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Health threat from cosmic rays

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The health threat from cosmic rays is the danger posed by galactic cosmic rays and solar energetic particles to astronauts on interplanetary missions. Galactic cosmic rays consist of high energy protons and other nuclei with extrasolar origin. Solar energetic particles consist primarily of protons accelerated by the sun to high energies via proximity to solar flares and coronal mass ejections. They are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft.[1][2]

The deep-space radiation environment

The radiation environment of deep space is very different from that on the Earth's surface or in low earth orbit, due to the much larger flux of high-energy galactic cosmic rays (GCRs), along with radiation from solar proton events (SPEs) and the radiation belts.

Galactic cosmic rays create a continuous radiation dose throughout the solar system that increases during solar minimum and decreases during solar maximum (solar activity). The inner and outer radiation belts are two regions of trapped particles from the solar wind that are later accelerated by dynamic interaction with the Earth's magnetic field. While always high, the radiation dose in these belts can increase dramatically during geomagnetic storms and substorms. Solar proton events are bursts of energetic protons accelerated by the sun. They occur relatively rarely and can produce extremely high radiation levels. Without thick shielding, SPEs are sufficiently strong to cause acute radiation poisoning and death.[3]

GCRs are the weakest of these effects and SPEs are the strongest with the radiation belts falling somewhere in between.[citation needed]

Life on the Earth's surface is protected from galactic cosmic rays by a number of factors:

  1. The Earth's atmosphere is opaque to primary cosmic rays with energies below about 1 GeV, so only secondary radiation can reach the surface. The secondary radiation is also attentuated by absorption in the atmosphere, as well as by radioactive decay in flight of some particles, such as muons. Particles entering from a direction close to the horizon are especially attenuated. At 15 km altitude, above not all yet most of the atmosphere's mass, radiation dose as an annual rate ranges from around 20 milli-Sieverts (mSv) at the equator to 50 - 120 mSv at the poles, varying between solar maximum and minimum conditions, but the world's population rather receives an average of 0.4 mSv of cosmic radiation annually (separate from other sources of radiation exposure like inhaled radon) due to the tons per square meter of atmospheric mass shielding.[4][5]
  2. Shielding by the bulk of the planet itself cuts the flux by a factor of two.[citation needed]
  3. Except for the very highest energy galactic cosmic rays, the radius of gyration in the earth's magnetic field is small enough to ensure that they are deflected away from Earth. Missions beyond low earth orbit leave the protection of the geomagnetic field, and transit the Van Allen radiation belts. Thus they may need to be shielded against exposure to cosmic rays, Van Allen radiation, or solar flares. The region between two to four earth radii lies between the two radiation belts and is sometimes referred to as the "safe zone".[6][7] See the impact of the Van Allen belts on space travel for more information.
  4. The solar magnetic field has a similar effect, tending to exclude galactic cosmic rays from the plane of the ecliptic in the inner solar system.[citation needed]
  5. The interplanetary magnetic field, embedded in the solar wind, also deflects cosmic rays. As a result, cosmic ray fluxes within the heliopause are inversely correlated with the solar cycle.[8]

As a result the energy input of GCRs to the atmosphere is negligible — about 10−9 of solar radiation - roughly the same as starlight.[9]

Of the above factors, all but the first one apply to low earth orbit craft, such as the Space Shuttle and the International Space Station. Average exposure on the ISS is a rate of 150 mSv per year, though crew rotations are shorter than that.[10] Astronauts on Apollo and Skylab missions received on average 1.2 mSv/day and 1.4 mSv/day respectively.[10] Since the durations of the Apollo and Skylab missions were days and months respectively rather than years, the doses involved were smaller than what would occur, for example, on a crewed mission to Mars (unless far more shielding could be provided).

Effects

Like other ionizing radiation, high-energy cosmic rays can damage DNA, increasing the risk of cancer, cataracts, neurological disorders, and non-cancer mortality risks.[11]

The Apollo astronauts reported seeing flashes in their eyeballs, which may have been galactic cosmic rays, and there is some speculation that they may have experienced a higher incidence of cancer. However, the duration of the longest Apollo flights was less than two weeks, limiting the maximum exposure. There were only 24 such astronauts, making statistical analysis of the effects difficult.

The health threat depends on the flux, energy spectrum, and nuclear composition of the rays. The flux and energy spectrum depend on a variety of factors: short-term solar weather, long-term trends (such as an apparent increase since the 1950s[12]), and position in the sun's magnetic field. These factors are incompletely understood. The Mars Radiation Environment Experiment (MARIE) was launched in 2001 in order to collect more data. Estimates are that humans unshielded in interplanetary space would receive annually roughly 400 to 900 milli-Sieverts (mSv) (compared to 2.4 mSv on Earth) and that a Mars mission (12 months in flight and 18 months on Mars) might expose shielded astronauts to ~500 to 1000 mSv.[12] These doses approach the 1 to 4 Sv career limits advised by the National Council on Radiation Protection and Measurements for Low Earth orbit activities.

