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Say 200-300 years in the future, humankind has colonized much of the solar system, using a combination of laser thermal drives and solid-core nuclear rockets (fusion never really panned out). Suddenly, astronomers spot an odd object entering the solar system. The object turns out to be a kilotonne-scale chunk of rock, ordinary in every way except for the fact that the thing is made entirely of antimatter. Collisions with the interplanetary medium, and the energy they give off, slow down the object sufficiently for it to be captured by the solar gravity field and enter a slowly-decaying orbit somewhere in the region of Neptune. Obviously, this rock is a potentially enormous fuel source for both antimatter drives and commercial power reactors. The question is, how exactly is one going to get antimatter out of this thing? You can't exactly land equipment on it and start mining, and it's not like you can just start chipping off pieces of it. So, how exactly would one go about mining an asteroid made of antimatter? Also, for extra points, how plausible is the scenario laid out in the question, where an antimatter asteroid enters a slowly decaying orbit in the near edge of the Kuiper belt, and how long could such an orbit last.

Note: It has been brought to my attention that collisions with the interstellar medium may not be sufficient to slow the asteroid. As a result, a gravity assist from Neptune or a Kuiper Belt Object may be involved to make the capture of this asteroid more feasible.

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    $\begingroup$ This sounds weird, but I think you've just subjected an asteroid to a Brownian Motion because of the constant random bombardment it gets due to how little matter there is in space. This wouldn't really have a stable anything, and have a trajectory impossible to precisely calculate. $\endgroup$
    – Halfthawed
    Sep 8, 2019 at 2:00
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    $\begingroup$ It's less Brownian motion and more likely to be a constant drag, due to the fact that the asteroid will be moving at orbital speeds. I'm also not sure how large the effect will be, as the density of the interplanetary medium is very very very very low. But that's the reason for the "extra points" section at the end of the question. $\endgroup$
    – Gryphon
    Sep 8, 2019 at 2:02
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    $\begingroup$ @Hypnosifl, no, I don't mean TNT equivalence. I mean that the asteroid has a mass of around a thousand tonnes (a kilotonne). This will have much, much, much more than a kilotonne of TNT equivalence. $\endgroup$
    – Gryphon
    Sep 9, 2019 at 2:11
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    $\begingroup$ 1 gram of normal matter hitting this thing will produce a 47kTon explosion. Even microscopic dust grains will generate huge blasts. Apart from the obvious risks of random explosions when working on the object, this also means that it isn't going to be a single object; it will be surrounded by a cloud of antimatter dust with some larger particles as well, of sizes up to multiple kilograms. I would not want to share a solar system with this object. $\endgroup$
    – Paul Johnson
    Sep 9, 2019 at 8:18
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    $\begingroup$ @AlmaDo why would this involve anything similar in mass? The interplanetary medium is a very sparse scattering of dust, the slowdown from collision with it would involve many microgram-sized collisions. An asteoroid just happening to hit a similar-sized asteroid is very, very, very improbable. The big issue, though, is that even a relatively miniscule particle hitting it could generate enough energy to blast that asteroid into smithereens and result in a scattered (and continously scattering) cloud of antimatter. $\endgroup$
    – Peteris
    Sep 9, 2019 at 16:14

9 Answers 9

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One thing which other posters have not mentioned is the effects of the interplanetary medium on the asteroid and mining operation.

Before you can do anything, it would be very wise to "clear" the area around the asteroid. A massive "wake shield" in an orbit slightly ahead of the antimatter asteroid will create a zone of extremely hard vacuum behind it, and the antimatter will be safely secured from further unwanted contact with the interplanetary medium.

Since you want to manipulate the antimatter without touching it, using a high energy laser to vapourize small sections is a good start. As each bit of antimatter is "puffed" away from the asteroid like a puff of smoke, another laser can supply the energy to ionize it, allowing for suitable electrostatic and magnetic fields to guide the plasma into a Penning trap for storage and later use.

enter image description here

Proposed antimatter harvester. The ionized antimatter is collected here and funneled into a Penning Trap

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Similar question but with a moon made up of antimatter. Is it realistic to have a moon made of antimatter?

