By pushing the start button?
Yes, you can. It destroys all the flying saucers that are currently on screen.
I will second the yes but be careful when you go back to playing Attack from Mars after playing a lot of Junk Yard and you clear all the saucers by resetting your game by hitting the start button instead of the launch button.
Cool thanks. I was afraid of trying it for that reason.
On AFM, yes. JY has a plunger.