Fobos-Grunt: decay estimate

From: Ted Molczan (
Date: Fri Dec 30 2011 - 15:06:19 UTC

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    The mean rate of decay of the semi-major axis of the twenty-nine (29) USSTRATCOM TLEs from epoch 11359.52026040 to
    11364.51702987 (2011 Dec 25 12:29 - Dec 30 12:25 UTC) was 1.7893 km/d. 
    Using the STOAG propagator and actual space weather during that period, I found that the mean rate of decay could be
    accounted for using A/m = 0.0011888 mē/kg and Cd = 2.2.
    Using those values with predicted space weather, STOAG propagates the orbit to decay on 2012 Jan 16. Estimated
    uncertainty is +/- 4 days, based on the rule of thumb of 20 percent of the time remaining to decay.
    On Dec 29, I noticed a small error in Celestrak's recent compiled values of 10.7 cm solar flux, such that observed
    values were taken to be 1 A.U.-adjusted values, from which erroneous observed values were estimated. It is too soon for
    the site to have responded to my alert, but assuming my finding is confirmed, and that I have not erred in my
    corrections, then the A/m values I reported prior to Dec 29 were nearly 3 percent too low. That is much less than the
    cumulative effect of the uncertainties in modelling of atmospheric density, orbit propagation, and space weather
    forecasting; therefore, it did not materially affect my decay estimates. This report reflects my corrections of the data
    in question.
    Below is the plot of estimated A/m since the orbit manoeuvres ceased, through Dec 27. Most of the points are at ~2 day
    intervals, and typically span the preceding ~2 day period. For Cd = 2.2, the mean A/m was 0.0012875 mē/kg; those values,
    with predicted space weather, would advance the estimated date of decay by about one day, to Jan 15.
    I extracted the actual and predicted space weather data required by STOAG from data provided by, which
    compiles data issued by official sources (mainly NOAA) and presents it in a convenient format:
    Ted Molczan
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