Why the Earth wears an Antimatter Belt!

“Magnetism, as you recall from physics class, is a powerful force that causes certain items to be attracted to refrigerators.” -Dave Barry

One of the first “invisible forces” people encounter in this world is when they’re first exposed to the humble magnet.

As the image above shows, you’re most familiar with what magnets do to other magnetic (or magnetizable) materials, like the paper clip.

But magnets also exert forces on electrically charged objects. They do it, though, in a way that you’re certainly not used to. Unlike gravity, where the gravitational field pulls you in the direction of your gravitational sources (like mass), magnetic fields treat charged particles quite differently.

Image credit: The Food and Drug Administration. No, really!

Rather than get pulled in the direction of the magnetic field, moving charged particles generally get dragged into helical shapes by magnetic fields, unlike the smooth parabolas and ellipses of gravity. In fact, at a fundamental (subatomic) level, this magnetic force — on electrically charged particles — is responsible for all the magnetic phenomena that you know in the world, from your refrigerator magnets to your hard drive.

It’s also, believe it or not, responsible for protecting the Earth.

Image credit: NASA, and I believe SDO.

The Earth’s magnetic field — the same field that causes your compass needle to point North — extends far out into space, and shields us from the high-energy, charged particles that come from not only the Sun, but also from powerful galactic and extra-galactic sources!

The Solar Wind, however, would by far be the most deadly to us, were it not for this magnetic “shield” that the Earth produces.

Image credit: NASA.

Instead, though, the Earth’s magnetic field bends charged particles (mostly) away from the Earth, with the rare exception of when particles get “funneled” into the polar areas, producing the beautiful light displays known as the aurorae, or the Northern (or Southern) Lights.

Image credit: International Space Station.

However, just because most of these charged particles don’t hit us doesn’t mean that we can’t trap them within our magnetic field! (In fact, we’ve known about this, and an excellent review can be found here.)

Image credit: Bickford et al., as are many subsequent images.

We know of two belts around our planet, in fact, where there are excesses of charged particles “trapped” in just such a fashion. Relatively close to the Earth, there’s a band of protons that lives in our magnetic “belt”, and a little farther out is a band of electrons.

Image credit: aero.org, J.E. Mazur and others.

Known as the Van Allen belts, these not only exist around Earth, but around presumably every planet with a substantial magnetic field. Since we see other planets with substantial aurorae, they likely have their own Van Allen belts.

Image credit: Cassini / NASA.

In the case of Saturn, for example, there are likely thousands of times as many protons and electrons trapped in the Van Allen belts.

But it isn’t just normal matter that gets trapped there. For many years, people have theorized that there should be antimatter trapped in belts around our planet as well!

How’s that possible?

Image credit: Bickford, as above.

Making antimatter is actually pretty easy, if you’ve got enough energy. Smash a proton into anything else — with enough energy — and via E = mc2, you can make extra particles and antiparticles. You can do this from any energetic enough particle, including ones that come from space!

The easiest way to get one into a stable (or quasi-stable) orbit around Earth, however, is to emit one of these antiparticles from the Earth itself in the proper direction.

Image credit: aero.org.

How do you get the Earth to do that? Believe it or not, high-energy particles from space striking our upper atmosphere all but make this inevitable. In particular, cosmic rays strike the Earth, and produce particles like neutrons (and antineutrons) in the upper atmosphere.

When they eventually decay, producing protons (and antiprotons), some of these particles will come out with just the right trajectories to get trapped in the Van Allen belts!

Image credit: Bickford again.

Well, that’s the theory, anyway. But this is news, now, because we’ve found this antimatter belt in space!

A team working with the PAMELA experiment has just published a paper called The Discovery of Geomagnetically Trapped Cosmic-Ray Antiprotons (free version here), where they’ve in fact discovered this “belt” of antiprotons in between the two Van Allen belts of normal matter!

And immediately, your imagination should start running wild.

