See on-screen instructions. You're defending the Earth/Moon System from Aliens who are using the Moon to transport an automated weapon to our system. Your primary weapon is Proximity Bombs - but the Aliens will eventually learn about and adopt the technology. You are limited to maneuvering thrusters, and fuel is also limited - so mind your orbit, aim carefully and conserve fuel.
System Defense is an upgrade to ED-III: -Photon Torpedoes can destroy the Alien Weapons. -So can Phasers, which now fire a spread and recharge more quickly. -Nine Proximity Bombs can be launched at a time. -Up to 9 Photon Torpedoes can be "flying" -There's a marker ring around he alien weapon - it will "attack" if you're inside the ring - the orbit predictor helps gage your orbit and the effect of thrust -marker beacons show L4 and L5 Lagrange Points. - the "h" key toggles between High/Normal thrust, and is indicated by a dot above the fuel "gage". -High thrust eats more fuel, be careful... Note: With careful navigation, you can "park" at either of the Lagrange Points and bomb from there without using thrust.