The quantitative biological effects of cosmic rays are poorly known, and are the subject of ongoing research. Several experiments, both in space and on Earth, are being carried out to evaluate the exact degree of danger. Experiments at Brookhaven National Laboratory's Booster accelerator revealed that the biological damage due to a given exposure is actually about half what was previously estimated: specifically, it turns out that low energy protons cause more damage than high energy ones.[citation needed] This is explained by the fact that slower particles have more time to interact with molecules in the body.

Mitigation

Shielding

Material shielding can be effective against galactic cosmic rays, but thin shielding may situationally actually make the problem worse for some of the higher energy rays, because more shielding causes an increased amount of secondary radiation, although very (arguably impractically) thick shielding could counter such too.[13] The aluminum walls of the ISS, for example, are believed to have a net beneficial effect. In interplanetary space, however, it is believed that thin aluminum shielding would have a negative net effect.[14]

Several strategies are being studied for ameliorating the effects of this radiation hazard for planned human interplanetary spaceflight:

  • Spacecraft can be constructed out of hydrogen-rich plastics, rather than aluminum.[15] Unfortunately, "[S]ome 'galactic cosmic rays are so energetic that no reasonable amount of shielding can stop them,' cautions Frank Cucinotta, NASA's Chief Radiation Health Officer. 'All materials have this problem, including polyethylene.'"[16]
  • Material shielding has been considered:
    • Liquid hydrogen, which would be brought along as fuel in any case, tends to give relatively good shielding, while producing relatively low levels of secondary radiation. Therefore, the fuel could be placed so as to act as a form of shielding around the crew. However, as fuel is consumed by the craft, the crew's shielding decreases.
    • Water, which is necessary to sustain life, could also contribute to shielding. But it too is consumed during the journey unless waste products are utilized.[16]
    • Asteroids could serve to provide shielding.[17][18]
  • Magnetic deflection of charged radiation particles and/or electrostatic repulsion is a hypothetical alternative to pure conventional mass shielding under investigation. In theory, power requirements for the case of a 5 meter torus drop from an excessive 10 GW for a simple pure electrostatic shield (too discharged by space electrons) to a moderate 10 kW by using a hybrid design.[14] However, such complex active shielding is untried, with workability and practicalities more uncertain than material shielding.[14]

Special provisions would also be necessary to protect against a solar proton event (SPE), which could increase fluxes to levels that would kill a crew in hours or days rather than months or years. Potential mitigation strategies include providing a small habitable space behind a spacecraft's water supply or with particularly thick walls or providing an option to abort to the protective environment provided by the Earth's magnetosphere. The Apollo mission used a combination of both strategies. Upon receiving confirmation of an SPE, astronauts would move to the Command Module, which had thicker aluminum walls than the Lunar Module, then return to Earth. It was later determined from measurements taken by instruments flown on Apollo that the Command Module would have provided sufficient shielding to prevent significant crew harm.[citation needed]

None of these strategies currently provides a method of protection that would be known to be sufficient[citation needed] while conforming to likely limitations on the mass of the payload at present (~ $10000/kg) launch prices. Scientists such as University of Chicago professor emeritus Eugene Parker are not optimistic it can be solved any time soon.[citation needed] For passive mass shielding, the required amount could be too heavy to be affordably lifted into space without changes in economics (like hypothetical non-rocket spacelaunch or usage of extraterrestrial resources) — many hundreds of metric tons for a reasonably-sized crew compartment. For instance, a NASA design study for an ambitious large spacestation envisioned 4 metric tons per square meter of shielding to drop radiation exposure to 2.5 mSv annually (+/- a factor of 2 uncertainty), less than the tens of mSv or more in some populated high natural background radiation areas on Earth, but the sheer mass for that level of mitigation was considered practical only because it involved first building a lunar mass driver to launch material.[13]

