Paul Thomspon wrote: <<I was curious however, about the brightness of SeaSat 1 ... was due for a pass ... [at predicted] 3.9 mag. This data is all from Heavens-Above. The satellite was easily visable, and not as dim as expected, appearing as bright as perhaps 3.2-3.3 mag.>> <<... I wondered if its orientation is variable, and with it, its brightness also>> Your speculation is correct. Seasat 1 is very irregular in shape and surface reflectivity, resulting in a large variance in magnitude for a given set of circumstances (range and solar illumination). Heavens-Above's magnitude estimates are based upon standard magnitude (aka intrinsic brightness) data derived by hobbyists from their observations. Seasat 1's standard magnitude is 3.9, when observed at a range of 1000 km, and a phase angle of 90 deg. But that value is an average, derived statistically from a large number of observations, as in the following graph of more than 500 of Russell Eberst's observations: http://satobs.org/seesat_ref/SeaSat_stdmag/78064A-stdmag.jpg The linear regression line on the graph confirms that the std mag is 3.9, but notice that the observed values range +/- 3 magnitudes from the line. As a result, predicted magnitudes are uncertain by +/- 3 magnitudes. Almost all satellites can flare several magnitudes on occasion, but most of them stay within about +/- 1 magnitude of the regression line, most of the time, for example, 93026B: http://satobs.org/seesat_ref/SeaSat_stdmag/93026B-stdmag.jpg Comparing the two graphs, it is clear that Seasat 1's magnitude is far less predictable than 93026B's. Perhaps, we should consider including statistical confidence limits with the standard magnitude. A simpler solution would for prediction software and services to prominently state that magnitude estimates are uncertain by at least 2 magnitudes. Ted Molczan ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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