NASA/TM–2011-216143

Use of International Space Station to Simulate 

Interplanetary Transit: Human Health and 

Performance Applicability of Current Increment 

Durations and Extended Durations

John B. Charles

NASA Johnson Space Center

Houston, TX 77058

Maneesh Arya

Wyle Integrated Science and Engineering

Houston, Texas, USA

Craig E. Kundrot

NASA Johnson Space Center

Houston, TX 77058

February 2011
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NASA/TM–2011-216143

Use of International Space Station to Simulate 

Interplanetary Transit: Human Health and 

Performance Applicability of Current Increment 

Durations and Extended Durations

John B. Charles

NASA Johnson Space Center

Houston, TX 77058

Maneesh Arya

Wyle Integrated Science and Engineering

Houston, Texas, USA

Craig E. Kundrot

NASA Johnson Space Center

Houston, TX 77058

February 2011
Available from:

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Hanover, MD 21076-1320 Springfield, VA 22161

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This report is also available in electronic form at http://ston.jsc.nasa.gov/collections/TRS/
i

Contents

1.0 Introduction.......................................................................................................................................1

1.1 Using the International Space Station to Mimic a Mars (or Near-Earth Asteroid) Mission.........1

1.2 Increasing Increment Duration......................................................................................................5

1.3 Concerns About Implementation ..................................................................................................6

1.4 Future Enhancement of International Space Station Analog ........................................................6

2.0 Recommendation ..............................................................................................................................7

3.0 Conclusion ........................................................................................................................................8

4.0 Workshop Attendees.........................................................................................................................9

5.0 References.......................................................................................................................................10
ii

Acronyms

ATV Automated Transfer Vehicle

D-RATS Desert Research and Technology Studies

HTV H-II Transfer Vehicle

ISS International Space Station

NEA Near-Earth Asteroid

NEEMO NASA Extreme Environment Mission Operations

SME Subject Matter Expert
1

1.0 INTRODUCTION

Preparation for future crewed missions to Mars will require utilization of space analogs for research (such 

as for feasibility and verification of monitoring technologies or countermeasures) and for training of flight 

crews and ground support personnel. Although several ground-based analogs, such as the Arctic 

Haughton-Mars Project (http://www.marsonearth.org/), NASA Extreme Environment Mission Operations 

(NEEMO) (http://www.nasa.gov/mission_pages/NEEMO/), and NASA Desert Research and Technology 

Studies (D-RATS) (http://www.nasa.gov/exploration/analogs/desert_rats.html) mimic various aspects of 

space exploration missions, the influence of Earth’s ever-present surface gravity cannot be ignored. Until 

recently, lunar missions were expected to be a stepping-stone for Mars expeditions,1but recent 

programmatic de-emphasis of lunar missions has encouraged consideration of piloted missions to NearEarth Asteroids (NEAs) instead. It has been suggested that in-space simulations of Mars (and now NEA) 

missions be conducted on the International Space Station (ISS). Some possibilities for, and space life 

sciences implications of, such an effort are described in this paper.

A brainstorming session involving space life sciences subject matter experts (SMEs) associated with the 

NASA Human Research Program, the Johnson Space Center Space Life Sciences Directorate, and the 

Ames Research Center Exploration Technology Directorate was convened on September 25, 2009, to 

respond to two questions:

1. How can the ISS be used to mimic Mars missions?

2. How can crew increment duration be increased to 9 to 12 months from the existing 6 months?

The positions and recommendations of the SMEs are specifically indicated in this white paper. The 

response to question 1 is addressed first, in the next section. The response to question 2 is addressed in the 

section titled “Increasing Increment Duration.” Additional recommendations are derived from a variety of 

other sources relevant to ISS, Mars, and NEA considerations.

1.1 Using the International Space Station to Mimic a Mars (or Near-Earth Asteroid) Mission

The ISS provides a space flight environment needed to enhance and validate technologies and 

countermeasures for long-duration missions to Mars. Many of them will be relevant to NEA missions, 

which will themselves serve as precursors to, and simulations of, Mars missions.

