There is considerable evidence that the Van Allen shields, which start at an altitude of between 250 miles to 750 miles, protect us from deadly solar and cosmic radiation. (Italics and bold in the below quotes are mine.)
Herbert Friedman, in his book Sun and Earth, describes Van Allen's global survey of cosmic-ray intensity: "The results from Explorer I, launched on January 31, 1958, were so puzzling that instrument malfunction was suspected. High levels of radiation intensity appeared interspersed with dead gaps ... Explorer III succeeded fully, and most important, it carried a tape recorder. Simulation tests with intense X rays in the laboratory showed that the dead gaps represented periods when the Geiger counter in space had been choked by radiation of intensities a thousand times greater than the instrument was designed to detect. As Van Allen's colleague Ernie Ray exclaimed in disbelief: 'All space must be radioactive!'." Herbert Friedman later explains that "Of all the energy brought to the magnetosphere by the solar wind, only about 0.1 percent manages to cross the magnetic barrier."
The April 28, 1997 HST Update: Recommisioning Status Report states that the Van Allen radiation belts, between 200 and 500 miles high, "act as a thin, protective skin for Earth, trapping charged particles before they bombard our planet and harm us."
Physics Textbook by the Space Physics Group of Oulu lists what high
energy particle radiation in the radiation belts does and is:
"-it degrades satellite components, particularly semiconductor and optical devices
-it induces background noise in detectors
-it induces errors in digital circuits
-it induces electrostatic charge-up in insulators
-it is also a threat to the astronauts".
If it is a threat then why were not animal experiments beyond the Van Allen belts done first?
"Today, we have far more knowledge of the space environment: from the turbulent surface of the Sun, with its continuous solar wind and periodic spewing of clouds of energetic ionized particles, to the protective boundary of the Earth's magnetic field, which provides a partial shield against deadly solar corpuscular radiation. The Earth's magnetic field is highly reactive to the onslaught of energy and pressure originating from the solar particles and fields."
"...Besides being a threat to satellite systems, energetic particles present a hazard to astronauts on space missions. On Earth we are protected from these particles by the atmosphere, which absorbs all but the most energetic cosmic ray particles. During space missions, astronauts performing extra-vehicular activities are relatively unprotected. The fluxes of energetic particles can increase hundreds of times, following an intense solar flare or during a large geomagnetic storm, to dangerous levels. Timely warnings are essential to give astronauts sufficient time to return to their spacecraft prior to the arrival of such energetic particles."
The average orbit altitude of the space shuttle is 185 miles, below where the Van Allen shields begin. How were the Apollo astronauts protected against these deadly energetic particles and solar flares?
"... the sun ... occasionally ejects high energy particles that can be deadly to electronic components and biological systems..."
"The area between the Sun and the planets has been termed the interplanetary medium. Although sometimes considered a perfect vacuum, this is actually a turbulent area dominated by the solar wind, which flows at velocities of approximately 250-1000 km/s (about 600,000 to 2,000,000 miles per hour). Other characteristics of the solar wind (density, composition, and magnetic field strength, among others) vary with changing conditions on the Sun. The effect of the solar wind can be seen in the tails of comets (which always point away from the Sun)."
"Intense solar flares release very-high-energy particles that can be as injurious to humans as the low-energy radiation from nuclear blasts. Earth's atmosphere and magnetosphere allow adequate protection for us on the ground, but astronauts in space are subject to potentially lethal dosages of radiation. The penetration of high-energy particles into living cells, measured as radiation dose, leads to chromosome damage and, potentially, cancer. Large doses can be fatal immediately. Solar protons with energies greater than 30 MeV are particularly hazardous. In October 1989, the Sun produced enough energetic particles that an astronaut on the Moon, wearing only a space suit and caught out in the brunt of the storm, would probably have died."
Why would NASA subject people to this kind of risk?
Once you are past the Van Allen shields, for example between the earth and the moon, or on the moon, you would be subject to the full brunt of solar flares. The Van Allen shields protect us here on Earth from this deadly radiation.