My proposition there:

  1. Cut some chunks off. You can do this using a laser. In the linked moon scenario you would have to blast stuff up and away from the moon because it would be too dangerous to approach. Your smaller chunk is more tractable in that respect.

  2. Confine small chunk using no-touch vacuum maglev jar. Magnetic levitation would be ideal. If a strong enough magnetic field can levitate a frog, you should be able to magnetically levitate and confine your antimatter chunk within a magnetic coil. Just about everything is diamagnetic or paramagnetic to some degree and magnets don't care what kind of matter you are.

  3. Keep jars someplace safe. The moon would be good.

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  • $\begingroup$ How exactly are you going to get the chunk you've sliced off with your laser a) into your jar, and b) charged sufficiently to allow for magnetic levitation. Trying to somehow siphon off positrons via charge induction sounds like a really good way to blow everything up, and any other way to charge a chunk of antimatter just seems even worse. $\endgroup$
    – Gryphon
    Sep 8, 2019 at 2:13
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    $\begingroup$ @Gryphon Charging is not required. Diagmagnetism and paramagnetism are inherent to neutral matter of the appropriate chemical composition--and as noted, that is most chemical compositions, with the exception of the few things that are ferromagnetic $\endgroup$
    – Logan R. Kearsley
    Sep 8, 2019 at 3:31
  • $\begingroup$ Re the jar: I assume the chunk you sliced off is floating near the parent mass, pushed away by the expansion of mass heated by the laser. You would then maneuver the levitation coil around it using a drone. Sort of like no-touch live capture of deep sea animals. $\endgroup$
    – Willk
    Sep 8, 2019 at 23:27
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    $\begingroup$ @Gryphon - I learned a lot about paramagnetism and diamagnetism from a YouTube called Brainiac75. If you are interested, check it out! youtube.com/… $\endgroup$
    – Willk
    Sep 9, 2019 at 0:37
  • $\begingroup$ @Willk Thanks! I'll see if I can find time to check it out. $\endgroup$
    – Gryphon
    Sep 9, 2019 at 2:18
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How exactly would one go about mining an asteroid made of antimatter?

In short:

  1. Laser ablation to blow off parts of the object. The laser can be put in orbit around the body, without physical contact with it.
  2. Channel the plasma to the "box". The produced plasma can be channeled via a suitable magnetic field into a magnetic storage. Again, no physical contact.
  3. Ship the "box". The magnetic storage can be sent to the following user.

Obviously you need a layer of safety: you don't want any interaction between any matter particle and the antimatter, so you need to provide an adequate shielding.

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    $\begingroup$ Does hot antimatter give off photons? ...or antiphotons? Is there such a thing as an antiphoton? If they do exist, might antiphotons interacting with the photons of your ablation laser cause problems? $\endgroup$
    – Arkenstein XII
    Sep 8, 2019 at 20:44
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    $\begingroup$ @ArkensteinXII I have an antiphoton generator in my kitchen - I use it for cooking! Unfortunately, it's just a microwave: Force carriers, such as photons, gluons, Z-bosons, et cetera, are their own antiparticles $\endgroup$
    – Chronocidal
    Sep 8, 2019 at 21:49
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    $\begingroup$ Instead of a laser consider using normal matter a simple lower power electron gun will cut this thing like a hot knife through butter, $\endgroup$
    – John
    Sep 9, 2019 at 13:51
  • $\begingroup$ @John, won't the electrons interacting with the antielectrons give a nice BOOM? $\endgroup$
    – L.Dutch ♦
    Sep 9, 2019 at 13:55
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    $\begingroup$ each electron annihilation is only generating 1.6 x 10^-13 j this is a lot on the atomic scale but pretty low on the macro scale. It essentially works like a high power laser while only requiring very small input energy (less than an old TV). as a bonus it does not produce a high energy cloud of antimatter, or at least produces less of it. It is also oddly a lot more precise than a laser, a laser is limited by the wavelength of the light this is limited by the width of an electron. You can use an ion gun for much the same effect albeit more energetic. $\endgroup$
    – John
    Sep 9, 2019 at 14:04
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You're not really mining, more like controlled exploding.

