the work of astronauts on the ISS?

However, there are real heroes of the 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 the 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 made such gorgeous images and was considered so successful that a whole series followed. NASA launched the last Landsat-8 in February 2013. The craft flew two Earth-observing sensors, the Operational Land Imager and the Thermal Infrared Sensor, collecting multispectral images of coastal regions, polar ice caps, 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 thunderstorms. Satellites are also used to estimate the amount of precipitation and snow accumulation, measure the extent of snow cover and track the movements of sea and lake ice. Since 1974, 15 GOES satellites have been launched into orbit, but at the same time only two satellites GOES "West" and GOES "East" are observing the weather.
3. Jason-1 and Jason-2 have played a key role in the long-term analysis of Earth's oceans. NASA launched Jason-1 in December 2001 to replace NASA / CNES's Topex / Poseidon satellite, which has been operating on Earth since 1992. For nearly thirteen years, Jason-1 has measured sea level, wind speed and wave height in over 95% of Earth's ice-free oceans. NASA officially retired Jason-1 on July 3, 2013. In 2008, Jason-2 entered orbit. 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. This data, in addition to being of value to oceanographers, provides an extensive insight into the behavior of global climate patterns.

How much do satellites cost?

After Sputnik and Explorer, satellites have become larger and more complex. Take TerreStar-1, a commercial satellite that was supposed to provide mobile data transmission in North America for smartphones and similar devices. Launched in 2009, the TerreStar-1 weighed 6,910 kilograms. And when fully deployed, it revealed an 18-meter antenna and massive solar panels with a 32-meter wingspan.

Building such a complex machine requires a ton of resources, so historically only government departments and corporations with deep pockets could enter the satellite business. Much of the cost of a satellite lies in hardware - transponders, computers, and cameras. A typical meteorological satellite costs about $ 290 million. The spy satellite will cost $ 100 million more. Add to this the cost of maintaining and repairing satellites. Businesses must pay for satellite bandwidth in the same way that phone owners pay for cellular. Sometimes it costs more than $ 1.5 million a year.

Another important factor is startup cost. Launching a single satellite into space can cost anywhere from $ 10 million to $ 400 million, depending on the vehicle. 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 could 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 an entire business out of it. Take Boeing, for example. In 2012, the company delivered about 10 satellites into space and received orders for more than seven years, generating nearly $ 32 billion in revenue.

The future of satellites

Almost fifty years after the launch of Sputnik, satellites, like budgets, are growing and getting stronger. The United States, for example, has spent nearly $ 200 billion since the start of the military satellite program and now, despite all this, has a fleet of aging vehicles awaiting replacement. Many experts fear that the construction and deployment of large satellites simply cannot exist on taxpayer money. The solution that could turn everything upside down remains private companies like SpaceX, and others that clearly will not suffer bureaucratic stagnation like NASA, NRO and NOAA.

Another solution is to reduce the size and complexity of the satellites. Scientists from Caltech and Stanford University have been working on a new type of satellite, CubeSat, since 1999, based on building blocks with a face of 10 centimeters. Each cube contains ready-made components and can be combined with other cubes to increase efficiency and reduce load. 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 and three CubeSats powered by commercial smartphones. The goal was to put the microsatellites into orbit for a short time and take some pictures with phones. The agency now plans to deploy an extensive network of such satellites.

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

As you know, geostationary satellites hang motionless above the ground over the same point. Why don't they fall? Gravity is not working at that height?

Answer

A geostationary artificial satellite of the Earth is an apparatus that moves around the planet in an easterly direction (in the same direction in which the Earth itself rotates), in a circular equatorial orbit with an orbital period 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 a 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 being put into geostationary orbit.

From the fact that a geostationary satellite hangs constantly over the same point on the Earth's surface, some make the wrong conclusion that the geostationary satellite is not affected by the force of gravity to the Earth, that the force of gravity at a certain distance from the Earth disappears, that is, they refute itself Newton. Of course it is not. The very launch of satellites into a geostationary orbit is calculated precisely according to Newton's law of gravity.

