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We present a study of comet C/2017 K2 (PANSTARRS) using prediscovery archivaldata taken from 2013 to 2017. Our measurements show that the comet has beenmarginally increasing in activity since at least 2013 May (heliocentricdistance of $r_{\mathrm{H}} = 23.7$ AU pre-perihelion). We estimate themass-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 minimumactive surface area of $\sim$10--10$^2$ km$^{2}$ for sublimation ofsupervolatiles 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 afew kilometers. Our Monte Carlo dust simulations show that dust grains in thecoma are $\gtrsim0.5$ mm in radius, with ejection speeds from $\sim$1--3 ms$^{-1}$, and have been emitted in a protracted manner since 2013, confirmingestimates by Jewitt et al. (2017). The current heliocentric orbit ishyperbolic. Our N-body backward dynamical integration of the orbit suggeststhat the comet is most likely (with a probability of $\sim$98\%) from the Oortspike. The calculated median reciprocal of the semimajor axis 1 Myr ago was$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).

Prediscovery Observations and Orbit of Comet C/2017 K2 (PANSTARRS)