To the various people who keep advocating unguided weapons for planetary bombardment:
Yes, it is possible to hit a planet from way out with an unguided weapon. The question is why? To do damage to a planet-sized target, you're looking at throwing asteroids. Normal railgun rounds are going to be next to useless, unless you use them in the same mass as the previously-mentioned asteroid.
So if you're going to do it, use nukes. You could probably end civilization as we know it by nuking the biggest 10 cities on the planet with a few megatons each.
Pretty much this entire post confuses me. When you talk about "doing damage to a planet sized target" it makes me think that you are talking about actually breaking the planet into a bunch of pieces Death Star style. We are just talking about making the biosphere unlivable for the current unpleasant occupants.
You compare normal railgun rounds to asteroids, and describe railgun rounds as "completely useless", however the typical railgun slug we have been talking about has been 1 kg, and a sizable--but not enormous-- asteroid could mass 200 million times that much. Maybe we can explore projectiles in between those two extremes?
The jump from here to choosing nuclear explosions as your weapon of choice is odd, the fact that you speak of "a few megatons" as something catastrophically powerful that would end civilization with a couple applications is just weird when juxtaposed with your dismissal of gigaton kinetic impacts in the paragraph before.
As an example if you take the impact effects site posted Elouda posted (
http://impact.ese.ic.ac.uk/ImpactEffects/ ). If we plug in a 10.6 meter diameter iron meter going 15,000 km/s (I keep using that speed because it is a very comfortable attack speed for the low-tech example destroyer Daring) we get a projectile that masses 5 tons, equivalent to a size-2 missile. According to the site this projectile explodes as a 60 Gigaton airburst. Also according to this site this will pretty much demolish any building and kill most people within 100 km, but it will continue doing damage out past that.
Now lets talk more about this site, I read the included research paper so that I could understand why the results it was giving me didn't seem destructive enough. We go beyond the bounds of what these equations can do in a couple of ways. The big one is that all of the equations for how strong projectiles are and how easily they break up are based off of meteors. Even iron meteors aren't going to be as strong as machined iron, there are going to be all sorts of imperfections in these naturally occurring objects that cause them to break apart faster than a slug of manufactured Iron, let alone manufactured Steel, or Tungsten, or Uranium, or Duranium. (Protip, in the paper they state that the relationship between density and structural strength they use comes from these meteors, they say that it is only accurate for up to the 8000 kg/m
3 iron density, so plugging in Tungsten densities (19300) or other materials won't be accurate).
Obviously a railgun slug would be much more durable than a meteorite, especially one made out of TN materials, so it would start breaking up far later than calculated by this site. The next problem is that they don't model anything after the airburst. Roughly half of the energy of an incoming object is expelled by the airburst, in a more typical collision, (for example our 5 ton iron meteorite at 50km/s) the remaining portion of the energy is only .6 Megatons of explosives, a fairly useless amount of energy when you are spreading it over 7-8 km drop zone. However when you are talking about a 15000 km/s entry you can no longer just ignore that energy, it's another 60 Gigatons in the strewn field, at that point all of the fireball effects are still going to occur, however at the site it doesn't model any impact or fireball effects if there is an airburst.
So that site gives a 5000 ton projectile a 60 Gigaton explosion, and there are many ways in which that's an extremely conservative estimate. I don't understand why sending a similar 5000 ton projectile to impart a 10 megaton explosion is a more desirable alternative.
