# Re: Simultaneous Iridium flares

From: Björn Gimle (b_gimle@algonet.se)
Date: Wed Mar 26 2003 - 09:59:57 EST

• Next message: Robert Holdsworth: "Re: Simultaneous Iridium flares"

```Using IridFlar and the current elset at Mike's site, I get the range
difference as 1665.6-1391.3 = 274.3 km and a flare center diff of 3.8
seconds.

Using SkyMap, their vertical separation was 800.0-693.3 = 106.7 km.

There are 17 satellites marked ? (engineering "spare" orbits - I am
surprised that HA predicted one of them) in iridium.tle. Their Mean
Motions are 14.36-14.93, vs the operational 14.34215.
This means that each one of those 17 will overtake one of the 11 in the
same plane 0.24-6.49 times per day, in total 47.6 encounters per day.

Actually, the higher the frequency, the shorter the duration of each
encounter.
"Your" Iridium 94 ? flare ground speed differs from the Iridium 28 ground
speed by about 0.27 km/s. If the definition of "close" is +- 1 s, this is
about 15 km, or 56 seconds.
Since Iridium 94 ? passes an operational one 3.56 times per day, there is
a close pass 3.3 minutes/day somewhere on Earth.

This duration (longer but less frequent, or in one case shorter but more
frequent) times 17 is almost one hour out of 24, or about 24000 km in
total.

The width of a good/visible flare depends on the elevation, say 150 km at
night, 10 km in daytime.
The length of the flare track may be almost the full orbit for twilight
orbits, a half orbit for "noon orbits" at 10 km width, and perhaps 1/10
orbit at 150 km width. At any time, there may be nearly two MMA flares
hitting the ground, on the average ??

Assuming an average (150,5,15 km) of 25 km, I get 600 000 square km
covered by double flares each day. The Earth is 510 000 000 km, so each
should have a one-second coincidence in three years.

Please prove my math wrong !

One way is to run a large number of two-month runs with IridFlar,
covering one or more years and a number of latitudes, and let a program
count the number of coincidences (or coincidences within a minute and
divide by 60).

> I have not had any response to my query so far.  However I have done a
web
> search and can find no mention of any flares in such close proximity
since
> 1999!  There are a few reports of double flares but these have been
much
> greater than 1 second apart.
>
> I would also appreciate it if someone could calculate the separation in
km
> or miles - it seems from the respective orbits that it may have been
less
> than 100 km.  I guess I should have stated my elevation in feet or
metres
...
> > This evening 25 March at 20:15 local (UTC plus 12) I observed with
the
> naked
> > eye simultaneous Iridium flares from a location at Wellington
waterfront
> > (41.2861 S; 174.779 E at 0 degrees elevation.)
> >
> > These were from Iridium 28 1997-051-E  and Iridium 94 2002-005-C
> >
...
> >
> > I would be interested to know how rare or otherwise sightings of
> > simultaneous flares are.
> >

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