Find_Orb's force model

Newtonian part :

   The sun is always included as a perturber.  Depending on which check
boxes you've ticked,  planets and the moon may be included as perturbers.

   If you tick the earth but not the moon,  Find_Orb computes the position
and mass of the earth-moon barycenter.  As a result,  including the moon
as a separate perturber usually doesn't matter all that much.

   For points within 0.03 AU of Saturn and Jupiter,  the effects of the
satellites of each of those planets is included separately.  Beyond that
distance,  the masses are "thrown in" to their primaries.

   Planets not ticked as perturbers are "thrown in" to the sun,  if
you are outside the orbit of that planet.  The process is a gradual one,
to avoid discontinuities:  if you are more than 120% of the planet's
mean semimajor axis from the sun,  the entire mass is thrown in.  If
you're less than 100%,  no mass is thrown in.  In between,  we ramp up
the mass linearly.  See the 'include_thrown_in_planets()' function.

   Up to 300 asteroids can be included.  Doing so can result in very
slow integrations.  Editing 'environ.dat' and resetting the BC405_ASTEROIDS
and/or ASTEROID_THRESH parameters can help a lot,  in the (usual) situations
where you are willing to ignore distant asteroids.

General relativity :

   GR is included only for the sun.  The method used is described at

http://www.projectpluto.com/relativi.htm#better

   At present,  this first-order approximation is more than ample.  But
if Gaia truly produces results as wondrous as claimed,  some smaller
effects will be noticeable.  GR from planets and second-order terms
may have to be included.

Non-spherical planets :

   The equatorial "bulge" (J2) terms,  and the smaller J3 and J4 zonal
terms,  are included for the Earth,  Mars,  and all four gas giants.
So far,  this has been sufficient.  But if I start dealing with lower
orbiting earth satellites,  that situation will change:  higher-order
terms and spherical harmonics will be required.

Solar radiation pressure

   At present,  SRP is treated as a simple radial force proportional
to the sun's gravity.  In other words,  a sufficiently "fluffy",
high area/mass object would cancel out the sun's gravity and travel in
a straight line relative to the sun (except for planetary
perturbations,  of course.)

Comet parameters

   By default,  the comet parameters A1 and A2 (or A1,  A2,  and A3)
follow the standard Marsden-Sekanina model.  In this model,  there
is almost no cometary force at 2.6 AU or more from the sun (because
almost no outgassing occurs),  and then immense forces close to the
sun as the comet boils away.

   For rocks,  one can edit 'environ.dat' and adjust the 2009BD
parameter to have the force dependence be a simple inverse square one.
That matches the physics for rocks such as 2009 BD and 101955 Bennu,
where there is an A1 outward force similar to SRP and an A2 tangential
force that is accelerating or decreasing the object's speed (i.e.,
increasing or decreasing the object's orbital period/semimajor axis).

Atmospheric drag

   This has only recently (October 2015) been added,  and is somewhat
experimental.  Only the earth's atmosphere is currently modelled,
though it would not be too difficult to add those of Mars,  Saturn,
Venus,  and Titan.  Drag only exists if SRP has been turned on.  If
the object has been tracked for long enough that Find_Orb can determine
its area/mass ratio,  that AMR is used in computing ephemerides for
the object's passage through the atmosphere.

   For meteors,  one can turn on SRP and do a seven-parameter fit.
The AMR will then be determined from the object's observed deceleration.
If you have no idea what the AMR might be from observations,  you can
enter a constraint such as "A=0.006" (m^2/kg),  do a full step,  and
then compute ephemerides based on that assumed AMR.

Discontinuities (short version:  there aren't any)

   Some care was taken in the force model to ensure that the accelerations
and their derivatives would be both finite and continuous.  The real problem
in that department was situations where objects pass inside of planets.  In
the real world,  this causes a very abrupt discontinuity,  i.e.,  an impact.
But in the initial stages of orbit determination,  it can make sense to compute
orbits passing inside planets.  Also,  abrupt discontinuities would be something
of a headache for numerical integration.

   The solution was this.  Accelerations are computed in their 'normal' manner
and left unchanged for distances greater than 90% of the planet radius.
Within about 60% of the remainder,  accelerations are zero;  i.e.,  there is
a "bubble" inside each object within which the planet exerts no force at all.

   Between those limits,  accelerations are multiplied by a factor computed
in the compute_accel_multiplier() function.  This factor is equal to one at
the 90% limit,  and zero at the inner "bubble" limit.  Its derivative at
both points is zero,  i.e.,  both this function and its derivative are
continuous across the outer and inner limits.  It is computed as a cubic spline.
Thus,  instead of having accelerations climb to infinity as you approach a
planet center,  they instead reach a peak,  then drop smoothly to zero.
