A low Earth orbit (LEO) is an orbit from roughly 100 to 1240 miles (160-2000km) above the Earth’s surface. Nearly all human spaceflight has taken place in the low Earth orbit, with a few notable exceptions. A great number of satellites are also in a low Earth orbit, as is the International Space Station.
In fact, what many people think of as space from photographs is still well within a low Earth orbit. The LEO itself is roughly contained by the innermost Van Allen radiation belt, which is held in place by the Earth’s geomagnetic field. There is some overlap between LEO and the Van Allen belt, with some satellites residing in the belt. The inner Van Allen radiation belt itself actually poses difficulties to satellite operation, because satellites have to be shielded against the high energy levels present. There is a proposal to drain the energy from this belt down substantially, reducing the amount of shielding that would be necessary, as well as the danger posed to human being by the energy levels.
There is a significant amount of drag exerted on objects within a low Earth orbit, depending on their altitude. Below about 310 miles (500km) objects reside within the thermosphere, while above this altitude they are within the exosphere. Various gasses are present in both, which exert drag on satellites, requiring them to expend some energy to remain in orbit. Because this drag increases as altitude diminishes, it is not common for objects to be placed at less than around 185 miles (300km) high.
A number of different human objects reside in LEO, from different time periods. The most notable of these is probably the International Space Station, which is situated about 200 miles (320km) above the Earth’s surface, well within the thermosphere. The International Space Station is visited regularly by the Space Shuttle, the Soyuz spacecraft, the Automated Transfer Vehicle, and the Progress spacecraft, all of which engage only in LEO missions.
A large number of satellites also reside in LEO, traveling around the world in roughly 90 minutes, at a speed of about 5 miles per second (8km/s). Launching a satellite into low Earth orbit takes much less energy than launching it into space, and the equipment needed to send a signal back to Earth can be much less powerful. For these reasons, LEO satellites are still widely used, even though they cannot remain situated over one part of the planet in the way geostationary satellites in space can. Debris also clutters the low Earth orbit, with some 8,500 objects larger than 10cm currently tracked. This debris poses a threat to satellites and missions, as even tiny objects traveling at that speed can cause enormous damage.
For all of the human activity in space, a surprisingly little amount of it has actually taken place outside of low Earth orbit. The amount of energy needed to bring a vehicle outside of this orbit is enormous, and returning can be tricky, making manned flights particularly daring. The Apollo program, which eventually sent men to the lunar surface, is probably the best known program to send humans outside of LEO, and since that time only a handful of other manned vehicles have passed the barrier.