Motivation
The positions, velocities, and other attributes of physical objects in our solar system are tracked by professionals around the world. NASA maintains two sources for solar system ephemeris data: the Horizons platform, and Generic SPICE Kernels updated (roughly) daily.
NASA provides generic SPICE kernels with the most common solar system bodies. These kernels, which are available for download at https://naif.jpl.nasa.gov/pub/naif/, allow you to use the SPICE Toolkit to query solar system ephemeris data locally in your kernel pool. You can fetch position and velocity data for spacecraft and solar system bodies, get shapes and mass parameters for celestial bodies, convert to and from different coordinate frames, and more. But how do you know which kernel to download? And how can you reliably access each kernel, and ensure your colleagues are downloading the same kernel?
NASA also provides ephemeris through the Horizons ephemeris platform's REST API. You can query for information about thousands of celestial bodies in our solar system, but the REST interface may be unfamiliar, and the query responses require custom parsing for numerical applications.
The EphemerisSources.jl super-package allows users to idiomatically fetch ephemeris data, and parse the results for Cartesian state vector information. The ability to parse the ephemeris data in other formats, including observer tables and osculating orbital elements, is not yet implemented. Pull requests are welcome!
JPL Horizons
Horizons allows for querying ephemeris for specific bodies at specific time points through a REST interface, a graphical web interface, a telnet command-line interface, and an email interface. Students commonly use the graphical web interface, but programmatically fetching and processing ephemeris data is useful for more formal and replicable analysis. The open-source packages below allow for programmatic and interactive ephemeris fetching from the JPL Horizons ephemeris platform. Packages marked external are not affiliated with EphemerisSources.jl.
| Package | Description | Location |
|---|---|---|
HorizonsAPI.jl | A precise JPL Horizons REST API client implementation, with keyword arguments for each acceptable parameter. | |
HorizonsEphemeris.jl | Convenience wrappers around the JPL Horizons REST API. | |
Horizons.jl | Functions for spawning the telnet interface, and querying files. | external |
SPICE Toolkit
SPICE data is packaged in binary files referred to as kernels. Kernel data can be parsed and processed with JPL's CSPICE library. Users of the Julia Programming Language can call CSPICE routines from within Julia with SPICE.jl.
| Package | Description | Location |
|---|---|---|
SPICEKernels.jl | All generic kernels exported as variable constants. | |
SPICEBodies.jl | Idiomatic wrappers around SPICE and SPICE.jl methods. | |
SPICE.jl | A Julia interface to the CSPICE library provided by NASA JPL. | external |
Ephemerides.jl | Ephemeris kernel reading and interpolation in pure Julia. | external |
Getting Started
These two ephemeris platforms — Horizons and SPICE — are both free to use, and have been incredibly helpful to students and researchers around the world. The Julia packages above — and others — available to help you fetch and parse ephemeris data. For more information on how to use these packages, continue on to the documentation examples.