“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.
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.
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.
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.
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.)
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.
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.
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?
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.
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!
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.
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.
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.
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!