Planet Centers SPK Generation #

LibEphemeris generates a compact SPK (SPICE kernel) file containing planet center positions for the outer planets, correcting for the barycenter-to-center offset inherent in JPL Development Ephemerides.

Table of Contents #

• Background
  • Barycenters vs Planet Centers
  • Barycenter Offsets
• Method
  • Analytical COB Correction (Current Approach)
  • Compact SPK Extraction (New Approach)
  • SPK File Format
  • Chaining with DE Ephemerides
• SPK Generation Process
  • Prerequisites
  • Source Files
  • Running the Script
  • Output
• Usage in LibEphemeris
• Precision and Validation
• Troubleshooting
• References
• Changelog

Background #

Barycenters vs Planet Centers #

JPL’s Development Ephemerides (DE440, DE441, etc.) provide positions for planetary system barycenters rather than planet centers for the outer planets. This is because these planets have significant moon systems whose combined mass affects the barycenter location.

Barycenter Offsets #

The barycenter can be significantly offset from the planet center:

For high-precision work, these offsets matter.

Method #

LibEphemeris employs a three-tier correction strategy to convert barycenter positions to planet center positions.

Analytical COB Correction (Current Approach) #

The current approach uses analytical moon theories to compute the Center of Body (COB) offset from the barycenter:

• Jupiter: E5 theory (Galilean moons)
• Saturn: TASS 1.7 (Titan-dominated)
• Neptune: Triton Keplerian elements
• Pluto: Charon two-body solution
• Uranus: NOT IMPLEMENTED (returns 0)

This approach requires no extra files but has limited precision (~0.02-0.15 arcsec).

Compact SPK Extraction (New Approach) #

The new approach extracts planet center segments from JPL’s satellite SPK files and creates a compact file containing only the planet centers. This provides maximum precision (<0.001 arcsec) with minimal file size (~5-10 MB).

SPK File Format #

The output file uses SPK Type 2 (Chebyshev polynomial) format, the same format used in DE ephemerides and satellite SPKs. This format stores polynomial coefficients that can be evaluated at any time within the segment’s coverage.

Type 2 SPK structure per segment:

• Chebyshev polynomial coefficients for X, Y, Z position
• Polynomial degree (typically 7-15)
• Record interval (typically 1-8 days)
• Time range (typically 1900-2100)

Chaining with DE Ephemerides #

Skyfield automatically chains SPK segments. When both DE440 and the planet centers SPK are loaded, Skyfield computes:

Earth -> SSB -> Jupiter Barycenter (from DE440)
                     |
              Jupiter Center (from planet_centers.bsp)

This chaining is transparent to the user. All segments use the J2000 reference frame (ICRF), consistent with DE ephemerides.

SPK Generation Process #

Prerequisites #

1. spiceypy >= 6.0.0 (Python wrapper for NAIF SPICE toolkit)
2. Internet connection (to download source SPK files)
3. ~500 MB temporary disk space (source files are deleted after extraction)

Source Files #

The script downloads these satellite SPK files from JPL NAIF:

These are older, more compact versions of the satellite ephemerides. Newer versions (jup365.bsp, etc.) are much larger (500 MB-1 GB each) but offer higher precision for satellite positions, which is not needed for planet centers.

Running the Script #

# Using leph CLI (recommended)
leph generate planet-centers-medium

# Other tiers
leph generate planet-centers-base
leph generate planet-centers-extended
leph generate planet-centers-all       # All three tiers

Output #

The script generates:

libephemeris/data/planet_centers.bsp

This file contains 5 segments:

Usage in LibEphemeris #

After generating the SPK file, LibEphemeris automatically loads it alongside the main ephemeris (DE440). The planet center positions are used instead of barycenter + COB correction.

Automatic Loading #

import libephemeris as eph

# planet_centers.bsp is loaded automatically if present
# Jupiter now returns planet center, not barycenter
pos, _ = eph.swe_calc_ut(jd, eph.SE_JUPITER, 0)

Verifying Planet Centers #

from skyfield.api import load

# Load both ephemerides
planets = load('de440.bsp')
centers = load('planet_centers.bsp')

# Get Jupiter center via chaining
ts = load.timescale()
t = ts.utc(2025, 1, 1)

earth = planets['earth']
jupiter_bary = planets['jupiter barycenter']
jupiter_center = centers['jupiter']

# Position of Jupiter center from Earth
astrometric = earth.at(t).observe(jupiter_bary + jupiter_center)
ra, dec, distance = astrometric.radec()

Precision and Validation #

Time Coverage #

The coverage depends on the source SPK files, typically:

Position Accuracy #

The extracted segments maintain the full precision of the source files:

• Position accuracy: sub-kilometer
• From Earth: sub-arcsecond (typically <0.001 arcsec)

Troubleshooting #

SSL Certificate Errors #

If SSL errors occur when downloading from JPL:

ssl.SSLError: certificate verify failed

The script automatically falls back to unverified SSL for the trusted JPL servers. If this fails, the source files can be downloaded manually.

spiceypy Not Found #

ModuleNotFoundError: No module named 'spiceypy'

Install spiceypy:

pip install "spiceypy>=6.0.0"
# or
uv pip install "spiceypy>=6.0.0"

Disk Space #

The script requires ~500 MB temporary space for downloading source files. These are automatically cleaned up after extraction.

References #

• JPL NAIF Generic Kernels
• SPK Required Reading
• Skyfield SPK Support
• SPICE Toolkit

Changelog #

• 2024: Initial implementation
  • Created generate_planet_centers_spk.py script
  • Added CLI command (leph generate planet-centers-medium)