  We present observations showing in-bound long-period comet C/2017 K2
(PANSTARRS) to be active at record heliocentric distance. Nucleus temperatures
are too low (60 K to 70 K) either for water ice to sublimate or for amorphous
ice to crystallize, requiring another source for the observed activity. Using
the Hubble Space Telescope we find a sharply-bounded, circularly symmetric dust
coma 10$^5$ km in radius, with a total scattering cross section of $\sim$10$^5$
km$^2$. The coma has a logarithmic surface brightness gradient -1 over much of
its surface, indicating sustained, steady-state dust production. A lack of
clear evidence for the action of solar radiation pressure suggests that the
dust particles are large, with a mean size $\gtrsim$ 0.1 mm. Using a coma
convolution model, we find a limit to the apparent magnitude of the nucleus $V
>$ 25.2 (absolute magnitude $H >$ 12.9). With assumed geometric albedo $p_V$ =
0.04, the limit to the nucleus circular equivalent radius is $<$ 9 km.
Pre-discovery observations from 2013 show that the comet was also active at
23.7 AU heliocentric distance. While neither water ice sublimation nor
exothermic crystallization can account for the observed distant activity, the
measured properties are consistent with activity driven by sublimating
supervolatile ices such as CO$_2$, CO, O$_2$ and N$_2$. Survival of
supervolatiles at the nucleus surface is likely a result of the comet's recent
arrival from the frigid Oort cloud.
