# How would you slow down a large object traveling through space?

I have a concept for an asteroid mining station. The metal ingots that are processed are fired out of a railgun towards Earth. How could I slow the ingots down before reaching Earth to avoid accidentally causing an extinction-level event?

Edit: The ingots are about 2-3 tons.

• Do you really NEED them to reach Earth? How about orbital foundries? Kinetic energy could be converted into heat.... Mar 29, 2021 at 18:34
• Shoot them at something that is much bigger, and does not mind getting whacked, repeatedly. But there is no "slow down" in space. There is only "speed up" and a chosen direction. slowing down by "speeding up" in the reverse direction is exactly as hard as speeding up directly. Mar 29, 2021 at 18:40
• hit the moon instead.--add in travel velocity as well so energy can be calculated Mar 29, 2021 at 19:11
• 2-3 tons of metal is not a very big slug. There are bullets used in naval guns and such that are bigger. Schwerer Gustav "...and could fire shells weighing 7 t (7.7 short tons) to a range of 47 km (29 mi).[1] " en.wikipedia.org/wiki/Schwerer_Gustav Mar 29, 2021 at 22:57
• 2-3 tons at planetary velocities does not produce an extinction level. The Chelyabinsk meteor massed an estimated 13-14 THOUSAND tons, and basically just broke a lot of windows: en.wikipedia.org/wiki/Chelyabinsk_meteor Mar 30, 2021 at 4:21

You need to dissipate kinetic energy, which can be accomplished in many different ways.

You could build a "reverse railgun" that catches the incoming ingots. Instead of using stored energy to accelerate the ingots to high speed, the reverse apparatus gently slows the ingots over a long barrel, capturing and storing the kinetic energy as electrical power for use elsewhere.

You could also build an identical railgun to the one that fired the original projectile, and just fire an identical projectile to collide with the first one. Two ingots of the same mass moving at the same speed in opposite directions will collide, and assuming the collision is inelastic, all the kinetic energy will be dissipated and both ingots will come to a stop. This might be a catastrophic collision that destroys the projectiles, though. Alternatively, you could fire lots of small projectiles that would have the same effect in aggregate, but have less energetic collisions.

Since you're firing a ballistic projectile that carries no propellant, you're going to need some external mass or force to affect the projectile. You need to either aim the projectile at whatever provides the mass/force, or bring the mass/force to the projectile.

• Neither of your suggestions is practical for objects moving 10s of km/s. Mar 30, 2021 at 2:01
• @puppetsock I agree the reverse railgun us a significant engineering challenge, but I don't think it's totally infeasible - it's basically the exact same apparatus you used to launch the projectile in the first place. If launching it is practical, so is catching it. Mar 30, 2021 at 13:25
• Well, one slight miscalculation, or one accidentally blown fuse and your reverse railgun will need some very quick repairs, otherwise all the mass that is already on the way will turn it into a crater.
– mlk
Mar 30, 2021 at 14:13
• The reverse railgun idea is very nice, especially because it reduces the energy spent for the transfer. I believe the problems with availability/precision (the fact that if the receiver malfunctions, the incoming ingot causes an "extinction-level event") can be avoided by missing deliberately - put the receiver in space somewhere near the earth and use less energetic (slower) means for the last bit. How near exactly depends on precision of emitter and tracking instruments. Depending on the tradeoff between time taken/cost of precise instruments you can vary the earth/receiver distance. Mar 30, 2021 at 16:43
• with a regular rail gun you only need to worry about the acceleration to get it up to speed with a decently sized gun. With an inverse rail gun you also need to worry about getting the projectile to hit the muzzle dead-on. Even if you don't need air-tight bore, we're talking about... what, a 1-m hole (really generous) in a target several astronomical units away, moving rapidly... when the slightest misalignment of one projectile means destruction of the entire capturing mechanism, and larger misalignment the loss of the cargo in Earth's atmosphere or in deep space. Mar 30, 2021 at 18:08

To steal an idea from one of Peter F. Hamilton's books: aerobraking!

Cast your metal chunks into foam metal blunt-body re-entry capsules. Coat the blunt end with spare rock and whatever else you can synthesise or scavenge from your asteroid mining activities. Attach a suitable solar-powered transponder to them, and shoot them into a suitable re-entry trajectory.

