ALERT: Iridium 48 ambiguity

Alan Pickup (
Tue, 13 Apr 1999 20:05:48 +0100

With regard to Iridium 48, Russell Eberst <>

>So the question arises, is the descent intended and planned (perhaps to
>start a plane change) or is it due to a malfunction such as a jammed
>valve that has used all the station-keeping fuel in one massive prolonged
>burst?  It will be interesting to see if the descent stops when the
>revs/day reaches around 15.16 (as with #25527and #25529, 98-66A and C)
>or if it continues until Irid48 reaches the denser layers of the atmosphere.

The descent appears to me more like an intentional manoeuvre than a
natural decay or a jammed valve.

The rate of descent has not followed the pattern I would have expected
for a decaying object; the "drag" has fluctuated but shows no sign of
the substantial increase (a factor of about 20) I would expect for the
drop from a 780x775 km orbit to one of 624x563 km (so far). Attempts to
"fit" a SatEvo evolution through groups of elsets tend to leave high
residuals, and the resulting prediction very quickly diverges from
reality as indicated by the elsets. 

A jammed valve might produce something similar to what is observed, but
only, I suspect, if the Iridium is actively stabilising its attitude.
Otherwise, it would likely set the object spinning and flashing wildly
like other failed Iridiums. I note that Russell's observation reports
that it was steady in brightness.

Another possibility is that the Iridium has failed and is being disposed
of via an intentional premature decay. Perhaps its communications
facilities have failed, though it is still under dynamic control.
Alternatively, a transmitter may be locked in an "on" state, generating
interference that poses a threat to the Iridium constellation.

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