Image credit: ESA/ITSF; Manchu.

The BBC imagines the ultimate fuel source for interstellar travel, while cynical PZ envisions antimatter bombs.

Hate to burst your bubble, but as much fun as a magnetic ramscoop would be, it’s not going to power your spacecraft. Yes, antimatter is the most efficient fuel source conceivable. I’m a big fan; per kilogram, nothing in the Universe will give you as much energy output.

Table credit: Bickford once more.

Unfortunately, although this is being reported as factors of thousands or tens-of-thousands times more than other antiproton sources, those are basically the sources you get in interstellar space. All told, we are talking about maybe a few nanograms of antiprotons in the entire space between the two Van Allen belts! You could collect it all — draining the region between the Van Allen belts of antiprotons — and it would contain about as much total energy as your car battery.

Even if you went to the most abundant antiproton-trapping planet in the Solar System, Saturn, here’s what you’d find.

Image credit: Bickford one last time, and see that paper for the explanation of why Saturn, and not Jupiter, traps the most antiprotons.

About a milligram of antiprotons. Yes, there’s about as much energy stored in a milligram of antiprotons (which is 1,000 times more than Saturn has; thanks Alex @4) as there is released in a large nuclear bomb, but it takes much more energy than that just to get to Saturn in the first place.

Still, we have an antimatter belt, and now you know why!

(Also, if you aren’t following me on Google+, you should be. And if you need an invite the first 150 clicks get one!)


  1. #1 Brownian
    August 9, 2011

    While I kept hoping that there would be a “to keep its antimatter pants up!” punchline somewhere along the way, I was fascinated by this post nonetheless! Thanks!

  2. #2 David Rutten
    August 9, 2011

    Does the Sun trap anti-matter as well?

  3. #3 David Rutten
    August 9, 2011

    When the magnetic field reverses itself, doesn’t it go very weak for a considerably long time? The geologists claim this reversal happens quite often, but clearly surface life has survived these cosmic ray bombardments over and over again. So what, did animals just get a lot of skin-cancer during these periods or are the cosmic rays and solar winds not quite as devastating as portrayed?

  4. #4 Alex Besogonov
    August 9, 2011

    “About a milligram of antiprotons. Yes, there’s about as much energy stored in a milligram of antiprotons as there is released in a large nuclear bomb, but it takes much more energy than that just to get to Saturn in the first place.”

    Uhm. No. One milligram is a large conventional explosion. One _gram_ of mass-energy is about 20 kilotons of TNT equivalent (about the same as in Hiroshima bombing).

    One milligram would be about the power of a truck loaded with explosives.

  5. #5 Ethan Siegel
    August 9, 2011


    Nice catch! Off to fix that… hate when I get my Joules and KiloJoules mixed up.

  6. #6 Vince whirlwind
    August 9, 2011

    Brilliant contribution to Science Blogs, as usual, thanks.

    Makes a nice change from all the other dross.

  7. #7 Daniel J. Andrews
    August 10, 2011

    Instead, though, the Earth’s magnetic field bends charged particles (mostly) away from the Earth, with the rare exception of when particles get “funneled” into the polar areas, producing the beautiful light displays known as the aurorae, or the Northern (or Southern) Lights.

    Is it really all that rare? I’ve lived and worked in the far north of Canada quite a bit, and seeing auroras in the winter was a regular thing. Sometimes it was just a blue glow, but many times it was flickering, moving and had more green or sometimes even red colours. From stories told to me by those who stayed in the high Arctic, they said they saw it most every night when the sky was clear.

    Seeing it further south is rare, but I strongly suspect aurora displays at the far north and south of the planet are common.

  8. #8 SCHWAR_A
    August 10, 2011

    Ethan, what do you think will happen to a matter-based spaceship, when passing through this anti-matter belt?

