Air-to-Ground Fire
You may fire an orbiting fleet at industry. If you do so, your fleet will first destroy any I-Ships, at the standard rate of two shots per one I-Ship, and then it will begin to destroy industry, at the rate of two shots per one industry.
You may also fire at population. If you do, your fleet will first destroy any P-Ships, at the rate of two shots per one P-Ship, and then it will begin to kill population, at the rate of two shots per one population. Note that, unless you are firing at a robotic population, this means you will be killing civilians; and if you are not a Positronix, or a Missionary firing at your Judged enemy, this will cost you some humanitarian points.
Finally, you may fire an orbiting fleet at home fleets, which is to say, at I-Ships and P-Ships. If you do this, you will first destroy any I-Ships, and then destroy P-Ships, and then if you have fired at least two more shots than necessary to destroy all I-Ships and P-Ships, the world will become neutral (but no damage will be done to actual industry or population). The world will remain neutral--unowned by any player--until there is only one player remaining with ships at the world, according to the standard capture rules.
This is a very useful tactic to deprive the world's owner of its use. It's particularly useful, for instance, if you happen to be lucky enough to be engaging your opponent at their homeworld.