#104 April 1997
Section 18.104.22.168.104.of the Artemis Data Book
by Richard Richardson
[Richard Richardson is founder and president of the Deschuttes Space Frontier Society [formerly the Central Oregon Space Society] in Bend, Oregon, home of the Oregon Moonbase. He has built a dome house in Bend, but also spends much time in Deering, Alaska, near Nome - a true frontiersman. Through the years, Richard has contributed several articles to MMM.]
Two things must be considered before any design aspects of radiation protection for a crewed vessel in interplanetary space can be intelligently discussed:
Until the early 60's little was known of the radiation environment in interplanetary space. Since that time, however, a rather detailed analysis has been conducted. The Apollo spacecraft and unmanned craft such as the Pioneers and the Voyagers as well as many others have spent extensive mission time in interplanetary space. Many of these craft were equipped with the appropriate sensors to allow scientists to determine the radiation environment of interplanetary space with considerable accuracy. Over the last eighty years or so, and especially since the creation of the atomic bomb and atomic fueled electrical power generators a great deal has been learned about the physiological effects of the various types of radiation.
The kinds of radiation that will be of concern to interplanetary travelers are:
Radiation damages cells in two ways: physical disruption and chemical disruption.
Physical disruption is the bullet effect whereby kinetic energy is transferred to the atoms of the cells at an impact site causing the shattering of the cell or cells involved.
Chemical disruption is the result of the atoms at and near the impact site becoming ionized and forming chemical bonds which impede or destroy the functioning of the chemical machinery of the cell or cells involved. This damage can spread over a several cell width due to secondary kinetic and/or ionization transfer as well as due to bremsstrahlung effect in the tissue itself.
It has been found that the background radiation level in interplanetary space (cosmic radiation) is at a level of about 10 rad per year. Although this is about 20 times the U.S. allowable rate for workers in certain fields with some risk of ionizing radiation exposure and about ten times the rate that an airline flight attendant might receive, it is, nonetheless, a level which probably would pose little danger over extended periods of time.
However, the small percentage of cosmic rays that are heavy primaries do pose a serious risk even over a period of time as short as months, or perhaps weeks. In addition, solar flares can and do fill interplanetary space with hundreds or thousands of times the radiation of the background level on a very unpredictable basis. As a result, for a mission of more than a few hours to be reasonably safe there is no escaping the need for some sort of radiation shielding.
It is important to bear in mind that placing physical material in a radiation environment like that of interplanetary space will result in the creation of bremsstrahlung effect radiation. This generates much more harmful radiation. However, the bremsstrahlung effect radiation is caused by the spreading out of the original particle's energy over many particles.
Each of these secondary particles will also spread their energy out over many other particles (if there are other particles for them to collide with in the direction they are going). And on, and on, and on. This chain reaction will come to a halt when the energies of the individual particles fall below the threshold necessary to disturb additional particles.
Therefore, a sufficiently thick shield will protect against most or all secondary radiation effects. Combined primary and secondary radiation effects are reduced to a level of about 0.5 REM per year by a radiation shield composed of regolith type material which is about two meters thick or a water shield of about five meters thickness. Less shielding and greater radiation exposure might be acceptable to a certain degree. But, in general, the less radiation exposure the better.
It is also conceivable that a force field of some sort could be used rather than, or in addition to a physical radiation barrier. However, the technology for such a force field is not presently developed to a point where this would be a viable option.
Because we are discussing shielding for a transportation device it is necessary to consider the implications of the radiation shield as an inertial mass. The greater the total mass of the spacecraft, including its radiation protection system, the greater will be the energy (and hence the fuel) required to operate it as it speeds up, slows down, and changes direction to move from one place to another. Therefore, from this perspective, the less shielding mass the better.
One possible way to try to minimize vehicle mass and still maintain the necessary degree of shielding might be to develop integrated spacecraft designs which rely on other vehicle components doing double duty as radiation shielding. Some such components are engines (though if they are nuclear engines the crew might need to be shielded from engine radiation as well), aero-brake shields, fuels, or cargo.
O'Neill, Gerard K., _The High Frontier_,
Space Studies Institute Press, 1989, pp. 103-108
Savage, Marshall T., _The Millennial Project_, Empyrean Publishing Ltd., 1992, pp. 138-145 and pp. 410-414
Contents of this issue of Moon Miners' Manifesto