57th International Astronautical Congress, Paper IAC-07-A5.2.04

Beyond astronaut’s capabilities: a critical reviewLuca Rossini†‡, Tobias Seidl†, Dario Izzo† and Leopold Summerer†

†ESA, Advanced Concepts Team, ESTEC Keplerlaan 1, Postbus 299, 2200 AG, Noordwijk‡Universita Campus Biomedico, Rome, Italy

contact: luca.rossini@esa.int, tobias.seidl@esa.int, leopold.summerer@esa.int, dario.izzo@esa.int

Abstract

Prolonged human presence in space has been studied extensively only in Earth orbiting spacestations. Manned missions beyond Earth’s orbit, require addressing further challenges: e.g. distancesexclude effective tele-operation; travel times, distances and the absence of safe abort and return optionsadd physiological stress; travel times require novel closed-cycle life support systems; robotic extrave-hicular activities require the development of hardware for semiautonomous exploratory, inspection andmaintenance tasks, partly tele-controlled by human operators inside the spacecraft. These few exam-ples suggest that if the endeavour of interplanetary manned space flight has to become a realistic futurepossibility, the technological support to astronauts will need to be substantially developed.

This paper critically reviews the current scientific maturity of a number of diverse and sometimecontroversial visions of possible solutions, and at the same time attempts to provide an overview onsome new key technologies potentially able to enhance astronauts capabilities. The status of researchon induced hypometabolic states is introduced together with the evaluation of its potential impactto space travel. Motor anticipatory interfaces are discussed as novel means to enable teleoperation,cancelling command-signal delays. Research results on brain machine interfaces are then presented andtheir applicability for space is discussed. Finally, liquid ventilation is assessed as a technology possiblysuitable to extend the astronauts capabilities to withstand acceleration loads of significant magnitude.

The paper attempts to address the critical parts of each one of these concepts showing that, whilesometimes considered science fiction, in some cases a number of scientific results already allow to bemore optimistic and in other cases to locate at least the showstoppers that will need to be removed inorder to successfully develop the corresponding technology.

Introduction

Recently, space agencies renewed their interest inexploration programs. In the white paper ”TheGlobal Exploration Strategy: The Frameworkfor Coordination”, drafted and endorsed by ASI(Italy), BNSC (United Kingdom), CNES (France),CNSA (China), CSA (Canada), CSIRO (Aus-tralia), DLR (Germany), ESA (European SpaceAgency), ISRO (India), JAXA (Japan), KARI (Re-public of Korea), NASA (United States of Amer-ica), NSAU (Ukraine), Roscosmos (Russia).[1], itis mentioned that Space exploration is essential tohumanity’s future. It can help answer fundamen-tal questions such as: Where did we come from?What is our place in the universe? What is ourdestiny? It can bring nations together in a commoncause, reveal new knowledge, inspire young peopleand stimulate technical and commercial innovationon Earth.

Following this vision, the present paper tries to

assess the technological levels of some of our en-gineering solutions for the design of manned mis-sions of prolonged duration. Allowing astronautsto “live in a tin can” is only one of the issues thatengineers are confronted with. The limited capabil-ities in terms of mobility and coordination duringany Extra Vehicular Activity (EVA), microgravityrelated loss of physical performance, the long com-munication isolation times, the signal delays dur-ing tele-operations add up, making manned explo-ration missions very demanding.

Some of these issues are expected to be alle-viated by current technical development, and themanned exploration of the Moon or Mars appeartoday within the capabilities of an ambitious explo-ration programme. On the other hand, completelynew technological solutions will be needed for thepursuance of human exploration of the solar systemand beyond.

With this in mind, the Advanced Concepts Teamof the European Space Agency [2] started in 2003

to address the areas of bio-engineering and bio-mimetics to find potentially revolutionary technolo-gies and concepts and to assess their developmentstatus and their potential benefits for the space sec-tor. This paper discusses a few of sometimes con-sidered controversial concepts, with the objectiveof establishing their scientific grounds, of criticallyassessing their development status and of under-standing their potential impact on future explo-ration programmes.