There is no real extraction just cutting off usable chunks. to do that you just throwing normal particles at it, a superfine stream of low pressure hydrogen will cut this thing like a high power laser through styrofoam. An electron gun or ion gun will cut better than any laser for very little energy input, switching between the two works even better and as others have pointed out you can use this to give the antimatter a charge thus making containment easier.

Just don't use anything powerful you don't want an industrial cutting electron gun, you want something closer to one used in a vacuum tube. You also don't want anything near it, Thucydides idea of a wake shield is great, breaking this thing up would be very very bad for the solar system. As it plows into the solar wind you will have issues with pieces breaking off as it is. you have to worry about anything coming near it hitting these near invisible particles.

As much as diverting to a more convenient location is, the problem is the flecks of matter you will hit doing a pass, steering will be near impossible when every scrap of dust hitting it produces huge amounts of thrust, you have this problem anyway but it gets worst the closer you get to a planet. you want this thing as far from other objects in the solar system as possible then you try to catch it.

The containment vessel will be massively expensive, (likely wiping out any gains for fuel, but for science it is worth doing) you need a magnetic bottle of ludicrous proportions. On the upside as long as you can keep it as a solid containment is fairly easy, expensive but easy. It may be worth refrigerating the entire contraption to prevent heat transfer, radiative heat transfer will still work after all, if it starts outgassing it becomes much harder to deal with. A cold solid lump of antimatter is a lot easier to deal with than a hot gas cloud of antimatter, to the point that putting a sunshade in front of the containment vessel would be a good idea.

The idea is to store it far away form everything then cut off tiny chunks for transport, you will not generate power at its location (not more than to run its containment systems) you transport less dangerous chunks to places to generate power for other purposes. This thing is far too dangerous even bring inside the asteroid belt no matter what safeties you have.

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  • $\begingroup$ There is still conservation of momentum. The explosion will not push it in the other direction. Neglecting relativistic speeds, if you put an equal amount of matter in front of it, you would slow it down by only 50% (assuming total inelastic collision), except everything's annihilated. $\endgroup$
    – infinitezero
    Sep 9, 2019 at 11:30
  • $\begingroup$ Secondly, you don't need much antimatter. Assuming perfect efficiency, 0.5kg has the same output as a small nuclear power plant running for more than 5 years. $\endgroup$
    – infinitezero
    Sep 9, 2019 at 11:30
  • $\begingroup$ Lastly, if you just let it slam into something, nothing about it is controlled. As just stated in my last comment, even 0.5kg of annihilating matter has devastating effects, to the point where the whole "asteroid" will probably just disintegrate. $\endgroup$
    – infinitezero
    Sep 9, 2019 at 11:32
  • $\begingroup$ @infinitezerowhich would be why I said not to use anything large, It is already in a solar orbit you are just trying to control said orbit. $\endgroup$
    – John
    Sep 9, 2019 at 13:37
  • $\begingroup$ Again, using matter to slow it down is cataclysmic $\endgroup$
    – infinitezero
    Sep 9, 2019 at 17:36
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Unfeasible due to frame challenge

If the thing is coming into the solar system, it is in a flyby trajectory. Deceleration due to contact with the interplanetary medium will reduce its velocity in two directions:

  • Radial out: this will rotate the orbital path, but not change it enough to have an apoapsis (and therefore cause a capture);

  • Retrograde: this can decellerate the object enough for a capture - but if a capture happens, this decelleration will never stop due to the object being made of antimatter. It will end up causing the object to go suborbital and eventually crash into the sun.

In either case, you won't have enough time to assemble a mining operation before it either exits the solar system or crashes into the sun.

