Nucl. Tracks Radiat. Meas., Vol. 20, No. 3, pp. 397401, 1992 ht. J. Radiat. Appl. Insfrum., Part D Printed in Great Britain

Pergamon Press Ltd

COSMIC RADIATION: CONSTRAINTS ON SPACE EXPLORATION

JAMES H. ADAMS JR

E.O. Hulburt Center for Space Research, Code 4154, Naval Research Laboratory, Washington, DC 20375. U.S.A.

(Received 11 November 1991)

Abstract-The hazard from cosmic radiation is examined using present cosmic ray models and the current definition of the quality factor. Current crew exposure limits are also assumed. Cosmic radiation is found to place strong constraints on manned missions to Mars. A workshop to discuss new measurements and models of galactic cosmic ray fluxes was held in August 1991. The motivation for the workshop will be explained by describing how galactic cosmic radiation constrains plans for manned space exploration and why an accurate knowledge of the absolute cosmic ray fluxes is needed to plan a manned mission to Mars.

1. INTRODUCTION

SEVERAL nations are contemplating manned missions to the Moon and to Mars. In the U.S. a presidential commission chaired by Thomas P. Stafford, a former astronaut, recently presented a report on plans for missions to the Moon and Mars entitled “America at the Threshold: America’s Space Exploration Initiative” (Stafford et al., 1991). In it the authors point out “the potential health hazard from galactic cosmic rays and solar flare events” and suggest some strategies to keep the radiation dose equivalent within the prescribed limits.

To obtain more accurate information on the intensity of galactic cosmic radiation a workshop entitled “Cosmic Radiation: Constraints on Space Exploration” was held at the 22nd International Cosmic Ray Conference on Friday 16 August 1991, in Dublin. The purpose of the workshop was to discuss measurements and models of absolute galac- tic cosmic ray fluxes. This special section contains papers presented at that workshop and papers on closely related topics.

2. CONSTRAINTS ON MISSIONS TO MARS

To understand the constraints due to cosmic radi- ation we must first understand how this hazard is measured and examine the limits placed on it. The physical quantity that can be calculated from cosmic ray measurements is the radiation dose. Radiation dose is the energy per unit mass that is deposited by ionizing radiation. Dose is measured in Grays, where 1 Gray equals 1 J kg-‘. Not all radiation doses produce the same biological results. To account for the relative effectiveness of different kinds of radi- ation, a quality factor, Q, has been introduced. Q is a function of the linear energy transfer of the radiation,

i.e. how intensely ionizing it is. The current definition of Q has been provided by the International Commis- sion on Radiation Units and Measurements (ICRU 60, 1991). The dose equivalent is defined as the dose multiplied by Q and it is measured in Sieverts (Sv). Table 1 gives the current limits imposed on NASA by the U.S. National Council for Radiation Protection for operations in low earth orbit (NCRP 98, 1989). Other spacefaring nations have adopted their own limits. No limits have yet been set for manned missions to Mars, so in the following we will assume that the limits in Table 1 will also be used for Mars missions. The limit in Table 1 that most constrains NASA’s plans is the 0.5 Sv limit on the annual dose at 5 cm depth in tissue.

Figure 1 shows four ways in which manned mis- sions to Mars can be constrained to avoid exceeding the exposure limits. Crew shielding provides some protection from cosmic rays. Unfortunately much of the cosmic ray flux is at high energies and the massive amount of shielding required to stop it is impractical. The cosmic ray intensity does vary with the 11 yr solar activity cycle so missions to Mars could be restricted to the phase of cycle when the galactic cosmic ray intensity is low. Unfortunately, the phase of lowest cosmic ray intensity coincides with the period when solar flares are most likely. One would have to rely on a massive onboard storm shelter to avoid additional radiation exposures from solar flares. Also such launch window restrictions would have an adverse programmatic impact. Limiting the travel time in interplanetary space will reduce the exposure, though there is some exposure on the Martian surface. The fourth option is to remain on Mars for a long period so that the interplanetary space exposures occur in two different years. The value of each of these mission constraints is investi- gated below.