First, the possibility of inducing a hy-pomethabolic state on humans as a mean of re-ducing psycho-physiological stress in long durationtravels is addressed, followed by the use of non-invasive brain machine interfaces and motion antic-ipatory interfaces as means to augment human ca-pabilities to tele-operate increasingly sophisticateddevices and finally to conclude by discussing theuse of liquid ventilation for water immersed astro-nauts as a mean to improve the astronaut responseto continous and impulsive high acceleration loads.

Induced hypomethabolic states

Motivation

When leaving Earth, humans have to cope withstress related to environmental conditions never be-fore experienced during their phylogeny [3]. Evenwith spacecraft offering the state of the art of envi-ronment control, both psychological and physiolog-ical issues arise. The most prominent of them re-gard social isolation, physical confinement, reducedafferent flow in the central nervous system (CNS),hypokinesia, loss of circadian rhythms, symptomsdue to increased radiation level, and loss of car-diopulmonary performance due to micro-gravity[4]. Furthermore, once in the space environment,it is rather difficult to discriminate the single ef-fects related with their specific stresses.

Therefore the study of the effects and their po-tential solutions is mainly addressed in Earth-basedexperiments. An example for such a terrestrialstudy on the effects of the space-environment onhuman physiology is the EXEMSI study [5]. It hasdemonstrated that prolonged isolation effects bodyweight, blood volume, the regulaton of hormonesrennin and aldosterone [6], and also the immunesystem [7]. On the other hand, a recent study on

crew members of ISS and MIR reported extremelylow levels of negative dysphoric, i.e. negative, emo-tions [8]. Having the “happiest employees” mostlikely results from the almost daily psychologicalsupport, euphoric attention from co-workers andthe public as well as regular communication withtheir families and friends. As a result, space trav-elling has even been reported to have positive emo-tional and salutary effects on most of its partici-pants [9, 10].

Obviously, the situation is rather different duringan interplanetary (and even more so hypotheticalinterstellar) travel as close to real-time communi-cation with ground control is impossible. Duringthese missions, the effects of isolation are likely tobe close to what was found in the EXEMSI study,unless they are counteracted by measures withinthe spacecraft. This will require far more and ex-tended facilities concerning psychological and phys-iological health onboard than hitherto establishedincluding a way of automated psychological sup-port to the individual [11, 12] with considerableconsequences on spacecraft design (e.g. 75m3 ofpressurised living space per astronaut for a mannedMars mission as assumed by a recent internalESA study [13]). A two years manned mission toMars with six crew members would lead to a masspenalty associated with providing life support anda suitable environment of approximately 40% of thetotal wet mass. Only for food stowage, the Equiv-alent System Mass (ESM) - a measure taking intoaccount both the quantity of consumable and theequipment required to maintain/deliver/manage it- is expected to be around 30 tonnes, not takinginto account the mass associated with water andatmosphere provision and waste management. Ittherefore seems evident that innovative solutionsfor the life support system (LSS) cost will be criticalfor the feasibility of missions involving long travels[14].

An approach to drastically reduce hu-man activity

During the time-consuming transfer to the finalgoal of an interplanetary mission, only minimal ifany human activity and control will be required.As a consequence it would be desirable to re-duce consumption of on-board resources duringthat time and somehow put the crew members “at

sleep”. Such behaviour can be observed in manyhot blooded animals including mammals, i.e. ani-mals related to humans, which hibernate in orderto minimize energy consumption during times of re-duced activity and food availability, i.e. winter [15].While poikilothermic (cold blooded) animals suchas fish or insects constantly adjust their body tem-perature to the ambient temperature, homoeother-mic (warm blooded) ones such as mammals haveto actively enter a state of reduced metabolism.This ability can be observed in different speciesbelonging to different families. Within the classof the mammalians, these range in size from theground squirrel (Spermophilus tridecemlineatus) tothe brown bear (Ursus arctos) [16].

If put into a hypomethabolic state, humanswould require less energy and food, they wouldproduce less “waste”, use less space, possibly faceless emotional stress by not being consciously facedwith its isolation, and finally could encounter a farreduced degradation in physical performance as itis usually observed during long times of inactiv-ity [4]. Furthermore, the reduced ventilation, heartrate, kidney filtration and CNS activity could makethe organism less sensitive to the deleterious effectsof microgravity; the radiation effects are expectedto be left unchanged instead.

