For this, you need a planet with one orbital shield (or megashield, for that matter) already built and at least one free population unit. As soon as some alien ship enters the system, go to the planetary screen and start building another orbital shield (or megashield). Go to the system screen and press the "time advance" button (a clock with an arrow). The alien ship will do one move. Press again, it will do another move. Continue pressing until it has fired the first shot at your planet. When the alien ship fires at the planet, the first thing that will be destroyed is the shield being built! (unfortunately, it will be destroyed by the first shot, no matter how lame the weapon being used) Since the alien ship will not fire again unless we press the "time advance" button, we go to the planetary screen again and start building our shield again (the population unit building the shield is not killed when the shield-in-construction is destroyed, it merely becomes free again). Now we press "time advance again", the ship fires, we rebuild the shield... repeat until the enemy ship has no more ammo or energy. You'll have to re-do this every turn, until reinforcements arrive (or until your own planetary weapons have destroyed the attacker)... quite time-consuming, but it can save your colony.