Suborbital Commercial Space Flight Crewmember Medical Issues
Aerospace Medical Association Commercial Space Flight Working Group
Jim Vanderploeg, M.D. Co-chairman/UTMB
Mark Campbell, M.D. Co-chairman
Melchor Antunano, M.D. FAA/CAMI
Jim Bagian, M.D. VA
Genie Bopp Wyle
Giugi Carminatti Weil, Gotshal & Manges
John Charles, Ph.D. NASA
Randall Clague XCOR
Jon Clark, M.D. BCM, NSBRI
John Gedmark Personal Spaceflight Federation
Richard Jennings, M.D. UTMB
David Masten Masten Space Systems
Molly McCormick Orbital Outfitters
Vernon McDonald, Ph.D. Wyle
Pat McGinnis, M.D. UTMSH
Vincent Michaud NASA
Michelle Murray FAA/AST
Jeff Myers, M.D. CHS Medical
Scott Parazynski, M.D. Wyle
Elizabeth Richard Wyle
Rick Scheuring, D.O. NASA
Rick Searfoss XCOR
Quay Snyder ALPA
Jan Stepanek, M.D. Mayo Clinic
Alan Stern Blue Origin
Erik Virre UCD, 0-G
Erika Wagner MIT
The operational experience for manned suborbital space flight (altitude greater than 100 km) is very limited, consisting of two Mercury-Redstone rocket flights in 1961, two X-15 flights in 1963, an inadvertent Soyuz launch abort in 1975, and three SpaceShipOne flights in 2004. All indications are that the sequence of acceleration - weightlessness - deceleration were well tolerated with minimal neurovestibular dysfunction. The problems that were encountered should be taken in the context of highly experimental, high performance vehicles flown in an environment that no one had ever experienced before. However, there are some indications that distraction and spatial disorientation can occur. Only further suborbital space flight experience will clarify if pilot performance is affected or will be an issue. It would be expected that experience in pilot performance during suborbital space flight will be obtained during the flight testing that will occur before space flight participants are carried on any of the future commercial space flight vehicles.
By definition, a suborbital space flight has to reach an altitude higher than 100 km (62 mi, 328,000 ft) above sea level. This altitude, known as the Kármán line, was chosen by the Fédération Aéronautique Internationale. The USAF and the FAA consider an altitude of 50 miles (80.47 km, 264,000 ft) as the altitude to qualify as space flight. The experience in human suborbital flight below the international standard of space flight (100 km altitude) is much broader (see Table I) and should be taken into account when discussing predictions of pilot performance.
Several documents have been produced discussing the medical standards for commercial space flight in regards to orbital space flight participants (1, 2), suborbital space flight participants (3, 4), and suborbital crewmembers (5). The content of this paper relies heavily on information from those previous papers. This paper describes the flight environment during a suborbital commercial space flight, identifies the possible medical risks, and discusses the mitigation strategies that could be used to lower those medical risks. In order to provide as much specific detail as possible, the projected flight profile of a single space craft and suborbital flight (SpaceShipTwo by Virgin Galactic) will be used as a baseline design reference mission. Note, however, that other suborbital flight vehicles under development will likely vary in their design and flight characteristics, and consequently the relative severity / risk of the medical issues discussed. We also limit the discussion to critical flight crewmember (as opposed to a space flight participant or a non-flight crewmember) medical issues. Finally, we will also provide as much referenced data that currently exists to help explain any rationale. The goal is to not focus on recommendations for standards or certification, but instead to develop evidence-based, referenced evaluations and guidelines of the medical risks and ways to mitigate those risks to improve safety. Additionally, we seek to better articulate what gaps exist in the current knowledge of commercial human space flight issues.
Medical certification of commercial suborbital space flight pilots is still in evolution and will continue to be better defined in the future by the FAA. A previous position paper from the Aerospace Medical Association recommended an FAA first-class airman medical certificate (5). Currently, the FAA requires “crewmembers who have a safety-critical role . . . possess and carry a second-class airman medical certificate.” Each member of a flight crew has to demonstrate the ability to “withstand the stresses of space flight, which may include high acceleration or deceleration, microgravity, and vibration in sufficient condition to safely carry out his or her duties so that the vehicle will not harm the public” (6). Policy and decision processes to be used for waivers and what functional tests (centrifuge, parabolic flight, altitude chamber) will be required to demonstrate that an individual can perform in the suborbital environment is still undefined by the FAA. As one of the largest potential customers, NASA may attempt to input into these requirements.
