USA 144: The Mystery Deepens - Flash Timings Needed

From: Ted Molczan (
Date: Mon Aug 05 2002 - 02:35:06 EDT

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    I have discovered that the object we have been calling USA 144:
    1 25744U 99028A   02215.06055556 -.00000029  00000-0 -45007-2 0    06
    2 25744  63.4406 138.5884 0237584 293.3003  64.3360  9.69785997    04
    responds to SRP (solar radiation pressure). SRP is the pressure exerted
    by photons of sunlight striking the surface of an object - the force
    that propels solar sails.
    Analysis of the SRP effects reveals that the object in question is 10 to
    20 times less dense than would be expected of a typical payload.
    Factoring in its apparent size, based on its brightness, suggests a mass
    of several hundred kilograms, no more than 10 percent of the expected
    mass of USA 144. Perhaps it is debris, or less likely, a decoy. If it is
    USA 144, then it must be a very unusual spacecraft.
    Another debris-like characteristic is the object's slow rotation.
    Photometric period observations may be helpful in solving the mystery of
    this object, as explained later in this message.
    Basics of SRP
    SRP produces a weak force which can significantly accelerate only
    objects having a high area to mass ratio; for example, solar sails, the
    Echo and Pageos balloons, and some pieces of debris.
    Objects in partially eclipsed orbits experience a net gain or loss of
    energy, depending upon their relative time during each revolution moving
    away from the sun or toward the sun.
    When moving away from the sun, SRP pushes from behind, causing a gain of
    energy. When moving toward the sun, SRP pushes from in front, causing a
    loss of energy. If more time per revolution is spent moving away from
    the sun than moving toward it, then there will be a net energy gain,
    manifested as a negative rate of decay. The reverse also is true.
    High objects, like the one in question, experience periods of constant
    sunlight lasting many weeks, during which there is no net change in
    energy, manifested by a zero rate of orbital decay.
    Method of Analysis
    Since tracking began in June 1999, the object has completed 26 periods
    when the effect of SRP on orbital decay should have been zero, positive
    or negative .
    During this time, hobbyists reported 632 precise positional
    observations, which I sorted chronologically and by SRP period.
    There were sufficient observations to produce a reliable orbit for 23 of
    the 26 SRP periods, and in each case the sign of the orbital decay was
    as predicted by SRP theory: positive, negative or near zero, 
    Decay rates generally were more positive than expected, i.e. where a
    zero value was predicted, it was often slightly positive; the magnitude
    of negative rates of decay was smaller than expected and the magnitude
    of positive rates of decay was greater than expected.
    A possible explanation is that the observations occurred near solar
    maximum, so atmospheric drag may have been present along with the SRP.
    If so, then this small constant decay should decrease as we move away
    from solar max over the next few years.
    SRP Analysis Reveals Area to Mass Ratio
    SRP analysis has yielded an accurate estimate of the object's area to
    mass ratio - more precisely, its kA/m value - area to mass ratio
    multiplied by a constant which accounts for its shape and reflectivity.
    The value of k can be between 1 and 2,
    A value of kA/m of about 0.135 m^2/kg appears to account for the
    object's historical rates of orbital decay. 
    Assuming k = 1.5, then A/m = 0.09 m^2/kg - at least an order of
    magnitude greater than that of most payloads and rocket bodies. For
    comparison, consider: 
    Compton GRO  0.004 m^2/kg
    Hubble ST    0.006 m^2/kg
    UARS         0.007 m^s/kg
    Judging by its standard magnitude of 3.6, the object should have a
    cross-sectional area of perhaps 50 m^2. Dividing by A/m of 0.09 m^2/kg,
    yields an estimated mass of only 560 kg.
    To be the payload, estimated to have a mass of at least 6,000 kg, it
    would require an area facing the sun of at least 540 m^2 - a solar panel
    perhaps - but what kind of satellite would require one that large? And
    there remains the problem of the slow rotation.
    Photometric Flash Period Measurements Needed
    Hobbyists have tracked the object since a couple of weeks after its
    launch, and all this time it has slowly rotated. Large LEO payloads tend
    to be three axis stabilized, so the rotation suggests that the object is
    Observations of other SRP objects, such as the fragments from the
    break-up of the Pageos balloon, revealed that their period of rotation
    varied considerably over time - sometimes increasing, other times
    There is some evidence that the object in question behaves similarly,
    Below is a compilation of data from positional observations, PPAS
    observations and from personal correspondence:
           Observer                    Photometric Period - seconds
    ------------------------------   --------------------------------
    1999 Jun 15 Peter Wakelin        "of the order of 1.5 minutes"
    1999 Jun 15 Pierre Neirinck      "maxima look flat and regular at 1 min
    1999 Jun 18 Pierre Neirinck      "several max at irregular intervals:
    158, 160, 61 sec. Maxima are also irregular in brightness"
    1999 Jul 02 Jim Nix              150
    1999 Jul 09 Tony Beresford       120
    1999 Jul 11 Tony Beresford       100
    1999 Aug 23 Russell Eberst       90 R
    1999 Sep 02 Pierre Neirinck      144 +/- 30
    1999 Sep 03 Pierre Neirinck      152 +/- 10
    1999 Sep 04 Pierre Neirinck      146.5 
    1999 Oct 01 Mike McCants         130 v2
    1999 Oct 13 Pierre Neirinck      145 +/- 6  
    1999 Oct 16 Tristan Cools        136 M'_M'
    1999 Dec 19 Tony Beresford       116
    2000 Aug 13 Bram Dorreman        steady or not measured
    2000 Aug 15 Bram Dorreman        steady or not measured
    2000 Aug 25 Bram Dorreman        steady or not measured
    2000 Sep 21 Jean-Pierre Rohart   steady or not measured
    2001 Nov 06 Russell Eberst       60 R
    2002 Mar 09 Russell Eberst       60 R
    2002 Jun 20 Ted Molczan          116
    2002 Jul 19 Pierre Neirinck      116.2 +/- 5
    2002 Jul 28 Pierre Neirinck      111.3 +/- 6
    Clearly the object now rotates about 30 s faster than it did in the Fall
    of 1999. Additional accurate timings are needed going forward, to
    determine whether or not it sometimes slows down.
    If its rate of rotation no longer varies, then could this be by design?
    Could a passive object behave that way?
    Perhaps flash period observers could help to discover the true nature of
    this object through long-term observations.
    A Lesson Learned
    While preparing this post, I ran across a fax message sent by Pierre
    Neirinck on 1999 Sep 25, in which he mentioned SRP as a contributing
    cause of the object's apparently high rate of decay. I never followed up
    on the idea, and quickly forgot about it. So strong was my belief that
    the object was USA 144, that I was not open to contrary evidence. That
    made it easy to ignore or explain-away inconvenient facts, like the high
    rate of decay, and the slow rotation.
    Ted Molczan
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