Astronomy: the low orbit, a small world above the world
Joining orbit is a difficult exercise. But what do states and companies do there? Between 250 and 1,500 km altitude, we are in low orbit. This is a very hierarchical area, which can be used for everything ...
And preferably not just anything.
There are very few active satellites at altitudes ranging between 200 and 300 km in altitude, This is for very simple reasons: because of the friction of the few atmospheric particles that remain at these heights, a satellite is quickly braked and desorbed in. It will only take a few weeks, even a few months. wants to test a new sensor, an innovative antenna, a super-light solar panel or electronics that works better and cheaper than the others, this is sometimes a great opportunity. Even even to try what to quickly desorb orbit a satellite, like a solar sail ...
Thelight launchers like Electron , or takeoffs in common with several dozen satellites under the hood, sometimes stop at low altitude to drop these small test satellites. They don't need an engine, and if they break down quickly, as long as their equipment has been tested, it doesn't matter. Some also come to test new satellite formats. After the advent in the 2010s of the CubeSat (a satellite standard from" 10 x 10 x 10 cm bricks), different competitors are testing their own even smaller platforms.
There is a fairly famous vehicle that we find at this altitude, it is the small American defense shuttle built by Boeing: X-37B . But it has an engine to bring it back to a more" calm "altitude if the atmosphere starts to get too hot. brake it.
If it is generally recommended to pay great attention to the problem of collisions and debris in orbit, special care must be taken of the one currently at 425 x 428 km altitude and occupied by five astronauts, c that is to say the ISS. Thousands of satellites and debris cross its orbit every day ... Which does not mean that it is obliged to maneuver permanently either! The ISS ejects it- even several dozen small satellites each year, in support of commercial programs, but also of projects of agencies which support universities or initiatives for states which do not have big budgets.
At about 400 km altitude , the satellite population is more diverse. There are larger CubeSats, Earth observation satellites, some "spies whose position is well known, but the capabilities jealously guarded. And some oldcarcasses of the 70s and 80s, on their slow path to desorbitation ...
Welcome to the Far West
Between 500 and 750 km of altitude, one meets a multitude different satellites. First there are those of most of the constellations in service or under construction today: Iridium communications, Earth observation with CubeSats from PlanetLabs ( but also many others), the Connectivity with Star , the satellites for tracking the ships of Spiers, the radars of Iceye .... We find satellites of all space powers and the" population is dense.
The great nations have something to do with it elsewhere certain units whose mission is not publicly disclosed (radar, electronic espionage, "visitor" satellites, etc.). It is at these altitudes that it is most likely that in the near future, a collision will take place ... Probably between old deactivated satellites. Because above 500 km altitude, friction with atmospheric molecules is low, and it often takes several decades for a satellite or a rocket stage to be desorbed.
There is also Hubble !
Heliosynchronous, follow the Sun
There are lower orbits more sought after than the others. For example, for a constellation of communication orEarth observation, it is useful to fly over the whole globe. Which looks easy… But requires what is called a polar tilt: on takeoff, the launcher is oriented towards the Pole (in practice, a little close to avoid the risk of collision). But there is even better: the "Rolls of low orbits, the heliosynchronous polar orbit, between 700 and 1000 km depending on the inclination. Because not only do you fly over the whole of the Earth, but no matter where you fly over, your satellite progresses at the same speed as the Sun around the Earth. Result, on the continents that we fly over, it is always the same time! Like noon (practical, there is no shadow) or ten o'clock, for example (practical, the shadows are always oriented the same).
These orbits, which are the most sought after for Earth observation like the constellateAmerican LandSat ions, European Copernicus or Chinese Gaofen. There are also weather satellites. To save space on these orbits, certain agencies form "trains", that is to say, arrange among themselves so that several satellites follow each other in the same orbit. In this way, not only do they not disturb each other, but their data can more easily be compared and exchanged.
And beyond?
There are far fewer active satellites above an altitude of 850 km, even if there is a lot of debris from the beginnings of space exploration: the satellites had few resources to maneuver, so being at 800 km of altitude represented the possibility of being in a stable orbit for a very long time. Even if that means that today, they represent debris that can remain in orbit for several centuries. This is the case of Vanguad-1, the second American satellite,or of Asterix-1 the first French satellite, always present in orbit. Of course, this region being "sparsely populated, it is seen by some as a suitable area to establish their constellations. This is the case of OneWeb which, although currently bankrupt , plans to send more than 600 satellites approximately 1,250 km above sea level.
We consider that beyond 1000 km and up to at 12,000 km at the equator, the presence of the internal Van Allen belt can disturb measurements and especially reduce the lifespan of the on-board electronics. But its distribution (centered around the equator) is not uniform according to altitude.
Of course, there remains bmany other interesting orbits ... but much higher!
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