Fobos-Grunt: decay estimate

From: Ted Molczan (
Date: Wed Jan 04 2012 - 19:45:19 UTC

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    The mean rate of decay of the semi-major axis of the twenty-nine (29) USSTRATCOM TLEs from epoch 11364.51702987 to
    12004.50254709 (2011 Dec 30 12:25 - 2012 Jan 04 12:04 UTC) was 2.2137 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 = 0.0012906 mē/kg and Cd = 2.2.
    Using those values with predicted space weather, STOAG propagates the orbit to decay early on 2012 Jan 16 UTC. Estimated
    uncertainty is +/- 2 days, based on the rule of thumb of 20 percent of the time remaining to decay.
    Below is the plot of A/m estimated from historical orbital and space weather data since the orbit manoeuvres ceased,
    through Jan 02. 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 0.0012789 mē/kg; those values, with predicted space weather, would delay the estimated date of
    decay slightly, to mid-day Jan 16 UTC.
    The above plot of A/m reveals no obvious long-term trend. Some of the apparent variation in A/m is due to imperfections
    in the orbital elements, orbital model and atmospheric density model; some of it may be due to actual changes in the
    object's orientation with respect to the velocity vector. In making decay estimates, the unpredictability of space
    weather (which is the basis of most atmospheric density models) adds to the uncertainty.
    In addition to STOAG, I used Alan Pickup's Satana and Satevo programs to estimate the decay, which yielded mid-day Jan
    16 UTC, based on the same span of historical orbital elements and 10.7 cm solar flux (mean = 135). 
    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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