I've just modified the asteroid pof filename.
Here the code :
#Asteroid Types
$Name: Small Asteroid
$POF file1: asteroid.pof
$POF file2: asteroid.pof
$POF file3: asteroid.pof
$Detail distance: (0, 530, 1580, 2150)
$Max Speed: 130.0
$Expl inner rad: 0.0
$Expl outer rad: 0.0
$Expl damage: 0.0 ;; set to zero for no area effect
$Expl blast: 0.0
$Hitpoints: 850 ;; Note, modified downward by Skill_level
$Name: Medium Asteroid
$POF file1: asteroid03.pof
$POF file2: asteroid03.pof
$POF file3: asteroid03.pof
$Detail distance: (0, 520, 1240, 2400)
$Max Speed: 90.0
$Expl inner rad: 25.0
$Expl outer rad: 75.0
$Expl damage: 20.0 ;; set to zero for no area effect
$Expl blast: 1000.0
$Hitpoints: 66 ;; Note, modified downward by Skill_level
$Name: Large Asteroid
$POF file1: ast01.pof
$POF file2: asta01.pof
$POF file3: astb01.pof
$Detail distance: (0, 585, 1350, 2600)
$Max Speed: 160.0
$Expl inner rad: 100.0
$Expl outer rad: 200.0
$Expl damage: 50.0 ;; set to zero for no area effect
$Expl blast: 3000.0
$Hitpoints: 147 ;; Note, modified downward by Skill_level