From: email@example.com (JamesOberg) Newsgroups: sci.space.history Subject: Re: Nodal Regression? Date: 21 Feb 1998 22:07:41 GMT <<They are unknown to me. What do they mean?>> Nodal regression refers to the shift of the plane of an orbit under the gravitational force of Earth's (or any planet's) equatorial bulge. For low orbit satellites, it can be as much as 6 to 8 degrees per day westward (for example, at inclinations of 52 degrees and 28 degrees respectively). For inclinations higher than 90 deg the shift is eastwards, so an orbit of say 96 degrees at about 300 miles high will have its plane shift about one degree eastward per day, which can be set to counteract the daily shift of the Sun wrt the celestial sphere (360 degrees every 365.26 days, natch!). This leaves the orbital plane constant wrt the Earth-Sun line, it's called sun-synchronous, and it's dandy for Earth surface observation missions. wrt= with respect to The regression rate depends on altitude (the higher, the lower the rate) and inclination (the higher, the lower the rate). There are equations for this if anyone wants to get really technical. Even Earth's moon's plane shifts in space due to various perturbations, mostly Earth's off-spherical bulge. It takes 19 years ("the saros") for the lunar line of nodes to shift 360 degrees -- that's why the dates of eclipses move through the calendar every year by about 18-20 days on average. It's a neat concept, understanding it opens up a LOT of insights to orbital motion. Jorge and I used to do this for a living. You guys paid. We're much obliged.
Newsgroups: sci.space.history From: Henry Spencer <firstname.lastname@example.org> Subject: Re: Nodal Regression? Date: Sun, 22 Feb 1998 00:46:59 GMT In article <email@example.com>, Rob Brown <firstname.lastname@example.org> wrote: >>The words "nodal regression" are unknown to them. > >They are unknown to me. What do they mean? To a first approximation, the plane of an orbit remains fixed in space as the satellite goes around the orbit and the Earth spins underneath. However, that's only a first approximation. In particular, because Earth has an equatorial bulge, it does *not* act like a point mass when looked at more closely. The biggest effect of this is that an orbit precesses, with its plane slowly rotating around the Earth's axis. For a typical orbit, the point in space where (for example) the satellite crosses Earth's equator going southward creeps westward a fraction of a degree per orbit. (The Earth is also spinning eastward underneath that point.) The points where an orbit crosses the equator are known as its nodes, and the precession is also called nodal regression. The rate of precession varies with altitude and inclination and other things, and it complicates life seriously if you want to keep two objects in the same orbit or bring them into the same orbit. For example, the quickie explanations of rendezvous all discuss how a spaceship in a lower orbit, going around the Earth in less time, "catches up" with a station in a higher orbit. What they don't mention is that the planes of those two orbits are precessing at slightly different rates, so the games you can play are sharply limited: you have to launch into a slightly different plane than that of the station, and then time the rendezvous maneuvers so that the spaceship moves up to the right altitude at just the time when its orbital plane has precessed to match that of the station. -- Being the last man on the Moon | Henry Spencer is a very dubious honor. -- Gene Cernan | email@example.com