Iridiums 20 (a.k.a. 18), 42, and 15

Ed Cannon (ecannon@mail.utexas.edu)
Tue, 09 Jun 1998 02:26:43 -0500

Monday evening (9 June UTC) Iridium observations (and a couple 
of others) from about 30.3086W, 97.7279W, 150m.

Around 3:29:35-45, I saw a very neat group of 8, 9, or 10 fast, 
bright one-power flashes (up to +0?, near Vega) from the Iridium 
with catalog number 24871 (apparently Ir 20 but identified as 18 
by Iridflar).  I counted 8 flashes but in the suddenness of the 
moment may have omitted the first and/or last flash that I saw.  
These flashes occurred just before the time it would have flared 
if it were operational.  They seemed quite regular, about every
second (or maybe faster?).  This satellite was only 12 seconds 
ahead of Ir 17, but Ir 17 didn't flare enough to be seen at one 
power from my location.  (I had a +4 flare prediction for it.)

Here are Ir 20, 18, and 17 elsets from Mike McCants' iridium.tle 
file:

Iridium 17 man
1 24870U 97034B   98159.14055882 +.00000408 +00000-0 +13848-3 0 01844
2 24870 086.3974 045.5291 0003825 089.2760 270.8876 14.34219965047908
Iridium 20 att
1 24871U 97034C   98159.14067606 +.00000176 +00000-0 +55146-4 0 01897
2 24871 086.4003 045.4858 0015074 088.7247 271.5682 14.34522891047935
Iridium 18
1 24872U 97034D   98159.10884767 -.00000250 +00000-0 -96410-4 0 01871
2 24872 086.3996 045.5046 0003980 115.9702 244.1906 14.34215128047890

About 6 minutes later I had a -1 prediction for Ir 42 also near 
Vega, but it only reached about +1.  (It's marked with a "?" in 
the annotations being provided by Mike and Alan Pickup.)  Less 
than 3 minutes after that I had a -2 prediction for Ir 15, and it 
was at least that bright if not -3.

Two other Monday night observations were the Cosmos 2351 rocket 
(25328, 98-27B), with maxima to about zero mag. about every 
10-12 seconds, and a pass of Seasat (10967, 78-64A) possibly as
bright as zero mag.  For new observers, note that Seasat has 
wide variances from predicted magnitudes, apparently due to its
configuration.  There's a small illustration of it at:

   http://www.jpl.nasa.gov/mip/seasat.html

By the way, June 28 is the 20th anniversary of Seasat's launch.

Terminology (ambiguities and equivocations, continued...).  For 
a (reverse?) "glint", see: 

   http://antwrp.gsfc.nasa.gov/apod/ap980608.html

Altitude v. elevation -- "elevation above sea level"?

Ed Cannon -- ecannon@mail.utexas.edu -- Austin, Texas, USA