be astronaut is not easy. Although today is a much less dangerous profession in the past, men and women dedicated to the conquest of space know that there are many things that could go wrong during a mission. But of all the dangers there is one that is often overlooked and yet is present in every mission. We refer to radiation.
A violent coronal mass ejection on the Sun generates a stream of energetic particles that can kill an astronaut (ESA / NASA).
All astronauts are exposed to significant doses of radiation during the course of a spaceflight. Although not a major obstacle to short-term missions, the radiation is converted into a huge problem if we want to live in space indefinitely, or travel the Solar System. In fact, for many is the space exploration problem par excellence. One would think that after several decades studying the effects of radiation on humans are able to accurately predict the impact of space radiation on humans. Not really. Not even close. The truth is we do not know many of their long-term effects.
The Origins of the space age, the radiation was a major concern of scientists. Many thought that any human being to venture beyond Earth's atmosphere would be subjected to dose deadly radiation that would kill him instantly or, perhaps, become a kind of mutant monster. The first space missions have demonstrated the existence of a steady stream of energetic particles in space, but at the same time it was found that doses were not lethal. By the way, several animals traveled into space before any man was put into orbit. As none of them was damaged by radiation and became a mutant freak, it was felt that manned space travel was safe.
Nearly five hundred men and women have traveled into space in the last fifty years, proving that space radiation not lethal. And yet there it is.
When performing an extravehicular activity (EVA) takes into account the level of radiation (NASA).
radiation in space
Unlike many people think, the near-Earth space is not a "vacuum" and immaculate, but is packed with all kinds of particles . Some of these particles have sufficient energy to cause damage to our body and break the DNA in our cells. And we all know what that means: cancer. Radiation ionizing radiation to which an astronaut is subjected has three possible sources: the Sun, cosmic rays and Earth's radiation belts. Let us briefly review the characteristics of each source of radiation in space.
The radiation damages the genetic material of our cells.
Sun
The Sun is a star static, but continually ejects material from its surface. This flow of particles called the solar wind , although it is a name that can be confusing. The solar wind is actually a plasma, ie, a flow of charged particles associated with a magnetic field, which is of great importance when analyzing their effects on health. It consists primarily of hydrogen nuclei (protons) and helium (alpha particles), the most abundant of our star and universe. There is a small proportion of heavy nuclei, but nothing spectacular. If it were only by the solar wind, the sun would not cause a hazard to astronauts.
solar wind as a function of solar latitude (NASA / ESA).
The problem is that the sun spits occasionally large amounts of highly energetic particles. These "solar storms" are called solar particle events or SPE (Solar Particle Events ) and its origin is quite complex. To oversimplify, we can say that the SPE is created from the interactions of the solar magnetic field and are associated with two very violent solar phenomena: the flares (flares ) and coronal mass ejections ( Coronal Mass Ejections , CME) . PES are composed mainly of protons with energies of a few megaelectronvoltios hundred (MeV) maximum, plus a few alpha particles and some other heavy nucleus.
SPE Events in recent years (NASA).
The effects of SPE in the human body are much worse than those caused by the solar wind. A lot. Let's say you would not like to be in outer space without protection during such an event, unless you want to be irradiated with potentially lethal doses (1-4 Sv). Fortunately, the SPE are very rare. Our star emits one or two major SPE every eleven years and only 20% come to affect the Earth-Moon system. Although unpredictable, the sun is more likely to generate a SPE when it is near the peak of its activity cycle of eleven years. Once unleashed, it takes twelve hours to two days to reach Earth orbit, which is usually more than enough time to alert the astronauts if they have an adequate detection network.
Cosmic Rays
Having a mysterious name, cosmic rays or GCR (Galactic Cosmic Rays ) are particles that originate in exotic corners of our galaxy. Most were created over millions of years by a supernova explosion or in the accretion disk of a black hole and have traveled thousands of light years before reaching our Solar System.
