Two Hyperbolic Baldheads in the Solar System: 2017 U7 and 2018 C2

We present a study of two newly discovered heliocentric hyperbolic objects-2017 U7 and 2018 C2. Both are possibly thermally evolved comets. Observations of the latter in 2018 March from Xingming Observatory revealed that it has a color similar to those of the long-period comets, Trojans, and D-type asteroids: mB − mV = 0.75 0.03, mV − mR = 0.41 0.02, and mR − mI = 0.37 0.03. A possible extremely faint coma of ∼9″ across was observed. The radial profile of 2018 C2 in comparison with those of the field stars helps confirm its cometary nature. Based on our convolution model, its mass-loss rate is estimated to be 0.7 0.2 kg s−1. Assuming geometric albedo pR = 0.04, its effective radius is 4.4 0.5 km, which means that the fraction of active area is merely ∼10−5-10−4. Our N-body dynamical simulations show that both objects are most likely dynamically old members from the Oort cloud. 2017 U7 has a ∼60% chance to escape the solar system in the 1.5 Myr following its current perihelion passage due to a moderate close encounter with Jupiter in 2020 May. On the condition that no disintegration occurs, 2018 C2 will revisit the inner solar system 0.13-0.14 Myr later, with perihelion distance ∼2 au.