Iridium flares as seen from orbit

From: Robert Reeves (reeves10@swbell.net)
Date: Thu Mar 20 2003 - 23:46:24 EST

  • Next message: Tom Wagner: "Historical info that includes magnitude information"

    Greetings.  A long post follows, but it regards a somewhat historical event
    in satellite observing, so please bear with me.
    
    A while back I mentioned that Rob Matson had perfected an Iridium flare
    prediction program for use by the ISS crew.  I reported the initial success
    with the program, but after that ISS Science Officer Don Pettit started
    reporting a long string of flare misses.  I was starting to get worried, but
    Rob was certain of the program's functionality.  The diagnosis of the
    problem centered on the ISS being one step ahead of the lastest orbital
    elements.  The station had been undergoing a series of reboosts and it seems
    it was always a hair off from the latest elements.  In spite of this, Don
    said he was having fun just looking for the flares and to keep the
    predictions coming.
    
    Things finally stabilized this morning and just before I left for work, Don
    sent me a note saying he had sighted flares for four out three predictions.
    Darn good odds, I'd say!  Actually, Don had seen an unpredicted fourth flare
    of an in-orbit Iridium spare.
    
    Later this evening about my dinner time, Don sent me the first photo of an
    Iridium flare taken from orbit.  The photo is embargoed by NASA, but by way
    of verbal description, it was taken with 50 mm f/1.2 Nikor on a Nikon D1.
    Assuming about a 30 X 40 degree field of view, shows a star field (after a
    little Photoshop work) down to about mag 5.5 or 6.  The flare is trailed
    aproximately one degree during the exposure and is not tapered at either
    end, showing that the exposure began and ended at mid flare.  There is a
    station solar panel from one corner about 1/3 the way into field of view and
    on the opposire side of the field is some dim, very out of focus equipment
    that is obviously located near the window on the outside of the Russian
    airlock module.  The view out the station is certainly cluttered!  Both Rob
    Matson and I have been trying to identify the star field, but it elludes us.
    I have been an astronomer for 40 years and this image star field mystifies
    me.
    
    The following is a clip from the note Don sent me about his flare
    observations.........
    
    *******************
    
    Robert and Rob:
    
    Good news.  We nailed nailed four out of three predictions!   I took note of
    times which you can see below.  I also saw one unpredicted flash, a bright
    one of mag  estimated at  -8 which was barely one minute before the
    predicted one of -1 mag.  See notes below.  I estimate that all times are
    +/- 2 seconds.  We keep accurate GMT clock to less than a second and I
    transfer this to the digital camera and use the start image time stamp for
    the flash.  Due to the lag between when I first can see the flash and I open
    the shutter (have to move my nose out of the window and place the camera on
    its mount),  I estimate the two second uncertainty.  The shutter speed was 5
    seconds with automatic dark current subtraction ON so I can not take a
    sequential shot until 10 seconds after the first was started.  I captured
    these flashes on two frames so the duration is at least 15 seconds each,
    probably more like 18 to 20 seconds with human delays.   This is great fun
    trying to catch these.   Currently I am using the windows in the Russian
    airlock which are on the 45 degree diagonal so they look aft-starboard and
    forward-port.  Sometimes I can not use the port and starboard facing windows
    in the service module due to too much ambient light and the work schedule
    which will not allow the lights to be shut off.   The lab window now points
    nadir directly at the two pointer stars in the big dipper and precesses
    about 1 degree per day.  Attached is a small image of the -5M flash at
    18:04.  Looks like you can see some structure on the satellites however it
    may be due to shooting out the airlock window at 45 degrees (double pain
    with some contamination on the outside surface).  I would like to try to
    photograph a bright flash out the lab window using a 180mm f2.8 telephoto to
    see if you could see any structure.  I would have to have good pointing
    information for that.   It looks like the nighttime predictions for the
    brighter flashes is working well.   Great job on the predictions!
    
    Don
    
    *****************
    
    And again, congratulations to Rob Matson for creating this program and
    taking the time to support the ISS with this "unofficial" observational
    experiment.
    
    Actually, I see great value in the Iridium search by the ISS, even if it
    isn't a real part of the ISS science progam.  These guys in orbit are pretty
    good engineers, scientists, pilots, etc., but they usually don't have a lot
    of seat time just observing the sky and looking for dim transient
    phenomenon.  I think the success rate in the flare observation program has
    ramped up partly because Don is now deliberately looking for different
    phenomena and in the process is unconsciously training training himself to
    see what he may have missed earlier due to inexperience.  He is indeed
    learning to be a better observer, and this will pay dividends in other ISS
    programs.
    
    Robert Reeves                      reeves10@swbell.net
    520 Rittiman Rd.                   www.robertreeves.com
    San Antonio, Texas 78209    210-828-9036
    USA                                     29.484  98.440  200 meters
    
    
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