Understanding Space Radiation
Outside the protective cocoon of the Earth's atmosphere is a universe full of radiation - it is all around us.
Space radiation is different from the kinds of radiation we experience here on Earth, such as X-rays or gamma rays. Space radiation is comprised of atoms in which electrons have been stripped away as the atom accelerated in interstellar space to speeds approaching the speed of light - eventually, only the nucleus of the atom remains.
Space radiation also has very different effects on human DNA, cells and tissues. This is due largely to the increased ionization that takes place near the track a particle of space radiation takes through a material. Ionizing radiation has so much energy it can literally knock the electrons out of any atom it strikes - ionizing the atom. This effect can damage the atoms in human cells, leading to future health problems such as cataracts, cancer and damage to the central nervous system.
It is very difficult to predict the long-term effects of space radiation on the human body, especially on our astronauts, who may spend
many months in space. Because of this uncertainty, NASA is funding research to determine how much radiation is in space and how
much damage it may cause - research that will help us to understand the risks astronauts face when they spend long periods of time in space, as well as to develop methods to mitigate those risks.
What is Space Radiation?
Space radiation is made up of three kinds of radiation: particles trapped in the Earth's magnetic field; particles shot into space during solar flares (solar particle events); and galactic cosmic rays, which are high-energy protons and heavy ions from outside our solar system. All of these kinds of space radiation represent ionizing radiation.
Image to right: CMEs typically drive shock waves that produce energetic particles that can be damaging to both electronic equipment and astronauts that venture outside the protection of the Earth's magnetic field. Credit: NASA
Flares and Coronal Mass Ejections
When a solar flare or a coronal mass ejection occurs (the two often occur at the same time, but not always), large amounts of high-energy protons are released, often in the direction of the Earth. These high-energy protons can easily reach the Earth's poles and
high-altitude orbits in less than 30 minutes. Because such events are very difficult to predict, there is often little time to prepare for their arrival.
Galactic Cosmic Rays
Galactic cosmic rays include heavy, high-energy ions of elements that have had all their electrons stripped away as they journeyed
through the galaxy at nearly the speed of light. Cosmic rays, which can cause the ionization of atoms as they pass through matter,
can pass practically unimpeded through a typical spacecraft or the skin of an astronaut. Galactic cosmic rays are the dominant source of radiation that must be dealt with aboard the International Space Station, as well as on future space missions within our solar system. Because these particles are affected by the Sun's magnetic field, their average intensity is highest during the period of minimum sunspots when the Sun's magnetic field is weakest and less able to deflect them. Also, because cosmic rays are difficult to
shield against and occur on each space mission, they are often more hazardous than occasional solar particle events. They are, however, easier to predict than solar particle events.
What Are the Effects of Space Radiation?
The energy that ionizing radiation loses as it travels through a material or living tissue is absorbed by that material or living tissue. The ionization of water and other cell components can damage DNA molecules near the path the particle takes - a direct effect of which is breaks in DNA strands including clusters of breaks near one another; breaks that are not easily repaired by cells. Such DNA break clusters are much less frequent, or do not occur at all, when cells are exposed to the types of radiation found on Earth. Because it can disrupt an atom, space radiation also can produce more particles, including neutrons, when it strikes a spacecraft or an astronaut inside a spacecraft - this is called a secondary effect.
Future research will develop the knowledge to understand how initial damage to DNA and cells from heavy ions relates to increased risks for cancer or other health effects, and how biological countermeasures to such risks can be developed.
How Much Space Radiation Do Astronauts Receive?
The amount of space radiation an astronaut may be exposed to while orbiting the Earth depends on a number of factors:
Protecting Current and Future Space Station Crews
- Orbital inclination - the closer a spacecraft's orbit takes it to the Earth's poles (where Earth's magnetic field concentrates ionizing particles), the higher the radiation levels will be.
- Altitude above the Earth - at higher altitudes the Earth's magnetic field is weaker, so there is less protection against ionizing particles, and spacecraft pass through the trapped radiation belts more often.
- Solar cycle - the Sun has an 11-year cycle, which culminates in a dramatic increase in the number and intensity of solar flares, especially during periods when there are numerous sunspots.
- Individual's susceptibility - researchers are still working to determine what makes one person more susceptible to the effects of space radiation than another person.
|Image above: Researchers are working hard to find ways to protect astronauts on long-duration spaceflights from cellular and molecular damage caused by space radiation. Credit: NASA
To determine acceptable levels of risk for astronauts, NASA follows the standard radiation protection practices recommended by the U.S. National Academy of Sciences Space Science Board and the U.S. National Council on Radiation Protection and Measurements.
Aboard the International Space Station, improving the amounts and types of shielding in the most frequently occupied locations, such as the sleeping quarters and the galley, has reduced the crew's exposure to space radiation. Materials that have high hydrogen contents, such as polyethylene, can reduce primary and secondary radiation to a greater extent than metals, such as aluminum.
Space station crew members each wear physical dosimeters, and also undergo a biodosimtery evaluation measuring radiation damage to chromosomes in blood cells.
Active monitoring of space radiation levels also can help reduce the levels of radiation an astronaut receives by helping the astronauts locate the best-shielded locations on the station. The monitoring also serves as a warning should radiation levels increase due to solar disturbances. Following a healthy diet and lifestyle, including the use of antioxidants following radiation exposure, should also reduce risks.
Excerpted from Understanding Space Radiation .pdf.