One of the many, many worries people had when first sending humans to the Moon had to do with the Van Allen radiation belts. These are layered, two-lobed areas of space around the Earth that have an unusually high density of high-energy charged particles, including electrons. These electrons damage electronics, penetrating deep into a spacecraft and often causing harmful releases of energy in semiconductors or electrical relays. When the Apollo missions sent humans through these belts of space, NASA simply had no idea what to expect since prior human flights had never gone far enough out to cross the fields. The astronauts zipped through unharmed, however, and today the Van Allen belts aren’t thought to pose a significant danger to living things so long as they are shielded and don’t spend too long inside.
However, even after all that, we still had no clear understanding of why the belts were so dangerous to electronics — an invisible force appeared to be ramping up these charged particles to nearly the speed of light, but where was that force coming from? We eventually developed two competing theories, one which said the source of accelerating energy was foreign, the other arguing that it was local. We know the particles mostly come from gusts of solar wind, but is there something intrinsic to our area of space that gives the particles a boost? This week the journal Science published the answer: it’s the Earth’s own magnetic field that makes the Van Allen belts so dangerous.
The cause, it seems, are lower-energy electrons that give off just the right frequency of electromagnetic radiation, in this case in the radio portion of the spectrum. It’s a powerful enough source of energy to be detectable with a hand-held antenna and headphones, though that can’t be too surprising given the level of energy it can impart to particles in the Van Allen belts. Lead researcher Geoffrey Reeves likens the effect to hitting a tether ball: “The waves have just the right frequency to hit that tether ball each time it comes around, at just the right time, so it goes faster.” Eventually, these electron tether balls approach relativistic speeds.
For decades it was believed that Earth had only two Van Allen belts, but just a few months ago the Van Allen probes discovered a third, much farther out than the first two. It turned out to be transient, eventually being blown away by a strong solar shockwave. Still, as we become increasingly dependent on global communications technology, a detailed understanding of these belts of space will become more important. Everything from GPS satellites to research telescopes shield their electronics and generally shut down when entering them to minimize the chance of damage.
Even then, solar storms and local geomagnetic phenomena can swell the fields dramatically, sometimes engulfing whole fleets of satellites with little warning. It’s only recently that scientists have truly appreciated how volatile these fields of space can be. Right now, their changes are often unpredictable — but this breakthrough might help us change that. Understanding the nature of the the space around our planet will be critical to predicting its actions.
Interestingly, there is a proposition to actually destroy the Van Allen belts with a program called the High Voltage Orbiting Long Tether (HiVOLT). This system of five 100 kilometer-long charged tethers would be deployed from satellites and magnetically deflect the charged particles. This would disburse them remarkably quickly, with projections putting the electron flux at just 1% of today’s level after two months of operation.
Regardless, understanding the Van Allen belts will be necessary if want to have any hope of continuing to advance our mechanization of the skies at the current pace.
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Research paper: doi:10.1126science.1237743 - “Electron Acceleration in the Heart of the Van Allen Radiation Belts”
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