# Does the International Space Station always travels in the same path?

From the 3d visualization http://www.satflare.com/track.asp?q=25544#TOP (the right image with the red line indication the path of the ISS) it looks like always the ISS takes the same path. Is it the case?

• Same path in what frame of reference? There is a frame of reference in which the ISS traces out approximately the same ellipse, again and again. That frame is centered on the Earth, but it does not rotate with the Earth. ("approximately" because the ISS orbit decays due to interactions with the Earth's magnetosphere and the outer reaches of Earth's atmosphere, and every month or so they have to fire motors to boost it back up to where it belongs.) Nov 24 '15 at 17:57

It shows the orbit of the station around the earth as a red line. From this view, the position of the line is approximately fixed around the center of the earth, with the angle almost fixed with respect to the stars (inertial frames).

In this view, the earth turns to the right (west to east) underneath the path, causing the ISS to pass over ground farther to the west every orbit.

If the earth were a perfect sphere and alone in space, the orbital path in this view would not drift. But it does move over time. The largest contributor to the shift is the fact that the earth is not a sphere. The squished earth has more mass around the equator than it does near the poles. This asymmetry causes the orbital path to precess (east to west) at about 5 degrees per day$^1$. If you were in the same position relative to the earth and the stars, you would see the orbital plane rotate around Antarctica over the course of two and a half months.

1 This document lists several parameters of the ISS orbit, including a calculation of precession. The calculation appears to match that found on the wiki page for nodal precession.

• Nice explanation. There are also smaller orbit changes as they sometimes fire thrusters to dodge debris. Nov 25 '15 at 13:11
• @BowlOfRed > This asymmetry causes the orbital path to precess (east to west) at about 5 degrees per day . Wow, that means every one on the earth will get a chance to see the ISS with naked eye. Is that correct? If yes what is the maximum distance of the hypotenuse b/w a view point on the earth and the ISS will considering that 5 degree deviation in the orbit of the ISS per day. Nov 25 '15 at 14:54
• @Talespin_Kit, you are correct that all locations at the correct lattitudes (mainly less than 52 degrees) can see it. Even without precession, this would happen due to earth's orbit around the sun and earth's rotation. Nov 25 '15 at 16:30
• Can you please point to the authentic source regarding this 5 degree deviation? Nov 25 '15 at 18:38
• @Talespin_Kit - The nodal precession of a satellite due to the oblateness of the planet has been known since the late 1700s thanks to work by Laplace on the inner moons of Jupiter. This nodal precession isn't of much use with regard to the ISS. It is of incredible use with satellites inclined by a bit over 98 degrees. The nodal precession that results is what enables things such as the A-train to exist. Nov 25 '15 at 20:53

No. The ISS is in low-earth orbit, so it won't maintain a specific ground-track along the earth. Wikipedia has a picture of the orbit at different times that I'll attach below (open source image):

• > they can generally get coverage of the whole earth every two days. Does this mean all the points on the earth will be covered every two days once? Nov 25 '15 at 8:30
• @Talespin_Kit that's something that is specific to the stuff that I work on, but there is overlap in what the satellites see. I'm going to delete that section of my answer because I think it will just cause confusion. Nov 25 '15 at 13:02
• More specifically, it's in an inclined low-earth orbit.
– user10851
Nov 25 '15 at 17:52