So how does one put together a "targeted" orbital bombardment? First, you need mass, and if that mass is to survive atmospheric re-entry it needs to be compact and dense, and you need enough separate chunks of that mass to reduce the objective, again assuming you're interesting in merely taking out military targets and not leveling and churning whole sections of the planet. In a planetary orbit, exactly where does one get such mass? If there's a moon, the mass has to be mined and broken into the right sizes and shapes. If there's no moon, such mass must be lifted off the surface [highly unlikely, because if invaders control the orbital area, the locals won't get that far, and if the invaders need bombardment to control the planetary surface, they obviously can't get to the surface to obtain the mass required for bombardment]. That leaves asteroid or other out-system mining, all of which require yet more equipment and transportation methods, adding time, cost, and yet more technology.

Second, the bombardment "projectiles" need to be of almost identical size and composition in order for there to be any chance of being dropped into a re-entry path that will get them anywhere near the target. There's also the problem and the need to compute such paths, and against a series of objects, such as defense installations, that amounts to considerably different computations... and the equipment and software to do so. Even so, in all probability, given all the variables involved, even precisely engineered objects will spread or shift in re-entry and descent so that they'll be unlikely to land within a kilometer of the target or targets, let alone within yards. An independent guidance system, with the equivalent of steering jets, is most likely required to assure impact near the target -- but that's effectively the definition of a missile, and would require rather large factories somewhere, plus miniature AIs and fuel, etc.

In short, orbital bombardment with "sharp stones" doesn't look too likely as a candidate for precision ground targeting, either for practical or technical reasons. And if you want to destroy the planet or the culture, you only need one smallish asteroid or comet.

While I've oversimplified somewhat, the point is that a number of so-called high-tech solutions advocated to replace more "conventional" weapons really won't work in practice. Some, of course, do, but that's when the economics, the technology, and the battlefield environment go hand in hand. When a modern jet costs upwards of $50 million, and when it costs $5 million plus to train the pilot, you can afford to lose a great number of far smaller and less expensive RPVs for the most dangerous missions, but you still can't afford to use them against individual soldiers or terrorists on a wide-spread basis.

Recent wars and conflicts, including the drug war in Mexico, continue to illustrate the same dichotomy as the orbital bombardment issue I outlined above. Focused military high-technology is extraordinarily good at annihilating discrete objects, often quite large objects, but it is expensive to develop and deploy and has considerable limitations in dealing with smaller targets, particularly those mixed in with objects and people you don't want to destroy. Also, using expensive high tech indiscriminately against multiple and numerous low-tech targets has a tendency to bankrupt the high-tech user.

While times and technology change, they change equally over time for the attacker and the defender, and several thousand years of military history suggests that every technology runs into limits and that both conquest and resisting conquest require soldiers with weapons, and that many wonderful ideas like targeted orbital bombardment remain wonderful ideas... and little else.

—L.E. Modesitt, Jr.:  3/06/2009 10:32:00 AM