Observing the International Space Station (ISS)

To observe the ISS, you must first determine whether it may indeed be viewed from your latitude. This depends on its orbital inclination and altitude as well as size, ability to reflect sunlight, etc. In general, the lower the orbital altitude, the more visible an object will be to the naked eye.

The ISS is certainly in a low earth orbit at 400 km altitude when compared with the nearly 40 000 km altitude orbits of meteorological and communications satellites. The ISS is in an equatorial orbit which encompasses orbital inclinations from approximately 0 – 70^o N or S.

What latitudes will ISS fly over encompass?

The ISS will orbit the Earth at an inclination of 51.6 o . This is about the same as the Russian Space Station Mir’s orbital inclination.

Here is an illustration of the ISS in orbit.

To further illustrate, here is the ISS ground track (a surface trace of the ISS orbit) on a Mercator Projection

We can see from these illustrations that the ISS may be viewed from all locations falling within its 51.6^o N to 51.6^o S orbital track. However, it is not only the inclination that determines the latitudes at which the ISS may be viewed. The visibility of an orbiting object depends on both inclination and orbiting altitude (as well as size and ability to reflect sunlight, etc.).

The ISS orbits at approximately 400 km above the surface of the Earth. For objects in such low Earth orbit, there is still about a 15^o window of corresponding latitude above the orbital inclination in which the orbiting object may be seen above the horizon. So, even observers to about 65^o N or 65^o S will be able to view the ISS.

Why? Let us illustrate

Any spacecraft below 400 km is below the horizon for latitudes of about 70^o N. The same would hold for 70^o S if we had illustrated the third or fourth Earth quadrant.

The visibility of an orbiting satellite, then, depends on both inclination and altitude above the Earth‘s surface. If the altitude of the object increases, the latitudinal visibility increases as well, all things considered (size, ability to reflect sunlight, etc.). This can be illustrated

(The effect an orbiting object’s altitude has on its visibility from Earth explains why the moon, which orbits at a 384 000 km altitude, is visible everywhere on Earth.)

Once you have determined whether your latitude falls within the ISS orbital inclination limitations, you can track it to see when the ISS will become visible in your area. Tracking software is widely available online at the Canadian Space Agency Internet site at the following address: http://www.space.gc.ca. By entering your latitude, longitude and viewing time, this program will provide the direction, maximum elevation, duration, etc. of the ISS appearance over your area. Also, check out Sky and Telescope’s magazine and Website (www.skypub.com).

Viewing Windows

Fortunately, there will be many opportunities to view the ISS in Canada. Most Canadians will just have to know when and where to look. The ISS is best observed when it passes high above the horizon. However, this will not occur at the same time each day. The time the ISS passes will vary. This is because our 24-hr. day is not evenly divisible by the ISS’ 90 min. orbital period. The space station will therefore appear in the sky a little earlier (or later) on subsequent days and thus move from daytime to night time passes cyclically. As mentioned, the Canadian Space Agency will provide specific information so that the best viewing times for all viable locations can be determined.