  9. #9 Jonas
    August 10, 2011

    Assume its all evenly distributed in a perfect sphere in the inner Van Allen belt for easier calculations
    Earth radius: 6000 km
    Inner belt radius: 100-10000 km beyond earth

    Volume of sphere where antiprotons are: 4pi/3 * (16000000^3 – 6100000^3) = 16198292880000000000000 m^3

    1 nanogram of anti-protons (assume same weight as proton): ~598802395209580 antiprotons
    which gives us 3,7 * 10^-8 antiprotons / m^3

    So you might see an extra pair of photons and lose a proton or two as your spaceshit crosses this “belt”, if you’re lucky…

  10. #10 OKThen
    August 10, 2011

    Amazing. I must confess that the accumulation in nature of any appreciable amount of antimatter never occurred to me. I frankly assumed that any antiproton or antielectron would interact away within milliseconds. And a nano gram is something.

    So thank you Ethan for putting my initial excitement into perspective. The BBC artile left me thinking that there were tons of the stuff just waiting to be scooped up.

    But I am left with a few other questions to help me put the natural occurrence of antimatter in perspective. (Oh and by the way, I never knew that one of the van Allen belts was protons and the other electrons. So thanks for that too! Even old news is exciting (i.e. new)when you never knew it before.)
    - are the normal matter antimatter belts mostly particle from earth that are trapped?
    - in theory are there two antimatter van Allen belts (one for positons and one for anti-protons? I assume so.
    - I always assumed that the van Allen belts were moving (as in a current of charged particles). Would the antiproton belt be moving in the opposite direction as the proton belt? And I assume that it would be at the same location.
    - Also, re Jonas’s remark, is the normal van allen belt more a belt than a sphere? and if a belt or sphere, now I’ve mixed myself up and will need to research why the northern and southern lights.

    Now to put this antimatter belt in perspective
    - I assume that the normal matter antimatter belt is much much more massive than the antimatter belt. Can anyone give me a mass of the proton belt in kilograms? Thanks.
    - Since I assume that the proton and antiproton belt occupy the same orbit; I assume that collision with protons (going in the opposite direction) eventually limit the number of antiprotons?
    - Assuming my assumptions are correct, is there maybe some (theoretical or observed) natural source of antimatter (maybe some kind of cosmic jets) that naturally separate the antimatter (e.g. north) from the matter (e.g. south)? I assume so but don’t know.
    - And how does the antimatter in the van Allen belt compare with the antimatter production of fermilab or CERN LHC? Amount but also are there any experiments that can be done with van allen antimatter that can’t be done in the lab?

    So you see how one little piece of news reveals how little I really know.

  11. #11 Jonas
    August 10, 2011

    OKThen: It’s a torus. http://en.wikipedia.org/wiki/Van_Allen_radiation_belt

    I just wanted to do some calculations to show how small the density of anti-particles really was and chose a sphere for easier calculations.

    The biggest hazard you would face by going through the Van Allen belt is normal radiation, not anti-particles annihilating your spaceship.

  12. #12 Joffemannen
    August 10, 2011

    I’d imagine the few nanograms are in some form of equilibrium – if we did harvest that car battery worth of energy, the belt would recreate itself? And if most those antiprotons are created in the upper atmosphere, they would be created in specific regions, i.e. rings on some height and latitude. So we could place some contraption there, which would change that equilibrium and/or increase the likelihood of sending new antiprotons in the right direction. So we could harvest a car battery worth of energy per day! Obviously not the most energy efficient way of getting car batteries worth of energy, just thinking

  13. #13 mikmik
    August 10, 2011

    I thought that anti-matter only existed in stable amounts in esoteric, secluded, and very distant regions of space as it would otherwise all mix with matter and be destroyed anywhere else. Great article! For some reason I am comforted knowing that there is an appreciable, natural occurring reservoir right around(pun) here.
    I wonder, though, why there isn’t two belts of anti-matter, one consisting of +e, and one of -p, because I thought neutron decay led to an electron/proton pair. I know it does because the total charge must be zero, and the mass equal to neutron mass.

