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For Millennia, people have gazed up at the night sky and dreamed of traveling to the stars. Now, when they visited the moon and lived in orbit at the space station, it seems that humanity is inevitably going: to Mars, to other places in the Solar system and beyond. This dream is in many cultures, and on its implementation working space agencies around the world.
But we know that space is dangerous. During any space flight, the astronauts are faced with severe cold, microgravity, lack of atmosphere and strong radiation. Still it seemed to be only a small technical problem that can be solved, as well as some risk, which is ready to go bold space travelers. However, my colleagues and I have conducted a new study which found that cosmic rays can be more damaging than previously thought, especially for fragile, but so important a body as the human brain. Although researchers for many decades know about radiation in space, only recently there is evidence, how seriously it affects the brain and how long the consequences might be.
Irradiating the mice, my colleagues and I has revealed significant and lasting cognitive impairment, which should probably occur in people, and this casts doubt on the possibility of successful space programs. Even during the flight to the International space station, which is relatively low, and astronauts largely protected from the adverse effects of the upper layers of Earth's atmosphere, they are risking some cognitive impairment. But the danger for those who will travel to Mars and beyond, can be fatal.
today, we have few opportunities to mitigate such risks. Improving the shielding of spacecraft can partially block the radiation, but yet there is enough light materials. The development of drugs that could fight the effects of radiation only begins now. If we don't find effective solutions, the mankind's dream to travel through the Solar system and beyond, may forever remain an impossible.
Cosmic radiation is destructive, we can't see or feel, but it fills every inch of empty at first glance, space and can seriously damage the fabric of the human body. The most dangerous for astronauts to galactic cosmic rays (GCR), consisting of charged atomic nuclei travelling near the speed of light. In addition to GCR, evenly distributed in space, there are more protons (hydrogen ions), which throws our Sun. Although most of the radiation in outer space is the protons, they are lightweight and therefore harm our body significantly less than the heavier particles. Most importantly, all of these particles of energy sufficient to penetrate through the armor of space ships and bodies of astronauts. The magnetic field around the Earth protect its inhabitants, deflecting much of the cosmic particles, but outside the magnetosphere inevitably leads to undesirable consequences of the interaction of particles with human tissues.
the Cosmic radiation is dangerous because when the particles pass through a person, they give part of its energy, which "ionizes" atoms in his body, that is, knocks electrons from atoms, turning them into charged particles. The particles then move on, knocking the electrons in the following atoms and thereby increasing the damage. The heavier the particle, the more it will have energy and the more it ionizes atoms. Motion of electrons leads to the fact that some molecular bonds between atoms are broken and damaged proteins, lipids, nucleic acids and other important molecules in cells and tissues of the body. Due to the movement of electrons appear free radicals — atoms or molecules with unpaired electrons on the outer orbitals, which have a very high reactivity and tend to complete the outer electron by the electrons from neighbouring atoms or molecules. Therefore, free radicals interact with other molecules in the body, turning them into new chemicals, not performing the initial functions. For example, when the radicals collide with DNA, they can break the bond between the deoxyribose and the phosphate group or to damage the nitrogen base.
To assess the effects of ionizing radiation, scientists use "absorbed dose" — the amount of energy transferred to the body (per unit mass). In the SI system absorbed dose is measured in gray (Gy), 1 Gy equals one Joule per kilogram. In addition, radiation can be of different "quality", that is, at one and the same dose may have different densities of ionization. For the characteristics of different types of radiation, scientists use the concept of linear energy transfer (let), i.e. the amount of energy loss per unit path. Thus, the dose of radiation with high let is much more dangerous than the same dose of radiation with low let as it leaves the body more energy and ionization of a greater number of atoms. Therefore, the cage will be harder to repair once damaged. Since many types of radiation found in cosmic rays, and the relatively high let, this feature is of great importance, if we are talking about far space flights.
In comparison with light particles heavier, flying, cause the formation of a larger amount of free radicals in its path, and more serious damage. At the molecular level, we find areas with a length of several nanometers, where the density of free radicals is so high that in a relatively small volume there is a lot of damage to important molecules. Therefore, heavy charged particles are much more multiple injuries compared with photon radiation (such as x-ray and gamma radiation). And because of the high density of damage to space radiation is more dangerous than conventional types of radiation with which we are dealing on the Ground.
