Health threat from cosmic rays

The health threat from cosmic rays is the danger posed by galactic cosmic rays and solar energetic particles to astronauts on interplanetary missions. Cosmic rays consist of high energy protons and other nuclei with extrasolar origin. Solar energetic particles consist primary 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.

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 and the radiation belts.

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.
2. Shielding by the bulk of the planet itself cuts the flux by a factor of two.
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 ("geomagnetic shielding");
4. The sun's magnetic field has a similar effect, tending to exclude galactic cosmic rays from the plane of the ecliptic in the inner solar system.
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.

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

Of the above factors, all but the first one apply to low earth orbit craft, such as the International Space Station. Therefore, the only astronauts who have ever been exposed to a significant radiation flux from galactic cosmic rays are those in the Apollo program. Since the durations of the Apollo missions were days rather than years, the doses involved were small compared to what would occur, for example, on a crewed mission to Mars.

Solar proton events, which occur relatively rarely, can produce much higher radiation levels than the continuous radiation dose from GCRs. Without shielding these events are sufficiently strong to cause immediate radiation poisoning and death.

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.

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 nearly impossible.

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), 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. 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. This is explained by the fact that slower particles have more time to interact with molecules in the body.

Mitigation

Shielding

Material shielding may be partially effective against galactic cosmic rays in certain energy ranges, but may actually make the problem worse for some of the higher energy rays, because more shielding causes an increased amount of secondary radiation. 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.

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.
• 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. Water, which is necessary to sustain life, could also contribute to shielding.
• Electromagnetic fields may also be a possibility.

None of these strategies currently provides a method of protection that would be known to be sufficient, while using known engineering principles and conforming to likely limitations on the mass of the payload. Scientists such as University of Chicago professor emeritus Eugene Parker are not optimistic it can be solved any time soon. The required amount of material shielding would be too heavy to be lifted into space - many hundreds of metric tons for a reasonably-sized crew compartment. Electromagnetic shielding has a number of problems: (1) the fields act in opposite directions on positively and negatively charged particles, so shielding that excludes positively charged galactic cosmic rays will tend to attract negative ions; (2) a very large power supply would be required in order to run the electrostatic and magnetostatic generators, and superconducting materials might have to be used for magnetic coils; (3) the possible field patterns might tend to dump charged particles into one area of the spacecraft; (4) the ultra-powerful magnetic fields required (perhaps as high as 20 teslas) can have deleterious effects on human biochemistry, necessitating heavier opposing-electromagnet designs to cancel the field in the crew sections of the spacecraft. 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 retard 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.

See also

• Space weather
• Space medicine: What are the effects of space on the body
• solar wind
• Solar proton event
• Proton: Exposure
• Solar flare: Hazards
• Magnetosphere
• Heliosphere
• Background radiation

References

• Eugene N. Parker, "Shielding Space Travelers," Scientific American, March 2006.
• John Dudley Miller, "Radiation Redux," Scientific American, November 2007.
• Space Studies Board and Division on Engineering and Physical Sciences, National Academy of Sciences (2006). "Space Radiation Hazards and the Vision for Space Exploration". NAP.

External links

• Booster Accelerator at Brookhaven National Laboratory.
• Space Radiation Laboratory at BNL.