Shape
Satellites orbit their primary body in a shape that is called an
ellipse. An ellipse can be thought of as a circle that is somewhat
"out of round," although the technical definition of an
ellipse is "a closed plane curve generated by a point moving
in such a way that the sums of the distances from two fixed points
is a constant." The characteristics of an ellipse are probably
best understood when compared to a circle.
A perfect circle has a single point in its center. Each point on
the circle is an equal distance from this point, known as the focus
of the circle. Any line that connects two sides of the circle and
passes through the focus are equal in length.
An ellipse is a circle that is slightly stretched in one dimension.
An ellipse has two focal points. The sum of the distances from any
point to each focal point will always remain constant. A line that
connects two sides of the ellipse and passes through both focal points
is called the major axis of the ellipse. A line perpendicular
to the major axis that passes through the point directly between the
focal points is the minor axis of the ellipse.
The degree to which an ellipse is stretched is described as the eccentricity
of the ellipse. The eccentricity can be described as the ratio between
the length of the major axis and the distance between the foci of
the ellipse. The foci in highly eccentric ellipse are spread farther
apart than those of an ellipse with a lower eccentricity. The value
of eccentricity ranges from 0 (a perfect circle) and approaches a
value of 1 as the ellipse becomes more eccentric.
A satellite's orbit around the Earth is in the shape of an ellipse,
and the Earth's center of mass is at one of the focal points of the
ellipse. A satellite orbit can be described by the eccentricity of
the orbit. Satellite orbits range from nearly circular orbits (with
a low eccentricity) to very highly elliptical orbits (with a high
eccentricity). A satellite with a nearly circular orbit maintains
a relatively constant altitude above the Earth's surface, while the
altitude of a satellite with a very highly elliptical orbit is constantly
changing.
Size
Another of the orbital elements used to describe a satellite's orbits
is the semi-major axis, which is defined as half the distance
of the major axis. In general, the larger the semi-major axis, the
larger the orbit. The larger the orbit, the greater the amount of
energy required to place the satellite into the orbit. Thus, satellites
with larger orbits with higher altitudes above earth are much more
expensive to launch and maintain.
__________________________________________________________________________