If your shooting isn't quite up to scratch, you can add some little rocket engines to them too... nothing expensive, as shipping them back is impractical. Just enough to do a bit of mid- and end-course correction.

On impact with earth's atmosphere they'll slow right down, and the stuff you've plastered the blunt end with will heat up and burn off in an exciting and pretty way, rapidly slowing the capsule down. Its terminal speed won't be exactly sedate, but it'll be quite restrained. Let it splash down in the ocean. Beauty of foam metal: it can be less dense than water! Send boats out to retrieve these in bulk, between volleys from your mining facilities.

The transponder helps with both keeping an eye on incoming metal packages, and recovering after splashdown.

Trying to use these as weapons would be futile as they'd just be too soft and slow to cause any serious property damage, and too hard to aim to hit anything in space or even any specific targets on the Earth, for that matter. Don't worry about exitinction-level events... the Chixulub impactor that killed off most of the dinosaurs was 1012 tonnes. Your multi-tonne slugs are billions of times smaller. Without clever engineering like I've just suggested, it'd be easy to have most of the metal simply burn up on re-entry.

• @puppetsock: Tell that to the folks who flew on the Space Shuttle, or any number of capsules, from Vostok & Mercury through the Apollos to SpaceX's Dragon. The sample return from Japan's Hayabusa 2 mission reentered at about 12 km/sec. Mar 30, 2021 at 4:29
• @puppetsock are you thinking of aerobraking on Mars, by any chance? Because the Earth's atmosphere is hella thick, and it is surprisingly hard to hit the ground even at supersonic speeds, if the impactor isn't suitably shaped and shielded (or just really, really big). The Apollo re-entry capsules had a terminal velocity under 200mph, I believe, and most small meteorites that hit the ground will also be travelling at subsonic speeds. My foam-metal capsules are might be denser than Apollo, but I doubt they'd sustain supersonic speeds down to the ground. Mar 30, 2021 at 9:03
• @StarfishPrime All the man-made re-entry capsules have been carefully aimed to "miss the Earth by a whisker", so that they hit the atmosphere instead. They spend a lot of time aerobraking only because they hit the atmosphere in a very low angle. If an Apollo re-entry capsule would dive into the atmosphere at a right angle, it would crash down in a matter of seconds, and at a high speed. Just like your regulat meteor. Near perfect aim required. Mar 30, 2021 at 19:50
• @JyrkiLahtonen and as has been clearly demonstrated on multiple occasions, it has been a trick acheivable by automated systems biult by humans for the last 50 years and should be a breeze by the time we're mining asteroids. Mar 30, 2021 at 20:06
• @JyrkiLahtonen see paragraph number 3, starting "If your shooting isn't quite up to scratch..." ;-) Mar 30, 2021 at 21:11

Fire them slowly.

You do not need your ingots to be cooking along like comets. They don't need to punch thru armor. You don't even need orbital speed. You hardly need any speed at all. They just have to get there. Its not like they are going to go sour if they are late.

Earth will accelerate them some on the way in. Earth actually might accelerate them a lot. The moon is a safer bet. It is smaller. Your slowpoke ingots will ease up to the moon then get a little frisky on the way down. You will not lose any mass to atmospheric friction because Luna is naked. Neither will you have induced eddy currents and ohmic heating because Luna does not go in for that magnetic field stuff.

Your ingots will land with a thump. It is a characteristic thump. Detectors on the moon will triangulate the landing site. Then your robot will go fetch the ingot.

Your foundry is on the moon too. Fewer environmental regulations.

• Orbital velocities are 10s of km/s. This is physics that cannot be fooled. Ever look at the moon and wonder why it's got all those craters? Orbital speeds of 10s of km/s. The "characteristic thump" of a 1 ton object coming from a transfer orbit from the asteroid belt will look like a half km crater. Mar 30, 2021 at 2:11
• To elaborate, in this idea, you're sending the ingots into a gravity well. It doesn't really matter if you send them fast or slow - their final speed will always be destructively fast regardless of starting speed, because of the "depth" of the well you're sending them down, which far outweighs the start speed. Mar 30, 2021 at 4:24
• @puppetsock: You're vastly over-estimating the size of the crater. A 1 ton object impacting the moon from the asteroid belt will produce a crater of about 10 meters. There are a number of impact crater calculators on the web, e.g. eaps.purdue.edu/impactcrater/crater_c.html Mar 30, 2021 at 17:02
• Q1: How will we find ingots that landed on the moon? A1: Look in the bottom of the ingot sized craters. Q2: What if there are a lot of ingot sized craters. A2: Then there were a lot of ingots. Mar 30, 2021 at 17:12
• @Willk, minimum impact speed for anything coming in from outside a body's sphere of influence is roughly equal to the surface escape velocity. A Hohmann transfer from the asteroid belt will be coming in quite a bit faster than that, but you can shed most of the speed through Earth aerobraking and good timing (or amplify it through bad timing).
– Mark
Mar 31, 2021 at 2:26

