13-01-2007 Manmade Satellite Orbits (Article 68)

[Simon Kenny]

Manmade Satellite Orbits (Article 68)

Following an earlier article on observing man made satellites; I was asked why satellites are only visible after sunset and before dawn for short periods of about 45 to 90 minutes. The attached diagram should display the reasons.

Satellites are described in general as having Low Earth Orbits (LEO) at 300 to 1000 Kms, Medium Earth Orbits (MEO) at around 10,000 Kms, Highly Elliptical Orbits (HEO) and Geostationary Earth Orbits (GEO) at 36000Kms approximately.

At present there are about 10,000 objects orbiting the earth. They include operational satellites, defunct satellites, rocket boosters and various other debris. About 450 objects can be seen with the naked eye.

LEO satellites are the ones most likely to be seen. The International Space Station (ISS) orbits at about 400Kms, is easily seen and rivals Jupiter in brightness. Check http://www.heavens-above.com for free orbital details on when you can see the ISS from your location. If you are lucky enough, the Space Shuttle can be seen docking with the ISS. Very bright flares lasting from a few seconds up to about 20 seconds distinguish Iridium communications satellites.

A special type of LEO is the Polar Orbit where satellites travel over the Earth’s poles and are easily distinguished, as they will move in a direct North/South path against the background of ‘fixed stars’. Each orbit takes about 90 minutes at a height of about 750 Kms. A satellite in polar orbit will cover every region of the earth.

HEO satellites can vary in height above the earth from 500 Kms at perigee, which is the nearest approach to earth, to 40,000 Kms at apogee, which is the farthest point from Earth.

Satellites in Geostationary orbit are too far (36,000 Kms approximately) to be seen with the naked eye. They are located over the Earth’s equator, have orbital periods equal to the rotation period of the Earth and consequently remain over the same geographical location on the Earth’s surface. These satellites are used for the continuous broadcasting of TV, Radio and general communications. Even though you may not be able to see them you can tell when a TV link up to a remote site goes through a GEO satellite — there is a noticeable delay between the interviewer posing a question and the interviewee responding. Clear skies! Simon Kenny