Space Tech

NASA Facts

National Aeronautics and

Space Administration

Lyndon B. Johnson Space Center

Understanding Space Radiation

FS-2002-10-080-JSC

October 2002

• 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.

Measuring Radiation

The absorbed dose of radiation is the amount of energy

deposited by radiation per unit mass of material. It is measured

in units of rad (radiation absorbed dose) or in the international

unit of Grays (1 Gray = 1 Gy = 1 Joule of energy per kilogram

of material = 100 rad). The mGy (milliGray = 1/1000 Gray) is

the unit usually used to measure how much radiation the body

absorbs. However, because different types of radiation deposit

energy in unique ways, an equivalent biological dose is used to

estimate the effects of different types of radiation. Equivalent

dose is measured in milliSieverts (mSv). The mSv, therefore,

takes into account not only how much radiation a person

receives, but how much damage that particular type of radiation

can do – the greater the possibility of damage for the same dose

of radiation, the higher the mSv value.

Crews aboard the space station receive an average of 80 mSv

for a six-month stay at solar maximum (the time period with the

maximum number of sunspots and a maximum solar magnetic

field to deflect the particles) and an average of 160 mSv for a

six-month stay at solar minimum (the period with the minimum

number of sunspots and a minimum solar magnetic field).

Although the type of radiation is different, one mSv of space

radiation is approximately equivalent to receiving three chest

x rays. On Earth, we receive an average of two mSv every year

from background radiation alone.

Crew members could receive higher doses of space radiation

during space walks while outside the protective confines of the

space station; however, NASA plans space walks to avoid the

trapped radiation belts around the Earth, and doses on previous

space walks have been kept very small.

Protecting Current and Future Space Station Crews

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 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 (see figure above).

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.

Radiation Measurements Aboard

the International Space Station

Below, in alphabetical order, are the many radiation measurement

devices and experiments that have flown to the

International Space Station.

Bonner Ball Neutron Detector

March – December 2001

A Japanese Space Agency experiment that measured the amount

of neutron radiation that entered the space station. Neutron radiation

can affect the blood-forming marrow in bones.

Charged Particle Directional Spectrometers – CPDS

2001 – present

There are three units mounted outside on the station’s S0 truss

that are designed to record the direction from which radiation

strikes. There is another unit inside the station.

Dosimetric Mapping – DOSMAP

March – August 2001

A German Space Agency/European Space Agency experiment

that consisted of four different types of radiation detectors

located throughout the space station, These measured the

amounts and types of radiation that entered the ISS.

Study of Radiation Doses Experienced by Astronauts

in EVA – EVARM

February 2002 – present

These sensors are being used to determine the levels of radiation

space walkers receive in their skin, eyes and blood-forming

organs. EVARM consists of three active dosimeters (placed on

Normal chromosome No. 2

and No. 4 in a postflight

metaphase sample

Damaged chromosome No. 2

in a postflight metaphase

sample

the leg, torso and near the eye) that are read before and after a

space walk. The EVARM data could be used to devise methods

of reducing the amount of radiation astronauts are exposed to

during space walks.

Passive Dosimetry

1999 – present

There are several types of radiation detectors aboard the space

station. The radiation area monitor (RAM) is a small set of thermoluminescent

detectors encased in Lexan plastic that respond

to radiation – the amount of radiation they absorb can be revealed

by applying heat and measuring the amount of visible light

released. RAM units are scattered inside the space station and

are returned to Earth for measurement after periodic space

shuttle visits. The crew passive dosimeter is very similar to

the RAM and is carried by each member of the crew. The

AN/UDR-13 radiac Set (a high-rate dosimeter) is a compact,

handheld or pocket-carried device capable of quickly measuring

doses of gamma or neutron radiation. Data readout and warning

messages are provided by a liquid crystal display on the set.

Phantom Torso

March – August 2001

This unique experiment

measured the effects of

radiation on organs inside

the human body by using

a torso equivalent in

height and weight to an

average adult male. The

torso contained radiation

detectors that measured

how much radiation the

brain, thyroid, stomach,

colon, and heart and lung area received on a daily basis. The

data are still being analyzed to determine how the body reacts

to and shields its internal organs from radiation, information

that will be very important during longer-duration space flights.

Tissue Equivalent Proportional Counter – TEPC

2000 – present

This radiation detector consists of a 2"-diameter by 2"-long

cylindrical cell that is filled with low-pressure propane gas. The

gas is used to simulate the hydrocarbon content of a human cell

that is two microns in diameter. A plastic jacket covering the cell

simulates the properties of adjacent tissue cells. Particles passing

through the gas release electrons, which are collected, helping to

identify the energy of the particles.

Measuring Space Radiation Between the Earth

and Mars

As the Mars Odyssey spacecraft made its way to Mars between

April and October 2001, the Mars radiation environment

experiment (MARIE) measured the amounts and kinds of space

radiation the spacecraft encountered along the way. These data

are essential to understanding how much and what kinds of

radiation future space travelers might encounter on a long trip

to explore the red planet.

Now in orbit around Mars, MARIE continues to measure the

amount of harmful radiation at the planet itself. Unlike Earth,

Mars does not have a global magnetic field to shield it from

solar flares and cosmic rays. Mars’ atmosphere is also less

than one percent as thick as the Earth’s. These two factors

make Mars very vulnerable to space radiation.

Aboard the International Space Station and in our own solar

system, NASA researchers continue to quantify the amounts

of space radiation our explorers face every day and will face

in the future. Understanding space radiation will not only protect

the crew currently aboard the International Space Station, but

those first humans who will continue the exploration of our

solar system.

Related Web Sites

http://srhp.jsc.nasa.gov/

http://spaceresearch.nasa.gov/research_projects/radiation.html

http://www.spaceflight.nasa.gov/station/science/bioastronautics/

http://marie.jsc.nasa.gov/main.html