# Satellite

Image of Sputnik 1 , the first artificial satellite in space

A satellite (from the Latin satelles "companion, bodyguard"), formerly also an artificial moon , is an artificial spacecraft in space travel that orbits a celestial body in an elliptical or circular orbit for scientific , commercial or military purposes.

## On the concept of the earth satellite

Satellites are, by extension, all astronomical objects which a celestial body - a star , planet or moon or other - orbit.

Artificial devices that orbit the earth are specifically called earth satellites in German . Artificial satellites that orbit and explore a body other than the earth, on the other hand, are referred to as orbiters , whereby a missile orbiting the sun is sometimes also called a "solar satellite". In contrast, there are the natural satellites of planets, which are also known as moons or satellites and - like the earth's moon  - are treated separately, as well as the natural satellites / satellites of the stars, planets, asteroids and other things. Artificial satellites that come from a park orbit around the earth into interplanetary space can be referred to as “artificial planetoids ”, we speak of space probes . Naturally, this also includes those who then enter orbit as orbiters at the target.

Missiles are only called satellites if they orbit the earth in space . A satellite lacks - even after it has reached its career path - a self-propulsion system, which distinguishes it from a spaceship . Simple brake rockets that lead to a controlled crash are not enough in the technical sense to turn a satellite into a spaceship.

## history

After the successful launch of Explorer 1, the project managers hold up a model: William H. Pickering , James A. Van Allen and Wernher von Braun

In 1955, US President Eisenhower commissioned the development of an American earth satellite, whereupon the Soviet Union announced a similar project four days later for propaganda reasons. Nevertheless, the successful launch of the Soviet Sputnik 1 satellite on October 4, 1957 (19:28 GMT, October 5, local time) surprised the world public and led to a real Sputnik shock in the West . Sputnik's radio signals indicated in coded form whether the satellite had been hit by matter. The first US American satellite Explorer 1 followed on February 1, 1958 and provided evidence of the Van Allen radiation belt at the beginning of the exploration of the ionosphere . Even so, the development of satellites was influenced by the Cold War for a long time .

In the field of international telecommunications, communications satellites have since the 1970s reduced the importance of other data connections such as the transatlantic telephone cable . Earth observation and weather satellites became just as important , while research satellites for astronomy , geodesy and cartography had been developed as early as the 1960s .

The United Nations Office for Outer Space has maintained an index (Index of Objects Launched into Outer Space) of all satellites launched into space since 1962.

According to the US American NASA, there were around 1950 artificial objects in space on May 31, 1969 , of which 1,889 orbited the earth, 17 were in an ellipse around the earth and 38 were in an orbit around the sun. In addition to burned-out rocket stages and other objects, there were 394 earth satellites and space probes on the reporting date , including 289 from the USA, 83 from the Soviet Union, 5 French, 3 Canadian, 2 British and 3 from the European Space Research Organization .

In 2016 the number of known active satellites was already over 1,400. In addition, there are several thousand other artificial objects (disused satellites, parts of rockets and other space debris ) in orbit: In 1996, according to ESA data, around 8,500 pieces of "space debris" are to be found. (from about 10 cm in size). In 2009, the Joint Space Operations Center of the United States Strategic Command knew of over 18,500 man-made celestial bodies.

Despite the large number, collisions are extremely rare. The first known collision of an active satellite with a disused object took place on February 10, 2009: the Russian satellite Kosmos 2251 , which had been in space since 1993 and probably out of operation since 1999, collided with the Iridium 33 communications satellite from the US company Iridium Satellite . Both satellites were completely destroyed. On January 22, 2013, the Russian small satellite BLITS (NORAD 35871) collided with fragments of the Fengyun-1C satellite, which was destroyed in 2007 by a Chinese anti-satellite missile, and was thrown out of its orbit. On May 23, 2013, debris from a Russian rocket caused the NEE-01 Pegaso satellite to tumble uncontrollably, causing it to spiral out of control.

Depending on the task of the satellite, a distinction is made between the following types:

Which satellite orbit is best suited in each case depends on the tasks. Observation satellites should fly as low as possible. The orbit of spy satellites is sometimes so low that the friction in the atmosphere limits their lifespan to a few months.

In contrast, communications satellites are given the highest possible orbits so that they can cover large areas. If they are to be stationary over a point on the earth's equator, they have to orbit the earth in a geosynchronous orbit at an altitude of about 36,000 km in the direction of the earth's rotation (special case: " geostationary ").

## construction

A satellite essentially consists of the scientific, commercial or military payload as well as the satellite bus , which contains the structures and subsystems necessary for their operation. This consists of the primary structure into which the other subsystems are integrated. This includes the energy supply ( solar cells , accumulators ), the temperature control system , the drive system for position and position control (path control) and the on-board computer system for control and data management .