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    $\begingroup$ I've got to disagree. The acceleration experienced by the asteroid will have a component that is radially outward (due to the movement of the interplanetary medium away from the sun, as you stated). However, the asteroid is also moving, and this will mean that the majority of the impacting material will hit the forward end (the end pointing in the direction of motion). This will cause retrograde acceleration (as the impacting material annihilates with the surface of the asteroid) and slow the asteroid, allowing a capture event. $\endgroup$
    – Gryphon
    Sep 8, 2019 at 2:11
  • $\begingroup$ The asteroid in this scenario is like a plane flying into a strong crosswind. It will be blown sideways, but that doesn't mean it won't experience drag. $\endgroup$
    – Gryphon
    Sep 8, 2019 at 2:15
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    $\begingroup$ @SRM no object can enter the solar system at less than escape speed, which is in the order of 40km/s; the object would quite some decelleration to have an orbit close to neptune's. $\endgroup$
    – The Square-Cube Law
    Sep 8, 2019 at 18:14
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    $\begingroup$ @Renan, escape velocity varies with altitude, and at a distance of Neptune's orbit is only 7.7 km/s. If the periapsis of the flyby is around Neptune's orbit, you "only" need to lose 2.25 km/s to get a circular orbit. You won't be able to do it using friction, but you might be able to do it with a gravity assist. $\endgroup$
    – Mark
    Sep 8, 2019 at 20:17
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    $\begingroup$ Actually the frame challenge gets the other way: won't it absorb too much energy while deceleration, and turn into a blob of plasma? 2 km/s delta-v means 1 MJ/kg kinetic energy, this is barely acceptable, but it grows with square of velocity. $\endgroup$
    – Juraj
    Sep 8, 2019 at 22:46
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Manipulate dust with electric field

First, shield the asteroid from further interaction from interplanetary matter as explained by other answers. Then, make it electrically charged using positrons. We can make positrons (anti-electrons) even with today's technology because they are product of one type of radioactive decay. Then, if there is any surface dust, it will start levitating because of electric repulsion, as it happens sometimes with the Moon regolith. The dust can be created artificially too with laser pulses.

Next, a computers will track all dust particles and use electric field to carefully guide one away and transport it to final destination, all the time by applying electric force (high voltage electrodes). If it detects the particle is moving toward one wall of container, it will put positive voltage on that wall and negative on the other side to stop or reverse the movement.

I believe managing charged antimatter dust particles will be much more efficient than taking care of antimatter plasma, because plasma particles must be moving at high velocities.

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  • $\begingroup$ A magnetic field is probably more feasible than an alternating voltage chamber $\endgroup$
    – infinitezero
    Sep 9, 2019 at 17:41
  • $\begingroup$ But it requires either magnetized antimatter, or very strong magnetic fields. $\endgroup$
    – Juraj
    Sep 9, 2019 at 18:20
  • $\begingroup$ It doesn't need to be magnetised, just charged. $\endgroup$
    – infinitezero
    Sep 9, 2019 at 18:24
  • $\begingroup$ While magnetic field does affect electrically charged objects, it's practical only for charged atomic particles. Not for microgram-sized specks of dust, the force is much weaker than using electrostatic field directly. $\endgroup$
    – Juraj
    Sep 9, 2019 at 18:50
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This really doesn't seem feasible.

I don't have hard numbers, but matter/antimatter annihilation is so incredibly energetic that this object wouldn't really have its own trajectory. any deviation in speed from the background medium would immediately be corrected by the increase in collisions on the front side. that means the object almost certainly wouldn't enter the heliopause, and if it did, it would immediately be repulsed by the solar wind. obviously if it were large enough, it could overcome those forces, but in that case, it would probably be so large that its surface reactions would probably give off more energy than the sun. good luck mining that.

