Presentation #409.03 in the session Planetary Defense! (iPosters).
Manx comets is a new class of comets with parabolic or near-parabolic orbits. Manx comets exhibit low or no activity, display little to no tail and are much fainter than normal long-period comets (LPC) of similar size due to very low volatile activity. The first to be characterized, C/2014 S3 (PANSTARRS), was discovered by Pan-STARRS and has a spectral reflectivity similar to inner solar system S-type asteroids, suggesting an asteroidal origin before being ejected into the Oort cloud, and eventually finding its way back into the inner solar system via the same mechanisms as LPCs . Earth impact velocities from Manx comets are similar to those from LPCs, and significantly faster than from asteroids but because of their low activity are much harder to detect.
Over 4000 synthetic Earth-impacting long-period parabolic orbits were simulated to show the evolution of brightness and motion of Manx comets as they approach (with their anomalous motion, rather than cometary nature, leading to discovery). Two, 3 and 4-detection tracklets were created for each night that the object is visible (V < 23.5) from the Pan-STARRS1 observatory. The tracklets were individually evaluated through the NEO Digest2 score  to determine warning timescales with our current tools [see figure]. The Digest2 score is considered to be a good indicator of the likelihood that an object is of a certain orbit class. Objects above the threshold score > 65 are considered to be NEO candidates.
The simulations demonstrate that Manx comets tend to be discovered 30 days before impact while they are only likely to be designated as NEOs 18 days before impact. The Manx population were generated with a size-frequency distribution (SFD) matching that of S-type asteroids – the current SFD for Manx comets is unknown though this work is in progress .
 Meech, K. et al. (2016). Inner solar system material discovered in the Oort cloud. Science Advances id e1600038
 Keys, S. et al. (2019). The Digest2 NEO Classification Code. PASP, 131, 1000, Pp. 064501
 Boe, B. et al. (2019). The orbit and size-frequency distribution of long period comets observed by Pan-STARRS1. Icarus, 333, 252–272