Background
The Oort Cloud is the largest reservoir of comets in our solar system, both in terms of population and physical size. These comets orbit at average distances of 1,000-100,000 au (one au is the Earth’s distance from the Sun). The first step in the formation of the Oort Cloud involved gravitational scattering of small bodies by the newborn, migrating giant planets into highly eccentric orbits. The number of comets and their orbital tilts at different distances from the Sun hold some memory of the original configuration of the planets, and so can shed light on where the planets formed. Galactic tidal forces and passing stars place some comets on elongated orbits that enter the planetary region, making them observable as long-period comets (LPCs). Determining accurate “original” orbits for LPCs before the planets perturb them is challenging because LPCs are so weakly bound to the Sun. For many comets, nongravitational (NG) recoil forces from the comet’s release of gas and dust must be modeled to achieve acceptable orbital fits. Indeed, some comets erroneously appear to be arriving from interstellar space if NG forces are neglected. NG effects have recently been detected in comets beyond Saturn’s orbit. The standard model for NG forces is inadequate for comets active that far from the Sun, as it predicts an abrupt cutoff beyond a certain distance at odds with observations. We are developing a code to fit the orbits of LPCs for a variety of models of how NG forces vary with distance from the Sun. We are determining the original orbits of hundreds of long-period comets and the uncertainties in their orbital elements and NG parameters. This work is timely because the Legacy Survey of Space and Time of the Vera Rubin Observatory in Chile is expected, beginning in 2025, to discover many more LPCs than the entire known population.
Approach
We are building a toolkit to determine the original orbits of long-period comets. This requires: (1) the collection, analysis, and weighting of observational data, (2) the development of a new NG model for comets active far from the Sun, and (3) fitting the comets’ orbits. Each fit involves an orbital integration that accounts for the gravitational attraction of the Sun and planets, as well as NG forces, on each comet.
Accomplishments
We identified 370 LPCs with at least 100 sky positions taken over a span of a year or more and obtained the data from the Minor Planet Center (MPC). We have compared all modern orbital determinations for these comets, including purely gravitational and NG fits. We have performed our own fits on some of these comets, using two different codes, OrbFit and Find_Orb. We are modifying OrbFit to incorporate new models of NG forces.