1 Copyright © 2004 by ASME
Proceedings of IMECE:
International Mechanical Engineering Congress and Exposition
November 13-19, 2004, Anaheim, CA
IMECE2004-59029
MITIGATING CREW HEALTH DEGRADATION DURING LONG-TERM EXPOSURE TO
MICROGRAVITY THROUGH COUNTERMEASURE SYSTEM IMPLEMENTATION







Jeremy M. Gernand/SAIC







ABSTRACT
Experience with the International Space Station (ISS)
program demonstrates the degree to which engineering design
and operational solutions must protect crewmembers from
health risks due to long-term exposure to the microgravity
environment. Risks to safety and health due to degradation in
the microgravity environment include crew inability to
complete emergency or nominal activities, increased risk of
injury, and inability to complete safe return to the ground due to
reduced strength or embrittled bones. These risks without
controls slowly increase in probability for the length of the
mission and become more significant for increasing mission
durations. Countermeasures to microgravity include hardware
systems that place a crewmember’s body under elevated stress
to produce an effect similar to daily exposure to gravity. The
ISS countermeasure system is predominately composed of
customized exercise machines. Historical treatment of
microgravity countermeasure systems as medical research
experiments unintentionally reduced the foreseen importance
and therefore the capability of the systems to function in a long-
term operational role. Long-term hazardous effects and steadily
increasing operational risks due to non-functional
countermeasure equipment require a more rigorous design
approach and incorporation of redundancy into seemingly non-
mission-critical hardware systems. Variations in the rate of
health degradation and responsiveness to countermeasures
among the crew population drastically increase the challenge
for design requirements development and verification of the
appropriate risk control strategy. The long-term nature of the
hazards and severe limits on logistical re-supply mass, volume
and frequency complicates assessment of hardware availability
and verification of an adequate maintenance and sparing plan.
Design achievement of medically defined performance
requirements by microgravity countermeasure systems and
incorporation of adequate failure tolerance significantly reduces
these risks. Future implementation of on-site monitoring
hardware for critical health parameters such as bone mineral
density would allow greater responsiveness, efficiency, and
optimized design of the countermeasures system.
INTRODUCTION
Human beings are adapted to live and work in the 1-g
environment of Earth. Once that biological system is placed in
the near-zero-gravity environment of low Earth orbit (LEO), it
reacts to the changing stimuli in ways that are deleterious both
to continued function on-orbit, and especially to return to
normal life back on Earth’s surface. As expected, some of the
most serious effects on the body involve those systems involved
in resisting the pull of gravity including the skeletal system and
skeletal muscle groups involved in posture and locomotion.
Effective countermeasures against the effects of
microgravity on human beings must be developed if we are to
continue to safely complete long-term missions in Space or
other worlds with reduced gravity. NASA research of the
effects on the human body of long-term living in Space dates
back to the Skylab missions of the 1970s, when U.S. astronauts
first lived in LEO for up to 84 days. As medical ethics require,
NASA has always provided crewmembers with the best
countermeasures available at the time, so little data is existent
on the wholly unmitigated effects of microgravity on humans.
However, the mitigated effects demonstrate the critical nature of
the risk:
• Skeletal muscle strength declines by as much as
30% in 3 months [1-3].
• Aerobic capacity decreases by as much as 30% in
30 days [4-5].
• Bone mineral density decreases by as much as
2.5% per month [6].

