JetPac was also available as a game on tape for the benighted Commodore Vic 20, for which, if I remember correctly, you had to buy a 5K RAM extension cartridge simply for the game to run properly.
The aliens came in several different shapes and sizes (and even colours!) and your hardy astronaut blasted away at them in glorious 2D with a horizontal laser beam, while assembling a spaceship at the same time. When the spaceship was complete, so was the level, and your astronaut would take off in stunning vertical flight, with static sounds to simulate the thrust of the engines.
When I was 8, I played this game until my eyes bled. If someone from the future had told me how bad it would look in 20 years time, I truly wouldn't have cared. I grew up on a diet of Pac-Man and Space Invaders, and even Frogger. Anyone remember Frogger? JetPac gave me many hours of dazed happiness.