When stargazing, you will occasionally notice a "star" moving steadily across the sky—one of the hundreds of artificial satellites orbiting our planet.
Artificial satellites are the wallflowers of the
sky. They are reserved, silent, and usually pass overhead without attracting a single glance. Yet, when they are seen, usually inadvertently, they can garner as much attention as a shooting star. At star parties, someone will often point up into the sky and shout, "Satellite!"
There are hundreds of satellites and spacecraft orbiting
Earth today. They are used for everything from telecommunications, meteorology, and navigation, to Earth-resources monitoring, geophysics, and astronomy. Not all satellites are visible to the naked eye, but many are, and you can usually spot at least one on any night.
Satellites are placed into one of three types of orbit: equatorial, polar, or geostationary. Equatorial satellites travel from west to east. Because they are usually placed at low altitudes, they tend to look like bright stars and take just a few minutes to move across the sky. Polar-orbiting satellites are placed in high-altitude orbits and travel north to south or south to north. Because of their greater altitude, they look like faint stars and seem to move more slowly than equatorial satellites do. Geostationary satellites are placed at extremely high altitudes and remain fixed over the same point on Earth. They can be glimpsed only in telescopes, and even then they are no brighter than a 14th or 15th magnitude star.
When a satellite or a piece of space debris reenters Earth's atmosphere, it may flare up so brightly that it casts a shadow. Such objects are often mistaken for fireballs, which, in a sense, they are, except that they are artificial.