2 Copyright © 2004 by ASME
Countermeasure system concepts currently employed on
board the ISS include a treadmill, a cycle ergometer, and a
resistive exercise device. Each of these modes is employed to
maximize the strengths and usefulness of all in counteracting
the effects of microgravity on the body.
Numerous other effects on the body as a result of living in
space could negatively affect crewmember performance. Some
of these effects include increased radiation exposure, decreased
immune response, cardiac arrhythmias, and decreased thermal
regulation [7]. Also, locomotor and neurovestibular de-
conditioning impacts crewmember safety upon return to Earth.
This paper focuses only on the risk of three principal effects
indicated above known to result specifically from lack of
gravitational stimuli and impact a person’s ability to function in
space and on return to Earth.
RISK ASSESSMENT
Severity
The available data suggest that without countermeasures,
any person placed in a microgravity environment long term will
experience degradation in functional capability and health. The
seriousness or severity of the risk must be assessed both for
continued operations in microgravity (nominal and emergency)
and for tasks and activities required during and after return to
the Earth. Additionally, there is a potential for chronic health
effects following long after the original space slight. Each of
the three principal effects examined in this paper to bone,
muscle and the cardiovascular system are assessed individually.
Decreased bone mineral density (BMD) results in increased
risk of bone fracture, increased risk of renal stones, risk of
permanent bone loss, and potentially increased risk of future
development of osteoporosis. Bone loss tends to be greatest in
the lower body since that part of the human structure
experiences the most significant reduction in load due to loss of
gravity. See Figure 1 below.
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Figure 1: Percent Change in Bone Mineral Density (BMD)
for Three Regions in NASA Mir Astronauts (n=7) [4]
Risk of bone fracture during nominal activities on-board
the ISS is very low due to the lack of severe loading on the
body, which causes the decreased bone mineral density in the
first place. However, the loads experienced by the body during
an emergency such as module isolation in the event of a leak or
evacuation of the ISS due to a crew medical problem are not as
benign or easily predictable. The de-orbit and landing of
spacecraft also present a case where skeletal loads would be
higher than normal. Shuttle landing profiles include sustained
accelerations of up to 3 gs [7]. The landing profile of the Soyuz
spacecraft can include sustained accelerations of up to 4.3 gs
and shock loads as high as 40 gs [8]. A 2%-4% loss of bone
mineral density may not result in a significant increase in these
risks, but longer missions such as extended ISS missions or
Mars exploration could carry substantial risk in this category if
the degradation rate is not slowed or halted. These injuries
could temporarily disable a crewmember or result in death when
combined with another emergency or failure condition.
The increased risk of renal stones results from secreted
calcium from bones increasing the concentration in the blood
stream. This risk is somewhat controlled by decreased dietary
calcium and medications [9], but reduced or halted bone
mineral loss would eliminate this concern as well. Renal stones
occurring during a mission could force an immediate return to
Earth for treatment, as well as temporarily disable the
crewmember involved.
Permanent bone loss and the increased risk of osteoporosis
impact the long-term health of the crewmember. While little
long term data exists on a person’s increased likelihood of
future osteoporosis, the similarity in characteristics between the
bone loss in microgravity and during osteoporosis suggests a
potential relation [10]. A portion of the bone loss that occurs
during long-term space travel could be permanent, and the
remaining bone loss, requires up to 3 to 4 times the mission
duration to recover [9]. For a crewmember in the most
susceptible category, a BMD loss of 12% could occur over the
course of a 6-month mission; most would recover in 2 years, but
some loss from pre-flight would remain. Beyond potential
disqualification from future spaceflight, this loss could
significantly impact the future life of a person in a high risk
group for osteoporosis.
Decreased muscle mass decreases a person’s strength and
endurance. This decline results in the person not being capable
of performing tasks one could previously perform. Although
crewmembers are required to have capabilities at the beginning
of a mission above the minimums necessary to complete the
required tasks, it is possible that the decrements in capability
could eventually reach the point of not being able to perform
certain mission critical tasks.
Among the most strenuous physical tasks are
Extravehicular Activities or EVAs. Since, EVAs are required at
certain times to maintain the ISS, they can be considered
essential for astronaut capability throughout the mission. In ISS
EVAs, only a person’s upper body is significantly involved in
mobility and performing tasks. EVAs become more difficult
when considering planetary exploration. In the situation of a
planetary EVA on the surface of Mars or the Moon a