Use a skyhook

It is a more feasible (and fun) version of the space elevator, where the big cable is rotating on itself instead of one side being attached to earth.

The skyhook has the advantage to be able to store energy easily when it receives ingots by rotating faster. You can then use this energy to send back supplies to the mining station.

You can also combine this approach to the other ones mentioned.

• Skyhooks are also awesome. Mar 31, 2021 at 19:48

# Magnetic braking.

The foundry is equipped with a "catcher" - a series of superconducting coils that generate a very strong magnetic field. This is the reverse of the famous magnet-in-the-copper-tube experiment.

As a result, a large portion of the incoming projectile's kinetic energy is converted into thermal energy inside the ingot, pre-heating it and making it easier to melt.

You would also not need to send them with excessive speeds, because all you're interested in is throughput. If you shoot one of them every ten minutes, they will arrive on average one every ten minutes - after several weeks or a whole year on a longer orbit, it's not that big a difference. And shooting slower ingots requires less power.

As @puppetsock pointed out, you don't get away from conservation of momentum: you have some three million kgm/s to go around, so if the smelter masses one million tons, it's going to be moving three m/s in the same direction as the incoming ingot after the catch. Even if it's just 1% of GEO speed, that's still too much (also because you won't be catching just the one ingot).

One economic possibility is having ingots coming from two opposite directions. Whatever the orbit it's in, the smelter is going to need some "downtime" when it's too risky or just impossible to send ingots towards it, so you're going to always need more than one smelter, and adjust its working cycle so it receives ingots just in some points of its orbit. At that point, if it lies e.g. on the ecliptic, it can receive ingots when it's at its greatest incoming elongation; its orbital speed will be reduced by each ingot. Then, the railgun will address the same number of ingots towards the opposite maximum elongation point, and the smelter will intercept them and have its orbital speed increased accordingly, compensating the earlier loss.

When the smelter, as seen from the incoming ingots, is in front or behind the Earth, another smelter will be approaching the maximum elongation points.

• Let us keep in mind both conservation of energy and momentum. Something gets hot. And the "catcher" gets pushed backward with each catch. You need to counter those effects or you will have disaster very quickly. Mar 30, 2021 at 2:06
• A catcher would require very exact targetting, which is much more expensive than the alternatives. Mar 30, 2021 at 8:10
• @puppetsock If the catcher is orbital, it must just alternate phases of orbit when catching the masses. One will be boosting, one will be decelerating, etc. So the net impact will remain the same. Exact navigation should not be a trouble for the sake of this site. Mar 30, 2021 at 10:34
• Great point on Throughput. It's why we use relatively slow container ships on earth to move stuff around. Yes, we have ways to move things faster, but reliability and consistent delivery schedules allow for planning, controlled expenses and so on. A smooth supply chain is a happy supply chain. Apr 1, 2021 at 18:06

The simplest way would be to use gravity assists between the moon and earth to cause the ingots to enter a high, stable, but probably fairly eccentric orbit. Basically, the process would go: metals get extracted and refined in asteroid belt => ingots get shot "backwards" (retrograde) from the perspective of the asteroid's orbit around the sun => the ingots "fall" (follow an orbit with a lower perihelion) towards earth => the ingots get close enough the the earth-moon system to be pulled off course => the ingots follow some precalculated path between the earth and moon, each pass of which slows them down a bit more => the ingots are picked up by an orbital craft and brought to a processing facility.

