A low Earth orbit is an orbit around Earth with an altitude between 160 kilometers,,
and 2,000 kilometers. Objects below approximately 160 kilometers will experience very rapid
orbital decay and altitude loss. With the exception of the manned lunar flights of the
Apollo program, all human spaceflights have taken place in LEO. The altitude record for
a human spaceflight in LEO was Gemini 11 with an apogee of 1,374.1 kilometers. All manned
space stations to date, as well as the majority of satellites, have been in LEO.
Orbital characteristics Objects in LEO encounter atmospheric drag
in the form of gases in the thermosphere or exosphere, depending on orbit height. Objects
in LEO orbit Earth between the atmosphere and below the inner Van Allen radiation belt.
The altitude is usually not less than 300 km for satellites, as that would be impractical
due to atmospheric drag. The orbital velocity needed to maintain a
stable low Earth orbit is about 7.8 km/s, but reduces with increased orbital altitude.
The delta-v needed to achieve low Earth orbit starts around 9.4 km/s. Atmospheric and gravity
drag associated with launch typically adds 1.5–2.0 km/s to the delta-v launch vehicle
required to reach normal LEO orbital velocity of around 7.8 km/s.
Equatorial low Earth orbits are a subset of LEO. These orbits, with low inclination to
the Equator, allow rapid revisit times and have the lowest delta-v requirement of any
orbit. Orbits with a high inclination angle to equator, are usually called polar orbits.
Higher orbits include medium Earth orbit, sometimes called intermediate circular orbit,
and further above, geostationary orbit. Orbits higher than low orbit can lead to early failure
of electronic components due to intense radiation and charge accumulation.
Use of LEO
Although the Earth's pull due to gravity in LEO is not much less than on the surface of
the Earth, people and objects in orbit experience weightlessness because they are in free fall.
A low Earth orbit is simplest and most cost effective for a satellite placement and provides
high bandwidth and low communication time lag.
Examples Earth observation satellites and spy satellites
use LEO as they are able to see the surface of the Earth more clearly as they are not
so far away. They are also able to traverse the surface of the Earth. A majority of artificial
satellites are placed in LEO, making one complete revolution around the Earth in about 90 minutes
The International Space Station is in a LEO about 400 km above the Earth's surface.
Since it requires less energy to place a satellite into a LEO and the LEO satellite needs less
powerful amplifiers for successful transmission, LEO is still used for many communication applications.
Because these LEO orbits are not geostationary, a network of satellites is required to provide
continuous coverage. Lower orbits also aid remote sensing satellites
because of the added detail that can be gained. Remote sensing satellites can also take advantage
of sun-synchronous LEO orbits at an altitude of about 800 km and near polar inclination.
ENVISAT is one example of an Earth observation satellite that makes use of this particular
type of LEO. Space debris
The LEO environment is becoming congested with space debris due to the frequency of
object launches. This has caused growing concern in recent years, since collisions at orbital
velocities can easily be dangerous, and even deadly. Collisions can produce even more space
debris in the process, creating a domino effect, something known as Kessler Syndrome. The Joint
Space Operations Center, part of United States Strategic Command, currently tracks more than
8,500 objects larger than 10 cm in LEO. However, a limited Arecibo Observatory study suggested
there could be approximately one million objects larger than 2 millimeters, which are too small
to be visible from Earth-based observatories. See also
Atmospheric reentry Escape velocity
Geostationary High Earth orbit
Highly elliptical orbit International Space Station
Medium Earth orbit List of orbits
Satellite phone Specific orbital energy examples
Suborbital spaceflight Notes
References
This article incorporates public domain material from websites or documents of the
National Aeronautics and Space Administration.