While a suborbital spacecraft’s stay in space will be short, it will have to be equipped to deal with issues like radiation and weightlessness. (credit: XCOR Aerospace)

Human factors in commercial suborbital flight: Radiation exposure and weightlessness

by John Jurist
Monday, October 3, 2005

Radiation exposure

The human radiation exposure during suborbital flight is considered for a typical winged (horizontal takeoff and landing) suborbital concept. These hazards are evaluated both in the context of the public at large (potential passengers) and occupational exposures (aircrew). The hazards are also considered in the context of typical background radiation exposures and medical exposures. Finally, recommendations are made regarding adherence to the ALARA (As Low As Reasonably Achievable) principle used by the Nuclear Regulatory Commission (NRC). For the purposes of this analysis, the following flight profile is assumed:

After motor ignition, the flight passes 7,500 meters (25,000 feet) altitude at 100 seconds, 15,000 meters (50,000 feet) at 120 seconds, 30,000 meters (100,000 feet) at 160 seconds and reaches apogee of 119,000 meters (390,000 feet) at 310 seconds. Descent below 30,000 meters occurs at 440 seconds, and 15,000 meters at 460 seconds. At 600 seconds post-ignition, an essentially constant descent rate is established from 12,000 meters (40,000 feet) to about 3,000 meters (10,000 feet) at 1,600 seconds. This phase of the flight is followed by approach and landing.

This baseline flight profile assumes a total of 920 seconds exposure above 7,500 meters, 340 seconds above 15,000 meters, 280 seconds above 30,000 meters, and 220 seconds above 60,000 meters.

In addition to possible on-board radiation sources, which are not a factor in commercial suborbital flights unless radioisotope fluid level sensors are used, radiation exposure comes from the following potential sources during space flight:

• Energetic cosmic photons (including gamma ray bursts): This source of radiation varies markedly in intensity. In general, the atmosphere provides effective shielding at low altitudes. Cosmic electromagnetic radiation contributes to the general increase in measured exposure with increasing altitude. Gamma ray bursts are poorly understood short-duration plane waves of gamma radiation which pass through the solar system at rare intervals. They are not considered a significant problem in LEO operations.
• Cosmic particulate radiation: Effective shielding (deflection) of charged particles of both cosmic and solar origin is provided by the terrestrial magnetic field and the atmosphere. Secondary (bremstrahlung) radiation resulting from charged particle interactions with the atmosphere is responsible for the general increase of background radiation with geomagnetic latitude at low altitudes. Occasionally, extremely energetic high-Z particles (the so-called “oh-my-God particles”) enter the atmosphere and produce intense localized secondary radiation fluxes.
• Energetic solar photons: This is considered an insignificant threat or source of exposure.
• Solar particulate radiation (including flare events): As with cosmic particulate radiation, the terrestrial magnetic field and atmosphere provide effective shielding from this radiation source at low altitudes and geomagnetic latitudes. Flare events, which are correlated with the solar cycle, can be predicted several hours in advance.
• Trapped particulate radiation belts: The Van Allen radiation belts are an insignificant source of radiation below altitudes of about 800 kilometers and are therefore an insignificant threat or source of exposure in suborbital operations.
• Terrestrial background: In addition to contributions of the above sources to ground level background radiation exposure, radionuclides present in the Earth’s crust provide some background radiation exposure. This is not regulated, nor is it biologically significant except under very limited circumstances. At ground level in the United States, the background dose for the general population typically averages 2 to 3 milliSieverts per year. This dose is the total from all natural sources.

The rare particulate and gamma ray events of cosmic origin can be ignored in this discussion. Since horizontal takeoff (HTO) suborbital flights are assumed to occur at low altitudes except for very short times (less than six minutes above 15,000 meters and less than 16 minutes above 7,500 meters) and at relatively low latitudes (centering around Mojave, California at 35° latitude and 760 meters altitude), solar flare events are also ignored. Radiation exposures will be converted into effective doses and expressed in Sieverts (Sv) or milliSieverts (mSv) in the following discussion.

As an approximation, the radiation background intensity tends to increase monotonically with geomagnetic latitude and with altitude up to about 20,000–25,000 meters. Above that altitude, intensity tends to decrease somewhat and then increase again (markedly if the Van Allen radiation belts are entered at altitudes above about 800 kilometers.

At low altitudes, the contribution of non-terrestrial sources to the daily background radiation dose rates can be summarized in the table below:

At higher altitudes, the following non-terrestrial daily dose rate contributions are observed:

These dose rates do not include shielding from vehicle structure. For example, the old Concorde supersonic airliner and other long-haul airliners, flying at altitudes of about 10,000–15,000 meters, expose passengers to dose rates of 0.103–0.233 mSv/day. The Skylab missions provide approximate dose rates in LEO with structural shielding. Those doses ranged from about 0.6–0.9 mSv/day. The average was 0.77 mSv/day for a total of 171 days over three missions. Skylab orbited at an altitude of about 435 kilometers at an inclination of 50°.

The following conservative non-terrestrial occupant dose rates, including vehicular shielding, are assumed for suborbital flights:

This daily dose rate versus altitude relationship used for the HTO suborbital vehicle is shown in the following graph:

These dose rates are based on high estimates in the absence of solar flares and on reasonable aircraft structural shielding capabilities.

