Radiation Protection Consequences of the Emerging Space Tourism Industry

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Radiation Protection Consequences of the Emerging Space Tourism Industry

Corresponding author: Dr. Joseph Bevelacqua, Bevelacqua Resources, 343 Adair Drive, Richland, WA 99352 USA, Tel: 509 6282240; Email: bevelresou@aol.com

Abstract

The emerging space tourism industry presents the possibility for a new type of public radiation exposure situation. However, there is no regulations or guidance to define allowable radiation exposures to members of the public. This paper provides an initial set of radiation protection guidelines and recommended practices for public space exposures associated with the low- earth orbit environment.

Keywords: Space tourism; Radiation guidelines; Low-earth orbit activities; Cosmic radiation; Solar particle events

Introduction

In this paper, space tourism refers to suborbital and orbital passenger flights provided by corporate organizations to members of the public. The initial phase of space tourism will involve primarily low-earth suborbital trajectories. As the technology matures, longer duration orbital flights and flights to orbital facilities may become possible.

The cost for a suborbital flight was initially projected at about

$200,000 with over 200 private persons having made an advance payment to fly on Virgin Atlantic’s Space Ship Two[1]. With a maturing technology, the number of tourists should increase with a corresponding decrease in price.

Space tourism flights introduce a new public radiation hazard that was previously restricted to a limited group of astronauts. The associated radiation exposure incurred by aircrews and space tourists present a potential public health concern. Solar flares and other sources of radiation affecting the low-earth orbit environment and their associated health issues are addressed in this paper. Since no space tourist radiation guidance currently exists, possible regulatory limits are also presented.

Because this paper may be used as a portion of the regulatory basis for radiation protection standards and guidelines for space tourism, it is written in a manner that will be understandable by regulatory and policy professionals.

The radiological data utilized in this paper focus on the haz- ards that will be encountered in the low-earth orbit environ- mental [28].These types of data were used to establish the as- sociated biological detriments for astronauts [8]. As such, this paper provides an overview of the various components of the low-earth orbit radiation environment and the associated bio- logical detriments. Moreover, it utilizes established astronaut guidelines [8] as a basis for subsequent space tourism recom- mendations.

Materials and Methods

The radiation characteristics associated with low-earth orbital activities by various space agencies is reasonably well char- acterized [2-28]. These assessments were performed in sup- port of government-sponsored space programs that involved well-trained personnel with considerable experience in flight operations and emergency procedures. Their training includ- ed knowledge of the sources and hazards associated with the low-earth orbit radiation environment. Space tourists will not have the training and experience of astronauts associated with these government programs. Given their limited experience and capabilities, protection from the space radiation environ- ment is essential. This protection will be based on recom- mended radiation limits for the space tourism activity.

Radiation Sources

Protons and alpha particles with smaller admixtures of light nuclei are the dominant space radiation types [2-28]. There are four dominant sources of space radiation incident on the earth and its atmosphere [19]. These sources are (1) energetic ions associated with solar particle events (e.g., solar flares), (2) cosmic rays coming from interstellar space at the edge of the heliopause, (3) galactic cosmic radiation (GCR) originating outside the Solar system, but within the Milky Way galaxy, and (4) extragalactic cosmic radiation.

The incident particulate radiation has sufficient energy to induce nuclear reactions and produce high-energy muons, electrons, photons, and neutrons [17,18,20,28]. Neutrons are attenuated by the atmosphere, contribute relatively low effective doses at sea level, and yield significant doses at higher altitudes. Space tourists and supporting aircrew mem- bers are dominantly exposed at these higher altitudes, and their possible doses are addressed in the subsequent discussion.

Solar-Induced Disruptions and Radiation Effects

Solar induced disruptions most frequently gather public at- tention when they interrupt communications and electrical power systems [28]. Interruptions of television and radio sig- nals are annoying, but relatively minor events. However, more significant disruptions have occurred and affected millions of people. For example, solar storms are known to disrupt cellu- lar phone service, global positioning systems, electrical power grids, and television and radio signals [28].

In March 1989, a Solar storm much less severe than the 1859 Carrington Event [15], disrupted the US, Canadian, and Sweedish power grids for several hours [28]. A 1994 Solar event caused communications satellite malfunctions that disrupted television and radio service throughout Canada [28]. The loss of communications capability during a low-earth orbital flight event could have a significant impact on flight emergency and support actions.

The Northeast Blackout of 2003 was another massive widespread power outage caused by a Solar event [28]. This event affected parts of the Northeastern and the Midwestern United States and Ontario, Canada. The blackout affected an estimated 10 million people in Ontario and 45 million people in eight U.S. states.

Given the number, magnitude, and influence of these Solar events, their characteristics and severity are addressed in the subsequent discussion.

