This would prove to be a masterstroke for Atari. When Space Invaders was released in cartridge form in 1980, sales of the 2600 went through the roof. Many people bought Atari's system just to play Space Invaders, and thus it became one of the earliest known instances of a "killer app". The VCS version of Space Invaders was an excellent rendering, quite authentic in most respects. While the design wasn't absolutely faithful, the gameplay was just as addicting on TV sets at home as it was in the arcades.
One of the challenges of adapting an arcade game to home console use is the fact that arcade titles, by design, are not meant to last long. The entire point was to keep fans plugging in more quarters in order to play longer. For a home console, short playtimes don't quite work, and so, to add value to the purchase, Atari included a staggering amount of variations to the basic Space Invaders theme. In all, 112 such variants were included. That may sound crazy to those of us who are used to a handful of game modes in modern consoles, but it wasn't uncommon at the time. As you might expect, the variations were often minor, but in a few cases, the changes were significant, and this was certainly the case for the cooperative gameplay types.
The first co-op variant was very much what you'd see in far-off sequels like Space Invaders Extreme. Two players controlled ships independently, and played at the same time. Though the manual technically listed this as a competitive mode, the only head-to-head elements were the individual high scores and the fact that players scored points when the other player lost a life. Lives were shared, so the game was over when three ships were lost by either player, or in combination.
The more innovative partnership modes were far more creative, and required a tremendous level of patience and a desire for teamwork in order to fend off the aliens with any degree of success. In one variant, the left player controlled the cannon until he fired a shot, and which point the right player took over, and so on. A second co-op variant had one player moving the cannon, while the other was in charge of firing it. Oddest of all was the variant which had both players controlling one cannon at the same time; the player on the left could move it left only, the partner, just to the right. Either could fire the cannon. I can only imagine the impromptu fights, heated arguments, trash talking, and giddy fits of laughter that would come about as the result of playing these strange yet intensely cooperative variants.
Video games in general owe a tremendous debt of gratitude to Space Invaders, but fans of co-op especially should recognize the tremendous impact it had on gaming. Shoot-em-ups like the Raiden series, Ikaruga, and countless others are the direct descendants of this innovative ancestor. The Atari 2600 port, along with Space Invaders II, were among the first cooperative video game experiences that can be found in video game history, and both are the immediate progeny of the original. Space Invaders is an unarguable classic that has literally shaped the face of cooperative video games as we know them today.