The Game: Commanding a fleet of ships, you use their combined firepower to wipe out an onslaught of alien ships (which, perhaps not at all surprisingly, are firing back at you). It only takes one hit to lose one of your own fleet, and when your fleet is completely wiped out, the game is over. Until then, do as much damage to the enemy armada as you can. (Sega/Gremlin, 1981)
Memories: I always admired games with novel ways of counting down how many “lives” a player had left until his quarter was declared a total loss. Moon Cresta had a three-stage rocket which could be destroyed stage-by-stage, and Lock ‘n’ Chase featured a getaway car full of extra crooks that could be deployed one-by-one into its Pac-Man-like maze. Tac-Scan gave the player one fleet – and only one fleet – of ships that would be wiped out as the game progressed. When the entire fleet was wiped out, thus ended the game.
Tac-Scan was one of Sega’s small number of vector games, and I always rather liked the wormhole/tube sequence – back then, that was some pretty cool stuff. But the only problem with commanding an entire fleet was that I tended to get wiped out pretty quickly – it takes a little bit of time for some players to adjust from watching out for their one little on-screen characters to watching out for a group of characters that took up most of the bottom of the screen…
Sega itself adapted Tac-Scan for the Atari 2600, ColecoVision, Atari 5200, and – unless I’m very much mistaken – I think there was a Commodore 64 port of the game too. Even 
the higher-end systems such as the Coleco and the 5200 couldn’t turn out a very good version of the game because of the difficulties (at least back then) of translating vector graphics games into raster games.