We are preparing to return humans to the Moon and setting the stage for exploration to Mars and beyond. However, it is unclear if long missions outside of Low-Earth Orbit (LEO) can be accomplished with acceptable risk. The central objective of our project, the Earth-Moon-Mars Radiation Environment Module (EMMREM), is to develop and validate a numerical module for completely characterizing time-dependent radiation exposure in the Earth-Moon-Mars and Interplanetary space environments.
Space radiation hazards pose one of the most serious issues to future human and robotic exploration to the Moon and beyond:
- Galactic Cosmic Rays (GCRs) are an ever-present background radiation that originate from outside our solar system and produce chronic but not acute exposures. GCRs are extremely difficult to shield against. Astronauts under nominal shielding (e.g., a few gm/cm2 of aluminum) could accumulate a career limit due to GCRs in roughly 3 years. We need to understand the current constraints imposed by GCRs as a function of mission transit time, shielding materials and thickness, and develop better techniques to shield them.
- Large Solar Energetic Particle (SEP) events defined here to include ions (also referred to as Solar Particle Events, SPEs) are extremely dangerous to astronauts. To mitigate the hazard they pose, we must develop the ability to predict when and where they will occur, and we must provide adequate shielding against them.
- There are unique radiation environments at each planet or satellite. At Earth, we have thoroughly characterized locations of the radiation belts, which allows us to mitigate the hazard they pose by transiting them rapidly. For future human and robotic exploration of other planets and satellites, we must characterize the planetary radiation environments so that appropriate mitigation strategies and adequate shielding are designed.
Primer for Radiation Exposure
The effects of energetic particle radiation on the human body are heavily dependent on the type and energy of the radiation, as well as the tissue being irradiated. These effects include cancer, degenerative tissue diseases, damage to the central nervous system, cataracts, and hereditary risks. The relative ability of energetic particles to cause biological damage is expressed as a quality factor, Q, which is a function of the Linear Energy Transfer (LET), or energy absorbed per distance traveled by a given particle through a medium. (For human tissue, this medium is approximated by water.) The LET is a function of particle atomic mass, charge (A and Z) and energy. For example, heavy elements such as Fe generally have large Q, even at relatively high energies, and therefore pose a serious safety hazard even though they are a fraction of the overall flux.
The following dose-related quantities(EMMREM output) are defined as follows:
- Dose (D): Mean energy absorbed per unit mass
- Dose Equivalent (H): Dose multiplied by a weighting factor (quality factor, Q) characterizes long-term radiation effects such as cancer
- Organ Dose (DT): Dose averaged over entire mass of a given organ or tissue (T)
- Equivalent Dose (HT): Organ Dose multiplied by a weighting factor characterizing long-term radiation effects depending on specific types of radiation (proton, neutron, alpha, etc)
- Effective Dose (E): The sum over all irradiated organs of the equivalent doses times a weighting factor for long-term radiation effects
- Linear Energy Transfer (LET): Mean energy loss by charged particle per unit distance traveled
- Dose-rate (DV): The radiation dose absorbed per unit time
Radiation Hazards from Cosmic Rays
Galactic Cosmic Rays (GCRs) pose the most serious chronic radiation hazard for long duration interplanetary missions to Mars, particularly in solar minimum activity conditions when approximately 10 cm of aluminum shielding may be needed to bring the radiation dose down to the current limit for astronauts in low-Earth orbit [Davis et al., 2001]. Most of the problem lies in the < 1 GeV/nuc GCRs, with a significant contribution from heavy nuclei, despite their low intensities. Although the GCR problem is less severe during solar maximum, large SEP events are more frequent, raising the frequency of acute exposure.
The following parametric tables for dose and dose equivalent in the Mars atmosphere from GCRs were calculated with the HZETRN 2005 model at the University of Tennessee:
Anomalous Cosmic Rays (ACRs) are less energetic and pose a lower radiation hazard, but they are trapped in a third radiation belt in the near Earth environment and may have sufficient energy to pose a threat to lightly shielded electronic systems, and possibly to astronauts during extra vehicular activity, if inside an ACR radiation belt.
