Satgen 614 Orbit Modification by GM4IHJ (BID SGEN614) 2000-12-30 Given the uncertainty attending recent events of this nature, it is perhaps interesting to take a broad look at what makes satellite orbit adjustment, a far from simple process. As radio amateurs we are normally concerned with effecting changes from original commercial launch orbits, to more suitable amateur radio orbits. Changes which generally involve, alterations to apogee and perigee height and ( much more difficult ), inclination of the orbit plane. A rough guide to how these changes are achieved and, how they affect several parameters simultaneously , is :- Thrust Reference Direction Will Modify 1. Line from planet to sat Argument of perigee Semi Major axis Perigee passage time Eccentricity 2. Line from planet Inclination perpendicular to orbit plane Argument of perigee Longitude of ascending node 3. Line from planet Same changes as 1. above perpendicular to 1 and 2 above Note.. other sets of axes for the direction of impulse are possible and desirable in some circumstances. Choice 2 above , often occurs in situations involving the orbits used by Phase 3 satellites. Where high orbital inclination angles are desirable. Unfortunately this can be extremely expensive in rocket fuel. Since the satellite behaves like a huge gyroscope, with a great deal of angular momentum, and change in inclination involves rotation of angular momentum. Which is expensive, Eg it is easy to drive an automobile straight , but thrust is needed to get it to change course. Interestingly enough. NASA has just run head on into a problem of this sort. Last September , a proposed mission to the outer most known planet, Pluto, was cancelled. But since then a lot of pressure has been forthcoming to reinstate the project. The reason being that if it is not done soon, it will simply not be possible . Noting that an interplanetary satellite with its inherent earth motion around the Sun, is fixed in inclination in the plane of this orbit. To launch to Pluto, a planet which differs from the rest of the planets , by travelling in a much higher inclination orbit, is enormously expensive in fuel. Unless that is, you fire it at its new inclination just as the sat crosses the line of nodes. That line between the planets ascending and descending orbital nodes . If that launch point is missed we have not got a rocket capable of putting a satellite out to Plutos highly inclined orbit. Hence NASAs , now or (almost ) never decision.