Satellite orbits
Depending on the function of the satellite or space probe, they are integrated in different orbits around the earth. A distinction is drawn here between low (LEO), medium (HEO) and 24-hour orbits (GEO), and planetary orbits. In addition, polar orbits are assigned to LEOs. Space rockets must be accelerated to a specified escape velocity in order to leave the field of gravity for the planned orbit. The inclination of the orbit also plays an important role.
LEO: Low Earth Orbits
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Low Earth Orbits are orbits for artificial satellites which orbit the earth at an altitude between 200 and 2000 km. The orbit time is approximately between 100 and 120 minutes. This means that a point on the earth loses contact with the satellite after 15 minutes. Examples of LEO satellites include civil and military earth observation satellites, meteorological satellites, research satellites for materials science and biological research, and astronomical research satellites. Manned space flight only uses the low earth orbit. Increasingly, communication satellites are also being placed in these orbits. However, in order to guarantee permanent coverage of a point on the earth’s surface by satellites in LEO, it is necessary for 25-75 satellites to orbit the earth. These satellites are then known as constellations. Examples of constellations are the Globalstar Fleet and the planned Galileo satellite navigation system.
HEO: High Earth Orbits
High Earth Orbits (also known as MEO – Medium Earth Orbit) are orbits at a distance of between 5000 and 20,000 km from the earth. These orbits are virtually circular. Navigation satellites are typical representatives of these orbits.
GEO: Geostationary Earth Orbits
Geostationary Earth Orbits are orbits in which satellites are stationed at an altitude of about 36,000 km above the equator. A geostationary satellite orbits the earth in the same direction and at the same angular velocity as the earth’s surface itself. It is positioned relative to the earth’s surface and always orbits over the same point on the earth. These orbits are mainly used for communication and TV satellites, and for meteorological satellites such as the Franco-German directly transmitting satellites DSF-Copernicus and TDF-1, and the meteorological satellites in the Meteosat Family.
PO: Polar Orbit
Polar Orbit is an orbit travelling over the poles of the earth. The altitude of the orbit is between 400 and 1000 kilometres. A satellite on a polar orbit always flies over the equator at the same local time and is therefore called a sun-synchronous orbit. For example, the orbit of most meteorological satellites orbiting the poles and many earth observation satellites is positioned at altitudes of approx. 850 km above the earth. The duration of the orbit is then approximately 100 minutes. During the flight from pole to pole, the earth rotates below the satellite and only strips of the earth surface are observed. These strips are then joined together to provide a complete picture of the earth’s surface. By contrast with geostationary satellites, the advantage of satellites with a polar orbit is that all areas of the earth can be observed using one satellite. The most well-known satellites for this orbit are the ERS satellites and Envisat.
Swing by manoeuvre
This technique relates to a flight guidance procedure used to enable a spacecraft to flyby a planet on its orbit such that the craft is either accelerated or decelerated by the gravitational field. A swing by manoeuvre allows a space probe travelling on a mission to outer space to be accelerated sufficiently to significantly cut down the journey time. An example of this kind of “planetary billiards” is the thrust generated by the comet probe Rosetta at the earth and at Venus on its journey to the comet Tschurjumow-Gerasimenko. This swing by manoeuvre can also be used to achieve a change in direction of the orbital plane. This was manoeuvre was carried out with the European space probe Ulysses around Jupiter in order to enter an orbit around the poles of the sun.
