Common Origin of Two Major Sungrazing Comets

Our extensive orbital calculations show that the motion of comet C/1965 S1 (Ikeya-Seki), a major member of the Kreutz sungrazing system, can be derived from the motion of its ``sister'' comet C/1882 R1 (Great September Comet) on the assumption that the two objects are fragments of a common parent that split in the year 1106, at the time when a very bright comet appeared near the Sun according to a number of historical records. Specifically, the orbit of Ikeya-Seki derived from astrometric positions in 1965-1966 is matched with remarkably high accuracy, well within the errors of observation, by (1) integrating the motion of comet C/1882 R1 back to 1106; (2) launching from the parent a fragment some 18 days after perihelion, 0.75 AU from the Sun, with a relative velocity of about 7 m s^-1 nearly in the antisolar direction; and (3) integrating the fragment's motion forward to 1965. We find that having the break-up closer to the Sun or before perihelion yields grossly inferior solutions. We conclude that the fragmentation event itself was not tidal in nature, but appears to have been due to rotational, and possibly other, forces acting on the parent comet, afflicted with cracks and fissures caused by the Sun's tidal forces a few weeks earlier. We note that the derived separation velocity is in the range established for nontidal fragmentation of minor sungrazers at large heliocentric distances and that there are obvious similarities with the behavior of comet D/1993 F2 (Shoemaker-Levy 9) following its close encounter with Jupiter in 1992 July.