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    $\begingroup$ Your supposition without hard numbers isn't exactly definitive. Unless you can come up with hard numbers to prove this is implausible, I don't exactly see why your blind guesswork carries more weight than anyone else's. $\endgroup$
    – Gryphon
    Sep 8, 2019 at 21:35
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    $\begingroup$ According to this paper on interplanetary dust, "The mass density of solid objects of mass m ≤ $10^{−3} kg$ at 1 AU is comparable to the solar wind mass density ~ $10^{−20} kg · m^{−3}$". If you approximate the asteroid as a cube of sides L meters traveling at v meters/sec in a direction orthogonal to one face, then in 1 second that side sweeps out a volume of v*L^2 meters^3 which would have a mass of about $10^{-20} v L^2$ kg of dust, colliding with the antimatter would release c^2 = ~ 10^17 m^2/s^2 times that amount of energy = $10^{-3} v L^2$ joules. $\endgroup$
    – Hypnosifl
    Sep 9, 2019 at 3:14
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    $\begingroup$ Say the asteroid has a density of 1 gram/cm^3 = 1000 kg/m^3, so in order to have a mass of about 1 kilotonne = 10^6 kg, the volume or L^3 would have to be about 1000 m^3, so L would be about 10 meters. So if the asteroid was initially traveling at about 10^5 m/s (within an order of magnitude of the escape velocity from the sun at 1 AU), the amount of energy released from matter/antimatter annihilation in the next second would be $10^{-3} vL^2 = 10^{-3} * 10^5 * 100$ = 10^4 joules. But the initial kinetic energy of the whole asteroid at that speed would be 1/2 * 10^6 * (10^5)^2 = 0.5*10^16 j. $\endgroup$
    – Hypnosifl
    Sep 9, 2019 at 3:22
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    $\begingroup$ So, even if the kinetic energy were being continually reduced by 10^4 joules every second it would take about 0.5 * 10^12 seconds (around 16,000 years) for it to go from the initial kinetic energy to zero...and that's more like a lower bound since in reality not all the energy from the M/AM collisions will go to reducing the asteroid's kinetic energy, and as the asteroid's speed decreases the number of collisions with dust particles will reduce to lower values on successive 1-second intervals. So, on human timescales the slowdown due to collisions with interplanetary dust is negligible. $\endgroup$
    – Hypnosifl
    Sep 9, 2019 at 3:26
  • $\begingroup$ thanks for confirming my theory with hard numbers. according to your math, a 10 meter asteroid would emit 10^4 joules of energy per second. that's not quite as energetic as the surface of the sun, but plenty hot to vaporize the entire asteroid in a matter of minutes, sending antimatter gas streaming away like the solar wind of a tiny star. $\endgroup$
    – teldon james turner
    Sep 27, 2019 at 22:33
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Magnetic confinement seems to be a fairly common suggestion and would work in the technical sense, but the amount of energy actually required to magnetically confine an asteroid worth (over the lifetime of the "mine") would be extraordinary. This is particularly concerning given that the containment field would not only have to contain the antimatter under constant acceleration, but would also have to have to account for the acceleration/deceleration of the transport device.

Keep in mind that the frog example mentioned above required a 16 Tesla magnetic field using a bitter magnet requiring ~20 kiloamperes of current. That's the electrical draw of 130-200 modern households to levitate the mass of a frog under constant acceleration. Add to that additional mass imparted through acceleration and you very quickly get to energy levels that are not reasonable (not even going to go into dealing with angular momentum). While technically the antimatter would produce substantially more energy than what is needed to cover this, transportation this way just feels like an inefficient/overly technical response to a problem.

A better option would be to build a power plant directly around the asteroid, and to perform the energy generation activities within relatively close proximity. The antimatter could be moved through short term containment, or even better, could be directly blasted into the reaction chambers (requiring only a short containment for sealing etc). The energy produced can then be converted into far more "docile" states which can be transported with much less complication (For example as Ultra-compressed gases/liquids etc etc).

I know you have disqualified fusion energy as an option, but I think it's important to consider that such a discovery would substantially improve the feasibility of fusion energy production as a means of energy production. One of the primary hurdles to fusion energy is generating enough energy to initiate the fusion reaction to begin with. Current technologies operate using a similar magnetic containment principle, only in this case we are containing ionized plasma either with external or intrinsic magnetic fields. A major hurdle in the process is the mind numbing amount of energy require to initialize a fusion reaction (something on the order of 100 million degrees kelvin). Short of discovering the miracle of cold fusion, the next best option of achieving such incredible temperatures would be antimatter. In this regard, using antimatter as the seed to fusion reactions, this asteroid could effectively generate unlimited energy so long as the fusion reactors have a steady supply of fuel.

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The IPM near Earth is 5 particles/cm^3. If we assumes this is 5 hyrdogen atoms we won't be far wrong.

First, lets do some Feynman math to see how plausible this is.

Everything is about as dense as water, give or take an order of magnitude. A water-density object that weights 1000 tonnes (or $10^6$ kg) is going to be about 10 meters on a side.

Solar escape velocity at Earth Orbit is about 40 km/s. So your asteroid will pass over a volume of 10 m * 10 m * 40 km / s or 4 * 10^6 m^3/s.

