Factory Design Pattern misunderstand

I tried to follow the factory design pattern but something is wrong. Maybe somebody could explain to me what I am doing wrong. I tried to create a class where the space to be populated with asteroids and spaceships but with no luck. I don't know how to create a concrete creator class.

<?php
  //concrete prod 1
  class Asteroid
  {
      private $size;
      private $speed;

      //bigger means faster
      const SPEED_COEFICIENT = 2;

      public function __construct( $size )
      {
          $this->size = $size;
          $this->speed = $this->setSpeed();

      }

      private function setSpeed()
      {
          return $this->size * self::SPEED_COEFICIENT;
      }

      public function getSpeed()
      {
          return  $this->speed;
      }
  }

  //concrete product 2
   class Spaceship
   {
        private $size;
        private $speed;

        //bigger means slower
        const SPEED_COEFICIENT = 0.5;

        public function __construct($size)
        {
            $this->size = $size;
            $this->speed = $this->setSpeed();
        }
        private function setSpeed()
        {
           return  $this->size * self::SPEED_COEFICIENT;
        }

       public function getSpeed()
       {
           return $this->speed;
       }
   }

   interface SpaceFactory
   {
       public function createSpaceObject(  $size);
   }


   class SpaceShipFactory implements SpaceFactory
   {
       public function createSpaceObject( $size)
       {
             return  new Spaceship($size) ;
       }
   }

      class AsteroidFactory implements SpaceFactory
      {
          public function createSpaceObject( $size)
          {
              return  new Asteroid($size) ;
          }
      }
        $asteroidFactory = new AsteroidFactory ();
        $asteroid = $asteroidFactory->createSpaceObject(50);
        var_dump($asteroid);

Thank you very much