I would disable the shields on the Thorun but they are very minimal and were included in 3.4b.
Okay, I have disabled their shields already anyway.
If you can confirm it is the individual asteroids and not the field ones file a mantis ticket and I will get it fixed.
They looks like this in the mission, maybe there is a variable that makes them explode like a ship:
$Name: Target 3 ;! Object #7
$Class: Asteroid (B, M)
$Team: Hostile
$Location: 7631.544434, -823.292542, 10625.733398
$Orientation:
0.354932, -0.736499, -0.575841,
0.685088, -0.214216, 0.696251,
-0.636143, -0.641623, 0.428535
$AI Behavior: None
$AI Goals: ( goals ( ai-play-dead 50 ) )
$Cargo 1: XSTR("Nothing", -1)
+Initial Hull: 100
+Initial Shields: 100
+Subsystem: Pilot
$Arrival Location: Hyperspace
$Arrival Cue: ( is-event-true-delay "Training" 8 )
$Departure Location: Hyperspace
$Departure Cue: ( false )
$Determination: 10
+Flags: ( "ignore-count" "no-shields" "no-arrival-music" "no-arrival-warp" "invulnerable" )
+Flags2: ( "targetable-as-bomb" )
+Respawn priority: 0
+Special Hitpoint index: 98
+Hotkey: 7 ; added by Wesp5
+Orders Accepted: 0 ;! note that this is a bitfield!!!
+Group: 0