Re: Decay

Alan Pickup (
Sat, 6 Dec 1997 11:20:00 +0000

In message <>, writes
>I enjoy reading the various posts about objects who's orbits are about to
>decay to a point of re-entry. I know there are various elements that effect
>when an object will re-enter but is there a specific or rough altitude at
>which an object will actually re-enter the atmosphere?

I think the usual convention is to take an altitude of 90-100 km as the
critical one, at least for an object in near-circular orbit - as most
are by the time they are on the verge of re-entry. The luminous firball
event which may result probably continues on a gently falling trajectory
to lower altitudes - perhaps (I'm guessing here!) to 50 km or lower.
Some objects, however, may begin to "burn up" but then survive
temporarily as they climb away from their low-point. For example, an
object in a circular inclined orbit will experience the strongest drag
as it moves closest to the surface as it crosses over the equator.

One exception to the 90-100 km rule concerns highly eccentric objects
whose perigees are forced down by gravitational perturbations. Such an
object may survive for many revolutions with a perigee below 100 km,
just as is happenning now for Cosmos 1172 whose perigee (above a
spherical Earth, see below) has been under 100 km for the past 20 days.
The latest two elsets for this are:
Cosmos 1172                                      32179 x 69 km
1 11758U 80028A   97337.93326027 -.00500496 -16114-4 -43290-4 0  1595
2 11758  61.8391 124.6147 7134839 246.2321  28.0563  2.57221836129352
Cosmos 1172                                      31808 x 88 km
1 11758U 80028A   97338.31842081  .02340191  00000-0  64161-3 0  1619
2 11758  61.8206 124.6189 7103544 246.2931  27.7582  2.60269106129778

I don't really believe the 69 km perigee and the negative drag term for
the first of these. In any case, the perigee height here is relative to
a spherical Earth of radius 6378 km and, with perigee deep in the
southern hemisphere of an oblate Earth, the true height of perigee above
sea-level is somewhat greater. Depending on the strength of the object,
the stresses it experiences near perigee may cause it to "burn up" well
before it is able to circularise its orbit by decreasing its apogee. In
fact, given the rate at which the perigee of Cosmos 1172 has been
plunging in recent weeks, this could happen at any time (if it hasn't

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