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You can fire it relatively slowly because of the low gravity of the moon, and take plenty of time bleeding v. Ideally there would be a space elevator and captur

You can fire it relatively slowly because of the low gravity of the moon, and take plenty of time bleeding v. Ideally there would be a space elevator and capture system to bring them back, but other methods work. You’re not just firing “at” Earth. Even if you were, Earth is mostly covered by oceans, and landing them there would be acceptable.

ben_w 8y ago
The low gravity of the moon isn’t the problem, it’s the high gravity of the earth: it will hit hard even if you had a lunar space elevator and gave it the minimum possible delta-v to reach Earth from that Lagrange point. The other problem is that even if you intend to use it safely, you have just built a massive gun which is too far away to shoot back at and which has the capability to target specific locations on this planet.
- resu_nimda 8y ago
  "The Moon is a Harsh Mistress" by Robert Heinlein involves some of that. But how difficult would it be to intercept the payload safely outside the atmosphere? Seems it would be easier than intercepting an ICBM.
  - ben_w 8y ago
    I get the impression it would be harder, because ICBM launches are really obvious (to current equipment) and you can at least make a good guess about their ballistic path, while anything of that size launched from the moon would be invisible until it had almost hit you. While it is often said that stealth is impossible in space, that tends to assume the appropriate tech has actually been manufactured rather than sitting in a lab somewhere waiting for a purchase order. Radar, even lidar, is going to have awful signal-to-noise ratios and almost all of the return signal will be stuff that bounced off the moon rather than any projectile. Even the lunar retroreflector turns 10^17 photons out into one photon back, according to Wikipedia, and that’s about as far from stealth as you can get. Edit: I suppose one could reasonably argue that any industrialisation of Luna would, as a result of my own argument, make governments choose to put loads of effort into appropriate defence technology. On the other hand, I’m told trench warfare was more accident than deliberate planning in WW1, so perhaps governments would be as myopic as ever even with a massive space gun on the moon.
    - piannucci 8y ago
      Once it gets within a few tens of megameters, pulsed radar systems could easily distinguish an incoming projectile thanks to the 2.4 second round-trip-time to the moon. Metallic objects have ideal visibility. If you are concerned about solar thermal radio emissions bouncing off the moon and reducing SNR, that could be an issue -- a phase-of-the-moon bug in the system, if you will. But based on what I’ve overheard from friends working on 21-cm radar teaching telescopes, I doubt the luminosity of the continuous background is high enough to pose a big problem, assuming you have a decently large antenna. I’m more concerned that there’s nothing much you can do to redirect such an object in the few hours it takes to fall on your head. By that time it will have reached 4km/s.
      - ben_w 8y ago
        Good to know. In that case, I’m not concerned by the first n (n<5000 ish, varies by nation), but the relative ease by which an attcker can just keep going until the target run out of missiles to shoot at the attacker’s projectiles.
- cfadvan 8y ago
  There is no significant advance in tech regarding colonization, mining, and manufacturing in space without significant dual uses. If “that could be a weapon!” is the end of it, then we’re stuck on this rock until we all die. It’s all pretty moot, since dropping rocks on a country won stop a proportional nuclear response.
  - ben_w 8y ago
    Nuclear responses aren’t possible if the rocks were dropped by unknown attackers who hacked or invaded the launch site, if the target isn’t a nuclear nation or in a defence treaty with one, or even if the attackers are the legitimate owners deciding to do a Sea Of Tranquillity Tea Party in a fight for independence because the nukes can’t easily get up there and are surprisingly rubbish in space anyway.
    - cfadvan 8y ago
      What makes you think that it would be possible to invade a Moon base, or hack it anonymously? We’ve gone past sci-fi here and into fantasy.
      - ben_w 8y ago
        First line of the story, “…The first trillionaire will be made in space”. At that scale, do you think it isn’t worth privately funding some mercenaries and your own BFR clone (or even Saturn V if the BFR isn’t copyable for whatever reason) for an incredibly hostile takeover of any infrastructure placed on the moon? And that’s Earth launch where it would be obvious who was behind it. If the same wealth leads to significant numbers of people going up, they don’t need anything like that kind of propulsion to do what has happened a thousand times before on land and sea, in the general area of strikes, secession, and mutiny. As for hacking being possible, that’s just the observation that basically everything is hackable. Even things that were given as examples of clueless parents fretting over nothing when I was a teen, such as making JPEG files execute when opened, turned out possible due to exploitable bugs.
        8y ago

ben_w 8y ago

The low gravity of the moon isn’t the problem, it’s the high gravity of the earth: it will hit hard even if you had a lunar space elevator and gave it the minimum possible delta-v to reach Earth from that Lagrange point. The other problem is that even if you intend to use it safely, you have just built a massive gun which is too far away to shoot back at and which has the capability to target specific locations on this planet.

