Or why satellites don't fall? A satellite's orbit is a delicate balance between inertia and gravity. The force of gravity continuously pulls the satellite towards the Earth, while the inertia of the satellite tends to keep its motion in a straight line. If there were no gravity, the inertia of the satellite would send it straight out of Earth orbit into outer space. However, at every point in the orbit, gravity keeps the satellite tethered.
To achieve a balance between inertia and gravity, the satellite must have a strictly defined speed. If it flies too fast, the inertia overcomes gravity and the satellite leaves orbit. (The calculation of the so-called second space velocity, which allows the satellite to leave Earth orbit, plays an important role in launching interplanetary space stations.) If the satellite is moving too slowly, gravity will win the fight against inertia and the satellite will fall to Earth. This is exactly what happened in 1979, when the American space station Skylab began to decline as a result of the growing resistance. upper layers earth's atmosphere. Having fallen into the iron tongs of gravity, the station soon fell to Earth.
Speed and distance
Insofar as gravity weakens with distance, the speed required to keep a satellite in orbit varies with altitude. Engineers can calculate how fast and how high a satellite needs to orbit. For example, a geostationary satellite, located always above the same point on the earth's surface, must complete one revolution in 24 hours (which corresponds to the time of one revolution of the Earth around its axis) at an altitude of 357 kilometers.
Gravity and inertia
Balancing a satellite between gravity and inertia can be simulated by rotating a load on a rope tied to it. The inertia of the load tends to move it away from the center of rotation, while the tension of the rope, acting as gravity, keeps the load in a circular orbit. If the rope is cut, the load will fly away along a straight trajectory perpendicular to the radius of its orbit.
What is the geostationary orbit? This is a circular field, which is located above the Earth's equator, along which an artificial satellite circulates with the angular velocity of the planet's rotation around its axis. It does not change its direction in the horizontal coordinate system, but hangs motionless in the sky. The geostationary orbit of the Earth (GSO) is a kind of geosynchronous field and is used to accommodate communication, television broadcasting and other satellites.
The idea of using artificial devices
The very concept of the geostationary orbit was initiated by the Russian inventor K. E. Tsiolkovsky. In his works, he proposed to populate space with the help of orbital stations. Foreign scientists also described the work of space fields, for example, G. Oberth. The person who developed the concept of using the orbit for communication is Arthur Clarke. In 1945, he published an article in the Wireless World magazine, where he described the advantages of the geostationary field. For active work in this area in honor of the scientist, the orbit received its second name - "Clark's belt". Many theorists have thought about the problem of implementing a qualitative connection. So, Herman Potochnik in 1928 expressed the idea of how geostationary satellites can 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 period of the Earth's revolution. 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 necessary 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 either an east or west shift, as the spacecraft moves differently at certain points in its orbit.
3. The "hovering point" of the space mechanism must be on the equator.
4. The location of satellites in geostationary orbit should be such that a small number of frequencies intended for communication does not lead to overlapping frequencies of different devices during reception and transmission, as well as to exclude their collision.
5. Enough propellant to keep the spacecraft stationary.
The geostationary orbit of a satellite is unique in that it is only by combining its parameters that it is possible to achieve the immobility of the apparatus. Another feature is the ability to see the Earth at an angle of seventeen degrees from those located on space field satellites. Each device covers 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 apparatus was created in the USSR and flew into space on October 4, 1957. He bore the name "Sputnik-1". In 1958, the United States launched a second device, the Explorer 1. The first satellite launched by NASA in 1964 was named Syncom-3. Artificial devices are mostly non-returnable, but there are those that return partially or completely. They are used to carry out scientific research and solving various problems. So, there are military, research, navigation satellites and others. Devices created by university employees or radio amateurs are also launched.
"Stop point"
Geostationary satellites are located at an altitude of 35,786 kilometers above sea level. This height provides a period of revolution that corresponds to the period of the Earth's circulation in relation to the stars. The artificial vehicle is stationary, so its location in geostationary orbit is called the “station point”. Hovering provides a constant long-term connection, once the antenna is oriented, it will always be directed to the correct satellite.
Movement
Satellites can be transferred from a low-altitude orbit to a geostationary one using geo-transfer fields. The latter are an elliptical path with a point at low altitude and a peak at an altitude that is close to the geostationary circle. A satellite that has become unusable for further work is sent to a disposal orbit located 200-300 kilometers above the GEO.