Projected Flight Profile of the Virgin Galactic "SpaceShipTwo"
According to publicly available information, SpaceShipTwo will have two pilots and up to 6 space flight participants. The cabin atmosphere will be pressurized to 8.000 ft (2440 m) altitude or lower with re-circulated atmospheric air (21% O2). The projected flight profile begins with a horizontal takeoff underneath the carrier aircraft "WhiteKnightTwo" with a flight to approximately 50,000 ft (15,240 m) where SpaceShipTwo will be launched. The boost phase will be 70 sec long and will have a maximum peak of 3.8 g (longest duration in +Gx with a brief spike in +Gz). Speeds will be Mach 1 at 8 sec and Mach 3 at 30 sec. Maximum speed will be 2600 mph (4180 km/h). The 0 g coast phase will last approximately 4 minutes and will reach an apogee of 110 km (361,000 ft). During the coast phase, space flight participants (but not the flight crewmembers) will be out of the seats and able to freely move around the 12 ft X 7.5 ft (3.7 m X 2.3 m) cabin cabin. The deceleration phase will have a maximum peak of 6 g, but the seats will recline to convert most of the forces to +Gx for the space flight participants. However, the flight crewmembers will experience most of the deceleration forces in the +Gz axis. The wings rotate to a feather position to increase stability and drag for entry. At 80,000 ft (24,380 m),, the glide phase will begin with a return to an unpowered horizontal runway landing that will occur after a glide of 25 min. Total flight duration will be 150 min. Radiation levels at high altitude would be 15 micro Sv/hr for less than 30 min. The noise environment and vibration forces to be experienced are still uncharacterized. Virtually every data point (altitudes, duration of various portions of the flight, speed, peak g forces, etc.) will in reality be ranges of values and not absolutes as stated above. The information is based on estimates from the SpaceShipOne flights with extrapolation to SpaceShipTwo. Until test flights of SpaceShipTwo are much further along the exact parameters will not be known. Other flight profiles that are used by other commercial space flight operators may have medical issues different than described in this paper. For example, XCOR with its Lynx spaceplane has one pilot and one space flight participant, both wearing pressure suits. They will make as many as four flights daily to 100km. Boost is projected to be more controllable and the occupants will experience +4Gx and four minutes of 0g.
Space flight exposes individuals to an environment that is far more hazardous than what is experienced by personnel that fly on current airline transports. Pre-existing medical conditions can be aggravated or exacerbated by exposure to stressors such as acceleration and microgravity. Most of the medical issues for suborbital space flight are relatively straight forward as compared to orbital space flight. The short duration of suborbital flights eliminates any concern for most of the medical problems associated with orbital flight such as deconditioning, fluid shifts, and acclimation to weightlessness or re-acclimation on return to Earth. There is also a large amount of experience and a large medical database concerning orbital space flight. It would be easy to conclude that the medical risks of suborbital space flight are well known and would be similar to orbital space flight, but less significant or less intense. However, the orbital space flight database is based upon medical standards for astronaut selection and certification that are very restrictive. Commercial suborbital flight crewmembers, under current regulations, will only be required to have a FAA second-class medical certification. Also, a critical aspect of suborbital space flight is the rapid change from the high g acceleration launch forces to 0 g weightlessness followed quickly by the high g deceleration of entry. These transitions could lead to both cardiovascular and neurovestibular effects that are currently unexplored. There is no way to completely simulate these forces and this total environment pre-flight except for the actual experience of suborbital space flight. Although the acceleration and deceleration forces can be simulated with centrifuge runs, the longest period of weightlessness that can be simulated with parabolic flight is only 25 sec. This amount of time is not sufficient for complete neurovestibular and cardiovascular reflex changes to occur, which upon entry into a deceleration environment may impact compensatory processes. More importantly, the total environment of acceleration-weightlessness-deceleration has never been simulated (except for the brief +1.8 g to 0 g to +1.8 g experienced in parabolic flights) and is quite different from the orbital space flight experience. It is important to emphasize again that the operational experience of manned suborbital space flight is very limited. The pilot experience on suborbital flights will be very time intense and probably repetitive with some pilots flying daily. The effects of repetitive exposures to the physiological stresses of suborbital flight have never been experienced.
Significant medical concerns exist with the application of sustained gravitoinertial forces (Gs) to the human body as a consequence of space vehicle launch acceleration or entry deceleration. Neurovestibular, cardiovascular and musculoskeletal problems are the primary health concerns associated with inflight acceleration exposure, with head-to-foot (“eyeballs down” or +Gz) acceleration causing the most harm. However, exposure to either +Gx or +Gz can have an impact on pulmonary function proportionally to its applied force magnitude by altering ventilation/perfusion ratios resulting in hypoxemia, airway closure, and atelectasis. To avoid the potential for compromising cardiovascular and neurological function, acceleration forces are preferably applied in the +Gx direction (eyeballs in). An individual is more tolerant to +Gx acceleration, and with the heart and brain located at approximately the same level within the acceleration field there is less risk for acceleration-induced loss of consciousness (G-LOC) or impaired cognitive performance with almost loss of consciousness (A-LOC). Acceleration stress is also known to be dysrhythmogenic (can cause changes in cardiac rate, rhythm, and conduction). Higher g forces or longer exposures to acceleration could potentially increase the frequency of dysrhythmias. As long as the head, neck and spine are stabilized before the acceleration exposure and remain so until the exposure is completed, the potential for musculoskeletal injury is markedly reduced (7).
Historically, space flight accelerations have been designed to be in the +Gx axis, until the Shuttle entry experience which is +Gz. The early Mercury, Gemini and Apollo flights had launch accelerations of 4.5 to 6.5 +Gx for 6 minutes and anywhere from 6-11 +Gx during entry. The Shuttle has a maximum of 3.0 +Gx during the 8.5 minute launch and 1.2 +Gz (briefly 2.0 +Gz during turns) for 17 minutes during entry (8). On entry, the astronauts are in a deconditioned state due to the long duration microgravity exposure and yet are required to maintain a high performance level in order to fly the Shuttle in for landing. For countermeasures they use fluid loading and anti-G suit protection which is mandatory for the commander and pilot.