The SMEs observed that there are two categories of activities to be considered: 

1. those that require the use of the ISS (or a comparable orbiting facility), typically because of the 

need for prolonged weightlessness, either as a stimulus or as a confounding factor; a high-risk 

environment that could not ethically be provided in ground-based simulations; and other 

fundamental environmental factors that cannot be produced on Earth; and, 

2. those that do not require ISS use in the strictest sense but can exploit its use to maximize their 

scientific return more efficiently and productively, due to higher fidelity, than in ground-based 

simulations. 

The first category includes studies of human physiological adjustments to the weightless environment of 

ballistic flight, which can be mimicked on Earth only poorly. Simulations such as bed rest and 
2

hypokinesia promote cardiovascular and musculoskeletal atrophy that is qualitatively similar to atrophy 

seen in weightlessness, but they provide only a redirected gravitational stimulus to the sensorimotor 

system. Any sensorimotor influence on other systems

2 is modified in ways different from those that occur 

in space flight. This category also includes weightless mobility and restraint, and interactions with tools 

and other hardware. These can be simulated during water immersion; but again, the influence of gravity is 

not eliminated, only redirected. In addition, the issue of increased viscosity must also be taken into 

consideration when using water immersion techniques. Weightlessness can be provided by parabolic 

aircraft flight, but it lasts approximately 20 seconds and is interspersed between bouts of hypergravity, so 

its usefulness in determining the effects of prolonged, uninterrupted weightlessness is limited.

The first category also includes countermeasures against the deleterious effects of weightlessness, 

especially those that require validation in a real-world space flight environment. Among the 

countermeasures, intermittent artificial gravity produced by the use of a short-radius (e.g., 2 m [~6.5 ft]) 

centrifuge could be tested on the ISS as a multisystem (i.e., bone, muscle, cardiovascular, and 

sensorimotor) countermeasure to combat the deleterious physiological effects of weightlessness. 

Demonstration of its potential benefits to the health and fitness of astronauts and thus to mission success 

would require considerable effort and expense to implement it aboard the ISS as currently configured. 

However, if this is not done aboard the ISS, there are no clear alternative flight venues in which to 

evaluate this potentially paradigm-changing capability.

To protect the crew from an unacceptable level of deconditioning, any new systems for exercise must be 

at least as effective as the latest systems on the ISS and must deliver their benefits in a more resourceefficient and reliable manner. In addition, there would need to be monitoring capability and secondary 

protection (like pharmaceuticals) that can be added if necessary. This work would be validated on the ISS.

The second category includes a much more varied list. Use of the ISS for these activities will provide 

necessary information without the expense of establishing lower-fidelity simulation facilities or scenarios 

duplicating the non-weightlessness aspects of space flight aboard the ISS (e.g., confinement, isolation, 

separation, workload, and astronaut-like individuals assembled as functioning teams).

Activities that can exploit the use of the ISS include:

• Acquisition of operational and medical experience in high-autonomy situations in which 

environmental constraints and conditions require that the individual crewmembers exercise their 

own decision making to complete tasks and mission objectives with little or no direction from 

Mission Control. One key aspect of an actual Mars mission will be the communication delay 

between the Mars-bound astronauts and the Earth-based support personnel. In an ISS analog 

mission, gradually increasing the one-way communication delay from zero at the start of the 

increment to about 7 minutes at the time of the simulated Mars arrival after 6 months will require 

crewmembers to function with increasing autonomy and will enhance the sense of crew and 

individual isolation due to the perceived remoteness. Figure 1 illustrates the variation in distance 

and communications delay between Earth and Mars.
3

• For behavioral health risks, further work to determine optimal crew selection, composition, and 

training criteria, and to ensure that effective crew support techniques are on board for monitoring 

and intervention of issues. The connections between physiological and neurobehavioral changes 

require further investigation. 

• Tests of lighting and other behavioral health countermeasures for circadian-rhythm disturbances 

and fatigue. The ISS orbits the Earth 16 times a day, with 16 sunsets and sunrises each day; 

therefore, crews living on the ISS rely on artificial lighting as an external cue for their circadian 

rhythms. A transit to Mars will also require that crews rely on artificial day-night cycles, as the 

vehicle will be continuously exposed to direct sunlight for months at a time.

