In This Issue
Winter Issue of The Bridge on Frontiers of Engineering
December 25, 2021 Volume 51 Issue 4
The NAE’s Frontiers of Engineering symposium series forged ahead despite the challenges of the pandemic, with virtual and hybrid events in 2021. This issue features selected papers from early-career engineers reporting on new developments in a variety of areas.

Mars Walking: Enabling Crew Health and Performance during Extravehicular Activity

Tuesday, January 4, 2022

Author: Andrew F.J. Abercromby

Capability gaps must be addressed to ensure astronauts’ safety and performance during extravehicular activity on missions to Mars.

The benefits, the opportunities, the challenges, and the risks of space exploration increase by orders of magnitude when sending humans beyond Earth’s orbit and out into the solar system. Keeping a healthy human alive and well in the most extreme of environments requires spacecraft with life support systems that provide a bubble of Earth-like atmosphere far out in the void of space. Even if that delicate bubble is successfully preserved all the way from the launchpad to the surface of Mars (at least 34 million miles from Earth—a voyage of 6–9 months) and back again, the astronauts living inside it are confronted with an array of challenges and stressors, some of which are immediately apparent while others may take weeks, months, or even years to manifest.

Physiological Challenges of Spaceflight

The transition from the strong pull of Earth’s gravitational field to the ­sudden experience of weightlessness of orbit can cause an array of symptoms, including disorientation, headaches, nausea, vomiting, and gastrointestinal discomfort (Barratt and Pool 2008; Buckey 2006). While these symptoms typically subside within 1–3 days as the central nervous system adapts, other long-term adaptations continue.

In response to weightlessness, altered sleep cycles, isolation, radiation, fluctuating carbon dioxide levels, and other environmental stressors, the body’s muscles and bones lose strength, and the ability to perform maximal (or prolonged submaximal) aerobic exercise decreases (Shen and Frishman 2019). Cephalad fluid shifts can cause congestion, affect taste and smell, and may contribute to structural changes in the eyes, causing loss of visual acuity (Lee et al. 2020). Radiation exposure in low Earth orbit is significantly greater than that experienced on Earth, causing cellular damage that contributes to lowered immune function during spaceflight and increased lifetime cancer risk (Benton and Benton 2001). When astronauts leave the protection of Earth’s magnetosphere, the radiation exposure and associated risks become greater still.

The myriad physiological stressors compound the psychological challenges associated with living in an isolated, confined, and extreme environment (­Palinkas 2001). Astronauts typically work as members of inter­national crews of up to seven people, with current missions lasting as long as a year; Mars missions lasting 2–3 years or more will far exceed previous human experience.

Long-duration human missions to the moon and Mars will require advanced technological capabilities, some of which do not yet exist. NASA refers to these as capability gaps (Burg et al. 2021).

In this paper I describe challenges, progress, and opportunities associated with four capability gaps relating to the maintenance of human health and performance during extravehicular activity (EVA). Often referred to as spacewalking, EVA will be among the most frequent, highest-workload, and highest-risk activities during human missions to the moon and Mars. It is also perhaps the most important element, both functionally and symbolically, that distinguishes human space exploration from robotic missions.

Capability Gap: Space Suit Fit and Injury

Injury prediction, monitoring, and mitigation technologies are needed to enable planning, training, operations, and system design for all suited mission phases and for all anticipated crewmember anthropometries.

Multiple injuries to astronauts have occurred while working in spacesuits, even with a relatively low frequency of EVAs (Chappell et al. 2017; Stirling et al. 2019). Reported injuries range in severity from blisters to fingernail delamination to shoulder injuries requiring surgery. In some cases, poor suit fit is believed to have been a contributing factor, and data have also shown that reduced recovery between successive EVAs also increases injury risk.

Future spacesuits must ensure that male and female astronauts of all shapes and sizes are not only accommodated by the suit but can perform all necessary mission tasks without discomfort or increased injury risk. Changes in anthropometry that occur during spaceflight must also be identified and accommodated (Young et al. 2021).

In addition, suits must protect astronauts expected to perform far more EVAs, with far less recovery time than ever before. Apollo surface stays on the Moon were up to 3 days in duration, with astronauts wearing custom-fit spacesuits for up to three EVAs; current mission designs considered by NASA call for as much as 24 hours of EVA per person per week throughout surface stays that may be months in duration.

EVA injury incidence during Apollo as well as in the current spacesuit used on the International Space ­Station (ISS) suggests that musculoskeletal injuries that affect mission objectives, and potentially long-term health, are not only possible but likely during these types of long-duration surface missions unless capabilities are developed and implemented to improve fit and mitigate injury risk. Computational anthropometric and injury modeling (examples of which are shown in figure 1) will be an important part of this capability development.

Abercromby figure 1.gif
FIGURE 1 Computational anthropometric and injury modeling capabilities are needed to help ensure health and performance for crewmembers of all shapes and sizes. Images show examples of model-predicted volumetric overlap (i.e., locations and degree to which suit may be too small for an astronaut).

Capability Gap: EVA Crew Capabilities and Constraints

Crewmember physical and cognitive state monitoring and prediction technologies will be essential to enable EVA planning, operations, system design, and decision support systems based on crewmember capabilities and constraints.

