  The $^{14}$C production of shock-accelerated particles is calculated in terms
of the total energy released in energetic particles. The recently reported 1.2%
jump in the $^{14}$C content of the atmosphere in the year C.E. 775, it is
found, would require $\gtrsim 10^{34}$ erg in energetic particles, less than
first estimates but far more than any known solar flare on record. It is noted
that the superflare from a large comet (comparable to C/Hale-Bopp) colliding
with the sun could produce shock-accelerated GeV cosmic rays in the solar
corona and/or solar wind, and possibly account for the CE 775 event. Several
additional predictions of cometary encounters with the sun and other stars may
be observable in the future.