The human response to sustained acceleration in the +Gx orientation has been well known for decades. These acceleration forces are very well tolerated as the hydrostatic fluid column is very short and so cerebral perfusion is well maintained. Sustained +Gx acceleration does increase the work of breathing (which is doubled at 4 +Gx) and does lead to some ventilation/perfusion mismatching, potentially leading to mild hypoxemia. For this reason, space vehicles are designed to keep as much of the acceleration forces in the +Gx axis as possible. Early in the U.S. space program (Mercury and Gemini), astronauts received 45 hours of +Gx centrifuge training, with some runs going up to 18 +Gx. This was later deleted as no medical or performance issues were discovered with the normal in-flight acceleration profiles experienced (+6.5 Gx for 6 minutes).
An individual’s tolerance to +Gz acceleration is dependent on the individual’s anatomic (height and weight) and physiologic characteristics and the nature of the acceleration profile. Conditioning, hydration, previous and recent exposure to +Gz forces, and recent centrifuge training all have the ability to influence the physiological response. The maximum +Gz level, exposure duration and the rate of onset of the +Gz are all important determinants of the risk of neurological compromise, cardiac rhythm disturbances and musculoskeletal (especially neck) injury. Rapid-onset rate (ROR) of acceleration is defined as increases greater than 0.33 G/sec. ROR tolerance limits are approximately 1 +Gz lower than Gradual-onset rate (GOR) tolerances. ROR tolerances are lower because they exceed the ability of the cardiovascular system to fully respond to preserve adequate central nervous system blood flow. The cerebral hypoxia reserve time is 4-6 sec, while the baroreceptor reflex (an increase in sympathetic tone resulting in increased heart rate, cardiac stroke volume and total peripheral resistance) takes 6-9 sec to initiate with restoration of blood pressure requiring up to 10-15 sec. Rapid-onset rates can also result in G-LOC without any of the usual visual warning symptoms (such as tunnel vision, gray-out or black-out). Anti-G suits increase the tolerance to +Gz by approximately 1 to 1.5 +Gz (7) by increasing the total peripheral resistance, shortening vascular column height, and increasing the venous return to the heart. Anti-G straining maneuvers can increase the tolerance to +Gz by as much as 3 +Gz by increasing intrathoracic arterial pressure, but is fatiguing and is generally used only for a relatively short period of time. Centrifuge data has allowed for the development of a model of +Gz tolerance limits which incorporate +Gz magnitude, duration, and rate of onset which is called the Stoll curve after the investigator who first described it in 1956 (9). Conservative relaxed, unprotected tolerance (no visual or performance dysfunction) of completely healthy humans to +Gz acceleration is considered to be approximately +3Gz (normal range 3.1 to 4.0) for rapid-onset profiles and increases to approximately +4.5Gz (normal range 3.7 to 5.6) with gradual-onset profiles.
A pilot experiencing -Gz (such as when flying an outside loop or to a lesser extent when in microgravity) will be in a state of enhanced parasympathetic tone after several seconds of exposure which results in bradycardia, decreased cardiac contractility, and decreased total peripheral resistance. Transition to +Gz can then cause a profound drop in cerebral blood pressure that may take 8-10 sec to compensate. +Gz tolerance is greatly decreased resulting in a shift of the Stoll Gz tolerance curve. The Stoll curve is based on prior exposure to +1 Gz and so is an inadequate model that overestimates +Gz tolerance when there is a prior exposure to a relative -Gz . The loss of +Gz tolerance has been estimated to be about 1.27 Gz, but some individuals have shown a loss as high as 3.9 +Gz. This "push-pull effect" occurs often in combat engagements and has been implicated in several combat training fatalities. It has also been identified as a possible cause of 30% of G-LOC events. The key factor is in performing a series of –Gz aerobatic maneuvers, and thus the effect can occur in any such aerobatic flight profile. There exists a knowledge gap in the complete understanding of this issue and no known countermeasures have been developed (10, 11). It is unclear if a "push-pull effect" will occur in transition from microgravity to entry deceleration, but it has been described in parabolic flight (11) and concerns have been expressed that it could occur in suborbital flights (7). The push-pull effect is prolonged with increasing the duration of the prior -Gz exposure (12). Normally, the -Gz exposure is only several seconds in combat or aerobatic flight. In typical parabolic flight profiles, the exposure is 20-30 seconds. It is simply not known if 4 minutes of microgravity would elicit the same response or further deterioration in the +Gz tolerance. It would be predicted that any effect would be transitory as the recovery period from the push-pull effect is typically less than 15 sec.
The conservative acceleration envelope recommended by the IAA for commercial aerospace vehicles (2) should not exceed +3Gz (-2Gz), ±6Gx and ±1Gy. These levels, if experienced as gradual onset (increases of less than 0.25 +G/sec), should be well tolerated by unprotected, relaxed healthy individuals. During the rocket engine boost of SpaceShipTwo, acceleration may peak as high as 3.8 +Gx and there will be a brief spike up to +3.8 Gz as the space vehicle rotates to a nose high attitude. On reentry, 6 g’s will be imposed predominantly in the +Gz-axis for flight crew members. Because of tilt-back seating and the flight profile, most of the acceleration during entry will be in the Gx axis for space flight participants. Duration of these g forces are expected to be no longer than 70 seconds on launch and 30 sec on reentry. The onset rate of the accelerative forces has not yet been defined, but is not expected to be a rapid onset rate (ROR is defined as greater than 0.33 G/sec). There are currently no plans to utilize anti-G suits similar to the Shuttle pilots during reentry on these flights, but could be considered for the pilots as the cost is minimal and a beneficial effect is possible. Recent centrifuge or other G-training modalities would also mitigate any difficulties in the adaptation to acceleration forces. Avoidance of any appreciable dehydration among the flight crew members is of key importance to avoid decrements in G-tolerance ,especially given the fact that many of the proposed and planned launch sites are in hot desert environments.
