ARTIFICIAL SATELLITES: WHAT ARE THEY FOR, HOW THEY WORK, TYPES, IMPORTANT - ARTE - 2025

The satellites are vehicles or devices built specifically to be released to the space without crew, in order to orbit around the Earth or other celestial body.

The first ideas about building artificial satellites came from science fiction authors, for example Jules Verne and Arthur C. Clark. The latter was a radar officer in the Royal Air Force and, at the end of World War II, conceived the idea of ​​using three satellites in orbit around the Earth to maintain a telecommunications network.

Figure 1. Artificial satellite orbiting the Earth. Source: Wikimedia Commons.

At that time, the means were not yet available to place a satellite in orbit. It took a few more years for the United States military to produce the first satellite communications in the early 1950s.

The space race between the United States and the Soviet Union boosted the artificial satellite industry. The first successfully placed into orbit was the Soviet Sputnik satellite in 1957 and it emitted signals in the 20-40 MHz range.

This was followed by the launch of the Echo I by the United States, for communication purposes. Since then, numerous launches into orbit were succeeded by both powers and, subsequently, many countries joined the new technology.

What are artificial satellites for?

-In telecommunications, for the retransmission of radio, television and cell phone messages.

-In scientific and meteorological research, including cartography and astronomical observations.

-For military intelligence purposes.

-For navigation and location uses, being the GPS (Global Positioning System) one of the best known.

-To monitor the land surface.

-In space stations, designed to experience life outside of Earth.

How do they work?

In his Principia, Isaac Newton (1643-1727) established what was necessary to place a satellite in orbit, although instead of a satellite, he used as an example a cannonball fired from the top of a hill.

Fired with a certain horizontal velocity, the bullet follows the usual parabolic trajectory. Increasing the speed, the horizontal reach becomes greater and greater, something that was clear. But will a certain speed cause the bullet to go into orbit around the Earth?

The Earth curves from a line tangent to the surface at the rate of 4.9 m for every 8 km. Any object released from rest will fall 4.9 m during the first second. Therefore, when firing the bullet horizontally from a peak with a velocity of 8 km / s, it will fall 4.9 m during the first second.

But the Earth will also have descended 4.9 m in that time, as it curves under the cannonball. This continues to move horizontally, covering the 8 km and would remain at the same height with respect to the Earth during that second.

Naturally the same thing happens after the next second and in all the successive seconds, turning the bullet into an artificial satellite, without any additional propulsion, as long as there is no friction.

However, friction caused by air resistance is unavoidable, which is why a booster rocket is necessary.

The rocket lifts the satellite to a great height, where the thinner atmosphere offers less resistance and provides it with the necessary horizontal speed.

Such speed must be greater than 8 km / s and less than 11 km / s. The latter is the escape velocity. Projected at this speed, the satellite would abandon the gravitational influence of the Earth, going into space.

Artificial satellite structure

Artificial satellites contain various complex mechanisms to perform their functions, which involve receiving, processing and sending various types of signals. They must also be light and have autonomy of operation.

The main structures are common to all artificial satellites, which in turn have several subsystems according to the purpose. They are mounted in a housing made of metal or other lightweight compounds, which serves as a support and is called a bus.

On the bus you can find:

- The central control module, which contains the computer, with which the data is processed.

- Receiving and transmitting antennas for communication and data transmission by radio waves, as well as telescopes, cameras and radars.

- A system of solar panels on the wings, to obtain the necessary energy and rechargeable batteries when the satellite is in the shade. Depending on orbit, satellites need about 60 minutes of sunlight to recharge their batteries, if they are in low orbit. More distant satellites spend much more time exposed to solar radiation.

Since satellites spend a long time exposed to this radiation, a protection system is required to avoid damage to other systems.

The exposed parts get very hot, while in the shade they reach extremely low temperatures, because there is not enough atmosphere to regulate the changes. For this reason, radiators are required to eliminate heat and aluminum covers to conserve heat when necessary.

Types of artificial satellites

Depending on their trajectory, artificial satellites can be elliptical or circular. Of course, each satellite has an assigned orbit, which is generally in the same direction that the Earth rotates, called asynchronous orbit. If for some reason the satellite travels the opposite way, then it has a retrograde orbit.

Under gravity, objects move in elliptical paths according to Kepler's laws. Artificial satellites do not escape this, however, some elliptical orbits have such a small eccentricity that they can be considered circular.

The orbits can also be inclined with respect to the Earth's equator. At an inclination of 0º they are equatorial orbits, if they are 90º they are polar orbits.

The altitude of the satellite is also an important parameter, since between 1500 - 3000 km high is the first Van Allen belt, a region to be avoided due to its high radiation rate.

Figure 2. Orbits, altitudes and speeds of artificial satellites. Disused satellites pass into the cemetery orbit, although there are remnants in all orbits. Source: Wikimedia Commons.

Satellite orbits

The orbit of the satellite is chosen according to the mission it has, since there are more or less favorable heights for different operations. According to this criterion, satellites are classified as:

- LEO (Low Earth Orbit), they are between 500 and 900 km high and describe a circular path, with periods of approximately 1 hour and a half and an inclination of 90º. They are used for cell phones, faxes, personal pagers, for vehicles and for boats.

- MEO (Medium Earth Orbit), they are at an altitude between 5000-12000 km, inclination of 50º and a period of 6 hours approximately. They are also employed in cell phones.

- GEO (Geosynchronous Earth Orbit), or geostationary orbit, although there is a small difference between the two terms. The former can be of variable inclination, while the latter are always at 0º.

In any case, they are at a high altitude -36,000 km more or less-. They travel circular orbits in periods of 1 day. Thanks to them, fax, long distance telephony and satellite television are available, among other services.