Geostationary orbit altitude
A satellite in a given field keeps at 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 height of the geostationary orbit is calculated by methods based on classical mechanics. This takes into account the correspondence of gravitational and centrifugal forces. The value of the first quantity is determined using Newton's law of universal gravitation. The centrifugal force index is calculated by multiplying the mass of the satellite by the centripetal acceleration. The result of the equality of the gravitational and inertial masses is the conclusion that the height of the orbit does not depend on the mass of the satellite. Therefore, the geostationary orbit is determined only by the height at which the centrifugal force is equal in absolute value and opposite in direction to the gravitational force created by the Earth's attraction 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 42164 kilometers. Given the equatorial radius of the Earth, we get a height equal to 35786 kilometers.
Calculations can be done in another way, based on the statement that the height of the orbit, which is the distance from the center of the Earth, with the angular velocity of the satellite, coinciding with the movement of the planet's rotation, gives rise to linear speed, which is equal to the first space at a given height.
speed in geostationary orbit. Length
This indicator is calculated by multiplying the angular velocity by the radius of the field. The value of the speed in orbit is 3.07 kilometers per second, which is much less than the first space velocity on the near-Earth path. To reduce the exponent, it is necessary to increase the radius of the orbit by more than six times. The length is calculated by multiplying pi times the radius by two. It is 264924 kilometers. The indicator is taken into account when calculating the "standing points" of the satellites.
Influence of forces
The parameters of the orbit along which the artificial mechanism circulates can change under the influence of gravitational lunar-solar perturbations, the inhomogeneity of the Earth's field, and the ellipticity of the equator. The transformation of the field is expressed in such phenomena as:
- The displacement of a satellite from its position along its orbit towards points of stable equilibrium, which are called potential holes in the geostationary orbit.
- The angle of inclination of the field to the equator grows at a certain rate and reaches 15 degrees once in 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. So, to compensate for the growth of the orbit inclination, the "north-south" correction is used, and to compensate for the drift along the field, the "west-east" correction is used. To regulate the path of the satellite during the entire period of its operation, a large supply of fuel on board is required.
Propulsion systems
The choice of device is determined by the individual technical features of the satellite. For example, chemical rocket engine has a displacement fuel supply and operates on long-term stored high-boiling components (diazote tetroxide, asymmetric dimethylhydrazine). Plasma devices have a significantly lower thrust, but due to the long operation, which is measured in tens of minutes for a single movement, they can significantly reduce the amount of fuel consumed on board. This type of propulsion system is used to maneuver the satellite to 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 the interaction with geostationary satellites are large delays in signal propagation. So, at a 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 transmission systems of terrestrial services, the total delay of the signal "source - satellite - receiver" reaches approximately 2-4 seconds. Such an indicator significantly complicates the use of devices in orbit in 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 the conduction of communications and television broadcasts in the regions of the Arctic and Antarctica. In situations where the sun and the transmitter satellite are in line with the receiving antenna, there is a decrease, and sometimes even a complete absence of the signal. In geostationary orbits, due to the immobility of the satellite, this phenomenon is especially pronounced.
Doppler effect
This phenomenon consists in changing the frequencies of electromagnetic vibrations with the mutual advancement of the transmitter and receiver. The phenomenon is expressed by a change in distance over time, as well as by the movement of artificial vehicles in orbit. The effect manifests itself as the instability of the carrier frequency of the satellite oscillations, which is added to the instrumental frequency instability of the onboard repeater and the earth station, which complicates the reception of signals. The Doppler effect contributes to a change in the frequency of the modulating vibrations, which cannot be controlled. In the case when communication and direct television broadcasting satellites are used in orbit, this phenomenon is practically eliminated, that is, there are no changes in the signal level at the receiving point.
Attitude in the world to geostationary fields
The birth of the space orbit created many questions and international legal problems. A number of committees, in particular the United Nations, deal with them. 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. States have stated that the geostationary orbit is a physical factor that is associated with the existence of the planet and depends on the gravitational field of the Earth, so the segments of the field are an extension of the territory of their countries. But such claims were rejected, since in the world there is a principle of non-appropriation outer space. All problems associated with the operation of orbits and satellites are resolved at the world level.
"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 placed an object into Earth orbit. And yet, the ancient Greek philosopher seems to have realized how valuable a view from space can be, although he did not know at all how to achieve this.
This notion of how to get an object "into and out of the atmosphere" 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 Earth's surface for a while, but will eventually succumb to gravity and fall back to Earth. Now imagine that you keep adding 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 a gravitational field, but never reaching the ground.