398 JAMES H.

Table 1. Ionizing radiation exposure limits (in Sieverts)

Depth Eye Skin (5 cm) (0.3 cm) (0.01 cm)

30 days 0.25 1.0 1.5 Annual 0.50 2.0 3.0 Career 1 s&4.0* 4.0 6.0

*The career depth dose-equivalent is based upon a maxi- mum 3% lifetime risk of cancer mortality. The total dose- equivalent yielding this risk depends on sex and on age at the start of the exposure. The career dose-equivalent limit is nearly equal to: 2.0 + 0.075 (age-30) Sv, for males, up to 4.0 Sv; 2.0 + 0.075 (age-38) Sv, for females, up to 4.0 Sv.

First let us explore the effects of shielding and the

phase of the solar cycle. Figure 2, taken from Letaw et al. (1988), shows the dose equivalent as a function of shielding depth. From this figure we can see that the radiation hazard diminishes with depth in shielding, but only slowly with increasing depth after about 7.5 cm of aluminum. This is due to the build-up of neutrons and other secondaries and the very slow attenuation of protons. Figure 3, also taken from Letaw et al. (1988) shows how the dose equivalent differs at the extremes of the solar cycle. If one chooses to employ 7.5 cm of aluminum shielding (as Letaw et al. recommended) to reduce the radi- ation hazard, then traveling to Mars only during the maximum of the solar cycle would reduce the dose equivalent by a factor of 2.

The results presented in Figs 2 and 3 were intended only to illustrate the effects of shielding and the solar cycle. These results are based on an older and less accurate model of galactic cosmic rays than those presented at this workshop and were calculated using an earlier definition of the quality factor. Figure 4 shows a recent estimate of the dose equivalent taken from the paper of Adams et al. (1991). It compares the dose equivalent from galactic cosmic rays with the 0.5 Sv annual exposure limit. The galactic cosmic ray flux was at a historic high in 1977, so Fig. 4 depicts

ADAMS Jr

LENGTHEN STAY ON

MARS

FIG. 1. The constraints that galactic cosmic radiation place on plans for manned missions to Mars.

a period when the hazard from galactic cosmic rays is near its maximum level. For the 7.5 cm of shielding recommended by Letaw et al., Fig. 4 shows that the dose equivalent rate is 0.5 Sv yrr’. Figure 4 also shows the 90% confidence level on the dose equival- ent rate, based on the uncertainties in the galactic cosmic ray flux. From these figures it appears necess- ary to either limit the free space exposure of the crew to less than 1 yr or to restrict missions to a more favorable part of the solar cycle. Let us explore the first alternative.

We now consider shortening the flight time to Mars. Stafford et al. have explored several “architec- tures” for missions to Mars. They have considered both short missions with 30 day stays on Mars and long missions with stays of more than 1 yr. Figure 5 displays the ranges of possible mission durations with 30 day stays on Mars. Both chemical propulsion and nuclear thermal propulsion systems are considered. The shortest missions employing chemical propulsion

50 I I I I I

SOLAR MINIMUM 1

ALUMINUM SHIELDING THICKNESS (cm)

FIG. 2. Dose equivalent at 5 cm tissue depth vs aluminum shielding thickness at solar minimum showing radiation components individually. Here the dose equivalent units are REM (100 REM = 1 Sievert). This

figure is taken from Letaw et al. (1988).

COSMIC RADIATION AND SPACE EXPLORATION 399

p 40 2 SOLAR MINIMUM

5 30 c- 3 a > 20

;

I I I I I I IO 20 30

ALUMINUM SHIELDING THICKNESS <cm)

FIG. 3. Dose equivalent at 5 cm tissue depth vs aluminum shielding thickness at solar minimum and solar maximum. Here the dose equivalent units are REM (100 REM = 1 Sievert). This figure is taken from

Letaw et al. (1988).