The physiological changes resulting from exposure to microgravity depend upon the total duration of the exposure, and can vary in magnitude from individual to individual. While the microgravity exposure will last only a short duration of four minutes, it is possible that inexperienced, non-adapted, or overly sensitive individuals might experience symptoms (neurovestibular or cardiovascular) associated with even short exposures to the space environment. Although no proof exists, parabolic flight experience might be a way to mitigate future suborbital flight symptoms by providing weightless experience and possibly serve to identify especially susceptible individuals. This rapid launch acceleration - weightlessness - entry deceleration profile can not be tested or simulated in continuity. The acceleration profile segments can be simulated on the centrifuge, but the closest analogue to the four minute weightlessness period is 25 sec of parabolic flight. More importantly, the total flight profile with the rapid changes from one environment to the next can not be reproduced. There is also only minimal operational experience with this flight profile in the Mercury-Redstone, the X-15 flights, and the recent SpaceShipOne flights.
An increase in central venous pressure (CVP) is initially seen in Shuttle astronauts while they are lying on their backs in preparation for launch. This is followed by a decrease in CVP to below normal levels on first reaching microgravity (13). This is surprising as the physiological prediction would be for an increase in CVP due to the shifting of body fluids cephalad in weightlessness (14). Several explanations exist; that this is compensatory for being slightly head down during the pre-launch period, possibly due to a relative state of dehydration during the pre-launch period, a reduction in intrathoracic pressure, loss of gravitational compression of the heart, or due to a change in microgravity in the pulmonary capacitance and peripheral resistance (15). In orbital flight, cephalad fluid shifting due to the loss of the hydrostatic gradient occurs immediately and a sensation of head fullness and facial edema occurs within minutes (16). However, post-flight orthostatic cardiovascular changes and urinary diuresis were not present on the early short duration orbital Mercury flights (17) and were only first noticed on the nine-hour flight of MA-8 (Schirra) and the 34-hour flight of MA-9 (Cooper). A decrease in plasma volume due to diuresis does occur, but over the next several days in orbital flight due to this cephalad fluid shift. In combination with cardiac deconditioning and blunting of the baroreflexor response, this results in increased risk of orthostatic intolerance after landing from an orbital flight.
All of these last physiological changes require time to develop in microgravity and would not be expected in suborbital flight. In addition, the SpaceShipTwo crew will not have a prolonged period of pre-launch horizontal position and should have even less shifting of fluids as compared to the Shuttle experience. The use of an anti G-Suit is mandatory for the Shuttle commander and pilot on entry and is an effective countermeasure against in-flight orthostatic hypotension. Although post-flight orthostatic hypotension should be minimal on suborbital flights, the risk of orthostatic hypotension during entry may be quite real. The enhanced parasympathetic tone that occurs after several seconds of exposure to –Gz leads to bradycardia, diminished cardiac contractility, and peripheral vasodilatation. This response increases the risk of a fall in head-level blood pressure on re-exposure to +Gz. A full compensatory response can take 8 to 10 seconds with the recovery period dependent on both duration and magnitude of relative –Gz. Given that the period of hypoxia latency for brain cells is 4 to 6 seconds (40), the risk for +Gz related symptoms is enhanced at lower than expected +Gz levels. There is no data on which to assess the risk of this push-pull effect after 4 to 6 minutes of micro gravity, but it may be prudent for pilots to use anti G-suits to countermeasure during the +6 Gz entry profile.
Although the neurovestibular effects of prolonged microgravity are well known, these prolonged adaptive changes are not considered a significant factor in that the exposure to microgravity will be less than five minutes duration for each suborbital flight. Neurovestibular dysfunction after orbital flight includes an altered ability to sense tilt and roll, defects in postural stability, impaired gaze control, and changes in sensory integration (18). These changes are dependent on the duration of weightlessness. However, there have been neurovestibular alterations observed in even short exposures to altered gravity environments in susceptible individuals. With rapidly changing gravitoinertial forces, compensatory eye movements may be inappropriate, leading to oculomotor dysfunction. Maintaining a "dual-adaptive" state by virtual reality based (see below under Space Motion Sickness) or centrifuged based training has been suggested to mitigate these effects or to attempt to identify susceptible individuals (19). However, there are only anecdotal reports that it is beneficial.
That pilot performance after brief exposure to 0 g and re-adaptation back to a hyper g environment (without the usual long period of adaptation as in orbital flights) could be degraded is of some concern. Somatogravic illusions with spatial disorientation had been reported on several of the high altitude X-15 flights. The total flight profile of rapid launch acceleration - weightlessness - entry deceleration profile with the rapid changes from one environment to the next can not be tested in continuity. There is also only minimal operational experience with this flight profile in the Mercury-Redstone and the X-15 flights followed by the SpaceShipOne more recent flights. An additional concern is that many pilots will be flying these suborbital profiles repeatedly and maybe on a daily basis. There is not any experience that indicates whether repeated and frequent suborbital profile exposures will be adaptive or cummulatively maladaptative to neurovestibular function. Obviously, more experience in suborbital flight is needed and will better define if this is even an issue. In the initial phases of flying suborbital missions, post-flight medical debriefs and data collection would be helpful until more experience has been obtained and there is more confidence that there will not be any performance medical issues. Frequent flights by the same pilot would also be another reason for close medical monitoring initially as there is absolutely no experience with frequent daily suborbital flights by the same pilot.
