Robert Heller writes:

I am writing a program that simulates a planetary system (a star and a group
of planets in orbit about it). I am writing the program in Tcl and I have
ported a C program, Stargen
(http://www.eldacur.com/~brons/NerdCo.../StarGen.html), to generate
the planetary system and part of the ORSA (http://orsa.sourceforge.net/), to
perform orbital calculations.

At this point I can generate a planetary system and compute the orbits of the
planets. I want to add in spacecraft, but I am not sure how to determine a
spacecraft's orbit -- for a given position and velocity, which planet (if any)
would the spacecraft be in orbit about? I don't know if I should create a
'body' (an ORSA data type/class), with a given mass, position, and velocity
for the spacecraft and then compute this body's orbit with each planet and the
star (the ORSA library has a method which computes the orbital parameters
given a pair of bodies). How do I tell which is the most likely orbit?

I'm not familiar with the software you're using, but the trajectory of
every body in the system is dependant on every other body. If ORSA
simplifies this to your spacecraft being in orbit around one other body,
it isn't going to be accurate enough to be of any use in most
circumstances. But, if you must assume you're only in orbit around one,
it'll be the one exerting the most force, ie the one for which M/r^2
(the mass of the body divided by the distance from your spacecraft to
that body squared) is greatest.