    Anyways, this blog you’ve created is by far my favourite source for physics information and beautiful and/or informative graphics and charts. You present ideas so elegantly, even a caveman would get it. :)

    As if that wasn’t more than I really could hope for, you make it even better!! I’ve been wanting a Google plus invite for a month with no hope of getting one, yet now I have an account, thanks very much to you, Ethan.

  14. #14 mikmik
    August 10, 2011

    Also, there would be two milligrams of matter converted to energy, for it takes a milligram of regular matter to combine with a milligram of antimatter to completely annihilate it all.
    Wouldn’t it?

  15. #15 SCHWAR_A
    August 10, 2011

    @Jonas (9):
    Thank you. I have read that the anti-protons are trapped between the belts and thus cannot escape from there. Thus I concluded lots more of them than you showed to us…

  16. #16 Eric Lund
    August 10, 2011

    Daniel @7: You are correct that auroras are common. The ones you saw in Canada (and I have seen on visits to Alaska) are caused by electrons coming in from the night side of the Earth. You are also correct that aurora is rarely seen much further south–I have seen only two or three auroral displays from the lower 48. (Cloud cover and light pollution further limit my opportunities.) Solar energetic particles do cause aurora at the cusps, and this is quite common (the Sun’s magnetic field interacts with the Earth’s, through what is called magnetic reconnection), but for obvious reasons is hard for humans to observe–Svalbard, at 79 degrees north, is the only inhabited place on Earth where you can see aurora at noon (for about six or seven weeks in the winter).

  17. #17 BenHead
    August 10, 2011

    @7 @10 Perhaps he meant rare on a per-particle basis, like only a tiny fraction of the particles are funneled in to form aurorae. Just a guess; it was a little unclear in the wording.

  18. #18 OKThen
    August 10, 2011

    Jonas thanks.

    Such excellent science especially considering the small amount of antimatter in the belt.

    Ethan’s link here also
    is a 77 page review, much of it is readable.
    “The worldwide output (of antimatter) is currently in the low nanogram per year range (from particle accelerators) at an estimated cost of up to $160 trillion per gram.” pg 5. So the quantity of antimatter in van Allen belt is about the same as are generated and captured in accelerators worldwide.

    But that $160 trillion per gram price tag has got to come down. I mean at a $1 billion per gram, i.e. per gram of controllable antimatter, we might have a business model.

    “the antiparticle with the opposite charge will spiral and drift in the opposite direction.” Yes. but the spiralling is complicated.

    “Saturn likely has a stable antiproton population of hundreds of micrograms based on extrapolations from the conservative Earth estimate.” pg 8. Wow, we have to put an automated antimatter “magnetic scoop” in orbit around Saturn.

    “Production in the space environment offers an intrinsic advantage for space exploration since the particles are generated and stored in orbit, therefore mitigating the need to transport them from the ground in bulky and heavy traps. A generator system placed in orbit will be far more efficient at collecting the antiparticles since the natural radiation trap will capture a wide angular distribution and energy range with minimal complexity.” pg 9. Wow, there we go.

    Thus, it would seem that the best place to do antimatter experiments is also in space. For example, wiki says, “the overwhelming consensus among physicists is that antimatter will attract (gravitationally) both matter and antimatter at the same rate that matter attracts matter, there is a strong desire to confirm this experimentally.” That’s an experimental result that I’d like to know one way or the other.

    “Scaling this to a 1 GWe power source would enable nearly 100 milligrams of antiprotons to be produced per year. This level of antiproton generation is sufficient to enable the first interstellar missions to nearby stars… The antiparticle can safely be stored in this artificial radiation belt which forms in the space surrounding the exterior to the vehicle.” pg 9. Wow, this is humanly doable. Where is my Hitchhiker’s Guide to the Galaxy?