3 Copyright © 2004 by ASME
crewmember’s lower body will be taxed with supporting the
mass of the person and the suit during walking, kneeling,
standing, and other activities.
Crew tasks during re-entry when the accelerations apply
additional load to the body beyond even what is experienced on
the surface of Earth can induce fatigue in crewmembers
acclimatized to microgravity conditions. Also, response to an
emergency such as the previously mentioned module isolation
or medical issue can require strength near the levels typical of
activities on the ground.
As seen in Figure 2 below, the decline in lower body
muscle strength is significant for long duration missions. Again,
the long-term data depicts the physiological response including
some countermeasures.
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Figure 2: Percent Change in Isokenetic Muscle Function
Tests from Pre-Flight for Knee Extension and Flexion
(Shuttle {short duration}, n=17; Mir {long duration}, n=5)
[1-3]
Aerobic and anaerobic capacity decreases by up to 30% in
the first 30 days and then reaches a plateau [5]. From the
earliest point in investigation of countermeasures, this effect has
proven to be the easiest from which to recover. Crewmembers
must retain aerobic capacity in order to complete EVAs and
maintain effectiveness during decent, especially important for
pilots. Responding to emergencies also requires a certain level
of aerobic capacity, probably in line with that of EVAs and
decent activities. Early fatigue and greater use of oxygen occur
in crewmembers who do not have adequate aerobic capacity.
Crewmembers also experience a drop in aerobic capacity
upon return to Earth. A person who has regained his or her pre-
flight capacity by the end of the mission will experience a
decrease following return. This leads to the conclusion that
someone who could not maintain an adequate level or only a
marginally adequate level during the mission could experience a
greater decline after return to the ground. For someone taking
part in a contingency landing scenario by Soyuz or another craft
not in an area of rapid medical response, this hindrance could
be a serious handicap.
The early fatigue caused by inadequate aerobic capacity
can result in serious complications compromising safety of the
crew. An inability to perform medical emergency response
procedures in a timely fashion increases the risk to other
crewmembers by eliminating the last line of defense.
As shown in Figure 3, aerobic capacity declines
significantly at the beginning of a mission, even in the presence
of the current countermeasures. The decline levels off, and is
recovered by the end of the mission with appropriate
countermeasure utilization.
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Figure 3: Decline in Crewmember Aerobic Capacity during
Mir Mission (n=1) [4]
All crewmembers experience these effects to some degree.
Significant variability occurs between individual crewmembers
and also between specific sites, regarding muscle and bone, on
the same individual [11]. Given that the effects outlined above
all occurred in the presence of some countermeasures, it seems
certain that these effects would eventually overcome all
crewmembers to this degree or greater given enough time.
Currently ISS missions last approximately 6-months, but future
missions could be longer, and other destinations beyond LEO
would almost certainly involve greater time periods. Some
limited Russian experience with longer missions, suggests that
with countermeasures these effects do not worsen from those
presented here [12].
Genetic factors as yet undiscovered are thought to affect
both a crewmember’s response to the removal of gravitational
stimuli, and their response to exercise countermeasures. No
currently available method accurately predicts a person’s likely
response to the microgravity environment. This may be
achievable in the future with more data and a greater sample
size.
It would not be without reason then, to suggest that all
crewmembers given a mission duration of 6-months without
countermeasures would experience several functional
impairments commensurate with a hazardous condition, and
some even with the countermeasures available to date would
experience a significant functional impairment in at least one
area of concern. This points to the need for continuing

