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Comment by wongarsu | Hacker News Reader

Getting from the moon to earth is trivial when compared to launching anything from earth. Using some back-of-the-envelope calculation a Atlas B ICBM from 1959 has enough fuel and thrust to deliver over 5 tons of warheads from the moon to earth. If you launched from the far side of the moon I imagine early warning and interception aren't on the table with 1960/70s tech, providing excellent first strike and second strike capability.

Of course actually getting ICBMs on the moon is a whole other endeavor.

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Early warning satellites would be a heck of a lot easier to install than the missiles. Basically once someone started building such a plant it would be impossible to hide what you're doing.

You'd never get first strike launching from the Moon, regular ICBMs would be all over you long before the moon IPBMs made the 1 day journey back to Earth. Even with the faster travel speed from only having to accelerate out of the Moon's gravity well, you're talking about a distance of 384,000km vs. 8,000km. Your missiles arrive to a planet that is already a radioactive moonscape.

It doesn't have to be a one day journey. With a 3 ton payload the Atlas B seems to have > 8000km/s delta-v. Getting from moon to earth took Apollo about 4800km/s delta-v, leaving our little Atlas 3200km/s spare fuel. If you use that to accelerate to 3200km/s travel speed you arrive on earth after about 2 minutes (if we ignore the time it takes to accelerate).

You are off by at least 3 orders of magnitude.

I think we can simplify this further. It seems reasonable to assume that any weapon deployed on the surface of the moon would be devolved and dedicated specifically for that purpose, so we can ignore playing theoretical math games :).

It would also be fairly obvious that someone was trying to move several thousand pounds of hardware onto the moon, and launch detection satellites would be even more effective at monitoring the moon. So you've got a first strike, that will be detectable basically immediately, and weapons that now need to travel ~385,000 km and survive even more serious re-entry than the ICBM's based on Earth. Or you've got a second strike capability that cost you billions of rubles/dollars.

Once you look at it like that, its pretty easy to see why the submarine launched weapons were the preferred method for threatening rapid first strike while providing a guaranteed second strike capability.

A two minute travel time is ridiculous. Even light takes more than a second to reach us from the moon.

The lunar orbit->earth delta-v is 4.8km/s. I didn't check your other numbers but they seem to be off by a factor 1000 too.

Besides acceleration at the moon side, wouldn't there need to be deceleration before entering the atmosphere? I'm thinking an IPBM traveling that fast would rip apart in the upper atmosphere and/or explode too high to cause any real damage.

Can a missile survive reentry at 3200km/s? From https://www.grc.nasa.gov/WWW/BGH/hihyper.html LEO reentry speed is 17500 mph = 28000 km/h = 7.7 km/s. It's 415 times the speed.

I didn't do the math but at that speed you don't need a warhead to destroy a target. Let's try with 1000 kg steel or whatever won't melt down

1/2 mv^2 = 500 * (3200 * 3600)^2 = 345,500,000,000 J

1 J lifts about 100 g mass by 1 m on the surface of Earth. That energy would lift 345,500,000 kg by 100 m.

If you can land a ship on the moon, you can land an ICBM on a house in Moscow. It was less about launching from the moon. It was more about getting it to land where you want.

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