  Comet C/2017 K2 (PANSTARRS) was discovered by the Pan-STARRS1 (PS1) Survey on
2017 May 21 at a distance 16.09 au from the Sun, the second most distant
discovery of an active comet. Pre-discovery images in the PS1 archive back to
2014 and additional deep CFHT images between 2013 May 10-13 showed the comet to
be active at 23.75 au. We derive an upper limit to the nucleus radius of
$R_N$=80 km, assuming a 4\% albedo. The spectral reflectivity of the comet
surface is similar to "fresh" regions seen on comet 67P/Churyumov-Gerasimenko
using the $Rosetta$ OSIRIS camera. Pre-discovery photometry combined with new
data obtained with Megacam on the CFHT show that the activity is consistent
with CO-ice sublimation and inconsistent with CO$_2$-ice sublimation. The ice
sublimation models were run out to perihelion in 2022 at 1.8 au to predict the
CO production rates, assuming that the outgassing area does not change.
Assuming a canonical 4\% active surface area for water-ice sublimation, we
present production rate ratios, $Q_{\rm CO}$/$Q_{\rm H2O}$, for a range of
nucleus sizes. Comparing these results with other CO-rich comets we derive a
lower limit to the nucleus radius of $\sim$14 km. We present predictions for
$Q_{\rm CO}$ at a range of distances that will be useful for planning
observations with JWST and large ground-based facilities.
