Phobos-Grunt: latest two TLEs suggest manoeuvring or venting

From: Ted Molczan (
Date: Fri Nov 11 2011 - 10:58:25 UTC

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    The mean motion and decay terms of USSTRATCOM's latest two TLEs are indicative of a small orbit-raising. Whether this is
    due to a deliberate manoeuvre or of venting from the propulsion or some other system is unclear. It also cannot be ruled
    out that the TLEs are spurious.
    Over the period from epoch 11313.39819696 through 11314.77184893, the decay term ranged between 0.00157546 rev/d^2 and
    0.00243885 rev/d^2. At the start of this period, they were closer to the higher value, but by the end had trended closer
    to the lower value. The values seemed to be settling down as tracking progressed, as would be expected. 
    The epoch 11314.95914282 value was an order of magnitude smaller than the previous range: 0.00019037 rev/d^2 Also, the
    mean motion had decreased slightly relative the previous TLE. This was quite unexpected, and the thought of a manoeuvre
    or venting occurred to me, but I could not exclude the possibility of a spurious TLE. I went to bed wondering what the
    next TLE would show.
    I awoke to the current TLE, epoch 11315.20886687, which has a decay term of -0.00002358 rev/d^2, and mean motion again
    slightly lower than that of the previous one.
    The net effect of the orbit change between the epoch 11314.77184893 and 11315.20886687 TLEs is an increase in mean
    altitude of 138 m (metres). Had the orbit continued to decay at the rate of the earlier TLE, its mean altitude would
    have decreased by 215 m. So the effect of the apparent manoeuvre or venting was to raise the altitude 353 m.
    This could all still turn out to be due in some way to spurious TLEs, but I suspect the effect is real.
    If venting is occurring, perhaps due to leakage from the propellant system, then the spacecraft could eventually begin
    to tumble. It is a phenomenon well-known to us with old upper stages, that after a long time in orbit undergo a small
    change of mean motion, accompanied by a spin-up, i.e. increase of rate of rotation of the object.
    A happier interpretation is a deliberate orbit manoeuvre by the spacecraft, but a quick check of the NSF and NK
    discussions where developments are being closely followed did not turn up any news consistent with that. Let's hope for
    the best.
    Yesterday, Brad Young reported the s/c steady in brightness, indicating no discernible rotation (tumbling) over several
    minutes of observation. It will be interesting to see what subsequent observations reveal.
    Ted Molczan
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