X-15 Neurovestibular Experience
Three X-15s were built, flying 199 test flights, with the first one flown on June 8, 1959 and the last one on October 24, 1968. Twelve test pilots flew the X-15; among them were future NASA astronauts Neil Armstrong and Joe Engle. During the X-15 program, 13 of the flights (by eight pilots) met the USAF space flight criteria by exceeding an altitude of 50 miles (80.47 km, 264,000 ft), thus qualifying the pilots for U.S. astronaut status. Of all the X-15 missions, only two flights (in July and August 1963, both piloted by Joe Walker) qualified as space flights per the international (Fédération Aéronautique Internationale) definition of a space flight by exceeding an altitude of 100 km (62.137 mi, 328,084 ft). Flight 90 on July 19, 1963 reached 105.9 km (65.8 mi, 347,440 ft) and Flight 91 on August 22, 1963 reached 107.8 km (67.0 mi, 354,200 ft). Physiological parameters were not measured on the X-15 flights, but pre-flight and post-flight flight surgeon examinations were never reported as abnormal. The best performance data possible; the requirement to fly the demanding X-15 aircraft showed that a high degree of pilot performance was obtained. Importantly, pilot performance was not impaired by launch acceleration - weightlessness - entry deacceleration. The disturbing exception to this was the crash of X-15-3 on November 15, 1967 on X-15 Flight 191 which killed the pilot, Major Michael J. Adams. This was due to a combination of system anomalies and pilot errors including display misinterpretation, distraction, vertigo, and loss of situational awareness. Pilot overload due to his attention being focused on troubleshooting the science payload was also a factor. The various g forces imposed on the pilot during the boost phase of the flight were very conducive to severe vertigo. Every X-15 pilot experienced this disorientation and sensed that he had over-rotated his climb angle. Mike Adams had reported severe vertigo during several of his X-15 flights during the boost phase. The accident investigation conducted after Mike Adams’ fatal X-15 flight concluded that the pilot suffered severe vertigo during climb-out which caused spatial disorientation. Small heading deviations caused by a degraded flight control system were made worse by incorrect pilot inputs at an altitude of over 20 km (65,000 ft). The pilot misinterpreted a roll indication for a slide slip indication and made control inputs in the wrong direction. Most puzzling was Adams' complete lack of awareness of major heading deviations in spite of accurately functioning cockpit instrumentation. An extreme heading deviation of 90 degrees developed which led to a hypersonic spin. Although recovery from the spin was made, a control system oscillation developed which increased in magnitude and eventually caused aerodynamic breakup of the aircraft. As a result of the accident investigation, it was recommended that all future X-15 pilots be medically screened for labyrinth (vertigo) sensitivity. It was also noted that a fixed based simulator was adequate to prepare for flight as long as the pilot had been exposed to centrifuge simulation training (20).
Space Motion Sickness
Microgravity exposure results in space motion sickness in about 70% of astronauts flying on orbital space flights for the first time. It is thought to be due to a sensory conflict between visual, vestibular, and proprioceptive stimuli. Susceptibility cannot be predicted by susceptibility to ground-based motion sickness or pre-flight testing. Symptoms typically occur within the first 24 hrs. However, symptoms have been reported immediately after main engine cut off with dizziness, pallor, sweating, and severe nausea and vomiting. Vomiting can crescendo quite suddenly without any prodromal symptoms. In a multipassenger vehicle, one passenger becoming nauseated can potentially trigger nausea in the other vehicle occupants.
Prophylactic use of anti-motion sickness medications might be considered for space flight participants, but would adversely impair pilot performance. Space Shuttle flight crew members (commander, pilot, flight engineer) are not allowed to take prophylactic medications for space motion sickness (21). The risk of nausea in reduced gravity is significantly abated if provocative motions, especially of the head, are avoided. Head movements generate conflicts between the semicircular canals and the otoliths. Pitch head movements are the most provocative (22). Intense concentration on task performance is also attenuating. Parabolic flight adaptation and experience in high performance jet aircraft do not appear to be protective. In the Russian program, vestibular training using Coriolis accelerations (rotating chair) is still used. However, it does not duplicate the sensory conflicts found in space motion sickness and there is no evidence that it has decreased the incidence. Space motion sickness might be reduced by pre-flight adaptation training in an attempt to make pilots "dual-adapted." One study has found a 33% decrease in the incidence of space motion sickness with this technique (23). Some examples of the training aids used in this effort to duplicate sensory conflict such as occurs in microgravity are the device for orientation and motion environment (DOME) which is a spherical virtual reality simulator and the tilt-translation device (TTD).
During suborbital flights, the risk will be reduced if flight crewmembers remain tightly strapped into their seats during the flight and limit head movements. However, suborbital flights may result in a novel manifestation of motion sickness, analagous to that sometimes experienced during parabolic flight. This phenomenon is well known and most people adapt to this after several exposures to parabolic flight.
