Reports are pouring in from The Netherlands, Britain, Ireland and other N-European countries about a very bright, extremely slow fragmenting fireball appearing around 21:45 - 21:55 GMT (23:45 -23:55 CEST) on the evening of 21 September 2012.
Various video's have been posted on Youtube, notably by observers from Britain (large parts of the Netherlands were clouded out, including the all-sky stations):
Because of the unusually long duration and slow movement, some people have suggested the possibility of a satellite reentry. For various reasons, this is however very unlikely.
Multiple reports make clear the object was moving from east to west. A report of observers from Bussloo Observatory, the Netherlands, for examples states that the fireball appeared in the north, moving from Perseus to Bootes, almost horizontally from east to west. Similar reports (e.g. here and here) come from Ireland.
Almost all non-polar satellites move prograde, from west to east (or north-south and v.v. for a polar orbit). An east to west movement would necessitate the object to have a retrograde orbit (meaning that it moves counter to the earth's direction of rotation). Such objects are extremely rare: they literally amount to only a handful of objects (including the US FIA Radar satellites, and the Israeli Ofeq/Shavit satellites/rb). For this reason, it is extremely unlikely that this fireball was a reentering satellite.
Update 24 Sep: in the comments to this blog post, the issue was raised of the potential reentry of a classified object. However, the larger classified pieces are tracked by us amateurs. We have no likely decay candidates among the retrograde objects that we track. We can account for and hence exclude the FIA's for example (the rocket bodies of that launch were deliberately de-orbitted right after launch so are no candidates either). The Israeli Ofeq/Shavit are no candidates as their orbital inclinations never take them over the Netherlands and the British Isles. And there are simply no other suitable retrograde objects -- end of update.
There are moreover no unclassified reentry candidates for this date listed by USSTRATCOM on their space-track portal. Given the brightness of the fireball, this should have been a seizable chunk of space debris, that really would have been tracked (and predicted). Again, this makes it very unlikely that this fireball was a satellite reentry.
While the duration of the fireball is unusual, it is not unprecedented. In many ways, the descriptions and video are reminiscent of the Peekskill fireball that dropped meteorites near Peekskill in 1992:
It is therefore my opinion that the 21 September fireball was most likely of meteoric origin: a chunk of asteroid. Alas, any surviving remains appear to have splashed down in sea (update: or possibly Scotland - N. Ireland).
The duration of the event, though not unprecedented, is certainly unusual and for this reason, I am saying "most likely not" rather than "certainly not".
UPDATE (12:45 GMT, 22 Sep): another bright fireball was widely seen from the US and Canada that same night near 20:30 GMT. There was at least one hour inbetween the two events, so they do not appear to be related (i.e. they do not concern the same fireball).
UPDATE 2 (13:30 GMT, 22 Sep): Suggestions that the fireball might be related to Chinese CZ-4 space debris, catalogue #26213, are plainly incorrect. That object (and any fragments of it) are in a 98 degree polar orbit. This is completely incompatible with the reported movement of the fireball. As seen from Bussloo in the Netherlands and Dublin in Ireland, the fireball moved perpendicular, not parallel, to the orbital plane of this Chinese space debris (and that of any related fragments).
IMPORTANT UPDATE 3 (24/9/2012): more and definite arguments that this was not a reentering satellite, can be read here.