4 Copyright © 2004 by ASME
development in the capability or functional envelope of the
countermeasures system.
Risk Control or Mitigation Requirements
These effects if evaluated on the ground for a worker,
would meet the definition of an occupational illness, which
under NASA safety guidelines is called a “critical hazard” and
must be protected against with a 1-fault-tolerant or equivalent
control approach [13]. Since sub-systems within the larger
countermeasures system are directed specifically at a sub-set of
microgravity effects, the fault tolerance of the system, is most
appropriately analyzed on a function (or target) by function
basis. As determined by this analysis, each identified function
(e.g., mitigating bone loss) should have redundant success
paths. As mentioned previously, NASA does permit an
approach without redundancy, if the reliability of that approach
would be comparable to systems with redundancy.
The limited nature of the data considering the small sample
size, high variability between individual crewmembers, and
limited success to date in mitigating some effects means that it
is not currently possible to determine a minimum exercise
protocol necessary to protect a person from these functional
decrements. Without a minimum protocol, capabilities of the
countermeasure system must be maximized to ensure the risks
are mitigated to the greatest possible extent. This statement
concurs with the medical ethics principle, which states that
decrements should be as low as reasonably achievable. Until,
the determination of the minimum countermeasures protocol
can be completed, the countermeasures must be assessed
against the presumed overall enveloping requirements for the
entire population.
In recognition of the importance of the countermeasures
system on-board the ISS, NASA has also implemented
requirements governing the maintenance and recovery activities
should any countermeasures sub-system fail. Currently, those
requirements state that if the entire countermeasures system
were to fail, all efforts would be made to restore at least one
sub-system within 5 days [14]. In the event of a single failure,
capability must be restored within 30 days, according to the
requirement. These requirements, while directed at flight
control team priorities, imply requirements for adequate supply
of hardware components necessary to affect repairs.
In addition to the requirements for countermeasures
function during a mission to maintain crewmember
effectiveness, NASA also has responsibility to maintain a
crewmember’s health to permit a speedy recovery. The flight
surgeon, or chief medical officer, has a responsibility to
rehabilitate a crewmember following a mission to full flight
status within 45 days [5]. So, losses during the mission must be
kept to a minimum to permit compliance with this requirement.
COUNTERMEASURES SYSTEM
The concept of microgravity countermeasures is to replace
the effects of 1-g on the body over a 24 hour period with 2 to 3
daily hours of countermeasures, in this case exercise.
History: Skylab, Shuttle and Mir
Experience developed through the history of the U.S. space
program informs current understanding of the benefits of
exercise countermeasures. From the beginning of long-term
space flight, NASA has sought to incorporate effective
countermeasures and improve those countermeasures as the
data warranted.
The first U.S. experience with long-term habitation in
Space included 3 missions on-board the Skylab space station.
Missions included durations of 28, 59 and 84 days, and
incorporated increasingly capable exercise countermeasures
hardware. The first mission included only a cycle ergometer, to
be augmented on the second mission by a handle/spring
resistive exercise machine and kinetic rope pull. The third
mission added a Teflon treadmill. Results showed that even
with increasing mission durations, increased modes of exercise
and increased exercise intensity on those later missions led to
less loss of muscle than on earlier missions [4]. The Skylab
crews demonstrated that it was possible to recover aerobic
capacity by end of mission to pre-flight levels. Crewmembers
with better post-flight functionality as compared to pre-flight
required less recovery time.
Extended duration Space Shuttle missions included
experiments to specifically evaluate the effects of microgravity
on crewmembers and their response to countermeasures. While
the flight durations only reached a maximum of 16 days, the
amount of data obtained allowed more insight into the
effectiveness of countermeasures. Shuttle exercise
countermeasures included a cycle ergometer, a rower, and a
treadmill. Results clarified that muscle atrophy occurs in
missions of less than 16 days primarily in anti-gravity
musculature [4]. Suited egress or EVA capability is dependent
on aerobic fitness, investigations found. Results also identified
the increase in aerobic stress to workloads post-flight. The
treadmill demonstrated a capability to maintain leg strength,
and aerobic exercise maintained aerobic capacity. Conclusions
from these investigations led to the recommendation that for
missions of 11 days or greater, exercise countermeasures should
be required.
Experience gained during the joint U.S.-Russian Shuttle-
Mir program substantially increased U.S. knowledge of long-
term operations in Space. The length of the missions including
7 U.S. crewmembers averaged 140 days, with a maximum of
188 days. Exercise countermeasures available based on
Russian flight experience included a treadmill, cycle ergometer,
expander straps (elastic exercise straps), and a penguin suit.
The penguin suit is a garment including elastic straps in specific
locations requiring constant force from the wearer in order to
maintain a specific orientation. Results from the Shuttle-Mir
missions indicated that all crewmembers showed significant loss
of bone mineral density in at least one region with significant
variability between regions and between crewmembers [4]. The
results also identified decreased muscle strength in the back and
legs, as well as alterations in stability following return to the
ground.

