The Nucleus and Dust Tail of Active Asteroid (248370) 2005 QN173

We report new observations and analyses of archival data of the newly discovered active asteroid (248370) 2005 QN₁₃₇. This object was found to have a long, straight comet-like dust tail by the Asteroid Terrestrial-Impact Last Alert System (ATLAS) survey on UT 2021 July 7, yet has orbital elements (a=3.067 au, e=0.226, i=0.067 degrees) that place it unambiguously in the outer main asteroid belt, and therefore among the class of objects known as active asteroids. Using the Canadian Astronomy Data Centre's Solar System Object Image Search tool and the Mikulski Archive for Space Telescopes, we have identified several archival snapshot observations of the object obtained in 2010 and 2011 by the Pan-STARRS1 survey in g'-, r'-, i'-, and z'-band (including some obtained at orbital positions close to the object's perihelion passage on UT 2010 August 24). The object has a stellar surface brightness profile and appears inactive in each of these images, where absolute magnitudes (assuming G=0.15) estimated from photometric measurements of the object in those data (mean values of H_g=16.47±0.03 mag, H_r=15.98±0.03 mag, H_i=16.02±0.02 mag, H_z=15.93±0.09 mag) are consistent with each other over the period of observations, and therefore with inactivity during each observation. Following the discovery of activity, follow-up observations using the Lowell Discovery Telescope (LDT) and Palomar Hale Telescope show that the near-nucleus region of the comet is approximately 1.3 mag brighter in g', r', and i' (as measured using 4"-radius photometry apertures) than expected from our absolute magnitude estimates, despite the nucleus having a nearly stellar surface brightness profile as measured perpendicularly to the dust tail. The dust tail is seen in the LDT and Palomar data to extend >9 arcmin from the nucleus along the object's orbit plane as projected in the sky. We will also report results from additional upcoming LDT and Palomar observations, as well as ongoing monitoring observations conducted as part of the Las Cumbres Observatory (LCO) Outbursting Objects Key (LOOK) Project (KEY2020B-009) and the Faulkes Telescope Project's Comet Chasers school outreach program (FTPEPO2014A-004). This work is supported by NASA SSO grant 80NSSC19K0869.