3 ms·

Surely reentering is very much not the problem?

kibwen 2y ago
Re-entering is a very large problem. Say you've got a few thousand tons of material hurtling towards Earth. How do you get it down to the surface in a useful way? You're not just going to shotgun it raw into the ocean, because then you still have to retrieve it somehow. You're not going to land it with rockets, SpaceX-style, because the fuel costs would be astronomical. You're not going to land it with a short atmospheric drag followed by parachutes, Apollo-style, because the weight makes the energies too great (the Apollo command module weighed about 6 tons upon re-entry). You're going to need something much more sophisticated, Space Shuttle-style, but on a grander scale than ever, and you're going to need to bear the cost of putting that thing back into orbit every time, or you're going to need to develop something brand-new (like a HUGE inflatable re-entry vehicle) or something that exists in the realm of sci-fi (like a space elevator).
- tzs 2y ago
  Does it have to be hurtling toward Earth? How about first parking it at the L4 or L5 Earth-Sun Lagrange point, and then nudging it so it leisurely meanders toward Earth instead of hurtles toward Earth? That would enter the atmosphere orders of magnitude slower than meteors do. Would that be enough for it to largely survive the fall to the surface either stay largely intact on impact or break up but the pieces would all be in the same general area?
  - DennisP 2y ago
    Heh that's an interesting point. It doesn't have to orbit the Earth, so it doesn't necessarily have lots of lateral velocity to burn off. It'd be interesting to work the numbers for just matching the Earth's velocity around the sun, at a distance of a few hundred kilometers. Edit: based on a quick chat with Claude Sonnet, reentry velocity would be about a fourth as high, but getting to that initial orbit in the first place makes the whole project significantly harder. But maybe if ablation from reentries became an environmental problem, and deep-space propulsion got really good, it'd be worthwhile.
  - kibwen 2y ago
    While the interplanetary transfer network is real (https://en.wikipedia.org/wiki/Interplanetary_Transport_Network https://en.wikipedia.org/wiki/Interplanetary_Transport_Netwo...), getting a payload onto the network itself requires a large and varying amount of energy based on where your asteroid is located. And then once you've successfully gotten your giant rock of solid platinum to some Earth-Sun Lagrange point, you still need to manage its descent to Earth, and beyond the technical difficulties of trying to steer it precisely, the superpowers of the world are unlikely to be peachy keen on the idea of a private company having global orbital bombardment capabilities, which is going to be a political headache of its own.
- DennisP 2y ago
  Starship has a return capacity of 50 tons.[0] If you have 5000 tons of material to return, that's a hundred Starship flights. I wouldn't expect asteroid mining to be viable until we have Starship or something like it at scale, doing thousands of launches per year. If most of those flights return empty, then that's a lot of cargo space already available. Starship uses $1 million in fuel to launch 100 tons to LEO, so half that much should be plenty to take 50 tons back down. But Starship's propellant mass for launch is 2600 tons[1] so that'd be up to thirteen launches to put the landing fuel in orbit. Actually it'd be less, since a lot of the orbital velocity is burned off by atmospheric breaking, not sure how much. Ideally though, get the fuel from the same asteroids you're mining already. Starship uses methane, so you're just looking for water ice and carbon, both abundant in asteroids. At current prices, 5000 tons of gold is worth about $400 billion, so it's not obvious that this wouldn't be economical. The world mines about 3000 tons of gold annually and the price of gold has still been going up, so if our asteroid miner returns a couple thousand tons per year it might not crash the price too badly. [0] https://space.skyrocket.de/doc_lau/super-heavy-starship.htm https://space.skyrocket.de/doc_lau/super-heavy-starship.htm [1] https://en.wikipedia.org/wiki/SpaceX_Starship https://en.wikipedia.org/wiki/SpaceX_Starship
  - kibwen 2y ago
    Gold is the worst example because the price of gold is almost 100% based on its scarcity. Doubling the supply of gold would just cause the price to roughly halve. Hilariously, if you actually had the capability to inundate the world with gold, you'd find it much easier and still extremely profitable to extort the people who own gold to pay you not do so.
    - DennisP 2y ago
      You'd make a lot of money on the way to doubling the world's supply. 3000 tons annually just doubles how much we mine, not how much we have; that's more like 190,000 tons. And even if you doubled the total supply instantly, you'd now own half the current value of the world's gold, which would be about $6 trillion. And of course gold has all sorts of really useful properties, so long term, it'd be worthwhile to make it abundant and cheap.