Geostationary satellites, like all other satellites, actually fall to the Earth, but do not reach its surface. They are acted upon by the force of attraction to the Earth (gravitational force), directed to its center, and in the opposite direction, the centrifugal force (force of inertia) which repels from 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 as a bucket, untwisted 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 attraction to it, but the satellites move, including geostationary ones (geostationary ones - with an angular velocity equal to the angular velocity of the Earth's rotation, i.e., one revolution per day, and the satellites of the lower orbits have a greater angular velocity, i.e., they manage to make several revolutions around the Earth per day). The linear velocity imparted to the satellite parallel to the Earth's surface during direct injection into orbit is relatively high (in low-earth orbit - 8 kilometers per second, in geostationary orbit - 3 kilometers per second). If there were no Earth, the satellite would fly at such a speed in a straight line, but the presence of the Earth makes the satellite fall on it under the action of gravity, bending its trajectory towards the Earth, but the Earth's surface is not flat, it is curved. As far as the satellite approaches the Earth's surface, as far as the Earth's surface leaves from under the satellite and, thus, the satellite is constantly at the same height, moving along a closed trajectory. The satellite is falling all the time, but cannot fall in any way.

So, all artificial satellites of the Earth fall to Earth, but - along a closed trajectory. The 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). Astronauts inside the ISS are in zero gravity not because the force of gravity to the Earth does not act in orbit (it is almost the same there as on the Earth's surface), but because the ISS freely falls to the Earth - along a closed circular trajectory.

Just as the seats in a theater provide different perspectives on a show, different satellite orbits provide perspective, each with a different purpose. Some seem to hang above a point on the surface, they provide a constant view of one side of the Earth, while others circle around our planet, sweeping over many places in a day.

Orbit types

How high do the satellites fly? There are 3 types of near-earth orbits: high, medium and low. On the high, farthest from the surface, as a rule, there are many weather and some communication satellites. Satellites rotating in medium-earth orbit include navigation and special ones designed to monitor a specific region. Most scientific spacecraft, including NASA's Earth Observing System fleet, are in low orbit.

The speed at which the satellites fly depends on the speed of their movement. As you get closer to the Earth, gravity becomes stronger and the motion accelerates. For example, NASA's Aqua satellite takes about 99 minutes to fly around our planet at an altitude of about 705 km, while a meteorological apparatus located 35 786 km from the surface takes 23 hours, 56 minutes and 4 seconds. At a distance of 384,403 km from the center of the Earth, the Moon completes one revolution in 28 days.

Aerodynamic paradox

Changing the altitude of a satellite also changes its orbital speed. There is a paradox here. If the satellite operator wants to increase its speed, he cannot simply start the engines to accelerate. This will increase the orbit (and altitude), resulting in a decrease in speed. Instead, the engines should be started in the opposite direction to the direction of the satellite's movement, i.e., perform an action that would slow down a moving vehicle on Earth. Doing so will move it lower, which will increase the speed.

Orbit characteristics

In addition to altitude, the satellite's path is characterized by eccentricity and inclination. The first relates to the shape of the orbit. A satellite with a low eccentricity moves along a trajectory close to a circular one. The eccentric orbit is elliptical. The distance from the spacecraft to the Earth depends on its position.

Inclination is the angle of the orbit in relation to the equator. A satellite that orbits directly over the equator has zero tilt. If the spacecraft passes over the north and south poles (geographic rather than magnetic), it tilts 90 °.

Together - height, eccentricity, and inclination - determine the satellite's motion and how the Earth will look from its perspective.

High near-earth

When a satellite reaches exactly 42164 km from the center of the Earth (about 36 thousand km from the surface), it enters the zone where its orbit corresponds to the rotation of our planet. Since the spacecraft moves at the same speed as the Earth, that is, its orbital period is 24 hours, it seems that it remains in place above a single longitude, although it can drift from north to south. This special high orbit is called geosynchronous.

The satellite moves in a circular orbit directly above the equator (eccentricity and inclination are zero) and stands still relative to the Earth. It is always located over the same point on its surface.

The Molniya orbit (inclination 63.4 °) is used for observation at high latitudes. Geostationary satellites are anchored to the equator, so they are not suitable for distant northern or southern regions. This orbit is quite eccentric: the spacecraft moves in an elongated ellipse with the Earth located close to one edge. Since the satellite is accelerated by gravity, it moves very quickly when it is close to our planet. When moving away, its speed slows down, so it spends more time at the top of the orbit in the edge farthest from the Earth, the distance to which can reach 40 thousand km. The orbital period is 12 hours, but the satellite spends about two-thirds of this time over one hemisphere. Like a semisynchronous orbit, the satellite follows the same path every 24 hours. It is used for communication in the far north or south.