-Doesn't require anything to be attached to the ingots. You could include a transponder for safety reasons, but assuming the railgun is reasonably accurate it should be pretty simple to plan the route beforehand and warn any ships to steer clear. Even if it veers off course for one reason or another, the ingots would be pretty trivial to detect and track, and if that didn't work for some reason, the chances of it hitting anything are still very low - think of how many satellites or space stations have actually been hit by meteors (i.e. none).

-Doesn't require aero- or lithobraking. Aerobraking is relatively safe (you probably won't end up nuking any major cities), but adding a chaotic, fluid atmosphere and multiplying that uncertainty over potentially millions of ingots means that some are bound to veer off course. Even de-orbiting objects from low earth orbit is inherently unpredictable. Using either type of braking to slow the projectile down into an earth orbit is better, but would lead to at least some loss of material from ablation or, well, from lithobraking. :p

-This method would add to the time it takes for the ingots to reach their final orbit. Given the already-considerable time between the asteroid belt and earth, I don't think it would be too substantial, but it is a factor.

-The ingots will probably be put into high, eccentric orbits, since that would take the least number of gravity assists to get into starting from a hyperbolic orbit. The downside would be that it would require more delta-V for the recovery craft the reach them, and even more delta-V to lower and circularize the orbit into something more useful afterwards. Given that your setting has at least some trans-martian travel, orbital maneuvers around earth shouldn't be much of a concern, but it's still something to consider.

Edit: Reading through the other answers, I've also realized that this method wouldn't require any infrastructure on the receiving end. Loss of funding, political upheaval, or even societal or environmental collapse can occur without having to worry about bombarding the planet with dense objects at orbital speeds or losing the payload into interplanetary space. Although I imagine that any rebuilt post-apocalyptic society is going to be very surprised to discover a bunch of refined metal ingots sitting around waiting for them.

• Gravity assist does not remove the need to change velocity of the ingots. They are coming in with a relative velocity of 10s of km/s. This needs to drop close to zero. Apr 5, 2021 at 19:57

Frame challenge. Don't send it to Earth. We don't need it down here.

The whole premise of asteroid mining (outside of some poorly researched sci-fi) has never been to acquire materials that we need down here on Earth. We've got more than enough metal down here to do everything we need. The point of mining metal in space is so that you can build big things in space.

Let's say you want to build a rocket that can send humans to Titan or Europa or whatnot. The amount of material you'd need to get off the the surface of Earth in order to make such a trip in a reasonable amount of time is prohibitively massive and expensive. It'd be far cheaper to build the rocket in orbit from materials that are already up there.

So what you'd be mining for would be materials to build a rocket, and something that will work as fuel for the rocket. You'd need far less fuel than for a rocket constructed on Earth since you don't need to escape our gravity well. So really all you'd need to get off-planet would be the astronauts, food, water, and other basic supplies for them, and enough equipment to be able to construct the rocket in orbit. (If you're doing this on the regular, you've probably been building up the rocket construction facility for some time now, so ultimately all you need are the people and supplies.

So you're firing metal ingots towards your orbital factory with rail-guns. Well, you only need to match the orbital speed of the factory (or close enough that someone can fly out and grab them), not slow them down enough to get them safely to the surface.

As for fuel, well, you're better off mining comets. Comets are made mostly of ice, which is of course frozen water. Water can be split into hydrogen and oxygen, and hydrogen (along with nitrogen) is perfect for making hydrazine, a powerful rocket fuel. You might also find enough ice on the moon for this purpose. (Launching materials from the moon is far less costly than from Earth due to its lower gravity.) And of course you can use the spare oxygen to breathe.

Just follow the Apollo/Soyuz landing system: ablation shield plus parachutes. The Apollo CM weighed around 5.8 tonnes, the Soyuz reentry section (middle section) 2.8 tonnes.
I have seen pre-parachute speed for Soyuz been quoted as 755 feet/s, that's about 820 km/h - that would likely bury the ingot in the center of an impact crater, so you probably want some parachute, though a smaller one than for Apollo/Soyuz might suffice.

Land or water?
At sea, you need a ship. That's more expensive to operate than a truck with a crane on land. OTOH moving the ingots once collected becomes much cheaper on water. (Coastline may be a good idea.)
If something goes wrong (e.g. the thing crashes into the ground due to parachute malfunction or because the ablation shield didn't properly position itself into harm's way) - well, on water, it just sinks, on land, you have to dig it up, but at least it's there.

