cosmic rays

cosmic rays, charged particles moving at nearly the speed of light reaching the earth from outer space. Primary cosmic rays consist mostly of protons (nuclei of hydrogen atoms), some alpha particles (helium nuclei), and lesser amounts of nuclei of carbon, nitrogen, oxygen, and heavier atoms. These nuclei collide with nuclei in the upper atmosphere, producing secondary cosmic rays of protons, neutrons, mesons, electrons, and gamma rays of high energy, which in turn hit nuclei lower in the atmosphere to produce more particles (see elementary particles). These cascade processes continue until all the energy of the primary particle is dissipated. The secondary particles shower down through the atmosphere in diminishing intensity to the earth's surface and even penetrate it. The size of the shower indicates the energy of the primary ray, which may be as high as 10²⁰ electron volts (eV) or more, almost a billion times higher than the highest energy yet produced in a man-made particle accelerator; however, cosmic rays of lower energy predominate. Cosmic rays were long used as a source of high-energy particles in the study of nuclear reactions. The positron, the muon, the pion (or pi meson), and some of the so-called strange particles were initially discovered in studies of this radiation. Cosmic rays were first found to be of extraterrestrial origin by Victor F. Hess (c.1912) when he recorded them with electrometers carried to high altitudes in balloons, an achievement for which he won the Nobel Prize in 1936. They were so named in 1925 by R. A. Millikan, who did extensive research on them. Since then much pertinent information has been collected that have been of use in studying the chemical composition of the universe, but the origin of cosmic rays remains a mystery. However, when they react with interstellar gases, the result is a gamma ray that can be traced back. Spacecraft results indicate that many of the gamma rays appear to come from the direction of supernova remnants. The nature of the acceleration processes by which the primary particles achieve great velocities (very nearly the speed of light) is also still highly speculative. Modern electronic detectors called charge coupled devices (CCDs) are effective cosmic ray detectors; a ray can strike a single pixel, making it much brighter than the surrounding ones. Cosmic rays play a significant role in the natural mutation and evolution of life on earth.

See B. B. Rossi, Cosmic Rays (1964); L. I. Dorman, Cosmic Rays (1974); M. W. Friedlander, Cosmic Rays (1989).

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Health threat from cosmic rays

The health threat from cosmic rays is the danger posed by cosmic rays generated by the Sun and other stars to astronauts on interplanetary missions. Cosmic rays consists of high energy protons and other nuclei. 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:

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.
Shielding by the bulk of the planet itself cuts the flux by a factor of two.
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");
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.

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 four factors, all but the first one apply to low earth orbit craft, such as the International Space Station (although the ISS crew gets most of its dose while passing through the Van Allen Belt). 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.

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 1950's), 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 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 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. The required amount of material shielding would be too heavy to be lifted into space. 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. 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 via the Forbush decrease effect. Coronal mass ejections (CMEs) can temporarily lower the local cosmic ray levels, and radiation from CMEs is easier to shield against than cosmic rays.