RADIATION PLAN FOR THE APOLLO LUNAR MISSION Jerry L. Modisette, Manuel D. Lopez, and Joseph W. Snyder Space Physics Division NASA Manned Spacecraft Center Abstract
The radiation protection plan for the Apollo Pro- gram is based on real-time monitoring of solar ac- tivity and radiation in the spacecraft to provide data on which to base estimates of the radiation to be expected. The major radiation hazard is from so- lar flare particle events, which are unlikely to occur during any given mission. The monitoring sys- tem, consisting of onboard dosimeters and the Solar Particle Alert Network, provides early warning through observation of solar flares and the associ- ated radio bursts and a continual updating of the radiation picture as particles arrive at the space- craft. Prediction criteria have been developed which are progressively revised as more data are received, with a corresponding reduction in the error limits on the prediction of radiation dose. The criteria are initially based on the energy in the radio burst, with flare classification, location on the sun, delay time between the flare and parti- cle arrival at the spacecraft, and particle flux measurements factored in as data become available.
Space radiation was brought to public attention as one of the unique problems of manned space flight when the Van Allen belts were discovered in 1958. At approximately the same time, researchers began to recognize that various ionospheric and solar dis- turbances which had been observed for many years were aspects of a greater phenomenon, the solar flare particle event. Although early conservative estimates indicated that radiation would be a major problem, observations from the ground and from spacecraft have demonstrated that the space radia- tion hazard is one of the lesser engineering prob- lems to be overcome in spacecraft design and mission planning. Flux maps of the Van Allen belts have be- come available, solar flare particle events have been subjected to intensive statistical analyses, and techniques have been developed to calculate ra- diation doses behind complex spacecraft structures. Van Allen belt radiation doses can be kept small by use of low-altitude orbits or by rapid movement through the belts. Only the very large (and conse- quently very rare) solar flare particle events con- stitute a hazard for moderately shielded spacecraft. Also, secondary radiation is not significant for such spacecraft. The radiation plan for the Apollo lunar mission calls for low-altitude earth orbits and rapid tran- sit to the moon to keep the Van Allen belt radiation dose below 1 rad. Most of the radiation protection activity is directed towards providing protection against major solar flare particle events which might occur while astronauts are in the lunar module or on the lunar surface. The events, which start at the sun, are detected by ground-based instrumenta- tion and are measured at the spacecraft by dosim- eters and particle spectrometers. A prognosis of the radiation dose is prepared and continually up- dated by radiation environment specialists using a console in the Mission Control Center. Dose esti- mates are then provided for the use of the medical officer, who advises the Flight Director of the ra- diation effects to be expected.
Real-Time Data Systems
Onboard Radiation Monitors The onboard radiation monitors measure both dose and particle flux and spectra. Each astronaut car- ries a personal dosimeter which measures the accu- mulated skin dose by integrating the current from a thinly shielded 10-cubic-centimeter ion chamber. The read-out is made by the astronaut from a digital register on the dosimeter (Fig. 1). Two additional ion chambers in the Apollo command module provide readings which are telemetered to the ground and fed into the data system at the Mission Control Center where the data are available for video display. One ion chamber measures skin dose; the other is shielded so that it measures the dose that would be received at a body depth of 5 centimeters. The depth dose is significant only for the relatively hard spectrum of Van Allen belt particles; therefore, this dosim- eter is called the Van Allen belt dosimeter (VABD) (Fig. 2). A portable dose rate meter (Fig. 3) is to be carrier into the lunar module and onto the lunar surface. Comparison of the dose behind the two different shield thicknesses of the VABD gives an indication of the particle spectrum. More detailed spectral information and discrimination between protons and alpha particles are provided by a solid-state spec- trometer mounted on the Apollo service module. Data from the particle spectrometer (Fig. 4) are also telemetered to the Mission Control Center where the data are used for the calculation of doses in the command module, lunar module, and space suits. The dose calculations are made automatically and are read out on the video data display (Table I). The relative biological effectiveness (RBE) of the pro- tons and alpha particles as functions of energy is introduced into the dose calculations so that the doses are given in rem. Solar Particle Alert Network The Solar Particle Alert Network (SPAN) (Fig. 5) monitors solar flares and associated radio emissions on a 24-hour basis. The solar flares are observed with optical telescopes equipped with filters that transmit a 1/2-angstrom band about the Ha line. Time of occurrence, area, and location of the flare are determined by SPAN observers and are teletyped to the Mission Control Center where the data are in- corporated into the estimate of the particle event size. Radio emissions associated with the flares are observed at 1420, 2695, and 4995 megahertz. The radio burst profile for each frequency is also tele- typed to the Mission Control Center. In approxi- mately 2 years of operation, SPAN has observed several hundred flares and radio bursts. Data from SPAN are augmented by data from the solar and iono- spheric monitoring systems operated by the Environ- mental Science Services Administration and the Air Weather Service.
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