Iridium 9 was replaced in the operational constellation, after some cosiderable delay, by Iridium 84:
Iridium 9 was lowered from its position in plane 5 to an
"engineering orbit" on 14 September 2000.
See Iridium 9 manoeuvre - on the way down? at http://www.satobs.org/seesat/Sep-2000/0248.html.
By 20 September 2000, it had been confirmed that this did not
necessarily represent the start of a de-orbit.
See Iridium 9 is not deorbiting at http://www.satobs.org/seesat/Sep-2000/0264.html.
On 11 October 2000, the orbit started to be lowered even
See Iridium 9 orbit alteration at http://www.satobs.org/seesat/Oct-2000/0145.html.
On 15 October 2000, it was confirmed that Iridium 9 *was* now
being deliberately de-orbited, but that this did *not* represent
a first step in de-orbiting the whole constellation.
See Iridium 9 de-orbit is NOT the beginning of the end at http://www.satobs.org/seesat/Oct-2000/0233.html.
On October 17, the character of the orbit adjustment changed,
evidently with the aim of reducing the perigee as much as
possible. See Descent of Iridium 9 (#24838)
The aim of reducing the perigee to 250km was not achieved. Presumably the fuel supply ran out.
By 2 November 2000, Iridium 9 had been observed to be tumbling
out of control.
See CoSaTrak Observations at http://satobs.org/seesat/Nov-2000/0026.html.
(Iridium 9 (24838, 1997-030C) eventually decayed on 11 March
See Iridium 09 decayed at http://www.satobs.org/seesat/Mar-2003/0116.html.)
The replacement of Iridium 9 was evidently delayed pending the confirmation of the handover of the Iridium system to new owners, even though there were two spare satellites in the relevant plane.
However, at the Iridium Satellite LLC press conference call on
12 December 2000
(see http://www.ee.surrey.ac.uk/Personal/L.Wood/constellations/iridium/conference-call-Dec-2000.html), it was stated that "Right now we have had one failure in 23 months and that's been de-orbited, out of the orbit. Beginning tomorrow, we will drift one of the spares up into that slot. That's been the only failure in 23 months and that's one that was identified quite a long time ago."
However, it was actually several weeks before the replacement was put into place:
As of 5 January 2001, Iridium 84 appeared to be in process of
being raised to fill the gap.
See Iridium 84 being raised at http://www.satobs.org/seesat/Jan-2001/0063.html.
However, no element sets for Iridium 84 were released by the
Orbital Information Group (OIG) for a long time.
See the discussion threads
Has OIG "lost" Iridium 84 ? starting at http://www.satobs.org/seesat/Jan-2001/0081.html and
iridium 84 mislaid? starting at http://www.satobs.org/seesat/Jan-2001/0178.html.
After a break of 13 days, the next element set released for
Iridium 84, on 18 January 2001, showed it on station.
See the discussion thread iridium 84 now on station starting at http://www.satobs.org/seesat/Jan-2001/0278.html.
Note: until the manouver of 05 January 2001, Iridium 84
appears to have had its orbit adjusted only very occasionally,
for instance on 22 September 2000 when its orbit was raised.
See Iridium 84 manouver at http://www.satobs.org/seesat/Sep-2000/0303.html.