Escape From Arcturus has two stages that can be played independently from each other.
The “Fortress” stage has you surrounded on all by attacking alien ships. You can shoot down the income photon torpedoes, But you can't destroy the ships. Survive as long as you can.
In the “Escape” stage you evacuate the Acturans from the planet by launching and guiding the evacuation ship. Shoot down the incoming invaders until the ship can enter hyperspace. There are three factories on the planet surface. If the invaders land they will destroy a factory. Get as many Arcturans off the planet as you can.