One drawing showing the shield provided by the the earth's magnetic field is available at http://home.texoma.net/~linesden/cem/geomag/geomag.htm. In this SOHO illustration at http://sohowww.estec.esa.nl/bestofsoho/PAGE2/magfieldG_b.gif the white lines represent the solar wind, the blue lines represent the earth's protective magnetosphere, and the purple line represents the bow shock line.
However, even in low-earth orbit, below the Van Allen shields, an August 1972 event could have been life-threatening had there been a space walk in low-earth orbit at the time.
According to Interplanetary crew exposure estimates for the August 1972 and October 1989 solar particle events , "...estimates of human exposure in interplanetary space, behind various thicknesses of aluminum shielding, are made for the large solar proton events of August 1972 and October 1989. A comparison of risk assessment in terms of total absorbed dose for each event is made for the skin, ocular lens, and bone marrow. Overall, the doses associated with the August 1972 event were higher than those with the October 1989 event and appear to be more limiting when compared with current guidelines for dose limits for missions in low Earth orbit and more hazardous with regard to potential acute effects on these organs. Both events could be life-threatening if adequate shielding is not provided."
Apollo 16 was in April 1972 and Apollo 17 was in December 1972. Why would NASA proceed with Apollo 17 just after the August 1972 event and risk astronauts' lives?
The hulls of the Apollo spacecraft were ultra-thin. They would have been unable to stop any significant amount of radiation. The same can be said for the spacesuits.
A calculation quantifies the radiation risk associated with solar flares beyond the Van Allen shields.
"Solar Flares are produced by 'storms' in the solar magnetosphere. These eruptions yield very high radiation doses within very short time periods (hours to days). There is a correlation with the 11 year solar cycle. The largest events occurring in the months following sunspot maximum. Solar flares are cataclysmic releases of energy resulting from processes that are poorly understood."
"During a solar maximum, about 15 flares per day emit detectable X-ray energies."
"...(1964 for solar minimum and 1970 for solar maximum)."
So the Apollo missions, from 1969 to 1972, were occurring during a solar maximum, when there would have been peak numbers of solar flares per day!
Edward P. Ney estimates the radiation risks in an article titled The Sun Under Surveillance in the 1967 World Book Science Year: "We have rough estimates of what the moon travelers can expect, based on a few observations made during the last solar maximum in 1957. The most violent flares probably will produce exposures of 100 roentgens each hour and may hold this level for several hours". The terms roentgen and rem (Roentgen Equivalent Man) are interchangeable.
This level of radiation dose is confirmed by Space Biomedical Research
Institute in Humans in
Very hazardous and intermittent but may persist for 1 to 2 days.
High energy protons travel at the speed of light so there is no time to get under cover.
Protected dose 10-100 REM/hr
Unprotected dose Fatal"
2020 Technical Report puts the space weather radiation hazard to human
life in perspective:
"...at geostationary orbit, with only 0.1 gm/cm2 of aluminum shielding thickness, the predicted radiation dose (REM) for one year continuous exposure, with minimum-moderate solar activity, is estimated to be about 3,000,000..."
At geosynchronous orbit, doses are "still low compared to interplanetary space due to geomagnetic shielding", according to Radiation Hardening In Space.
A radiation dose value from a low energy flare is provided from NASA Mooned America, p. 134: "On page 256 of 'Astronautical Engineering' there is a chart that shows the dosage of four different flares. On August 22, 1958 there was a low energy flare that could have been reduced to 25-rem with 2-cm of water shielding."
So, being conservative and using 25 rems per flare, we have 25 rems x 15 flares/day = 375 rems / day for the Apollo astronauts.
For occupational exposure dose limits, the International Atomic Energy Agency states that the "occupational exposure of any worker shall be so controlled" that the limit of an "effective dose of 50 mSv" "in any single year" "be not exceeded". 50 mSv converts to 5 rems.
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