### Energy supply system

The satellite is mostly supplied with electricity (energy) by solar cells with the support of accumulators if there is sufficient brightness of the sun in the area close to the earth, or by batteries if only short periods of use are planned. The considerably smaller radioisotope generators are used for satellites that are further away from the sun and so the supply of radiant energy is too low .

After the satellite has started, its continued operation must be guaranteed. This includes not only on-board control and monitoring systems, but also corresponding ground stations (e.g. Mission Control Center ) that take over ground control, remote control and evaluation or provision of data from the satellites or their payload.

## Speeds

For a near-earth, circular orbit, the first cosmic speed of v 1 = 7.9 km / s applies .

When starting in an easterly direction, the rotation of the earth contributes a maximum of 0.46 km / s to the orbit speed. However, the earth's rotation is not fully exploited, since the missile is slowed down due to air particles (winds) moving in other directions. A v 1 of 7.44 km / s can thus suffice for a rocket . In a westerly direction, the share would have to be raised, so almost all satellites are launched in an easterly direction. The circular speed of polar orbits remains unaffected by the rotation of the earth.

If you want to leave the earth's gravitational field , the satellite has to be accelerated to the second cosmic speed of about 11.2 km / s. It corresponds to times the first cosmic speed. ${\ displaystyle {\ sqrt {2}}}$

## Observation from the earth

Numerous larger earth satellites can be observed with the naked eye as points of light moving across the night sky. With telescopes specially equipped for solar observation , it is also possible to observe the passage of satellites in front of the sun. The ISS , as the largest artificial object in Earth orbit, can achieve an apparent brightness of up to −5 mag. The satellites of the Iridium system can reflect the sunlight with their antenna surfaces as an Iridium flare and then briefly achieve an apparent brightness of up to −8 mag. In contrast to an airplane, a satellite does not have any flashing colored lights. However, in some of the objects, the brightness changes due to rotation or a tumbling movement. Sometimes satellite light reflections are mistakenly mistaken for stars.

When it comes to observation with the naked eye, this is usually only possible shortly after sunset or shortly before sunrise. This is due to the fact that the satellite in its (not too) high orbit still has to be illuminated by the sun so that it can be seen on the ground (where it is already / still dark) in front of the dark sky; in the middle of the night it also flies in the shade and remains invisible. The orbit must not be too high either, because the distance will make the satellite too small to be visible even when exposed to radiation.

A satellite can be recognized by the high speed at which it moves across the sky; it typically only takes a few minutes to completely fly over the visible part of the sky.

A large object like the ISS can be particularly bright. But even it is rarely seen in our latitudes. This is due to several points that also apply to other satellites:

• The object must have a sufficiently inclined orbit to the equatorial plane so that it even penetrates into our latitudes at all; if the object always circles exactly over the equator, it can only be seen there. The ISS in particular only barely and rarely reaches our latitudes.
• As stated above, the orbit must lead the object to our latitudes at a suitable time around sunset or sunrise. Accordingly, there are websites with date previews, when and for which object the next sightings will be possible.
• The lower the orbit of the object, the larger it appears and the brighter it is visible, but also the shorter it is in the visible field of view and has to hit its own location more precisely.
• The higher the orbit of the object, the smaller and less bright it appears, but it is longer and visible from a larger area.

### Long-term recordings from geostationary satellites

While stars move in the night sky, geostationary satellites are always in the same place. This is how they appear as points on long-term recordings:

## Transport and course

Symbols used:

 ${\ displaystyle \ gamma}$ : Gravitational constant = 6.673 · 10 −11 m 3 / kg · s 2 r : Orbit radius or distance between the centers of mass of the earth and the body in orbit = 6.378 x 10 6 m M. : Mass of the earth = 5.9722 x 10 24 kg m : Mass of the body in orbit v : Orbital speed of the body in orbit

An earth satellite must be given such a high orbital speed when it is launched that its centrifugal force (or radial force ) is at least equal to its weight .

The centrifugal force is calculated as follows:

${\ displaystyle F_ {r} = {\ frac {m \ cdot v ^ {2}} {r}} \}$.

The force of gravity is calculated as follows:

${\ displaystyle F_ {G} = \ gamma \ cdot {\ frac {m \ cdot M} {r ^ {2}}} \}$.

There must be, after insertion: ${\ displaystyle F_ {r} = F_ {G}}$

${\ displaystyle \ \ {\ frac {m \ cdot v ^ {2}} {r}} = \ gamma \ cdot {\ frac {m \ cdot M} {r ^ {2}}} \}$.