Several active shielding methods have been considered for lesser mass than passive mass shielding, but they remain in the realm of uncertain speculation at the present time.[14][19] Since the segment of space radiation penetrating farthest through thick material shielding, deep in interplanetary space, is GeV-level positively charged nuclei, a repulsive positively charged electrostatic shield has been hypothesized, but issues include plasma instabilities and power needs for an accelerator constantly keeping the charge from being neutralized by deep-space electrons.[20] A more common proposal is magnetic shielding using superconductors (or plasma currents), although, among other complications, if designing a relatively compact system, magnetic fields up to 10-20 Tesla could be required around a manned spacecraft higher than the several Tesla in MRI machines. The employment of magnetic structures which expose crew to such a high magnetic field may further complicate matters, though, since high-field (5 Tesla or more) MRIs have been noted to produce headaches and migraines in MRI patients, and high-duration exposure to such fields has not been studied. Opposing-electromagnet designs might cancel the field in the crew sections of the spacecraft, but such would raise mass. A hybrid of an electrostatic shield and a magnetic shield has also been conceived, charge neutral at large distances and theoretically much reducing the individual weaknesses of each, yet complex to design if doable.[14]

Part of the uncertainty is that the effect of human exposure to galactic cosmic rays is poorly known in quantitative terms. NASA has a Space Radiation Shielding Program to study the problem.

Drugs

Another line of research is the development of drugs that mimic and/or enhance the body's natural capacity to repair damage caused by radiation. Some of the drugs that are being considered are retinoids, which are vitamins with antioxidant properties, and molecules that cell division, giving the body time to fix damage before harmful mutations can be duplicated.

Timing of missions

Due to the potential negative effects of astronaut exposure to cosmic rays, solar activity may play a role in future space travel. Because galactic cosmic ray fluxes within the solar system are lower during periods of strong solar activity, interplanetary travel during solar maximum should minimize the average dose to astronauts.

Although the Forbush decrease effect during Coronal mass ejections (CMEs) can temporarily lower the flux of galactic cosmic rays, the short duration of the effect (1–3 days) and the approximately 1% chance that a CME generates a dangerous solar proton event limits the utility of timing missions to coincide with CMEs.

Orbital selection

Radiation dosage from the Earth's radiation belts is typically mitigated by selecting orbits that avoid the belts or pass through them relatively quickly. For example, a Low Earth orbit, with low inclination, will generally be below the inner belt.

The orbit of the Earth-Moon system Lagrange Points L2 - L5 takes them out of the protection of the Earth's magnetosphere for approximately two-thirds of the time.

The orbits of Earth-Sun Lagrange Points L1 and L3 - L5 are always outside of the protection of the Earth's magnetosphere.

See also

Notes

  1. ^ Can People go to Mars?
  2. ^ Shiga, David (16 September 2009), "Too much radiation for astronauts to make it to Mars", New Scientist (2726)
  3. ^ Biomedical Results From Apollo - Radiation Protection and Instrumentation;[1]
  4. ^ Evaluation of the Cosmic Ray Exposure of Aircraft Crew
  5. ^ Sources and Effects of Ionizing Radiation, UNSCEAR 2008
  6. ^ "Earth's Radiation Belts with Safe Zone Orbit". Goddard Space Flight Center, NASA. Retrieved 27 April 2009.
  7. ^ Weintraub, Rachel A. "Earth's Safe Zone Became Hot Zone During Legendary Solar Storms". Goddard Space Flight Center, NASA. Retrieved 27 April 2009.
  8. ^ Cosmic Rays by R. A. Mewaldt;
  9. ^ Jasper Kirkby; Cosmic Rays And Climate CERN-PH-EP/2008-005 26 March 2008
  10. ^ a b Space Radiation Organ Doses for Astronauts on Past and Future Missions Table 4
  11. ^ NASA Facts: Understanding Space Radiation
  12. ^ a b The Cosmic Ray Radiation Dose in Interplanetary Space – Present Day and Worst-Case Evaluations R.A. Mewaldt et al., page 103, 29th International Cosmic Ray Conference Pune (2005) 00, 101-104
  13. ^ a b NASA SP-413 Space Settlements: A Design Study. Appendix E Mass Shielding Retrieved 3 May 2011.
  14. ^ a b c d e Magnetic Radiation Shielding: An Idea Whose Time Has Returned? - G.Landis (1991)
  15. ^ NASA - Plastic Spaceships
  16. ^ a b Cosmic rays may prevent long-haul space travel - space - 01 August 2005 - New Scientist
  17. ^ Morgan, P. (2011) "To Hitch a Ride to Mars, Just Flag Down an Asteroid" Discover magazine blog
  18. ^ Matloff, G.L. and Wilga, M. (2011) "NEOs as stepping stones to Mars and main-belt asteroids" Acta Astronautica 68(5-6):599-602
  19. ^ Simulations of Magnetic Shields for Spacecraft. Retrieved 3 May 2011.
  20. ^ NASA SP-413 Space Settlements: A Design Study. Appendix D The Plasma Core Shield Retrieved 3 May 2011.

12. http://www.techsciencedaily.com/general/juno-probe-built-to-study-jupiters-radiation-belt-gets-a-titanium-suit-of-interplanetary-armor/

References

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