Although the atmosphere acts as a huge protective shield, the further we move away from the Earth's surface, the more energetic particles from space hit our bodies and can cause serious damage there. Especially for astronauts this becomes a problem.
On Earth as well as in space, we are exposed to cosmic radiation at all times. High-energy particles with energies of a few megaelectronvolts up to 10 20 electronvolts hit the earth. For comparison: The largest particle accelerator to date, the Large Hadron Collider at the CERN research center near Geneva, accelerates the particles to just a few 10 12 electron volts. This energy is roughly equivalent to the kinetic energy of a flying mosquito – albeit concentrated to the size of a proton – while the energy of one of the most energetic particles of cosmic radiation is already equivalent to the energy of a tennis ball dropped from a height of ten meters onto the ground. However, very few of such high-energy particles hit the earth: about one particle per square kilometer per century.
Energy spectrum of cosmic radiation
Cosmic ray particles are primarily protons, helium nuclei, and electrons. Especially with the highest energies one finds however also very many iron cores. In addition to our sun, star explosions in the Milky Way. Active galactic nuclei outside the Milky Way are traded as possible sources. However, the origin of the high-energy particles in particular has not yet been clearly clarified. While the number of high-energy particles from distant sources remains nearly constant over the years, the amount of low-energy particles from the sun can fluctuate, sometimes wildly. The cause is usually eruptions on the surface of the sun.
Today cloudy with particle showers
The primary cosmic radiation hits the earth's atmosphere from all directions of the universe and is decelerated there by oxygen and nitrogen atoms. In the process, various secondary particles, especially neutrons, protons and pions, are created by sometimes complex physical processes. Because of these reactions, the highest radiation intensity is at an altitude of about twenty kilometers above the earth's surface, below which it decreases again. However, the radiation exposure also depends on the geographical latitude, because the cosmic radiation does not hit the earth's atmosphere evenly distributed: At the geomagnetic poles, the intensity is greater than at the equator. This is caused by the earth's magnetic field, which deflects electrically charged particles from their original path. The Earth's orbit also contains the Van Allen radiation belt – a ring of high-energy charged particles that are trapped, as it were, by the Earth's magnetic field. The radiation belt extends over a range of about 700 to 6000 kilometers above the Earth's surface and must be taken into account because of the high radiation exposure for astronauts, for example, when positioning space stations.
On the earth's surface, the natural background radiation from the rocks is predominant, because the cosmic radiation is shielded by the atmosphere. However, people are now increasingly exposed to it due to space travel and air traffic. In general, the farther away from the earth's surface, the stronger the influence of cosmic radiation becomes. Their effect can sometimes be quite different from natural radioactivity. On the one hand, this is due to the fact that cosmic radiation is much more energetic. On the other hand, cosmic radiation is not absorbed through food or through the air we breathe.
Damage is done by radiation ionizing atoms and molecules in our bodies: The energy introduced can cause an electron, for example, to leave the atom or molecule, leaving behind a positively charged ion. Processes of this type can trigger chemical or biochemical reactions in the affected cells. In this way – especially in the genetic material – lead to damage. Due to their penetration depth, the different types of radiation have different target organs. Alpha particles (helium nuclei) are rapidly decelerated in matter. The depth of penetration into the human body is a few micrometers, so damage primarily affects the skin. The penetration depth of beta particles (electrons) depends on the energy and is about 0.5 centimeters per megaelectronvolt. Gamma rays, on the other hand, penetrate into all organ systems of the body and can cause damage here. A measure of radiation exposure for humans is the effective dose. It takes into account both the different biological effectiveness of the various types of radiation and the sensitivity of the various organs and ties to ionizing radiation. The unit for the biologically weighted radiation dose is the sievert (see blue box).
In a space station in space, the effective radiation dose is about 200 millisieverts per year, while the radiation exposure from cosmic rays on Earth is only about 0.3 millisieverts per year (at sea level). Compared to the total effective dose from natural radiation sources, which adds up to one to six millisieverts per year in Germany, depending on the location, cosmic radiation is only a fraction. During a spacewalk, astronauts experienced the annual radiation dose on earth after only one day. So when planning long-term missions, radiation-related health damage is a factor to be considered. A special problem for space travel are solar eruptions, which cannot be predicted with certainty so far. During these events, the radiation dose can increase many times, causing short- and long-term health problems.
Effect on the genetic material
Cosmic radiation means a chronic burden for the organism. When the high-energy particles or the high-energy electromagnetic radiation hit the body and penetrate it, the absorption of the energy there can set off a chain of reactions. If, for example, the energetic state of a molecule changes, in particular the DNA as the carrier of genetic information, this may lead to the death of a cell or to cell mutations. But ionizing particles or secondary electrons can also cause great damage indirectly: If, for example, they hit a water molecule in the body and destroy it, so-called radicals may be formed – atoms or molecules that are particularly reactive. Radicals are also able to damage cells and cause diseases, including cancer. The biological effects of ionizing radiation show a considerable time lag between primary, direct physical interactions (immediate) and late-onset tumors (a few years) to genetic changes in subsequent generations (many years).
