When I wanted to calculate the heliocentric position of asteroids from the elements provided by the JPL, I startet writing these Lisp functions.
One can use download-element-files-from-jpl to download the orbital
elements from the Jet Propulsion Laboratory. They come in two files,
ELEMENTS.UNNUM and ELEMENTS.UNNUM, which contain more than one million
elements of asteroids.
Because of small differences in the two files and me to lazy to
abstract from them, there are two functions to read them into lists of
asteroid objects, read-elements-numbered and
read-elements-unnumbered.
Asteroid object are defined by defclass asteroid. The class is only
used to store the information contained in the element files. The id
of unnumbered asteroids is the same as there designation.
To calculate the heliocentric ecliptic position of an asteroid one can
use the function asteroid-heliocentric-position-at-date with an
asteroid object and a modified julian date.