• Evaluations in situ of analysis techniques for medical specimens and samples from biological 

experiments. It will not be useful to the crew involved in an ongoing mission to postpone analysis 

of such specimens and samples from a Mars mission until return to Earth. In situ analysis 

techniques taking advantage of the microgravity environment can be thoroughly evaluated during 

simulated Mars missions on the ISS to understand their areas of greatest usefulness.

• Evaluations of acceptability of outdated food in controlled circumstances. Experience with longduration missions has shown that both nutrition and well-being depend on an adequate food 

Figure 1. Variation in Distance and Communications Delay between Earth and Mars (2001-

2005). In an actual Mars mission, the one-way communication delay will continue to increase to 

about 22 minutes at the midpoint of the 18-month Mars surface stay, with a 2-week 

communication blackout at opposition, then decrease to about 7 minutes at departure for Earth, 

reaching zero again at Earth arrival. NEA missions are expected to have much more variability, 

but a 2025 mission to one well-studied target, 1999 AO10, would have a one-way 

communications delay increasing gradually to 30 seconds at NEA arrival after 110 days, then 

decreasing back to zero for the remainder of the 155-day round-trip mission.
4

system. Today, the shelf life of foods and the packaging efficiency of the food system are 

inadequate and inefficient for 1000-day exploration-class missions. Food prepared with current 

methods will reach their expiration dates before the end of a 900-day Mars mission. Evaluating 

food during an ISS simulation can be accomplished by delivering appropriately aged food in 

routine deliveries by resupply vehicles such as Progress from the Russian Federal Space Agency, 

Automated Transfer Vehicle (ATV) from the European Space Agency, H-II Transfer Vehicle 

(HTV) from the Japan Aerospace Exploration Agency, and commercial orbital transportation 

services.

• Tests of robust, highly reliable life support systems in a microgravity environment required for 

Mars missions.

• Tests of expanded data management systems designed for use on a Mars mission during a 

simulation. 

• Studies and refinement of concepts for stowage and inventory control, which will be challenging 

during a lengthy Mars mission.

• Reliance on human research hardware only as maintained on orbit, not refurbished on Earth, for a 

higher fidelity simulation.

• Finally, demonstration of appropriate onboard training facilities and protocols that will be 

required for maintaining astronauts’ technical skills for lengthy periods or will provide just-intime training without the aid of current Earth-based support infrastructure. In particular, extensive 

simulation capabilities will ideally be provided so planetary landings and other operational 

procedures can be practiced during the transit phase. 

Again, none of the foregoing suggested activities requires the ISS for implementation. However, their 

incorporation into the ISS missions, whether or not a Mars expedition is being simulated, will enhance the 

applicability of those missions to Mars-forward planning. 

The degree of fidelity of ISS-based activities to activities in a genuine interplanetary mission varies 

significantly, and selected mission elements are listed in Table 1.

Table 1. Low- and High-Fidelity Mission Elements to be Investigated during Transit Simulations on ISS

Low-Fidelity Mission Elements High-Fidelity Mission Elements

Circadian Shift and Desynchronization Autonomy

Confinement Communications Delay

Isolation Crew

Operational Mode Ground Support

Proximity to Earth (visual) Task Design

Radiation Weightlessness

Risk Workload (type and quantity)
5

1.2 Increasing Increment Duration

Current 6-month ISS mission operations and human biomedical research are already relevant to Mars 

transit because the duration of every increment mimics portions of future Mars missions. 

Studying issues related to weightlessness drives most of the activities requiring the use of the ISS, and the 

combination of long-duration confinement, realistic workload, separation from familiar surroundings and 

support mechanisms, and, of course, the unique characteristics of the astronauts themselves greatly 

magnify its applicability to Mars mission simulations. The current ISS increment duration is 6 months, 

which is also the nominal Earth-Mars transit duration for baselined conjunction-class missions3 using 

anticipated propulsion capabilities.

4 Thus, the physiological, behavioral, and operational experiences of 

astronauts on current ISS missions are at least qualitatively comparable to those of Mars-bound 

crewmembers. 