5 Copyright © 2004 by ASME
In 1995, NASA convened an expert panel, the Life and
Microgravity Sciences and Applications Advisory Committee
(LMSAAC) to evaluate the countermeasures program [15]. The
panel’s findings included that the available countermeasures did
not adequately counteract the decline in muscle strength and
endurance, which occurred to such a degree as to impair
performance during EVA, especially on a planetary surface.
The panel recommended heavy resistive exercise be
incorporated to increase the effectiveness of the
countermeasures program in reducing losses in muscle strength
and bone mineral density in the most susceptible areas. The
panel found the rate of loss of bone mineral density predisposed
the crew to kidney stones, and an increased risk of lumbar
spinal injury and disk herniation during post de-orbit activities.
NASA incorporated these recommendations into design of the
ISS countermeasures system.
Design of ISS Countermeasures System
The design of the ISS countermeasures incorporates
knowledge gained from the previous experience with long term
space flight. As mentioned before, the three primary
components in the countermeasures system are a treadmill, a
cycle ergometer, and a resistive exercise machine (See Figure
7). Each of these systems is similar to equipment used in fitness
centers on the ground, but each has been modified to meet the
unique requirements for transport to and operation in the ISS.
The countermeasures design is best analyzed in light of the
functions of providing loading for bone and skeletal muscle,
and the capacity for aerobic exercise. The countermeasures
system must also mitigate effects other than the three discussed
in this paper, such as decline in locomotor function and balance,
which would impact a comprehensive evaluation of ISS
countermeasures, but not be evident here. The three primary
countermeasures sub-systems must satisfy together the three
functions identified. The various training modes of running,
cycling, and weight lifting, however, are not each specifically
directed at a single function. Each contributes to several
functions, and more efficiently towards some than others.
Mitigation of loss of muscle mass and subsequent loss of
strength and endurance is accomplished by loading of the
crewmember’s muscles. All training modes provide this
function; however, the resistive exercise machine is the most
effective, since it provides the highest loads across all muscles
of the body. The cycle and treadmill provide muscle loading of
the lower body with a different set of characteristics that
mitigate muscle loss to a lesser degree when employed alone.
The resistive exercise machine, again, provides the most
effective mitigation to loss of bone mineral density, followed by
the treadmill.
Both the treadmill and the cycle equally provide aerobic
conditioning to crewmembers. This function is the only one for
which a true redundancy exists, although even this redundancy,
an unlike redundancy, is contingent on a crewmember’s training
protocol prior to flight. Since, a person’s body adapts to the
training method received, crewmember’s must train roughly
equally between running and cycling prior to a mission in order
to fully have the redundancy in this function.
As a whole the countermeasures system must accommodate
crewmember capabilities from the 5th percentile Japanese
female to the 95th percentile American male, the range of
potential ISS crewmembers [16]. This wide functional
envelope presents an engineering challenge to countermeasures
design. These requirements together with restrictions on power
use, load transmitted to ISS structure and total mass make
design of an effective and reliable system difficult. These
requirements with cost and schedule limitations led to
compromises on the functional envelope in some cases. The
resistive exercise machine, for example, provides only 300 lbs.
of load, significantly less than the originally specified 500 lbs
[17]. The cycle met its performance requirements. The
treadmill achieved the majority of its functional envelope, but
as we are to see in the next section, sacrificed reliability.

Treadmill Cycle Ergometer Resistive Exercise Device
Figure 7: ISS Countermeasures Sub-systems
Evaluation of Effectiveness
The failure and anomaly reports provide a measure with
which to evaluate the effectiveness of the countermeasures
hardware in meeting their requirements in the operational
environment. The availability of those hardware systems can be
calculated using the dates in the problem reports for detection
of the problem, the immediate impacts, and the implementation
date for the resolution. Historical tracking logs provide
important data on any operational constraints put in place
following an anomaly or failure until resolution to determine the
relative functionality during this period. The number of
exercise opportunities or days is the basis of the calculation.
Data is then categorized based on the hardware being
nominally functional, functionally degraded, or non-functional.
Those categories can be more fully described as the hardware
meeting at least 90% of its performance requirements, more
than 10% of its performance requirements or less than 10% of
its performance requirements respectively.
The figures 4, 5 and 6 below display the results of that
analysis. Availability is calculated based on the number of
exercise opportunities where hardware is functional divided by
the total number of opportunities. Exercise opportunities are

6 Copyright © 2004 by ASME
analogous to mission days, since exercise is prescribed 6 days
per week.
The ideal measure of effectiveness would be to evaluate the
pre- and post-mission condition of ISS crewmembers, which is
conducted on a regular basis. However, due to the small crew
size, medical privacy concerns prevent this data from being
presented mission by mission in comparison to the
countermeasure availability at the time. This data is considered
by the ISS flight surgeons, who then make recommendations for
countermeasure system design and prescribe utilization by
future crewmembers.

