  We present a study of comet C/2017 K2 (PANSTARRS) using prediscovery archival
data taken from 2013 to 2017. Our measurements show that the comet has been
marginally increasing in activity since at least 2013 May (heliocentric
distance of $r_{\mathrm{H}} = 23.7$ AU pre-perihelion). We estimate the
mass-loss rate during the period 2013--2017 as $\overline{\dot{M}} \approx
\left(2.4 \pm 1.1 \right) \times 10^{2}$ kg s$^{-1}$, which requires a minimum
active surface area of $\sim$10--10$^2$ km$^{2}$ for sublimation of
supervolatiles such as CO and CO$_2$, by assuming a nominal cometary albedo
$p_V = 0.04 \pm 0.02$. The corresponding lower limit to the nucleus radius is a
few kilometers. Our Monte Carlo dust simulations show that dust grains in the
coma are $\gtrsim0.5$ mm in radius, with ejection speeds from $\sim$1--3 m
s$^{-1}$, and have been emitted in a protracted manner since 2013, confirming
estimates by Jewitt et al. (2017). The current heliocentric orbit is
hyperbolic. Our N-body backward dynamical integration of the orbit suggests
that the comet is most likely (with a probability of $\sim$98\%) from the Oort
spike. The calculated median reciprocal of the semimajor axis 1 Myr ago was
$a_{\mathrm{med}}^{-1} = \left( 3.61 \pm 1.71 \right) \times 10^{-5}$ AU$^{-1}$
(in a reference system of the solar-system barycentre).