    “The total rate of collection (in earth’s van allen belt) with a very large 100 km coil system could be as high as 100 micrograms per year for the worst case flux… This is three orders of magnitude faster than the current capabilities of Earth based accelerator production.” pg 46, Yes tough science and difficulties are described. But those difficulties are the opportunity of business, technical and economic spinoff.

    “Magnetic scoop” does not exactly fire the imagination; but mark my words, within 50 years this will be a trillion dollar business. What government wouldn’t pay $1 billion dollars for a gram of antimatter; even if it was delivered 500 miles up in space. Even North Korea would spring a counterfeit $1 Billion for a gram. “The proposed collection system does not require the development of any fundamentally new technology to make it work.” pg 51. “Technology Gap… Technology Gap… Technology Gap.” OK, I get the point.

    “Antimatter has an energy density more than ten orders of magnitude higher than the best chemical propellants currently used in rocket systems.” pg 54, Ethan couldn’t have said it better. NOT ten times, “ten orders of magnitude “!!!! And yes we understand “extreme difficulty (and cost)”

    “Artificially generating antiprotons in magnetospheres (natural or otherwise) would be very valuable and efficient. By effectively locating the particle accelerator within the magnetic ‘bubble’, the system can produce and trap antiparticles within high efficiency which can then be used for propulsion.” pg 58. There you have it. If I understand this next step is to put “a space qualified nuclear reactor (Project Prometheus)” in space and generate an enhanced antimatter van Allen belt. Cool!! It is really nice to find crazy ideas that make sense and might actually get done!!

    Yeah, environmental concerns and cautions; but if we don’t the Chinese will!!

    Wiki, “Project Prometheus was established in 2003 by NASA to develop nuclear-powered systems for long-duration space missions… Its budget shrank from $430 million in 2005 to only $100 million in 2006, $90 million of which was for closeout costs on cancelled contracts.” Oops, lack of vision.

    A couple of other searches turned up a few other antimatter tidbits I did not know:

    “the mysterious cloud of antimatter in the central regions of the Milky Way… The observations have debunked the chances that the antimatter is coming from the annihilation or decay of astronomical dark matter… The cloud itself is roughly 10,000 light-years across, and generates the energy of about 10,000 Suns.” Wow.

    “the hypothesis of the gravitational repulsion between matter and antimatter can be tested at the Ice Cube, a neutrino telescope, recently constructed at the South Pole.” Nice but repulsion of matter and antimatter is not the only one hypothesis.

    My personal speculation is that matter and antimatter are gravitationally neutral; while antimatter and antimatter gravitationally repulse. But experiment and observation will decide; I await patiently because gravitational experiments are extremely difficult.

  19. #19 Jonas
    August 10, 2011

    If you want to read a good sci-fi trilogy re: antimatter harvesting you should check you Peter F Hamilton’s Night Dawn Trilogy

  20. #20 OKThen
    August 10, 2011

    Thanks, I’ll check it out. Amazing how science and science fiction feed off one another.

  21. #21 Vicki
    August 10, 2011

    (Will following your on Google+ get me additional content, or just mean I don’t need to click over here to see your posts?)

  22. #22 MRC
    August 10, 2011

    Interesting read..

  23. #23 Michel
    August 11, 2011

    psssst ethan…. don´t tell Phil Plait about this… he´ll make us go POOF!!

  24. #24 forrest noble
    August 15, 2011

    I would expect that although the theory seems quite sound, the longevity of such anti-particles I believe entirely based upon our belief that anti-protons are long-lived particles in the absence of matter, but I believe present theory is wrong and that they are short lived particles unless we could someday figure out how to artificially extend their lives by re-enforcing their spin.

  25. #25 SCHWAR_A
    August 15, 2011

    @forrest noble (24):

    “…someday figure out…”

    Please look here, the yellow mark-up. How is that compatible with your hypothesis?

  26. #26 Johnny
    August 25, 2011

    Only if the heavens above can be measured and the foundations of the earth below be searched out will I reject all the descendants of Israel.