The long-term cosmic-ray (CR) modulation cycle has a well known ~11-year variation with solar cycle, and a 22-year cycle coinciding with the polarity cycle of the solar magnetic field. The CR time profiles are more flat-topped (sharply peaked) around solar minimum when the interplanetary magnetic fields have a positive (negative) polarity in the northern hemisphere. This phenomenon is likely due to CR gradient, curvature, and current sheet drift transport, which depends on the sign of the magnetic field polarity [e.g.,Kota and Jokipii, 1983; Potgieter and Moraal, 1985]. In the beginning of a positive polarity cycle, the cosmic-ray intensity can increase quickly over a 1-2 year time scale so that relatively early in the cycle, the CR intensity and associated radiation hazard reach maximum levels.
Abrupt steps in GCR intensities occur due to outward propagating clusters of merged interaction regions (MIRs) formed from interplanetary CMEs and shocks that merge into global merged interaction regions (GMIRs) beyond ~ 10 AU. GMIRs act as barriers against CRs causing short-term (~1 year) variations in radiation exposure.
Radiation Hazards from Solar Energetic Particles (SEPs)
The following parametric table for proton dose in the Mars atmosphere from SEPs was calculated with the HZETRN 2005 model at the University of Tennessee:
SEPs are accelerated either in coronal flares or at shocks driven by coronal mass ejections (CMEs) [see review by Reames, 1999a]. Assuming that distinct physical processes are responsible for accelerating particles in flares and at shocks, the SEP events observed at 1 AU are traditionally grouped into two classes, namely impulsive and gradual. Gradual or CME-related events typically last several days and have larger fluences, while the impulsive or flare-related events last a few hours and have smaller fluences. Impulsive events are typically observed when the observer is magnetically connected to the flare site, while ions accelerated at the expanding CME-driven shocks can populate magnetic field lines over a significantly broad range of longitudes [Cliver et al., 1989]. Energetic ions accelerated in large gradual events [e.g., Reames, 1999a] arrive within minutes to hours (depending on observation distance and particle energy) of the onsets of the associated flare and the CME and provide limited advanced warning. These sudden events pose significant radiation hazards for unprepared humans and technological systems in space [e.g., Feynman and Gabriel, 2000].
Unfortunately, as shown in Figure 9, many large SEP events are also accompanied by further increases in the intensities of ions in the ~10-100 MeV energy range, and occasionally up to ~500 MeV [Reames 1999; Lario and Decker 2003], that peak when the corresponding CME-driven shock arrives at the observer [e.g., Cane et al., 1991]. The interplanetary shock-associated ions are called Energetic Storm Particle (ESP) events because of their strong association with geomagnetic storms [Bryant et al., 1962]. Although the exact origin of the earliest arriving high energy ions is still under debate, it is now established that the overall ion intensity enhancements during simultaneous SEP and ESP events pose the most significant radiation hazards to unshielded humans and technological systems near Earth and the Moon [e.g., McKinnon, 1972; Shukitt-Hale et al., 2004; Rabin et al., 2004], and near Mars [e.g., Cleghorn et al. 2004; Saganti et al. 2004].
We currently have limited ability to accurately predict key properties during a solar particle event (e.g., time of onset, peak intensity at high energies, total fluences, the extent and shape of the energy spectra, the heavy ion composition, and whether the SEP event is likely to be accompanied by a strong ESP event). We need a detailed physics-based understanding of particle acceleration and transport out to 1 AU (§A.3.2.2) to develop predictive models. Specifically, we need to model the propagation of CME-driven shocks through the interplanetary medium by characterizing properties of the ambient solar wind plasma, the magnetic field, and the suprathermal seed population that CME shocks accelerate (e.g., Desai et al., 2003).
This material is based upon work supported by the National Science Foundation under Grant No. NNX07AC14G.
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.