An interface to JPL HORIZONS system and other Solar System Dynamics APIs in Julia.

• Jorge A. Pérez Hernández, Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México (UNAM)
• Luis Eduardo Ramírez Montoya, Facultad de Ciencias, Universidad Nacional Autónoma de México (UNAM)

Comments, suggestions, and improvements are welcome and appreciated.

HORIZONS.jl is a registered Julia package and may be installed from the Julia REPL doing

] add HORIZONS

Current stable release is v0.4.1, which is compatible with Julia >= 1.6.

The horizons() function is a shortcut to the HORIZONS telnet interface prompt from the Julia REPL:

julia> using HORIZONS

julia> horizons() # get Horizons prompt from the Julia REPL
JPL Horizons, version 4.70
Type '?' for brief help, '?!' for details,
'-' for previous prompt, 'x' to exit
System news updated June 08, 2020

Horizons>

To run this function, the telnet command line utility should be locally installed and enabled.

HORIZONS.jl also has Julia functions for some of the scripts authored by Jon D. Giorgini for automated generation of small-body binary SPK files and tables. These scripts were originally written in expect, and can be found at the JPL's Solar System Dynamics group ftp server ftp://ssd.jpl.nasa.gov/pub/ssd/SCRIPTS/. Below, we describe the functions smb_spk, smb_spk_ele and vec_tbl.

The smb_spk function automates generation and downloading of Solar System small-bodies binary SPK files from HORIZONS:

using HORIZONS, Dates

# Generate a binary SPK file for asteroid 99942 Apophis covering from 2021 to 2029
local_file = smb_spk("DES = 20099942;", DateTime(2021,Jan,1), DateTime(2029,Apr,13))

isfile(local_file) # Check that the binary SPK file `local_file` exists

Binary SPK files (i.e., extension .bsp) can be read using e.g. SPICE.jl:

# ] add SPICE" # uncomment this line to add `SPICE.jl` to current environment
using SPICE, Dates
furnsh(local_file)
et = 86400*(datetime2julian(DateTime(2024,3,1)) - 2.451545e6)
pv = spkgeo(20099942, et, "J2000", 0)

HORIZONS.jl function smb_spk_ele generates .bsp binary SPK files for small-bodies from a set of osculating orbital elements at a given epoch:

using HORIZONS, Dates

epoch = 2449526.5 # Osculating elements epoch, in Barycentric Dynamical Time (TDB)
ec = 0.6570220840219289 # Orbital eccentricity
qr = 0.5559654280797371 # Perihelion distance
tp = 2449448.890787227 # Julian date of perihelion passage
om = 78.10766874391773 # Longitude of ascending node
w = 77.40198125423228 # Argument of perihelion
inc = 24.4225258251465 # Inclination

start_time = DateTime(2021,Jan,1)
stop_time = DateTime(2022,Jan,1)

# Generate a binary SPK file for asteroid 1990 MU at `epoch`
local_file = smb_spk_ele("1990 MU", start_time, stop_time, epoch, ec, qr, tp, om, w, inc)

isfile(local_file) # Check that the binary SPK was downloaded

HORIZONS.jl function vec_tbl allows the user to generate vector tables for designated objects and save the output into a file:

# Date variables for start and stop times
t_start = DateTime(2029,4,13)
t_stop = Date(2029,4,14)

# Step size (allowed types: Period, Int, String)
δt = Hour(1) # 1 hour step size

# Generate tables and save output to Apophis.txt in current directory:
vec_tbl("Apophis", t_start, t_stop, δt; FILENAME = "Apophis.txt", CENTER = "@ssb", REF_PLANE = "FRAME", OUT_UNITS = "AU-D", CSV_FORMAT = true, VEC_TABLE = 2)

Note that CENTER, REF_PLANE, etc., are keyword arguments. If they are omitted from the vec_tbl call, then they will take default values:

δt = 1 # Return only one step

# Generate tables with default values and save output to Apophis.txt in current directory:

vec_tbl("Apophis", t_start, t_stop, δt; FILENAME = "Apophis.txt")

More details about default values of keyword arguments are available in the vec_tbl docstrings.

If the output file is not specified, then vec_tbl returns the output as a String, which may be then used for further processing within Julia:

δt = "2 hours" # 2 hour step size

# Save into `apophisvt::String` the output from HORIZONS
apophisvt = vec_tbl("Apophis", t_start, t_stop, δt)

# Do stuff with `apophisvt` inside julia...

HORIZONS.jl function sbdb fetchs data for a specific small-body in JPL's Small-Body DataBase (SBDB) and returns the output as a Dict{String, Any}:

# Fetch data of asteroid 433 Eros
sbdb("sstr" => "Eros")

# Fetch data of asteroid 99942 Apophis, including close-approach information
sbdb("sstr" => "Apophis", "ca-data" => "true")

HORIZONS.jl function sbradar searches for radar astrometry of asteroids/commets and returns the output as a Dict{String, Any}:

# Search Apophis' radar astrometry
sbradar("spk" => "20099942")

# Add observer information
sbradar("spk" => "20099942", "observer" => "true")

HORIZONS.jl is licensed under the MIT "Expat" license.

This software package is not affiliated, associated, authorized, endorsed by, or in any way officially connected with NASA, JPL, or any of its subsidiaries or its affiliates.

JAPH is thankful to Dr. Jon Giorgini for his helpful comments and feedback towards the first release of this Julia interface to the HORIZONS system, and to Yuri D'Elia for their help with the telnet interface via Expect.jl. Special thanks to LERM (@LuEdRaMo) for the implementation of the JPL APIs via HTTP. The HORIZONS system itself is the work of several people at JPL:

• Design/implementation :
  • Jon Giorgini
  • Don Yeomans
• Cognizant Eng.:
  • Jon Giorgini
• Major body ephemerides:
  • William Folkner (Planetary ephemerides)
  • Bob Jacobson (Satellites)
  • Marina Brozovic (Satellites)
• Contributors:
  • Alan Chamberlin (web interface, database)
  • Paul Chodas (some subroutines)
  • The NAIF group (SPICELIB) (esp. Chuck Acton, Bill Taber, Nat Bachman)

• JPL Solar System Dynamics APIs
• HORIZONS documentation (HTML)
• Giorgini, J.D., Yeomans, D.K., Chamberlin, A.B., Chodas, P.W., Jacobson, R.A., Keesey, M.S., Lieske, J.H., Ostro, S.J., Standish, E.M., Wimberly, R.N., "JPL's On-Line Solar System Data Service", Bulletin of the American Astronomical Society, Vol 28, No. 3, p. 1158, 1996.