karn (08/06/82)
It is a well-known (and utilized) fact that a satellite in orbit about an oblate central body does NOT follow classic two-body motion. This is because the gravitational field around an oblate body is dependent on position as well as radius distance from the body's center. For example, the earth's oblateness causes the "regression of nodes" in which the plane of a satellite precesses (rotates) about the earth's axis; the exact rate is a function of the orbital inclination (angle with the equator) and the orbit period. The so-called "sun synchronous" orbit, in which the NOAA and LANDSAT satellites are launched, along with amateur radio Oscars 6-9, consists of an orbital inclination and period such that the plane precesses eastward exactly once around the earth each year. This compensates for the earth's movement around the sun, hence the satellite's orbit plane remains in a fixed angle with the day/night side terminator. This means that the satellite cameras always see the same sun angle on the earth below. ALL earth satellites except those in EXACTLY polar orbits (inclination = 90 degrees) will precess in this manner. In addition, the "line of apsides" (orientation of the semi-major axis of the orbit ellipse WITHIN the orbit plane) rotates as a function of inclination, orbital period and eccentricity. The only way the line of apsides will remain fixed in earth orbit (other than by using a perfectly circular orbit, in which case the "line of apsides" is undefined) would be to set the inclination exactly equal to 63.4+ degrees. This is used by the Russian Molniya satellite series, in order to fix apogee at a high latitude over northern Siberia. In the case of the planet Mercury, its line of apsides would also rotate if the sun is indeed oblate, and I suspect that it is simply due to its slow rotation (~25 day period.) Phil Karn Bell Labs Murray Hill