Lesson 3 - Radiation

On Earth, our magnetic field and atmosphere protects us from most of the space radiation coming from the Sun and other parts of the galaxy. Astronauts on the International Space Station don't have the same protection and have an increased lifetime risk of developing cancer or other degenerative diseases. In fact, an astronaut on the International Space Station receives the same amount of radiation exposure in one day as an average person experiences on Earth in one year!

There are three main types of radiation that astronauts have to deal with in space.

1) Particles from the sun

Our sun regularly emits bursts of energy and particles, which are called solar storms. These solar storms can fly off in almost any direction. The sun actually varies in how many solar storms it has over an 11 year cycle.

The surface of the sun appears as a mosaic of dark browns, deep reds and warm orange, yellow hues. It curves away into the inky blackness of space, but just at the very edge a large solar storm eruption glows white hot, and pours from the sun as water pours from a glass. (Credit: NASA)

If the Earth happens to be in the path of the solar storm, the particles hit our magnetic field. Our magnetic field diverts most of the particles around the Earth and towards the North and South Pole. This phenomenon is why auroras form.

The Sun is shown radiating a wave of particles towards the Earth, which appears surround by concentric curved lines. The curved lines act as a barrier to the Sun's particles.(Credit: DataCenterKnowledge.com)

Any particles that are not diverted end up getting blocked by the atmosphere before they make it to the Earth's surface. The particles can also get trapped in a special zone called the Van Allen radiation belts, which also helps keep particles from reaching the surface.

2) Particles trapped in the Van Allen radiation belts

There are two zones around the Earth where radiation particles can get 'stuck'. The inner Van Allen radiation belt generally stretches from 600 to 1000 kilometers above Earth's surface and the outer belt stretches from 13500 to 58000 kilometers above Earth's surface.

The Earth is shown in the middle of the two Van Allen belts, that circle the Earth as thin donuts. (Credit: NASA)

This is higher than the orbit of the International Space Station, which is usually only between 330 to 435 kilometers, so astronauts onboard do not usually have to worry about these particles. In fact, the only humans to ever pass through these radiation belts were the Apollo astronauts who visited the Moon!

3) Galactic Cosmic Rays

Galactic Cosmic Rays are extremely high-energy, high-speed particles that have originated from somewhere outside our solar system, but (mostly) within our Milky Way galaxy. They travel at nearly the speed of light and can pass easily through a spacecraft and the skin of an astronaut. Galactic Cosmic Rays are stopped from reaching the Earth's surface by the inner Van Allen radiation belt and the Earth's atmosphere.

Why is radiation bad?

Since as far back as the Apollo missions to the Moon, astronauts have reported occasionally seeing bright flashes of light inside their eyeballs. NASA believes these are triggered by Galactic Cosmic Rays passing through an eyeball at near light-speed, causing a false signal which the brain interprets as light.

It also seems that astronauts have a much higher rate of developing cataracts than usual. Cataracts are cloudy lens that cause vision to become blurry. There is a known link between radiation exposure and cataracts, although scientists haven't figured out exactly why this happens.

A close-up view of a person's face, showing one eye with a clear black pupil, and one eye with a milky, cloudy pupil. The milky eye has a cataract. (Credit: Mayo Clinic)

The greatest problem that radiation causes for humans is DNA damage. DNA is a double-stranded molecule inside each and every one of the cells inside our bodies. DNA contains the code for all the functions in our bodies, and is essential for our cells to be able to divide and grow. Radiation can cause damage to DNA in different ways, such as completely breaking the strand, breaking one side of the strand, or even completely changing some of the code written on the strand.

A piece of DNA, shown as a ladder that has been twisted around itself, has completed shattered as if cut by a pair of scissors, due to radiation. (Credit: UPI.com)

Our bodies are pretty good at repairing DNA, but once in a while the repair process can make a mistake. These mistakes are called mutations, and this is the biggest threat to human health in long-term space missions. The more mutations a human body gets, the greater the risk of developing cancer and other degenerative diseases.

Surviving Radiation in Space

There are currently two main strategies for dealing with radiation in space. First, astronauts are warned if a solar storm has been tracked to be heading towards Earth. In this case, they can have as much as two days warning to shelter inside the parts of the space station with the thickest walls.

Secondly, as many of the health problems with radiation exposure are caused by the amount of repeated exposure over time, astronauts have a lifetime radiation limit that they cannot exceed. This means they can only fly for in space for a limited amount of time during their life.

The current NASA radiation limit for astronauts is around 0.50 Sieverts per year and 1 Sievert over their spaceflight career. This limit changes depending on the astronauts age and sex. A dose of 1 Sievert of radiation corresponds to about a 5% increased risk of developing cancer within a lifetime. For comparison, the yearly limit for radiation workers on Earth (such as people working in nuclear power plants) is 0.05 Sieverts per year.

If humans want to live for long periods of time in orbit, or if they want to live on the Moon or Mars, we will need to invent better ways of protecting humans from radiation. Some researchers suggest covering Moon or Mars habitats with thick layers of Moon dust or Mars dust to provide extra protection.

A collection of small habitat modules joined together to form a larger Mars base. Half of the modules are submerged under a huge mound of brownish red Mars dust. (Credit: Hackaday.com)

Another future strategy could be covering spacecraft or habitats with materials that have lots of hydrogen. Hydrogen is great at blocking particle radiation and makes up a lot of some common materials such as water and polyethylene plastic (the same plastic used in grocery bags).

NASA is developing a material called 'Hydrogenated boron nitride nanotubes', which are tiny tubes made of carbon, boron, and nitrogen, with hydrogen spread out throughout the empty spaces left in between the tubes. They have been able to make yarn out of these nanotubes, and are currently testing to see whether they could be used to make spacesuits better at protecting astronauts from radiation!

A web of stringy, off-white colored hydrogenated boron nitride nanotubes. The stringy nanotubes are being pulled from one end to stretch and gather them into a yarn. (Credit: SigmaAldrich.com)

Resources:
https://cosmosmagazine.com/space/how-much-radiation-damage-do-astronauts-really-suffer-in-spacehttps://www.nasa.gov/analogs/nsrl/why-space-radiation-matters

https://www.nasa.gov/content/goddard/van-allen-probes-spot-impenetrable-barrier-in-space

https://www.express.co.uk/news/science/1148407/weather-forecast-space-news-solar-storm-2019-noaa-latest-update-space-weather-forecast

https://spaceplace.nasa.gov/solar-cycles/en/

https://www.researchgate.net/publication/273988604_Galactic_cosmic_radiation_risk_in_human_space_missions

https://www.popsci.com/exomars-radiation-astronauts/

https://www.nasa.gov/feature/goddard/real-martians-how-to-protect-astronauts-from-space-radiation-on-mars

https://science.nasa.gov/science-news/science-at-nasa/2004/22oct_cataracts