Like in Defender, there are humans (stranded pilots here) who show up periodically. Rescuing them will net you a fair number of points, but unlike Defender, you CAN shoot them if you feel evil enough to do so. Missile bases appear immediately after you rescue a pilot and they're pretty easy to eliminate. But after the third one is destroyed the screen becomes scattered with debris which stands still for a moment and then moves quickly into the path of your ship. The bits and pieces need to be avoided for a while, then a black landing strip appears. This is where you dock to refuel. Once refueling is complete, you start over again but now you're up against faster enemy weapons and more objects to avoid.
Judging the game on its own merits, Planet Patrol is a serviceable -- if not thrilling -- shooter. The problem is, there's nothing in this game that hasn't been done before, and there are other games on the 2600 that do it better. The graphics approach Activision quality in terms of sharpness and vividness, but you'll notice some flickering going on (a-la 2600 Defender). Overall, Planet Patrol is a simpler (although not necessarily easier) version of Atari's Defender.