There are many satellites revolving around earth, then why any $2$ of them collide to each other.$$OR$$ Why all of them have different orbits? What the scientists do to separate their orbits so that they could not collide?


closed as unclear what you're asking by uhoh, James Jenkins, Hohmannfan, ForgeMonkey, Brian Tompsett - 汤莱恩 Dec 31 '16 at 15:30

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  • $\begingroup$ The satellites are used for very different tasks and require very different orbits. GPS navigation would not be possible if all satellites should use the same orbit. The height and inclination of a satellite must be carefully and individually selected for each satellite. There are groups of communication satellites in a geostationary orbit in a very close distance to each other, but a lot of other satellites use a much lower orbit. $\endgroup$ – Uwe Dec 31 '16 at 13:20

The short answer is simply the vastness of space between the satellites. Here is a recent satellite orbit map showing the somewhat ordered madness of active satellites. enter image description here The first thing to notice is that there are groups of satellites at much larger orbital distances from earth than the cluster of satellites near earth. The common satellite orbit altitudes are:

  1. Low Earth Orbit (LEO): 160-2,000 km
  2. Medium Earth Orbit (MEO): Includes geocentric orbits ranging from 2,000 km to just below geosynchronous orbit at 35,786 km. These are most commonly at 20,200 km or 20,650 km with orbital periods of approximately 12 hours.
  3. High Earth Orbit (HEO): Geocentric orbits above the altitude of geosynchronous altitude of 35,786 km.

Obviously, these satellites are small in nature, but are spaced in such a way that the odds of colliding with another is very slim, since they are all mostly moving in the same direction at various altitudes. Once again, this is simply due to the vastness of space for which the satellites orbit within. Additionally, as noticed in the picture, there are satellites at various inclination angles at similar altitudes. This is done primarily for covering more of Earths northern and southern surfaces, however, also aids in avoiding collisions. But honestly, the chances of a collision are extremely small, even in LEO.

  • $\begingroup$ You might want to add that a very large fraction of the satellites at a particular orbital altitude and inclination and moving in the same direction (and that a particular orbital altitude pretty much implies a particular velocity). So it's a bit like a car highway: Lots of cars moving at relatively high speed, but they are all moving in roughly the same direction at pretty much the same speed, so the relative speed between cars, or in this case satellites, is very low. Just because two satellite are blazing along at 7 km/s doesn't mean they are approaching each other at a relative 14 km/s. $\endgroup$ – a CVn Dec 30 '16 at 22:25
  • $\begingroup$ Good point. I figured the notion that they are all mostly moving in the same direction was obvious, but probably not for the OP. Thanks. $\endgroup$ – TRF Dec 31 '16 at 0:17
  • $\begingroup$ While you are at it, it's a good idea to give credit to the source of the image. If it's a screenshot from a website, mention the site. $\endgroup$ – uhoh Nov 10 '17 at 7:43

Space is big. Really big. As TRF pointed out, satellites are tiny compared to the vastness around them.

But there is a valid worry. From Wikipedia -

the Kessler Syndrome proposed by the NASA scientist Donald J. Kessler in 1978, is a scenario in which the density of objects in low Earth orbit (LEO) is high enough that collisions between objects could cause a cascade where each collision generates space debris that increases the likelihood of further collisions. One implication is that the distribution of debris in orbit could render space activities and the use of satellites in specific orbital ranges unfeasible for many generations.

But even that worry is really only a concern for LEO - which has the highest density of satellites - and various programmes are looking at solutions.


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