a spacecraft's orbit is essentially fixed, although effects, such
as venting and atmospheric drag, can cause orbital perturbations.
OMS or RCS thrusting periods can be used to correct or modify the
orbit, as required, for mission operations. The direction and magnitude
of the thrusting period, as well as the time of application, determine
the resulting shape of the orbit.
thrusting period increases the speed at the point of application
and will raise every point of the orbit except the thrusting point.
A retrograde thrusting period decreases the speed at the point of
application and will lower every point of the orbit except the thrusting
thrusting period alters the inclination of the spacecraft's orbital
plane. It does not change the vehicle's period of orbit or height
above the Earth.
A radial thrusting
period is one in which the thrust is applied in a direction perpendicular
to the spacecraft's velocity vector and in the vehicle's orbital
plane. With the vehicle in a circular orbit, a radial thrusting
period would be applied along the radius vector either toward or
away from the center of the Earth.