Orbits of Satellites
Jered B. / Physics 338 / 19 May 1997
A satellite is a device which resides in space, intended for observation,
research, or communication that is in orbit around the earth. In order
for it do a specific job it requires a certain orbit.
Different Types of
Most satellites are placed into orbit by multistage booster rockets
that are released after the fuel is consumed. Once the satellite is about
two hundred miles from the earth's surface it is free from the earth's
drag on it. Some orbits are circular while others are elliptical, or oval
shaped. Satellites that have an elliptical orbit, have both an apogee and
a perigee. An apogee is the satellites farthest point from the center of
the earth. The perigee is when it is closest to the earth's center. The
altitude of the orbit decides on the period of time it takes to complete
one orbit, this is called the orbital period. In order for the satellite
to remain in orbit, the satellite needs to maintain a constant velocity
while the earth's pull of gravity keeps it in orbit. The velocity helps
the satellite to try to escape the atmosphere but the earth's gravity holds
it in orbit.
Satellites that have low altitude orbits are a couple hundred miles
above the earth's surface. The satellites are in the earth's atmosphere,
but in the highest level and can orbit the earth in ninety minutes. The
satellite can be very large with this type of orbit and less propulsion
is needed. There is little air resistance against them but they only stay
in orbit for a short time, about a couple of weeks to a month.
An orbit that takes about twenty-four hours to revolve around the earth
is called a geosynchronous orbit or a high altitude orbit. It is about
22,282 miles from the earth's surface and stays in the same spot over the
earth at all times, because it is moving at the same speed as the earth.
A satellite in this orbit moves in the same direction as the earth along
the equator. While moving along the equator line, the satellite moves slightly
up and down along the line forming a sort of figure eight. For a satellite
to be put into this type of orbit a very powerful launch vehicle is needed.
Another type of orbit that is similar to the geosynchronous orbit is
the geostationary orbit. A satellite in the geostationary orbit circles
the earth at the same altitude as the geosynchronous orbit, at 22,282 miles
but orbits directly over the equator with out moving up or down. To get
a satellite to maintain this orbit is very difficult and often impossible.
Both the geosynchronous and the geostationary orbits are good for communication,
broadcasting, and weather satellites.
A third type of orbit is the sun-synchronous orbit or the polar orbit.
It passes over the North and South poles. Its "orbit requires one
entire earth year to make a full revolution about the sun. As observed
from the position of the sun, the satellite orbit plane remains in the
same apparent orientation throughout the year" (Parker 42). In a day
a satellite in this orbit makes about fifteen revolutions and a good use
for it is reconnaissance satellites.
Placing a Satellite
"In order for a satellite to be orbited, it must be propelled
at a velocity that imparts enough energy to keep it in that orbit with
out the application of additional force. If the orbit is low, the resistance
of the outer most atmosphere will cause the satellite to lose orbital speed
and reenter the atmosphere" (Encyclopedia America 288). Once a satellites
velocity decreases it will start to fall from orbit and the gravitational
force of the earth pulls it down into a dense part of the atmosphere. Eventually
the satellite is pulled down far enough into the atmosphere. When this
happens the satellite rapidly compresses the air in front of it. This causes
the air to become so hot, that all or most of the satellite will burn up.
For a satellite the type of orbit and the amount of velocity are two
key roles in order for a satellite to do its job. The type of orbit the
satellite takes depends upon the job it has to do. The velocity plays a
key role in a satellites orbit, in that the speed that the satellite is
going will decide if it will remain in orbit.
Parker, Sybil P. "Satellite" McGraw Hill Encyclopedia
of Science and
Technology. New York: McGraw Hill Inc., 1992. Volume 16, pgs.
Satellite, Artificial. Encyclopedia America International Edition.
Grolier Incorporated, 1994. Volume 24, pgs. 286-290.
Satellite, Artificial. The World Book Encyclopedia. Chicago:
World Book Inc., 1995. Volume 1, pgs. 150-150c.
Space Satellites. New book of Popular Science. Danbury: Grolier
Incorporated, 1994. Volume 1, pgs. 304-305.
VanDoren, Charles. "Satellite" Science and Technology
Chicago: Encyclopedia Britannica, 1984. Volume 23, pgs. 2830-2831