The Case for Activity on and in Gateway Objects — Analytical and Modeling Considerations

Gateway objects are small solar system bodies transitioning from Centaur-space in the outer solar system, beyond Jupiter’s orbit, to the active-comet space within the inner solar system (Sarid et al. 2019). The Gateway is a dynamical designation, but this marked transition may have important thermo-physical consequences as well. It can drive short-term evolution and significant processing of volatile material on the surface and in the interior of any given object.

Currently observed inhabitants of the Gateway region are: 29P/ SW1, P/2010 TO20, P/2008 CL94, 2016 LN8, and 2020 MK4. P/2019 LD2 is among another small group of objects suspected to be very recent occupants of the Gateway as well (Steckloff et al. 2020). Some of the Gateway objects seem quiescent, while other have observed cometary activity, as a sustained dust coma with conjectured volatile outgassing. LD2’s activity has been persistent for at least 5 years (discovery in 2019 and precovery observations from 2017). SW1 has been consistently observed to be active for nearly a century (since discovery in 1927), with repeating periods of large outburst and low-level activity, some of which may be related to sublimation of water ice and more volatile species (such as CO and CO2, Womack et al. 2017).

The residence time of objects entering the Gateway is on the order of 100-1000 years, which means these objects go through 10s to a few 100s of orbits. While it is a short timescale, the thermal environment is rather unique. The heliocentric distances are such that they straddle the regimes of water ice sublimation, vigorous crystallization of amorphous water ice, and potential activation of deeper buried pockets of other volatile ices. Nuclei shapes and spins, surface topography, and outgassing patterns can also evolve on timescales comparable to a few orbits, depending on heliocentric distance and nucleus physical parameters. As such, Gateway objects may exhibit a wide variety of activity patterns, durations, and drivers, also including rapid evolution of surface features and internal composition. Once out of the Gateway, either as a Jupiter-family comet or as an outer solar system distant Centaur, that short-term physical and chemical evolution determines each body’s subsequent evolution, and because of that active evolution – Our ability to observe and characterize that comet or Centaur.

We discuss here the conditions and varied ways Gateway objects can experience activity ramp up, sustained gas and dust coma, and subsurface processing. We also discuss the case for observing such features, with current and future observing facilities.