Photocenter Correction

Optical comet astrometry can measure a center of light that is not the comet center of mass.¹ In DiffOrb, the propagated state still represents the center of mass. A photocenter correction changes only the modeled optical direction before right ascension and declination are formed.

DiffOrb uses a scalar global form of the S0 offset model used in comet orbit fitting:²¹

offset_distance = S0 / r_h**2

S0 is in km. r_h is the heliocentric distance in au. A positive S0 moves the modeled optical point away from the Sun along the Sun-comet direction. JPL SBDB also exposes S0 as an orbit model parameter field for small-body solutions.³

The correction applies to optical right ascension and declination. It does not change the propagated orbit, force model, radar delay, or radar Doppler prediction. S0 is therefore an optical observation-model parameter, not a dynamical acceleration parameter.

Read Next

• Read Light-Time Model for the optical one-way light-time model used before the angular prediction is formed.
• Read Differential Correction for how observation-model parameters can be estimated together with the orbit.
• Use Estimate A Comet Photocenter Offset In Differential Correction for the concrete S0 fitting path.

References

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1. Farnocchia, D., Bellerose, J., Bhaskaran, S., Micheli, M., & Weryk, R. (2021). High-fidelity comet 67P ephemeris and predictions based on Rosetta data. Icarus, 358, 114276. https://doi.org/10.1016/j.icarus.2020.114276 ↩↩

2. Yeomans, D. K. (1994). A review of comets and nongravitational forces. In A. Milani, M. di Martino, & A. Cellino (eds.), Asteroids, Comets, Meteors 1993, IAU Symposium, Vol. 160, 241-254. https://doi.org/10.1017/S007418090004657X ↩

3. NASA/JPL Solar System Dynamics. SBDB API documents orbit model_pars; SBDB Query API lists S0 and S0_sigma as query fields. https://ssd-api.jpl.nasa.gov/doc/sbdb.html and https://ssd-api.jpl.nasa.gov/doc/sbdb_query.html ↩