Re: Favored decay latitude?

Alan Pickup (alan@wingar.demon.co.uk)
Wed, 2 Dec 1998 23:57:17 +0000

Mark Hanning-Lee <markhl@prodigy.net> writes
>I've noticed that many reentry predictions on the list seem to predict
>decay soon after an equator crossing.
>
>Why would a sat in a roughly circular orbit tend to decay at any
>particular latitude? Does a decaying sat feel significantly more drag at
>low latitudes? If so, why is the atmosphere number density higher there;
>is it just the greater insolation?

The Earth's atmosphere, like the Earth itself, is oblate. Imagine an
object in a circular orbit 6,557 km from the Earth's centre and passing
over the poles (ie it has an orbital inclination of 90 degrees). When it
passes over the poles it will be exactly 200 km high and feeling the
atmospheric drag appropriate to a height of 200 km. Ninety degrees
further around its orbit, it will still be the same distance from the
Earth's core, but it will be only 179 km above the Earth's surface. To a
first approximation, that 21 km closer to the surface means that the
atmospheric drag it meets over the equator is double the drag it
experiences over the poles. Since most objects (Molniyas excluded) decay
from near circular orbits, there is a greater chance that they will
receive their fatal dose of drag over equatorial regions than over the
poles. Of course, if the dose is not quite fatal, they can still drop
along a different arc of their orbit and (unless the re-entry is
observed) we may never know.

However, there are day-night variations in density too, with the drag
over the night-time hemisphere less than that over the sunlit one. And,
just to keep things interesting, the drag varies seasonally and with
solar activity.

>
>(I appreciate that a sat in a very eccentric orbit decays near perigee,
>which makes sense to me.)

Yes, a highly eccentric satellite like Molniya 3-11 was essentially free
of drag over most of its orbit right until the end, except near perigee.
It is possible to conceive of a situation when it could just emerge from
perigee and re-enter halfway around its orbit, below the former apogee,
but this is much less likely than a decay at (or close to) perigee
itself. 

Alan
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