Low Earth

Most scientific satellites, many meteorological and space stations are in almost circular low Earth orbit. Their slope depends on what they are monitoring. TRMM was launched to monitor rainfall in the tropics, so it has a relatively low inclination (35 °) while remaining close to the equator.

Many of NASA's observational satellites have near-polar, highly inclined orbits. The spacecraft moves around the Earth from pole to pole with a period of 99 minutes. Half of the time it passes over the daytime side of our planet, and at the pole it goes over to the night side.

As the satellite moves, the Earth rotates beneath it. By the time the vehicle enters the illuminated area, it is above the area adjacent to the zone of its last orbit. In a 24-hour period, polar satellites cover most of the Earth twice: once during the day and once at night.

Sun-synchronous orbit

Just as geosynchronous satellites must be above the equator, which allows them to stay above one point, polar-orbiting satellites have the ability to stay at the same time. Their orbit is sun-synchronous - when the spacecraft crosses the equator, the local solar time is always the same. For example, the Terra satellite crosses over Brazil always at 10:30 am. The next crossing after 99 minutes over Ecuador or Colombia also takes place at 10:30 local time.

A sun-synchronous orbit is essential for science, as it allows sunlight to be stored on the Earth's surface, although it will change with the season. This consistency means scientists can compare images of our planet at the same time of year over several years without worrying about too large jumps in lighting that could create the illusion of change. Without a sun-synchronous orbit, it would be difficult to track them over time and gather the information needed to study climate change.

The satellite's path is very limited here. If it is at an altitude of 100 km, the orbit should have an inclination of 96 °. Any deviation will be unacceptable. Since atmospheric drag and the gravitational pull of the Sun and Moon alter the craft's orbit, it must be adjusted regularly.

Launching into orbit: launch

Launching a satellite requires energy, the amount of which depends on the location of the launch site, altitude and slope of its future trajectory. Getting to a distant orbit requires more energy. Satellites with significant inclination (for example, polar ones) are more energy intensive than those that circle above the equator. Launching into orbit with low inclination is assisted by the rotation of the Earth. moves at an angle of 51.6397 °. This is necessary to make it easier for space shuttles and Russian rockets to reach it. ISS altitude - 337-430 km. Polar satellites, on the other hand, do not receive help from the Earth's impulse, so they need more energy to climb the same distance.

Adjustment

After launching a satellite, efforts must be made to keep it in a specific orbit. Since the Earth is not a perfect sphere, its gravity is stronger in some places. This unevenness, along with the attraction of the Sun, Moon and Jupiter (the most massive planet in the solar system), alters the inclination of the orbit. The GOES satellites have been corrected three or four times throughout their lifetime. NASA LEOs must adjust their tilt annually.

In addition, Earth's satellites are affected by the atmosphere. The uppermost layers, although thin enough, offer strong enough resistance to pull them closer to Earth. The action of gravity causes the satellites to accelerate. Over time, they burn up, spiraling lower and faster into the atmosphere, or fall to Earth.

Atmospheric resistance is stronger when the Sun is active. Just as the air in a hot air balloon expands and rises when it heats up, the atmosphere rises and expands when the sun gives it extra energy. The thinner layers of the atmosphere rise, and denser ones take their place. Therefore, satellites in Earth's orbit must change their position about four times a year to compensate for atmospheric drag. When solar activity is at its maximum, the position of the apparatus must be corrected every 2-3 weeks.

Space debris

The third reason forcing the change in orbit is space debris. One of the communication satellites Iridium collided with a non-functioning Russian spacecraft. They shattered, forming a debris cloud of over 2,500 pieces. Each element was added to the database, which today includes over 18,000 man-made objects.

NASA carefully monitors everything that may be in the path of satellites, since space debris has already had to change orbits several times due to space debris.

Engineers track the position of space debris and satellites that could interfere with movement and carefully plan evasive maneuvers as needed. The same team plans and performs maneuvers to adjust the tilt and height of the satellite.