  Distant long-period comet C/2017 K2 has been outside the planetary region of
the solar system for 3 Myr, negating the possibility that heat retained from
the previous perihelion could be responsible for its activity. This inbound
comet is also too cold for water ice to sublimate and too cold for amorphous
water ice, if present, to crystallize. C/2017 K2 thus presents an ideal target
in which to investigate the mechanisms responsible for activity in distant
comets. We have used Hubble Space Telescope to study the comet in the
pre-perihelion distance range 13.8 to 15.9 AU. The coma maintains a logarithmic
surface brightness gradient $m = -1.010\pm$0.004, consistent with steady-state
mass loss. The absence of a radiation pressure swept tail indicates that the
effective particle size is large (0.1 mm) and the mass loss rate is $\sim$200
kg s$^{-1}$, remarkable for a comet still beyond the orbit of Saturn.
Extrapolation of the photometry indicates that activity began in 2012.1, at
25.9$\pm$0.9 AU, where the blackbody temperature is only 55 K. This large
distance and low temperature suggest that cometary activity is driven by the
sublimation of a super-volatile ice (e.g.~CO), presumably preserved by K2's
long-term residence in the Oort cloud. The mass loss rate can be sustained by
CO sublimation from an area $\lesssim 2$ km$^2$, if located near the hot
sub-solar point on the nucleus. However, while the drag force from sublimated
CO is sufficient to lift millimeter sized particles against the gravity of the
cometary nucleus, it is 10$^2$ to 10$^3$ times too small to eject these
particles against inter-particle cohesion. Our observations thus require either
a new understanding of the physics of inter-particle cohesion or the
introduction of another mechanism to drive distant cometary mass loss. We
suggest thermal fracture and electrostatic supercharging in this context.
