Scientific American March 1959 (PDF)
Scientificamerican.com/issue/sa/1959/03-01/

Radiation Belts around the Earth

by Dr. James Van Allen

Conclusion on page 47
Click for Complete PDF

Our measurements show that the maximum radiation level as of 1958 is equivalent to between 10 and 100 roentgens per hour, depending on the still-undetermined proportion of protons to electrons.  Since a human being exposed for two days to even 10 roentgens would have only an even chance of survival, the radiation belts obviously present an obstacle to space flight.  Unless some practical way can be found to shield space-travelers against the effects of the radiation, manned space rockets can best take off through the radiation-free zone over

The hazard to space-travelers may not end even when they have passed the terrestrial radiation belts.  According to present knowledge the other planets of our solar system may have magnetic fields comparable to the earth's and thus may possess radiation belts of their own.  The moon, however, probably has no belt, because its magnetic field appears to be feeble.  Lunar probes should give us more definite information on this point before long. 

Dr. Earl S. Harold  on his Science In Action series.
California Academy of Sciences
"The Earth is surrounded by intense radiation."

Dr. R. Stephen White,  the Leader of the
Nuclear Effects Group,  Lawrence Livermore National Laboratory
Inner and Outer Radiation Belts

Rocket launched from balloon from a ship
into Earth's orbit

"Rock-oon" put geiger counters into orbit
ranging from 200 to 1500 miles

Radiation increased, then dropped to zero

Above 800 miles the radiation was so intense...

the Roentgens per hour dropped to zero
as the Geiger counter got over-loaded
When the instruments got below the belt they recovered
 

Pioneer 3 confirmed the results at 65000 miles
and found a 2nd radiation belt
Sr. Staff Physicist Dr. Stanley Freden and Dr. Al Oliver

Low inner belt, about 800 miles, has 1 Roentgen/hour
Inner belt is maximum at 2000 miles out, 10 Roentgens/hour
Outer belt is maximum at 10000 miles out, up to 100 Roentgens/hour
A human should only get 1 Roentgen per month

To be safer, astronauts should go out and in
through the lower radiation regions at the Earth's poles
Inner belt: high energy electrons, low energy protons

Charged particles go through the film plates

Lines created by charged particles are studied on a numbered grid

Cosmic ray particles are absorbed by the atmosphere, protecting us
What shielding is needed to protect astronauts?
 

1 inch of lead to stop 90% of protons
6 inches of lead to stop 99% of protons

A typical dental ionizing X-ray for a split second
exposes patients to .15 - 0.5 Roentgens per film.
www.paradisedental.us/patient-education/dental-x-rays

Challenges of Spaceflight: Dealing with Space Radiation

Orion, America’s next-generation spacecraft, will take astronauts to destinations beyond low-Earth orbit and thus outside the protective shield of Earth’s atmosphere and magnetic field. Without the protection Earth provides, the crew and systems of Orion will be exposed to the full spectrum of space radiation. To prepare for the effects of that exposure, scientists and engineers are working now to develop methods to protect them.

THE DIFFERENCES BETWEEN RADIATION ON EARTH AND IN SPACE

Radiation on Earth

On Earth’s surface, we are protected from most of the radiation of the universe and the Sun by our atmosphere and magnetosphere.

Non-ionizing Radiation

Non-ionizing radiation refers to a less energetic – but still harmful – radiation in which particles impart energy on to the atoms and molecules with which it interacts, but does not strip off electrons. One form of non-ionizing radiation is ultraviolet, or UV, radiation, which you may be familiar with – we use sunscreen on Earth to shield ourselves from harmful UV rays.

Radiation in Space

Ionizing Radiation

There are several forms of ionizing radiation in space. Ionizing radiation deposits energy onto the atoms and molecules with which it interacts, causing electrons to be lost. The resulting ions, or charged particles, give this form of radiation its name.

One form of ionizing radiation is galactic cosmic radiation. This type of radiation is thought to come from supernovae, or exploding stars. While this radiation is low intensity, the particles associated with galactic cosmic radiation have a high level of energy and cannot be shielded with current spacecraft design technologies.

The second form is trapped radiation, which occurs when radiation becomes trapped in Earth’s magnetic field. This type of radiation is not a problem outside of Earth’s magnetic field. Finally, solar energetic particles are released by the Sun in what are called solar particle events. This type of radiation is lower in energy, and is easier to protect astronauts and electronics from with shielding materials.

EFFECTS OF SPACE RADIATION ON HUMANS

Space radiation can cause radiation sickness and other health problems. These effects are classified as either acute, which have immediate impacts on humans, or chronic, which affect humans over longer periods of time.

Acute Effects

Acute effects can be felt almost immediately when a large dose of radiation is accumulated in a short amount of time. These effects can include acute radiation syndrome, which causes nausea, vomiting and fatigue.

Chronic Effects

Chronic effects are the results of an accumulated dose of radiation over longer periods of time. Chronic effects include an increased risk of cancer, for example. The onset of these diseases can happen decades after the exposure to radiation occurs.

COUNTERMEASURES

NASA protects astronauts from radiation in several ways during space flight. One that requires no special equipment is to limit the time humans spend exposed to radiation. For example, space walks are scheduled so that they do not occur during times of intense solar activity. Also, NASA limits the amount of radiation astronauts are exposed to during their careers. Every astronaut is required to wear a dosimeter during missions to keep track of the radiation to which they have been exposed.

ORION’S SPACE RADIATION PROTECTION

Orion is designed for both short missions to nearby destinations and long-distance missions to Mars that will last more than three years. As the average long-duration space flights for Americans have lasted about six months and have taken place entirely within low-Earth orbit, NASA must be prepared to deal with significantly more radiation than astronauts have experienced in the past.

Orion will use the mass that is already on board to protect its crew by creating a temporary shelter in the aft bay of the spacecraft, which is the inside portion closest to the heat shield. This location minimizes the amount of equipment to move around while maximizing the amount of material that can be placed between the crew and the outside environment. The mass that will be used includes supplies, equipment and launch and re-entry seats, as well as water and food. By using the items already on board, the astronauts benefit from additional shielding without adding to Orion’s mass.

Radiation Sensors Flying on Orion’s First Mission

The Orion spacecraft will have two types of radiation sensors on board during its first mission, Exploration Test Flight-1. The first type of sensor is called a Radiation Area Monitor, or RAM. There will be several of these sensors on board to record the peak level of radiation Orion is exposed to during the flight. The second type of radiation sensor being flown is the Battery-operated Independent Radiation Detector (BIRD). The BIRD sensor will be recording radiation levels throughout the mission for comparison to the mission telemetry after the completion of the flight. This will reveal where the radiation was encountered.

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