Constant Velocity?

I have tried to control my object with my mouse using this code:

 using UnityEngine;
 using System.Collections;
 
 public class AstronautScript : MonoBehaviour {
 
     public float SPEED;    //In units per frame
 
     Vector3 destination;
 
     // Use this for initialization
     void Start () {
     }
     
     // Update is called once per frame
     void Update () {
         if (Input.GetButtonDown("Fire2")) {
             Vector3 pos = Input.mousePosition;
             pos.z = 0;
             destination = Camera.main.ScreenToWorldPoint(pos);
         }
     }
 
     void FixedUpdate () {
         Vector3 movement = destination - transform.position;
         GetComponent<Rigidbody2D>().velocity = movement.normalized*SPEED;
     }
 }
 

As you can see I've normalized the movement vector, but when I play my game, the speed of the object is higher the farther away from the destination that it is. Anyone have an answer?