Post-flight Medical Problems
The most likely post-flight medical issues to be expected involve the nervous system and sensory organs (including motion sickness, vestibular disturbances, vertigo, and postural instability), and post-landing orthostatic intolerance. As all of these problems are very dependent on the duration of time spent in weightlessness, it is predicted that they will not be issues for suborbital flight unless in-flight motion sickness has occured. Medical debriefs post-flight are highly recommended, not only for collection of critical medical data, but also for the resolution and follow up of any health issues resulting from space flight.
Entry Motion Sickness
Entry motion sickness can occur on return from an orbital space flight, and can be severe following long duration missions. It is less frequent and less severe on shorter duration Shuttle flights. It is a concern because it would adversely affect the ability of a pilot to control a complex vehicle during entry and landing. It could also impair the ability of any crewmember to perform an emergency egress after landing (21). We anticipate that entry motion sickness will not likely be a significant issue on very short duration suborbital space flights.
Emergency Egress Capability
The major risk to health and safety of passengers and crew are launch and landing accidents, and emergency egress capability from a survivable accident will be an important consideration. It is an operational assumption that crewmembers will be capable of performing an emergency evacuation without assistance. Five to fifteen percent of Shuttle astronauts were judged to be too impaired post-landing to perform an unaided egress (19). This was due to a combination of entry motion sickness, post-flight neurological dysfunction, and post-flight orthostatic intolerance. However, as all of these problems are dependent on the duration time of weightlessness, they should not present themselves as frequently following suborbital flights, unless persistent motion sickness were to be present in any given crewmember. It is recommended that emergency egress training be performed as this would be a mitigating factor in maintaining performance even if an individual were symptomatic.
Environment Medical Issues
Spacecraft Cabin Environment
Cabin temperature and humidity will vary depending on the vehicle design. In most orbital space flight vehicles, the cabin temperature is typically 21-26°C (70 -79°F) with a relative humidity of 30-40%. Inappropriate control or a malfunction of the cabin heating, air circulation, and/or cooling systems could result in an uncomfortable cabin environment that affects cognitive and psychomotor performance. Space motion sickness is known to be exacerbated by over-heating. Cabin pressure also may vary depending upon the design of the space vehicle. In current orbital space flight vehicles, the cabin pressure is maintained at a sea level pressure of 14.7 psi (101 kPa), unless it is decreased for a specific reason such as EVA pre-breathing. This allows for essentially a shirt-sleeved environment. Airline transport aircraft are designed to maintain a cabin altitude below 8,000 ft (2400 m) while flying at their operational altitude. Ear and sinus blocks are possible with rapid changes in cabin pressure. Suborbital space vehicles will operate at such high altitudes that there is a potential risk for an inflight decompression (rapid or explosive) to very low or even absent atmospheric pressures. Such an exposure could result in hypoxia or even death (due to either hypoxia or ebullism) among the occupants. Pressure suit use could be chosen by individual commercial space flight operators as an additional safety option for mitigation of the risk of cabin depressurization hazard. There are disadvantages to the use of a pressure suit, including weight, expense, thermal loading, and decreased pilot performance. It is noted that a pressure suit was not used on the SpaceShipOne flights. However, without a pressure suit the crew is absolutely reliant on cabin integrity being mantained as there is no redundancy and depressurization would be a catastrophic event. Historically, there have been two periods where a pressure suit was not required for orbital flights. In the Soviet space program, the Voskhod and the early Soyuz flights did not have a pressure suit due to extreme weight and volume limitations. This was changed after the three cosmonauts died on Soyuz 11; which is described as a space craft malfunction causing depressurization, but in reality was also a program failure to provide pressure suits as a backup. In the US program, the pre-Challenger Shuttle flights did not have the Launch and Entry Suit (LES), which was not developed until after a recommendation from the Challenger Accident Investigation Board. Physiologic training using an altitude chamber should be utilized to mitigate the hazards of a partial depressurization event as it results in better recognition with a more rapid response to hypoxia and depressurization.
The cabin atmosphere composition (O2 and CO2) will also need to be controlled within safe levels realizing that cabin designs may be variable and may incorporate either an open or a closed loop system. Fire detection, prevention, and suppression will be designed into the vehicle and could limit the maximum O2 concentration. A redundant backup O2 supply will probably be available. Air circulation in the cabin must account for avoidance of CO2 accumulation about the head of immobile, seated crew and passengers so as to not impact performance.
Ionizing radiation consists of subatomic particles that can interact with biological tissues and cause genetic damage that can lead to cellular death or dangerous mutations. The sources of ionizing radiation in space are galactic cosmic radiation, solar radiation, solar flares and the trapped radiation from the Van Allen belts. Galactic cosmic radiation is omnidirectional and originates outside of the solar system. It consists of hydrogen nuclei protons (87%), helium nuclei alpha particles (12%), and high energy heavy nuclei such as iron and lithium (1%). Solar cosmic radiation is a proton-electron plasma ejected from the surface of the sun at very high velocities that varies in magnitude according to the sun’s 11-year activity cycle. Solar flares are magnetic disturbances on the sun’s surface generating electromagnetic radiation and high-energy protons that result in solar particle events (SPEs). The Van Allen belts contain trapped protons, heavy ions and electrons. These magnetically trapped high-energy particles can also produce significant levels of radiation. Protection from cosmic radiation for the Earth’s inhabitants is provided by three variables; the sun’s magnetic field and solar wind (solar cycle dependent), the Earth’s magnetic field (latitude dependant), and most importantly, the Earth’s atmosphere (altitude dependant). There is a rapid increase in the radiation dose as the altitude increases due to reduced atmospheric shielding.
