var myVector = GameObject.Find("middle").transform.position - transform.position;
rigidbody2D.velocity += 0.2 * Time.deltaTime * myVector;

if(Input.GetKey(moveLeft)) {
        transform.Rotate(Vector3.back * -turnSpeed * Time.deltaTime);
        //this value is something like (0, 0, -8)
    }

    if(Input.GetKey(moveRight)) {
        transform.Rotate(Vector3.back * turnSpeed * Time.deltaTime);
        //this value is something like (0, 0, 8)
    }

var myVector = GameObject.Find("middle").transform.position - transform.position;
lastVector = myVector;
//for example:
myVector is (-1, 1, 0)
lastVector is (-1, -1, 0)