Figure 3. Diagram of the orbits of artificial satellites. 1) Earth. 2) LEO. 3) MEO, 4) Geosynchronous orbits. Source: Wikimedia Commons.

Geostationary satellites

At the beginning the communication satellites had different periods than the Earth's rotation, but this made it difficult to position the antennas and communication was lost. The solution was to place the satellite at a height such that its period coincided with that of the Earth's rotation.

In this way the satellite orbits together with the Earth and appears to be fixed with respect to it. The height required to place a satellite in geosynchronous orbit is 35786.04 km and is known as the Clarke belt.

The height of the orbit can be calculated by establishing the period, using the following expression, derived from Newton's Law of Universal Gravitation and Kepler's laws:

Where P is the period, a is the length of the semi-major axis of the elliptical orbit, G is the universal constant of gravitation and M is the mass of the Earth.

Since in this way the orientation of the satellite with respect to the Earth does not change, it guarantees that it will always have contact with it.

Most important artificial satellites of the Earth

Sputnik

Figure 4. Replica of Sputnik, the first artificial satellite in orbit in history. Source: Wikimedia Commons.

It was the first artificial satellite in the history of mankind, put into orbit by the former Soviet Union in October 1957. This satellite was followed by 3 more, as part of the Sputnik program.

The first Sputnik was quite small and light: 83 kg of aluminum mainly. It was capable of emitting frequencies between 20 and 40 MHz. It was in orbit for three weeks, after which it fell to Earth.

Replicas of Sputnik can be seen today in many museums in the Russian Federation, Europe and even America.

The space shuttle

Another well-known manned mission was the Space Transport System STS or Space Shuttle, which was in operation from 1981 to 2011 and participated, among other important missions, in the launching of the Hubble Space Telescope and the International Space Station, in addition to missions of repair of other satellites.

The Space Shuttle had an asynchronous orbit and was reusable, since it could come and go to Earth. Of the five ferries, two were accidentally destroyed along with their crews: the Challenger and the Columbia.

GPS satellites

The Global Positioning System is widely known for accurately locating people and objects anywhere on the globe. The GPS network consists of at least 24 high altitude satellites, of which there are always 4 satellites visible from Earth.

They are in orbit at an altitude of 20,000 km and their period is 12 hours. GPS uses a mathematical method similar to triangulation to assess the position of objects, called trilateration.

GPS is not limited to locating people or vehicles, it is also useful for cartography, surveying, geodesy, rescue operations and sports practices, among other important applications.

The Hubble Space Telescope

It is an artificial satellite that offers incomparable never-before-seen images of the solar system, stars, galaxies and the distant universe, without the Earth's atmosphere or light pollution blocking or distorting distant light.

Figure 5. View of the Hubble Space Telescope. Source: NASA via Wikimedia Commons.

Hence, its launch in 1990 was the most remarkable advance in astronomy in recent times. Hubble's huge 11-ton cylinder is at an altitude of 340 miles (548 km) orbiting Earth in a circular motion, with a period of 96 minutes.

It is expected to be deactivated between 2020 and 2025, being replaced by the James Webb space telescope.

International Space Station

Known as ISS (International Space Station), it is an orbiting research laboratory, managed by five space agencies around the world. So far it is the largest artificial satellite in existence.

Unlike the rest of the satellites, in the Space Station there are human beings on board. In addition to the fixed crew of at least two astronauts, the station has even been visited by tourists.

The purpose of the station is primarily scientific. It has 4 laboratories in which the effects of zero gravity are investigated and astronomical, cosmological, and climate observations are carried out, as well as various experiments in biology, chemistry and the influence of radiation on various systems.

Chandra

This artificial satellite is an observatory to detect X-rays, which are absorbed by the Earth's atmosphere and therefore cannot be studied from the surface. NASA put it into orbit in 1999 via the Space Shuttle Columbia.

Iridium communication satellites

They make up a network of 66 satellites at an altitude of 780 km in LEO-type orbits, with a period of 100 minutes. They were designed by the Motorola telephone company to provide telephone communication in inaccessible places. However, it is a very high cost service.

Galileo satellite system

It is the positioning system developed by the European Union, equivalent to GPS and for civil use. It currently has 22 satellites operating, but it is still under construction. It is capable of locating a person or an object with a precision of 1 meter in the open version and is interoperable with the satellites of the GPS system.

Landsat series

They are satellites specially designed for observing the earth's surface. They began their work in 1972. Among other things, they are in charge of mapping the terrain, recording information about the movement of ice at the poles and the extent of the forests, as well as mining prospecting.

Glonass system

It is the geolocation system of the Russian Federation, equivalent to GPS and the Galileo network.

Observation of artificial satellites

Artificial satellites can be seen from Earth by amateurs as they reflect sunlight and can be seen as points of light, even if the Sun has set.

To locate them, it is advisable to install one of the satellite search applications on the phone or consult internet sites that track satellites.

For example, the Hubble Space Telescope can be visible with the naked eye, or better yet, with good binoculars, if you know where to look.

Preparations for observing satellites are the same as for observing meteor showers. The best results are obtained on very dark and clear nights, without clouds and without moon, or with the moon low on the horizon. The farther away from light pollution the better, you also have to bring warm clothes and hot drinks.

References

1. European Space Agency. Satellites. Recovered from: esa.int.
2. Giancoli, D. 2006. Physics: Principles with Applications. 6th. Ed Prentice Hall.
3. Maran, S. Astronomy for Dummies.
4. POT. About the Hubble Space Telescope. Recovered from: nasa.gov.
5. What are artificial satellites and how do they work? Recovered from: youbioit.com
6. Wikiversity. Artificial satellites. Recovered from: es.wikiversity.org.