In 2005, x-rays from another solar storm disrupted satellite-to-ground communications and Global Positioning System (GPS) navigation signals for about 10 minutes [28]. Since spacecraft utilize GPS and satellite information for maneuvering and docking, disruptions have serious consequences for the space tourism industry.

Solar Event Characteristics

Solar flare radiation or Solar particle events are ejections of matter from the Sun. Their composition reflects the mass constituent characteristics of Solar plasmas. Therefore, they are composed predominantly of protons with admixtures of alpha particles and heavier nuclei [2-28]. The intensi- ty and composition of Solar flare radiation vary with the specific event. Carbon, nitrogen, and oxygen dominate the Z > 2 particles and constitute about 1% of the Solar flare fluence rate [17,19,20,28].

Typical flare events last from one to four days although somewhat longer durations have been observed. On an annual basis, 8 – 11 significant Solar flares occur [17,28]. Solar physics models are not sufficiently advanced to predict the timing, duration, and intensity of a flare event. This uncertainty and the magnitude of these Solar particle events present a significant radiation hazard to space tourists residing in the low-earth orbit environment.

Electronic technologies and their associated components that comprise critical space vehicle systems are vulnerable to solar particle events (SPEs). The effects of a massive solar flare are similar to the electromagnetic pulse effects [28] following a nuclear detonation. For example, cell phone communica- tions, global positioning systems, and radar transmissions are vulnerable to large SPEs. In addition, commercial satellites are at risk from large-scale events such as the Carrington flare [15].

Humans working in space would also be at risk. Space crews and space tourists would have limited time from the initial indication of a major SPE to find shelter from energetic solar particles and photons. Although it takes hours to days for an SPE to reach the earth, the time from detection to required action is often much shorter [17,28]. Accordingly, spacecraft should have adequate shielding to attenuate the various SPE radiation types [13,15,22].

In space, SPE doses can be quite large [14]. An August 1972 SPE was one of the largest dose events of the space era and it occurred between two Apollo missions [14,17,28]. However, ice core data from Antarctica indicate that the largest SPE in the past 500 years was probably the Carrington Flare of 1859 [15]. A comparison of the Carrington Flare to other large SPEs is summarized in Table 1. These data are further evaluated and their implications explored in subsequent discussion.

Date >30 MeV Proton Fluence (109 protons/cm2)
August – September 1859 18.8
1895 11.1
November 1960 9.7
1896 8.0
1894 7.7
1864 7.0
July 2000 6.3
1878 5.0
August 1972 ~5
a Cliver and Svalgaard (Ref. 14)

Low-Earth Orbit Radiation Environment

Manned low-earth orbit (LEO) activities are influenced by the various components of the space radiation environment. The relative importance of each of the components depends on the specific LEO parameters including the spacecraft trajectory (e.g., altitude, orientation, and orbital characteristics), travel timing relative to periodic Solar activity, travel duration, and spacecraft shielding characteristics [11]. Space tourism activi- ties are also affected by the LEO radiation environment.

Table 1. Large solar energetic proton events during 1859–2000a.

Particle Type Source Energy (MeV) L

(keV/μm)

Ability to Penetrate Comments
EVA

Suit

Spacecraft
Electrons Trapped particlesb 0.5 to 6 ~ 0.2 Yes Yes Electrons dominate the effective dose for aluminum shields with a density thickness < 0.15 g/cm2
Electrons Decay products from interactions with trapped GCR ionsb

Atmospheric scattering

1 to >

1,000

0.2 to > 3 Yes Yes The effective dose contribution is about 10 times greater than the trapped electron effective dose
Protons Trapped particlesb < 10 > 5 No No At low energies, protons pose a limited effective dose concern
Protons Trapped particlesb

SPEs

10 to 400 0.3 to 5 Yes Yes As proton energies increase, the effective dose increases
Light ions SPE 10 to 400 0.3 to 5 Yes Yes The effective dose depends on the nature of the SPE and the specific ions and their energies
Ions (Z>1) and charged secondary fragments GCR > 50

MeV/ nucleon

1 to 1,000 Yes Yes Pion production occurs, but the pion contribution to the effective dose is not well characterized
Charged target fragments Nuclear interactions from all sources < 10

MeV/ nucleon

2 to 1,200 Yes Yes Large effective doses are possible
Neutrons Nuclear interactions 0.1 to 500 c Yes Yes Large effective doses are possible

an NCRP 142 (Ref. 11).

b Particles are trapped in the earth’s electromagnetic field.

c Neutrons interact with atomic nuclei to produce highly ionizing charged particles.

Table 2. Characterization of the LEO Radiation Environment.

LEO environments are normally dominated by energetic charged particles including electrons, protons, and heavy ions.

The environment is also significantly influenced by large emissions of Solar flares and the temporal and spatial fluctuations of the particles trapped by the earth’s magnetic field [11].