resu_nimda 8y ago
"The Moon is a Harsh Mistress" by Robert Heinlein involves some of that. But how difficult would it be to intercept the payload safely outside the atmosphere? Seems it would be easier than intercepting an ICBM.
- ben_w 8y ago
  I get the impression it would be harder, because ICBM launches are really obvious (to current equipment) and you can at least make a good guess about their ballistic path, while anything of that size launched from the moon would be invisible until it had almost hit you. While it is often said that stealth is impossible in space, that tends to assume the appropriate tech has actually been manufactured rather than sitting in a lab somewhere waiting for a purchase order. Radar, even lidar, is going to have awful signal-to-noise ratios and almost all of the return signal will be stuff that bounced off the moon rather than any projectile. Even the lunar retroreflector turns 10^17 photons out into one photon back, according to Wikipedia, and that’s about as far from stealth as you can get. Edit: I suppose one could reasonably argue that any industrialisation of Luna would, as a result of my own argument, make governments choose to put loads of effort into appropriate defence technology. On the other hand, I’m told trench warfare was more accident than deliberate planning in WW1, so perhaps governments would be as myopic as ever even with a massive space gun on the moon.
  - piannucci 8y ago
    Once it gets within a few tens of megameters, pulsed radar systems could easily distinguish an incoming projectile thanks to the 2.4 second round-trip-time to the moon. Metallic objects have ideal visibility. If you are concerned about solar thermal radio emissions bouncing off the moon and reducing SNR, that could be an issue -- a phase-of-the-moon bug in the system, if you will. But based on what I’ve overheard from friends working on 21-cm radar teaching telescopes, I doubt the luminosity of the continuous background is high enough to pose a big problem, assuming you have a decently large antenna. I’m more concerned that there’s nothing much you can do to redirect such an object in the few hours it takes to fall on your head. By that time it will have reached 4km/s.
    - ben_w 8y ago
      Good to know. In that case, I’m not concerned by the first n (n<5000 ish, varies by nation), but the relative ease by which an attcker can just keep going until the target run out of missiles to shoot at the attacker’s projectiles.
cfadvan 8y ago
There is no significant advance in tech regarding colonization, mining, and manufacturing in space without significant dual uses. If “that could be a weapon!” is the end of it, then we’re stuck on this rock until we all die. It’s all pretty moot, since dropping rocks on a country won stop a proportional nuclear response.
- ben_w 8y ago
  Nuclear responses aren’t possible if the rocks were dropped by unknown attackers who hacked or invaded the launch site, if the target isn’t a nuclear nation or in a defence treaty with one, or even if the attackers are the legitimate owners deciding to do a Sea Of Tranquillity Tea Party in a fight for independence because the nukes can’t easily get up there and are surprisingly rubbish in space anyway.
  - cfadvan 8y ago
    What makes you think that it would be possible to invade a Moon base, or hack it anonymously? We’ve gone past sci-fi here and into fantasy.
    - ben_w 8y ago
      First line of the story, “…The first trillionaire will be made in space”. At that scale, do you think it isn’t worth privately funding some mercenaries and your own BFR clone (or even Saturn V if the BFR isn’t copyable for whatever reason) for an incredibly hostile takeover of any infrastructure placed on the moon? And that’s Earth launch where it would be obvious who was behind it. If the same wealth leads to significant numbers of people going up, they don’t need anything like that kind of propulsion to do what has happened a thousand times before on land and sea, in the general area of strikes, secession, and mutiny. As for hacking being possible, that’s just the observation that basically everything is hackable. Even things that were given as examples of clueless parents fretting over nothing when I was a teen, such as making JPEG files execute when opened, turned out possible due to exploitable bugs.
      - 8y ago