If the positive effects of using hypomethabolicstates appear obvious, there are still several diffi-culties to be understood better and mastered. Oneof the big open challenges is to artificially inducehypomethabolic states in beings that don’t natu-rally enter into them.

First steps towards that goal have already beenachieved: so far it is possible to induce hiberna-tion in animals that are known as non-hibernatorsbut that belong to the same family as hibernators.The synthetic compound DADLE (Ala-(D)Leu-Enkephalin), once injected, induces hibernation bymimicking the action of the Hibernation Induc-tion Trigger (HIT), the natural hibernation triggermolecule in hibernators like ground squirrels, bats,and black bears [17]. The DADLE molecule is amodified form of enkephalin, a natural opiate usu-ally found in the brains of mammals. Opiates causeeffects similar to those observed during the mam-malian hibernation, like bradicardia, hypotension,respiratory suppression, and lowering of set-pointin thermoregulation. But there are still no proofsof hibernation on animals whose family never goes

in hibernation state. However, the uniformity ofDADLE as a trigger substance for several differ-ent taxa indicates that hibernation did not evolveparallel but has a uniform ancestor despite the in-dividual differences. This suggests that the geneticprogram for allowing initiating hibernation mightindeed be present in all mammals and only deacti-vated by a block of genetic transcription. Advancesin genetic therapy and a more profound knowledgeof the genome relevant for hibernation could en-able to genetically turn on the hibernation-programwhich then is triggered by treatment or even am-bient conditions. Support for this theory lies inthe fact that at least some of the genes associatedwith hibernation, such as those involved with fatmetabolism (PL and PDK-4) are already known tobe present in the human genome [18].

The human-hibernaculum

Obviously, during hibernation an automated con-trol of physiological parameters and the possibilityto actively de-hibernate an astronaut would be nec-essary. This would be realized with what Ayre etal. described as a human-hibernaculum [4]. Ayre etal. argue that as the hibernation technique is stillunknown, it is impossible to determine the exactrequirements placed on the human-hibernaculum,but it is possible to have an idea on how such adevice would look like assuming its main functionwould be to monitor and maintain a given corporealcondition.

Ayre et al. then proceed evaluating the hypo-thetical scheme of a space human hibernator and itsimpact on the global life support, which is reportedschematically in Figure 1. As hibernating beings donot eat, drink, urinate or defecate, the componentsof a typical life support system are downscaled andpartially omitted (see grey areas in Figure 1, takenfrom [4]).

For example, the atmosphere revitalisation com-ponent is still required to support the astronautsrespiration, which continues in astronauts in anhypometabolic state but with a breathing and ametabolic rate that are drastically lowered if com-pared with normal sleeping humans. This alsolowers the moisture transfer to the atmospherethrough respiration/perspiration, with associatedsmaller quantities of waste generated from the at-mospheric and water management functions. Then,

water management and waste processing compo-nents would be positively downscaled too. Theprincipal qualitative benefits in terms of payloadmass and complexity for long term missions appearobvious but have still to be quantified.

Figure 1: Human hibernation impact on a life sup-port system as taken from Ayre et al. [4]

Neuro-Inspired Interfaces

Motivation

The high versatility of the human motor coor-dination allows for a huge range of elaboratedbehaviours, not even related with “evolutionary”taks, such as dancing, playing soccer, doing acro-batics or playing music instruments. On the otherhand, one of the big limitations of human motorperformance is that it is strictly bound to the phys-ical conditions that govern our planet. Our percep-tion and planning of movement, as well as the per-ception of the external movement in our environ-ment, for example, is strictly related to the identi-fication of the gravity axe. Inevitably, our sensory-motor system will encounter a loss of performancein situations of changed or annihilated gravity [3].This loss is so important that, from certain perspec-tives, we can assume that astronauts are in a sim-ilar situation to people affected by motor disabili-ties [19]: both of them have, for opposite reasons,a deficit in the performances required to accom-plish their motor tasks. On Earth, disabled people

can take advantage of assistive systems, which aretechnologies designed to fill the gap between user’sresidual abilities and required ones [20, 21]. So,the answer to the reduction of physical and mentalability suffered by astronauts can be addressed (andit already partially is) with assistive technologies,once they are redesigned for functioning in space.

