Re: Why do Iridiums Flare?

Brian Hunter (bkh@chem.QueensU.CA)
Tue, 26 Aug 1997 15:29:53 -0400

As has been stated earlier by Paul Maley, these satellite have several flat
surfaces; solar panels, antennas, and perhaps the body panels of the
satellite itself.   These are all common features of payloads.  There is a
difference here.  These objects are designed to communicate with users more
or less directly below them and with other satellites nearby.  At least
some of the antennas are gimbaled.

For details of the satellite and the launch see the Iridium mission book at
Boeing:

  http://www.boeing.com/defense-space/space/delta2/iridiumbook.htm

The unusual aspect of the flare behavior is not that they flare but that
they always seem to flare for particular sun angles and the very bright
reflection lasts for such a long time.  These are not particularly large
satellites; five fit on a Delta II.  Whatever is producing the reflection
is not a very large surface but it must be highly reflective.  

Now some speculation.  

The above comments probably require that the orientation requirements are
not the same as for most other payloads and the antennas are moving all the
time to satisfy pointing requirements.  If we were dealing with a flat
mirror surface fixed in space, the flare would last only long enough to
sweep the solar image across us.  I haven't done the arithmetic but I am
sure that the Iridium flares last too long for that.   However, if the flat
surface is being reoriented at the same time, then the bright image can
last much longer.  Since most satellites keep their solar panels oriented
toward the sun and do not produce such spectacular flares, I suspect as
suggested earlier by Paul that the reflections come from the antennas.  

In fact, these objects can be fairly bright over a wide range of sun
angles. They are usually about 6th magnitude but it is not uncommon (baased
on about 25 observations of the first group launched, the 97020 group) for
them to be easy naked objects even when they do not exhibit the extremely
bright flares.

Is there anyone out there from Iridium or Motorola who can tell us how the
various flat bits of the payload move with respect to the payload itself?

BKH


----------
> From: jwilson <jwilson@scsn.net>
> To: Satellite Observing <SeeSat-L@cds.plasma.mpe-garching.mpg.de>
> Subject: Why do Iridiums Flare?
> Date: Tuesday, August 26, 1997 1:42 PM
> 
> Can't find anything in the documentation on their web site as to why they

> flare.  Anyone out there know?
> 
> 
> 
> 
> 
> James -
> 
> "Fall 7 times, stand up 8." Japanese Proverb  
> 
> 
>