Hacker News Reader

Comment by c048 | Hacker News Reader

Building 'cylinders' gives us an environment that's a lot easier to control, manage and safeguard compared to a whole planet. Most planets will be entirely hostile to complex earth based life to begin with. They'll require either complex terraforming or some sort of isolated environment, and that's not even bringing up issues concerning gravity.

Furthermore, in future wars planets would also be far easier, bigger and immovable targets compared to asteroids and space-stations. Just look up what happens when you drop a 10ton tungsten rod from outer-space. Thanks to gravity, when it hits the ground it has the same impact of a small nuclear device.

When humanity starts to create colonies outside of earth, I expect most planets will only become fully automated mining colonies. Colonizing planets is a Star Trek fantasy that, most likely, will not be worth it.

View on HN

> Just look up what happens when you drop a 10ton tungsten rod from outer-space. Thanks to gravity, when it hits the ground it has the same impact of a small nuclear device.

Is that actually true? I mean, you could hoist two and a half of those tungsten rods into LEO with a single Falcon 9 expendable launch, and if we're just looking at kinetic energy, you can't get more out than you put in. So, the tungsten rod's re-entry wouldn't create any more energy than it took to lift it into orbit in the first place (assuming tungsten doesn't do anything weird like combust), and there'd be a lot of energy lost in the process. I wouldn't consider a Falcon 9 launch to be "equivalent to a small nuclear device" in terms of energy.

Another way to look at is is that a 10,000 kg mass at a height of 1,000 km above a planet with gravity of 9.8 m/s^2, is 10,000 * 9.8 * 1,000,000 = 9.8 * 10^10 joules. 1 kilowatt hour is 3.6 megajoules, so that comes out to 27,222 kilowatt hours of energy. A kiloton is (according to a google search) equal to 1,162,200 kilowatt hours, so that comes out to about 0.023 kiloton TNT equivalent. With some of that energy lost as atmospheric heating.

That isn't to say that dropping tungsten rods from orbit isn't a serious threat, or that you couldn't impart a lot more energy by flinging them at the Earth with much greater velocity. I just don't think the comparison to nukes is quite right.

The original idea was dubbed "rods from God" and it was proposed by the U.S. Air Force to target bunkers, silos, and other hardened targets without using nuclear detonations. The rods would have orbital velocity—7 km/s at LEO—giving them, at 10,000kg, about the momentum of a jumbo jet at cruise (an unfortunate comparison, to be sure). Except the kinetic force would be targeted at an area the diameter of a telephone pole, so they would penetrate deeply (100m or more) and cause enough of a shockwave in the earth to collapse almost any buried structure. Their thin profile and solid tungsten construction ensured most of their mass would survive re-entry at close to their original speed.

They also evaded the 1967 Outer Space Treaty, which forbade "objects carrying nuclear weapons or any other kinds of weapons of mass destruction"[0].

I'm not sure they actually reach the yield of a nuclear device, but "bunker-buster" nukes are relying on overkill in yield since they can't achieve that kind of ground penetration in delivery.

[0] https://history.nasa.gov/1967treaty.html

You're calculating the resting potential energy, aren't you?

"Dropping a tungsten rod on earth" is not picking it up and dropping it. It's firing it at the earth. Depending on what body you're firing it from, that's a lot of additional delta v.

Something coming at the earth from the Moon, for instance, is moving at least 2.3 kps, plus another kps if you fire it forward. This calculator is telling me that's 55.2 gigajoules, about 15 thousand times higher.

Assuming your math is right, and then my math is right, .35 megatons is still an exceedingly bad day.

Yeah, I was ignoring sideways velocity. I was taking "just look up what happens when you drop a 10ton tungsten rod from outer-space" perhaps a bit too literally.

If we add in 1/2 * 10,000 kg * (7500 m/s)^2, to account for sideways LEO-equivalent momentum, I have it coming out as about 0.0906 kilotons: about 4 times more energy. Which isn't too surprising, since the hardest part about getting into orbit isn't the going up part, it's the going sideways fast enough enough part.