- kragen 2y ago
  getting raw materials from space onto the surface of earth intact is such an easy problem that dumb rocks do it literally every day entirely by accident. you just need a low enough sectional density that it loses most of its kinetic energy to the atmosphere rather than exploding when it hits the ground (i.e., not much higher than 1kg/cm²), a high enough sectional density that it doesn't burn up entirely given whatever materials are on the front side of it, a shape that's aerodynamically stable in the supersonic regime, aerodynamics and structural integrity such that what doesn't ablate lands in one piece rather than exploding into a fireball, and good enough targeting that it lands on land in friendly territory rather than at the bottom of the ocean i mean i'm not an expert in the area, but i think this is a well-understood set of problems. passive reentry vehicles have been used for decades for spy satellite film canisters, v-2 ballistic missiles, scud ballistic missiles, icbms, mirvs, and space capsules like those used in the vostok, apollo, and mercury programs. it gets easier in this case because you don't care if it breaks when it hits the ground, so you don't need parachutes or retrorockets; you just don't want it to vaporize specifically, a hunk of mass at rest at the von kármán line of 100 km has 981 kilojoules per kilogram of gravitational potential energy with respect to the ground. one degree of temperature is roughly one kilojoule per kilogram, so that's roughly 981 degrees of heating if it falls to the ground from that height with no further air resistance—not enough to melt steel, nickel, gold, platinum, or even mafic rocks. and it's still 981 kilojules per kilogram whether it's a kilogram of platinum or a thousand tons of platinum. but it gets better! that heating is shared between the reentry vehicle and a somewhat larger mass of whatever it hits, roughly in inverse proportion to their stiffness. and of course there is atmospheric resistance, even without a parachute—quite a significant amount of it at the comparatively low sectional densities you need to lose most of your escape velocity from hitting the atmosphere. so, typically, when meteorites land, they aren't even hot. the arguments above explain why in most cases they can't be hot unless they hit so hard as to blow open a crater 980 kilojoules per kilogram is 1400 meters per second, about mach 4, so you really don't want it to hit you. but it won't even melt, and newton's penetration depth approximation assures you it won't penetrate very far into the ground, like, less than a meter so basically you're worried about a nonproblem

Re-entering is a very large problem. Say you've got a few thousand tons of material hurtling towards Earth. How do you get it down to the surface in a useful way? You're not just going to shotgun it raw into the ocean, because then you still have to retrieve it somehow. You're not going to land it with rockets, SpaceX-style, because the fuel costs would be astronomical. You're not going to land it with a short atmospheric drag followed by parachutes, Apollo-style, because the weight makes the energies too great (the Apollo command module weighed about 6 tons upon re-entry). You're going to need something much more sophisticated, Space Shuttle-style, but on a grander scale than ever, and you're going to need to bear the cost of putting that thing back into orbit every time, or you're going to need to develop something brand-new (like a HUGE inflatable re-entry vehicle) or something that exists in the realm of sci-fi (like a space elevator).

tzs 2y ago
Does it have to be hurtling toward Earth? How about first parking it at the L4 or L5 Earth-Sun Lagrange point, and then nudging it so it leisurely meanders toward Earth instead of hurtles toward Earth? That would enter the atmosphere orders of magnitude slower than meteors do. Would that be enough for it to largely survive the fall to the surface either stay largely intact on impact or break up but the pieces would all be in the same general area?
- DennisP 2y ago
  Heh that's an interesting point. It doesn't have to orbit the Earth, so it doesn't necessarily have lots of lateral velocity to burn off. It'd be interesting to work the numbers for just matching the Earth's velocity around the sun, at a distance of a few hundred kilometers. Edit: based on a quick chat with Claude Sonnet, reentry velocity would be about a fourth as high, but getting to that initial orbit in the first place makes the whole project significantly harder. But maybe if ablation from reentries became an environmental problem, and deep-space propulsion got really good, it'd be worthwhile.
- kibwen 2y ago
  While the interplanetary transfer network is real (https://en.wikipedia.org/wiki/Interplanetary_Transport_Network https://en.wikipedia.org/wiki/Interplanetary_Transport_Netwo...), getting a payload onto the network itself requires a large and varying amount of energy based on where your asteroid is located. And then once you've successfully gotten your giant rock of solid platinum to some Earth-Sun Lagrange point, you still need to manage its descent to Earth, and beyond the technical difficulties of trying to steer it precisely, the superpowers of the world are unlikely to be peachy keen on the idea of a private company having global orbital bombardment capabilities, which is going to be a political headache of its own.
