# Decay height (for ISS)

From: Alan Pickup (alan@wingar.demon.co.uk)
Date: Sat Apr 29 2000 - 17:04:44 PDT

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```DeHBeaver0@aol.com writes
>From the New York Times: As of April 29 (2000), the ISS was at a height of
>227 miles above earth. Whether this information was in standard miles of
>knautical miles it did not say.
>

The latest elset, with the height in sensible units in line zero, is:
ISS             20.0  4.0  0.0  2.5 d   70       346 x 336 km
1 25544U 98067A   00120.41481481  .00079113  00000-0  49311-3 0  6005
2 25544  51.5855  29.0207 0007586 119.8206  46.1416 15.76177361 82268

>    Does anyone know what the approx. height, in both standard and knautical
>miles, the Station or ANY object would decay, AS OF NOW, considering the
>solar max.

At current rates of drag, the ISS might decay in September this year. It
is not in my SatEvo Decay List because I am confident that something
will be done to avoid this. I guess you could say that most objects re-
enter soon after their perigee height drops below about 100 km,
irrespective of whether we are near sunspot maximum (as at present) or
sunspot minimum. However, how quickly any object gets to that height
depends on its current height, the atmospheric drag it is experiencing
currently and the vagaries of the atmospheric density during its

The effect of drag on an object depends on the ratio between its cross-
sectional area and its mass. Put simply, given two objects of the same
size and shape in the same orbit, but one twice as massive as the other,
then the more massive one will enjoy a life twice as long. Because of
this, it is impossible to estimate the lifetime from the height alone.

The effect of solar activity on atmospheric density increases with
height. The effect is small, probably negligible, at heights below about
125 km, which is why the "decay height" of an object does not vary
between sunspot maximum and minimum. However, at a height of 250 km the
atmosphere is some 3 times denser at an average sunspot maximum than at
minimum and the factor increases with height to about 8 at 400 km and 20
at 600 km. You might surmise that at the ISS's current height, the
atmosphere is perhaps 5 times denser at present than it would have been
near sunspot minimum. This implies that if its putative lifetime is say
5 months now, it might have been (5 x 5 =) 25 months if we had been near
sunspot minimum.

Alan
--
Alan Pickup | COSPAR 2707:  55d53m48.7s N  3d11m51.2s W   156m asl
Edinburgh   | Tel: +44 (0)131 477 9144     Fax: +44 (0)870 0520750
Scotland    | SatEvo page:   http://www.wingar.demon.co.uk/satevo/

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