Asteroid
Asteroids are large, celestial rocks found in Hyperspace. In the FreeSpace universe, asteroids are generally mobile, and found in large groups termed asteroid fields.
Mobile asteroids are usually harmful to any vessel that comes near, as they could collide with the ship, and the resulting impact would deal damage to the ship's hull integrity. As such, asteroids are a secondary target of warship turrets and fighters, to protect important vessels crossing hazardous fields.
Asteroids on a collision course towards any vessel are highlighted with white brackets in a ship's HUD. These must be destroyed before they impact the victim ship. At worst, multiple collisions from several asteroids could cause even large capital ships to experience hull failure and, ultimately, destruction. A notable mission which involves defending an important ship from asteroid collisions is Rebels and Renegades, where the player's squadron must shoot down asteroids on a collision course towards the NTF Iceni.
Although generally larger than fighters, asteroids are very easy to destroy as only several hits from even the most basic weapons would be required to take them down. When asteroids explode, smaller fragments are formed; if the fragments are large enough, they may still continue the collision course to the victim ship, and therefore must be taken out as well.
Notes for mission designers
Asteroids are usually generated with the "Asteroid Field" function. However, there is also a ship called "Asteroid" in the Ships list. This asteroid is very large, but it does not move and cannot collide with objects. Furthermore, the texture of this asteroid seems to be less detailed than the ones generated with the "Asteroid Field" function; so unless you're determined, it is recommended that you use the "Asteroid Field" function instead; if you want only one, immobile asteroid, make the number of asteroids "1" and check the "Passive" box.