Earth Orbits
Sub-orbital spaceflight
refers to an object that reaches
space but exhibits a trajectory that still intersects the Earth’s
atmosphere.
The orbital decay experienced by such a path prohibits the object from
completing one orbital revolution. The requirements for such an orbit
is that
the object launched from Earth reach an altitude of at least 62 miles
above sea
level. During the time of freefall, the trajectory is still part of an
elliptical orbit with the path merely in intersection with the Earth.
Low Earth Orbit (LEO) refers to
orbits generally in the range of 160 to 2000 km above the Earth’s
surface. Such
an orbit lies high enough above the surface of the Earth to experience
relatively low amounts of drag from the atmosphere while still lying
inside of
the Van Allen radiation. It is because of this that the vast majority
of human
spaceflights have taken place in LEO.
A fourth type of Earth orbit is GEO,
or
Geostationary Earth Orbit. GEO is a geosynchronous orbit that exists
above the
Earth’s equator. The orbit has a period that is exactly the same as
that of
Earth’s own rotational period and exhibits an orbital eccentricity of
practically 0. What results is any object or satellite placed in such
an orbit
maintaining a constant presence in one part of the Earth’s sky. This
effect
makes the orbit perfect for communication and weather satellites needed
for
coverage of areas around the Earth’s equator.
The Molniya orbit refers
to an orbital trajectory exhibiting
a highly elliptical path with a planal inclination of 63.4 degrees.
Such orbits
typically have an orbital period of about 12 hours. Most Molniya orbits
are
used for communication satellites for the polar regions of the planet.
The highly
elliptical orbit provides maximum coverage time for northern regions
while simultaneously
limiting the amount of time spent covering the other side of the
planet. The following
diagrams illustrate the dynamics of the Molniya orbit.
