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The ISS orbits West to East covering almost every part of the land on Earth, but excludes the polar regions. Why was this orbit chosen?

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The decision to orbit (and assemble) the International Space Station (ISS) in 51.65° prograde Low Earth Orbit (LEO) was driven by its accessibility and utility. You see, inclination changes are terribly expensive for visiting spacecraft delivering astronauts / cosmonauts, consumables, science experiments and other cargo, and ideally, you'd always launch rockets in orbit with its inclination matching that of the launch site's azimuth to maximize rocket's carrying capability. I.e. you'd launch directly due East to take as much advantage of the Earth's rotation as possible to reach required orbital velocity and the resulting orbital inclination is then a direct result of the Earth's own rotation on its axis.

Since most launches to the station are done from Baikonur Cosmodrome, Kazakhstan (Progress and Soyuz rockets), with the launch site roughly 46° North of the equator, ISS' orbit nearly matches that of the launch site. Nearly, but not quite, since directly due East and downrange of Baikonur the land relatively soon (for a rocket flight) enters the territory of China, so to avoid that, the target orbital inclination was increased to 51.65° and the rockets don't overfly China at all. This is also important during crew landings in case they under / overshot the targeted landing site:

   enter image description here

   Rockets coming from the Baikonur Cosmodrome, Kazakhstan are sent into orbits inclined 51.65 degrees to avoid overflying China.

This decision was made jointly by NASA and Roscosmos and is also convenient for Earth observations and covers 75 percent of the Earth and 95 percent of its population under its direct line of sight. I explain this a bit more in Why did the recent Sirius FM6 launch use such a high inclination? and Can I see the ISS from the surface with the naked eye?

Now, if the station instead orbited in a prograde polar orbit (overpassing polar regions, but not in marginally retrograde and permanently sunlit Sun-synchronous Orbit, e.g. the so-called dawn-to-dusk SSO where satellites circumnavigate the Earth riding the solar terminator, which would be a thermal management nightmare for the station), on top of requiring more launches and keeping the station more expensive with it, it would also further increase the time interval at which it would orbit over the launch sites. In its current orbit, this time interval is a bit more than a day, or about 16 of its circa 93 minutes long orbits. If this took significantly longer, it would also make launch windows less frequent and impact traffic to and fro the station. It would also limit crew escape options in an emergency, since you wouldn't want to do a deorbit burn over the polar regions, and it would expose the crew onboard the station to more radiation over those regions too - which would mean it would have to orbit lower still to decrease overall radiation exposure, which would in turn increase required propellant mass to periodically re-boost its orbit due to higher atmospheric drag, and make it even more expensive to maintain / service.

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If you are asking the mechanics of the orbit, then only a polar orbit will ever pass directly over the poles. Think of the orbit as a circle like a hula hoop around a basketball (the ISS's orbit would be very close to the ball). You can tilt the hoop any way you want, but it will only pass over the poles if you stand it on end.

If the Earth did not rotate, the ISS would appear to travel the same path over the surface of the Earth each orbit, but as the Earth rotates the ISS's orbit falls over different parts of the Earth:

                                           orbits
                     The lighter blue area is where the Space Station can be seen from the ground. Image: NASA

If it's orbit were at geostationary distance (might be a little different if not over the equator) but inclined to the equator at the same angle, it would appear to follow a longitudinal line on the Earth's surface from 51.65 degrees north to 51.65 degrees south and back each day.

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The orbit if the ISS stays within the envelope of the lower Van Allen belt. If the inclination were greater, above 52°, the radiation exposure of the crew would be elevated severely, to the point where the health risk would be unacceptable. As is, the exposure is four times that permitted of nuclear plant workers. If the orbit were higher or the inclination greater, the crew would only be able to stay aboard the station for a few days before the radiation exposure would force their return to the surface.

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The greatest exposure of radiation to the crew of the station in the current orbit is from South Atlantic Anomaly and EVA. The SAA is only a light concentration of protons at the current orbit, a mere grazing of the belt compared to the full density of the protons in the lower belt. And the station/proton collisions generate a small amount of hull penetrating gamma radiation on three to four orbits a day, with varying intensity. To venture into the full density of the lower Van Allen belt thirty two times a day (twice an orbit) would bring at least a fifty fold exposure, if not several hundred to thousands of times the exposure, depending on the time spent in the belt each orbit. It is a very serious constraint on any manned orbit path.

Traveling once through the Van Allen to higher orbit or escape velocity is not nearly as hazardous as facing it thirty two times a day. It is however a concern, and manned craft leaving low earth orbit is something that is no longer done without sufficient reason. The ISS covers the maximum range of space that is considered an acceptable radiation risk.

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  • $\begingroup$ How did the Apollo guys survive? $\endgroup$ – Organic Marble Feb 18 '16 at 14:19
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    $\begingroup$ I believe this is an very secondary contributor to the choice of orbit, and TidalWave's answer about inclination is the main factor. The original US "Space Station Freedom" would have been in the max efficient (for KSC) 28.6 orbital inclination. When Russia was joined to the program it was a real struggle to get STS to that high of an inclination with a useful payload. Many changes were made including a major redesign of the ET to use aluminum-lithium. Higher inclinations would have been impractical to the point of impossible. $\endgroup$ – Organic Marble Feb 18 '16 at 19:37

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