The JPL HORIZONS on-line solar system data and ephemeris computation service provides access to key solar system data and flexible production of highly accurate ephemerides for solar system objects ( 381106 asteroids, 2435 comets, 168 planetary satellites, 9 planets, the Sun, L1, L2, select spacecraft, and system barycenters ). HORIZONS is provided by the Solar System Dynamics Group of the Jet Propulsion Laboratory.

High precision ephemerides for solar-system bodies are available on-line using JPL's HORIZONS system. Most optical and radar observers will be best served by using our HORIZONS system to generate custom observer-table ephemerides. Our on-line HORIZONS system can also be used to output ephemerides in the form of osculating orbital elements and/or Cartesian vectors (position, velocity, and acceleration). However, there are some ephemeris data files available for export (see below) for those rare cases when HORIZONS will not suffice.

Professionals requiring high-precision ephemerides readable from their own software may find ephemeris data files (in SPK format) and a software toolkit needed to read those files from JPL's NAIF web-site. SPK files for planetary and planetary satellite ephemerides are available from NAIF. Comet and asteroid SPK files may be generated using our HORIZONS system. Visit the HORIZONS web site for more details on accessing data via the telnet, email, or web-interface.

  • Physical Data & Dynamical Constants

Selected physical characteristics of planets, planetary satellites, comets and asteroids are available. In addition, there is a table representing astrodynamic constants (both defining and derived) useful in the field of solar system dynamics. Radar astrometry for selected solar system bodies is also provided.

  • Planets

Selected physical parameters for the planets such as mass, mean radius, density, absolute magnitude, geometric albedo, surface gravity, and escape velocity are available in a table with references.

  • Planetary Satellites

Selected physical parameters such as GM, mean radius, mean density, magnitude, and geometric albedo are available for some planetary satellites. These parameters are presented in a table with references. In addition, there is a JPL lunar constants and models technical document available in PDF format.

  • Comets and Asteroids

Physical parameters for asteroids are not well known primarily because these bodies are so small and there are so many. The only parameter determined for nearly all asteroids is the absolute magnitude (H) which can be used to obtain a very rough estimate of an asteroid's size. However, a few asteroids have other parameters determined including rotation period, geometric albedo, colors (B-V, U-B, I-R), spectral taxonomic type, mass (GM), and bulk density.