are more than 1 yr in duration. For such missions, the equivalent of 347.6 days in space. If nuclear there will be a year in which 335 days will be spent thermal propulsion is employed then it becomes in transit to or from Mars and 30 days will be spent possible to shorten the mission to 280 days with 30 on the surface of the planet. Once on Mars, the of them spent on Mars. The equivalent exposure is atmosphere provides 18 g cme2 of CO and CO2 262.6 days, but there will also be some radiation as shielding from above and Mars itself provides exposure from the nuclear thermal rocket. Ward massive shielding from below. The Mars lander will (pers. commun.) has estimated this to be 0.1 Sv for be much lighter than the mother ship (which will be the mission. If the mission were conducted under the left in orbit). If we suppose that the lander provides conditions that existed in 1977, then the dose equiv- 3 g cme2 of aluminum shielding and that the mother alent rate, at a 90% confidence level, would be less ship provides the 7.5 cm (20.25 g crn2) of aluminum than 0.575 Sv yr-’ or 1.58 mSv day-i. The exposure shielding, as recommended by Letaw et al., then a during the shortest mission employing chemical crew member who spends 1 day on the surface of propulsion would be < 347.6 days x 1.58 mSv day-’ Mars will have the same exposure as if he had spent or co.548 Sv. If nuclear thermal propulsion is used 0.42 days in transit. The shortest mission employing then the shortest mission will result in a dose equiv- chemical propulsion would then expose the crew to alent of < 0.414 Sv from cosmic rays and 0.1 Sv from

1977 solar minimum (galactic cosmic rays only)

Annual limit

------____.

Present work

I I 10 20

Aluminum shielding (cm)

1 30

FIG. 4. The dose equivalent at a depth of 5 cm in tissue vs aluminum shielding depth during 1977. Results from Adams et al. (1991) are compared with those of Letaw et al. (1988). Adams et al. predict higher dose equivalents because their cosmic ray flux model more accurately represents the galactic fluxes in 1977 and because they used the new definition of Q (ICRU 60, 1991). The dashed curve is an upper bound

on the dose equivalent at the 90% confidence level reported by Adams et al..

400 JAMES H. ADAMS Jr

Mars short duration stay missions

All chemical propulsion (specific impulse = 475 s)

Nuclear thermal propuls% (specific impulse = 925 s)

2 2000- b

St? w" .P & 1500 -

.E g)3 orst opportunities

2 8.: IOOO- CSZ EUG

07J ‘B a, .v, 5 500 - Ez -iS Best opportunities

I- O I I I I I I 250 300 350 400 450 500 550

Total mission duration (days, with 30 days on the surface)

FIG. 5. Reproduced from America at the Threshold (Fig. 6. p. 23), by the Synthesis Group published by the U.S. Government Printing Office, Copyright 1991, used by permission.

Table 2. Upper limits on radiation exposures for Mars missions (in Sieverts)

Chemical Nuclear thermal propulsion propulsion

30 day stay missions* 0.548 0.514 Long stay missions* 0.342 0.355

*In each case the mission parameters were chosen that minimize the radiation exposures.

the rocket for a total exposure of co.514 Sv. The resulting radiation exposures are shown in Table 2. They indicate that when the galactic cosmic ray intensity is at its highest no mission plan will reduce the annual exposure below 0.5 Sv at the 90% confi- dence level. It may, therefore, be necessary to restrict

short stay missions to a more favorable part of the solar cycle.

Mars missions near periods of maximum solar activity will be exposed to a reduced cosmic ray flux as noted above. To benefit from this reduction, the exposures to radiation from solar flares must be avoided. In principle, this can be done by equipping the spacecraft with a massive storm shelter that will protect the crew from solar energetic particle radiation. The present flare warning system operated by NOAA is adequate to warn of radiation from a flare so that the crew could take shelter. Flight during solar maximum periods seems a viable plan.

Now let us consider the effect of extending the stay on Mars. Figure 6, also taken from Stafford et al., shows the range of one-way transit times for long stay

Transit times for Mars long duration missions

All chemical propulsion (specific impulse = 475 s)

Nuclear thermal propuls% (specific impulse = 925 s) 2 2000 x r

P 8 01 I I I I

50 100 150 XXI 250

One-way transit times to and from Mars (days)

FIG. 6. Reproduced from America at the Threshold (Fig. 5, p. 22), by the Synthesis Group published by the U.S. Government Printing Office, Copyright 1991, used by permission.

