From: vince@offshore.ai (Vincent Cate)
Newsgroups: sci.space.tech,sci.space.science,sci.space.history
Subject: Simple Atmospheric Model for Space?
NNTP-Posting-Host: 209.88.68.230
Message-ID: <9186edb5.0310052108.54744457@posting.google.com>

Jan Philips on sci.space.history asked "How low can you orbit?".
I have a space simulator that could be used to play around
with this kind of problem.  I had not really worried about 
orbital decay yet.  I tried some tests to see if I could match 
a graph in "Space Planners Guide" but my decay times are order of 
5 times shorter for orbits under 250 miles.  I also get much 
shorter times for the Vostok and Mercury examples given by
Chris Jones.

I am using the atmospheric model at:
http://www.grc.nasa.gov/WWW/K-12/airplane/atmosmet.html
but I don't think this model is really supposed to be used at orbital 
altitudes.  

I found some real space atmospheric models like:
  http://nssdc.gsfc.nasa.gov/space/model/atmos/atmos_index.html
  ftp://nssdcftp.gsfc.nasa.gov/models/atmospheric/met/met.for
  http://nssdc.gsfc.nasa.gov/space/model/atmos/msis.html

But these need far more detail to run than I want to deal with.
For example the last URL above says:
 > The model expects as input year, day of year, Universal Time, 
 > altitude, geodetic latitude and longitude, local apparent solar time, 
 > solar F10.7 flux (for previous day and three-month average), and 
 > magnetic Ap index (daily or Ap history for the last 59 hours). 

I have a graph on page 16.18 of "The Standard Handbook for Aeronautical
and Astronautical Engineers" but I want something I can code up.

Does anyone know a simple atmospheric model that just takes an 
altitude as an input but would be closer to reality than what I have
now? 

My simulator is at http://spacetethers.com/spacetethers.html

See sample inputs:
  "62  Decay of Circular Orbits" 
  "63  Decay of Eccentric 160 km Orbits"

Thanks,

  -- Vince