a question on space debris

Message Yeah, that's why I backed up to add the "well, for geological time-scales at least" disclaimer. Also, I probably overstated the case in saying re-entries could be predicted to within a few minutes. Might have been better to have just said "to within the hour". Learned some new things. Thanks.

Regards,
Mike Combs

Combs, Mike wrote: The only debris which will return to Earth is that which, at some point in its orbit, at least slightly grazes the top-most parts of the atmosphere.Trivially true. But actually, due to perturbations from the Earth, Moon, Sun and other planets it's a lot more complicated than that; since the orbits are not fixed. Even light pressure plays a part.
Any debris not doing this (for example, all the junkclose to GEO) is up there for good (well, OK, up there for geological time-scales at least), which is part of the problem.Yes, however the problem isn't as bad at that altitude- the orbital speeds are lower and the volume is bigger. It's low/medium earth orbit where there are potentially the biggest problems.
NASA and the Russian space agencies seem to have gotten pretty good at predicting re-entry times, at least to within a few minutes.
When debris will re-enter is largely a function of its orbit. Rotation rate won't really enter into it much.Largely true, but only up to a point. Rotation affects the average ballistic coefficient of the object which in turn affects its decay. If it's not spinning it may take up a lower drag aspect ratio.