   Find_Orb supports (at present) eight different non-gravitational force
models.  More may be added later.

   'Solar radiation pressure (radial, 1/r^2)' is the simplest : the model
assumes that the object is pushed outward from the sun by a force that
is proportional to the amount of sunlight hitting it,  i.e.,  the inverse
square of the distance to the sun (except when in the earth's shadow).

   (Pro tip : hit the '* key,  then '1' to turn simple SRP on,  '*' then
'0' to turn non-gravs off,  and so on.  This can save you a lot of mouse
activity.)

   'Solar radiation pressure (two-parameter)' adds a force in the direction
of the object's motion around the sun,  proportional to the radial force.
This is sometimes seen with small asteroids (a few meters across),  and is
quite often seen with space junk.  It is a simplification of the Yarkovsky
effect : if the object has prograde rotation,  it will absorb some sunlight,
emit it as it rotates, and be (very gently) accelerated.  If its rotation
is retrograde,  the opposite will occur.

   For most asteroids larger than a few meters across,  though,  the
radial term is negligible (the object is too big/massive to be forced
"outward" noticeably).  Unless an asteroid is really small,  you should
use the 'Yarkovsky (A2-only)' model described below.

   'Solar radiation pressure (three-parameter)' adds a force in the
out-of-plane direction,  normal to the other two forces.  This can result
in a better fit to the data for space junk;  I've yet to see it apply to
a natural one,  except for 2006 RH120 (very small,  temporarily captured
object).  Even with space junk,  it does not really match the actual
physics and should be used with caution.

   'Comet model (A1 and A2)' applies the standard Marsden-Sekanina force
model;  see page 3 of

http://www.lpi.usra.edu/books/CometsII/7009.pdf

   'Cometary Orbit Determination and Nongravitational Forces',  D. K. Yeomans,
P. W. Chodas, G. Sitarski, S. Szutowicz, M. Krolikowska, "Comets II";  see
'runge.cpp' in the Find_Orb source for more detail.  Also,  note that water
ice is assumed by default,  but this can be changed by editing 'environ.dat'.

   'Comet model (A1, A2, A3)' is usually only used for comets observed at
many apparitions,  and adds an out-of-plane component to the A1/A2 model
(almost identical to the way the three-parameter SRP model works.)

   Note that the SRP and comet models are virtually identical,  except that
for SRP,  the force is just that of sunlight and is simply expressed
as proportional to 1/r^2.  For comets,  a rather complicated scheme is
required to account for the rate of outgassing as a function of distance
from the sun,  as described at the above reference.

   On occasion,  we get comets where the activity lags the comet's position
by days or weeks or months.  It takes the comet a while to warm up in
the sun to get to the point where it's really pushed around,  and then
it will take it a while before it cools back down.  Such cases can be
modelled with the A1, A2, A3, DT model : the 'DT' corresponds to a time
lag in days.  It takes at least a few oppositions to get enough data
to determine this value reliably.  This model only makes sense if you
know the object to be a comet.

   The 'Yarkovsky (A2-only)' model is used for objects where the
Yarkovsky effect can be modelled,  to decent accuracy,  as being only a
tangential ("along-orbit") force proportional to the amount of solar
radiation hitting the object : i.e., it's an inverse-square force,  like
the various SRP models.  This is basically the same as the 'solar
radiation pressure (two-parameter)' model, except with A1 constrained to
be zero.  (In fact, you can set that model, constrain A1=0,  and get
exactly the same result as with the Yarkovsky (A2-only) model.)

   The 'delta-V' model is used almost entirely for spacecraft,  where you have
reason to believe that the object had a change of velocity at or near a
specific time,  and want to determine more precisely when it occurred and the
direction and amount of the velocity change.  Select this option, and Find_Orb
will ask for your estimate of when the maneuver occurred. Subsequent
least-squares fits will determine the time of the maneuver and the components
of the velocity vector,  in meters per second,  in J2000 ecliptic coordinates.
