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Answer from Robert Frost, Instructor and Flight Controller at NASA:
The orbit of the ISS is a product of requirements and compromise. The primary characteristics are altitude and inclination, let's look at the considerations for those.
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Altitude
The program set a payloads requirement that said we must be able to provide a microgravity environment for at least thirty day increments for at least 180 days a year. Firing thrusters interferes with the microgravity environment. Atmospheric drag can force the ISS to drop 100 meters in a day. The minimum altitude of the ISS is, therefore, the altitude at which the vehicle can support the nominal 30 days of descent due to atmospheric drag, plus an additional contingency 60 days of orbital day without endangering the vehicle or crew. That turns out to be about 280 km (174 miles).
The upper limit is bounded by multiple constraints. The Russian supply vehicles are certified to an altitude of 460 km (285 miles). So that sets our upper bound. But, even if they were not, above 500 km (310 miles) radiation levels climb sharply, particularly in the South Atlantic Anomaly (SAA), because of the Van Allan Belts. Radiation exposure limits for both crew and equipment would prohibit spending long periods above 500 km. And finally, we had to be able to get the modules to the orbit. Modules like the US Lab are massive. The Space Shuttle had to work hard to get them to the selected altitude.
Inclination
Traditionally, NASA vehicles launched from the Cape go into orbits with an inclination of ~28 degrees (the latitude of Cape Canaveral). Russian vehicles typically launch into orbits with an inclination of 51.6 degrees (I'll explain why in a moment).
Technically, we can get a spacecraft to any orientation, but when we launch a vehicle, we like to take advantage of the velocity it already has from the rotation of the Earth. The Earth spins fastest at the equator and slowest at the poles. Therefore, it is significantly cheaper to orbit with an inclination equal the launch latitude. Anything else requires a maneuver that uses additional fuel. It takes more fuel to head towards the equator and less to head towards the poles.
Therefore, when NASA and the Russians negotiated placement of the ISS, it was agreed that it would be more economical to put it at an inclination favorable to the Russians. It also allows a lot more Earth observation experiments, since a greater amount of the Earth is traversed.
Now, as to why specifically 51.6 degrees. This seems a little weird, at first, because we know the Russians launch from Baikonur. But if we look up Baikonur on a map, we see its latititude is at about 46 degrees.
But, sometimes rockets fail on ascent. And having a neighbor that could see rocket bodies falling on it as an attack, it becomes prudent to try to avoid flying over that neighbor on ascent.
Launching at an inclination of 51.6 degrees ensures that the spacecraft does not pass over China until it is clearly in space.
How was the orbit of ISS decided? originally appeared on Quora - the knowledge sharing network where compelling questions are answered by people with unique insights. You can follow Quora on Twitter, Facebook, and Google+. More questions:
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