A Remarkable Inbound Long-Period Comet at Record Heliocentric Distances

C/2014 UN271 (Bernardinelli-Bernstein) is a recently discovered long-period comet that was observed at unprecedentedly large inbound distances from the Sun (~29 au). Here we seek one HST mid-cycle orbit using the WFC3 camera to observe the remarkable comet at heliocentric distance ~20 au. Our primary scientific objective is to determine the nucleus size by photometrically isolating the nucleus signal from the surrounding coma. The proposed HST program will enable us to study an inbound comet at a rarely-seen distance from the Sun in good detail.