The most applicable duration greater than 6 months is the 9- to 12-month interval, which corresponds to 

the worst-case Earth-Mars transit duration. The longest applicable duration is 22 months, which 

approximates an intentional Mars flyby, an aborted Mars orbit insertion and subsequent return to Earth, or 

a Mars-orbit mission, perhaps to explore the Martian moons, without landing on Mars. This duration 

would involve continuous weightlessness for a period about 8 months longer than any individual has ever 

experienced in the history of human space flight. 

Of all the ISS-based scenarios, the one that seems to make the best use of available resources and existing 

operational expertise with minimal impact on other “non-Mars” applications is mimicking the Mars 

outbound transit, whether nominal or worst-case. The outbound transit has the advantages of a recent 

departure from Earth and the increasing acclimation to space flight as the mission progresses, as would be 

the case for an actual Mars-bound mission. Those advantages are absent from other possible mission 

phases, such as inbound to Earth.

An increase in mission duration can provide opportunities for enhanced and focused research; however, 

several problematic issues must be considered as well. The SMEs agreed that increasing the ISS crew 

increment duration to 9 or even 12 months from the current 6 months would pose little additional risk to 

crewmembers; however, after the workshop, evidence surfaced to suggest that recently identified ocular 

changes, usually associated with increased intracranial pressure in clinical patients on Earth, were 

becoming more frequent among crewmembers after as little as one month in space. Therefore, some 

additional medical monitoring capabilities seem to be required beyond those currently used for ISS 

operations. It is also advisable that an increase in mission duration occur gradually over several 

increments due to psychological concerns.

From both physiological and psychological perspectives, the primary benefit of an extended mission is 

the ability to document the adapted state of some human systems in weightlessness. In addition, such 

missions may uncover unsuspected gaps (“unknown unknowns”) that would not have been found in 

missions of shorter duration. An increase in mission duration for each astronaut subject means obtaining 

data on fewer unique astronauts than in the same time span of shorter missions. This would encourage life 

sciences investigators to prioritize the variables of primary importance and ensure that the results for 

those variables will have statistical significance, instead of depending on larger numbers of subjects to 
6

provide enough statistical power to detect significance of secondary variables. The longer duration might 

also allow more precise characterization of effects that steadily increase or even accelerate with time. In 

addition, it would enable researchers to examine team dynamics and individual psychology over an 

extended duration, both with astronauts in space and support personnel on Earth—an option that does not 

currently exist in either space flight or high-fidelity analogs. 

Several negative aspects are associated with extended missions and should be taken into account before 

planning begins. An important finding of such extended missions might resolve the question of whether 

bone loss continues unabated. In addition, the SMEs expect an increased likelihood of vision changes and 

kidney stones in longer missions and increased dependence on countermeasures to prevent behavioral 

health problems. Finally, repeat fliers may reach their lifetime radiation limit while participating in longer 

missions. 

1.3 Concerns About Implementation 

If the ISS is to be used for high-fidelity Mars mission simulations, several issues concerning the fidelity 

and utility of the simulations should be considered. Since crew safety is the paramount concern on any 

space flight mission, emergency medical interventions and system failures will obviously break the 

simulation. The most important factor in deciding to terminate the simulation will be the severity of the 

emergency or failure. In addition, as there is great difficulty in isolating “Mars” crewmembers from 

“regular” ISS crewmembers, it is crucial that International Partners (Russia, Canada, Japan, and the 

member states of the European Space Agency) be fully involved in the simulations, and not merely 

observers. 

All of the partners in the ISS are already obligated to use the space station’s unique characteristics to 

answer fundamental questions in the physical sciences, life sciences, and technology development, in 

fulfillment of national commitments. Those obligations need not be compromised by re-purposing the ISS 

as a Mars-transit analog. A range of Mars-related simulations could be conducted depending on 

requirements and resources and could include simply adding a maintenance task to a regular mission to 

targeting the penultimate ISS increments as dedicated simulation increments about 2 years before 

decommissioning the station.

Finally, unlike in a real Mars mission, Earth will always be outside the ISS windows. This phenomenon is 

unavoidable, and should temper the effort dedicated to maximizing the realism of other aspects of the 

Mars transit simulation. 