Unlike the solar wind, their energies are very variable, but what interests us is that they can reach up to 10 GeV per nucleon, between ten and twenty times more than a proton emitted by the Sun This means that some particles move nearly the speed of light. Most cosmic rays are also protons (90%) and alpha particles (8%), but about 2% are heavy nuclei. As we shall see, that 2% is particularly problematic in the face of human spaceflight. Of course, the number of cosmic rays per unit time, ie the flow-is much lower than the solar wind protons in the SPE, which greatly minimizes dangerous. Fortunately, a large number of cosmic rays are deflected by magnetic fields of the Sun and Earth.
Modulation cosmic ray flux as the solar activity cycle. LEO was observed in the flow is very low because of the magnetosphere (NASA).
radiation belts
Strictly speaking, the belts are not a "source" of radiation itself, as they are formed by energetic particles trapped in Earth's magnetic field. The origin of these particles are cosmic rays and solar wind, which explains that the majority are protons with maximum energy a few hundred MeV. Other radiation belts consist of electrons, but these are less dangerous. The shape and intensity of the radiation belts vary with solar activity cycle, but most of protons in a ring that has a maximum density of 6000 kilometers.
Earth's radiation belts. In
early enough to stay in orbit below 500 kilometers high if we are to avoid the effects of the radiation belts. Unfortunately, our planet's magnetic field has a distortion that allows the penetration of radiation belt protons at lower altitudes over a region located off the coast of Brazil (35 º S and 35 º W). This region receives the appropriately named "South Atlantic Anomaly" (SAA South Atlantic Anomaly ) and affects all manned space missions whose orbital inclination exceeds 30 degrees, as is the case Space Station (ISS). Most of the radiation received by the crew of the ISS is due to this anomaly.
levels of radiation in low Earth orbit. You can see the South Atlantic Anomaly (NASA / JAXA).
Earth's magnetosphere protects us from cosmic rays and the PES (NASA).
In a way, it seems as if the universe conspired to prevent astronauts from radiation can defend. If we limit ourselves to the missions in low Earth orbit (LEO), the Earth's magnetic field protect us from the SPE and cosmic rays, but we suffer the effects of the radiation belts. By contrast, the missions beyond Earth will resist the invitation, the SPE and cosmic rays. During solar activity minimum the chance of an SPE are minimal, but instead decreases the intensity of solar magnetic field and doubles the number of cosmic rays into the inner solar system.
flow distribution belt protons (NASA).
The flow of different types of particles of space radiation according to their energy. Fortunately, the most energetic particles are also those with lower flow (NASA).
dose and effects
How to measure radiation doses? In the International System of units is used gray (Gy) to measure radiation absorbed dose , a unit that replaces the traditional rad (1 Gy = 100 rad). Radiation from a gray deposit July 1 (1 J) of energy per kilogram of matter. Not all types of radiation have the same penetrating power and the absorbed dose depends strongly on the nature of the incident particles.
If our interest is to measure the effects of radiation on humans, the absorbed dose is a quantity not particularly useful, since the effects of radiation vary with the type of organ irradiated. Therefore, we use the concept of equivalent dose , which is similar to the absorbed dose but adjusted to take account of damage to living tissue. Its unit is the sievert (Sv, 1 Sv = 100 rem). As is known, the effects of radiation are stochastic. A nice way of saying that there is no minimum dose that can cause damage. A priori, any radiation dose can to cause cancer, although obviously the probability depends on the dose. Hence the panic caused by the mere mention of the word "radiation", but we must not forget that we are all subject to natural sources of radiation. A person normally receives over one year a dose of about 3.6 millisievert (mSv)-ie, 0.0036 Sv-natural causes. Including cosmic rays. Moreover, as important as the dose is the exposure time. To say that a person has suffered a dose of 1 mSv means nothing if we do not specify the duration of irradiation.
The vast majority of astronauts traveling to low Earth orbit (LEO), dominated effects due to the radiation belts. The doses in LEO depend both solar activity. Within the ISS are normally kept in the range of 0.4 to 1.1 mSv per day , including the effects of shielding. The long-term expeditions remain six months in orbit, so the equivalent dose reaches values \u200b\u200bof 70-500 mSv per year.
daily dose of radiation received in several missions. The green dots are the Apollo missions. You can see how the missions in LEO to higher altitudes and inclinations are subject to radiation doses similar to those of Apollo (NASA).