In October 1957 Soviet Union finally confirmed Newton's guess by launching Sputnik 1, the first artificial satellite in Earth's orbit. This initiated the space race and numerous launches of objects meant to fly around the Earth and other planets in the solar system. Since the launch of Sputnik, some countries, mostly the US, Russia and China, have launched more than 3,000 satellites into space. Some of these man-made objects, such as the ISS, are large. Others fit perfectly in a small chest. Thanks to satellites, we get weather forecasts, watch TV, surf the Internet and make phone calls. Even those satellites, whose work we do not feel and do not see, serve the military well.
Of course, the launch and operation of satellites has led to problems. Today, with more than 1,000 operational satellites in Earth orbit, our nearest space region has become busier than Big City at rush hour. Add to that non-working equipment, abandoned satellites, pieces of hardware, and fragments from explosions or collisions that fill the skies along with useful equipment. This orbital debris, which we are talking about, has accumulated over the 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 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 the planet. The Moon is a natural satellite of the Earth, and next to the Earth there are many satellites made by human hands, so to speak, artificial. The path followed by the satellite is an orbit, sometimes taking the form of a circle.
To understand why satellites move in 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 their movement 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 looks like an ellipse, a flattened circle that goes 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 is not distributed uniformly throughout its path, and the speed of the satellite is constantly changing. It moves fast when it is closest to the planet - at the point of perigee (not to be confused with perihelion), and slower when it is farthest from the planet - at the point of apogee.
Satellites are the most different forms and sizes and perform a variety of tasks.
- Meteorological satellites help meteorologists to predict the weather or see what is happening with it at the moment. The Geostationary Operational Environmental Satellite (GOES) is a good example. These satellites usually include cameras that display the Earth's weather.
- Communications satellites allow telephone conversations to be relayed via satellite. The most important feature of a communications satellite is the transponder, a radio that receives a conversation on one frequency, then amplifies it and transmits it back to Earth on a different frequency. A satellite usually contains hundreds or thousands of transponders. Communications satellites are usually geosynchronous (more on that later).
- Television satellites transmit television signals from one point to another (similar to communication satellites).
- Scientific satellites, like the once Hubble Space Telescope, carry out all kinds of scientific missions. They watch everything from sunspots to gamma rays.
- Navigation satellites help aircraft fly and ships sail. GPS NAVSTAR and GLONASS satellites are prominent representatives.
- Rescue satellites respond to distress signals.
- Earth observation satellites are noticing changes, from temperature to ice caps. The most famous are the Landsat series.
Military satellites are also in orbit, but much of their operation remains a mystery. They can relay encrypted messages, monitor nuclear weapons, monitor enemy movements, warn of missile launches, listen to land radio, and carry out radar surveys and mapping.
When were satellites invented?
Perhaps Newton launched satellites in his fantasies, but before we actually accomplished this feat, a lot of time passed. One of the first visionaries was science fiction writer Arthur C. Clarke. In 1945, Clark suggested that a satellite could be placed in orbit in such a way that it would move in the same direction and at the same speed as the Earth. So-called geostationary satellites could be used for communications.
Scientists did not 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 shape of a ball. Although it was a remarkable achievement, the content of Sputnik was meager by today's standards:
- thermometer
- battery
- radio transmitter
- nitrogen gas that was pressurized inside the satellite
On the outside of the Sputnik, four whip antennas were transmitting at shortwave frequencies above and below the current standard (27 MHz). Tracking stations on Earth picked up a radio signal and confirmed that the tiny satellite had survived the launch and was successfully on 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 on cold war, American scientists tried to put the satellite into orbit along with the planet Vanguard. Unfortunately, the rocket crashed and burned down at the takeoff stage. Shortly thereafter, on January 31, 1958, the US repeated the USSR's success by adopting Wernher von Braun's plan to launch the Explorer-1 satellite with the U.S. redstone. Explorer 1 carried the instruments to detect cosmic rays and found, 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 magnetic field Earth.
Encouraged by these successes, some companies started developing and launching satellites in the 1960s. One of them was Hughes Aircraft along with star engineer Harold Rosen. Rosen led the team that brought Clarke's idea to fruition - a communications satellite placed in Earth's orbit in such a way that it could reflect radio waves from one place to another. In 1961, NASA awarded Hughes a contract to build a series of Syncom (synchronous communications) satellites. 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 with the Nigerian Prime Minister in Africa. Syncom-3 soon took off, which could actually broadcast a television signal.
