Re: Fobos-Grunt: analysis of orbit evolution

From: William Thompson (
Date: Thu Nov 17 2011 - 10:17:12 UTC

  • Next message: Paul Salanitri: "Re: Fobos-Grunt: analysis of orbit evolution"

    Is the Phobos probe itself alive and well?
    Maybe it doesn't "know" it's still attached to
    the escape stage, and is trying to hold its
    attitude and maneuver.
    --Bill Thompson
    ----- Original Message -----
    > From: Ted Molczan <>
    > To:
    > Cc: 
    > Sent: Thursday, November 17, 2011 2:58 AM
    > Subject: Fobos-Grunt: analysis of orbit evolution
    >T he orbit of Fobos-Grunt (11065A / 37872) continues to evolve in unexpected 
    > ways. To the previously noted low rate of
    > decay and possibly propulsive orbit change(s), we add the puzzling rise of its 
    > perigee altitude. These observations
    > relate to USSTRATCOM's TLEs, which remain the only orbital data available to 
    > the public.
    > In an effort to determine whether the perigee rise could be explained by 
    > gravitational perturbations not modeled by SGP4
    > (the orbital model of TLEs), I compared the actual orbital elements against 
    > those yielded by propagating the earliest
    > reliable TLE data using the STOAG (Semi-analytic Theory of mOtion under Air drag 
    > and Gravity) software. I propagated the
    > epoch 11313.39819696 TLE through day 11319, varying the area to mass ratio, A/m, 
    > by trial and error to match the actual
    > rate of decay of the semi-major axis.
    > The value of A/m found to explain the observed rate of decay of the semi-major 
    > axis, 0.654 km/d, was 0.000634 m^2/kg,
    > one half the expected value of approximately 0.00129 m^2/kg, based on the 
    > object's known dimensions and mass. Despite
    > their forced agreement on the overall rate of decay, STOAG and the TLEs differed 
    > greatly regarding the evolution of the
    > perigee and apogee, as summarized:
    >                    STOAG    TLEs    Diff
    >                    km/d     km/d    km/d
    > Semi-major axis   -0.654   -0.654   0.000
    > Mean perigee      -0.247   +0.386  +0.633
    > Mean apogee       -1.062   -1.694  -0.632
    > It is evident that in addition to drag, some unknown force raised the perigee at 
    > the rate of +0.633 km/d, but lowered
    > the apogee at nearly the identical rate.
    > It should be noted that running STOAG with A/m set to zero, reduced the overall 
    > rate of decay to zero, and revealed
    > negligible non-drag perturbations of perigee and apogee of -0.025 km/d and 
    > +0.025 km/d, respectively; therefore, the
    > observed evolution probably does not have a natural cause.
    > The analysis also revealed that the argument of perigee is precessing at more 
    > than 1.3 times the expected rate. Since
    > the STOAG rate is close to the value predicted by SGP4, the observed rate 
    > probably is a side-effect of the decreasing
    > eccentricity.
    > Comparison of the rate of precession of the RAAN revealed close agreement among 
    > STOAG, SGP4 and the observed TLE values,
    > which tends to confirm the uniqueness of the findings with respect to the 
    > argument of perigee.
    > I have plotted the above results on two graphs, available here:
    > The unexpected low rate of decay and the unusual perturbation affecting 
    > eccentricity and argument of perigee probably
    > are manifestations of the same unknown force acting on the orbit. The STOAG 
    > model is sufficiently complete to rule out
    > natural forces that could have such a large effect over so short a time; 
    > therefore, I am left with the possibility of
    > thruster firings or venting. The former seldom have such a large effect; 
    > however, on an NSF forum, contributor Patchouli
    > wondered whether the Fregat-derived propulsion stage could be firing thrusters 
    > to settle propellants - an interesting
    > idea that I intend to follow up.
    > I remain open to other possible explanations. I doubt that errors in the TLEs 
    > could explain all of the observed effects,
    > but could the perturbation be creating some side-effects in the elements that 
    > are more apparent than real?
    > In closing, I caution that my experience using STOAG (and programs like it) is 
    > extremely limited. I ran a few tests to
    > familiarize myself with its operation and accuracy. I was impressed with its 
    > ability to accurately propagate USA 193
    > (06057A / 29651) from 2007 Jan 1 until just prior to its unnatural demise in 
    > 2008 Feb, using a value of A/m well within
    > 10 percent of what I believe to be correct. It also handled the object's 
    > frozen orbit very well, with argument of
    > perigee remaining in the vicinity of 90 deg at all times. The program appears 
    > not to propagate mean anomaly, which
    > somewhat limits its usefulness, but it appears to be helpful in evaluating 
    > long-term perturbations of LEO orbits. A
    > description of STOAG, as well as source code and binaries is available here:
    > I would be interested to learn the results of similar evaluations, using 
    > semi-analytic and numerical models.
    > Ted Molczan
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