Phobos-Grunt: a small surprise in ESA's confirmation of decay estimate

From: Ted Molczan (
Date: Wed Jan 25 2012 - 19:12:04 UTC

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    ESA's report confirming the decay estimate contains a small surprise. ESA reports that the 17:46 UTC decay time in
    USSTRATCOM's final TIP (Tracking and Impact Prediction) message corresponds to descent to altitude 80 km, but it was my
    understanding that their estimates correspond to descent to 10 km altitude:
    "Before the reentry of Russia’s Phobos–Grunt Mars probe on 15 January, ESA established a final prediction for reentry of
    17:45 GMT, referring to an altitude of 10 km. Within the expected uncertainties, the prediction has been largely
    confirmed by observations.
    Refining the reentry estimates
    Following the 15 January Phobos–Grunt reentry, the US Strategic Command confirmed a reentry time of 17:46 GMT, referring
    to an altitude of 80 km at 46°S and 87°W, near the South American coastline. This corresponds to a pass at 10 km
    altitude about seven minutes later – very close to ESA’s prediction."
    NASA's Handbook for Limiting Orbital Debris mentions the reference altitude of 10 km:

    "Another important function of the SSN is to monitor the orbital decay of satellites and to predict
    when and where they will reenter the atmosphere. This process is called Tracking and Impact
    Prediction (TIP), sometimes referred to as Reentry Assessment (RA). The TIP process typically
    begins two weeks before the expected reentry date. The SCC normally increases sensor tasking
    on the object because the orbit can change significantly between sensor tracking opportunities.
    Official TIP messages, including the time and location of reentry at a reference altitude of 10 km,
    are issued at the following times prior to reentry: 96 hours, 72 hours, 48 hours, 24 hours, 12
    hours, 6 hours, and 2 hours. However, due to several inherent uncertainties, including
    atmospheric density and stability of the object, the accuracies of these predictions are less than
    desired and can lead to significant errors in the predicted location of the nominal impact location.
    A final, post-event assessment is issued a few hours after reentry."
    The 1991 master of science thesis, "An Analysis Of Tracking And Impact Predictions", includes a table derived from USAF
    records for 1987-1990, that lists the decay altitude as 10 km. See top of report pg.16 (pdf pg.27), second column from
    Before anyone expends effort trying to estimate how much farther downrange P-G would have travelled in the descent from
    80 km to 10 km, it would be useful to obtain confirmation that USSTRATCOM did indeed base the decay time on 80 km. If
    they did, then it would be useful to know why, and if it represents a permanent policy change, then when did it come
    into effect?
    Ted Molczan
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