  Asteroid (3200) Phaethon, a B-type asteroid, has been active during its
perihelion passages. This asteroid is considered to be a source of the Geminid
meteor stream. It is reported that this asteroid is spinning at a rotation
period of $3.60 \ hr$ and has a top shape (an oblate body with an equatorial
ridge) with a mean equatorial diameter of $6.25 \ km$. Here, we report that
Phaethon's rotation state may be close to or above its critical rotation period
when the bulk density is $0.5 \ - \ 1.5 \ {g/cm^3}$ (a typical bulk density of
a B-type asteroid). We found that in this condition, the structure of Phaethon
is sensitive to failure unless the cohesive strength is ${\sim}50 \ Pa \ - \
{\sim}260 \ Pa$. This result implies that if there are some surface processes
driven by, for example, thermal waves, large-scaled deformation may happen and
cause mass shedding. From this interpretation, we propose the processes that
produced the Geminid meteor stream in the past and dust tails recently.
Phaethon initially rotated at a spin period shorter than the current period.
The magnitude of structural deformation at this stage was higher than the
present spin condition, and a large mass shedding event, i.e., the Geminid
meteor stream, occurred. After this deformation process, the body became more
oblate, and its spin slowed down. At this point, while the spin was high enough
for the body to have mass shedding events, the magnitude of these events became
small.
