Adding Velocity to a 2D rigidbody

Question

I have just started using Unity and I decided to make a 2D asteroids clone, and so far I have the rotation and movement down but I can't seem to figure out how to make the ship save its velocity (so you continue moving until you give an equal force in the opposite direction) ... This is what I have so far...

using UnityEngine;
using System.Collections;

public class movement : MonoBehaviour {

void Update () {
    GameObject facer = GameObject.Find("facer");
    //Facer is another object I am using as a sort of aiming reticle

    if(Input.GetKey(KeyCode.UpArrow))
    {
        this.transform.position = Vector3.MoveTowards(this.transform.position, facer.transform.position, 0.1f);
    }
    if(Input.GetKey(KeyCode.DownArrow))
    {
        this.transform.position = Vector3.MoveTowards(this.transform.position, facer.transform.position, -0.1f);
    }
    if(Input.GetKey(KeyCode.LeftArrow))
    {
        RotateLeft();
    }
    if(Input.GetKey(KeyCode.RightArrow))
    {
        RotateRight();
    }
}
void RotateLeft() {
    transform.Rotate (Vector3.back * -5f);
}
void RotateRight() {
    transform.Rotate (Vector3.forward * -5f);
}

}

Answer 1

Is this what your're asking?

[yourigidbody].velocity = transform.forward * speed;

or

[yourigidbody].velocity = transform.TransformDirection(Vector3([xspeed], [yspeed], [zspeed]));