your code
shuttle(500,200)
mothership(50,50)
wall(400,100,400,300)
planet(50,150,"earth")
alien(400,300,400,450)
asteroid(400,50)
asteroid(250,150)
wall(300,300,300,500)
alien(100,200,200,200)
alien(200,200,350,200)
planet(200,250,"venus")
fuel(250,100,500)
fuel(50,300,500)
wall(100,350,200,350)
planet(150,400,"neptune")
asteroid(150,250)
asteroid(100,300)
planet(350,100,"mars")
alien(500,50,500,170)
wall(150,0,150,170)
wall(220,50,320,50)
planet(500,350,"jupiter")