Pedagoguery

Space is a hostile environment. The lack of gravity and an atmosphere create huge problems for human habitation. Perhaps the most insidious problem, however, is posed by radiation, particularly cosmic rays.

Cosmic rays hare high energy atomic nuclei, mostly protons, that fly through space at nearly the speed of light. Earth is constantly bombarded by cosmic rays, but we feel very little in the way of effects on the Earth's surface. This is not due to Earth's magnetic field, which is far too weak to have much of an effect on particles with energies as high as those of cosmic rays. Rather, it is the bulk of Earth's atmosphere that provides the bulk of the shielding.

When a cosmic ray enters Earth's atmosphere, it typically travels about 1/14 of the way in before hitting the nucleus of an atom in the air. The collision will typically knock a proton or neutron loose from the nucleus and unleash a shower of gamma rays and particles called pi mesons, or pions. The gamma rays will propagate deeper into the atmosphere and generally produced electron positron pairs, which annihilate and produce lower-energy gamma rays. That cycle will continue until the gamma rays have too little energy to produce electron-positron pairs. The pions, however, will quickly decay into mu mesons, or muons, which penetrate to the ground. It is this shower of secondary radiation which makes shielding against cosmic rays so difficult. At sea level, however, it amounts to about the level of a couple of chest x-rays every year – enough for our bodies' natural self-repair mechanisms to deal with.

In space, however, this cascade of reactions takes place in the spacecraft and the astronauts, yielding about 5000 ions zipping through a person every second. These ions can cause a lot of damage, leaving trails of broken chemical bonds and damaging free radicals it its wake. Plus, heavier ions carry much greater potential for damage, since their ability to break chemical bonds is proportional to the square of their electrical charge. So, an iron nucleus does 676 times the amount of damage as a single proton.

The effects of radiation on the body are in many cases not well known. Most of the data we have comes from those unfortunate people exposed to massive doses in a short period of time, such as those in close proximity to nuclear tests or nuclear accidents. While an astronaut on a trip to Mars would suffer a similar radiation dose, it would be spread out over a much longer period of time, and no one knows is the comparison is valid. Even so, there is little reason to take any risk, so shielding astronauts from cosmic radiation is essential. Such shielding would also be effective against solar radiation as well, although solar storms are much more intermittent.

Three mechanisms have been devised for such shielding. The first uses the same mechanism as the Earth: matter. Earth's atmosphere has a mass of about one kilogram per square centimeter. Astronauts could probably make due with half that, the equivalent of an altitude of 5500 meters (17,800 feet). Any less than that and the shielding would fail to absorb all of the “shrapnel” of the cosmic ray collisions. Assuming you use water, the shell has to be 5 meters deep (about 16 feet). That amounts to about 500 tons of water for a small capsule. When you consider that the space shuttle has a cargo limit of 30 tons, this starts to look infeasible, especially if you want to scale up. Water works well because the astronauts will need it anyway, and hydrogen works best as a shielding agent for a given mass, because in heavier nuclei, the protons and neutrons shield each other, giving the nucleus a smaller collision cross section for the mass than a proton. You can increase the quantity of hydrogen by using ethylene (C₂H₄) which can be polymerized into polyethylene, which is a solid. That reduces the shielding down to 400 tons, still not feasible.

A second possibility is magnetic shielding. If you could generate a sufficiently strong magnetic field, you could deflect the incoming cosmic rays. The problem is, it needs to be very strong: about 600,000 times the strength of the Earth's magnetic field. Such a strong field would require superconducting wires to handle the strong current. A suitable scheme for this has been devised that mases only 9 tons – an improvement but still pretty massive. Naturally, it has some drawbacks. First of all, magnetic fields would provide no shielding at the poles. This means that the living quarters of the spacecraft would have to be dough nut shaped, to leave the magnetic axis empty. Secondly, a magnetic field that strong could have serious biological effects and no one knows what they might be. People experiencing a field 1/40th the size have reporting flashing lights and a string acid taste in the mouth when they move in such a field. One possibility is to use a second superconducting magnet to partially counter the field in the living quarters. Unfortunately, this does not completely cancel the field and it greatly increases the complexity of the system.

The third possibility involves electrostatic shielding. If you can fling away electrons from the ship, you leave the ship with a net positive charge. That can repel incoming cosmic ray particles if it is strong enough. The advantage of this is that it has no coverage gaps and produces no hazardous effects for the crew. However, to be effective, the ship would have to be charged up to two billion volts, requiring a gargantuan electric current. Plus, it would server as a powerful attractor to any negatively charged particles in the vicinity. It is the solution that has had the least study, however, so perhaps the issues can be overcome.

In the final analysis, interplanetary travel will require some major problems to be overcome if the safety of the crew is to be assured.

Next issue: what were the first stars of the universe like?