Interfacing Naturally

The required assistive technologies for astronautsare, essentially, robotic hands, arms, rovers, andso forth. Most of them are already technologicallywell developed, but are still lacking in operability.In fact, the reduction in motor coordination relatedwith weightlessness conditions affects also the useof any kind of physical interface [22]. Moreover,even when complex robots are operated by experi-enced users on Earth, the open-loop control in real-time has been proved several times to be inefficientwith the traditional physical interfaces [23].

The reason is that the interfaces are operatedby predefined user’s motor actions, and typicallydifferent action kinematics and geometries are as-sociated with different interfaces (i.e. computerkeyboards and mouse). The use of these inter-faces requires a re-modulation of connections be-tween the user’s brain motor areas involved withthe device utilization (i.e. the neuronal networksinvolved with the actual formulation of a sentence),and those dedicated to the motor task for the in-terface operation (i.e. the hand and fingers controlto obtain a correct typing of this sentence). Obvi-ously, operating a complex interface involves a highcognitive load which - when several different inter-faces have to be operated within a short period -can easily decrease the operator’s mental perfor-mance.

In consequence, there is a tendency to suggestthe adoption of natural interfaces aiming to makethe communication and control of the device easierfor the user, and allow him to fully focus on the taskinstead of on the interface use [24]. A natural inter-face, with respect to the traditional ones, exploitsuser’s natural communicative channels and henceallows intuitive and universal use without elongatedtraining sessions [25]. The commonly studied natu-ral interfaces integrate speech recognition, gesturerecognition, facial expression recognition, and gazetracking. Even though these interfaces have the

potential to allow the control of complex systems,their usefulness for severely disabled people and - inan analogue way - for astronauts is arguable. Thegesture and facial expression recognition cannot beefficient if the impaired user is not able to performthem precisely and speech recognition faces prob-lems due to background noise (64 dBA for the airconditioning to 100 dBA for some vent relief valvesin case of space stations)[3, 26, 27]. Gaze trackingalone is not enough precise and rich in contents topermit the control of complex systems.

Interfacing the brain

A new approach for control interfaces is based onthe prediction of user’s motor intentions. These in-tentions are not at all related with the user’s abil-ities but can be read and translated in machineactions via a feedback loop. There are two basicapproaches to “read” a user’s motor intention.

One is the monitoring of involuntary movementsconstantly proceeding in time the intentional fi-nal movements [28, 29, 30] using a so called mo-tor anticipatory interface (MAI). Another and moreknown technique is the identification of the user in-tention as it naturally origins in the brain, by usingbrain-machine interfaces (BMI) [31, 32, 33].

Brain Machine Interfaces

With brain-machine interfaces it is theoreticallypossible to restore or augment human communica-tion and control skills [31], by directly interfacingthe brain activity with the controlled devices. Witha traditional interface, the messages and commandsthat a subject wants to express are organized inhis dedicated brain structures and sent to the ex-ternal world through the physiological pathways ofcorticospinal fibres and muscular fibres of periph-eral nerves that connect the central nervous system(CNS) with the subject’s muscles. The activity ofmuscles produces the movement required to oper-ate the interface in order to obtain the physical ex-pression of the intended message or command (seeFigure 2 taken from [34]).

A BMI records the neural activity related withthe message/command intention at the corticallevel by an invasive or non-invasive brain activity’smeasurement system. This activity is decoded intothe intended message/command, and finally actu-

ates the user’s intention by means of an externalactuator (like a cursor on a PC monitor, a robotichand, or a wheelchair) (Figure 2). Invasive systemsdirectly record the electrical activity of a group offew neurons (usually about one hundred) via elec-trodes inserted in the user’s brain cortex [35]. Asdemonstrated by Nicolelis [36, 37], by recording theactivity of even such a small number of neuronsrandomly selected in the motor cortex, it is possi-ble to decrypt the user’s intended movement in a 3dspace, and then to make a robotic arm replicatingit in the real world. The monkeys used for these ex-periments succeeded in operating the robot withoutmoving their own arms, as if the robot arms hadbecome extra limbs of their own bodies. The inva-sive techniques require severe and currently still po-tentially dangerous neurosurgery operations, whichare unsuitable on subjects not affected by severeparalysis. But paralyzed subjects don’t have anynatural feedback anymore, which makes the firsttraining phase of implementation of these interfacesimpossible, and their subsequent use largely ineffec-tive [38].

