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.

Introduction

   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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