Pre-discovery Observations and Orbit of Comet C/2017 K2 (PANSTARRS)

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 pre-perihelion). We estimate the mass-loss rate during the period 2013–2017 as 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 . 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 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 au−1 (in a reference system centered on the solar-system barycenter).