Figure 2: Different pathways from intention to ac-tion between tradicional and brain machine inter-faces as taken from [34].

As a consequence, non invasive systems, whichoperate by recording the brain activity from “theoutside” of the skull, appear today as the moresuitable platform for studies on healthy subjectsand, therefore, on astronauts [?]. The basicrecording of brain activities takes place via wellknown clinical non invasive diagnostic devicessuch as electro-encephalogram (EEG), magneto-encephalogram (MEG), functional magnetic res-

onance (fMRI), positron emission tomography(PET), and near red spectroscopy (NIRS). Mostof these devices have a poor temporal resolution ofthe signals (e.g. fMRI, PET, the current NIRS)and hence do not suffice for real-time operations.Both EEG and MEG however, do measure brainactivity with the desired temporal resolution.

The EEG records the projection of the electricfield generated by the activity of a large group ofneurons on the scalp of the subject by surface elec-trodes. The MEG monitors, instead, the magneticfield associated with the same activity. The dif-ferences between them are apparent. The electricfield produced by neural activity is shielded by thelayers of protecting tissues and fluids that sepa-rate the brain from the outside world before beingrecorded by EEG instruments and in consequence,the spatial resolution is drastically limited, makingthe precise identification of the group of neuronsgenerating the signal impossible. For this reasons,BMIs based on EEG did not even come close toachieve the capabilities of Nicolelis invasive BMI[33].

MEG-BMI Space Module

The same layers of living tissues and fluids are how-ever “transparent” to the magnetic field. Accord-ingly, MEG instruments can identify with high spa-tial definition the topological origin of the detectedactivity. However, the magnetic field is so weakthat, in order to detect it, the device has first to bewholly insulated from any other magnetic source(even Earth) by enclosing it in a room delimited bythick lead walls; secondly the signal can be success-fully amplified only by mean via an extremely sen-sitive device such as cooled superconducting quan-tum interference devices (SQUIDs).

For the current general aim of studying BMIs asassistive technology platforms for disabled people,MEG-BMIs are too expensive, large, heavy, andnot enough portable for most cases. Some of theseconstraints might be less unfavourable in a spaceperspective. The insulation chamber could be lessthick and massive; future high temperatures super-conductors might require less, or even no coolingat all, reducing the overall dimensions of the de-vice. The portability is probably less problematicby implementing e.g. a MEG-BMI dedicated room.

Motor Anticipatory Interfaces

A rather unknown but potentially interesting ap-proach to natural interfaces is the anticipatory one.The study of anticipatory interfaces originated inwork on multimodal interfaces and natural inter-faces. Generally anticipatory interfaces are definedas all those control apparatus able to identify user’sintention not by explicitly given commands, butby monitoring involuntary user’s behaviours and/orenvironmental parameters that are coupled withthe intention but anticipating it. The subclass ofmotor anticipatory interfaces exploits any involun-tary motor behaviour of the user that anticipatesthe intended command.

Involuntary movements do not impose any cog-nitive load to users as they form an integral partof the related sensory-motor task. There is a widegroup of known involuntary movements that an-ticipate the related motor intentions [39], such asgaze and head movements which contain most ofthe information on human desired actions. Theycan be tracked by a multitude of devices such asgaze tracking systems, inertial measuring units,and optokinetic/photogrammetric/ultrasound ap-paratus for movement analysis. The only class ofanticipatory movements that was exploited in thevery first prototype of anticipatory interface is thehead rotation anticipating the steering action dur-ing active locomotion [40, 41]. This movement waschosen due to its wide anticipatory time (approx 1s) and easy to detect rotation (approx. 25-30), asshowed in Figure 3, taken from [40]. The interfaceis potentially able to drastically reduce the com-munication delays in teleoperation tasks (compareFigure 4 and Figure 5). Although research on an-ticipatory interfaces is only at the beginning, theirpotential for space application is apparent.