• Wow, this seems to be a difficult point for people. Planetary transfer orbits put the ingots in near-Earth space at a relative speed of 10s of km/s. You cannot use aero-braking for that. The ingots are only in the atmosphere about 10 seconds. You need to slow them down in space before they re-enter. Apr 5, 2021 at 19:59
• @puppetsock Ah ok, I didn't think too much about the difference between moon and interplanetary, and I'm not convinced that aero-braking is useless but I agree you need to do more (slingshot maneuvers with the moon, solar sails, etc.) I'm wondering what would happen if an ingot were just allowed to impact - it wouldn't bury very deep, but would it be shallow enough to be extracted easily, or would it be less expensive to do an in-flight slowdown? Apr 6, 2021 at 7:40
• Kinetic energy is v-squared. Artillery rounds are less than 1 km/s, usually less than 0.8 km/s though the fastest push 1 km/s. Orbital transfer thus has at least 100 times the energy of an artillery shell of the same mass. How practical do you think impact recovery will be? Apr 6, 2021 at 14:03
• @puppetsock well Apollo already had 11 km/s reentry speed, so that's already beyond what you believe doable - the well-known trick is to put your trajectory into enough of thin atmosphere to slow down sufficiently. Asteroid incoming impacts at roughly 18 km/s (claims Google), so the energy multiplier would be 2.7. I suspect the higher energy makes the re-entry window narrower, but being more shock, deceleration, and heat resistant considerably widens it - now I'm wondering whether there will really be cratering or not. Apr 6, 2021 at 17:22
• Apollo 11 starter their maneuver at 11 km/s. Then they used retro-rockets to slow. Then they hit the atmosphere. Apr 7, 2021 at 18:09

Shooting material in your manner is expensive from an energy, infrastructure, and safety stand point. The more you have the harder it is to transport also. What if there was a way to reverse all of those things?

Solar sails

solar sails are not made out of gold, and any metal can be used to make solar sails theoretically. So instead of making a transport system for the materials, you can make the materials the transport system. just take your ingots and turn them into large flat solar sails. They won't be as good as NASA's sails but that isn't the point. solar sails require little to no energy besides a small gyro scope and a computer to pilot the craft. The solar sail will need a a solar sail factory, but that is essentially a machine that flattens the material to the needed width and then sends that to a machine that links those thin sheets. The solar sails are very safe, they accelerate slowly and even if they do get into the atmosphere they will burn up. The more material you need to transport the larger you can make your sails, making them more effective. further more, you can transport other things back to earth that can't be made into solar sails using the sails. you can even throw the material with an accelerator and use sails that deploy mid flight to decelerate it. Solar sails are slow, with an acceleration of 0.058 mm/s^2 acceleration it will take 5 and a half years to accelerate to the needed 10 km/s speed and 5 and a half years to decelerate. If you are willing to wait a decade or two you can get it basically for free. If you are willing to spend energy you can use lasers to speed up the acceleration. However, this will be difficult as in order to transfer the power effectively you would need a very big sail or a very accurate laser. However, if you are transferring a lot of material you can make a very large sail without any problem, making the laser not much of a concern.

• Can you please estimate the time a 10 km wide solar sail (even of nearly zero mass) would require to change the velocity of a 1 ton ingot by 10 km/s? Make reasonable assumptions about solar wind or look it up. Mar 30, 2021 at 2:08
• magnetic sail tugs would probably work better, as they are especially ideal for deceleration and can be used against the magnetic field of the target planet. Mar 30, 2021 at 2:43
• A one ton ingot of iron has a volume of 0.115227326 meters cubed. With a solar sail depth of 100 nanometers (a bit thick but still feasible) that gives us a sail of 1152273.26 meters squared. That isn't large enough for a 10 km wide solar sail but is enough for a one km solar sail. That might make it slower than some larger solar sails but won't disqualify it from laser acceleration. Breakthrough Starshot is trying to use solar sails 4 meters wide, which is less than one kilometer. It will then take about eleven years to accelerate and decelerate.
– user64888
Mar 30, 2021 at 6:31
• @CharlieHershberger, Breakthrough Starshot is planning to use batteries of gigawatt lasers to to launch featherweight spacecraft.
– Mark
Apr 2, 2021 at 5:22
• @CharlieHershberger Does 11 years sound feasible to you? Does keeping a laser focused on perhaps thousands of ingots (at 1 ton each it takes a lot of them to build anything useful) for 11 years sound practical? Apr 5, 2021 at 20:03

Have you thought about using the railgun as an engine? Instead of shooting the valuable stuff towards Earth, shoot the mining waste in what ever direction physics needs to bring the asteroid closer to Earth. Once its in a closer orbit to Earth it will be easier to transfer the valuable stuff off the asteroid and onto the Earth with other spacecraft (or point the rail gun at Earth but now you'll be better able to target things to avoid hitting something valuable).

