(Low-Earth Orbit) is a
communications satellite in orbit 400 to 1600 miles above the earth. Being much
closer than 22,282 mile-high geosynchronous satellites (GEOs), LEO signals make
the round trip from earth much faster. Thus, low-powered "pizza
dishes" and handheld devices can be used. LEOs are also better suited to
interactive conferencing. Unlike GEOs, which travel at earth speed, LEOs
revolve around the globe every couple of hours, and any single LEO is in view
for only a few minutes. In order to maintain continuous communications,
multiple LEOs must be used. From 48 to 66 LEOs are needed to cover the earth.
Orbital characteristics
Objects in LEO encounter
atmospheric drag in the form of gases in the thermosphere (approximately 80–500 km up) or exosphere (approximately 500 km and up), depending on orbit
height. LEO is an orbit around Earth between the atmosphere and below the inner Van Allen radiation belt. The altitude is usually not less than 300 km
because that would be impractical due to the larger atmospheric drag. Equatorial
low Earth orbits (ELEO) 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 are usually called polar orbits.
Higher orbits include medium earth orbit (MEO), sometimes
called intermediate circular orbit (ICO), and further above, geostationary orbit (GEO).
Orbits higher than low orbit can lead to early failure of electronic components
due to intense radiation and charge accumulation.
Human use
Low Earth Orbit has been used for both military and aeronautical purposes. Military rocketry and missiles have long taken advantage of this orbit to launch missiles and rockets over long distances. A missile launched in low earth orbit follows three stages. First it would launch into a suborbital path using its engine. The second stage would be where the thrust and momentum created would allow the missile to reach cruising speeds. In final stage the influence of gravity brings it back to Earth towards it target. In space flight the majority of human spaceflight occurs here. Right now the cost of human spaceflight are astronomical so most space agencies are funded by governments and need to work within set budgets for missions. This is why Low Earth Orbit is still the destination of choice for missions.
Advantages
and Disadvantages of LEO
Low
Earth Orbit is used for things that we want to visit often with the Space
Shuttle, like the Hubble Space Telescope and the International Space Station.
This is convenient for installing new instruments, fixing things that are
broken, and inspecting damage. It is also about the only way we can have people
go up, do experiments, and return in a relatively short time.
There
are two disadvantages to having things so close, however. The first is that
there is still some atmospheric drag. Even though the amount of atmosphere is
far too little to breath, there is enough to place a small amount of drag on
the satellite or other object. As a result, over time these objects slow down
and their orbits slowly decay. Simply put, the satellite or spacecraft slows
down and this allows the influence of gravity to pull the object towards the
Earth.
The
second disadvantage has to do with how quickly a satellite in LEO goes around
the Earth. As you can imagine, a satellite travelling 18,000 miles per hour or
faster does not spend very long over any one part of the Earth at a given time.
So what happens if we want a satellite to spend all of its time over just one
part of the Earth? For instance, a weather satellite wouldn't be very effective
for us in North America if it didn't have a long dwell time over us. (Dwell
time = the time a satellite sits over one part of the globe.) Also, a
communications satellite wouldn't work very well for us in North American if it
spent most of its time over Africa or Asia.
There
are two ways to accomplish this. One solution is to put a satellite in a highly
elliptical orbit and the other is to place the satellite in a geosynchronous
orbit.
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