There are 100^3 cm^3 in a m^3, so this is 10^6 * 4 * 10^6 * 5 hydrogen atoms/second, or 4*10^12 hydrogen atoms/second.

There are 6*10^26 hydrogen atoms per kg, so this is 0.7 * $10^{-14}$ kg/s of hydrogen swept up.

Times two, times c^2, gives us about 1.25 kwatts.

The kinetic energy of a 1 kilotonne object moving at 40 km/s is 8*10^20 J. The gravitational binding energy of such an object is 8 J. This is a problem.

Assuming the energy released is 100% efficient at slowing the asteroid, it would take about a million light years at that speed to reduce the kinetic energy of this asteroid by 50%. Collisions with the ISM won't, on average, do anything detectible other than make the asteroid glow with high-energy photons (and deposit some heat).

If the asteroid hit something more substantial (say, a mote of dust or whatever), the first problem is that anything that can slow it down from solar escape velocity will also blow it apart.

The problem is that the binding energy of any natural asteroid is going to be tiny, and the KE of a comet is huge; any impact that is enough to soak the KE in a short period of time is going to shatter the asteroid.

Can chemically bound might survive such an impact? A pure anti-Tugsten has a chemical binding energy of 850 kJ/mol. That mol has a weight of a bit under 200 g. 1/2 (40km/s)^2 * 200 g is 160,000,000 J. So an impact that would slag the asteroid would shave off half-a-percent of the KE of the asteroid.

So no impact/ism based energy deposit is going to do anything useful.

For the asteroid to be captured, it will have to get extremely lucky with orbital mechanics. It falls inward, does a close pass by a gas giant that "slingshots" it. That might be enough to kick it into a long elipitical orbit. If the slighshot (through insane luck) then swung by another large planet "just right", it could shed more KE and have a less elongated orbit.

A traditional capture would involve doing this dozens of times over millions of years, where a a few failures results in the asteroid being ejected from the system or hitting a planet.

In your situation, the best you could hope for is losing 5-10 km/s from a gas giant flyby. It has to get lucky and avoid hitting anything larger than a mote of dust (even a mote of dust will create an explosion, but maybe not enough to destroy the asteroid).

Then you might be able to use a gravity tug, over decades, to move it into a better orbit.

Once you have it reachable...

To cut is, fire a stream of electricaly neutral particles (to avoid giving it a net charge; any electron you shoot at it will cause a random proton to be attracted to the object as it seeks to become electrically neutral). As even a ghostly amount of such particles hit, it will produce an extremely vigorous energy emission; far more than anything you could generate for anywhere near the same amount of effort. I might fire separate proton and electron beams to cut different spots, or have them merge into a neutral beam.

Once you have defeated whatever chemical bonds are keeping the asteroid together, they'll drift apart easily.

You won't want to get anywhere near the object. Even rocket exhaust should be avoided near the target; it will be insanely more dense than the ISM. So you might want solar sails, avoid being either up or down "wind" of it (downwind, because it will shed antimatter; upwind, because you'll shed matter). You'd cut a piece off, then can move that piece by hitting it very gently with a beam.

Capturing an isolated piece becomes extremely tricky. Tricks with magnetic bottles etc are fine, but probably the easiest is to throw near-vacuum matter at it and let it glow and move itself.

You'd maintain some kind of defence system around the anti-asteroid, to avoid even dust particles from hitting it. As noted, a few microgram dust particle is enough to give it a kick enough to utterly change orbits; a defence system powerful enough to detect [b]every little bit[/b] of microgram dust and deflect it would be an extreme challenge.

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  • $\begingroup$ "Solar escape velocity at Earth Orbit is about 40 m/s" are you quite sure about that? $\endgroup$
    – Starfish Prime
    Sep 9, 2019 at 21:40
  • $\begingroup$ @starfish gove or take a factor.of 1000 $\endgroup$
    – Yakk
    Sep 9, 2019 at 22:22
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    $\begingroup$ @StarfishPrime Fixed. That change did eliminate the "mote of dust" braking possibility. $\endgroup$
    – Yakk
    Sep 10, 2019 at 17:52

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