In terms of radiation, the main threat to the health of people in space comes from particle radiation (as opposed to electromagnetic radiation). As we saw in Chapter 6, this is essentially made up of energetic (rapidly moving)
subatomic particles, such as electrons, protons, and sometimes ions (the nuclei of atoms stripped of their attendant electrons). This type of radiation comes from a number of sources. For people in Earth-orbiting spacecraft, the main offending source is the Van Allen radiation belt, which contains high-energy electrons and protons that have been captured from the Sun and trapped by Earth's magnetic field. However, in low Earth orbits of altitudes less than about 1000 km (620 miles), spacecraft are well below the most intense parts of the Van Allen belt, and are also protected from the majority of direct solar particle radiation by the Earth's magnetic field. As a consequence, astronauts living long-term in the ISS, at an altitude of around 350 km (220 miles), suffer a relatively low level of potentially damaging radiation. However, the use of higher orbits for long-term habitation, for example in the most intense part of the proton radiation belts, which is at a height of about 4500 km (2800 miles), would result in a fatal radiation dose for the crew.
Once the spacecraft leaves Earth orbit and the shelter of Earth's magnetic field, we have another situation entirely. Future trips to other planets will involve astronauts traversing large distances, taking hundreds of days to reach their destination. In these cases, the crew is at the mercy of direct particle radiation from the Sun. At times of solar maximum, solar storms can occur that fling huge quantities of particle radiation across the solar system. If the spacecraft happens to be in the path of one of these outbursts, the level of radiation can be potentially lethal for an unprotected crew. Thus the problem of providing adequate radiation protection would appear to be a potential roadblock for future manned flights to the planets. However, there is a cost-effective two-part solution. First, there must be an effective early warning system that monitors the Sun's output to detect the solar storms, probably using a system of spacecraft sensors in orbit around the Sun. A storm warning can then be communicated to the distant manned spacecraft. Second, there must be a storm shelter, which is a small pressurized compartment onboard the manned vehicle where the crew can stay during the solar storm. To protect the crew, you might expect that this shelter needs to be lined with a considerable thickness of lead. However, a better solution, in terms of reducing mass, is to reorient the spacecraft to put a significant mass of existing spacecraft hardware or propellant between the shelter and the Sun. For example, it is estimated that about a half-meter thickness of liquid hydrogen propellant would provide adequate radiation protection for the crew.
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