A satellite doesn’t necessarily have to be a tin can spinning through space. The word “satellite” is more general than that: it means a smaller, space-based object moving in a loop (an orbit) around a larger object. The Moon is a natural satellite of Earth, for example, because gravity locks it in orbit around our planet. The tin cans we think of as satellites are actually artificial (human-built) satellites that move in precisely calculated paths, circular or elliptical (oval), at various distances from Earth, usually well outside its atmosphere.
To understand why satellites move this way, we must revisit our friend Newton. Newton proposed that a force – gravity – exists between any two objects in the universe. If it weren’t for this force, a satellite in motion near a planet would continue in motion at the same speed and in the same direction – a straight line.
What do they do?
Communications satellites are “space mirrors” that can help us bounce radio, TV, Internet data, and other kinds of information from one side of Earth to the other.
One of the most surprising things about satellites is the very different paths they follow at very different heights above Earth. Left to its own devices, a satellite fired into space might fall back to Earth just like a stone tossed into the air. To stop that happening, satellites have to keep moving all the time so, even though the force of gravity is pulling on them, they never actually crash back to Earth. Although there are many different types of satellite orbits, they come in three basic varieties, low, medium, and high—which are short, medium, and long distances above Earth, respectively.
Many satellites have orbits at a carefully chosen distance of about 36,000 km (22,000 miles) from the surface. This “magic” position ensures they take exactly one day to orbit Earth and always return to the same position above it, at the same time of day. A high-Earth orbit like this is called geosynchronous (because it’s synchronized with Earth’s rotation) or geostationary (if the satellite stays over the same point on Earth all the time). Communications satellites—our “space mirrors”—are usually parked in geostationary orbits so their signals always reach the satellite dishes pointing up at them. Weather satellites often use geostationary orbits because they need to keep gathering cloud or rainfall images from the same broad part of Earth from hour to hour and day to day (unlike LEO scientific satellites, which gather data from many different places over a relatively short period of time, geostationary weather satellites gather their data from a smaller area over a longer period of time).
The higher up a satellite is, the longer it spends over any one part of Earth. GPS Navistar satellites are in MEO orbits roughly 20,000 km (12,000 miles) above our heads and take 12 hours to “loop” the planet. Their orbits are semi-synchronous, which means that, while they’re not always exactly in the same place above our heads, they pass above the same points on the equator at the same times each day. It’s just the same as jet planes flying over your head: the slower they move through the sky, the higher up them are. A medium-Earth orbit (MEO) is about 10 times higher up than a LEO.
Scientific satellites tend to be quite close to Earth—often just a few hundred kilometers up—and follow an almost circular path called a low-Earth orbit (LEO). Some follow what’s called a polar orbit, passing over both the North and South poles in a “loop” taking just over an hour and a half to complete. Since they have to be moving very fast to overcome Earth’s gravity, and they have a relatively small orbit, they cover large areas of the planet quite quickly and never stay over one part of Earth for more than a few minutes.
Allow telephone and data conversations to be relayed through the satellite. Typical communications satellites include Telstar and Intelsat. The most important feature of a communications satellite is the transponder — a radio that receives a conversation at one frequency and then amplifies it and retransmits it back to Earth on another frequency. A satellite normally contains hundreds or thousands of transponders. Communications satellites are usually geosynchronous.