gistfile1.cs

//All this should be in the GameManager Script

// initialise the list
public List<Transform> asteroids;

//this function will all you to use the ship's transform and the asteroid list to find the closest asteroid
	void SortCharactersByDistance(Transform myTransform, List<Transform> characterList)
	{
		characterList.Sort (delegate(Transform t1, Transform t2){
			return Vector3.Distance(t1.position, myTransform.position).CompareTo(
				Vector3.Distance (t2.position, myTransform.position));
		});
	}

// this function will be called in the ship's update function to find the closest asteroid
public Transform ClosestAsteroid(Transform myTransform)
	{
			bool isEmpty = !asteroids.Any();
			if(!isEmpty)
			{
				SortCharactersByDistance (myTransform, asteroids);
				return asteroids[0];
			}
	}
	
	
	
// Then in the ship's update function you call
// closestAsteroid = GameManager.instance.ClosestAsteroid(transform);