The applicable radiation dose standard for radiation exposed workers is 20 mSv/yr (averaged over 5 years). Exposure to 20 mSv/yr over a work life of 40 years results in an excess lifetime fatal cancer risk of 3.2%. Concerns have been expressed over increased rates of breast cancer, thyroid cancer, leukemia, and cataract formation in people exposed to even low doses of ionizing radiation. However, no studies (24) have shown a statistical increase in any of these diseases at the limits described above for occupationally exposed workers (20 mSv/yr). NASA astronauts have established monthly, 1-year, and career exposure limits based upon a maximum of 3% excess lifetime cancer mortality (25). These limits are recommended by the National Council on Radiation Protection or NCRP (26). The recommended maximum limits were decreased further on the more recent 2000 NCRP report (27). These are individually adjusted, since younger age and female sex are at an increased risk. The ten year career effective dose limits are 0.4-3.0 Sv depending upon sex and age (28). Planned exposures to radiation (such as during an EVA or passage through the South Atlantic Anomaly) are kept as low as reasonably achievable (ALARA principle). Orbital space flight results in an extremely variable radiation dose exposure dependent on orbital altitude and solar activity and ranges from 0.01 - 0.1 Sv/month.
Radiation levels at an altitude of 350,000 ft would be similar to high altitude Concorde flights (there is minimal additional protective effect of the atmosphere above 60,000 ft) and, therefore, should be less than 15 microSv/hr (24, 29) for a total duration of less than 30 mins. The occupational exposure limit recommended by the International Commission on Radiological Protection (ICRP) for commercial aircrews such as on the Concorde supersonic transport is 20 mSv per year, averaged over 5 years with a maximum in any one year of 50 mSv (30, 31). This is in contrast to the ICRP recommendation for the general public to be less than 1 mSv/yr. Professional aircrews are considered to be occupationally exposed and employers have a duty of care to conform with the ICRP recommendation, even if their particular national authority does not have appropriate regulation.
For the most part, there is no concern regarding the acute effects of ionizing radiation because of the short duration of the flight and the fact that launch can be controlled depending upon atmospheric conditions. However all flight crewmembers should be required to wear personal dosimeters to track an individual's accumulated dose for each mission, as do radiation workers and medical imaging personnel, to ensure compliance with OSHA standards. Radiation exposure during pregnancy could have significant adverse effects on the developing fetus and should be avoided. The U.S. NCRP recommends that the total radiation dose received by a pregnant woman not exceed 5 mSv during the entire pregnancy, while the ICRP recommends at total dose during pregnancy not to exceed 1 mSv. Over one hundred suborbital flights would still result in exposure below this level.
The intense combustion and powerful thrust required to launch a vehicle into a suborbital space flight generates a large amount of noise which is transmitted through the whole vehicle. As a space craft is an enclosed space, the noise is reflected multiple times off the walls, floor and ceiling. These noise levels are of short duration but can be quite intense. The physiological effects of extreme acute noise (unprotected) is reduced visual acuity, vertigo, nausea, disorientation, ear pain, headache, temporary hearing threshhold shift, and degradation in pilot performance. Loud noise can also interfere with normal speech, making it difficult to understand verbal communication and affecting team interaction. Noise is a distraction and can increase the number of errors in any given task, but especially in tasks requiring multiple information sources, information processing, and vigilance (32, 33). Noise levels in the crew compartment during a Shuttle launch reach close to 120 dB. Some of the space vehicles being proposed will generate loud noise levels for brief time periods although the exact decibel level is not currently known. NASA had set a goal of a noise level of less than 105 dB for the Constellation Program (34, 35). Auditory protection will be required during suborbital space flight launch by the crew (by helmet or headset) to prevent sensorineural hearing loss (permanent threshhold shift) and to facilitate communication. Hearing standards for pilots should be congruent with the current FAA hearing medical standards for all classes (Audiometric speech discrimination test: Unaided discrimination of pure tones with thresholds in the worse ear no worse than - 35 dB at 500 Hz, 50 dB at 1,000 Hz, 50 dB at 2,000 Hz, and 60 dB at 3,000 Hz, conversational voice test).
Vibration is oscillatory motion in a dynamic system (such as the human body) and is characterized by frequency, amplitude, resonance, direction, spectrum and duration. Most aerospace vibration exposures remain well below injury levels. The vibration associated with launch and aerodynamic loading of a space vehicle, however, can be significantly greater than standard aircraft operations. Minimal tolerance occurs between the frequencies of 4-8 Hz (due to whole body resonance). Symptoms commonly elicited to vibrations include general discomfort, fatigue, headache, and back pain. Cardiopulmonary response to vibration in the 2-12 Hz range is similar to aerobic exercise. Manual tracking errors increase in the 2-16 Hz range causing impaired psychomotor coordination. Compensatory eye movement is a physical response to vibration and affects visual performance. Blurred vision may occur at high frequencies. Transient vibrational loads of >0.5G for < 1 minute, especially at critical frequencies or in the Gz axis, sudden onset of vibrational experiences, and cumulative vibration loads of longer duration can interfere with the ability of the pilot to visually track displays, maintain situational awareness and could interfere with pilot performance (36). This was a transient problem on Mercury-Redstone 3. Vibration was also noted on the in-cabin videos of several of the SpaceShipOne flights during both ascent and entry. SpaceShipOne Flight 16P experienced significant thrust oscillations at 5-10 Hz towards the end of the two phase flow portion of the boost which produced an impressive amount of vibration with the pilot's head being slammed against his headrest for several seconds as seen on the in-cabin video.
