[meteorite-list] OT: Iridium flares

From: Matson, Robert <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Thu Apr 22 10:07:02 2004
Message-ID: <AF564D2B9D91D411B9FE00508BF1C86901B4E45F_at_US-Torrance.mail.saic.com>

Hi Marco and List,

For those not interested in more info on Iridium flares, delete now.
(This is a bit off-topic for the Meteorite List, but since many
meteorite afficionados are also interested in bolides, there is
a tenuous connection.)

Marco wrote:

> For those who do not know, this is a series of artificial
> communication satellites, initiated some years ago (I believe
> we started to first see them in 1997) ...

This is correct -- summertime of '97.

> ... in order to provide for (expensive) satellite telephone
> from anywhere in the world.

Expensive is right. The marketing plan for Iridium was questionable
at best. The phones are bulky and very expensive themselves, and
the per-minute telephone charges were many dollars per minute. Of
course, if you're working in Antarctica on some research project,
you aren't likely to have a nearby cellular site! ;-)

> The network entailed as much as 70 satellites, most of which
> have indeed been launched.

Over 90 have been launched -- perhaps around 95 by now. About
a dozen of these have failed, and of that dozen several have
already reentered the atmosphere. 91 remain in orbit at the
present time.

> The project failed as the company in question got bankrupt,
> but the satellites are still there although they should be
> phased out and re-enter as was the plan some time ago when
> the company was dismantled and the network closed.

Actually, the DoD bought out Iridium ILC's interest in these
satellites (for pennies on the dollar) and they continue to
operate to this day. The DoD was the primary customer of
the system, so they had a vested interest in keeping it alive.

> They carry a large antenna panel which reflects beams of
> sunlight.

There are 3 main mission antennae (MMA), any of which is
capable of reflecting solar glints down to the ground depending
on the geometry. These MMAs are quite large -- door-sized (and
a large door at that). Under the most favorable geometry, an
antenna can produce a glint as bright as magnitude -8.5, and
thus are quite visible in daytime. (I've seen several dozen
of these daytime Iridium flares).

Back in the summer of 1997, shortly after the first 5 satellites
were launched, the "flare phenomenon" was serendipitously
discovered by an amateur satellite observer. The flares were
totally unforseen by the satellite builders. Working with a
network of observers and the spacecraft designers, I was able
to quickly cobble together a program that would predict future
flares for any observer. As more flare observations came in,
the brightness curves were tweaked until I finally had a very
reliable flare predictor. Indeed, you can set your watch by
these flares -- they're that reliable.

What makes the flares predictable is that the satellite orientation
is very accurately maintained and thus predictable for weeks into
the future. Marco mentioned that there is a website for predicting
flares -- this is the Heavens-Above site. Its predictions should
be quite compatible with those of my program (IRIDFLAR) as the
program author used my model for photometric brightness. The
Heavens-Above site is handy in that you don't need to know how
to run a flare prediction model. You just plug in your coordinates
or your city name, and it does all the work for you. The only
drawback is that you have to be logged onto the internet.

Anyway, for the few people left in the world who have never seen
an Iridium flare, make the effort to see one. They are fabulous,
and great for getting kids interested in astronomy.

Cheers,
Rob
Received on Sat 19 Oct 2002 03:00:19 PM PDT