The era of satellites has begun.
What's the difference between a satellite and space junk?
Technically, a satellite is any object that orbits a planet or smaller celestial body. Astronomers classify the moons as natural satellites, and over the years they have compiled a list of hundreds of such objects orbiting the planets and dwarf planets our solar system. For example, they counted 67 moons of Jupiter. And so far.
Man-made objects such as Sputnik and Explorer can also be classified as satellites, since they, like the moons, revolve around the planet. Unfortunately, human activity has led to the fact that in Earth's orbit it turned out to be great amount garbage. All these pieces and debris behave like large rockets - revolve 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, as a rule, consider as satellites those objects that perform a useful function. Fragments of metal and other trash fall into the category of orbital debris.
Orbital debris comes from many sources:
- The rocket explosion that produces the most junk.
- The astronaut relaxed his arm - if an astronaut is repairing something in space and misses a wrench, that wrench 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 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 launched a special satellite called LDEF to study the long-term effects of space debris impacts. Over the course of 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 in Japan there is already a giant network for catching space debris.
What's inside an ordinary satellite?
Satellites come in many shapes and sizes and perform a variety of various functions, but they are all basically the same. All of them have a metal or composite frame and a body that English-speaking engineers call a bus, and Russians call a space platform. The space platform brings everything together and provides enough measures to ensure that the instruments survive the launch.
All satellites have a power source (usually solar panels) and batteries. Solar arrays allow batteries to be charged. The latest satellites include fuel cells. Satellite energy is very expensive and extremely limited. Nuclear power cells are commonly used to ship space probes to other planets.
All satellites have an onboard computer to control and monitor various systems. All have a radio and an antenna. At a minimum, most satellites have a radio transmitter and a radio receiver so that 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 is not an easy task. It takes years. It all starts with defining the purpose of the mission. Determining its parameters allows engineers to assemble the right tools and install them in right order. Once the specification (and budget) is approved, the assembly of the satellite begins. It takes place in a clean room, in a sterile environment that maintains the correct temperature and humidity and protects the satellite during development and assembly.
Artificial satellites are usually made to order. Some companies have developed modular satellites, that is, structures that can be assembled to allow additional elements to be installed according to the specification. For example, the Boeing 601 satellites had two basic modules - a 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 not from scratch, but from a blank.
How are satellites launched into orbit?
Today, all satellites are launched into orbit on a rocket. Many transport them in the cargo department.
In most satellite launches, the rocket launches directly upwards, which allows it to pass through the thick atmosphere faster and minimize fuel consumption. After the rocket takes off, the rocket's control mechanism uses inertial system guidance to calculate the necessary adjustments to the rocket nozzle to achieve the desired inclination.
After the rocket enters the rarefied air, at a height of about 193 kilometers, the navigation system releases small rackets, which is enough to flip the rocket to a horizontal position. After that, the satellite is released. Small rockets are fired again and provide a difference in distance between the rocket and the satellite.
Orbital speed and height
The rocket must reach a speed of 40,320 kilometers per hour to completely escape Earth's gravity and fly into space. Space velocity is much greater than what a satellite needs in orbit. They do not escape the 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 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 orbital speed is 27,400 kilometers per hour. To maintain an 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 make one pass 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. The geostationary orbit is ideal for meteorological and communications 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. On the high altitude there is practically no resistance, and a satellite, like the moon, can be in orbit for centuries.
Satellite types
On the ground, all satellites look the same - shiny boxes or cylinders adorned with solar panel wings. But in space, these clumsy machines behave very differently depending on their flight path, altitude, and orientation. As a result, the classification of satellites becomes a complex matter. One approach is to determine the orbit of the vehicle relative to the planet (usually the Earth). Recall that there are two main orbits: circular and elliptical. Some satellites start in an ellipse and then go into a circular orbit. Others move in an elliptical path known as the "Lightning" orbit. These objects typically circle north-south across the Earth's poles and complete a complete orbit in 12 hours.
Polar-orbiting satellites also pass through 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 polar orbit. Since polar orbits give 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 in 12 hours.
You can also classify satellites by their height above 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 move close to the Earth's surface are ideal for observational, military and weather information gathering purposes.
- 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 move 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 along the equator, which has created a lot of "traffic jams" in this region of space. Several hundred television, communications and weather satellites use the geostationary orbit.
