RE: magnitude question Agena-D

From: Ted Molczan (ssl3molcz@rogers.com)
Date: Tue Aug 24 2010 - 17:00:28 UTC

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    Ralf Vandebergh wrote:
    
    > The coming days I will be hunting to capture HR images of an 
    > Agena upper stage Agena D Rocket (00733 1964-002-A).
    > 
    > Problem is I get 2 different magnitude-predications. 
    > Calls-sky predicts mag 4.4 and Heavens Above says mag 3.7.
    > Generally it appears, the Heavens above predications are 
    > considerably more positively.
    > 
    > I would like to hear from earlier observations of this object 
    > to get an impression which of the 2 predictions is more realistic.
    > 
    > The reason that this is important for me is, that it makes a 
    > lot of difference and difficulties in the settings for the used
    > imaging-setup in the case if it is fainter then mag 4.0 - 4.1.
    
    You raise an important point. I cannot comment specifically on the difference between those
    predictions, but it is small in relation to the inherent uncertainty.
    
    For the majority of satellites, brightness predictions have an uncertainty of at least 1 magnitude,
    often more.
    
    As a result of the efforts of several of our colleagues, I have in my archive more than 1500
    brightness observations of Agena D rocket bodies. Below is a plot of eight Agena D rockets in LEO,
    with magnitude normalized to 1000 km range, versus phase angle:
    
    http://satobs.org/seesat_ref/misc/Agena_D_stdmv.pdf
    
    (Note: we have more than 1000 obs of 71110B / 5679 alone - far more than of any of the others, so to
    avoid over-weighting its data, I used only every 20th data point.)
    
    The standard magnitude of the eight Agena D stages is 5.4 +/- 2 (1000 km, 90 deg phase angle).
    Coefficient of phase is 0.0065 mag/deg.
    
    Included among the eight objects in the above plot, is 64002A /733, which I have also plotted
    separately:
    
    http://satobs.org/seesat_ref/misc/64002A_stdmv.pdf
    
    The sample is very small, but comparing both plots, 64002A occupies more or less the same region.
    Standard magnitude is 5.3 +/- 2 (1000 km, 90 deg phase angle); coefficient of phase is approximately
    zero.
    
    In your follow-up message, you wrote:
    
    > Heavens above seems to predict fainter then mag 4 for the lower passes as
    > well which is very plausible. For the favorable 86 pass this evening it
    > predicts mag 3.7 and Calsky mag 4.4)
    
    For your pass of 64002A tonight, both results yield estimated maximum brightness of magnitude 4.9
    +/- 2, in other words, between about 2.9 and 6.9, so it may be necessary to observe the object many
    times, before it cooperates with your brightness requirements.
    
    Ted Molczan
    
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