Question about orbital mechanics

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?

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