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 debris. 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 decreasing. 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 interval" 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 ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www.satellite.eu.org/seesat/seesatindex.html
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