Liquid Ventilation and WaterImmersion

Motivation

The presence of human beings inside of moving ma-chines in general greatly limits the variety of ma-noeuvers the machine can take in order not to harmthe fragile human body. In airfighters, pilots wearspecial suits to compensate momentarily appearing

Figure 3: Head anticipatory movement duringwalking steering. Steering begins at time 0. Thewide anticipatory rotation is apparent, as well asthe re-alignment of head after steering. [40]

acceleration (see below), in manned spacecraft, thecontrol architecture is far more complex than inunmanned ones. Overcoming the physical fragilityof the human body would allow for maneuvers atmuch higher G levels, and hence would allow forpreviously unthinkable mission concepts.

The human body consists of different parts withdifferent specific weight. When subjected to accel-erations, each of these elements experiences a rela-tive change in weight compared to the other bodyelements. This effect is particularly pronouncedwith the skeletal system and the fluidic compo-nents, which have the highest density values in hu-man body. While the musculo-skeletal system isfirmly linked, the fluidic components are indeedsurrounded by soft tissue. Hydrostatic pressurechanges result in shifts of the fluids and deforma-tion, with expansion of surrounding tissues [42, 43].For example, under headwards (+Gz) accelerationthe inertial response is directed footwards, thenblood pools in the splanchnic region and in the su-perficial vessels of the lower extremities. The car-diac return is decreased which then also reduces theheart’s filling and its subsequent arterial outflowand pressure of the next beat. It becomes increas-ingly difficult for the heart muscle to maintain ar-terial blood circulation at the level of the eyes andbrain. If these conditions are prolonged for morethan three to five seconds, insufficient oxygen and

User decides to turn right

User sends command “turn right”

Robot executes the command “turn right”

User rotate his head t? ta td

T

Figure 4: Functioning time line of a standardcontrol interface with an intrinsic communica-tion/execution delay present (td). The time elapsedfrom the user first intention to the execution of thecommand is equal to the sum of the time betweenintention and anticipatory movement (t? - ingored),the time between anticipatory movement and theintended movement (ta - anticipation), and the in-trinsic time delay (td - delay).

electrolyte supply occurs, resulting in lack of vision,and, after few more seconds, loss of consciousness[44]. At this stage, if the acceleration is not imme-diatly reduced, firstly irreversible brain damages,and subsequent death, will follow.

Figure 6, taken from [44], illustrates the hydro-static pressure differences in the systemic and pul-monary circulations of a seated pilot at 1 Gz (cen-tre panel) and at 5 Gz (right panel). Supposingthat blood pressure imposed by the heart pumpingdoes not change, just at +5Gz blood pressure athead level is already reduced to zero, clearly show-ing that without any countermeasures, each subjectwould experience the above described symptoms.

Anti G Suits

More than 70 years ago the development of anti-G suits was initiated in order to reduce the prob-ability of in-flight incidents related to the loss ofconsciousness phenomena and its often fatal con-sequences, [45, 46, 47, 48]. These devices applycounter-pressures to the lower extremities and ab-dominal region, preventing blood from pooling inthese regions. By using these suits in combinationwith special respiratory straining techniques, pilotscan sustain Gz values of up to 9 G. A rather simpleapproach to facilitate blood reflux during prolongedphases of acceleration is to simply orient the longaxis of the body perpendicularly to the thrust ofacceleration, most suitably with the subject in thesupine or prone position [49, 43]. In such an orien-

User decides to turn right

User sends the command “turn right”

Robot executes the command “turn right”

User rotate his head

Translation of the anticipatory head movement in the command “turn right”

t? ta Apparent Cause-Effect relationship

T

td

Figure 5: Functioning timeline of an antici-patory interface with an intrinsic communica-tion/execution delay present (td). The time elapsedfrom the user first intention to the execution of thecommand is equal to the sum of the time betweenintention and anticipatory movement (t? - ignored),and the intrinsic time delay (td). The timeline hasbeen shortened from the anticipation time. If thedelay between the translation of user intention fromits anticipatory behaviour and the execution of theintended command is set to be equal to that ofnatural anticipation (ta), the user experiences anapparent cause-effect relationship between its in-terface operation and the corrisponding commandexecution

tation, the maximum intravascular pressure devel-oped under transverse acceleration is considerablyless than it would be under longitudinal accelera-tion, since the anterior-posterior dimension of thebody is only of a small fraction of the sitting height.But this technique is not free of side effects: in thehorizontal supine position, the heart is abruptlydisplaced in dorsal direction and hence threatensthe lungs with severe over-distension which maylead to lung injury [50].

