Low Earth orbit


A low Earth orbit is an Earth-centred orbit with an altitude of or less, or with at least 11.25 periods per day and an eccentricity less than 0.25. Most of the manmade objects in outer space are in LEO.
There is a large variety of other sources that define LEO in terms of altitude. The altitude of an object in an elliptic orbit can vary significantly along the orbit. Even for circular orbits, the altitude above ground can vary by as much as due to the oblateness of Earth's spheroid figure and local topography. While definitions based on altitude are inherently ambiguous, most of them fall within the range specified by an orbit period of 128 minutes because, according to Kepler's third law, this corresponds to a semi-major axis of. For circular orbits, this in turn corresponds to an altitude of above the mean radius of Earth, which is consistent with some of the upper altitude limits in some LEO definitions.
The LEO region is defined by some sources as the region in space that LEO orbits occupy. Some highly elliptical orbits may pass through the LEO region near their lowest altitude but are not in an LEO Orbit because their highest altitude exceeds. Sub-orbital objects can also reach the LEO region but are not in an LEO orbit because they re-enter the atmosphere. The distinction between LEO orbits and the LEO region is especially important for analysis of possible collisions between objects which may not themselves be in LEO but could collide with satellites or debris in LEO orbits.
All crewed space stations to date, as well as the majority of satellites, have been in LEO. From 1968 to 1972 the Apollo program's lunar missions sent humans beyond LEO. Since the end of the Apollo program there have been no human spaceflights beyond LEO.

Orbital characteristics

The mean orbital velocity needed to maintain a stable low Earth orbit is about, but reduces with increased orbital altitude. Calculated for circular orbit of it is, and for it is. The delta-v needed to achieve low Earth orbit starts around 9.4 km/s. Atmospheric and gravity drag associated with launch typically adds to the launch vehicle delta-v required to reach normal LEO orbital velocity of around.
The pull of gravity in LEO is only slightly less than on the Earth's surface. This is because the distance to LEO from the Earth's surface is far less than the Earth's radius. However, an object in orbit is, by definition, in free fall, since there is no force holding it up. As a result objects in orbit, including people, experience a sense of weightlessness, even though they are not actually without weight.
Objects in LEO encounter atmospheric drag from gases in the thermosphere or exosphere, depending on orbit height. Due to atmospheric drag, satellites do not usually orbit below. Objects in LEO orbit Earth between the denser part of the atmosphere and below the inner Van Allen radiation belt.
Equatorial low Earth orbits are a subset of LEO. These orbits, with low inclination to the Equator, allow rapid revisit times of low-latitude places on Earth and have the lowest delta-v requirement of any orbit, provided they have the direct orientation with respect to the Earth's rotation. Orbits with a high inclination angle to the 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.
In 2017, "very low Earth" orbits began to be seen in regulatory filings. These orbits, referred to as "VLEO", require the use of novel technologies for orbit raising because they operate in orbits that would ordinarily decay too soon to be economically useful.

Use of LEO

A low Earth orbit requires the lowest amount of energy for satellite placement. It provides high bandwidth and low communication latency. Satellites and space stations in LEO are more accessible for crew and servicing.
Since it requires less energy to place a satellite into a LEO, and a satellite there needs less powerful amplifiers for successful transmission, LEO is used for many communication applications, such as the Iridium phone system. Some communication satellites use much higher geostationary orbits, and move at the same angular velocity as the Earth as to appear stationary above one location on the planet.

Disadvantages

Satellites in LEO have a small momentary field of view, only able to observe and communicate with a fraction of the Earth at a time, meaning a network of satellites is required to in order to provide continuous coverage. Satellites in lower regions of LEO also suffer from fast orbital decay, requiring either periodic reboosting to maintain a stable orbit, or launching replacement satellites when old ones re-enter.

Examples

The LEO environment is becoming congested with space debris because of 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.