• That's a very good idea! Apr 1, 2021 at 13:41
• Interesting idea. You start with a rock full of undifferentiated ores, and reach Earth with a ball of refined metal.
– Mark
Apr 2, 2021 at 5:14
• How does a rail-gun operate on non-metallic stuff? Apr 5, 2021 at 19:59

Shoot them in a cargo bay of your space truck, which is accelerated beforehand obiviously.

if you just shoot them out, without guidance and an orbit correction system - you probably will have a hard time even shoot nearby, not to mention hitting the earth.

with guiding block, you provide the energy and general direction and delta-v for your ingots, space truck catches them and its role is to fine-adjust the orbit to make gravitational reverse slingshot maneuver and insert into the orbit of the earth.

also, to cause an extinction event you need to shoot a very big lump of metals as if you shoot them as 1-10-100kg pebble even if it coincidently crosses earth's atmosphere it will be funny sparks in the sky, and people will make wishes when they see them.

## if it is big, really big

Let's say you have the means to accelerate big objects, 1000's of tons, and more.

Then you have all the means for making a canister with cold thrusters, which works on the oxygen you extract from oxides of those metals, which is about 40% by mass of those materials. And using that mass of oxygen you can maneuver to make a proper insertion.

so your asteroid contains everything you need to make a self-delivering package if you are able to provide it with an initial kick.

• I was thinking more about 1-5 tons, but thanks for the idea. I should specify the size and weight. Mar 29, 2021 at 19:05
• @randominternetperson it also makes sense to add how far apart are those orbits in terms of delta-v, as about half of that delta is the difference between earth and those ingots. may or may not be relevant for someone. Mar 29, 2021 at 19:13
• Orbital speeds are 10s of km/s. What do you use to move your space truck? Mar 30, 2021 at 2:07
• @puppetsock it is the same problem as in "how all that mining and refining equipment got to that asteriod" there are multiple ways. Space truck clearly space to space construction, so it is not 10km/s, depends where that asteriod is. Structural part of it can be build on same asteroid, what it needs to be send is brains and navigation comunication equipment. So there are different ways. I do not hold asteriod mining as a great idea, at initial stages of space exploration, but all that is up to OP. Space truck is just a solution which does not bring more handwavium than there is already. Mar 30, 2021 at 6:49
• @puppetsock judging by other comments, main point of space truck is in getting in orbit around earth, not land on it, unloading can be a different problem. It needed to make gravitional deaceleration maneuver and make enough kick to close orbit around earth. So it for making precise orbit intersections Mar 30, 2021 at 6:58

shape your copper ingots like a coil, your iron ingots like a cylinder, and give the iron ingots a magnetic field. Then shoot them such that they "collide" on the other side of the Earth. By "collide" I mean the cylinder passes through the center of the copper coil. The eddy currents on the copper coil will serve as a dampener and this will result in a controlled inelastic collision.

• It's a ridiculous accuracy story, but let that pass. Do the following math. How much energy is required to slow a 1 ton projectile from 10 km/s to zero? And what temperature will copper be heated to if it absorbs that much heat? AND what acceleration is required to do this in, say, 100 meters? Does any part of this thing make it to the ground, or is it very small shrapnel even if it works as intended? Apr 5, 2021 at 20:07

Before I get into my answer to your question, I'd just like to request that you give me a little warning before you begin your bombardment. I'd like a chance to not be here when death starts raining from the skies, thanks.

Anyway...

Unguided masses shot in the general direction of Earth are going to wander, no matter how tightly you control the firing mechanism. You'd need to spend a tremendous amount of effort on balancing them for starters, and you've already allowed for a 50% variance in mass, each 'shot' would require a fairly large amount of computation, etc. It's a big, expensive job.