And this much or too little? Well, let's say that is enough. Legislation in the U.S. and other countries like Spain limits 50 mSv annual dose maximum a worker can receive in an environment subjected to radiation, although usually not normally exceed 2 mSv / year in these jobs. There are other professions more "conventional" who are also exposed to radiation. For example, airline pilots flying on intercontinental routes can receive 1-5 mSv / year because of cosmic rays. NASA decided in 2000 to revise downwards the maximum he could receive an astronaut in the course of his career, so that the likelihood of cancer mortal throughout his life because of the radiation does not exceed 3%.
maximum radiation dose for NASA.
As we see, the maximum permitted increase with age. Women have a higher risk of cancer because the mammary glands, hence the maximum will be less than in the case of men. Is this dose safe? For now, do not know. The number of astronauts is not high enough for meaningful data from the statistical point of view. Moreover, the problem is that these doses have been set apart from the data obtained by exposure to gamma rays or X rays, but we know very little about the effects of heavy nuclei in the human body. These cores are an essential part of cosmic rays and is very difficult to reduce their effects, unlike what happens with the SPE. Unfortunately, available data suggest that the tumors generated by the action of heavy nuclei are more aggressive and tend to occur earlier.
Some daily doses in several space missions.
astronaut maximum dose along the space race (NASA).
Comparison of doses received by astronauts performing EVAs in low orbit and those without. No great differences (NASA).
radiation detectors on the ISS (NASA).
How to defend ourselves from radiation?
it sounds a platitude, the best defense is to make short-term spaceflight. This simple technique helped to limit the dose received by astronauts on the Apollo missions even though they traveled outside the protection of the radiation belts. The odds of an SPE takes place on a mission a few days is minimal. This is what is called "statistical protection."
However, in the case of a Mars mission we have no choice but to deal with these problems. Unless we use revolutionary methods of propulsion, the duration of a trip to the red planet is large enough to easily get one or two SPE during the course of the mission. To make matters worse, cosmic rays, practically negligible orbital flights, takes on enormous importance in these interplanetary missions long duration.
The Apollo missions were not large doses of radiation because of its short duration (NASA).
Mars missions are divided into two types: conjunction and opposition. Conjunction missions include a long stay on the surface (300-600 days) and a round trip of 150-250 days (the duration depends on the relative position of the planets). Opposition missions covered short stay of only 20-60 days, and travel tempos of 100-400 days. The effects of radiation would be lower in the case of the missions of conjunction, because during the stay in Mars doses significantly reduced thanks to the mass of the planet and its thin atmosphere.
Radiation Dose on the Martian surface due to cosmic rays. The highest regions are the least protected, being outside the atmosphere (NASA).
Therefore, it is obvious that a Martian spacecraft should be equipped with a "safe haven" especially to protect astronauts from the SPE. The best braking materials formed by proton radiation are those with low atomic number elements such as hydrogen. But we have the heavy nuclei of cosmic rays. And here is the problem. Heavy nuclei from cosmic rays moving at relativistic speeds, so when they hit the metal frame of a spacecraft generate a cascade of secondary particles, including neutrons, alpha particles and mesons. These secondary particles are a source of additional radiation very worrying. As a result, sometimes the structure of steel or aluminum spacecraft does not diminish the radiation dose, but increases.
Therefore, the use of several layers of polyethylene (hydrocarbon rich in hydrogen) and water is believed to be the best way to protect the crew of a ship, at least in the case of SPE. Another option would be to include an active shielding using magnetic or electrostatic fields. However, this system is energy intensive, requiring the use of nuclear reactors or huge solar panels. Furthermore it offers complete protection against the most energetic heavy nuclei.
as radiation shielding materials of different density (NASA).
shielding effectiveness of various test on two shuttle missions (NASA).
Effect on radiation dose to use several shields (NASA).
electrostatic shield proposal for a lunar base (NASA).
Without active shielding, the radiation dose on a trip to Mars would be one or two sievert least well above the current limits of NASA. With these figures in mind, it is not surprising that the first human to set foot on the red planet is a man over sixty years.