Radiation damage to the DNA
Considering the molecules inside a cell, the damage of enzymes, proteins, RNA molecules or biomembranes by ionizing radiation is less crucial than radiation damage of DNA, which can be of different types. These include single or double strand breaks, base damage or loss, and faulty crosslinking of base pairs. Chromosomal damage is also possible: if a DNA strand is interrupted, this can lead to the loss of a chromosomal fragment and thus to a loss of genetic information. In addition, ionized radiation-induced cross-linking of base pairs can lead to incorrect connections within a chromosome or even to the connection of two chromosomes.
Every living organism has the ability to repair or compensate for radiation damage to some degree. At the molecular level, single-strand breaks or single base damage can be repaired better than double-strand breaks or multiple damage. However, there can also be a misrepair, which may activate genes that were previously inactive. In the best case, this leads to cell death; in the worst case, the cell undergoes genetic changes and forms a tumor cell with uncontrolled cell division.
Ties and cells that divide rapidly are particularly sensitive to radiation, while those with a low division rate are less so. But also the phase of the cell cycle and external factors such as temperature and oxygen partial prere play an essential role in the radiation sensitivity of a cell. The hematopoietic stem cells of the bone marrow represent one of the most sensitive ties to radiation due to high division rates. If these cells are damaged, the formation of blood cells can be disrupted, making the body more susceptible to infections or prone to bleeding. The mucous membrane of the digestive tract and the skin also belong to the division-active ties. Whether a tumor ultimately develops, however, depends on many factors – such as the growth rate of the cells in this tie, the type of cell, and which gene is affected. Tumors in slow-growing ties, for example in the prostate, sometimes do not develop to clinical relevance.
DNA damage of sperm or egg cells can also lead to genetic changes in future generations. In the testes, the sperm-producing stem cells are particularly sensitive, while the sperm themselves are quite resistant. In females, all oocytes are already present at birth. Damage to them accumulates over time. Even a fertilized egg can be damaged in the womb by ionizing radiation. The less advanced the development, the greater the consequential damage. Damage in the first two weeks often results in embryo death.
Radiation doses in spacecraft and aircraft
The higher the radiation dose, the greater the probability that ionizing radiation will damage the cells in the body. Therefore, the probability of developing cancer increases, especially during long-duration flights. Increased mutation rates of cells have indeed been demonstrated in astronauts, but for aircrew the data is controversial. Other risks are also discussed, such as an increased likelihood of developing cataracts, a clouding of the eye lens, and an increased risk of arteriosclerosis (arterial wall changes). However, due to the low number of cases among astronauts, an accurate assessment of the risks is currently only possible to a limited extent.
However, you certainly do not have to give up occasional flights in airplanes due to cosmic radiation, because the effective dose here is still very low calculated on the year and lies with a few microsieverts below the alarming range. A short-haul flight increases the average annual effective dose from natural radiation exposure by less than one percent, while a long-haul flight increases it by about five percent. Depending on the flight route, duration and altitude as well as the current solar activity, the radiation exposure varies. Even for pregnant women, the health risk when flying is judged to be low according to current knowledge. However, there are no clear figures on this. However, a space flight should rather be postponed to a time after the pregnancy.
To better assess the radiation risk in space, researchers measure radiation doses there, for example, using the Matroshka experiment. A special manikin weighing seventy kilograms and equipped with sensors records the radiation exposure inside and outside the International Space Station (ISS). As part of the project, scientists are also investigating how best to shield humans from cosmic radiation. This ie plays a crucial role, especially for longer space missions, such as flights to Mars, and must be taken into account in the future when building spacecraft and especially when realizing visionary ideas such as "generation ships". In this case, however, radiation is only one of various risk factors that have so far been difficult to control, due to factors such as the lack of gravity or the confinement and monotony on board. These include problems such as massive bone and muscle loss, mental illness, difficulty with social interaction and nutritional ies, to name a few.
The unit for the radiation dose is called Gray, abbreviated Gy. One gray is equal to one joule of energy absorbed by one kilogram of body weight. Acute exposure of more than four grays is usually fatal to humans.
Since the different types of radiation ionize to different degrees, each of them is assigned a radiation weighting factor. For X-rays, gamma and beta radiation the factor is one, alpha radiation reaches a factor of twenty, and for neutron radiation it is between five and twenty, depending on energy. Multiplying the radiation dose in gray by the weighting factor of the type of radiation gives the organ dose, expressed in sieverts (Sv). In some cases, the term equivalent dose is also used. In contrast to organ dose, however, dose equivalent is not based on the absorbed absorbed dose of an organ or body part, but on an average value for a soft tie with defined properties.
At an organ dose of about 0.2 Sv, the probability of genetic damage and cancer risk increases. The value corresponds to about one hundred times the radiation exposure measured in Germany per year on average.
Further weighting factors are set for the organs in the human body, as for example many of the internal organs are much more sensitive to radiation than the skin. This gives the effective dose, which is also expressed in Sv.