DennisP 2y ago
Starship has a return capacity of 50 tons.[0] If you have 5000 tons of material to return, that's a hundred Starship flights. I wouldn't expect asteroid mining to be viable until we have Starship or something like it at scale, doing thousands of launches per year. If most of those flights return empty, then that's a lot of cargo space already available. Starship uses $1 million in fuel to launch 100 tons to LEO, so half that much should be plenty to take 50 tons back down. But Starship's propellant mass for launch is 2600 tons[1] so that'd be up to thirteen launches to put the landing fuel in orbit. Actually it'd be less, since a lot of the orbital velocity is burned off by atmospheric breaking, not sure how much. Ideally though, get the fuel from the same asteroids you're mining already. Starship uses methane, so you're just looking for water ice and carbon, both abundant in asteroids. At current prices, 5000 tons of gold is worth about $400 billion, so it's not obvious that this wouldn't be economical. The world mines about 3000 tons of gold annually and the price of gold has still been going up, so if our asteroid miner returns a couple thousand tons per year it might not crash the price too badly. [0] https://space.skyrocket.de/doc_lau/super-heavy-starship.htm https://space.skyrocket.de/doc_lau/super-heavy-starship.htm [1] https://en.wikipedia.org/wiki/SpaceX_Starship https://en.wikipedia.org/wiki/SpaceX_Starship
- kibwen 2y ago
  Gold is the worst example because the price of gold is almost 100% based on its scarcity. Doubling the supply of gold would just cause the price to roughly halve. Hilariously, if you actually had the capability to inundate the world with gold, you'd find it much easier and still extremely profitable to extort the people who own gold to pay you not do so.
  - DennisP 2y ago
    You'd make a lot of money on the way to doubling the world's supply. 3000 tons annually just doubles how much we mine, not how much we have; that's more like 190,000 tons. And even if you doubled the total supply instantly, you'd now own half the current value of the world's gold, which would be about $6 trillion. And of course gold has all sorts of really useful properties, so long term, it'd be worthwhile to make it abundant and cheap.
kragen 2y ago
getting raw materials from space onto the surface of earth intact is such an easy problem that dumb rocks do it literally every day entirely by accident. you just need a low enough sectional density that it loses most of its kinetic energy to the atmosphere rather than exploding when it hits the ground (i.e., not much higher than 1kg/cm²), a high enough sectional density that it doesn't burn up entirely given whatever materials are on the front side of it, a shape that's aerodynamically stable in the supersonic regime, aerodynamics and structural integrity such that what doesn't ablate lands in one piece rather than exploding into a fireball, and good enough targeting that it lands on land in friendly territory rather than at the bottom of the ocean i mean i'm not an expert in the area, but i think this is a well-understood set of problems. passive reentry vehicles have been used for decades for spy satellite film canisters, v-2 ballistic missiles, scud ballistic missiles, icbms, mirvs, and space capsules like those used in the vostok, apollo, and mercury programs. it gets easier in this case because you don't care if it breaks when it hits the ground, so you don't need parachutes or retrorockets; you just don't want it to vaporize specifically, a hunk of mass at rest at the von kármán line of 100 km has 981 kilojoules per kilogram of gravitational potential energy with respect to the ground. one degree of temperature is roughly one kilojoule per kilogram, so that's roughly 981 degrees of heating if it falls to the ground from that height with no further air resistance—not enough to melt steel, nickel, gold, platinum, or even mafic rocks. and it's still 981 kilojules per kilogram whether it's a kilogram of platinum or a thousand tons of platinum. but it gets better! that heating is shared between the reentry vehicle and a somewhat larger mass of whatever it hits, roughly in inverse proportion to their stiffness. and of course there is atmospheric resistance, even without a parachute—quite a significant amount of it at the comparatively low sectional densities you need to lose most of your escape velocity from hitting the atmosphere. so, typically, when meteorites land, they aren't even hot. the arguments above explain why in most cases they can't be hot unless they hit so hard as to blow open a crater 980 kilojoules per kilogram is 1400 meters per second, about mach 4, so you really don't want it to hit you. but it won't even melt, and newton's penetration depth approximation assures you it won't penetrate very far into the ground, like, less than a meter so basically you're worried about a nonproblem