[Celestia-developers] Apollo 11 Orbit

[Clint W. <cwe...@co...>]

Hi All,

With 3 kids running around now, it's hard to contribute much to Celestia =
but I did notice something I thought I'd investigate.

I suspect the Apollo 11 elliptical orbit parameters are significantly =
inaccurate. I was following Apollo 11 above the moon and noticed that =
the altitude was too high. Also noticed the orbital speed was very slow, =
almost imperceptibly so. A single orbit took nearly 5 hours or 0.2 days =
as is currently set in apollo.ssc.

I visited a few sites, gathered some information and made some =
calculations to adjust the orbit parameters in this file.

http://www-pao.ksc.nasa.gov/history/apollo/apollo-11/apollo-11.htm =
quotes the Apollo 11 lunar orbit with apogee of 186 km and perigee of =
183 km. Using the volumetric mean radius of the moon (1737.1 km) as =
quoted from =
http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html, one can =
compute the eccentricity of the orbit as about 0.0558. The semimajor =
axis of the orbit is 1923.1 km. Assuming the orbit to be roughly =
circular, the period of the orbit is about 2.099 hours or 0.0875 days.

Updating apollo.ssc with these values yielded some unexpected results, =
however. I expected the Apollo 11 lunar orbiter to stay between 183 and =
186 km above the moon. Instead, it varied between 293 km and 78 km. =
Interestingly, the sums of these two sets of altitudes (183 + 186, 293 + =
78) are roughly equal. This suggests to me that somehow the center of =
the orbiters' orbit is not at the center of the moon. If the center of =
the moon were located about 107 km away from Apollo 11 orbits' center =
and on the semimajor axis, you would get the results these values =
produce. Don't know how that's possible, though.

I'm going to investigate the code a little bit, but if anyone has any =
insight into this problem, please don't hesitate to share it.

Best regards,

Clint Weisbrod.