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Low Earth orbit (LEO) is an orbit 100-1240 miles above Earth’s surface, where most human spaceflight and satellites are located. The International Space Station is in LEO and is regularly visited by spacecraft. LEO has drag from gases in the thermosphere and exosphere, and debris poses a threat to satellites. The Apollo program is the best-known program to send humans outside of LEO.
A low Earth orbit (LEO) is an orbit approximately 100 to 1240 miles (160 to 2000 km) above the Earth’s surface. Nearly all human spaceflight has taken place in low Earth orbit, with a few notable exceptions. A large number of satellites are also in low Earth orbit, as is the International Space Station.
In fact, what many people think of as space from the photographs is still well within low Earth orbit. LEO itself is approximately 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 internal Van Allen radiation belt itself actually poses challenges for satellites to function, because satellites must be shielded against the high energy levels present. There is a proposal to drain the energy from this belt substantially, reducing the amount of shielding that would be required, as well as the danger posed to humans by the energy levels.
There is a significant amount of drag exerted on objects within low Earth orbit, depending on their altitude. Below about 310 miles (500 km) objects reside within the thermosphere, while above this altitude they are within the exosphere. In both, various gases are present, which exert drag on the satellites, requiring them to expend some energy to stay in orbit. Because this drag increases as altitude decreases, it is not common for objects to be placed lower than about 185 miles (300 km) in elevation.
A number of different human objects have resided in LEO, from different time periods. The most notable of these is probably the International Space Station, which sits 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 engaged in LEO missions only.
A large number of satellites also reside in LEO, traveling around the world in about 90 minutes, at speeds of about 5 miles per second (8 km/s). Launching a satellite into low Earth orbit requires much less energy than launching it into space, and the equipment needed to send a signal to Earth can be much less powerful. For these reasons, LEO satellites are still widely used, even though they cannot remain located on one part of the planet as geostationary satellites in space can. Debris also litters low Earth orbit, with about 8,500 objects larger than 10cm currently being tracked. This debris poses a threat to satellites and missions, as even small objects traveling at that speed can cause massive damage.
For all of human activity in space, a surprisingly small amount of it has actually taken place outside low-Earth orbit. The amount of energy required to lift a vehicle out of this orbit is enormous, and return can be complicated, making manned flights particularly daring. The Apollo program, which eventually sent humans to the lunar surface, is probably the best-known program to send humans outside of LEO, and only a handful of other manned vehicles have cleared the barrier since that time.
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