We don’t think about them that often, but above us are hundreds of flying robots that play a large part in our lives on Earth:
In 1957, lonely Sputnik circled the Earth by itself, but today, the worlds of communication, weather forecasting, television, navigation, and aerial photography all rely heavily on satellites, as do many national militaries and government intelligence agencies.
The total market for satellite manufacturing, the launches that carry them to space, and related equipment and services has ballooned from $60 billion in 2004 to over $200 billion in 2015. Satellite industry revenue today makes up only 4% of the global telecommunications industry but accounts for over 60% of space industry revenue.
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About two-thirds of active satellites are in Low Earth Orbit. LEO starts up at 99 miles (160 km) above the Earth, the lowest altitude at which an object can orbit without atmospheric drag messing things up. The top of LEO is 1,240 miles (2,000 km) up. Typically, the lowest satellites are at around 220 miles (350 km) up or higher.
Most of the rest (about one-third) of the satellites are much farther out, in a place called geostationary orbit (GEO). It’s right at 22,236 miles (35,786 km) above the Earth, and it’s called geostationary because something orbiting in it rotates at the exact speed that the Earth turns, making its position in the sky stationary relative to a point on the Earth. It’ll seem to be motionless to an observer on the ground.
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A small percentage of other satellites are in medium Earth orbit (MEO), which is everything in between LEO and GEO. One notable resident of MEO is the GPS system that most Americans, and people from many other countries, use every day. I never realized that the entire GPS system, a US Department of Defense project that went live in 1995, only uses 32 satellites total. And until 2012, the number was only 24—six orbits, each with four satellites.