Nuclear interactions of neutrons, protons, and heavy ions with the spacecraft, earth’s atmosphere, and the human body produce secondary particles that contribute to a space tourist’s effective dose. In contrast, most of the electrons do not penetrate the wall of a spacecraft but could penetrate suits worn during an extravehicular activity (EVA) [11].

Table 2 summarizes the LEO radiation environment by particle type, the source of the particle, particle ener- gy, and ability to penetrate an EVA suit and the spacecraft.

Date Fluence (protons/cm2)
E > 10MeV E > 30 MeV
February 23, 1956 2 x109 1×109
July 10-11, 1959 5 x109 1×109
July 14-15, 1959 8 x109 1 x109
July 16-17, 1959 3 x109 9 x108
November 12-13, 1960 8 x109 2 x109
November 15, 1960 3 x 109 7 x 108
July 18, 1961 1 x109 3 x108
November 18, 1968 1 x109 2 x108
April 11 – 13 1969 2 x109 2 x108
January 24-25, 1971 2 x109 4 x108
August 4 – 9, 1972 2 x1010 8 x109
February 13 – 14, 1978 2 x109 1 x108
April 30, 1978 2 x109 3 x108
September 23 – 24, 1978 3 x109 4 x108
May 16, 1981 1 x109 1 x108
October 9 – 12, 1981 2 x109 4 x108
February 1 – 2, 1982 1 x109 2 x108
April 25 –26, 1984 1 x109 4 x108
August 12, 1989b 8 x109 2 x108
September 29,1989 b 4 x109 1 x109
October 19, 1989 b 2 x1010 4 x109
November 26, 1989 b 2 x 109 1 x108

a Wilson et al. [7].

b The listed 1989 SPEs had an extended duration.

Table 3. Proton Fluence Levels of Significant Solar Events of Cycles 19–22 Likely to Exceed the NCRP 132 Recommendations a.

The unrestricted linear energy transfer (L∞) in water [11] is also provided. Additional details regarding the LEO radiation environment are provided in Appendix A.

The proton fluence for energies greater than 30 MeV [7] is typ- ically in the range of 106 to 1010 protons/cm2. Table 3 provides a summary of SPEs from Solar Cycles 19–22 that are likely to exceed the NCRP 132 dose recommendations for LEO activi-

ties [8]. These fluence values illustrate the variation that can be encountered during a Solar cycle. The variations have a significant impact on the doses delivered to space tourists in low-earth orbit. Specific NCRP 132 dose recommendations are provided in subsequent discussion.

A comparison of Tables 1 and 3 illustrate uncertainties in the evaluation of SPE fluence data. For example, the August 1972 proton (E > 30 MeV) data vary by a factor of about 2 (~5 to 8×109 protons/cm2) [7,11].

Low-earth Orbit Dose Limits

In the 21st Century, public access to low-earth orbit will sig- nificantly increase [1]. With expanded access, space tourist LEO radiation protection limits should be established. The radiation environment description and dose limit recommen- dations such as those published in NCRP 132 [8] are consid- erations for establishing dose limits and regulatory standards for space tourist activities in low-earth orbit. In order to deter- mine the direction of these standards, current astronaut limits are reviewed.

The NCRP 132 LEO recommendations are established for short-term exposure, limiting health effects, and career doses [8]. Included in the NCRP 132 recommendations are career whole-body exposure limits for lifetime excess risk of total cancer of 3% (Table 4), 10-year career limits based on 3% ex- cess lifetime risk of cancer mortality (Table 5), and dose limits for all ages and both genders (Table 6).

Age (y) Female (Sv) Male (Sv)
25 1.0 1.5
35 1.75 2.5
45 2.5 3.25
55 3.0 4.0

a NCRP 132 [8].

Table 4. Career Whole-Body Exposure Limits for a Lifetime Excess

Risk of Total Cancer of 3% as a Function of Age at Exposurea.

Age at exposure (y) Effective Dose (Sv)
Female Male
25 0.4 0.7
35 0.6 1.0
45 0.9 1.5
55 1.7 3.0

a NCRP 132 [8].

Table 5.Ten Year Career Limits Based on Three Percent Ex- cess Lifetime Risk of Cancer Mortalitya.

Time Frame Blood-Forming Organs (Gy-Eq) Eye (Gy-Eq) Skin (Gy-Eq)
Career b 4.0 6.0
1 y 0.50 2.0 3.0
30 d 0.25 1.0 1.5

a NCRP 132 [8]

b The career stochastic limits in Table 5 are adequate for protection

against deterministic effects.

Table 6. Recommended Dose Limits for all Ages and Both Gendersa.

Tables 4-6 form the basis for developing radiation protection standards for space tourism activities. These limits are based on the wealth of accumulated data including the information summarized in Tables 1-3. Subsequent discussion develops radiation protection recommendations for space tourists and establishes an initial basis for developing specific regulatory guidance and recommended practices.