My whole formula is this: (((10000 * 9.8 * 1000000) + (0.5 * 10000 * (7500 * 7500))) / 3600000) / 1162200

Basically, (mgh + 1/2 mv^2) converted from joules to kilowatt hours to get it into a larger unit I can sort of relate to, and then converting that in turn to kilotons equivalent TNT in order to compare with nuclear yields.

That's assuming I didn't leave off some zeroes somewhere or apply a formula incorrectly.

> Something coming at the earth from the Moon, for instance, is moving at least 2.3 kps, plus another kps if you fire it forward.

What do you mean "fire it forward"? If you fire it forward, it will never fall on Earth. To get from Moon to Earth, you need to fire it backward in Moon orbit, to slow it down. Essentially, you need to slow it down a lot if you want it to get to Earth surface, as opposed to orbiting it alongside Moon.

Not if it’s in an elliptical orbit with a low enough apogee.

10^10 Joules converted to TNT directly is 2 ton TNT. All the energy should be released at the time of impact in a short period of time. Even assuming >50% energy loss due to atmospheric interference, 1ton TNT should be left.

Living in a asteroid belt, makes the poorest person capable of getting a ion-engine on a rock the ruler over all the well-dwellers by default - after all you have the high ground.

Thus speaks the lord thy over-lord:

I. You shall not erect asteroid defences.

II. You shall not send others to space to rule beside me.

III. Once every aeon i shall send a rock to test your faith. You shall fight one another, to proof your worthiness.

IV. Be nice to one another.

V. Game-theory applies, but in a similar way for everyone living in system, to everyone else out-system.

Things are going to get weird once private orgs start doing asteroid redirect activities (as in: massive build up of orbital railguns, hypervelocity nukes etc. once someone points out that in 10 years or 50, it's trivial to setup a rock that'll end a moderate sized nation state).

> Just look up what happens when you drop a 10ton tungsten rod from outer-space. Thanks to gravity, when it hits the ground it has the same impact of a small nuclear device.

That’s... not as good as a nuclear device? Susceptibility to 10 ton tungsten rods is pretty lame to worry about when a 10 ton actual nuclear bomb would be so much worse.

You're missing the point. The point is how easy it becomes to destroy planetary targets once you start having an extra-earth presence as a society. You literally drop heavy stuff from orbit and you have a nuke. You need an actual, guidable, nuke (or self propelled object) to have the same effect on a space station or asteroid.

I don't think Earth is substantially more vulnerable than anywhere else in the solar system.

A heavy object picks up some kinetic energy as it falls into a gravity well, but realistically a tungsten rod moving at orbital velocities could destroy a space station or asteroid habitat just as effectively as it could destroy an Earth target. And there'd be somewhat of a difference: with Earth, we have an atmosphere to slow fast-moving objects down and provide an early warning that something is coming at us. (Probably not enough early warning to do usefully react, but still. A couple seconds is more than 0 seconds, and maybe there's some way to use that to a tactical advantage.) In space you'd have no warning at all, unless you can track it on radar or telescope, in which case Earth could do the same.

Earth's big advantage is that everything we do is so geographically distributed that any damage done is a temporary setback. It would be a sad day, but the human race will go on. Unless it was a really big rock, in which case it would be hard to redirect one into Earth's orbit without anyone noticing.

I suppose if you really want to be protected from flying rocks, the best bet may be to find some moon or minor planet with weak gravity, and tunnel deep into the surface and set up shop there. You could do something similar on Earth, though there's a limit to how deep you can go before it gets too hot. (In Consider Phlebas by Iain M Banks, there is planet whose original inhabitants had built a deep underground subway system where train cars house the military command structure, and in time of war the trains would always be moving around so that a nuclear-armed adversary wouldn't know where to strike. That approach could work too.)

Realistically, though, I think if humanity develops the technology to start moving large heavy objects around the solar system, then almost everyone is going to be very vulnerable. The best defense is to be on friendly terms with all your neighbors, and have very good visibility into where everyone is and what they're doing.

If you live at the bottom of a well, don't make fun of the guy peering down at you from the top.