On Thursday, I believe I should be able to make a (hopefully) accurate forecast of the June 8 ISS / Venus transit track. The basic idea occurred to me a while back, but it required an orbit boost such as that scheduled for today. The projected TLE for day-of-year 149.7 (May 28) published by the MCC yesterday gives an orbital period corresponding to about 15.6839 orbits per day (there are 86400 seconds in a year, so that precision corresponds to about 1/2 second per day): 1 25544U 98067A 04149.70875706 .00020000 00000-0 20000-3 0 9120 2 25544 51.6304 65.0080 0005811 166.6668 193.4647 15.68392604 35268 This TLE corresponds closely to the TLE for DOY 109.5 (April 18), 1 25544U 98067A 04109.51565041 .00020000 00000-0 20000-3 0 9033 2 25544 51.6252 269.8838 0010857 45.7908 314.4142 15.68393153 28958 With some offset in time (within half the orbital period, or about 46 minutes), it should be possible to transform a position (such as crossing the equator, northbound- at longitude 0, we'll pretend) predicted by the historic DOY 109.5 TLE to a position (also crossing the equator, northbound- at longitude 3.1416 E, we'll pretend) predicted by the DOY 149.7 TLE, namely by adding 3.1416° of longitude. The transit of Venus begins at DOY 160.231192, or 10.5224351 days = 909138.39 seconds after DOY 149.708757. So, by using the historic TLE for DOY 109.5 + 10.5 = 120.0; the nearest to which is DOY 119.5 (April 28), 1 25544U 98067A 04119.51743710 .00020000 00000-0 20000-3 0 9024 2 25544 51.6283 218.9203 0011040 78.8338 281.4063 15.68949863 30528 with the appropriate time offset & longitudinal correction, it should be possible to use the well-known position of the ISS at that earlier time to forecast the position during the Venus transit, and so to produce an acceptably accurate ISS / Venus transit track for June 8! ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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