Now you can see that the mass of the body on the circular path has no relevance, since it is omitted in the equation. The orbit speed required for a certain orbit therefore only depends on the orbit height:

${\ displaystyle v ^ {2} = {\ frac {\ gamma \ cdot M} {r}} \}$, It follows: .${\ displaystyle \ v = {\ sqrt {\ frac {\ gamma \ cdot M} {r}}} \}$

The first cosmic speed or orbital speed :

With this speed it is possible for a body to keep this orbit on a circular orbit around the earth:

${\ displaystyle v_ {1} = {\ sqrt {\ frac {\ gamma \ cdot M} {r}}} \}$, insert results
${\ displaystyle v_ {1} = {\ sqrt {\ frac {6 {,} 673 \ cdot 10 ^ {- 11} \, \ mathrm {\ tfrac {m ^ {3}} {kg \, s ^ ​​{2 }}} \ cdot 5 {,} 976 \ cdot 10 ^ {24} \, \ mathrm {kg}} {6 {,} 378 \ cdot 10 ^ {6} \, \ mathrm {m}}}} = 7905 {,} 4 \, \ mathrm {\ tfrac {m} {s}} = 28459 {,} 6 \, \ mathrm {\ tfrac {km} {h}} \ approx 7 {,} 9 \, \ mathrm { \ tfrac {km} {s}} \}$.

With the second cosmic speed or escape speed he can leave the gravitational field of the earth. It amounts to:

${\ displaystyle v_ {2} = v_ {1} \ cdot {\ sqrt {2}} = 11 {,} 18 \, \ mathrm {\ tfrac {km} {s}} \}$.

The transport into orbit takes place with the help of rockets , which are designed as step rockets for technical and energetic reasons . The satellite is placed on the top (mostly third) rocket stage and is aerodynamically disguised. It is either shot directly into the orbit or released by another spacecraft . As long as the rocket is working, it runs on what is known as the “active path”. After the rocket motors burn out , the "free flight path" (or passive path) follows.

### Satellite orbits

The motionless motion of a satellite obeys the laws of the two-body problem of celestial mechanics - but other forces cause orbital disturbances . If the earth were an exact sphere without an earth's atmosphere and if there were no other celestial bodies , the satellite orbit would follow a more or less eccentric ellipse around the earth according to Kepler's laws . The orbital planes of the earth satellites go through the center of the earth and are approximately fixed in space, i.e. unchanged compared to the fixed stars , while the earth rotates below.

Depending on their altitude, satellites are divided into different types:

• GEO (Geostationary Orbit): geostationary satellites with an altitude of about 35,790 km. Here the cycle time is exactly one day. These satellites are stationary with respect to the earth's surface. Examples: Astra , Eutelsat , Inmarsat , Meteosat etc.
• MEO (Medium Earth Orbit): satellites with an altitude of 6,000–36,000 km and an orbital period of 4–24 hours. Examples: GPS , GLONASS , Galileo etc.
• LEO (Low Earth Orbit): satellites with an altitude of 200–1500 km and an orbital period of 1.5–2 hours. Examples: Iridium , Globalstar , GLAST etc.
• SSO (Sun Synchronous Orbit): ERS , Landsat , Envisat

Due to the flattening of the earth and the inhomogeneity of the earth's surface and the earth's gravity field , the satellite orbits deviate from the ideal elliptical shape by a few kilometers. From the observation of these deviations, the satellite geodesy can determine the exact shape of the earth - the geoid deviates from the fictitious earth ellipsoid by up to 100 m. For these deviations (on a radius of 6357–6378 km only 0.001%) the somewhat unfortunate terms potato and pear shape were coined.

In addition, the earth's atmosphere causes constant slight braking of the satellites, so that orbits below an altitude of about 1000 km spiral closer to the earth. The lifespan also depends on the surface / mass ratio and ranges from a few weeks or years (LEOs) to millennia (MEOs). Further orbital disturbances are caused by the gravitation of the moon , the radiation pressure of the sun and effects in the ionosphere . The satellite orbit must therefore be constantly monitored and, if necessary, readjusted ( Attitude Determination and Control System ). When the gas supply for the correction nozzles is used up, the satellite leaves its orbit and is usually worthless.

## literature

• Michel Capderou: Satellites - orbits and missions. Springer, Paris 2005, ISBN 2-287-21317-1
• Louis J. Ippolito: Satellite communications systems engineering - atmospheric effects, satellite link design and system performance. Wiley & Sons, Chichester 2008, ISBN 978-0-470-72527-6
• R. Bender: Launching and operating satellites - legal issues. Nijhoff, Dordrecht 1998, ISBN 90-411-0507-7
• Bruno Pattan: Satellite systems - principles and technologies. Van Nostrand Reinhold, New York 1993, ISBN 0-442-01357-4
• CB Pease: Satellite imaging instruments - principles, technologies and operational systems. Ellis Horwood, New York 1991, ISBN 0-13-638487-0