Results

The NCRP 132 risk estimates are subject to large uncertainties [8]. Part of this uncertainty is inherent in the nature of SPEs. These uncertainties include limits of scientific knowledge, risk model limitations, and lack of data to adequately character- ize the risk. In addition, these uncertainties lead to shielding requirements that place significant limitations on space vehicle design and flight duration. Given these uncertain- ties, risk estimates suggest that for each week in space outside the earth’s magnetosphere there isa 1 in 500 chance that unshielded space tourists will receive a lethal dose from Solar flare radiation [17].

Near Term Considerations

Near term space tourist activities will be dominated by suborbital flights of relatively short duration. These initial fights will incur a less significant radiation hazard that an orbital flight.

The recommendations of Tables 4 – 6 greatly exceed current international recommendations [29] and US public dose limits for nuclear facility operations, which are both 1 mSv/y from normal licensed activities [30,31]. This limit is unrealistic given the radiation levels associated with the LEO environment. LEO space tourism limits must consider the anticipated radiation environment, the voluntary nature of public space tourist participation, and possible health effects. Given these conditions, regulatory limits for space tourists will likely exceed the occupational limits for radiation workers as embodied in US Federal Regulations (10CFR20 and 10 CFR835) [30,31].

Public space tourist participation must involve informed consent. This consent is based on radiation protection training including a review of the LEO radiation environment, the SPE

hazard, and possible biological effects from this environment. Following this training, a hazards acceptance statement should be signed to eliminate future legal action related to the space tourist’s radiation exposure.

Space tourism regulatory requirements and guidance should

incorporate a number of considerations including:

  1. All space tourists must wear dosimetry to measure the vari- ous radiation types in the LEO environment.
  2. Space tourists must receive training to ensure the radiological hazards are understood. This training should also include measures to reduce radiation exposures during an emergency event.
  3. Given their susceptibility to the biological effects of ionizing radiation, no minors are permitted to utilize an LEO space tourist service.
  4. Pregnant individuals are excluded from LEO space tourism

to protect the developing embryo/fetus.

  1. Given the voluntary nature of public space tourist participation, passenger radiation dose limits should be based on the NCRP 132 limits specified in Tables 4, 5 and 6. The NCRP 132 limits should be periodically evaluated based on space tourism flight experience.
  2. The recommendations summarized in Table 5 suggest that no individual younger than age 25 participate in the voluntary space tourism activity.

In addition to these radiological items, space tourists must be physically and mentally capable of meeting the challenges of the LEO environment. Physical and psychological examinations should be performed. These examinations will certify that the space tourist can withstand a variety of factors including the physiological stress associated with the gravitational forces encountered with the launch, low-gravity environment of low-earth orbit, and psychological impacts of the close quarters of the spacecraft.

In flight, procedures should address the expected operating, abnormal, and emergency radiation conditions that could be encountered during the range of space tourism activities. To characterize emergency radiation hazards of the LEO envi- ronment, a worst case SPE is selected. The 1859 Carrington Flare is defined as this bounding occurrence. This selection represents a 500-year frequency flare event [15,17].

Before, the Fukushima Daiichi accident, the author would have selected a 50 – 100-year frequency flare event (e.g., about 10 times September 29, 1989, flare [7]). However, the Fukushi- ma Daiichi power reactor accident suggests that improbable, but historically viable events should be selected as a credible design basis assumption to characterize the most severe emer-

gency condition that could credibly occur.

Absorbed doses from Carrington-type SPEs as a function of aluminum shield thickness are summarized in Table 7. For the Carrington Flare, bone marrow doses of 1-3 Gy are possi- ble inside a spacecraft. A shielded room with about 18 cm of aluminum is needed to reduce the Carrington Flare absorbed doses to the applicable NCRP 132 recommended deterministic doses (30 d blood forming organ (BFO) dose limit of 0.25 Gy- Eq) [17].

Shielding (g/cm2 Al) Skin (Gy) Eye (Gy) BFO (Gy)
1 35.4 23.4 2.81
2 6.65 6.02 1.71
5 2.82 2.73 1.09

a Derived from Townsend [15].

Table 7. Carrington Flare Absorbed Dose Estimatesa.

It is unlikely that a space tourist vehicle can accommodate the weight required to provide 18 cm of aluminum or equiv- alent shielding required to mitigate a Carrington-type flare. Therefore, other measures must be provided to minimize the radiation dose received by space tourists.

A practical solution to minimize passenger doses is to utilize satellite radiation warnings or onboard radiation instrumentation to indicate elevated radiation levels. These indications would signal the pilot to abort the LEO flight trajectory and reenter the atmosphere for immediate land- ing. The combination of reduced altitude and timely landing significantly reduces passenger and crew radiation expo- sures. The earth’s atmosphere provides significant shielding to attenuate the effects of the SPE [17,28].

Long-Term Considerations

An established space tourism industry will involve longer orbital flights and possibly permanent facilities including hotel and resort facilities. These flights and facilities present an additional risk because they involve longer durations of potential exposure to elevated radiation levels.

The dose that could be received during a massive Carrington-type Solar event is of considerable concern. Early detection of a Solar event is a critical factor in mitigating its radiation effects. If detection occurs before the launch, the hazard is eliminated by delaying departure until the event ends.

If the SPE occurs after launch, orbital satellites and onboard radiation monitors would indicate a radiation hazard. Flight emergency procedures should provide dose reduction methods that include (1) aborting the launch trajectory and returning to earth, or (2) altering the planned trajec- tory to change the orbit or return the spacecraft to earth’s atmosphere. It would also be desirable to provide a shield-

ed enclosure to minimize crew and passenger doses. The shielding thickness needs to consider the bounding Solar event, launch weight limitations, and the number of passen- gers and crew.

If tourist facilities reside in earth orbit, the radiation hazard becomes more significant because the exposure time is longer than the flight abort scenario. These tourist facilities should be equipped with a shelter to mitigate the radiation effects of a Solar particle event. The facility shelter can be more substan- tial than the spacecraft shelter.

In the event of a Carrington-type event, additional shield- ing (e.g., food stores, repair materials, station infrastructure, metal structures, and water storage tanks) should be locat- ed around the emergency shelter to minimize the SPE doses. These placements should be part of the orbital facility design requirements. Facility dose assessments should determine the tourist dose for the basic shielded shelter and the additional items surrounding the shelter. These dose values will be a key safety parameter for an extended duration Solar event.

Other orbital facility emergency actions that could be evaluat- ed for a massive Solar event include:

  1. Relocating all space tourists to the shielded emergency

structure

  1. Assessing if a change in the orbit of the facility is feasible
  2. Administering radio protective agents [18,20,28] to mitigate

the biological effects of the event

  1. Determining if an evacuation of the tourist facility is feasible
  2. Performing a dose assessment to determine the best option to minimize tourist doses
  3. Implementing medical measures to minimize deterministic effects that may occur following exposure of the facility inhab- itants
  4. Obtaining continuous radiation levels and particle fluence

values in the facility

Based on the specific SPE characteristics, ground personnel should assist in determining additional measures to min- imize tourist facility doses. Close coordination and clear communications between ground and space facility personnel and space tourists should be maintained. The risks from the event should be presented in a clear, logical manner. Dialogue should be encouraged, and all tourist concerns should be ad- dressed.

Conclusion

Space tourism offers a considerable challenge to ensure the health and safety of public participants. Suborbital flights present a less severe radiological hazard than orbital activities. Initial tourist dose limits should be based on current astronaut guidance provided in NCRP 132. Subsequent limits should consider experience gained as the space tourism industry matures. A number of emergency actions are available to mitigate the radiological consequences of a large Solar particle event. These actions include orbital changes, the use of shielded shelters, medical interventions, and returning the tourists to earth.

Appendix

Overview

This appendix provides a review of the low-earth orbit radiation environment and corresponding health risks associated with space tourism. It provides supplemental information that more completely defines the LEO radiation environment and its major components [8,10,11,16,32-35]. These effects and associated data were developed to support LEO missions that were successfully completed by astronauts. Although the general radiation characteristics of the LEO environment are relatively well established, there is considerable variability, and these variations present a chal- lenge for the emerging space tourism industry.

Basic Physics Overview

Space radiation is grouped into three broad components or source terms. These components involve the source of the radiation, namely (1) the earth’s van Allen belts (VABs) in which charged particles are trapped by the planet’s magnetic field, (2) galactic cosmic rays (GCRs), and (3) Solar particle events (SPEs) [32-35]. The radiation characteristics of planetary VABs, GCRs, and SPEs are addressed in subsequent sections of this appendix.

Radiation Protection Limitations

The space radiation environment and dose limit recommen- dations such as those published in NCRP 132 [8] are consider- ations for limiting the duration of space missions. The NCRP 132 recommendations only apply to activities in low-earth orbit, and NCRP 153 [16] outlines information needed to make radiation protection recommendations for space missions beyond low earth orbit.

The NCRP 132 LEO recommendations are established for short-term exposure, limiting health effects, and career doses. Included in the NCRP 132 recommendations are career whole-body exposure limits for lifetime excess risk of total cancer of 3% (Table 4), 10-year career limits based on 3% excess lifetime risk of cancer mortality (Table 5), and dose limits for all ages and both genders (Table 6).

The NCRP 132 risk estimates are subject to large uncertainies. Part of this uncertainty is inherent in the nature of SPEs. These uncertainties include limits of scientific knowledge, risk model limitations, and lack of data to adequately character- ize the risk. In addition, these uncertainties lead to shielding requirements that place significant limitations on space vehicle design and mission duration. Given these uncertainties, risk estimatessuggest that foreachweekinspaceoutside the earth’s magnetosphere there is a 1 in 500 chance that unshielded astronauts will receive a lethal dose from Solar flare radiation. Missions on the order of 2 years would correspond to approximately a 20% chance of exceeding a lethal dose. Considering these doses and their associated probability of oc- currence, the potential for adverse health effects due to radia- tion exposure introduces a hazard that must be addressed for the development of a successful space tourism industry.

Overview of the Space Radiation Environment

The characteristics of the three dominant space radiation source terms are summarized in Table A.1. A more detailed description of the LEO environment is provided in Table 2 and subsequent discussion.

Characteristic VAB

Trapped Radiation

GCR SPE
Proton energy

range (MeV)

Up to several

hundred

Up to several

thousand

Up to several

hundred

Energetic, highly

charged nuclei (MeV/nucleon)

No significant contribution Up to several thousand No significant contribution
LET range

(keV/μm)

0.25 – 10 0.25 – 1000 0.25 – 10

a Derived from [33].

Table A.1. Characteristics of Space Radiation a.

An examination of Tables A.1 and 2 suggests that the space radiation environment is complex, and the individual source terms are dominant at different spatial locations. The van Allen belts are important for low earth orbit. At higher latitudes, GCRs are also important. SPEs and GCRs are important for missions outside the earth’s magnetosphere. These source terms also vary with Solar and extra-Solar conditions.

The sun is constantly releasing protons and electrons as a result of fusion reactions. These particles are trapped in the earth’s magnetic field where they circulate between the north and south magnetic poles [33]. This physical process produces zones of radiation or belts of trapped radiation referred to as the VABs.

Protons, with smaller admixtures of helium ions and heavy ions, are the dominant components of GCRs and SPEs. There are a number of differences in the particle types, energy

distribution, and emission frequency of GCRs and SPEs [33].

The first difference between GCRs and SPEs is the energy distribution of the emitted particles. GCRs are extremely high-energy events that originate outside the Solar system. SPEs are lower energy events governed by Solar dynamics. SPEs may be produced by coronal mass ejections or Solar flare events.

The second difference lies in the relative periodicity of SPEs and GCRs. SPEs are sporadic and governed by stellar dynamics and Solar plasma instabilities. It is difficult to forecast the onset, duration, and magnitude of a Solar mass ejection event. This uncertainty is likely to remain until advances in Solar physics and observational capabilities improve. In addition, GCRs are usually more slowly varying events because the initial violence of the big bang is damped by the long time period since that event.

A final difference involves the presence of high Z particles. Energetic, highly charged nuclei (HZE particles) are principally found as part of GCRs. The range of GCR nuclei extends from protons to isotopes of iron and heavier nuclei.

General Characterization

Although the GCR background is reasonably well character- ized by existing models, the amount of shielding that can be incorporated into a space vehicle is only of limited effective- ness against HZE particles. The biological effectiveness of HZE particles is not well understood, but they are one of the most significant radiological hazards in space [8,16,33].

Physics issues also exist for trapped radiation. The trapped radiation environment is dynamic. The short-term variation in the VABs is not well understood and is difficult to accurately forecast. However, average values of trapped radiation are rea- sonably well understood [33].

On the positive side, shielding is effective in attenuating low-en- ergy SPE protons and trapped radiation. Judicious scheduling and the selection of low dose trajectories effectively manage the hazards of LEO activities.

Trapped or van Allen Belt Radiation

Charged particles are influenced by the electromagnetic fields of planets, moons, and other bodies. These particles originating either within or outside a Solar system can be trapped by these electromagnetic fields. This trapped radiation forms radiation belts that have a significant influence on objects that orbit these planetary bodies. Rapid transit through trapped radiation belts is an effective means to limit doses. The subsequent commentary is derived primarily from Ref. 33.

For the earth, there are two VABs (i.e., an inner and an outer belt). In terms of the earth’s radius (Re = 6.4×103 km), the in-

ner belt extends to about 2.8 Re. The outer belt occupies the region between 2.8 Re and 12 Re.

The inner belt is composed primarily of protons having energies up to several hundred MeV. Protons with energies of 400 MeV peak in intensity at about 1.3 Re, and the 4 MeV protons peak is at about 2 Re . The time-averaged spatial distribution of protons, with energies greater than 100 MeV, exhibit a sharp rise at about 1.16 Re and reach a maximum at about 1.5 Re with an integral fluence of 104 protons/cm2. Beyond 1.5 Re, the proton fluence decreases slowly, then rises to a maximum at about 2.2 Re, then drops to about 100 protons/cm2 at about 2.8 Re.

Electrons with energies greater than several MeV dominate the outer belt. Electrons are also found in the inner belt, but their intensity is only about 10% of the outer belt intensi- ty and their energy is lower. The electron fluence, for ener- gies greater than 40 keV, peaks at about 3.5 Re with a value of 109 electrons/cm2. In the outer belt, large variations in the electron fluence (2 – 4 orders of magnitude) occur over periods of hours to days.

The VABs have a distorted toroidal shape and lie in the plane of the geomagnetic equator. The energy and spatial distribution of the particles in the belts, particularly the lighter electrons, vary with time.

The fluence for both protons and electrons are each a strongly varying function of altitude and location above the Earth. In the vicinity of 35o South latitude and 325o East longitude, the fluence of trapped particles is largest. This region is known as the South Atlantic Anomaly (SAA). At 370 km altitude, the proton fluence in the SAA for energies between 40 and 100 MeV is as much as 1000 times larger than other proton fluences in the belt at the same altitude. Similar spatial behavior is observed for electrons.

Galactic Cosmic Ray Radiation

Missions in LEO are not exposed to the full intensities of the GCR and SPEs. The Earth and its atmosphere provide shielding to attenuate GCR and SPE radiation. Earth’s geomagnetic field deflects lower energy protons and heavier ions into deep space. Therefore, particle fluence rates from GCR and SPE sources are much lower in LEO than will be encountered in missions beyond earth.

Galactic cosmic rays constitute a major radiation source outside the magnetosphere and consist of protons (88%), alpha particles (10%), electrons and gamma rays (1%), and heavy ions or HZE particles (1%) [34]. In view of its abundance and high linear energy transfer, iron is one of the most important HZE particles. Table A.2 summarizes the distribution of radiation types by intensity and abundance of galactic cosmic radiation. The maximum GCR total particle fluence rate is about 4 particles/cm2-s.

Radiation Type Fluence Rate (particles/cm2-s) Abundance (%)
protons 3.6 88
alpha particles 0.4 9.8
electrons and gamma rays (E>4 GeV) 0.04 1
C, N, O, and F nuclei 0.03 0.75
Li and B nuclei 0.008 0.2
10 ≤ Z ≤30 nuclei 0.006 0.15
Z ≥ 31 nuclei 0.0005 0.01

a Derived from Ref. 34.

Table A.2. Distribution of Galactic Cosmic Radiation a .

The GCR energy spectra decrease rapidly with increasing energy with energies extending to 1020 eV [34]. Since GCR sources lie well outside the Solar system, their spatial distribution is essentially isotropic. Gamma ray bursts and supernovas can lead to perturbations in the GCR source term.

The GCR proton energy spectrum exhibits a broad maximum at about 1 GeV, and the spectrum of alpha particles and HZE particles peaks at about 300 MeV/nucleon. At Solar maxima, the GCR intensity is a minimum and slowly increases until its maximum value is reached during Solar minimum conditions.

Solar Flare Radiation or Solar Particle Events

Solar flare radiation or Solar particle events are ejections of matter from the Sun. Their composition reflects the mass constituents’ of Solar plasmas. Therefore, they are composed predominantly of protons with admixtures of alpha particles and heavier nuclei. The intensity and composition of Solar flare radiation vary with the specific event. Carbon, nitrogen, and oxygen dominate the Z > 2 particles and constitute about 1% of the Solar flare fluence rate.

Typical flare events last from one to four days although somewhat longer durations have been observed. On an annual basis, 8 – 11 significant Solar flares occur. Solar physics models are not yet able to predict the timing, duration, and intensity of a Solar flare event. This uncertainty and the magnitude of these SPEs present a significant radiation hazard to tourists in the LEO environment.

When the sun is very active, such as the periods near sunspot maxima that occur about every 11 years, SPEs can deliver doses of 0.3 to 3.0 Gy over a period of about 3 days. These absorbed doses are significant and merit attention.

Given the general characteristics of the trapped radiation (VAB), GCR, and SPE radiation sources, the radiological consequence of space tourism can be addressed. This discussion is based on historical radiation data supporting astronauts’ missions in LEO [8,10,11,16,32-35].

Historical Space Missions

Manned space missions have occurred in low-earth orbit in a variety of space vehicles. This section addresses the radiation environment in low-earth orbit.

Low-Earth Orbit Radiation Environment

LEO missions are influenced by all three dominant components of the space radiation environment. The relative importance of each of the components depends on the specific LEO parameters including the spacecraft trajectory (e.g., altitude, orientation, and orbital characteristics), mission timing relative to periodic Solar activity, mission duration, and spacecraft shielding characteristics [11].

LEO environments are normally dominated by energetic charged particles including electrons, protons, and heavy ions including alpha particles and ions with Z ≤ 92. The environment is also significantly influenced by large emissions of Solar particles and the temporal and spatial fluctuations of the particles trapped by the earth’s magnetic field.

The LEO radiation environment is influenced by spatial and temporal factors including the 11-year Solar cycle, the Solar wind, and the earth’s magnetic field that traps some particles and deflects others. The earth’s magnetic field varies in strength and configuration over timescales from days to years. Solar events also alter the distribution of trapped particles in the earth’s VABs.

Nuclear interactions of neutrons, protons, and heavy ions with the spacecraft, space suit, earth’s atmosphere, and the human body produce secondary particles that contribute to the space tourist’s effective dose. In contrast, most of the electrons do not penetrate the wall of a spacecraft but could penetrate suits worn during extravehicular activity (EVA) resulting in eye and skin equivalent doses [11]. Table 2 summarizes the LEO radiation environment by particle type, the source of the particle, particle energy, and possible impact during EVA or inside the spacecraft.

the Space Radiation Environment outside Earth’s Magnetic Field

Prior to outlining dosimetric data from historical missions, the projected dose equivalent rates outside the earth at 1 AU and in LEO are reviewed. Table A.3 provides dose equivalent val- ues that could be experienced at 1 AU outside the earth’s mag- netic field. The values in Table A.3 quantify the GCR and SPE radiation fields and provide a credible bound on potential low earth orbit doses that could be encountered by a space tourist. Table A.4 summarizes dose equivalent values appropriate for entry into the earth’s VABs. Doses of the type presented in Ta- ble A.3 and A.4 exhibit variation in the literature.

Source Radiation Type Effective Dose or Effective Dose Rate
Unshielded Space Suit Spacecraft
GCR positive ions 0.02 mSv/h 0.02 mSv/h 0.02 mSv/h
Solar Wind positive ions 10-4 mSv/h 0 0
Medium

Solar Flare

positive ions 1,000 mSv 500 mSv 3 mSv
Maximum

Solar Flare

positive ions 106 mSv 5×105 mSv 3,500 mSv

a Derived from [35].

Table A.3 Sol System Radiation Fields Exterior to the Earth at 1 AUa .

Source Radiation Type Effective Dose Rate (mSv/h)
Unshielded Space Suit Spacecraft
van Allen Belt positive ions/protons 600 300 3
van Allen

Belt

electrons 106 100 10

a Derived from [35].

Table A.4. Low-Earth Orbit van Allen Belt Radiation Fieldsa.

A.6 Biological Effects from SPE Doses

Given the current level of knowledge of Solar Physics, it is not possible to forecast key SPE parameters with any degree of accuracy. These key parameters include predicting the timing, magnitude, duration, and fluence rate of the SPE. The unpredictability of SPEs adds to their inherent radiation hazard.

The intensity, energy spectra, and angular distributions of SPE protons and alpha particles vary considerably with individual Solar flares and are a function of time within any given event. A typical flare has a duration of about 1– 4 days although longer duration flares have been observed. Normally, a flare’s intensity increases rapidly over the first few hours and then decreases.

The biological effects of ionizing radiation from the various sources encountered during a space mission are not completely understood. These effects are broadly categorized as either deterministic or stochastic. Deterministic effects include threshold effects such as acute somatic effects resulting from solar particle events. Stochastic effects include cancer and hereditary effects. The major risks of space flight are cancer induced by HZE particles, immune system detriment, and neurological and behavioral effects that would jeopardize an extended duration mission. The cancer risk resulting from these effects is compounded because synergistic effects may exist with other hazards encountered in space. As an example, the possible synergistic effect of low gravity and high dose rates resulting from an SPE is not fully understood [16,20].

Historically, mitigating the early effects of a crew’s radiation exposure were addressed by shielding the spacecraft, and the focus has been the estimation of the risk of late radiation effects such as cancers. However, the true risk includes both early and late radiation effects [16,20].

Assessments of mission radiation risk are difficult to perform because SPEs can be large and unpredictable. In addition, SPE events offer the potential for rapid and progressive exposure to the primary charged particles representing a wide array of atomic numbers, atomic masses, energies and dose-rates and any resulting secondary radiation.

The absorbed doses from an SPE summarized in Table A.3 are sufficient to cause deterministic effects. These effects (and associated threshold) include nausea, vomiting, and diarrhea (1 Gy); death without medical intervention LD50,30 (4 Gy); gastrointestinal syndrome (10 – 20 Gy); and central nervous system syndrome (20 – 100 Gy) [20]. The actual doses depend on the time the SPE is active. Long duration flares have the potential to deliver the aforementioned deterministic effects.

The health effects from GCR radiation are less severe than the SPE detriments. Considering the nominal effective dose rate from GCR radiation of 0.02 mSv/h provided in Table A.3 and a 1-week space tour, the total dose delivered from GCR radiation is

E  0.02mSv / h1week 7days / week 24h/ day  3.36mSv (A.1)

The dose calculation provided by Eq. A.1 presumes the space tourism trajectory minimizes entry into the intense VABs.

This dose is comparable to the annual background radiation received by US citizens [19,20]. Given the variation in doses across the US, no health effects are likely for this effective dose [20].

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