ELi5: How exactly does the ISS avoid some 2,800 satellites while orbiting Earth?
In: Engineering
2,800 may sound like a large number, but it’s not when compared to the size of the Earth and the space above it. Think of how many planes there are each day flying through the sky, none of them hitting each other. [Not literally all, there are accidents with the majority human error]
In the context of space, the satellite does not take up a lot of room. Additionally, you have to consider they are not all on the same plane. [You can think of plane as how many miles they are from the Earth, along with the path of their orbit]
The ISS has a static, and well known, orbit. When satellites are launched they are released in such a way that their orbit does not collide with the ISS orbit, similar to a plane setting a course so that it does not collide with another plane.
The ISS is in a relatively low orbit that’s not that good for most satellites, so many of them are a complete non issue.
For the rest…planning. Orbits are *extremely* predictable. Space agencies track everything and identify any potential conflicts really far in advance. When you can see it coming days or weeks or months away, it only takes a *tiny* correction to make sure things stay apart.
Everything larger than a few inches is tracked by radar all the time. It’s not satellites you worry about, it’s tiny space junk too small to track that’s a more realistic threat.
Satellites are small, and spread out over the whole world, and separated into many different altitudes. The only thing that makes them at all likely to collide, is the fact they move at thousands of mph / kph.
Long answer: Size, altitude, planning, and finally avoidance.
Size: Satellites are typically no larger than a bus, and usually much smaller. The smallest satellites are no larger than a shoebox. The odds of two relatively small objects in vast space colliding is low, but still worrying if you have 1000s of them. Except…
Altitude: The ISS orbits at ~400 km. This is a compromise between being easy to reach, and avoiding drag from the Earth’s upper atmosphere. Satellites operate at different altitudes relating to their mission. For Earth observation satellites this may be low (500-1500 km); for geostationary comms satellites it is much higher (35,786 km); for SpaceX’s planned 10,000-satellite Starlink constellation it will be low (550 km); and some specialist missions go lower, e.g. JAXA’s Tsubame at 167 km. Note that these are all 10s to 1000s of km from the station height. So the vast majority of satellites will *never* operate anywhere near the ISS.
Planning: As one of very few manned stations in space, you are **not** allowed to go anywhere near the ISS as a satellite operator. I can’t say for sure but I highly doubt you can even operate a satellite at the same height as the ISS (excluding cubesats that are sometimes launched from the ISS). Rocket launches to higher altitudes need to be planned so that they do not come close to the ISS, satellites deorbiting need to avoid the ISS, etc.
Avoidance: Finally, the ISS is able to (very slowly) maneuver in order to avoid objects. However, this has only ever been to avoid space junk AFAIK, i.e. scraps of material cast off by other spacecraft. Generally what it does is raise its orbit slightly to go over the hazard. Since the ISS must already frequently re-raise its orbit to account for upper-atmospheric drag, this doesn’t really add cost in the long run.
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