Given the projected flight altitude versus time profile for the assumed suborbital vehicle, the conservative whole body dose per flight estimate is no more than 0.0053 mSv. A typical two-view chest x-ray examination provides a dose of about 0.06–0.25 mSv. Therefore, the typical chest x-ray corresponds to more than 11 suborbital flights, and the general population background dose of perhaps 2 mSv annually corresponds to more than 300 suborbital flights annually.

The NRC limits for the general public from radiation operations are 1 mSv per year and 0.1 mSv per year for minors. The ICRP limit for the general public is also 1 mSv per year. These limits are not exceeded for minors at 18 flights annually, and they are not exceeded for adults at a flight rate of 188 flights annually.

Another issue is the dose for suborbital crews. If the crews were limited to 188 flights annually, they would not exceed the dose limits for the general adult public.

NASA considers astronauts as radiation industry workers. Thus, their annual dose limit is considered to be 0.5 Sv. This is equivalent to more than 94,000 flights annually. Assuming a maximum of eight flights daily over a five-day working week and a 50 week working year, a crew member would be exposed to a maximum of less than 11 mSv annually.

There are also career limits for radiation workers. They are:

As shown above, the career limits are not approached by suborbital crew member doses. Radiation exposure during orbital operations, particularly if prolonged, must be considered carefully in contrast to suborbital operations. A suborbital vehicle designer and operator might take cognizance of the following points when considering radiation exposure issues:

• Some states have adoption agreements with the federal government regarding radiation exposures of all citizens.
• In other states, x-ray exposure is regulated locally and the NRC regulates radionuclide-related exposures. In these states, the NRC exposure regulations could well not be applicable unless the FAA’s Office of Commercial Space Travel adopts them.
• Depending on the regulatory environment under which suborbital operations will take place, voluntary adoption of the NRC general population and occupational dose limits may be advisable.
• Treating suborbital crew as radiation workers would entail establishing a monitoring program, which would cost perhaps $1,000 annually for up to 10 workers exclusive of personnel time.
• Fly a dosimeter (either TLD or film) on the first 10 flights without passengers to verify the estimates given above.
• Do not fly in times of predicted high solar radiation exposures at altitude.

Weightlessness

In the suborbital flight regime, weightlessness or microgravity is not a significant issue. First, a suborbital flight might subject crew and passengers to a maximum of perhaps 3½ minutes of microgravity. Second, the most significant risk related to brief exposure to reduced gravity is motion sickness or nausea. The remaining biological effects of reduced gravity conditions typically take exposures of hours to days to manifest themselves and of concern only during orbital or interplanetary operations.

The risk of space motion sickness or nausea is most significant during the first few days of orbital space flight and tends to manifest itself within an hour or so in susceptible people. Recovery generally occurs within 1½ to 2 days of flight.

The risk of nausea in reduced gravity is significantly abated if provocative motions, especially of the head, are avoided. During suborbital flights, the risk will be reduced if vehicle occupants remain strapped into their seats during the flight. During a short exposure of a few minutes, allowing passengers to unstrap from their seats and then return to their seats before deceleration commences may be impractical in any event.

Incipient motion sickness can be countered by holding the head in a fixed position. Odds of nausea can be reduced by various medications taken in advance of the flight and by prior familiarization with exposure to reduced gravity in aircraft flights. In a multipassenger vehicle, one passenger becoming nauseated can potentially trigger nausea in the others.

If passengers are in pressure suits instead of a shirtsleeve environment during suborbital flight, response to nausea would require opening the helmet face plate to get a waste bag into position. Vomiting into a closed pressure suit helmet and/or oxygen mask is not only unpleasant but also dangerous.

A note to the reader

This completes my discussion of the major human factors considerations in commercial suborbital flight. If there is sufficient interest, these discussions could be extended to orbital and interplanetary flight. Readers with an interest in extending the discussions or wishing to discuss topics related to suborbital flight in more detail are encouraged to contact me at JMJSpace@aol.com.

References

1. Berry, Charles A.: Aeromedical preparations. Chapter 11 in Mercury Project Summary, NASA SP-45.
2. Bottollier-Depois, J. F.; Chau, Q.; Bouisset, P.; Kerlau, G.; Plawinski, L.; and Lebaron-Jacobs, L.: Assessing exposure to cosmic radiation during long-haul flights. Radiation Research.
3. Cool, D. A.; and Peterson Jr., H. T.: Standards for Protection Against Radiation, 10 CFR Part 20 (NUREG-1446), October, 1991.
4. Dow, Norris F.: Structural implications of the ionizing radiation in space. Proceedings of the Manned Space Station Symposium, IAS, Los Angeles, 1960.
5. Levedahl, Blaine H.: A survey of radiobiology for engineers. Human Factors, pp. 1-68, August, 1959.
6. Nicogossian, Arnauld E.; Huntoon, Carolyn L.; and Pool, Sam L.: Space Physiology and Medicine, 3rd Ed., Lea & Febiger, 1994.
7. Schaefer, Hermann J.; and Golden, Abner: Solar influences on the extra-atmospheric radiation field and their radiobiological implications. Pp. 157-181 in Physics and Medicine of the Atmosphere and Space. Benson Jr., Otis O.; and Strughold, Hubertus (editors), John Wiley & Sons, New York, 1960.

John Jurist is a biophysicist with a long-standing interest in human factors. He currently resides in Billings, Montana and can be contacted at JMJSpace@aol.com. Any of the opinions expressed in these columns are his own and do not necessarily reflect the opinions or policies of XCOR Aerospace, Montana State University, or Santa Clara University.

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