Steve Brezenoff 
Final Mission [PDF ebook] 

Twelve-year-old Amelia and the dog-boy Greeny have disobeyed their captain’s orders. They stowed away to join a mission where they’ll gather valuable junk from a debris field. But soon the life signs of the adult crew members blink off. Alarms start blaring, "Intruder alert!" Aliens have boarded their ship. Can Amelia and Greeny escape the deadly invaders, or will they be destined to join the lifeless mass of space scrap?