RE: USA193 decay

From: Ted Molczan (ssl2molcz@rogers.com)
Date: Wed Jan 30 2008 - 09:32:40 UTC

  • Next message: Ted Molczan: "Updated elements of USA 193"

    Vossinakis Andreas wrote:
    
    > Hi to all. I am a little bit confused about this:
    > Playing around with SatEvo and inputing various -F values, 
    > the result is: 
    > the bigger the value (which means strongest solar activity), 
    > the decay time moves later.
    > But reading articles about space debris, the result of strong 
    > solar activity is faster decay of the satellite.
    
    I too noticed this behaviour. Using a recent elset of USA 193 (06057A / 29651),
    with Satevo's default 10.7 cm solar flux of 150, resulted in a decay prediction
    15 d later than using a value of 70. I believe what is happening is this. The
    program uses the solar flux to estimate atmospheric density at the altitude of
    the satellite at the epoch of the initial elements, then combines this
    information with the object's rate of decay to estimate its ballistic
    coefficient, which is its cross-sectional area divided by its mass, denoted by
    A/m. A formula for the ballistic coefficient is available here:
    
    http://www.satobs.org/columbia/STS107mysteryobject.html#Ratio
    
    Note that the rate of decay is in the numerator, and the atmospheric density is
    in the denominator. Since rho increases with solar flux, using lower values of
    solar flux will result in a greater ballistic coefficient, resulting in a
    quicker decay than with greater values of solar flux.
    
    Ted Molczan
    
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