/** This is a simple model of Mars's atmosphere for modeling re-entry.

    See:
    https://www.grc.nasa.gov/WWW/K-12/airplane/atmosmrm.html

    These equations give nonsensical results (e.g. negative Kelvin
    temperatures) above 100 km.

    A better option might be to find the Mars-GRAM data files which
    require getting through broken NASA webpages.
*/
class MarsAtmosphere
{
   /** Calculates temperature, pressure, and density of Mars's atmosphere at
       the given altitude.

       parameters:
         altitude

       returns:
         [temperature, pressure, density]
   */
   class getTPD[altitude] :=
   {
      if altitude < 7000 m
         t = C[-31 - 0.000998 (altitude/m)]
      else
         t = C[-23.4 - 0.00222 (altitude/m)]

      if t < 0 K
         t = 0 K

      p = .699 kPa exp[-0.00009 * (altitude/m)]
      if t > 0 K
         rho = (p / kPa) / (.1921 m^3/kg (t/K))
      else
         rho = 0 kg/m^3
      
      return [t, p, rho]
   }
}