Space Radiation

The space radiation environment is mainly determined by galactic cosmic radiation (GCR), solar particle radiation (SPR), as well as geomagnetically trapped protons and electrons populating the so-called Van Allen radiation belts. Outside the protection afforded by the Earth’s magnetic field and atmosphere, the main sources of concern are large but infrequent solar particle events (SPEs) and the chronic background provided by the energetic heavy charged particles in the GCR spectrum consisting of all of the naturally-occurring elements.

Galactic Cosmic Radiation

GCR comprises particles of charge from hydrogen to uranium arriving from outside the heliosphere. These particles range in energy from ~10 to ~1012 MeV per nucleon, with fluence-rate peaks around 300 to 700 MeV per nucleon. Because of the vast energy range, it is difficult to provide adequate shielding, and thus these particles provide a steady source of low dose-rate radiation. The cyclic variation of the solar magnetic field and changes in the solar wind velocity cause modulation of the GCR spectrum within the heliosphere. Energetic heavy charged particles, whose fluxes change only slowly during the 11-year solar cycle, present new and significant problems for radiation protection, that have not yet been resolved. Unlike SPEs, GCR appears as a low-level background exposing the astronaut 24 hours each day and requiring shielding during normal mission activity. Our knowledge about the interactions of high energy nuclei with matter and the associated biological response is far from being complete. Although the GCR intensity is low, its accumulated exposure is a limiting factor in long-term space operations.

Solar Particle Events

If adequate shielding is not available, large fluence rates of high-energy particles originating on or near the sun will pose the greatest radiation risk to space travellers outside the geomagnetosphere and, in any case, will impose important operational constraints on human interplanetary space exploration. There is a general association of SPE frequencies observed at Earth with the solar activity cycle, but it is still debated whether a repeatable systematic pattern in the SPE occurrence can be found in sequential solar cycles. A large SPE could, in a short time period (hours or days), increase the proton fluence rate by four or five orders of magnitude. There is a very small, but nonzero, possibility that an extraordinarily-large SPE would expose an interplanetary spacecraft to as much fluence at energies above tens of MeV as an entire solar cycle of exposure to GCR. SPEs of this magnitude occurred in November 1960, August 1972, and October 1989. In an unshielded or lightly shielded environment, SPE particle fluxes have the potential to cause acute radiation effects, but several techniques, such as seeking refuge in a well shielded ‘storm shelter’, can be used to keep the dose from SPE particles within acceptable limits.

References and Notes

  • G. Reitz, Characteristic of the radiation field in low Earth orbit and in deep space. Zeitschrift für Medizinische Physik 18, 233-243 (2008).
  • National Council on Radiation Protection and Measurements. Information needed to make radiation protection recommendations for space missions beyond low-Earth orbit. NCRP Report 153 (2006).

The HAMLET project is funded by the European Commission under the EU’s
Seventh Framework Programme (FP7) and coordinated by the
German Aerospace Center (DLR)

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