Ultimately, the radiation in space has not proved an obstacle to reach Earth orbit. But if in the future we want to live on other planets, we have no choice but to learn to protect yourself against this invisible enemy.
A violent coronal mass ejection on the Sun generates a stream of energetic particles that can kill an astronaut (ESA / NASA).
All astronauts are exposed to significant doses of radiation during the course of a spaceflight. Although not a major obstacle to short-term missions, the radiation is converted into a huge problem if we want to live in space indefinitely, or travel the Solar System. In fact, for many is the space exploration problem par excellence. One would think that after several decades studying the effects of radiation on humans are able to accurately predict the impact of space radiation on humans. Not really. Not even close. The truth is we do not know many of their long-term effects.
The Origins of the space age, the radiation was a major concern of scientists. Many thought that any human being to venture beyond Earth's atmosphere would be subjected to dose deadly radiation that would kill him instantly or, perhaps, become a kind of mutant monster. The first space missions have demonstrated the existence of a steady stream of energetic particles in space, but at the same time it was found that doses were not lethal. By the way, several animals traveled into space before any man was put into orbit. As none of them was damaged by radiation and became a mutant freak, it was felt that manned space travel was safe.
Nearly five hundred men and women have traveled into space in the last fifty years, proving that space radiation not lethal. And yet there it is.
When performing an extravehicular activity (EVA) takes into account the level of radiation (NASA).
radiation in space
Unlike many people think, the near-Earth space is not a "vacuum" and immaculate, but is packed with all kinds of particles . Some of these particles have sufficient energy to cause damage to our body and break the DNA in our cells. And we all know what that means: cancer. Radiation ionizing radiation to which an astronaut is subjected has three possible sources: the Sun, cosmic rays and Earth's radiation belts. Let us briefly review the characteristics of each source of radiation in space.
The radiation damages the genetic material of our cells.
Sun
The Sun is a star static, but continually ejects material from its surface. This flow of particles called the solar wind , although it is a name that can be confusing. The solar wind is actually a plasma, ie, a flow of charged particles associated with a magnetic field, which is of great importance when analyzing their effects on health. It consists primarily of hydrogen nuclei (protons) and helium (alpha particles), the most abundant of our star and universe. There is a small proportion of heavy nuclei, but nothing spectacular. If it were only by the solar wind, the sun would not cause a hazard to astronauts.
solar wind as a function of solar latitude (NASA / ESA).
The problem is that the sun spits occasionally large amounts of highly energetic particles. These "solar storms" are called solar particle events or SPE (Solar Particle Events ) and its origin is quite complex. To oversimplify, we can say that the SPE is created from the interactions of the solar magnetic field and are associated with two very violent solar phenomena: the flares (flares ) and coronal mass ejections ( Coronal Mass Ejections , CME) . PES are composed mainly of protons with energies of a few megaelectronvoltios hundred (MeV) maximum, plus a few alpha particles and some other heavy nucleus.
SPE Events in recent years (NASA).
The effects of SPE in the human body are much worse than those caused by the solar wind. A lot. Let's say you would not like to be in outer space without protection during such an event, unless you want to be irradiated with potentially lethal doses (1-4 Sv). Fortunately, the SPE are very rare. Our star emits one or two major SPE every eleven years and only 20% come to affect the Earth-Moon system. Although unpredictable, the sun is more likely to generate a SPE when it is near the peak of its activity cycle of eleven years. Once unleashed, it takes twelve hours to two days to reach Earth orbit, which is usually more than enough time to alert the astronauts if they have an adequate detection network.
Cosmic Rays
Having a mysterious name, cosmic rays or GCR (Galactic Cosmic Rays ) are particles that originate in exotic corners of our galaxy. Most were created over millions of years by a supernova explosion or in the accretion disk of a black hole and have traveled thousands of light years before reaching our Solar System.
Unlike the solar wind, their energies are very variable, but what interests us is that they can reach up to 10 GeV per nucleon, between ten and twenty times more than a proton emitted by the Sun This means that some particles move nearly the speed of light. Most cosmic rays are also protons (90%) and alpha particles (8%), but about 2% are heavy nuclei. As we shall see, that 2% is particularly problematic in the face of human spaceflight. Of course, the number of cosmic rays per unit time, ie the flow-is much lower than the solar wind protons in the SPE, which greatly minimizes dangerous. Fortunately, a large number of cosmic rays are deflected by magnetic fields of the Sun and Earth.