Finally, one can think of satellites in the sense of where they are "looking for". Most of the objects sent into space over the past few decades are looking at the Earth. These satellites have cameras and equipment that can see our world in different wavelengths of light, allowing us to enjoy a breathtaking spectacle in our planet's ultraviolet and infrared tones. Fewer satellites turn their eyes to space, where they observe stars, planets and galaxies, and also scan for objects like asteroids and comets that could collide with the Earth.
Known satellites
Until recently, satellites have remained exotic and top-secret devices used primarily for military purposes for navigation and espionage. Now they have become an integral part of our Everyday life. Thanks to them, we will know the weather forecast (although weather forecasters, oh, how often they are wrong). We watch TV and work with the Internet also thanks to satellites. GPS in our cars and smartphones allows us 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.
- Landsat satellites have been photographing the Earth since the early 1970s, and in terms of observations of the Earth's surface, they are champions. 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 took 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 flew two Earth-observing sensors, Operational Land Imager and Thermal Infrared Sensor, collecting multispectral images of coastal regions, polar ice, islands and continents.
- 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 patterns that can lead to tornadoes, hurricanes, floods and lightning storms. Satellites are also used to estimate the amount of precipitation and snow accumulation, measure the degree 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 GOES West and GOES East satellites are monitoring the weather at the same time.
- Jason-1 and Jason-2 have played a key role in the long-term analysis of the Earth's oceans. NASA launched Jason-1 in December 2001 to replace the NASA/CNES Topex/Poseidon satellite that had been orbiting Earth since 1992. For nearly thirteen years, Jason-1 has 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 precision instruments to measure the distance from the satellite to the ocean surface with an accuracy of a few centimeters. These data, in addition to being valuable to oceanographers, provide an extensive look at the behavior of the world's climate patterns.
How much do satellites cost?
After Sputnik and Explorer, satellites have gotten bigger and more complex. Take, for example, TerreStar-1, a commercial satellite that was supposed to provide mobile data transmission to North America for smartphones and similar devices. Launched in 2009, TerreStar-1 weighed 6910 kilograms. And when fully deployed, it revealed an 18-meter antenna and massive solar arrays with a wingspan of 32 meters.
Building such a complex machine requires a lot of resources, so historically only government departments and corporations with deep pockets could get into 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. The spy satellite will cost $100 million more. Add to this the cost of maintaining and repairing satellites. Companies must pay for satellite bandwidth in the same way that phone owners pay for cellular communications. It sometimes costs more than 1.5 million dollars 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 craft. The Pegasus XL rocket can lift 443 kilograms into low Earth orbit for $13.5 million. Launching a heavy satellite will require more lift. An 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. 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 US, for example, has spent almost $200 billion since the start of the military satellite program and now, in spite of all this, has a fleet of aging vehicles waiting to be replaced. 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 won't be caught in bureaucratic stagnation like NASA, NRO and NOAA.
Another solution is to reduce the size and complexity of the satellites. Scientists at Caltech and Stanford University have been working since 1999 on a new type of CubeSat satellite, based on building blocks with a 10-centimeter edge. Each cube contains ready-made components and can be combined with other cubes to increase efficiency and reduce workload. By standardizing designs 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 based on commercial smartphones. The goal was to put microsatellites into orbit at a short time and take some pictures with your phones. The agency now plans to deploy an extensive network of such satellites.
Whether big or small, the satellites of the future must be able to communicate effectively with ground stations. Historically, NASA has relied on RF communications, but RF has reached its limit as demand for more power has arisen. To overcome this hurdle, 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 received a record transfer rate of 622 megabits per second.
It may seem that satellites in Earth's orbit are the simplest, most familiar and dearest 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 in its movements. But if we ourselves launch satellites into Earth orbit, they stay there for only a few or decades, and then re-enter the atmosphere and either burn up or fall into the ocean and onto the earth.
Moreover, if you look at natural satellites on other planets, they all last much longer than the man-made satellites that orbit the Earth. International space station(ISS), for example, revolves around 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 - are stable in their orbits.
Io is expected to remain in Jupiter's orbit for the rest of the solar system's life, but the ISS, if no action is taken, will be in its orbit for less than 20 years. The same fate holds true for virtually all satellites present in low Earth orbit: by the time the next century rolls around, almost all of today's satellites will enter the Earth's atmosphere and burn 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 the water.
