BOX 2-1

Main Characteristics of Space Radiation

Solar Particle Events

  • Composed largely of protons, generally with low to medium energies (tens to a few hundred MeV per nucleon).

  • More likely at solar maximum. Onset, duration, dose rate, and dose are at present unpredictable.

  • With adequate warning and access to shelter (>10 g/cm2 aluminum-equivalent), radiation hazard can be reduced to acceptable levels.

  • Biological effects are similar to those from x-rays or gamma rays.

  • Major research questions involve the prediction of onset and relevant characteristics and the health risks due to the residual low dose rates when under shielding.

Galactic Cosmic Rays

  • Composed of protons, alpha particles, and heavy ions, up to very high energies exceeding tens of GeV per nucleon.

  • Steady background varying over the 11-year cycle roughly by a factor of 2.

  • Shielding is ineffective because ions penetrate hundreds of centimeters of material and produce secondary radiation.

  • Biological effects are poorly understood, with large uncertainties in projections because there are no human data on which to base estimates.

  • Major research questions involve solar cycle variations and the need for understanding of mechanisms linking radiation exposure to health risk.

There have been several solar system measurements of the GCR at distances at and beyond Mars, providing confidence in extrapolations of more detailed GCR measures taken near Earth. There have also been limited radiation measurements taken from satellites in orbit around Mars (relevant instruments on Mars Odyssey include the Mars Radiation Environment Experiment [MARIE], the High Energy Neutron Detector, and neutron information from the Gamma Ray Spectrometer; there have also been indirect measurements using instruments on Mars Global Surveyor). Use of these data for estimates of the surface radiation environment requires transport code estimates of particles created in or scattered from the martian atmosphere and calculations of particle transport from the surface through the atmosphere.

The Radiation Assessment Detector (RAD) on the 2009 Mars Science Laboratory will characterize the broad spectrum of radiation at the martian surface. RAD will measure high-energy charged particles coming through the martian atmosphere. In addition to identifying neutrons, gamma rays, protons, and alpha particles, RAD will measure galactic cosmic ray heavy ions up to iron on the periodic table.

Finding 2-1. Current knowledge of the radiation environment on the Moon. Data from many satellites have enabled the characterization of GCR and SPEs near Earth, and these results serve to characterize the radiation incident on the surface of the Moon. Knowledge of the secondary radiation, which is produced by galactic cosmic rays and SPEs interacting with material on the lunar surface, is currently based on data from Apollo, Lunar Prospector, and Clementine and on calculations.

Finding 2-2. Current knowledge of the radiation environment on Mars. The radial extrapolation of the GCR environment from Earth to Mars is well understood, based on measurements made by numerous scientific satellites as they traveled outward through the solar system. To within a few percent or so, the GCR environment at the top of the martian atmosphere is expected to be the same as that near Earth. There are very few simultaneous measurements of SPEs at Earth and at Mars, and current models are inadequate to extrapolate near-Earth measurements of SPEs to Mars. Knowledge of the secondary radiation environment on the surface of Mars is currently based on calculations and measurements taken by spacecraft in Mars orbit.

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