The effects of a long term exposure to radiation

The argument

So far the only human missions outside low Earth orbit have been the Apollo missions to the Moon — short journeys by the standards of anything now proposed. Everything known about human beings in deep space rests on that narrow base, and radiation is the clearest reason this argument treats longer crewed missions as too dangerous to attempt. Within low Earth orbit, crews remain substantially sheltered by Earth's magnetic field, whose Van Allen radiation belts deflect much of the charged particle flux that would otherwise reach them. Beyond it that protection is absent, and it is not restored at the destination: a planet such as Mars has no comparable magnetic field to shield anyone standing on its surface. A crew travelling there would be exposed continuously — on the outbound journey, throughout the stay, and on the return. Engineering a substitute might not be feasible. Radiation shields heavy enough to matter must themselves be carried, and mass is the binding constraint on any mission profile; a shield adequate to the dose may simply be too heavy to launch, which is a different kind of problem from one further development can be expected to solve. Compounding this, there is no sufficient prediction of the dose astronauts would actually receive. Exposure depends on the duration of the mission, on solar activity across that period, and on conditions that cannot be forecast years in advance. Without a reliable figure, nobody can say whether a given design keeps a crew within survivable limits or well outside them. Sending people into an environment whose principal hazard cannot be reliably estimated and may not be shieldable is, on this argument, not an acceptable risk. Space flights of that kind are too dangerous, and so we do not need manned missions.

Context

During space segments of a mission and also on planet surface (like Mars) astronauts might not be adequately protected from radiation.

Premises

Counter-arguments

Some reasonable measures can be put in place. Water can be used as a shield on the vessel. On Mars surface astronatus could live in an under-ground habitat and use terrain itself as a shield Some estimation shows radiation to be at an acceptable level.

Rejecting the premises

[Rejecting P1] The premise conflates two things. The Van Allen belts are trapped radiation held by the magnetosphere, not the shield itself, and Mars — while lacking a global magnetic field — does retain an atmosphere and, more importantly, offers its own surface material as mass shielding. [Rejecting P2] The counter-argument on record identifies feasible measures rather than novel ones: water and stored consumables can be arranged as shielding around a crew compartment on the outbound vessel, and habitats on Mars can be placed under regolith so the terrain does the work. Neither requires technology that does not exist. [Rejecting P3] The claim that exposure cannot be predicted is the weakest of the three. Instrumented spacecraft have measured the radiation environment on the transit to Mars and at the surface, so the dose for a reference mission is estimated rather than unknown — which is what the counter means in saying some estimates put it at an acceptable level. [Rejecting C] The conclusion also treats a quantified occupational hazard as disqualifying, which is not how comparable risks are handled elsewhere. Whether the dose is acceptable is a judgement about limits and mission design, and the argument argues from uncertainty rather than from a figure. (The body is also badly typo-affected and is a candidate for replacement.)