Playing space to the Earth
Despite the fact that charged particles are distributed throughout the space, to reproduce this type of radiation on earth to study its effect on the body is very difficult. One of the few places where we can conduct experiments with simulated space radiation Laboratory NASA space radiation, which was created by NASA and Brookhaven national laboratory on long island in 2003, There are accelerators of charged particles acceleration of different ions to velocities close to those typical of space radiation. Researchers, including myself, is subjected to objects (in our case, mice) exposure to such radiation and evaluate its impact. Using these tests we find out how certain types of space radiation in different doses affect the tissues of a living organism.
we Recently took a six-month mice, subjected to exposure to low radiation doses (from 0.05 to 0.3 G) charged particles (including oxygen and titanium), and then evaluated their behavior. To understand how radiation affected the memory and cognitive ability of the mice, we conducted tests to determine the novelty of the object (novel object recognition, NOR) and the definition of the novelty of the location of the object (object in place, OiP). First, the rodents examined the empty box. Then we put the details of LEGO, rubber ducks and other toys and give the mice one more time there is to run. Then, after a few minutes, and in other cases a few hours or days, we replaced the toy with a new one (NOR), or moved it to another place (OiP). Smart healthy animal, it will take longer to explore a new toy in a new place.
the Mouse is the same, damages will be less to pay attention to the changes. These tests allow to reliably evaluate the work of the hippocampus (it is responsible for memory and learning) and cortical (cognitive ability). To assess the behavior of the animal, we used the preference index (discrimination index), to calculate which calculated the ratio of time spent around the animals a new facility or location, to the total time spent on the survey of all objects.
by means of tests NOR and OiP, we found a significant decrease of the preference index in mice exposed to radiation. Six weeks in rodents that received exposure equal to 5 and 30 cGy (centigray), the index fell by about 90%, and changes were remarkably uniform regardless of the dose received. Moreover, subsequent studies revealed that the effect was maintained for 12, 24 and even 54 weeks after exposure.
The results indicate that such levels of cosmic radiation can create problems the astronauts who must solve problems, make important decisions and to engage in other essential activities.
the Hurricane for neural tree
After it was carried out behavioral tests, my colleagues looked at what happens in the brain of irradiated mice. Passing through the brain, the charged particles may seriously affect the communication between neurons. We wanted to find physical damage, which could be associated with the detected changes in behavior. For this, we used genetically modified mice in the brain were neurons with a fluorescent protein, clearly visible under a microscope with powerful magnification.
We have received a series of sections of certain brain structures at different levels and combined them, creating a three-dimensional reconstruction. We found significant changes in the dendrites of nerve cells. Dendrites — fingerlike appendages of the cell that receive chemical signals from other neurons (like the processes involved in transmitting signals called nerve). In studies recently conducted in our laboratory, was found to lead to a weak ionization (with low let) x-rays and gamma radiation causes a significant decrease in the length, area and branching of dendrites in ten and 30 days. All together indicates a decline in dendritic complexity. This is an important parameter, which may be compared with the degree of branching of the tree. In recently conducted studies, the results of which are published in 2015 in the journal Science Advances, we also found that exposure to even very low doses of charged particles can cause significant and irreversible simplification of the dendritic tree.
moreover, changes occurred in a specific area of the brain — the medial prefrontal cortex, which are known to be involved in memory formation, and based on our behavioral tests, we assumed that it can be damaged. This does not mean that other areas of brain damage, and that no other neural system remained undisturbed. But in our study due to a combination of behavioural research with neuroimaging, we have identified the Association between cognitive deficits and structural changes in these brain areas. We examined sections obtained at high magnification to find evidence of other structural changes in spines — small, less than a micron, outgrowths on dendrites, providing learning and memory. If the dendrite — like branches on a tree, the dendritic spines — like leaves on the branches.
They allow to form synapses with which a neuron receives signals from other cells, and can be of different shapes and perform different tasks. In our old job where we used x-rays and protons and later with charged particles it turned out that dendritic spines are extremely sensitive to radiation. We found that the density of spines, i.e. their number per unit length of dendrite in mice was reduced after a short period of time after exposure (ten days), and after a longer period (six weeks). Such serious and lasting consequences mean that charged particles can cause structural changes in the brain, reducing the number of synaptic connections and disrupting the ability of neurons to transmit signals.
to make sure that the changes in mouse behavior is caused by the changes that have been detected in neurons, we compared the behavioral results of each animal with the density of dendritic spines in his brain. We got confirmation that the lower density of spines and deteriorating cognitive abilities. Animals, worst of all performed tests (that is, losing interest or desire to explore new) also had a lower density of spines, and thus, the impairment in cognitive abilities was, at least partly, due to a decrease in their number. This was the first evidence of a connection between the structural and behavioural abnormalities in animals exposed to space radiation.
