I have analyzed Russell Eberst's observations of space shuttle and ISS to determine their standard visual magnitude. 1. Shuttle During 1990-2001, Russell observed shuttles in free flight on nine missions. The following chart plots the relationship between visual magnitude normalized to 1000 km range and phase angle: http://satobs.org/seesat_ref/Shuttle_and_ISS/Shuttle_stdmag.jpg The standard magnitude is 0.2 (1000 km range, 90 deg phase angle). As with most satellites, there is a statically weak correlation between brightness and phase angle, but the shuttle's variation is quite a bit greater, no doubt due to it's unique shape and pattern of reflectivity. When seen nose-on or tail-on, it presents a much smaller cross-section than when seen cargo bay-on or heat shield-on. And of course, the heat shielded side is far less reflective than the cargo bay and the top of the wings. 2. ISS Russell cautioned me about using his ISS magnitude data: "There are very few comparison stars or planets that have magnitudes in the range around -2 or so, that I.S.S. frequently exhibits. Really only Jupiter (or rarely Mars) are in the right magnitude range. So this means that if these planets are not on view at the time of the transit, all that you have to go on is your "magnitude memory". As a result, I would expect my estimates to be quite unreliable, and I would not be surprised if any analysis based on them were to be described as 'flimsy'." "However, if you want to proceed in the absence of other estimates, then it might be worthwhile including a statement that the figures are 'indicative' and should not be regarded as quality data." I accept Russell's cautionary comments; however, I have no qualms in relying on his "magnitude memory", developed by making more than 200,000 precise observations over more than four decades. His accuracy and reliability have been confirmed as a result of standard magnitude evaluations across numerous examples of identical satellite buses and rocket bodies. Much of our hobby's ability to accurately predict satellite visual magnitudes derives directly from Russell's observations. One complication in the ISS analysis is that Russell roughly apportioned the total observed magnitude over the various docked components. I recombined them on the assumption of equal reflectivity among all of the components. I computed a pseudo reflecting surface area assuming the relationship, mag ~ 2.5 log10(1/area), then applied this formula to the sum of those areas. Russell confirms that my results are within several tenths of a magnitude of his original observations. The following plot is of 47 observations during 2002 December to 2005 July, when ISS was in its present configuration, plus or minus a Progress supply ship: http://satobs.org/seesat_ref/Shuttle_and_ISS/ISS_stdmag.jpg The standard magnitude is -1.3 (1000 km range, 90 deg phase angle). Note the much lower variation in brightness for a given phase angle, compared with the shuttle. Based on the difference in their standard magnitude, most of the time ISS can be expected to be about 1.5 mag brighter than a shuttle; however, there is sufficient variation to frequently allow a shuttle to outshine ISS. Ted Molczan ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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