void FixedUpdate()
    {
        Vector2 input = new Vector2 (Input.GetAxis ("Horizontal"), Input.GetAxis ("Vertical"));
        rigidbody2D.AddForce (input * speed);

        if (rigidbody2D.velocity != Vector2.zero) {
            var a = rigidbody2D.rotation / 360.0f * 2.0f * Mathf.PI;

            var v = rigidbody2D.velocity.normalized;
            var orientation = new Vector2(-Mathf.Sin (a), Mathf.Cos (a));

            var angleDiff = Vector2.Angle (v, orientation);
            angleDiff = Mathf.Min(20.0f, angleDiff);
            var cross = Vector3.Cross (v, orientation);

            if (cross.z > 0) {
                angleDiff = -angleDiff;
            }

            var torque =
                (angleDiff * turnSpeed); // / (1.0f + Mathf.Abs(rigidbody2D.angularVelocity));

            rigidbody2D.AddTorque(torque);
        }
    }