the results of the experiments confirm what NASA had suspected for many years: radiation can disrupt cognitive abilities of astronauts. So far such fears have largely been based on the description of clinical cases disorders in patients with brain cancer, survived radiation therapy. But previously, scientists could not be sure to transfer these results on astronauts, because it was different and people, and types of radiation and its dose. In the treatment the usual daily dose is 2 Gy, which exceeds the dose that would be obtained, to fly to Mars and stayed there for a long time. For interplanetary flights the astronaut will receive a dose approximately of 0.48 mGy (milligray) per day for about 360 days of the flight there and back and half of this dose during the stay on Mars in a year or more (due to the big mass planet partially blocks the radiation). Although the total dose of radiation in the clinic are much higher than those in space, x-ray and gamma radiation, typically used to combat tumors, clubionidae (low let), and charged particles, which we fear in space, strongly ionizing (high let). It would therefore be incorrect based on the results of irradiation of cancer patients to draw conclusions about the consequences for the astronauts.
Our study confirms the assumptions about the dangers of space radiation for the brain of astronauts, but then you need to make an important remark. Although we used doses of radiation, comparable to those observed in space, we could irradiate the mice with the same rate, which will only be exposed to the astronauts. During the flight, people are exposed to radiation continuously for many months or even years. But since we were able to work with the accelerator only a limited time, mice received the same dose in a few minutes. Such a big difference in speed may put our conclusions into question, since we can assume that if the irradiation to produce slow, the cells will have time to recover. But in fact, it's not like such a distinction was of great importance, since the total dose is small (in other words, the particles fly not often) and greatest concern is caused by cosmic particles with high let, causing severe damage to the cells, which are difficult to correct no matter what speed the irradiation. Finally, in most of the brain is no formation of new neurons, which greatly complicates recovery. Although our results are obtained by irradiation of rodents, not people, there is no reason to believe that human neurons will react to cosmic radiation differently than a mouse in our experiment.
is There a future long space flights?
to send the people in flight through the Solar system, it is necessary to overcome great difficulties. To send to Mars and other parts of the Solar system, astronauts will need more powerful rockets than those used now. Upon arrival, they will have to live somewhere, to get water and fuel from local sources. Now to the list of issues has increased the need to protect people from radiation, which can pass through most solid obstacles.
the First method of overcoming this problem is to use protective materials that can stop radiation before it can cause harm. They should be covered with space ships and houses, or suits and clothing. At present, the only known scholar the way of protection against radiation — the use of thick and heavy materials such as lead. It's effective, but highly impractical, because the materials are too heavy and will need to use a lot of rocket fuel to move such weight. Now trying to create a superior protective materials and technical control systems to strengthen the protection of the individual compartments of the spacecraft. The astronauts will be able to stay in these more protected bays during periods of high solar activity, and during sleep or spacewalk, will wear helmets and pressure suits to provide maximum protection from radiation. This will require new materials that protect better modern.
Scientists are also developing a food and medicinal Supplement. The astronauts will have to take them regularly, or after acute exposure (for example, after a strong solar storm) to mitigate the effects from radiation on the brain. For example, when testing on mice antioxidants have proven as a promising tool to partially prevent damage during irradiation. In addition, scientists have moved towards the creation of drugs that strengthen the nervous system and helping the brain keep working after the injury. However, all these studies are now in the early stages and none of the drugs will not be a panacea.
We cannot avoid damage in the best case we can hope to reduce them. In addition, it is necessary to continue to study the effect of cosmic rays on the brain and the whole body below in more detail to figure out what the health risk in the short and long term can carry a long exposure. Our research has shown the aspect of the dangers of long space flights, which was probably greatly underestimated. For example, about the danger of cancer from the radiation known much better but it might be not so important, because most radiogenic cancers develop quite a long time. And we have shown that even weak exposure to cosmic radiation causes neuronal damage and cognitive impairment in mice and it is likely that in humans.
Another reason for concern is that caused by irradiation changes. It is too early to assert that the irradiation leads to irreversible consequences, but at the moment scientists have not found any indication that the damaged dendrites and disappeared after exposure, the spines can themselves recover. Therefore, while researchers will find ways to ensure healing of the tissues of the brain damaged by radiation, the best we can do is to protect the still intact structure.
Cosmic rays may be one of the most serious obstacles in the flight to Mars and in more remote areas. Although our findings are not indisputable, it will be difficult to ignore the available data and their possible implications for the future of space exploration. Does this mean that we are forever tied to the Earth? Probably not. This is just another obstacle that we must overcome humanity before we can see our main problem or perhaps our greatest success.
Author: Charles Limoli — neuroscientist and specialist in radiation biology, working at the Medical school of the University of California, Irvine. He studies cognitive deficits that occur with some cancer treatments and because of cosmic radiation.
Translation: M. S. Bagotsky
Source: the Magazine "world of science" 2017.14
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