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Distant long-period comet C/2017 K2 has been outside the planetary region ofthe solar system for 3 Myr, negating the possibility that heat retained fromthe previous perihelion could be responsible for its activity. This inboundcomet is also too cold for water ice to sublimate and too cold for amorphouswater ice, if present, to crystallize. C/2017 K2 thus presents an ideal targetin which to investigate the mechanisms responsible for activity in distantcomets. We have used Hubble Space Telescope to study the comet in thepre-perihelion distance range 13.8 to 15.9 AU. The coma maintains a logarithmicsurface brightness gradient $m = -1.010\pm$0.004, consistent with steady-statemass loss. The absence of a radiation pressure swept tail indicates that theeffective particle size is large (0.1 mm) and the mass loss rate is $\sim$200kg s$^{-1}$, remarkable for a comet still beyond the orbit of Saturn.Extrapolation of the photometry indicates that activity began in 2012.1, at25.9$\pm$0.9 AU, where the blackbody temperature is only 55 K. This largedistance and low temperature suggest that cometary activity is driven by thesublimation of a super-volatile ice (e.g.~CO), presumably preserved by K2'slong-term residence in the Oort cloud. The mass loss rate can be sustained byCO sublimation from an area $\lesssim 2$ km$^2$, if located near the hotsub-solar point on the nucleus. However, while the drag force from sublimatedCO is sufficient to lift millimeter sized particles against the gravity of thecometary nucleus, it is 10$^2$ to 10$^3$ times too small to eject theseparticles against inter-particle cohesion. Our observations thus require eithera new understanding of the physics of inter-particle cohesion or theintroduction of another mechanism to drive distant cometary mass loss. Wesuggest thermal fracture and electrostatic supercharging in this context.

Distant Comet C/2017 K2 and the Cohesion Bottleneck