Recently I've started learning Unity3D. I'm working on a game as an exercise in which you control a space ship and have to dodge asteroids. If you feel like it's getting a bit too much you can hit the space bar, emitting a blast in all directions that repulses nearby asteroids. To create this blast I have the following code:

public class PlayerBlastScript : MonoBehaviour {

public ParticleSystem BlastEffect;

// Update is called once per frame
void Update () {
    if (Input.GetKeyUp(KeyCode.Space)) {
        Fire();
    }
}

public void Fire() {
    ParticleEmitter effect = (ParticleEmitter) Instantiate (BlastEffect, transform.position, Quaternion.identity);
    effect.Emit();

    Vector3 explosionPos = transform.position;
    Collider[] colliders = Physics.OverlapSphere(explosionPos, 25.0f);

    foreach(Collider hit in colliders) {
        if (!hit) {
            continue;
        }

        if (hit.rigidbody) {
            hit.rigidbody.AddExplosionForce(5000.0f, explosionPos, 100.0f);
        }
    }   
}
}

Even though the blast effect appears, the asteroids are not affected at all. The asteroids are all rigid bodies so what's the problem?