C/2018 F4 (PanSTARRS)

Lore & Background

C/2018 F4 was discovered on 17 March 2018 when it was 6.4 AU from the Sun, beyond Jupiter's orbit. At that time it appeared asteroidal, with no cometary activity. As it approached perihelion inside Jupiter's orbit, activity began, and in April 2018 it was reclassified as a hyperbolic comet. The observation arc of 35 days suggests that before entering the planetary region (epoch 1950), its orbital period was on the order of a hundred thousand years. The heliocentric eccentricity became greater than 1 in November 2016, when the comet was 9.6 AU from the Sun. The comet fragmented around August 2020.

Reader's Guide

C/2018 F4 is notable for its hyperbolic orbit and the uncertainty surrounding its classification. The orbital eccentricity of 1.0038±0.0072 is decently constrained, but a parabolic orbit could also fit the data, as was common for short-arc comets. Its velocity uncertainty of ±0.2 km/s and an inbound velocity at 200 AU of roughly 3 km/s—matching the escape velocity of 2.98 km/s—strongly indicate an Oort cloud origin rather than interstellar. The short observation arc of 35 days limits precise orbital determination, and further observations are needed to confirm its hyperbolic nature. Its fragmentation in August 2020 marks the end of its observable activity. The comet's legacy lies in demonstrating how early asteroidal appearances can mask cometary behavior, and in reinforcing the distinction between Oort cloud objects and interstellar visitors like ʻOumuamua.

Did You Know?

• It was discovered on 17 March 2018 at a distance of 6.4 AU from the Sun, beyond Jupiter's orbit.
• Its incoming velocity at 200 AU from the Sun matches that of a typical Oort cloud object, not an interstellar one.