The demise of this satellite was covered for over a year on this blog: as the satellite was weighing 1.2 tons and as it had an unknown amount of remnant fuel onboard, the uncontrolled reentry raised some questions and initial concerns (see coverage here).
The last amateur observations of the object were done by Mike Waterman (USA) on July 24th and Alain Figer (France) in evening twilight of July 25th. The last amateur orbital update by Ted Molczan based on a.o. Mike Waterman's observations, showed it to have descended to a 211 x 213 km orbit on July 24th and analysis of this dataset by this author using Alan Pickup's SatEvo suggests reentry on July 26, somewhere between approximately 9:50 and 10:50 UTC.
USSTRATCOM published a final TIP for IGS 1B on July 26th (that they did so for a classified object is unusual), placing re-entry at 26 July 2012, 09:52 +/- 2 min UTC, near 25 S, 186 E, which is near New Zealand. This is at the start of the reentry window given above and hence seems very reasonable even though the reentry coordinates are a verbatim copy (down to one decimal) of a pre-decay prediction issued at 7:34 UTC (only the uncertainty value has changed, from 2 hours to 2 minutes). No details on the orbital development in the final few revolutions were given.
The map below shows the USSTRATCOM determined reentry location and final trajectory. In principle, the re-entry could have been observed from the northern islands of New Zealand and potentially the Fiji-Tonga area. Note that only half a revolution later (about 30 minutes later) it would have passed over NW Europe and next west Africa.
The diagram below shows the orbital evolution in terms of apogee and perigee altitudes, from malfunction early 2007 to decay on 26 July 2012. It is based on orbital element sets calculated by Mike McCants and Ted Molczan from amateur observations, including mine:
IGS 1B was a nice object to observe over the past years: it was bright, and it was interesting to follow its orbital evolution towards decay. The observation that remains the most vivid imprint in my memory is the one that resulted in the picture below: on 2 September 2011, while I was watching and photographing a pass in a slightly hazy sky, the satellite brightly flared to at least magnitude -8 if not more: the brightest satellite flare I have ever seen. I was jumping up and down and yelling "WOOOOOWWWW!!!!" when this happened. It resulted in this wonderful, eerie picture: