Making Deep Space and Nuclear Rockets Safe for Astronauts: PART I  

  As far back in human history as one would care to go the exposure to radiation has always existed with Homo Sapiens thus the need to protect people whose occupation requires them to work or travel among natural or man-made excessive sources of ionizing radiation anywhere in the Solar System.

  It was only in the later part of the 19^th century that scientists found the existence of accelerated particles as cathode rays by Geissler and Pluecker in 1859 these were later extracted as an external beam by Lenard in 1893. Roentgen would repeat Lenard's experiments and discovered more diffuse penetrating x-rays produced at the fluorescent impact site of a cathode rays on the tube wall or external target.

  This began a history of subatomic particle use in service to medical diagnostic and therapeutic tools. In those early days x-ray penetrating power came with a price as in misuse and reports of adverse biological effects like dermatitis, sore eyes and hair falling out followed by reports of cancer within an x-ray produced ulcer and other adverse biological consequences. It wasn't till Becquerel in 1896 that he convinced the scientific community of his day that these same x-rays were inherent properties of the uranium atom. Rutherford discovered two radioactivity rays as alpha and beta of different penetrating power as emissions from uranium Becquerel showed that Rutherford's beta ray was the same as cathode rays. The Curies discovered that uranium ores contain other radioactive elements as radium and polonium. If the radioactive decay times of ²³⁵U, ²³⁸U and ²³²Th (and to a lesser extent ⁸⁷Rb and ⁴⁰K) were not on the order of the age of the universe, there would be no significant natural radioactivity, the development of nuclear physics would have been delayed for decades, and nuclear energy based on fission would not have developed ¹.It was their mental and physical sacrifice that led them to win the first of the Noble prizes and the beginnings of an atomic age.

  Along with this novus technology came the understanding that these "Rays" cause ions to be generated in the air and other matter enhancing the conduction of electrical charge and producing chemical change. It was also known that ions present in the atmosphere could discharge condensers (electrometers/electroscopes) over a period of time and the rate of increased discharge was caused by these known radioactivity rays: cathode rays, x -rays.

Brief History of Ambient Radiation Detectives

  Between 1912-1913 V.F. Hess developed electrometer experiments by carrying them aloft in a balloon he found that radiation did indeed arrive from the top of the Earth's atmosphere His studies found ionization rates to decrease with altitude up to 500m followed by a steady increase at higher altitudes to where the ground level rate is matched at 1,500m. For this discovery he received a Noble prize in physics in 1936. The term 'cosmic radiation' came into being in 1926.

Another important contributor to the understanding of natural charge particles was the term 'cosmic ray' was coined by Robert Millikan in 1925.

  The discovery of his law of motion of a particle falling towards the earth after entering the earth's atmosphere, together with his other investigations on electrical phenomena, ultimately led him to his significant studies of cosmic radiation (particularly with ionization chambers). The last piece of important evidence from a human exposure perspective was the discovery of heavy ion tracks by Phyllis Frier and co-workers using nuclear emulsion track detectors in high-altitude balloon flights. Although the initial emphasis of this discovery was the ability to sample cosmic matter, attention would turn to the possibility of human exposure by these ions in high-altitude aircraft and future space travel.³ The trapped radiation were directly observed by the first United States satellite launched in 1958 with a Geiger-Mueller tube.

BASIC CONCEPTS

  Human activity has increased human exposures to natural radiation because of technological development.

The concept of field refers to the spatial distribution of a physical quantity which may vary with time of observation. Ionizing radiation refers to the field associated with a physical agent which has the property of producing ions within a material substance placed within the field.

  The ionizing radiation field itself consists of particles streaming from some source and could be either charged or neutral particles. The ionization within substances is ultimately associated with the transfer of kinetic energy of the field particles to bound electrons of the substance by which molecular changes are initiated. In the case of charged particle fields (electrons, muons, pions, protons, alpha particles, and other ions), the ionization is caused directly through coulombic interaction between the radiation field constituents and bound electrons in the substance. In the case of neutral particle fields (photons and neutrons), the ionization is mainly indirect by the release of an energetic charged particle within the substance. Ionization induced by photons is mainly the result of transfer of photon energy to a bound electron providing sufficient kinetic energy to the electron resulting in additional ionizing events. Ionization induced by neutrons is through a nuclear collision event in which a recoiling nucleus or energetic charged nuclear reaction products are the agents of ionization. The ionization process in living tissues consists of ejecting bound electrons from the cellular molecules leaving behind chemically active molecular species (radicals) which are the source of adverse changes. Cellular processes are able to overcome most of this injury as many of the radicals resulting from radiation injury are similar to those produced in ordinary metabolic processes for which the cell has developed recovery mechanisms needed for long term survival (Billen 1990)⁴.

  Conventional wisdom considers changes in the DNA structure as the substantive target of radiation injury which may also be injured directly by the passing ionizing particle. The ability of the cell to repair the effects of ionization depends in part on the number of such events occurring within the cell from the passage of a single particle and the rate at which such passages occur. The number of ionization events per particle passage is related to the physical processes by which particle kinetic energy is transferred to the cellular bound electrons. Charged particles have a long range interaction and tend to loose energy through small discrete events over short mean free paths on the order of atomic dimensions and appears as a continuous slowing down process similar to motion of an object through a viscous media. The rate of energy loss increases rapidly with increasing charge of the particle and decreasing speed. The rate of energy loss of electrons and singly charged ions at high energies are more like photon exposures in their biological consequences. The distance traveled depends on the inertia and massive particles are more penetrating than lighter particles of the same charge and speed. Uncharged particles have longer free paths and for neutrons larger energy transfers per event resulting in energy losses which appear as isolated occurrences along the particle’s path. The electrons resulting from photon interactions produce few ions in isolated cells and the rate at which such events occur in a given cell is important since repair of a single event is relatively efficient unless many events occur within the repair period (Wilson et al. 1993). Indeed, the single photon event may do little more than add a few free radicals to the melee of metabolic related radicals and have little or no added consequence (Billen 1990). The effects on health risks of dose rate of the exposures is included in protection through a Dose Rate Effectiveness Factor DREF. The massive low energy ions resulting from neutron interactions always produce copious ions in the struck cell and repair is less efficient for such events. Unlike photon doses which are ineffective in causing injury unless many interactions occur within the cell’s repair period (i.e., high dose rates), neutron exposures under most conditions will be singular events in a few cells and will not be so dependent on the dose rate as is true for high LET radiation events of heavy ions (Schaefer 1952, Curtis et al. 1995, Wilson et al. 1995a).⁵

  Allowable exposures depend on the radiation risk coefficient, an assessment of acceptable risk levels, the exposed individual’s knowledge of the exposure, and relationship to the individual’s possible benefit as related to the exposure. For example, exposure of the general public is kept low since they may not even be aware of the exposure and may not directly benefit. Such public exposures are limited to be on the order of natural background levels (ICRP 1991). Those occupationally exposed derive a direct benefit from their work activity and are informed of their exposures. Occupational exposure levels are limited to risk levels comparable to risk of accidental death in moderately safe industries. Astronaut annual exposures may be quite high since they are informed, receive benefit from their occupation, start careers in mid-life, and are limited to a short career duration so that their lifetime risks are comparable to career lifetime risks of death in moderately safe industries.

Pregnancy is a separate issue since the exposed individual is highly sensitive, receives no direct benefit, and is allowed to be exposed by decision of the parent. The current regulatory limits and proposed new limits are shown in table 1.⁶

Terrestrial Background Exposures

  The term terrestrial background exposure is used here in the sense that these exposures are the normal result of everyday living in our present culture. Primordial matter consists of a vast array of elemental components, many of which were unstable to radioactive decay and have long since disappeared in the 4.5 billion years of earthly existence.

Only those primordial elements with unusually long decay times (U-238, Th-232, U- 235, Rb-87, and K-40) remain within the Earth's crust (Halliday 1962). These are the principal sources of exposure on the Earth's surface. There are three main decay chains headed by the long lived isotopes U-238, Th-232, and U-235 which decay by sequential emission of alpha, beta, and gamma rays. The rates of decay within these chains are dominated by the lifetime of the longest lived isotopes at the head of the chain. A fourth decay chain Np-237 exists but was of sufficiently short decay time to have since disappeared from the Earth’s environment.
The emitted particles are in order of increasing range in matter as alphatens of micrometers, beta-few centimeters, and gamma-many meters. Unless internalized, only the gamma rays are of consequence in human exposures. A detailed summary of terrestrial exposures is given in NCRP Report 93 (1987).

  Among the decay products of these radioactive chains is the noble gas radon which can become airborne and inhaled (along with its airborne decay products) resulting in exposures of lung tissues. Rn-222 produced in the U-238 decay sequence is the main source of such exposures although Rn-220 from the Th-232 sequence makes non-negligible contributions. Radon related exposures are mainly through alpha emission and the bronchial epithelium is primarily exposed. Even then the main exposures result from breathing the trapped air within buildings and depends on location and construction techniques. The US average annual radon decay effective dose equivalent to lung tissues is given in table 3 as 2 mSv and consists of high LET alpha particles with an assumed quality factor of 20 (Q=20).

Terrestrial Occupational Exposures

  Typical exposure of any individual within the US population from the several background sources in table 3 are within a factor of two of the average values shown. In addition to these exposures, there are those whose work activity result in an occupational radiation hazard. The number of individuals for whom occupational exposures are part of the workday have increased dramatically with developing technology from 216,000 in 1958 to 1.1 million in 1975 to an estimated 1.3 million in 1980. The estimated average exposure for this class of people had declined from 2.4 mSv to 1.1 mSv over the same period. Typically half of the identified radiation workers had only negligible exposures so that the average of those with significant exposures was 4.8 mSv in 1958 dropping to 2.2 mSv in 1980. The remainder of the radiation workers experienced only (or nearly) background levels as typified by table 3. Note that the average occupational effective dose equivalent is on the order of the background levels in table 3. Even so there are a few radiation workers who are near maximal exposure levels in table 1 but have only small impact on averages due to their small number. Most individual radiation worker exposures in 1980 are within a factor two of the 2.2 mSv.⁷

Past Commercial Air Transportation Studies

When the possibility of high-altitude supersonic commercial aviation was first seriously proposed, Foelsche brought to light a number of concerns for the associated atmospheric radiation exposure due to penetrating cosmic rays (CR) from the galaxy (GCR) and the sun (SCR also referred to as solar particle events, SPE) including the secondary radiations produced in collision with air nuclei (Foelsche 1961, Foelsche and Graul 1962). Subsequently, a detailed study of the atmospheric ionizing radiation components at high altitudes was conducted from 1965 to 1971 at the Langley Research Center (LaRC) by Foelsche et al. (1974 ).⁸

Space Exposure Limits

  The Earth’s protective atmosphere is a massive 1 kg/cm2 or equivalent mass of ten meters of water. There is little wonder that the cosmic ray levels are low on the surface and still modest at aircraft altitudes where only 25 percent of the atmosphere remains to protect subsonic aircraft and only 5 percent remains to shield the HSCT. Even so, 5 percent is equivalent to 50 cm of water. Even if one is above the atmosphere, there is still the geomagnetic field which provides protection from extraterrestrial radiations near the equator (low inclination orbits) but also poses a new hazard from those particles trapped in the geomagnetic field itself (see figure 2). In addition to the greater intensities of the space radiation environment, the astronaut is committed to 24 hours of exposure time for each work day unlike any other occupational exposure. For these reasons, astronaut exposure limits have always been considered outside the realm of other radiation related occupations.⁹

Deep Space Mission Exposures

  Outside the geomagnetosphere, astronauts are subject to the full galactic and solar cosmic ray environments except for what protection is afforded by the vehicle or spacesuit. The GCR are of low intensity and were not of concern in the Apollo mission which were of short duration (less than two weeks). The main concern was the possibility of a solar particle event in which acute effects may occur as was reflected in the design exposure limits in table 6. As future space exploration is towards Mars and there is the possibility of a lunar colony, the long term exposures to galactic cosmic rays are a critical issue (NCRP 1989).
There are no exposure standards for such missions established since the biological cancer risks and other adverse effects of exposures to the HZE ions are largely unknown (NAS 1996). Galactic background exposures: The galactic cosmic rays are more intense outside the solar system and must overcome the outward convection of the solar wind. As the solar wind changes intensity over the changing solar cycle (see sunspot numbers in figure 11) there is a corresponding inverse variation in cosmic ray intensity within the solar system. The solar modulation zone extends to 100 astronomical units but intensities vary only slightly between Earth and Mars (about 10 percent or less, Fujii and McDonald 1997).¹⁰

Concluding Remarks

  Exposures in low Earth orbit with low inclination have a large contribution from low LET components for which the risk coefficient is relatively well defined. The reverse is true for high inclination where a significant fraction of the exposure is from HZE ions (table 14). Even the shield design based on dose equivalent as in the case of the International Space Station may be misleading with respect to control of the health risk.

  Deep space mission exposures from background are dominated by HZE ions and the corresponding risk uncertainties are large (NAS 1996). Traditional construction materials like aluminum probably increase the health risks but polymeric materials show some usefulness in shielding the astronaut from the HZE ions.

  Solar particle event (SPE) exposures in LEO are limited by the natural precessional motion of the spacecraft even in the event of a concurrent large geomagnetic storm. Crew rotation may be required after such an event but health risks are manageable. In deep space, solar events can present a serious health hazard and adequate monitoring with a warning system and storm shelter design is required. Large exposures are possible if adequate shelter is not acquired in a timely fashion. Still lethality is not likely since tissue repair greatly reduces the effect of exposure and especially if adequate medical planning is part of the mission design. Space stress could greatly alter the biological response to such exposures and an adequate understanding of space stress affects is necessary to mission planning.¹¹

Shielding Humans in Deep Space

In 1995 a workshop was conducted by NASA, DOE Lawrence Berkeley Laboratory, NOAA Space Environmental Laboratory, the National Academy of Science Space Studies Board, aerospace industries and several universities to tailor strategies with regard to shielding humans required to live and work on lunar and martian environments from hazardous levels of radiation.¹²

The workshop's object was to devise strategies to conduct missions to reduce the levels of radiation to as low as reasonably achievable so that astronauts could conduct safely their missions and return to conduct future missions. The radiation exposure risks health risks to the astronaut had to be kept at acceptable levels currently taken at :

• Not more than 3% lifetime excess fatal cancer risks.
• Prevention of radiation sickness which may impact mission safety (lethality, vomiting, nausea etc.).

This workshop dealt with manipulation of lunar and martian regolith to provide the least most efficient expenditure of cost and energy to conduct base operations. Habitats were designed, vehicles and soil moving equipment were examined space suits were evaluated, solar event early warning system, radiation shielding material and design issues were assessed to find the best method to protect human and microelectronics from the effects of dangerous space radiation.

Candidate shielding materials like:

• Lunar Regolith 56% SiO2 | 19.8% FeO | 17.5%W% Al2O3 | 10% MgO | density 0.8-2.15g/cm2*
• Martian Regolith 58.2% SiO2 | 23.7% FeO2O3 | 10.8%MgO | 7.3%CaO | density 1.0-1.8g/cm2
• Aluminum
• water
• Lithium hydride
• Magnesium hydride (good medium because its use as a hydrogen storage medium (non cryogenically)
• Polyethylene
• Borated polyethylene
• Regolith/epoxy
• Epoxy
• Polyetherimide
• Polyamide
• Polysulfone
• Polytetrafluoroethylene
• Hydrogen

* studies show lunar regolith at 75g/cm2 of regolith will reduce the annual GCR dose solar event minimum/large flares to LEO limit operation limits.

As the study pointed out the need to provide a comfortable margin of radiation mitigation is the goal since transporting around sick or dying astronauts isn't good human exploratory space policy - as the saying goes, "Better safe than sorry."

SPACE RADIATION BUSTERS

*FEATURE INTERVIEW*

INTERVIEW WITH JOHN W. WILSON
RADIATION SHIELDING AND MITIGATION STUDIES
LANGLEY RESEARCH CENTER NASA

 Hello ...I'm Bruce Behrhorst writer for the online publication nuclearspace.com...
I would like to thank you, Dr. John W. Wilson for volunteering your time to speak with us this October 2003 day.
I understand that you have been involved with Dr. Frank Cucinotta in Space Radiation Health Risk Mitigation Technologies at Langley NASA for a number of years is that correct.?

JW: We started back in the mid eighties working together for I guess twenty years now that we've been working this problem.

BB: Could you explain your office?

JW: The main thing that we do at Langley is to look at the development of computational procedure for estimation of cancer risk within various material configurations and seeking what materials give the optimum amount of protection to the astronaut. And in particular we look for materials that we can use in some sort of multifunctional mode so that we don't think of shielding so much as adding something into a design that's already been pretty well established. Actually integrating good materials in the overall design as a way to try and optimize the spacecraft. So primarily the focus we have here and Frank [Cucinotta] was part of this group up until about 1990-91 more like ' 95 and we had begun a process looking at biological models he at Langley trying to understand what the impact of the biological models were on the shield optimization process and as the radiation biology program was expected to gear-up to try and pin down risk coefficients much better than what we have right now.
There is still a lot of very large uncertainties in terms of cancer risk. Frank went to Johnson [NASA] to head up radiation health protection program and he also serves there as the radiation safety officer for the astronaut core.

BB: Last week NASA Director O'Keefe was before a Senate Committee on Space Science and Transportation describing how deep space radiation can create health risks for astronauts exposed to radiation sources. Like Galactic Cosmic Rays (GCR), accelerated proton and electrons to high energy (E) and high charge (Z) heavy Ions called HZE particles and last week's Solar Particle Event (SPE). All background radiation throughout interplanetary space and some trapped inside Van Allan belts. He also described physiological changes in regards to bone density and muscle mass loss even through astronauts kept strict health and exercise regimes in the weightless environment.
How do you see your work as beneficial to protecting astronaut's long periods of exposure to these health risks in space?

JW: If we are going to have a human exploration program of course this is a critical issue that has to be solved. O'Keefe recognized when he first took over the helm of NASA that basically NASA had two high priority issues that it needed to resolved. One of those is the question of propulsion providing more efficient propulsion for interplanetary travel and the other is to provide the necessary protection for the health of the astronaut against primarily Galactic Cosmic Rays (GCR). It turns out that Solar Particle Events(SPE) are all that typical to protect from. The GCR's are very high energy particles that are streaming into the Solar System and they are very high energy and very difficult to shield against. And they also consist of particles for which we have very little information. At one time we had very little information both from the shielding properties but also from the biological risk and biological risk are very uncertain for these particles.

BB: You describe in your paper, "Radiation Shielding Analysis for Deep Space Missions" as the "Interplanetary Cruise Phase" where Galactic Cosmic Rays (GCR) and Solar Events Particles (SEP) are encountered. Could a space vehicle propulsion system providing the best thrust for speed in-transit to Mars for example reduce astronaut exposure to "Interplanetary Cruise Phase" GCR and SEP health risks as described in Cucinotta's operational functional category on risk mitigation? (Since comparing the "NASA Design Reference Mission" meaning Earth-to-Mars leg in chemical propulsion would take 6 months as opposed to NTP propulsion of only 2.5 months or a round trip total of 5 months as compared with 12 months for the NASA plan.)

JW: Once you get on the Mars surface you have two things helping you out. The first thing is; you are well shielded from GCR's or SPE's either one because of below the horizon, because of the presence of the planet itself. So, your main problem of penetration through the atmosphere which is not very dense on Mars. So you do have a bit of protection from the atmosphere from the point of view of radiation protection in space but it's nothing like the Earth's atmosphere which provides a great deal of protection for us living on the Earth's surface. On Mars you will see the break-p of a number of the heavy ions in their penetration through the Mars atmosphere and this is very helpful within itself. It does generate through a lot of neutrons and it turns out a lot of those neutrons are produced in the soil underneath you. The neutrons of course are also very damaging biologically and it really depends what the ground underneath you is like. If your up in the polar regions where you have CO2 ice, a lot of CO2 in the ground underneath you; that is helpful. If you have water ice especially underground underneath you that's even more helpful. But if your in the low latitudes planes were it's mainly rock and regolith underneath you then the neutrons that you see on the surface are mostly produced in the rock and regolith below the site where your [standing] at.

BB: In transit if you were to take. Let's say...implement a "Direct Mission Plan" to Mars with a crew of four. The "Powers" that be decided, they were not going with chemical propulsion. They decide to use a nuclear thermal propulsion system or even nuclear electrical propulsion system with on-board neutron sources. Would the tradeoff [to exposure] be better, using a faster propulsion system so that the exposure in-transit would be shorter for the astronauts?

JW: Oh, definitely...I was just pointing out that you do have a problem once you get there to. But it does help a great deal.

BB: You broke down Environment radiation shielding analysis for manned deep space mission into 4 phases:

• Interplanetary Cruise Phase
• Final planetary approach
• Orbit insertion
• The surface Phase

In your estimation from your previous answer; 'The surface phase' would be detrimental of the four?

JW: It depends on the stay time on the surface and if it's a relatively short stay then most of your exposure is going to be in-transit. But, if you have a very long stay on the [Mars] surface like there have been some scenarios were people may spend as much as a year in a habitat on surface and then the surface conditions now become a very important design problem and that is were the neutron problem come in and play a very important role.

BB: Can you evaluate briefly the MARIE experiment since it was a platform on the 2001 Mars Odyssey orbiter, in as far as Cruise Phase and Orbit Phase radiation analysis around Mars ?

JW: The main thing MARIE has been used for is to validate the models that we use in exposure estimates. Once you place in the corrections for the structure of the spacecraft itself the agreement between MARIE and the interplanetary environment and here I'm referring to just the galactic cosmic ray component have been very very accurate. If you go to the website: MARIE down at Johnson Space Center [NASA] often the differences the models and experiments. We're talking several percent or less.
So, the MARIE instruments has been very helpful providing confidence in what we're doing. It's mainly been validating the environment models that we have been using. So far it has been very successful.

BB: So...you've got a good match ?

JW: We have a good match, of course when you have a solar particle event (SPE) it's different and MARIE has seen a number of SPE events now. Remember SPE events are almost statistical in nature, it's difficult to predict ahead of time what an SPE event is going to be like. So, there are no models there that we can really do any detailed comparisons with.
The best you can hope for is MARIE [data] and the group is working on this now. Of the solar events that have occurred we have both platforms are near 1AU of course we have MARIE near Mars and between those two you can begin to sort out hopefully some of how the particles propagate from 1AU on out to Mars to help you relate all of the observations that we have seen near Earth and there is a fairly large database on SPE events near Earth. But how you relate those to what those events would look like on Mars is still an issue.

BB: You state the Jovian satellite Callisto not very exciting when compared to Io, Europa, Ganimede but, it is the safest from a radiation point of view since it's quite distant from the other moons and Jupiter itself. And that an older male astronaut seem to fair better career dose limits compared to female astronauts, does gender make that big of a difference in choosing astronauts for example, Jovian system human exploration and why?

JW: This has to do with risk coefficients and Frank could discuss more on this. There's a lot of individual differences in radiation response of which gender is were we have some of the best studies which really comes from the Japanese survivors in WWII and there are significant differences in the cancer induction rate between men and woman. A lot of this is driven by breast cancer alone which is fairly rare in men. And it isn't induced by radiation in men as it is in women. There is a factor of almost two difference. If you look at the paper, "Emerging Radiation Health Risk Mitigation Technologies" it gives the exposure for a 3% risk as a function of age for males and females and you'll see that it gets up to almost a factor of two.

BB: NASA for the last 35 years has collected and monitored the radiation doses received by all NASA astronauts that traveled into space in fact some elderly astronauts have returned to space and one Apollo astronaut had to physically restrain an irate intruder. Proving in some respects he's in top physical condition. In your estimation have you known an astronaut that has exhibited out of the ordinary health maladies attributed to prolonged exposure to space borne radiation?

JW: The problem you have there is the number of astronauts involved is a very small sample. Even if you look at very large number of workers say...in the nuclear industry or nuclear weapons development where you have really large numbers of people involved even then it's difficult. Of course their exposure levels are much smaller than the astronauts. The point is, you do need these really large populations to do these types of studies. Lief Peterson and Frank Cucinotta down at Johnson Space Center have been doing those type of studies for the astronauts but the samples are just too small in order to relate to risk, to cancer risk.

BB: Along those lines what do you think about "Radiation Hormesis" ? (low doses of irradiation can have a beneficial physiological effect on the human body for example; chronic exposure exhibited by nuclear workers had lower cancer mortality rates among general public of the same profile)

JW: Very controversial...The place where Hormesis has been clearly seen is in the early radiation response of biological systems. And it does appear even in single cells but also in whole animals that if you give them a very low initial exposure of something like x-ray or gamma rays that you elicit some repair mechanism that start. So, then if you come in a little while later with a much larger perhaps a lethal exposure. Then what you find is, they tend to survive better if they got this early exposure of low dose gamma rays. There is a lot of people that have seen that type of effect both on a cellular as well as a whole animal level.
The question is in the case of cancer induction. Which is the primary risks to astronauts?

The risk coefficients we're using are primarily driven by a cancer risk and their more driven by mutations and not early death or survival of the cell. There is the concern even-through the survivability of the biological system maybe enhanced that you're adding additional mutations even with these low doses and that is a concern.

BB: I noticed you mention "radiation vaccine" be close to "radiation hormesis" ?

JW: It's sort of a broader issue and again you find in terms of looking at the pharmaceuticals. There is dichotomy between the early effects radiation exposure at very high doses and the effect of long term exposure. Frank is really the guy to talk more about this. For pharmaceuticals generally in the past they have been driven mostly by the military and they're worried about survival on the battlefield and they're worried about early radiation effects and not so much about cancer later in life. Where as in astronauts it's about cancer later in life which is the primary concern.

BB: Excluding the Earth's poles where on Earth are "hot spots" of natural radiation emission among our continents? Along those lines, is there tiny specks of decay products of Uranium in space around Earth ?

JW: There were some high altitude tests both by Russia as well as by the U.S. In which both fission products as well as uranium and plutonium went into outer space. Whether there's any still there or not - I don't know. There is, of course, a small uranium compound in GCR's but it's very small.

BB: I would think this outer space uranium would have come out of some kind of volcano or high altitude explosion or supernova maybe stars.

JW: But, those fluxes are very small. In terms of natural radiation on Earth there are Thorium sands of the coast of India and some places in Brazil that's "very hot". In Egypt the delta [Nile River Basin].

BB: Does all this bother human beings?

JW: Well, people don't live in those regions most of the time.
Denver is a "Hot Spot" in the U.S. Not as high as the beaches in India. Even over there in that region their exposure is much higher than most people around the world and of course they are a real target of doing studies. Some people who argue Hormesis will point to these areas as no irradiation effect.

BB: Are you doing some of these studies? (see abstract, "Overview of Atmospheric Ionizing Radiation (AIR) Research: SST- Present" available at: www.sciencedirect.com Also "Overview of Radiation Environments and Human Exposures".)

JW: At the annual meeting of the NCRP (National Council on Radiation Protection / ICRP International Commission on Radiological Protection) in 1998 I was asked to do a summary of human exposure from the ordinary person, on the terrestrial surface to Astronauts and what astronauts may be experiencing in the future. That includes air crew and passengers on airplanes and everything in between. I tried to cover a fairly broad spectrum.
We have been doing some atmospheric ionizing radiation studies associated with high speed civil transport. In this report it discusses some of these issues where the "hot spots" are.
One of the important things from our point of view is. Who is being exposed by neutrons on the Earth's surface? There's a few places where the neutron sources are quite a bit higher. That's the capital of Bolivia.

BB: La Paz.

JW: Same way in Tibet, Lhasa and Denver here in the U.S. Also Leadville which is the highest village in the U.S.

BB: Uh...Leadville?

JW: Yeah, Leadville, Colorado...It's 11,000 -11,500 feet? They're right up there. And most of those neutrons are coming from the atmosphere.

BB: You mentioned in your latest paper "Emerging Radiation Health Risk Mitigation Technologies" that if the body as a magnetic field the dipole component would make an effective magnetic bottle in which the neutron charged decay products electrons and protons could be trapped forming a belt of ionizing radiation centered on the magnetic equator such as for Earth and Jupiter.
Could a 'mini' electromagnetic Van Allen Belt be artificially manufactured around a human Mars vehicle in transit thus, funneling most ionizing radiation on two spots providing crew protection or would the energy requirements be to expensive to produce?

JW: If you go for a straight dipole and try and use a like super conducting magnets and so on, you find that's a very difficult problem to try and protect against GCR rays and the main reason is that they are so energetic.
The Earth's magnetic field gives us a lot of protection mainly because it's very extensive. And it isn't that it's a very intense field but, that it covers a large volume of space that even the small amount of curvature that it provides to GCR rays does give a lot of protection inside the magnetic field. You could just see that on the Earth if you go near the poles substantially higher ionization rates than you do down at the equator if your on a ship or plane and you see those effects readily.
The problem is trying to build a large field like that associated with a spacecraft. There have been some suggestions of using a plasma to release at the center of the magnetic field to carry it out. Plasma is a very good conductor and it will grab the "field lines" and tend to pull them out to very large distances. This has been proposed and the system is called the "M2P2". So people are suggesting that sort of thing it's not in the final issue...isn't done yet. But they're both proponents and those who don't propose that it's going to be helpful.

BB: In Cucinotta's Functional category on radiation health risk mitigation shielding it states, "At the present time shielding is the most single counter-measure available." What sort of lightweight shielding material and their properties qualify and are these categorizes like: Aluminum, styrofoam, polyethylene, lithium hydride, liquid hydrogen, hydrogenated graphite nano fibers, water, ice, urine to name a few and do these scatter radiation enough to render secondary radiation manageable ?

JW: When we started, his work started in about 1980 and one of the questions that we wanted to try and resolve at that point was what kind of material should we be using and of course it largely was in a nuclear physics issue. So the early part of his work really focused on trying to define the nuclear physics and defining models that were sufficiently accurate that you could begin to characterize what shielding materials were. By 1990 we had a pretty good picture of what was good and what was not good. Of course the prevailing technology right now is aluminum alloys. You can apply them well in low Earth orbit and that's mainly because the protection problem is driven by trapped radiation, once you get outside the Earth's magnetic field for long periods of time it appears that aluminum is actually more hazardous. You would think of it as a shielding material, it actually generates an interior environment that is even more hazardous than if you didn't have [shielding] anything there. And that's because of secondary radiation being produced in collision with the aluminum.
The best material is Liquid Hydrogen .

BB: Gosh...You can use that as fuel ?

JW: Oh yeah...In fact if you look at the study we did for the mission to Callisto that's basically what we were doing is utilizing hydrogen there in the fuel tank or in trying to develop a low pressure material and there's been some preliminary experiments that's been looking promising.
But basically high hydrogen content material, unusually high either from liquid hydrogen or some sort of nano fiber structure...So that's what we used there. This is looking forty years out . Now that's the best material to have.
The question is, if you have to add something with hydrogen what is best?
Of course Lithium Hydride looks like a very promising material but the problem is it's not a very good material structurally and so you can build things out of it. So you try to find things you can build things with and that brings you into aliphatic polymers (organic compound in which the carbon atoms are joined in straight chains; as in hexane.) of which polyethylene is one example were looking at. How can you utilize polyethylene to build future spacecraft out of and build structures that are both, damage tolerant to withstand meteorite impact? They have to have enough strength to withstand the loads from propulsion [production] and under impact.
Polyethylene has limited structural properties over a fairly limited range of temperature-this is a problem.
If you look at aromatic polymers these are polymers that have a lot of ring structures and their fairly good, like polyamides they have a broad range of temperatures over which they maintain good properties.
So what we're looking into is how we can combine aliphatic and aromatic into hybrids systems where you can design the temperature and the strength characteristics to meet your requirements?
So you might think of a craft that's maybe a pure aromatic structure outside so it has all the good characteristics you need on the outside to withstand temperature and still support the loads. Then as you come toward the inside of the craft your changing through various grades of hybrid structures until you get to the inside and then there would be an almost pure aliphatic polymer which has good radiation protection properties as well as structural properties.

BB: What about just plain water or ice?

JW: Well... Water or ice wouldn't withstand loads but water is very helpful. If we're going to take people you'll have a lot of water on board so utilizing water tanks is part of the protection system. It is of course one of the options you want to build into the design process.
One of the options if we went to Mars would be to set-up a methane production site for the return fuel so you don't have to transport all that fuel for the vehicle. Methane has very good protection properties and it's also easy to liquefy much easier than hydrogen.
Lithium Hydride that's a material that normally is integrated into nuclear reactor shielding designs for space so you're gonna have plenty of lithium hydride anyway. So you do have that to utilize as well.
The real key is when you build these crafts and your choosing materials. Chose the materials that meet all your requirements that have the best radiation protection properties and you have a lot of choices there and we need to expand those choices so the engineering community when they do their designs can integrate protection factors into it.

BB: How much of an advantage does the vacuum of space contribute to the limited reflectability of let's say...On-board radiation sources of low energy neutron exposure?

JW: When you're using reactor power the streaming especially in space is very important. Because if you had to build a shield completely enclosing the reactor the masses get out of hand.
Normally what people do is build a shadow shield and there you have to minimize the amount of scatter in the shield design that's part of the design process. Outside that shadow shield the streaming of particles on out into space is not considered a problem. As long as you don't bring a human in the vicinity. You have to be very careful in what zone your in. So you do have a protection problem from that point of view basically, you work behind a shadow shield.

BB: Do you feel that space radiation mitigation is a new field and will the need to provide skilled technicians in verification, testing and safety/risk grow as space mission requirements become more complex in the future?

JW: Yes, because mostly what NASA has been funding so far is really small basic research programs to understand what factors are important in terms of materials. Behind that is a whole engineering process that requires a lot of technicians, a lot of testing, a lot of development of how to apply those materials in specific technologies and that problem is not resolved yet.
That is a big problem, example: polyethylene was identified almost fourteen, fifteen years ago as probably the best structural type of material or at least an Aliphatic compound as the best structural material. But still, there's a huge gap between that identification and putting it into engineering practice. And it's the engineering practice that is going to take a fairly long term development and we're going to need people to work that problem.

BB: Do you think you have some of those people now?

JW: No we don't. We're just in the process of defining things you have to do, in terms of engineering development. There's really nothing that's been done yet and there's going to have to be a lot of new people.
You know...How do you practically fabricate some of these designs concepts?

It's a real issue.

Because they maybe relatively uniform structures chemically but when you look at the physical properties they're very in-homogenous. Like, if you're worried about damage tolerance basically you need a polyethylene structure that has a hard shell on the outside and a hard shell on the inside and then it has a lightweight foam in between and the purpose of that is , the outer shell will fragment meteoroids and once they fragment they begin to follow cone expansion and as they penetrate past the first shield you have the fairly lightweight foam to allow them to expand. So when they hit the inner hard surface the energy is dispersed over a very large area and it's not likely to penetrate. That's typical of a wipple shield type structure. But, we have to learn how to build these things using these new materials, that is not a solved problem yet.

BB: Could you explain briefly how you got started in this field ?

JW: I became interested in physics when I was a sophomore in college taking the typical sophomore physics courses.

BB: Which college?

JW: I went to a junior college in my hometown at that point in my junior year I transferred to Kansas State University in Manhattan, Kansas (the little apple). That's were I learned computers; computers were pretty new when I went there. I'm almost sixty three years old, this was back in the very early sixties and computers were still quite new. I earned a living, working on a cow project on [Dairies] milk production. And learned about computers in the process and NASA just happen to come through interviewing when they found out I had been doing a lot of research with computers even though on counts they where happy because few knew what a computer was back then.
Starting with NASA I worked on real time simulation but then became aware of a fellow named Trutz Foelsche he was a German who came here after World War II and was interested in the super sonic transport and I had identified a number of issues with respect to human exposure on super sonic transport. I started working with him on that problem I then realized that the transport problem bridges almost all areas of physics from atomic to nuclear particles so I figured that would be a good problem to work on.
Soon after that I started to work on a graduate degree from the College of William and Mary and finally got a Ph.D. degree in heavy ion interactions and that was the beginning of this work because of the cosmic ray problem so it just sort of grew out of there.

BB: Thank you, very much...Dr. Wilson for a good interview, it's refreshing to get NASA technical people to come forward and explain to the public their work.

JW: The thing I found particularly exciting is the website's student outreach, I think that's important. Because we need to start worrying about who's going to fill the ranks of the next generation of technology people.

BB: I have been told that same thing, a worry that the United States could fall behind in space technology.

JW: Well...I'm fortunate in my group here [NASA Langley] I have Robert in his mid forties and another person in his early fifties.
But then I have two young people who are in their early thirties and mid thirties. So...We have people that are moving up into this area of expertise now, that are fairly young so we have a pretty good chance of continuing this effort.

Because I'm not going to continue to work here forever.

BB: Oh...Yeah, I know you need to pass on the [technology] torch, essentially that's one of the functions of nuclearspace.com is to try and interest american students to at least train for some of these endeavors since any up close tangible exhibitions would require someone to travel to for example; Los Alamos, New Mexico and visit the Bradbury Science Museum. Because there is no exhibits dedicated to the use of nuclear power and propulsion in space.

JW: At the "Space 2003 conference" I just got back from there in late September. That was a very big topic at the conference, basically the question. Who's going to prepare the next generation of technologies to carry on space activity? Not only civilian they were also concerned about military as well as a commercial space activity.

There is a lot of exciting things going on in space science for the young today I would urge students to look into this as a career.

[  PART II ]

RELATED LINKS:

SPACEWeather.com | Space Radiation Shielding | NASA Space Radiation Laboratory |

| Space Radiation Analysis Group NASA | Spaceflight Radiation Health Program |

| NASA Engineering Safety Center | Today's space weather - NOAA |

HEALTH PHYSICS TERMS LINKS:

NRC Glossary | Radiation Related Definitions | What is dose? | Luckey98 Radiation Hormesis |

| Bobby's Radiation Glossary |

REFERENCES:

1.John W.Wilson, Overview of Radiation Environments and Human Exposures, Health Physics Society vol 79 #5 Nov. 2000, pg.470

2. Kalevi Mursula and Ilya Usoskin, Heliospheric Physics and Cosmic Rays, pg.1

3.John W.Wilson, Overview of Radiation Environments and Human Exposures, Health Physics Society vol 79 #5 Nov. 2000, pg.471

4.John W.Wilson, Overview of Radiation Environments and Human Exposures, Health Physics Society vol 79 #5 Nov. 2000, pg.472

5.John W.Wilson, Overview of Radiation Environments and Human Exposures, Health Physics Society vol 79 #5 Nov. 2000, pg.473

6.John W.Wilson, Overview of Radiation Environments and Human Exposures, Health Physics Society vol 79 #5 Nov. 2000, pg.474

7.John W.Wilson, Overview of Radiation Environments and Human Exposures, Health Physics Society vol 79 #5 Nov. 2000, pg.475

8.John W.Wilson, Overview of Radiation Environments and Human Exposures, Health Physics Society vol 79 #5 Nov. 2000, pg.476

9.John W.Wilson, Overview of Radiation Environments and Human Exposures, Health Physics Society vol 79 #5 Nov. 2000, pg.483

10.John W.Wilson, Overview of Radiation Environments and Human Exposures, Health Physics Society vol 79 #5 Nov. 2000, pg.486

11.John W.Wilson, Overview of Radiation Environments and Human Exposures, Health Physics Society vol 79 #5 Nov. 2000, pg.490

12.John W.Wilson, J.Miller, A. Konradi and F.A Cucinotta, Shielding Strategies for Human Space Exploration, NASA CP-3360 publication, Dec.1997, pg iii, 15-306 (conference publications available at: NASA STI)

OTHER REFERENCE:

J.W. Wilson, I.W. Jones, D.L. Maiden, and P. Goldhagen DOE Environmental Measurements Lab., Atmospheric Ionizing Radiation (AIR): Analysis, Results, and Lessons Learned From the June 1997 ER-2 Campaign, NASA CP-2003-212155 publication, February 2003 (conference publications available at: NASA STI)

J.W. Wilson, F.A. Cucinotta, W. Schimmerling, Emerging Radiation Health-Risk Mitigation Technologies, [Space Technology and Applications International Forum] STAIF 2004

J.W. Wilson, W. Schimmerling, F.A. Cucinotta, J. S. Wood, Effects of Radiobiological Uncertainty on Vehicle and Habitat Shield Design for Missions to the Moon and Mars. 1993 NASA technical paper 3312 (available at: NASA STI)

Myung-Hee Y. Kim, Sheila A. Thibeault, Lisa C. Simonsen, and J.W. Wilson, Comparison of Martian Meteorites and Martian Regolith as Shield Material for Galactic Cosmic Rays, October 1998 NASA TP-1998-208724 (available at:(NASA STI)

G. De Angelis, M.S. Clowdsley, J.E. Nealy, R.C. Singleterry, R. T. Tripathi, J.W. Wilson, Radiation Shielding Analysis for Deep Space Missions, [Space Technology and Applications International Forum] STAIF 2003 (available at AIP or local public library)

J.W. Wilson, M.S. Clowdsley, F.A. Cucinotta, R.K. Tripathi, J.E. Nealy, G. De Angelis, Deep Space Design Environments for Human Exploration (available at: LARC search)

J. Wilson photo courtesy: John Wilson