Low Earth orbit: Low-Earth-orbiting spacecraft of significance

In simple terms, a low Earth orbit (LEO) is a circular orbit in which an object orbits the Earth at a lower altitude than is possible in any other orbit. It can be as low as 100 miles (160 kilometers) above the Earth, which is approximately 1,200 miles (2,000 kilometers) above.

Satellites and the International Space Station (ISS) are located in low Earth orbit (LEO). The influence of gravity on a body increases the closer you get to a large mass, like the Earth.

So that might not sound very 'low,' but remember that the closer you get to a large mass, the greater its influence. The Earth's mass center will quickly begin to pull such bodies if they get too close and do not have enough momentum. With a parachute or retro thrusters, that journey would not end well. 

It is because of this reason that a satellite must move at an average speed of 17,500 miles per hour (7.8 kilometers per second) to remain in the orbit. It takes about 90 minutes to complete one orbit of the planet at this speed.

We are drawn to the surface of the planet by gravity, which also makes orbits possible. As humans would float out into space if gravity weren't present, a satellite would fly in a different direction if it weren't orbiting the Earth.

The effect can be felt by a spacecraft traveling at speeds exceeding the Earth's escape velocity of 25,000 mph (11.2 km/s). As with leaping into the air, an object that is moving considerably slower will inevitably fall back to Earth.

A tangent flight of an object is prevented by gravity when it is traveling at 17,500 mph (7.8 km/s). The Earth will continue to circle an object traveling at this speed. The planet's surface is parallel to this horizontal speed.

As you've probably seen, rockets appear to shoot straight up vertically when they go into space. However, this is because they must immediately ascend above (or at least most of) the atmosphere to avoid drag forces. Upon reaching the atmosphere, however, they start moving horizontally.

On a clear day, the vapor trail of a rocket can be seen arcing or curving as it rises. LEO satellites are considered in orbit when their orbital velocity reaches 17.500 mph (7.8 km/s).

At higher altitudes, however, the speed required to keep a satellite in orbit changes. It turns out that speed actually decreases as altitude increases. Despite this, launching a satellite into a higher orbit does not require less energy. Getting to that higher altitude requires a lot of energy.

The additional work required to reach higher altitudes, together with the higher resolution images Earth-observing satellites can acquire from closer proximity, and the fact that there are more available routes for satellites in LEO is one of the reasons most satellites are deployed in LEO.

Geostationary or geosynchronous equatorial orbit (GEO) is one high-altitude orbit worth the extra effort to reach. A satellite in LEO orbits the Earth 16 times per day/per full rotation. Nevertheless, satellites in GEO move from west to east above the equator, following Earth's rotation and taking 23 hours, 56 minutes, and 4 seconds to travel around the globe.

In GEO, it appears as if satellites have a fixed position over a fixed timeframe. When GEO reaches an altitude of about 22,000 miles (36,000 km), its orbital speed slows down, and a single orbit corresponds to exactly one Earth rotation.

Since satellites at that height hover over a single area of the Earth's surface, they are particularly suitable for satellite TV and other communications systems.