Re: Decay watch: Oct 10 #1

Alan Pickup (
Mon, 11 Oct 1999 22:25:59 +0100

Hello Marco,

Marco Hahn <> writes
>Given the rather accurate position of decay, I wonder how you arrive at them?
>What is the shape of the last "orbit" so much deformed that somewhere (where
>exactly?) near the perigee the satellite loses almost all its forward kinetic
>energy due to friction and thus is falling almost vertical due to gravity?

The prediction you mention had a quoted uncertainty of +/-4 hours, which
implies that I thought it could decay at any time over a period of 8
hours, from about October 10 19:00 UTC to October 11 03:00 UTC. As it
is, my conclusion today is that it actually decayed at October 10 21:22
UTC -30m+90m, or between 20:52 UTC and 22:52 UTC. SpaceCom, on the other
hand, says October 11 00:40 UTC +-2h. This is an unusually large
discrepancy, but we are unlikely to know which analysis is the more
accurate unless an observation of the re-entry is reported, or SpaceCom
comes up with further elsets to support its conclusion.

I say that the object could have decayed "at any time" during those 8
hours. In fact, there are reasons why re-entry is more likely over some
parts of the orbit than others. An object in an eccentric orbit is most
likely to decay near (or a little beyond) its perigee; if it survives a
perigee passage it is likely to be "safe" until it nears the next one.
An object in a circular orbit meets most air resistance as it passes
over the equator twice on each orbit, so we might expect it to re-enter
near one of these points. It turns out that #25916 was in a moderately
eccentric orbit with its perigee close to the point where it crossed the
equator moving northwards, so there are two reasons for judging this the
most likely point for re-entry. Incidentally, the orbit of a decaying
object does not get very deformed over its final orbits - rather, it
gets more and more circular as the apogee falls more quickly than the
perigee. Of course, this ever-more-circular orbit is contracting more
and more quickly, so that the actual path followed by the object is a
downwards spiral.

The time I quote for #25916 is the minute of the equator crossing, but
the actual re-entry event could last many minutes and cover a thousand
km or more. During that period, the object is moving almost parallel
with the ground and descending only slowly. If parts of it survive the
fireball, they will fall almost vertically only over the final few km,
by which time the fireball will be extinguished and only blackened metal
scrap will be falling through the lower atmosphere. I understand that
SpaceCom's decay point is their estimate of the ground impact point for
any such debris. My estimate, on the other hand, is for the visible re-
entry phenomenon, the fireball, as the object descends through
approximately the 100-70 km range.

 Alan Pickup | COSPAR 2707:  55d53m48.7s N  3d11m51.2s W   156m asl
 Edinburgh   | Tel: +44 (0)131 477 9144     Fax: +44 (0)870 0520750
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