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 .