Maneuvering in Space
The point at which the spacecraft goes into orbit is generally the high point of its orbit, called the apogee. It then ‘falls’ around the earth to a low point, called the perigee. Here is where Kepler’s laws come into play again. At apogee, the spaceship’s speed is at its slowest. At perigee it is at its fastest. Very often, it is necessary to raise the perigee, and sometimes to change the elliptical orbit into a circular orbit.
How is this done?
The speed at apogee must be increased. Firing a rocket motor there will raise the perigee without affecting the apogee. By the same token, firing a motor at perigee will raise the apogee without affecting the perigee. This is how a LEO orbit can be changed into a higher orbit. Usually it is desired to raise both the apogee and perigee, and circularize the orbit. This requires two burns—one at each of the critical points in the orbit.
These burns are called delta v, or changing velocity. In those we’ve discussed, the burns are done in the direction of travel and are called posigrade burns. It is also possible to move a spacecraft from a high orbit to a lower orbit with retrograde burns. In this case the burns are done opposite to the direction of travel. This of course is how the shuttle ends its flights and reenters the atmosphere.
It is not just altitude and shape that can be changed by engine burns. A spacecraft enters orbit at a particular orbital inclination—that is, an angle to the equator. Inclination can also be changed with delta v, but it must be applied perpendicular to the direction of travel and in the direction of the desired inclination change. In low orbits, only very small inclination changes can be made. At very high orbits, inclinations can be changed as much as 30 degrees. This is possible because the orbital velocity is low, as low as around 6000 mph in some, and the pull of gravity is much less than in LEOs. Therefore, less energy is required to change the inclination.