Friday, January 3, 2025
Thursday, January 2, 2025
A little info on the Van Allen Belts relevant to space travel through them
The Van Allen belts, are not actually part of our atmosphere. They’re well beyond it, extending hundreds of miles outwards into space. There are two, both donut-shaped rings surrounding our planet, and are a consequence of our planet’s magnetic field. The Space Shuttle typically orbited at a height of 190 miles to 330 miles above the surface, and the International Space Station orbits at a height of somewhere between 205 and 270 miles above the surface of the Earth.
The innermost Van Allen belt sits somewhere between 400 to 6,000 miles above the surface of our planet. Even if the innermost belt is at its closest, the ISS (and the space shuttle in its day) are more than 100 miles away from the Van Allen Belts. For near-Earth missions, the Van Allen belts are not a hazard to spacefarers.
It was, however, a hazard for the Apollo missions. The Van Allen belts are not a physical barrier to spacecraft, and so, in principle, we could have sent the Apollo spacecraft through the belts. It would not have been a good idea. The Van Allen belts are a kind of trap for charged particles like protons and electrons. They’re held in place by the magnetic field of the Earth, and so they trace the shape of the magnetic field itself. The problem with the Van Allen belts lies not in them being impassable, but in the charged particles they contain.
Charged particles are damaging to human bodies, but the amount of damage done can range from none to lethal, depending on the energy those particles deposit, the density of those particles, and the length of time you spend being exposed to them.
In the case of the Apollo missions, the solution was to minimize the second two factors. We can’t control the energy of those particles, though they can be large. The density of the Van Allen belts is well known (from sending uncrewed probes through them), and there are hotspots you can definitely avoid. In particular, the innermost belt is a rather tightly defined region, and it was possible to stay out of it for the trip to the Moon. The second belt is much larger, and harder to avoid, but there are still denser regions to avoid. For the Apollo trips, we wanted to send the astronauts through a sparse region of the belts, and to try and get through them quickly. This was necessary in any case; the crafts had to make it to the Moon in a reasonable amount of time, and the shorter the trip, the less exposure to all sorts of radiation the astronauts would get.
In the end, it seemed that these tactics worked; the on-board dose counters for the Apollo missions registered average radiation doses to the skin of the astronauts of 0.38 rad. This is about the same radiation dose as getting two CT scans of your head, or half the dose of a single chest CT scan; not too bad, though not something you should do every week.
The Van Allen belts can be dangers to space exploration, but with careful observations, orbital maneuvering, and inventiveness, we’ve navigated our way beyond them many times. Hopefully, we'll continue to do so in the future many times more.
