Iridium flare magnitude comments

Wed, 4 Mar 1998 11:54:11 -0600

I noticed Richard Keen's comments on the Iridium brightnesses so I would
like to append my own here. Most seem to be within 2 or 3 magnitudes of
predicted brightness. Because of attitude orientation variances, it is
possible for some to be off considerably more. In Curacao I observed a
predicted -1.9 magnitude flare as observed to be -4 on 25 February, for
example.  In another case a predicted -4 magnitude flare seen from Seabrook
Texas was observed to be -8 at night. These are relatively typical

For daylight flares, I have failed to observe them about 90% of the time.
The geometry is so critical for overcoming background sky brightness in the
daytime that the satellite brightness must be on the order of at least -6 or
-7, I think, in order to be visible to the unaided eye. The duration must
also be long enough to be recognized by the eye. Two seconds is about as
long a duration for a daytime flare that I have seen. Earlier in February, I
used a 1:250,000 scale topographic map to estimate where I needed to be
situated in order to observe a very bright daylight flare at Eagle Point,
Texas. After arriving there I encountered a rather scruffy looking person
wandering about with a beat up van. He walked over to where I was observing
and I explained what I was doing. It was then that he pullled out (not a gun
or knife...)  a GPS receiver of all things. Using that instantaneous reading
I found out that I was exactly where Iridflar indicated I should be. The
flare was predicted to be -8.3 magnitude at 77 degrees elevation. The sky
was completely clear and no flare was observed.  Three more similar
geometries occurred in the day for different (older) Iridiums that week and
no flares were seen.

Iridium flares should not be expected from Iridiums 11,21, and 27 since they
are not properly oriented. Those satellites which are relatively newly
launched and/or are in parking orbit are generally aligned in the proper way
and you should be able to observe flares from them using the available
programs. Again, this is not always guaranteed since those vehicles in
parking orbits can be experiencing maneuvers.  

Paul D. Maley
United Space Alliance
DO5/Cargo Operations
NASA Johnson Space Center
Houston TX 77058 USA

tel. 281-244-0208
latitude 29.6049 north, longitude 95.1086 west