Human factors in commercial suborbital flight: Radiation exposure and weightlessness
by John Jurist
|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 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:
|Equivalent whole body dose (mSv/day)|
|Sea Level||2,000 meters|
At higher altitudes, the following non-terrestrial daily dose rate contributions are observed:
|Equivalent whole body dose (mSv/day)|
|Altitude (m)||Equator||55° Latitude|
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:
|Altitude (m)||Equivalent whole body dose (mSv/day)|
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:
|Age (yrs)||Limit (Sv)||Male Career Flight Limit at 0.0053 mSv each|
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:
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.
|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.|
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.
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.