Observing Satellites


Many people have probably noticed satellites passing overhead on a clear night and wondered what they were. Heavens-above.com maintain a set of dynamic Web pages which generate predictions of visible satellite passes for any point on the Earth's surface. To the best of my knowledge, this site is unique on the Internet because it is the only one to offer dynamically generated predictions for ANY location using the latest orbit data.

There are over a hundred objects orbiting the Earth which can be seen with the naked eye, and this means that there will be several passing over your location every evening, wherever you are. Due to its large size and relatively low orbit, the Russian space station Mir is a very bright object and can be easily seen, even under poor viewing conditions. Satellites can be seen as star-like objects moving quite rapidly across the sky, and most newcomers are surprised at how fast they move. They can be distinguished from a high-flying aircraft because they normally don't blink, make no sound, and occasionally can be seen entering or leaving the Earth's shadow. The apparent speed is similar to a high-flying aircraft, because Mir, for example, travels about 30 times faster but is 40 times higher up. Satellites in higher orbits will have a slower apparent motion which is roughly inversely proportional to their height. Sometimes, a satellite will suddenly become much brighter for a few seconds, when the angle between the observer, the satellite and the sun is just right and the sunlight glints off the solar panels or some other part of the structure. Many of the visible objects are spent rocket upper stages which are tumbling and this causes a periodic increase and decrease in brightness with a period of a few seconds which can easily be observed.

The best way to observe satellites is to generate the predictions page for the particular evening you are interested in as late as possible, and then print it out if you can. That way, you will have the latest, most accurate predictions available to refer to. It is always a good idea to give your eyes a minute or so to adjust to the darkness before you start looking, and if possible, find a spot away from street lights which can spoil the viewing conditions.

For a good introduction to observing visible satellites, see the Visual Satellite Observer's Home Page.

My favorites are Iridium flares!
An Iridium flare is caused by the sun being reflected from one of the three main mission antennae (MMA) of an Iridium satellite. The MMAs are flat, highly polished aluminum surfaces, and when the angles are just right, they can reflect the sun just like a mirror. There are over 80 of these communications satellites in orbit, and they are operated by the Iridium LLC Consortium. For more information, please see the Heavens Above Iridium flare help page.

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