When returning to Earth after long-duration stays in microgravity, crewmembers are nauseated and have sometimes extremely reduced ability to perform even simple tasks such as walking, until their vestibular system has readapted to Earth’s gravity (Mulavara et al. 2018). Unlike landing on Earth where a support team can lift astronauts out of their capsule, Mars astronauts must adapt to the transition from the micro­gravity of the voyage to Mars to the partial gravity of Mars (about 3/8 of Earth’s gravity), get into their EVA suits, and perform any necessary postlanding tasks themselves.

Uncertainty and variability in what functions each astronaut will be capable of performing at any given point in a mission present many chal­lenges, compounded by uncertainty in the exact physical and cognitive demands and cost associated with many of those functions when performed in the lunar or Mars environment.

As another example, the full-body physical workload associated with planetary EVA, especially in Mars gravity, is expected to be higher than for EVAs in microgravity (Norcross et al. 2010), where instances of fatigue-related performance decrements are already anecdotally apparent. In addition to potential performance implications, this increased workload may impact consumables usage, caloric expenditure, heat storage, CO2 exposure, decompression sickness risk, and hydration. Human testing using prototype spacesuits in reduced-gravity simulations (e.g., figure 2) is essential to understanding these effects and developing the capabilities to accommodate them.

Abercromby figure 2.gif
FIGURE 2 Testing to characterize crewmember capabilities and constraints in spacesuits often uses test environments such as the Active Response Gravity Offload System (ARGOS) shown here, which allows for simulation of reduced-gravity EVA conditions using a system similar to an overhead bridge crane.

Capabilities to predict and monitor physical and cognitive state before and during EVA are currently very limited but will be essential to the safe planning and execution of EVA during Mars missions.

Capability Gap: EVA Bioinformatics and Decision Support

Ground-based and in-flight EVA technologies need to be developed to maintain EVA crew health and performance monitoring and decision making during increasingly Earth-independent operations.

The unavoidable communications latency between Earth and Mars will require that many of the support, monitoring, command, and control functions currently provided by a large team of flight controllers in Mission Control Center (MCC) on Earth be performed by astronauts and their spacesuits or spacecraft. This paradigm shift will require a significant evolution of technology, training, and operations. It is also likely to increase the cognitive workload for EVA crewmembers.

Capabilities to enhance EVA scientific exploration under communications latency have been the subject of previous studies (e.g., Abercromby et al. 2013; Beaton et al. 2019; Rader and Reagan 2013), but the critical role of EVA medical operations support to continually monitor and protect crew health and performance during EVA with communication latency is not well understood. The physical and cognitive state prediction capability described above will have to integrate with life support system models and purpose-built operational EVA planning and execution tools that protect and provide for health and performance during the planning, execution, and real-time replanning of EVAs.

These capabilities are enabling for inflight crew­members on Mars to fulfill the many critical EVA support functions currently performed by humans and systems in MCC, but they are also enhancing and possibly even enabling for high frequency EVA during lunar surface missions. Hundreds and sometimes thousands of person-hours go into planning and preparing for a single EVA on the ISS; significantly improved planning capabilities are required to enable high frequency exploration EVA.

Capability Gap: Decompression Sickness Mitigation

The fourth capability gap is the decompression stress prediction and mitigation technologies that will be needed to enable EVA planning, training, operations, and system design for planetary surface missions where existing microgravity decompression sickness countermeasures are not applicable.

Apollo missions used a 100 percent O2 cabin atmosphere, which effectively eliminated the risk of decompression sickness (DCS) during EVA on the moon. NASA’s future missions to the moon and Mars are expected to use nitrox gas mixtures of up to 34 percent O2, 66 percent N2; this will reduce flammability risk compared with Apollo, but will necessitate oxygen prebreathe prior to EVA to reduce DCS risk to acceptable levels (Abercromby et al. 2015). Prebreathe protocols used on the space shuttle and ISS are validated for microgravity EVAs, but the significantly increased risk of DCS during equivalent ambulatory EVAs makes these protocols inapplicable to planetary EVA (Conkin et al. 2017).

An “exploration atmosphere” of 56.5 kilopascals (kPa) (8.2 psia), 34 percent O2, 66 percent N2 has been recommended by NASA as a compromise that ­balances prebreathe duration, hypoxia, and flammability risk, assuming a 29.6 kPa (4.3 psi) spacesuit. However, this atmosphere may not be used for vehicles that do not support frequent EVA; and with commercial and inter­national providers expected to deliver landers, pressurized rovers, habitats, and spacesuits, different combinations of vehicle and spacesuit atmospheres are possible and will each require validated prebreathe protocols.

A validated DCS risk prediction tool, encompassing the range of potential spacecraft and spacesuit pressures and atmospheres, will enable risk-informed development and operation of all future spacecraft and spacesuit operations, including contingency scenarios such as cabin depressurizations.


The specific research, development, and testing necessary to close each of these capability gaps is captured in the Crew Health and Performance (CHP) EVA ­Roadmap, a multiyear strategic planning document that is updated on an ongoing basis. The most recently published version is organized around seven gaps, which have since been consolidated to the four gaps identified here (Abercromby et al. 2020). The updated roadmap is expected to be published in 2022.


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human missions to the moon and Mars will require advanced technological capabilities, some of which do not yet exist.




Capabilities to predict and monitor astronauts’ physical and cognitive state before and during extravehicular activity will be essential for safe Mars missions.



About the Author:Andrew Abercromby is lead of the Human Physiology, Performance, Protection, & Operations (H-3PO) Laboratory at NASA Johnson Space Center in Houston.