Modulation cosmic ray flux as the solar activity cycle. LEO was observed in the flow is very low because of the magnetosphere (NASA).
radiation belts
Strictly speaking, the belts are not a "source" of radiation itself, as they are formed by energetic particles trapped in Earth's magnetic field. The origin of these particles are cosmic rays and solar wind, which explains that the majority are protons with maximum energy a few hundred MeV. Other radiation belts consist of electrons, but these are less dangerous. The shape and intensity of the radiation belts vary with solar activity cycle, but most of protons in a ring that has a maximum density of 6000 kilometers.
Earth's radiation belts. In
early enough to stay in orbit below 500 kilometers high if we are to avoid the effects of the radiation belts. Unfortunately, our planet's magnetic field has a distortion that allows the penetration of radiation belt protons at lower altitudes over a region located off the coast of Brazil (35 º S and 35 º W). This region receives the appropriately named "South Atlantic Anomaly" (SAA South Atlantic Anomaly ) and affects all manned space missions whose orbital inclination exceeds 30 degrees, as is the case Space Station (ISS). Most of the radiation received by the crew of the ISS is due to this anomaly.
levels of radiation in low Earth orbit. You can see the South Atlantic Anomaly (NASA / JAXA).
Earth's magnetosphere protects us from cosmic rays and the PES (NASA).
In a way, it seems as if the universe conspired to prevent astronauts from radiation can defend. If we limit ourselves to the missions in low Earth orbit (LEO), the Earth's magnetic field protect us from the SPE and cosmic rays, but we suffer the effects of the radiation belts. By contrast, the missions beyond Earth will resist the invitation, the SPE and cosmic rays. During solar activity minimum the chance of an SPE are minimal, but instead decreases the intensity of solar magnetic field and doubles the number of cosmic rays into the inner solar system.
flow distribution belt protons (NASA).
The flow of different types of particles of space radiation according to their energy. Fortunately, the most energetic particles are also those with lower flow (NASA).
dose and effects
How to measure radiation doses? In the International System of units is used gray (Gy) to measure radiation absorbed dose , a unit that replaces the traditional rad (1 Gy = 100 rad). Radiation from a gray deposit July 1 (1 J) of energy per kilogram of matter. Not all types of radiation have the same penetrating power and the absorbed dose depends strongly on the nature of the incident particles.
If our interest is to measure the effects of radiation on humans, the absorbed dose is a quantity not particularly useful, since the effects of radiation vary with the type of organ irradiated. Therefore, we use the concept of equivalent dose , which is similar to the absorbed dose but adjusted to take account of damage to living tissue. Its unit is the sievert (Sv, 1 Sv = 100 rem). As is known, the effects of radiation are stochastic. A nice way of saying that there is no minimum dose that can cause damage. A priori, any radiation dose can to cause cancer, although obviously the probability depends on the dose. Hence the panic caused by the mere mention of the word "radiation", but we must not forget that we are all subject to natural sources of radiation. A person normally receives over one year a dose of about 3.6 millisievert (mSv)-ie, 0.0036 Sv-natural causes. Including cosmic rays. Moreover, as important as the dose is the exposure time. To say that a person has suffered a dose of 1 mSv means nothing if we do not specify the duration of irradiation.
The vast majority of astronauts traveling to low Earth orbit (LEO), dominated effects due to the radiation belts. The doses in LEO depend both solar activity. Within the ISS are normally kept in the range of 0.4 to 1.1 mSv per day , including the effects of shielding. The long-term expeditions remain six months in orbit, so the equivalent dose reaches values \u200b\u200bof 70-500 mSv per year.
daily dose of radiation received in several missions. The green dots are the Apollo missions. You can see how the missions in LEO to higher altitudes and inclinations are subject to radiation doses similar to those of Apollo (NASA).
