Iridium flare and flasher observed

Frank Reed (
Fri, 12 Jun 1998 22:46:35 -0700

I sighted a predicted mag -8 flare of Iridium 39, right on schedule,
predicted for 1998 June 13 04:18:11 UTC (9:18:11 6/12/98 MST).  Very nice,
maybe not quite as bright as -8, but I don't know how to estimate these.

With 10x50 binoculars I sighted 24871 (Iridium 18 aka 20) blinking at about
mag 5 as it rose in the North near Gam Cep.  It appeared rather steady at
about mag 5 for about 2/3 second, then disappeared for about 1/3 second,
repeating this pattern for more than a minute, then the phase appeared to
shift, and the blinking resumed much the same as before.  Then suddenly the
flashes became much brighter, I watched at 1x and saw two very bright
flashes at perhaps mag -3, then a fainter one at perhaps mag. 0.  I watched
again through the binoculars and the flashing/blinking stopped as the
satellite moved south at a steady estimated mag. 5 or so.  I clocked the
blinking and flashing with a lap memory stopwatch.  The times reduced to
UTC were as follows:

Date 1998 June 13 UTC


04:32:39.36	Begin counting flashes (flash # 0)
04:33:08.20	Flash #30, delta t = 28.84 seconds
04:33:37.19	Flash #60, delta t = 28.99 seconds
04:34:06.34	Flash #90, delta t = 29.15 seconds
				a few seconds later there appeared to be a phase shift
04:35:01.09	Much brighter flash at 10x
04:35:04.05	Very bright flash at 1x (est. mag. -3)
04:35:05.04	est. mag. -3 flash
04:35:06.03	est. mag. 0 flash

There were 90 blink periods over a measured time of 86.98 seconds.  I
calculate a period of approximately 0.966 seconds.

The above UTC times were measured with reference to a WWVB controlled clock
and should be accurate to within a second or so.

-- Frank Reed
Scottsdale, Arizona, USA  111.898W  33.484 N  1227 feet