Once you solve those problems there are a slew of others just waiting for you at the other end. Firing into a gravity well speeds up your projectiles, so they're going to arrive going faster than they were when you launched them. The Sun's gravity will speed them up while they approach, then the Earth-Moon system will pull them in even faster. You're looking at a very large amount of kinetic energy to try to bleed off once they arrive.

Most of the normal solutions for the problem will fail in the face of this massive kinetic energy. Aero-braking for instance exchanges kinetic energy for heat, which is likely to result in your deliveries vaporizing in the atmosphere. While some of that heat will radiate away into space, the majority of it will get added to the Earth's total heat. Keep it up long enough and... well, we won't have to worry about the next ice age, I guess.

Magnetic capture might be your best option, especially since you can use the process to generate energy. You're going to need a series of large hoops along the expected path, with enough maneuvering capability to get in the right place. A 2-ton projectile moving at high speed will cause a lot of damage to any structure it encounters, so make sure your timing is perfect. Of course the expense of running these gigantic field generators is going to be pretty high, but you're the one that wants to throw kinetic devastators at a populated planet.

Of course you could always just throw them at the moon, assuming you're not using it for anything else. You'll need to mine the ores out once the dust settles, but that's OK.

It would be far more productive to simply strap a drive system on the packages and have them maneuver themselves into an appropriate orbit around the Moon or Earth. Think more drone, less doomsday cannon.

• If you were firing them from the vicinity of Mercury however, you could calculate an initial velocity that yields high parking orbits in the neighborhood of the Earth-Moon system. The suns Gravity would slow and bend their trajectories. If carefully planned, the Earth-Moon system and the payload arrive at the predicted points in space, such that gravity from the Earth-Moon system prevents the payload from falling into the Sun. There's a lot of Iron and free solar power on Mercury. Apr 1, 2021 at 20:50
• @jwdonahue, I don't know if the Moon's big enough to do a gravity capture for something coming from Mercury. You need a heck of a velocity change to bring it up to the Earth's orbital speed, and since a gravity capture is a three-body process, you can't use the Earth's gravity to do it.
– Mark
Apr 2, 2021 at 5:13
• The Earth and Moon combined is plenty large enough. Think what happens when you toss a baseball up in the air. If falls back down. Toss something at the correct velocity from a lower point in a gravity well, to a higher point where it starts falling again. Somewhere near that point, you can find a relative velocity to the Earth-Moon system, that it would easily captured by their combined gravity. Then it's just a matter of capturing and redirecting it to where you want to be. And you don't actually need high delta-v to accomplish it. Apr 2, 2021 at 18:24
• If the projectile is moving slow enough, relative to Earth, for instance, it can be captured into one of it's Lagrange points. It would take a lot of computer simulation runs, to find the right transfer orbit, but when you have the mass of the Sun at your back, you don't need a hefty breaking system to hit capture velocities. Though you might need a few low energy coarse corrections, but those can be provided by a network of solar orbiting lasers. Apr 2, 2021 at 18:55
• @jwdonahue, the problem is that "if": anything coming up from Mercury is going to be going a minimum of 7.5 km/s Earth-relative at apoapsis, and a ballpark estimate is that the Moon can only give you 2 km/s of acceleration. If you're going for Earth orbit, Earth can't give you anything: orbital mechanics obeys the law of conservation of energy, so if you're trying to adjust your energy relative to a body (such as the Earth), you need to give any excess to a third body (eg. the Moon, or your rocket exhaust).
– Mark
Apr 5, 2021 at 22:13

## Make them foldable gliders.

Using rigid origami and a keen knowledge of the mathematics of paper folding, your ingots can sprout stubby wings and guide themselves down with all the dubious grace of a Space Shuttle. (No, I'm not actually saying to have landing gear, but make a landing slow enough that the ingot can be picked up off a sandy erg in solid form) The Space Shuttle can also inform your heat shielding for the way down; cobble it together out of plates of slag from your refining process. (Yes, you'll end up with craters and shed tiles of space slag all over the desert - let the environmentalists whine! It's still better than making a mine on Earth... probably, at least according to the PR department. The thickness of the metal ingot should ensure that not too much is lost if the heat shielding starts coming apart during reentry.)