Water immersion to overcome the ef-fects of acceleration

Once completely immersed in a physiological watersolution within a non expandable container, humanbeings are able to sustain acceleration with a mag-nitude of 24 Gx without any noticeable pain; or inother words the double of the acceleration whichusually causes strong pain, unfeasibility of breath-

Figure 6: Hydrostatic pressure differences in thesystemic and pulmonary circulations of a seated pi-lot at 1 Gz (centre panel) and at 5 Gz (right panel).Already at +5Gz, blood pressure is reduced to zeroat head level. Physiological countermeasures arenot taken into account. Taken from [44]

ing, and after few seconds the phenomena of gray-out, blackout and unconsciousness [44]. Water im-mersion augments tolerance to acceleration as theacceleration forces are equally distributed over thesurface of the submerged body [46]. This abruptlyreduces the magnitude of localized forces and a ho-mogenous hydrostatic response of the whole bodyis induced. The increased fluid pressure developedwithin the cardiovascular system during accelera-tion is approximately balanced or even cancelledout by the gradient of pressure developed in theliquid tank outside the body [51, 52], with evidentbenefits for blood and lymphatic circulation.

The first limiting factor for acceleration toler-ance under water immersion appears to be the re-lated thoracic compression [53, 54, 55, 56, 51, 52].The thorax, with its air-filled lungs, has a meandensity considerably lower than that of the restof the body which results, when accelerated dur-ing water immersion, in orthogonal homogeneousthoracic squeezing. This compression causes severedifficulties from pain, lung hemorrhage, pneumoth-orax, alveolar collapse, to even death depending on

the magnitude of acceleration. Besides, in orderto breathe under exposition to the elevated extra-thoracic pressure, the inhalating gas has to be overpressurized. This procedure produces the typicalscuba diving issues, related with the high level ofnitrogen solved in blood which induces alterationsin behaviour and consciousness, reduction of intel-lective properties, ammonia intoxication and em-bolism.

If, however, the air is removed from the lungs,the sustainability to linear acceleration reportedlyincreases significantly [55, 56]. Animal studieswith mice showed that, where the acceleration-timelethal threshold for water immersed mice is around1300 Gx for 15 seconds, when their lungs are emp-tied from air, the maximum acceleration reaches3800 Gx for more than 15 minutes without anyphysical impairment.

The current technique for achieving respirationwith air-free lungs is the extracorporeal circulation.While it is an approved method commonly appliedin surgical operations such as open heart surgeries,the extracorporeal circulation is complex to man-age, and imposes the administration of anticoag-ulant drugs in high doses. To improve accelera-tion tolerance, it seems too risky, difficult and com-plex as procedure to be practically implemented forspace applications.

Oxygen supply for immersed astro-nauts

If it is not possible to empty the lungs, there isstill the option of filling them with another liquid:experiments showed that, during water immersion,the liquid inside the lungs reacts to pressures pro-duced by any accelerations with an equal hydro-static pressure, and directed in the opposite direc-tion, counterbalancing the thoracic squeezing, andavoiding pain and difficulties to breathe [57]. Itis also possible to breathe liquid instead of air byfilling the lungs with a specially prepared fluid inwhich both respiratory gases oxygen and carbondioxide are highly soluble [58, 59, 60, 61]. The per-fluorocarbon (PFC), a liquid carrier medium forgas exchange, has been demonstrated to be free ofnegative side-effects for both short and long termuse, and to be easily removable from alveolar sacs.Under such circumstances, it can be expected toincrease the current 24 Gx limit of human toler-

ance during water immersion. Direct theoreticaland experimental data on humans supporting theseassumptions are however still lacking.