1.4 Future Enhancement of International Space Station Analog

A future enhancement of the Mars mission simulation experience on the ISS is possible, albeit at 

significantly greater cost. To increase the fidelity of the Mars analog scenario, one could incorporate a 

planet-side phase into a full mission simulation. In the most extensive version, this would involve backto-back ISS tours separated by 6 to 18 months on Earth in a simulated Mars surface environment, perhaps 

Antarctica, or in some specially designed facility. This configuration would provide the best simulation of 

an entire Mars mission for a variety of reasons. In particular, we speculate that human factors and some of 

the behavioral health issues do not asymptote, peak and recede, or increase linearly with mission duration. 

Rather, they are expected to arise only past the 6-month mark, especially when communication and 
7

support from Earth are reduced or lost. Risks associated with an enhanced analog mission include crew 

safety and health ethical issues, especially concerning nonintervention during a medical event. In 

addition, a detailed cost-benefit analysis would have to be performed before commitments were made by 

crewmembers, the ISS Program, and International Partners.

2.0 RECOMMENDATION

The ISS provides components of a high-fidelity, cost-effective simulation of an eventual Mars (and NEA) 

mission: personnel (both in flight and on the ground) representative of likely Mars mission participants; a 

vehicle in space, with all of its operational ramifications; exposure to—and threat from—the actual space 

environment; perceived risk to life and limb that is qualitatively similar to that of an exploration mission, 

and one that could not ethically be duplicated in ground-based studies; and meaningful work. 

Any such simulation is limited by the constant presence of Earth outside the window, aspects of 

infrastructure (e.g., resupply timing, real-time Mission Control monitoring) that cannot easily be altered, 

potential confounding influences of ongoing non-Mars experiment activities, and the ability to break 

simulation when necessary. The benefits of conducting such missions are so great that the limitations can 

be tolerated; therefore, the SME group recommends using the ISS for Mars simulations. 

A logical progression of activities during ISS increments would permit the orderly development of Marsforward activities with minimal disruption of ongoing non-Mars activities, at least in the near term. In 

general terms, a four-stage approach is indicated (suggested dates are notional, and assume U.S. 

commitment to the ISS through 2025):

1. 2010-2012: Current ISS operations and activities in which operational and experimental protocols 

intended to protect the safety, health, and efficiency of ISS crewmembers are evaluated for their 

applicability to Mars (and NEA) missions.

2. 2013-2016: ISS increments into which are inserted discrete Mars-forward activities, such as 

intermittent multiday periods exploring different degrees of bounded autonomy by the ISS crew, 

including communication delays typical of Mars missions. Sets of assigned tasks will be 

accomplished with minimal intervention by Mission Control, but with few alterations to onboard 

procedures and Mission Control monitoring of ISS systems. Minimal impact to non-Mars 

onboard science operations will be required. Flight rules will specify the threshold at which the 

simulation is broken in case of emergency or system malfunction.

3. 2017-2020: ISS increments in which crew procedures and Mission Control oversight are modified 

to provide more realistic experience in in-flight autonomous operations to both crew and ground 

personnel. More rigorous, longer periods of autonomy are planned. There will be some inevitable 

impact to non-Mars-related scientific activities. 

4. 2021-2025: ISS increments in which transits to Mars (and NEAs) are simulated as rigorously as 

feasible in low-Earth orbit with the existing infrastructure. Progressively increasing 

communication delays may be introduced, reaching a maximum of 6 to 8 minutes after 6 months 

to mimic arrival in Mars proximity. Onboard science operations must be compatible with Marslike mission parameters.
8

Given the need for a series of incremental demonstrations of high autonomy and other Mars-related 

procedures and technologies, preparations for full opposition-class4 simulated missions must commence 

soon. Preparations, including crew recruiting, informed consent, and long-term planning for simulations 

of even the 6-month Mars transit must commence soon to maximize their value and minimize their impact 

on ongoing use of the ISS for other purposes. 

Results from these and other possible simulations can then be used to inform Mars crew selection and 

composition for missions circa 2030.