And this much or too little? Well, let's say that is enough. Legislation in the U.S. and other countries like Spain limits 50 mSv annual dose maximum a worker can receive in an environment subjected to radiation, although usually not normally exceed 2 mSv / year in these jobs. There are other professions more "conventional" who are also exposed to radiation. For example, airline pilots flying on intercontinental routes can receive 1-5 mSv / year because of cosmic rays. NASA decided in 2000 to revise downwards the maximum he could receive an astronaut in the course of his career, so that the likelihood of cancer mortal throughout his life because of the radiation does not exceed 3%.
maximum radiation dose for NASA.
As we see, the maximum permitted increase with age. Women have a higher risk of cancer because the mammary glands, hence the maximum will be less than in the case of men. Is this dose safe? For now, do not know. The number of astronauts is not high enough for meaningful data from the statistical point of view. Moreover, the problem is that these doses have been set apart from the data obtained by exposure to gamma rays or X rays, but we know very little about the effects of heavy nuclei in the human body. These cores are an essential part of cosmic rays and is very difficult to reduce their effects, unlike what happens with the SPE. Unfortunately, available data suggest that the tumors generated by the action of heavy nuclei are more aggressive and tend to occur earlier.
Some daily doses in several space missions.
astronaut maximum dose along the space race (NASA).
Comparison of doses received by astronauts performing EVAs in low orbit and those without. No great differences (NASA).
radiation detectors on the ISS (NASA).
How to defend ourselves from radiation?
it sounds a platitude, the best defense is to make short-term spaceflight. This simple technique helped to limit the dose received by astronauts on the Apollo missions even though they traveled outside the protection of the radiation belts. The odds of an SPE takes place on a mission a few days is minimal. This is what is called "statistical protection."
However, in the case of a Mars mission we have no choice but to deal with these problems. Unless we use revolutionary methods of propulsion, the duration of a trip to the red planet is large enough to easily get one or two SPE during the course of the mission. To make matters worse, cosmic rays, practically negligible orbital flights, takes on enormous importance in these interplanetary missions long duration.
The Apollo missions were not large doses of radiation because of its short duration (NASA).
Mars missions are divided into two types: conjunction and opposition. Conjunction missions include a long stay on the surface (300-600 days) and a round trip of 150-250 days (the duration depends on the relative position of the planets). Opposition missions covered short stay of only 20-60 days, and travel tempos of 100-400 days. The effects of radiation would be lower in the case of the missions of conjunction, because during the stay in Mars doses significantly reduced thanks to the mass of the planet and its thin atmosphere.
Radiation Dose on the Martian surface due to cosmic rays. The highest regions are the least protected, being outside the atmosphere (NASA).
Therefore, it is obvious that a Martian spacecraft should be equipped with a "safe haven" especially to protect astronauts from the SPE. The best braking materials formed by proton radiation are those with low atomic number elements such as hydrogen. But we have the heavy nuclei of cosmic rays. And here is the problem. Heavy nuclei from cosmic rays moving at relativistic speeds, so when they hit the metal frame of a spacecraft generate a cascade of secondary particles, including neutrons, alpha particles and mesons. These secondary particles are a source of additional radiation very worrying. As a result, sometimes the structure of steel or aluminum spacecraft does not diminish the radiation dose, but increases.
Therefore, the use of several layers of polyethylene (hydrocarbon rich in hydrogen) and water is believed to be the best way to protect the crew of a ship, at least in the case of SPE. Another option would be to include an active shielding using magnetic or electrostatic fields. However, this system is energy intensive, requiring the use of nuclear reactors or huge solar panels. Furthermore it offers complete protection against the most energetic heavy nuclei.
as radiation shielding materials of different density (NASA).
shielding effectiveness of various test on two shuttle missions (NASA).
Effect on radiation dose to use several shields (NASA).
electrostatic shield proposal for a lunar base (NASA).
Without active shielding, the radiation dose on a trip to Mars would be one or two sievert least well above the current limits of NASA. With these figures in mind, it is not surprising that the first human to set foot on the red planet is a man over sixty years.
Ultimately, the radiation in space has not proved an obstacle to reach Earth orbit. But if in the future we want to live on other planets, we have no choice but to learn to protect yourself against this invisible enemy.
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