Furthermore, liquid ventilation faces some tech-nological challenges that make it, for the time be-ing, not yet applicable in humans. The currentlybiggest show-stopper appears to be the PFC liquiditself which has to be oxygenated and freed fromcarbon dioxide at least four times per minute [62].As the high density of this liquid makes it impos-sible to be breathed naturally, PFC has to be me-chanically exchanged continuously, which is moreproblematic than conventional gas ventilation [60].The adjustment and regulation of expiratory or in-spiratory pressures has to be finely tuned as liq-uids - contrary to gasses - are far less compressible.Any slight overpressure would charge the lung tis-sues and seriously threaten their integrity. At thecurrent state, only prototypes of liquid ventilatorsfor small animals exist and human experiments arenot foreseen [63].

Conclusions

A number of concepts that could substantiallychange the way we approach the design of hu-man solar system exploration in a far fetched fu-ture are being studied. This papers intends topresent the status of some of these concepts: hu-man hibernation for long duration interplanetarytravels, machines interfacing directly to the astro-nauts brains or to their involuntary reflexes, astro-nauts immersed in a thin water layer surroundinguniformly their bodies, whilst they are breathingliquid instead of air.

Even if so far hibernation has not been in-duced yet in mammals belonging to non hiber-nating families, artificially induced hibernation hasbeen achieved using the chemical compound DA-DLE. Before evaluating the real potential of induc-ing hypometabolic states in human, more scientificresults on the effects of microgravity on hibernatingbeings are needed. If the protective mechanisms ofhibernators concerning physiological atrophy dur-ing torpor will prove to be effective also during mi-crogravity conditions, hypometabolic states duringspace flight could become an option to approachphysiological and psychological stress of crew mem-bers. But it can also well be that the presence of

microgravity adds other insurmountable problemsunless artificial gravity can be provided at reason-able cost. So far there is no theoretical model andhence only experimental data on animals hibernat-ing under microgravity conditions, as planned bythe Canadian Arrow Project, will elucidate the fea-sibility of the proposed techniques.

The benefits of neuroinspired interfaces for spaceapplications could also be significant, allowing as-tronauts to operate in an ‘unnatural’ environmentwithout handicaps and to reduce the communica-tion delay during robots teleoperations. Based oncurrent scientific knowledge and progress, a dedi-cated MEG-BMI module might be the method ofchoice in the far future. For immediate implemen-tation trials of space dedicated BMIs, the EEG-BMI platforms seem to be the only practical solu-tions, due to their technical simplicity and porta-bility. Moreover, an EEG-BMI system could be agood testing platform for a number of studies onthe modifications of brain activity caused by mi-crogravity exposition.

Finally, liquid ventilation has been assessed. Liq-uid ventilation is a well developed technolgy. Pre-mature infants are already treated with a partialfilling of the lungs with PFC and the other halfwith atmospheric air, and prototypes of total ven-tilators for animals have already been realized. Asa consequence, an application on adult humans canbe expected in the near future.This technology willallow the design of liquid-filled capsules for astro-nauts, inside which the stress experienced duringhigh-G acceleration will be reduced to the hydro-static pressure gradient all long the accelerationaxis and a homogeneous hydrostatic pressure thatdepends on the thickness of the layer of liquid watersurrounding the astronaut. Using a liquid film ofonly one centimetre thickness between subject andshell, the hydrostatic pressure would reach valuesclose to zero and the only stress experienced shouldbe that caused by the pressure gradient.

It is difficult to estimate an ultimate accelerationlimit possible with this set-up, but it presumablycan be higher than hundreds of G. The weight asso-ciated to such a liquid shock absorber would scaleconsiderably with the amount of liquid required.Nevertheless it would allow the spacecraft to un-dergo far higher accelerations and hence, possibly,for a simpler design that could overcompensate theweight of the liquid ventilator. Completely new

concepts, such as magnetic railguns, could also beconsidered for manned missions, should it be exper-imentally confirmed that the physiological stressesdue to high acceleration loads vanish using this typeof set-up. Most of the components of such a sys-tems have already been realized, the developmentof experimental demonstrators with animals couldbe initiated right away.

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