Standards for whole body vibration are published by the American National Standards Institute (37) and the International Standards Organization (38) and are based upon frequency, amplitude and duration. These do not address the more complex issue of pilot performance. Mitigation strategies for reducing vibration would be to aggressively decrease vibration in the design of the vehicle, isolate the pilot by seat design, and the use of a helmet to isolate the head which has been shown to improve display reading performance and vibrational tolerance (39).
Many gaps in knowledge remain concerning the medical issues discussed in this paper. The effects from acceleration, cardiovascular and neurovestibular microgravity effects, space motion sickness, ionizing radiation, noise, vibration, spacecraft environment, and post-landing performance are unlikely to be obstructions or impediments to the flight crew of commercial suborbital space flights that are similar to the anticipated flight profile of Virgin Galactic's SpaceShipTwo. Flight profiles other than SpaceShipTwo will probably be similar, but may have different medical issues. However, without an evidence base on which to draw, it is prudent to incorporate a vigilant observation process to expand our knowledge base, fill in the gaps in knowledge, and adjust flight crew training and medical standards as necessary.
We would suggest the following recommendations for operationally critical flight crewmembers participating in suborbital space flight:
A FAA first-class medical certificate using the same age-based schedule as is required for ATP pilots. An FAA first-class medical certification (as recommended by the AsMA position paper (5) instead of the current FAA requirement for an FAA second-class certification) differs from a second-class only in that it requires an EKG and has to be renewed every 6 instead of 12 months over the age of 40.
Pre-flight medical evaluations would be beneficial in the very early developmental flights to reduce risk and liability if any unpredicted medical issues occur.
Post-flight medical debriefs with data collection, especially in the early stages of suborbital space flight experience.
Consideration should be given to establishing an independent data repository of medical findings to enable analysis of findings and periodic reevaluation of medical standards with recommendations for changes to respond to medical issues that may be discovered.
Periodic reevaluation of the current medical standards during the early stages of developmental flights to respond to any medical issues that may be discovered.
Passive ionizing radiation dosimeters worn by each flight crewmember.
Auditory protection in the helmet or headset for all crew members.
Emergency egress training for all crewmembers.
Physiologic training (altitude chamber) to ensure flight crew recognition of signs and symptoms associated with decompression including hypoxic changes.
Recent centrifuge or other G-training may be beneficial if there is significant (> +3) Gz acceleration forces in the flight and the flight crewmembers do not have adequate +Gz training in other environments.
Anti-G suit use on early flights until more experience has been obtained as there will be significant (>3) +Gz acceleration forces in the flight profile and deterioration of +Gz tolerance may occur due to the "push-pull effect" after several minutes of 0g. There is no data concerning +Gz tolerance following four minutes of 0g.
Parabolic flight training may be beneficial as it does provide some experience to the acceleration-weightlessness-deceleration environment, although no studies have shown that it contributes to establishing a "dual adaptive" state. Some personnel have experienced motion sickness with the initial exposure to parabolic flight, but develop tolerance with adaptation to the changing gravitational fields.
Pressure suit use may be adopted by some commercial space flight operators as it would be beneficial in the case of failure of the pressurized vehicle. Without a pressure suit the crew is absolutely reliant on cabin integrity being mantained as there is no redundancy and depressurization would be a catastrophic event.
Further investigation should be conducted on the effects on pilot performance from the rapid changes in the acceleration - microgravity - entry deceleration flight profile as this can not be simulated or trained for and there is little operational experience. Especially of concern is the impact on an individual involved with repetitive flights. Current data suggests that this may be well tolerated, but only actual flight experience will show if this is actually true.
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The definitions of suborbital rocket flight is the criteria used by the FAA to differentiate civil aircraft subject to aircraft certification from a suborbital rocket launch subject to licensing under the Commercial Space Launch Act (49 U.S.C. Subtitle IX, chapter 701).
‘Suborbital rocket’ means a vehicle, rocket-propelled in whole or in part, intended for flight on a suborbital trajectory, and the thrust of which is greater than its lift for the majority of the rocket-powered portion of its ascent.
The internatinal definition (Fédération Aéronautique Internationale) of space flight is an altitude of 100 km, while the FAA and USAF definition of space flight is an altitude of 50 miles.
Suborbital rocket flights by vehicle:
Vehicle Suborbital Altitude Altitude Altitude
trajectory > 60,000 ft > 50 miles > 100 km
NF-104A 302 302 0 0
X-15 (XLR-99) 146 143 13 2
Trident II 100 98 0 0
Trident IISE 96 94 0 0
F-84G ZELMAL 28 0 0 0
X-15 (XLR-11) 28 8 0 0
SM-30 ZELL 26 0 0 0
Trident I 25 0 0 0
X-24B 24 17 0 0
X-15A2 22 21 0 0
M2-F3 22 22 0 0
HL-10 20 14 0 0
X-24A 18 9 0 0
F-100D ZEL 18 0 0 0
X-2 13 8 0 0
F-104G ZLL 13 0 0 0
SpaceShipOne 6 6 3 3
Mercury 2 2 2 2
Ba 349 Natter 1 0 0 0
Soyuz 18a 1 1 1 1
Total 911 745 19 8