Why is this happening? Why don't these satellites care about the laws of Einstein, Newton and Kepler, and why don't they 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 go out of orbit, but not so fast. 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, the particles of the atmosphere stretch far and high, it's just that their density is getting smaller and smaller. Eventually the density drops - below a 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.
Particles of the solar wind
The sun is constantly emitting a stream of high-energy particles, mostly protons, but there are also electrons and helium nuclei that collide with everything they meet. These collisions in turn change the momentum of the satellites they collide with and gradually slow them down. After enough time, the orbits begin to break down as well. And although this is not the main reason for the deorbit of satellites to LEO, for satellites farther away it has more importance, as they get closer, and with it, atmospheric resistance increases.
Earth's imperfect gravitational field
If the Earth didn't have an atmosphere like Mercury or the Moon, would our satellites be able to stay in orbit forever? No, 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 non-permanent gravitational field. This field and changes as the satellites orbit the planet result in tidal forces acting on them. And the closer the satellite is to the 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 is born on 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, compressed into a non-rotating black hole - without an atmosphere, and the satellites were 100% shielded from the solar wind, these satellites would gradually begin to spiral into the center of the Earth. They would have remained in orbit longer than the Sun itself would have existed, but this system would not be perfectly stable either; the orbits of the satellites would eventually be disrupted.
Relativistic effects
Newton's laws - and Keplerian orbits - aren't the only thing that governs motion celestial bodies. The same force that causes Mercury's orbit to precess an extra 43" per century causes orbits to be disrupted by gravitational waves. The rate of this disruption is incredibly slow for weak gravitational fields (like those we have found in the solar system) and for long distances: It will take 10,150 years for the Earth to spiral down to the Sun, and the degree of disruption of 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 general theory relativity, effectively manifesting itself on the closer satellites of the planet.
All this not only affects the satellites we have created, but also the natural satellites that 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 atmosphere of the Red Planet. Despite having an atmosphere that is only 1/140 of Earth's, Mars's atmosphere is large and diffuse, and moreover, Mars has no protection from the solar wind (unlike Earth with its magnetic field). Therefore, after tens of millions of years, Phobos is all. 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 the closest satellite of Jupiter is not Io: it is Metis, according to mythology, the first wife of Zeus. Closer to Io there are four small satellites, of which Metis is the closest - only 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 is experiencing formidable tidal forces that are responsible for this moon's spiraling descent toward Jupiter.
As an example of what happens when powerful tidal forces predominate, comet Shoemaker-Levy 9 and its impact on Jupiter in 1994, after it was completely torn apart by tidal forces, can be noted. Such is the fate of all satellites that 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 violated. After all, all orbits are unstable, but some are more unstable than others.
To launch a satellite into near-Earth orbit, it is necessary to give it an initial velocity equal to the first space velocity or slightly higher than the latter. This does not happen immediately, but gradually. A multi-stage satellite-carrying rocket slowly picks 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 depends on the initial velocity given to it and its direction: its dimensions and eccentricity.
If there were no resistance of the environment and the perturbing attraction 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 under the influence of various reasons.
The main force that changes the satellite's orbit is the deceleration that occurs due to the resistance of the rarefied medium through which the satellite flies. Let's see how it affects his movement. Since the orbit of a satellite is usually elliptical, its distance from the Earth changes periodically. It decreases towards the perigee and reaches its maximum distance at the apogee. The density of the earth's atmosphere rapidly decreases with increasing altitude, and therefore the satellite encounters the greatest resistance near the 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. The decrease in perigee also occurs, but much more slowly than the decrease in apogee. Thus, the dimensions of the orbit and its eccentricity gradually decrease: elliptical orbit approaches a circle. 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. With large sizes, it can reach the surface of the Earth.
It is interesting to note that the deceleration of the satellite does not decrease 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 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, let us take the satellite orbit as circular. Moving at a constant speed υ, the satellite passes the distance υ Р = 2 πа along the orbit during a complete revolution, whence Р = 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 and 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, i.e. light and corpuscular streams (solar wind). For small satellites, this force practically does not affect, but for such satellites as Pageos, it is very significant. At launch, Pageos had a circular orbit, and 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 perturbing. The main force holding the satellite in its orbit is the force of gravity. And here we meet with some features. We know that as a result axial rotation the figure of the Earth differs from a spherical one and that the earth's gravity is not directed exactly to 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 Earth's axis of rotation. This phenomenon is clearly visible from observations made over a period of one week. All these changes in orbits are of great scientific interest, and therefore systematic observations are being made of the movement of artificial satellites.
