  I call attention to extraordinary features displayed by the genetically
related long-period comet pair of C/1844 Y1 (Great Comet) and C/2019 Y4
(ATLAS). The issue addressed most extensively is the fragmentation and
disintegration of the latter object, itself a thousands-of-years-old fragment.
Of the four fragments of C/2019 Y4 recognized by the Minor Planet Center -- A,
B, C, and D -- I confirm that B was the principal mass, which stayed undetected
until early April. The comet's 2020 fragmentation is proposed to have begun
with a separation of B and A near 22 January, when the nuclear condensation
suddenly started to brighten rapidly. From late January to early April, only
Fragment A was observed. The remaining Fragments C and D split off most
probably from A in mid-March, but they too were detected only in April. A new
fragment, E, is proposed to have been observed on three days. Further addressed
are the issues of the orbital period and antitail of C/1844 Y1 and the extreme
position of this pair among the genetically related groups/pairs of long-period
comets.