3.0 CONCLUSION

The use of the ISS to simulate aspects of Mars missions seems practical. Current ISS operations and 

human biomedical research are already analogous to Mars missions because every 6-month ISS mission 

mimics portions of future Mars missions. Longer durations, up to 9 to 12 months, may be possible 

without significantly increased risk to crewmembers beyond that of 6-month missions. Incrementally 

increasing Mars-specific focus is achievable, but will require programmatic decisions. Higher-fidelity 

Mars simulations involving the ISS may foreclose other options for ISS use, requiring International 

Partner buy-in. 

A group of NASA flight surgeons at Johnson Space Center has observed that “... an Expedition-class life 

science research program would allow the ISS Program to not only further our understanding of 

deconditioning and countermeasures, but pave our way to Mars.”5 The ISS can and should be a valuable 

resource to prepare for future human Mars missions, and the highest-possible fidelity simulation of the 

Earth-to-Mars transit phase is the best fit to ISS capabilities.
9

4.0 WORKSHOP ATTENDEES

Workshop on International Space Station Crew Increment Extension Beyond 6 Months,

For Use as Mars Analog

Center for Advanced Space Studies

Houston, Texas

September 25, 2009

Maneesh Arya

Yael Barr

Regina Buccello-Stout

Michael Chandler

John Charles

Tamara Durham

Stephen Hart

Cindy Haven

Judy Hayes

Albert Holland

Mary Kaiser (by telephone)

Lauren Leveton

Linda Loerch

Ronald Moomaw

Joe Neigut

Michele Perchonok

Bob Pietrzyk

Michael Rapley

Richard Scheuring

Edward Semones

Mark Shavers

Camille Shea

Gwyn Smith

Tom Sullivan

Terrance Taddeo

Mary Van Baalen

Robert Welch (by telephone)

Mark Weyland

Sandra Whitmire

Barbara Woolford
10

5.0 REFERENCES

1 Anon. The Vision for Space Exploration (NASA/NP-2004-01-334-HQ), 

http://www.nasa.gov/pdf/55583main_vision_space_exploration2.pdf (accessed 30 Nov. 2010).

2 Monos E, Lorant M. Vestibular control of the cardiovascular system. Orv Hetil 139: 1851-1855, 1998.

3 Drake BG. Human Exploration of Mars Design Reference Architecture 5.0 (NASA/SP-2009-566), p. 

47, http://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf (accessed 21 Oct. 2010).

4 Drake, BG. Decadal Planning Team Mars Mission Analysis Summary (NASA/TM-2007-214672), p. 

23, http://ston.jsc.nasa.gov/collections/TRS/_techrep/TM-2007-214762.pdf (accessed 21 Sep. 2010).

5 The Skylab Medical Operations Project (NASA/TM-2009-214790), p. 6.

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Use of International Space Station to Simulate Interplanetary Transit: Human Health and 

Performance Applicability of Current Increment Durations and Extended Durations

6. AUTHOR(S)

John B. Charles, NASA Johnson Space Center

Maneesh Arya, Wyle Integrated Science and Engineering

Craig E. Kundrot, NASA Johnson Space Center 

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13. ABSTRACT (Maximum 200 words)

Preparation for future crewed missions to Mars will require utilization of space analogs for research (such as for feasibility and 

verification of monitoring technologies or countermeasures) and for training of flight crews and ground support personnel. Although 

several ground-based analogs mimic various aspects of space exploration missions, the influence of Earth’s ever-present surface 

gravity cannot be ignored. Until recently, lunar missions were expected to be a stepping-stone for Mars expeditions, but recent 

programmatic de-emphasis of lunar missions has encouraged consideration of piloted missions to Near-Earth Asteroids (NEAs) 

instead. It has been suggested that in-space simulations of Mars (and now NEA) missions be conducted on the International Space 

Station (ISS). Some possibilities for, and space life sciences implications of, such an effort are described. A brainstorming session 

involving space life sciences subject matter experts associated with the NASA Human Research Program, the Johnson Space Center 

Space Life Sciences Directorate, and the Ames Research Center Exploration Technology Directorate was convened on September 25, 

2009, to respond to two questions: how can the ISS be used to mimic Mars missions; and how can crew increment duration be 

increased to 9 to 12 months from the existing 6 months. Positions and recommendations are indicated. 

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