COSMIC RADIATION AND SPACE EXPLORATION 401

missions to Mars. Again, both chemical and nuclear thermal propulsion systems were considered. For these long missions, we will suppose that the journeys to and from Mars do not occur in the same year. In this case, the missions with the lowest exposures will be those with the shortest transit times. The most favorable mission employing chemical propulsion will require 110 days to reach Mars with the remain- der of the year spent on the Martian surface. In this case the equivalent space exposure will be 217.1 days for a radiation exposure of co.342 Sv at the 90% confidence level in 1977. If a nuclear thermal system is used then Mars could be reached in 70 days. In this case the equivalent space exposure is 193.9 days. At the 90% confidence level for 1977, the cosmic ray exposure would be < 0.305 Sv and the exposure from the nuclear propulsion system would be 0.05 Sv (because it was used only to reach Mars) for a total exposure of < 0.355 Sv. These results are summarized in Table 2 and show that long stay missions to Mars are possible at any phase of the solar cycle provided the exposure from solar energetic particle events can be limited to co.15 Sv.

The radiation hazard from galactic cosmic rays was among the considerations that led Stafford et al. to recommend the development of nuclear thermal rocket technology for missions to Mars. Table 2 compares the radiation exposures using chemical and nuclear thermal propulsion for both short and long stay missions to Mars. The differences are smaller than the other uncertainties in the estimation of the radiation exposures. We conclude that the use of nuclear thermal propulsion offers no clear advantage in reducing the radiation exposure of the crew on manned missions to Mars.

3. CONCLUSIONS

Figure 4 shows that, at the 90% confidence level, there are periods when the galactic cosmic rays alone may deliver dose equivalent at a rate which would exceed the annual exposure limit for reasonable amounts of shielding. This figure makes clear the need for accurate knowledge of the absolute galactic cosmic ray flux since the 90% confidence level is determined primarily by the uncertainty in the galac- tic cosmic ray flux. It also makes clear the necessity of exploring mission constraints that will reduce the annual exposure.

Missions with 30 day stays on Mars were con- sidered. With the currently available cosmic ray data and models and the current exposure limits for low earth orbit, it was not possible to show that such missions could be conducted safely during periods of maximum cosmic ray intensity if reasonable amounts

of shielding were used. This conclusion was reached without considering the additional radiation ex-

posure that may result from solar energetic particle events. It may be necessary to restrict the plans for such missions to favorable phases of the solar cycle.

It was argued that restricting missions to periods near the maximum of solar activity is a viable option, though the attendant launch window constraints are undesirable.

Missions with long stays on Mars were also con- sidered. It was shown that these missions could be accomplished safely, under the assumptions stated above, using either chemical or nuclear thermal propulsion systems. It was shown that this could be done even when the galactic cosmic ray intensity was assumed to be at its historic high.

Radiation exposures on missions employing both chemical and nuclear thermal propulsion systems were considered in this paper. It was found that nuclear thermal propulsion systems offer no clear advantage in reducing the radiation exposure of the crew on manned missions to Mars.

The purpose of this workshop was to encourage the publication of measurements and models of the absolute cosmic ray flux so that these could be used to determine the radiation hazard that must be managed on a manned mission to Mars. At the workshop new results were reported on the absolute cosmic ray fluxes and two models of the galactic cosmic ray fluxes were presented. The new models reported in these proceedings represent as much as a five-fold improvement over earlier models. The measurements reported here are a valuable addition to the cosmic ray data base.

REFERENCES

Adams J. H. Jr, Badhwar G. D., Mewaldt R. A., Mitra B., O’Neill P. M., Ormes J. F., Stemwedel P. W. and Streitmatter R. E. (1991) The absolute spectra of galactic cosmic rays at solar minimum and their impli- cations for manned space flight. In Proc. 22nd hf. Cosmic Ray Conf. (Dublin), Paper No. OG 5.2.7 (in press).

ICRU 60 (1991) ICRU Report 60. International Commis- sion on Radiation Units and Measurements.

Letaw J. R., Silberberg R. and Tsao C. H. (1988) Galactic cosmic radiation doses to astronauts outside the mag- netosphere. In Terrestrial Space Radiation and its Biological Effects (Edited by McCormack P. D., Swenberg C. E. and Biicker H.), pp. 663. Plenum, New York.

NCRP 98 (1989) NCRP Report No. 98. National Council on Radiation Protection and Measurements.

Stafford T. P. et al. (1991) Report of the Synthesis Group on America’s Space Exploration Initiative. Super- intendent of Documents, U.S. GPO, Washington, DC 20402, U.S.A.