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Figure 4: Availability of Treadmill during ISS Expeditions
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Figure 5: Availability of Cycle Ergometer during ISS
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Figure 6: Availability of Resistive Exercise Device (RED)
during ISS Expeditions 1-7
Incorporation of fault tolerance or functional redundancy
cannot always be considered to be more effective. Although all
other things being equal, redundancy does reduce the
probability of loss of function. An example of this can be
observed in the availability data for the resistive exercise device
(Figure 6). Following Expedition 3, once engineers observed
that the hardware reliability did not meet expectations, a spare
set of hardware was maintained on board the ISS. The effects
of this change are evident in the increased average availability
following Expedition 3.
Desired availability is for no losses of function greater than
11 days per increment crew (to match recommendation for
extended shuttle missions). That results in an availability of
0.9389 for a 180-day mission (0.9083 for a 120 day mission).
Since many exercise systems can also operate successfully
in a degraded mode (50% of maximum exercise load or speed
for example), a measure to gauge the availability of a functional
plus degraded mission profile is desired. Assuming that 50%
functionality extends the acceptable duration without nominal
performance by a factor of 4, one arrives at a desired
availability of 0.7556 for 180-day missions (0.6333 for a 120
day mission) for nominal performance with the remainder
period sustained in a degraded mode.
To more appropriately consider the effectiveness of the
countermeasure system in light of the overlapping functionality
of the three main hardware systems, one must evaluate the
functional availability. The availability for three
countermeasure system functions: aerobic exercise, muscle
loading and bone loading, appears below in Table 1. It should
be noted that the countermeasures system must provide
additional functions to be wholly successful such as locomotor
and neurovestibular conditioning, but those functions are
beyond the scope of this paper. The data for muscle and bone
loading functions were combined, since they were identical.
This functional availability incorporates both primary and
secondary capabilities of the various hardware systems. For
example the treadmill provides aerobic exercise and also lower

7 Copyright © 2004 by ASME
body muscle and bone loading. In the area of muscle and bone
loading, however, the treadmill is not as capable as the resistive
exercise device (RED), so during periods of complete RED
loss, the fully functional treadmill is counted as a 50%
functional muscle and bone loading countermeasure. The cycle
ergometer has similar overlapping functions with the treadmill
and RED. This partially satisfies the 1-fault tolerance
requirement identified earlier.
Only the aerobic exercise function was lost completely at
any point, and it occurred briefly at the end of Expedition 1 and
beginning of Expedition 2 (before installation of the cycle
ergometer). Fifty percent (50%) functional capability was
maintained in all other cases where failures occurred.
According to the criteria established here, no missions show
hazardously insufficient countermeasure availability. Two
occasions warranted additional investigation for muscle and
bone loading during Expedition 2 (0.7197 required) and aerobic
exercise during Expedition 3 (0.6333 required), but both were
acceptable, at least marginally.

Table 1: Availability of ISS Countermeasures by
Function per Expedition Crew
Function
Mission
Aerobic
Exercise
Muscle and
Bone Loading
Exp. 1 0.9716 0.8652
Exp. 2 0.9427 0.7261
Exp. 3 0.7000 0.8500
Exp. 4 0.8777 1.0000
Exp. 5 1.0000 0.9467
Exp. 6 1.0000 1.0000
Exp. 7 1.0000 1.0000

