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| orion spacecraft |
Interplanetary space is bathed by radiation from two distinct sources.
The Sun mainly blasts us with waves of high-energy protons accelerated by
solar flares, while cosmic rays consist of a thinner stream of extremely highenergy
atomic nuclei propelled by supernova explosions from beyond
the Solar System. Here on Earth we’re safely protected by the atmosphere and
even astronauts in low orbit also receive some shielding from the Earth’s magnetic field. Once you
travel beyond that, radiation levels quickly build up. Solar protons can
be blocked by the aluminium skin of the spaceship itself, but the strange
thing about galactic cosmic rays is that physical shielding can actually
increase the radiation dose. This is because the heavy particles have
so much energy that they trigger a cascade of secondary radiation from
the atoms of the aluminium shield. Hydrogen atoms don’t generate
secondary radiation, however, so materials that are rich in hydrogen,
like water, work much better. Some of this could be water that’s needed
for life support in any case, stored as cavity insulation in the hull.
To provide a thick enough shield, spacecraft would need to bring along
a lot of extra water, which is more mass that must be accelerated by
the engines. The alternative is to use active shielding to deflect the path of
dangerous particles. Galactic cosmic rays are positively charged, so they
could be deflected by a positively charged electrostatic shield. However,
this would require a huge amount of energy to offset the constant
neutralising effect of deep-space electrons. Magnetic shields are more
promising, but magnetic fields five to ten times stronger than those in an
MRI scanner would be needed. The long-term effects of human exposure
to magnetic fields this strong aren’t known and might actually be worse than the radiation.
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