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Pairs and Groups of Genetically Related Long-Period Comets and Probable Identity of the Mysterious Lick Object of 1921

We present the history of investigation of the dynamical properties of pairs and groups of genetically related long-period comets (other than the Kreutz sungrazing system). Members of a comet pair or group move in nearly identical orbits and their origin as fragments of a common parent comet is unquestionable. The only variable is the time of perihelion passage, which differs from member to member considerably due primarily to an orbital-momentum increment acquired during breakup. Meter-per-second separation velocities account for gaps of years or tens of years, thanks to the orbital periods of many millennia. The physical properties of individual members may not at all be alike, as illustrated by the trio of C/1988 A1, C/1996 Q1, and C/2015 F3. We exploit orbital similarity to examine whether the celebrated and as yet unidentified object, discovered from the Lick Observatory near the Sun at sunset on 1921 August 7, happened to be a member of such a pair and to track down the long-period comet to which it could be genetically related. Our search shows that the Lick object, which could not be a Kreutz sungrazer, was most probably a companion to comet C/1847 C1 (Hind), whose perihelion distance was ~9 R_sun and true orbital period approximately 8300 years. The gap of 74.4 years between their perihelion times is consistent with a separation velocity of ~1 m/s that set the fragments apart following the parent's breakup in a general proximity of perihelion during the previous return to the Sun in the 7th millennium BCE.