Why do satellites spin or spin
The ISS is not alone
The most famous object orbiting the earth is the International Space Station (ISS). But she is by no means alone in space. More than 2000 fully functional satellites are added, plus tens of thousands of defective satellites and debris. Satellites buzz around the earth - almost like mosquitoes in the evening sun.
The ISS is on a fairly low orbit at an altitude of 400 kilometers. A satellite can hardly fly much lower. Because below an altitude of around 300 kilometers, the friction with the thin residual atmosphere ensures that a satellite sinks deeper and deeper within a few months and ultimately crashes.
Many satellites that observe the earth move at an altitude of around 500 to 800 kilometers. These satellites monitor what is going on on the ground and in the atmosphere for scientific, commercial or military purposes.
The navigation satellites of the US "Global Positioning System" (GPS) or its European counterpart "Galileo" orbit at a height of over 20,000 kilometers. They send their signals to the navigation systems in our cars and smartphones.
While the ISS or the low-flying earth observation satellites orbit the earth in just over an hour and a half, a navigation satellite needs around 14 hours for one orbit. In space, the higher a satellite flies, the slower it travels and the longer it takes for a lap. Because the further away a satellite is from the earth, the lower the earth's gravitational pull for it. An ever slower pace is enough to stay on track and not crash.
The orbit on which satellites stand still
This leads to a very interesting effect: at an altitude of 36,000 kilometers, the orbital time of a satellite is 23 hours and 56 minutes. That is as long as it takes the earth to make one revolution. If satellites are at this height above the equator, they rotate exactly with our planet.
Only at this altitude do these spacecraft stand still over a point on earth. Experts therefore speak of the geostationary orbit. Most of the TV and communications satellites are located there. Because only satellites on this orbit can be found with fixed satellite dishes. If the TV satellites were on other orbits, the antennas would always have to follow them across the sky.
Many weather satellites are also located at this altitude. There they always have the same area of the earth in focus. Europe's "Meteosat" provides, for example, the images for the cloud film in the ARD news program Tagesschau from this height.
Research and military
However, there are also a number of weather satellites in low orbits. More details can be seen from a height of 800 kilometers. For this, a fast-moving satellite down there has a certain area on earth in focus only a few times a day.
Other satellites observe what it looks like on earth, how far the ice sheets are, where there is what pollution in the atmosphere and in the oceans, how high the sea level is, whether fields are well irrigated, where the rainforest is being cleared, how metropolitan areas expand, etc.
The satellites in orbit don't just take classic photos. Special filters also in the infrared or thermal radiation range show the state of the vegetation or certain substances in the soil. The radar satellites in space need neither daylight nor good weather; because radar waves record the processes on the ground even in darkness and thick clouds.
Hundreds of satellites from the USA, Russia, China, France, Germany and many other nations serve military purposes. They take photographs or radar images of the ground, establish contact with units abroad or monitor telephone and radio connections.
Some astronomical satellites are also in orbit around the earth. They are not aligned towards earth, but upwards into space. The best known is the Hubble Space Telescope, which has been circling since 1990.
Internet from space: gold rush in orbit
A lot of money is made from observing the earth: entire swarms of satellites, which are only the size of a shoebox, provide a complete picture of the earth's surface every day. For example, farmers use these images to check the condition of their fields, insurance companies record the extent of storm damage, security authorities monitor what happens near national borders, etc.
The really big business in space is communication: telephone, television and Internet connections run via geostationary satellites. An increasingly important market are cruise ships on the seas, whose passengers also want to use the Internet on the high seas.
But to receive the signals from a satellite at an altitude of 36,000 kilometers, an antenna dish is necessary. Reception with a cell phone is not possible. Therefore, some companies rely on the Internet from space, which is also accessible to small end devices.
For this purpose, thousands of new satellites should orbit the earth at an altitude of only about 1200 kilometers. These networks provide fast internet anywhere on our planet at any time.
Some astronomical satellites are also in orbit around the earth. They are not directed towards the earth, but upwards into space. The best known is the Hubble Space Telescope, which has been circling since 1990.
Is the end of space travel looming? The garbage problem in space
When a satellite is launched, parts of the rocket also reach orbit and then orbit the earth permanently. If a satellite is defective, it can no longer fulfill its task. But then it does not crash, but remains on its path. The tanks of many disused rockets and satellites have exploded, leaving huge swarms of scrap pieces behind.
All of this has resulted in more than 100 million debris populating the earth's orbits that are larger than a millimeter. There are more than 900,000 parts larger than a centimeter. There are around 20,000 objects in the range of five to ten centimeters and above. Only they can be followed well from the ground using radar. The smaller pieces move unrecognized through space.
If a piece of space debris collides with an intact satellite, the satellite is usually destroyed. Usually it breaks up into hundreds of pieces of debris, which in turn pose a threat to other satellites. Experts fear an avalanche effect that could make space travel extremely difficult, if not impossible, within a few decades.
The ISS already has to evade larger pieces of garbage several times a year. Again and again there are hits from small parts. Several satellites have failed that are believed to have been hit by space debris.
There is no simple solution to the garbage problem in space; because a kind of "vacuum cleaner" does not work in a vacuum. Research institutions and space agencies such as NASA, ESA and many others therefore focus on avoiding waste. In the future, satellites will be deliberately crashed at the end of their mission so that at some point they do not leave numerous pieces of garbage behind when they collide with other objects.
However, it is unclear whether all states and companies adhere to this recommendation. If the space around the earth continues to be littered as before, then space travel is threatened in a few decades. Because then the debris would quickly destroy almost every satellite - and the launch of humans into space would then be far too dangerous.
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