2018AJ....155...25H

We present a study of comet C/2017 K2 (PANSTARRS) using pre-discovery 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_H=23.7 au pre-perihelion). We estimate the mass-loss rate during the period 2013-2017 as ~(2.4±1.1)×10² kg s^–1, which requires a minimum active surface area of ∼10-10² km² for sublimation of supervolatiles such as CO and CO₂, by assuming a nominal cometary albedo p_V=0.04±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 0.5 mm in radius, with ejection speeds from ∼1 to 3 m s^–1, and have been emitted in a protracted manner since 2013, confirming estimates by Jewitt et al. 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 ∼98%) from the Oort spike. The calculated median reciprocal of the semimajor axis 1 Myr ago was a_med^–1=(3.61±1.71)×10^–5 au^–1 (in a reference system centered on the solar-system barycenter).