ASSIST: An ephemeris-quality test particle integrator

We introduce ASSIST, a software package for ephemeris-quality integrations of test particles. ASSIST is an extension of the REBOUND framework (Rein & Liu 2012) and makes use of its IAS15 integrator (Rein & Spiegel 2015) to integrate test particle trajectories in the field of the Sun, Moon, planets, and 16 massive asteroids, with the positions of the masses coming from the JPL DE441 ephemeris. The package incorporates the most significant higher order gravitational effects, namely the Sun’s J2 zonal gravitational harmonic, Earth’s J2, J3, J4 zonal harmonics, and relativistic corrections using a choice of either the Einstein-Enfield-Hoffman PPN or Damour & Deruelle (1985) approximations. It also accounts for position- and velocity-dependent non-gravitational effects according to the Marsden et al. (1973), although other parameterizations of non-gravitational forces can be easily included. All components in the equations of motion have been verified to machine precision in a term-by-term comparison with output from JPL’s small-body integrator. The analytic first order variational equations are included for all terms and have been verified numerically. This new framework is intended to facilitate the study of short-term solar system dynamics, with the goals of accurately fitting orbits, predicting close Earth approaches and potential impacts of near-Earth objects, guiding spacecraft to solar system targets, and performing detailed tests of gravity. ASSIST is open source and is freely distributed under the GNU General Public license (version 3). This work was supported by grants from the Smithsonian Scholarly Studies Program and NASA.