• "probably, at least according to the PR department" - lol))))) Apr 5, 2021 at 10:35

Calculate an orbit around earth or moon, where the orbital speed is approximately the speed your ingots will achieve by the time they reach that orbit. That's an easy enough task.

The ingots will need small thrusters to fine tune orbital insertion, and orbital timing (eg to not collide with other ingots or processing stations, or be conveniently close), but those can be recovered and reused.

• Won't work. Orbital mechanics are symmetric, so if you come in to Earth at a given speed, you'll go right back out at that same speed (but in a different direction -- this is the basis of gravity assists).
– Mark
Apr 2, 2021 at 5:16
• Is that necessarily so? Earth moves in orbit at 30 km/sec, so we can reach earth vicinity with a relative velocity anything from -30 to +30 km/s by choosing to insert into orbit from different directions ("behind", "in front" or radially, or any in between). We can also insert into orbits that have about 1 - 10 km/s instantaneous orbital speed on top of that, also from any direction. So I'd have thought by judicious choice we can insert into earth orbit matching the gravity well from the asteroid belt to a suitable point in earths orbit, direction of insertion, and some Molniya/larger orbit? Apr 2, 2021 at 5:49
• Yes, it is necessarily so. If you're approaching Earth from behind at a Sun-relative 30 km/s (Earth-relative 0 km/s), you'll get accelerated by Earth's gravity as you approach Earth, reaching low Earth orbit at around 41 km/s Sun-relative (11 km/s Earth-relative), somewhat faster than the Earth-relative 7.7 km/s of a low-Earth orbit or even the Earth-relative 10.5 km/s of an atmosphere-grazing Molniya orbit at periapsis. The key term here is "hyperbolic excess velocity", and unless you're already in an Earth orbit, you'll have some you need to get rid of to enter Earth orbit.
– Mark
Apr 2, 2021 at 20:47

Without some kind of complicated deceleration device attached to each ingot it becomes a complicated problem in orbital mechanics. If time is not of the essence however it should be possible to work out a trajectory that will take your ingots past a series of other gravity wells on the way to Earth. You then use these bodies for a reverse gravity assist.

Gravity assists (or slingshots) are currently used routinely by space agencies to accelerate probes into the outer solar system by having them pass close to a large planet on an angle that causes them to gain velocity from that planet's own orbital momentum around the sun - effectively 'stealing' the velocity from the planet. That planet in turn loses orbital momentum (minutely) by an amount equal to what the probe gained - so that the energy exchange balances out.

Point is the energy exchange works in reverse as well a probe (or ingot) can lose velocity by traveling in a reverse trajectory, 'dumping' velocity into the planet. Again the planes orbital velocity increases to balance out the energy exchange.

The downside is;

1. you would need to attach maneuver thrusters to each ingot together with a navigation computer and radio link.

2. It will take years for each ingot to reach Earth but after the first shot arrives every subsequent shot sustains a continuous supply chain with every new ingot arriving in a predicable and reliable sequence.

The plus side is it is very fuel efficient and your ingots can shed most of their velocity while using very little fuel in return.

• The problem with gravity assists is timing: things only line up right at long intervals. For a transfer from the asteroid belt, the only thing in position to assist is Mars, which works out to roughly one shot every 18 months (and Mars is too small to get much of a gravity assist -- most assists use Earth, Venus, or Jupiter).
– Mark
Apr 3, 2021 at 23:47
• Yep, I said the process would be slow. You have to calculate multiple using different target planets at different times. The stations supercomputer will be doing lots of overtime. Still doable I think though. The key takeaway?There's no 'easy' solution to this problem.
– Mon
Apr 4, 2021 at 5:14

Why fire them as ingots? Eject them out of the rail gun as plasma, which would condense into metallic dust clouds on the way.

• A cloud of metal would be very difficult to refine once it got back to earth, and you run the risk of scorching the entire planet. Apr 1, 2021 at 13:42

You need to fire them with just enough speed to be able to take a trajectory to Earth. And/or, fire them at powerful arrays of magnets that reduce the speed of the ingots to a more reasonable velocity.

• "Just enough speed to be able to take a trajectory to Earth" means an impact velocity of around 11 km/s minus atmospheric drag, which for a multi-ton metal ingot won't be very much.
– Mark
Apr 2, 2021 at 5:10