This module computes positions and velocities of a celestial small body, from a
NASA SPICE SPK ephemeris kernel file of data type 21 (Extended Modified
Difference Arrays).
See http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/FORTRAN/req/spk.html

You can get SPK files for many solar system small bodies from HORIZONS system of
NASA/JPL.
See https://ssd.jpl.nasa.gov/?horizons

This module reads SPK files of data type 21, one of the types of binary SPK
file. At the point of Oct. 2018, HORIZONS system provides files of type 21 as
binary SPK files by default. You can get type 21 binary SPK file for celestial
small bodies through TELNET interface by answering back 'B' for 'SPK file
format'. Also you can get type 21 binary SPK file from:
https://ssd.jpl.nasa.gov/x/spk.html