HST targets, and Iridium 11 flares

Ed Cannon (ecannon@mail.utexas.edu)
Thu, 12 Feb 1998 01:39:48 -0600

Tonight I found the site that gives planned Hubble telescope


I had been wanting to find that for purposes of attempting 
to determine, by knowing where it was pointing at the time 
of such an observation, what makes it flare brightly (-3 at 
least) on occasion.  We've had evening passes here the last 
few nights; one pass had a very bright flare, and another 
pass was fainter than most.  Wednesday evening about 1:35:40 
UTC, when HST was about 40 degrees above the horizon SSW of 
here, it briefly flashed to what I very roughly -- given 
there were some clouds -- estimate was about -2.  In the 
Weekly Timeline, at that time it was supposed to be aimed 
at NGC4472 (M49) in Virgo, which was below the eastern 
horizon.  The Sun at that time was at azimuth 264, altitude 
-18.  I *think* this narrows down what surface on it could 
be the source of such flashes -- if only I could figure it 

Now also due to the issue Russell raised of how Iridium 
flares might affect HST's instruments, we at least can know 
where it's aimed.  Hopefully HST operators know how to find
out if an Iridium flare might ruin their observations, if
not the instruments themselves!

In my message last night about Iridium 11 flares, I should 
have mentioned that it's one of the ones that is not in a 
standard operational orbit, and flares from it have not yet 
been successfully predicted.  Rob Matson says in his 
Iridflar.exe documentation that he programmed it to produce 
flare predictions for Iridium 11 based on some assumptions 
about the satellite's situation, and he wanted to receive 
reports of Iridium 11 flares.  Even though my observations 
were not related to flare predictions, it appeared to me 
that they offered some data as to Iridium 11's situation, 
for the purposes of possibly predicting flares from it.  

Since those two flares looked much like ones from other 
Iridiums, it seems that it must be fairly stable, with its 
MMAs being held more-or-less in position.  In a message 
earlier today, Rob says that they believe that it's near 
the correct orientation but apparently has much more 
variance from the norm, perhaps as much as 10 degrees.  
Randy John told me that his program, SkySat, allows for 
experimenting with possible MMA orientations.  If Iridium 
11 is reasonably stable, even if it's not [yet?] in a 
standard orbit, perhaps some more flare observations will 
lead to its flares becoming predictable, to some extent at 
least.  It's a small issue, since it's only 1 out of 46 
now and only 1 of 72 when the constellation goes into 
operation (and how many after years of operation, when 
some have expired?), but still I've enjoyed trying to 
catch those unpredicted flares!

Ed Cannon
Austin, Texas, USA