Based on these results, one can identify failures that could
have resulted in a hazardous loss of function, had the
circumstances been different. These potential hazard causes or
failure precursors identify the “weak links” in the overall
countermeasures system and can be utilized to improve the
reliability of the hardware design and the system failure
tolerance or robustness.
A true investigation of the countermeasure effectiveness
should be made by detailed examination of the capability of
each crewmember with respect to the capability of the
countermeasures equipment to determine functional availability,
and then that data must be analyzed against the post-mission
results in bone, muscle, and cardiovascular function loss for the
specific crewmember. The criteria utilized here are arbitrary,
although based on previous mission experience. They
encompass a proposed measure of effectiveness with which to
focus resources on future improvement in ISS countermeasures.
Also, other countermeasure goals not investigated in this paper,
such as mitigation of loss in locomotor and neurovestibular
function play a significant role in the overall system design.
Analysis including the comprehensive set of countermeasures
functions must be complete to guide future decisions.
The hazardous situations mitigated by the ISS
countermeasures hardware are chronic in nature, and make
assessment of effectiveness difficult. Improvement in
functional availability for the countermeasures system during
later missions is the result of adapted operational management
of those sub-systems which exhibited weaknesses earlier in the
ISS experience. Additionally, the limitations of re-supply mass
and volume mean that on-time arrival of spares and refurbished
components may not occur in time to adequately protect for
hardware failures. This limitation contributes a large amount of
uncertainty in predicting future availability of countermeasures.
Future Improvements
Several efforts are currently underway to continue
improvement of the microgravity countermeasures system for
ISS and future long-term missions. Top among those project
priorities are increased reliability and availability of the
hardware systems, as well as increased capability in terms of
expanding the physical training envelope for the crewmembers.
Two new countermeasure sub-systems are planned to
advance the current ISS countermeasures suite. The first among
these improved systems will be a new resistive exercise
machine, with twice the load capability of the current system,
and improved reliability. The second improved system is
planned to be a new treadmill, which will increase reliability
and expand the functional envelope beyond what the current
treadmill can provide. These upgrades will alleviate the
discrepancy of having better crewmember training capability
than the machines can provide.
The current ISS countermeasure system provides
redundancy in cardiovascular function by use of the cycle
ergometer and the treadmill, and in bone and muscle loading
function by the provision of spare RED hardware maintained
on-board. This redundancy has thus far prevented a hazardous
loss of function for the system while certain components have
underperformed. Redundancy does not exist for all
countermeasure functions outside of those presented in this
paper, however, and future selection and design of new systems
should take this into account. Priority among these should be
the provision of locomotor and neurovestibular conditioning.
Those functions are currently only provided by the treadmill,
which has performed poorly. In addition to prioritization of a
more reliable system, sufficient on-orbit supply of spare
hardware to mitigate all single failure points or the future
inclusion of a dissimilar system providing the same function
should be pursued.
Future implementation of on-site monitoring hardware for
critical health parameters such as bone mineral density would
allow greater responsiveness, efficiency, and optimized design
of the countermeasures system. While these parameters are
routine to measure on the ground the systems are large and not
readily adaptable to space travel. Currently in evaluation are
instruments capable of providing this critical feedback to the
management of countermeasures during the mission. This
capability, once realized, would allow optimized use of the

8 Copyright © 2004 by ASME
countermeasures system for each individual’s needs, both
reducing risk for the overall population and possibly increasing
the amount of available work-time for those found to be at
lower risk.
CONCLUSIONS
Experience from the days of early spaceflight to the current
ISS program demonstrates significant advancement in
microgravity countermeasures. However, crewmembers
continue to experience losses of functional capability, which in
some cases approaches the level of an occupational illness.
Safety hazards and risks to long term health increase in
probability as the time spent in that condition increases.
Current data points to the need for greater functional capability
in the ISS countermeasures hardware in order to prevent crew
functional decrements as currently experienced.
The availability of the ISS countermeasures hardware,
while not falling below the criteria established in this paper,
could be marginal at times. More intensive investigation of the
hardware capability against crewmember capability and post-
mission evaluations should lead to a better measure of risk to
current ISS crews.
In looking at the ISS as a step in a broader human
exploration program, the limitations on countermeasure success
data currently prevents conclusions on appropriate
countermeasures system design for longer term missions in
microgravity (transportation to Mars) or in condition of 0.14 g
(Moon) or 0.38 g (Mars).
ACKNOWLEDGMENTS
I would like to acknowledge the NASA JSC Space and Life
Sciences Directorate personnel for providing insight into the
health effects of humans living in microgravity and historical
countermeasures performance information.
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