Effects of ionizing radiation in spaceflight

Galactic cosmic rays (GCRs) from outside the Milky Way galaxy consist mostly of highly energetic protons with a small component of HZE ions.

[8] The effects of changes in oxygen levels or in immune dysfunction on cancer risks are largely unknown and are of great concern during space flight.

[20] In the United States, the legal occupational exposure limits for adult workers is set at an effective dose of 50 mSv annually.

[21] The relationship between radiation exposure and risk is both age- and sex-specific due to latency effects and differences in tissue types, sensitivities, and life spans between sexes.

These relationships are estimated using the methods that are recommended by the NCRP [10] and more recent radiation epidemiology information [1][20][22] The as low as reasonably achievable (ALARA) principle is a legal requirement intended to ensure astronaut safety.

Mission programs and terrestrial occupational procedures resulting in radiation exposures to astronauts are required to find cost-effective approaches to implement ALARA.

[20] For organ dose calculations, NASA uses the model of Billings et al.[23] to represent the self-shielding of the human body in a water-equivalent mass approximation.

Consideration of the orientation of the human body relative to vehicle shielding should be made if it is known, especially for SPEs [24] Confidence levels for career cancer risks are evaluated using methods that are specified by the NPRC in Report No.

The average loss of life-expectancy, LLE, in the population is defined by:[28] The loss of life-expectancy among exposure-induced-deaths (LLE-REID) is defined by:[28][29] The low-LET mortality rate per sievert, mi is written where m0 is the baseline mortality rate per sievert and xα are quantiles (random variables) whose values are sampled from associated probability distribution functions (PDFs), P(Xa).

126, defines the following subjective PDFs, P(Xa), for each factor that contributes to the acute low-LET risk projection:[30][31] The accuracy of galactic cosmic ray environmental models, transport codes and nuclear interaction cross sections allow NASA to predict space environments and organ exposure that may be encountered on long-duration space missions.

After a sufficient number of trials have been completed (approximately 105), the results for the REID estimated are binned and the median values and confidence intervals are found.

[32] Age-and sex-dependent mortality rate per unit dose, multiplied by the radiation quality factor and reduced by the DDREF is used for projecting lifetime cancer fatality risks.

[10] The radiation mortality rate is defined as: Where: Identifying effective countermeasures that reduce the risk of biological damage is still a long-term goal for space researchers.

[15][16][17][18] There are three fundamental ways to reduce exposure to ionizing radiation:[32] Shielding is a plausible option, but due to current launch mass restrictions, it is prohibitively costly.

This observation is readily understood by noting that the average tissue self-shielding of sensitive organs is about 10 cm, and that secondary radiation produced in tissue such as low energy protons, helium, and heavy ions are of high linear energy transfer (LET) and make significant contributions (>25%) to the overall biological damage from GCR.

Several strategies are being studied for ameliorating the effects of this radiation hazard for planned human interplanetary spaceflight: Special provisions would also be necessary to protect against a solar proton event, which could increase fluxes to levels that would kill a crew in hours or days rather than months or years.

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 provide a method of protection that would be known to be sufficient[42] while conforming to likely limitations on the mass of the payload at present (around $10,000/kg) launch prices.

[42] 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 space station 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 millisieverts 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.

[35][43][44] Since the type of radiation penetrating farthest through thick material shielding, deep in interplanetary space, is GeV positively charged nuclei, a repulsive electrostatic field has been proposed, but this has problems including plasma instabilities and the power needed for an accelerator constantly keeping the charge from being neutralized by deep-space electrons.

[49] A collaborative effort between the Israeli Space Agency, StemRad and Lockheed Martin was AstroRad, tested aboard the ISS.

The product is designed as an ergonomically suitable protective vest, which can minimize the effective dose by SPE to an extent similar to onboard storm shelters.

[citation needed] Another line of research is the development of drugs that 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.

Due to the potential negative effects of astronaut exposure to cosmic rays, solar activity may play a role in future space travel.

[citation needed] Although the Forbush decrease effect during coronal mass ejections 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.

There is continued controversy for doses that are below 5 mSv, however, and for low dose-rate radiation because of debate over the linear no-threshold hypothesis that is often used in statistical analysis of these data.

[10][20][51][52] This article incorporates public domain material from Human Health and Performance Risks of Space Exploration Missions (PDF).

The Phantom Torso, as seen here in the Destiny laboratory on the International Space Station (ISS), is designed to measure the effects of radiation on organs inside the body by using a torso that is similar to those used to train radiologists on Earth. The torso is equivalent in height and weight to an average adult male. It contains radiation detectors that will measure, in real-time, how much radiation the brain, thyroid, stomach, colon, and heart and lung area receive on a daily basis. The data will be used to determine how the body reacts to and shields its internal organs from radiation, which will be important for longer duration space flights.
Comparison of radiation doses - includes the amount detected on the trip from Earth to Mars by the RAD on the MSL (2011 - 2013). [ 15 ] [ 16 ] [ 17 ] [ 18 ]
Standard spacecraft shielding, integrated into hull design, is strong protection from most solar radiation. This usefulness is defeated by high-energy cosmic rays, which it effectively splits into showers of secondary particles. This shower of secondary particles may be reduced by the use of hydrogen-dense materials or light elements for shielding.