Reactivation of 2017 QN84, a Jupiter-Family Comet Discovered through Citizen Science

We report the discovery of recurrent cometary activity from minor planet 2017 QN₈₄. Cometary activity on 2017 QN₈₄ was first identified by volunteers of our NASA Partner Citizen Science program, Active Asteroids, hosted on the Zooniverse platform. In the project, volunteers examine archival images of minor planets, searching for cometary activity like comae and tails. Project volunteers identified activity on 2017 QN₈₄ in one Dark Energy Camera (DECam) image from UT 2017 December 23 (Prop. ID 2017B-0307, PI: Sheppard) in the form of a coma and tail (Figure 1). This represents the Active Asteroids project’s first activity discovery by participants, and was originally reported by volunteers on the project's chat forum (Chandler 2022; Chandler et al. 2024)

Further archival searches of images of 2017 QN₈₄ did not discover additional evidence of activity. However, images of the field directly behind 2017 QN₈₄ in the DECam image did not reveal any background source that could have caused 2017 QN₈₄’s apparent coma and tail. This suggests that the UT 2017 December 23 image shows true cometary activity on the object, not an image artifact. With only one known epoch of activity, the underlying cause of the outburst (e.g., sublimation, rotational distribution, impact) could not be constrained. Although, the observed activity was present at heliocentric distance r_H = 2.62 au and true anomaly angle f = 38°, consistent with sublimation near perihelion.

As part of our Active Asteroids campaign, we routinely perform follow-up observations of project discoveries at various observatories, including the Astrophysical Research Consortium (ARC) 3.5-meter telescope at Apache Point Observatory (New Mexico, USA) and the Lowell Discovery Telescope (LDT) at Lowell Observatory (Arizona, USA). We observed 2017 QN₈₄ as it approached its December 2024 perihelion passage. On UT 2024 July 14, we detected a short tail emanating from 2017 QN₈₄ in images we acquired with the ARC 3.5-m (Figure 1; observers C. Chandler, M. Frissell) when 2017 QN₈₄ was at f = 312° (Figure 2). We continued to monitor activity through perihelion passage with the ARC 3.5-m and LDT (Figure 3; observer K. Farrell). 2017 QN₈₄ continued to display cometary activity as of UT 2025 April 10, at f = 36°.

We have observed two epochs of cometary activity near perihelion passage from small body 2017 QN₈₄ through the Active Asteroids project and follow-up observations. This suggests that the activity is caused by volatile sublimation. With a Tisserand parameter with respect to Jupiter T_J = 2.943 in the prescribed 2 < T_J < 3 range, we classify 2017 QN₈₄ as a Jupiter-family comet (JFC). JFCs are a dynamically unstable class of comets that originate from the outer solar system and are disturbed inward by interactions with the giant planets. Thus, they hold clues to the current distribution of volatiles in the solar system and how those volatiles are transported into the inner solar system.

Figure 1. This is the DECam image from UT 2017 December 23 (Prop. ID 2017B-0307, PI: Sheppard) where volunteers identified activity. At the time of the exposure, 2017 QN₈₄ was at f = 38°. The yellow and red markers indicate the anti-solar and anti-motion directions, respectively.

Figure 2. The ARC 3.5-m telescope image of  2017 QN₈₄ from UT 2024 July 14 (observers C. Chandler, M. Frissell), where we discovered recurrent activity. In this image 2017 QN₈₄ is at true anomaly angle f = 320°. The yellow and red markers indicate the anti-solar and anti-motion directions, respectively.

Figure 3. An LDT image of 2017 QN₈₄ from UT 2024 October 31 (observer K. Farrell) from our activity monitoring program. At the time of the exposure, 2017 QN₈₄ was at f = 350°. The yellow and red markers indicate the anti-solar and anti-motion directions, respectively.