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Jupiter-family comet (JFC) P/2021 HS (PANSTARRS) only exhibits a coma withina few weeks of its perihelion passage at 0.8~au, which is atypical for a comet.Here we present an investigation into the underlying cause using serendipitoussurvey detections as well as targeted observations. We find that the detectionof the activity is caused by an extremely faint coma being enhanced by forwardscattering effect due to the comet reaching a phase angle of $\sim140^\circ$.The coma morphology is consistent with sustained, sublimation-driven activityproduced by a small active area, $\sim700~\mathrm{m^2}$, one of the smallestvalues ever measured on a comet. The phase function of the nucleus shows aphase coefficient of $0.035\pm0.002~\mathrm{mag/deg}$, implying an absolutemagnitude of $H=18.31\pm0.04$ and a phase slope of $G=-0.13$, with colorconsistent with typical JFC nuclei. Thermal observations suggest a nucleusdiameter of 0.6--1.1~km, implying an optical albedo of 0.04--0.23 which ishigher than typical cometary nuclei. An unsuccessful search for dust trail andmeteor activity confirms minimal dust deposit along the orbit, totaling$\lesssim10^8$~kg. As P/2021 HS is dynamically unstable, similar to typicalJFCs, we speculate that it has an origin in the trans-Neptunian region, andthat its extreme depletion of volatiles is caused by a large number of previouspassages to the inner Solar System. The dramatic discovery of the cometarynature of P/2021 HS highlights the challenges of detecting comets withextremely low activity levels. Observations at high phase angle where forwardscattering is pronounced will help identify such comets.

Comet P/2021 HS (PANSTARRS) and the Challenge of Detecting Low-Activity Comets