Review Article | Open Access
Andrew Terhorst, Jason A. Dowling, "Terrestrial Analogue Research to Support Human Performance on Mars: A Review and Bibliographic Analysis", Space: Science & Technology, vol. 2022, Article ID 9841785, 18 pages, 2022. https://doi.org/10.34133/2022/9841785
Terrestrial Analogue Research to Support Human Performance on Mars: A Review and Bibliographic Analysis
Terrestrial analogues can provide essential scientific information and technology validation to assist future crewed missions to the Martian surface. This paper analyses the recent literature since 2010 in this area, highlighting key topics, authors, and research groups. It reviews analogue locations, missions, the scientific impact from research activities. The findings indicate that permanent analogue sites enable reproducible science and objective comparison between studies. A standard, open registry of analogue facilities, and associated peer-reviewed research may lead to accelerated and better targeted analogue research.
Sending humans to Mars is an extremely challenging undertaking. With current propulsion technology, a round-trip to Mars will take at least 18 months to complete and may expose crew members to serious health risks during transit and on the surface of Mars. Risks include extended exposure to ionizing radiation, impacts of microgravity on the musculoskeletal system and visual acuity, psychological effects of prolonged isolation and confinement, lack of medical facilities, dietary impacts from limited food choices, and failure of life-support systems.
Terrestrial analogues allow controlled and safe simulation of some of the environmental conditions which crew members will experience traveling to and living on Mars. They provide a way to assess human performance, experiment with different habitats, test life-support systems, practice extravehicular activities, evaluate remote medical procedures, and trial robot technology [1, 2]. The term terrestrial analogue can mean different things. For some, it refers to sites that resemble the geology or geomorphology of Mars. Such sites allow one to test equipment and assess extravehicular activity in a simulated environment. Others are interested in sites with life forms that live and survive in extreme conditions similar to those found on Mars. Arctic and Antarctic sites mimic the extreme cold and remote isolation that a crew on Mars will experience. There is evidence suggesting frozen water may exist on Mars. Arctic and Antarctic sites also allow testing of ice-drilling equipment for extracting water. Not all terrestrial analogues are in remote or Mars-like places. Experiments to assess how crews cope with living together in a small confined habitat over an extended period have been done on university campuses, space facilities, and in research institutes. Different approaches to in-situ resource utilization have been trialed in an urban setting. The location of a terrestrial analogue site depends on what its primary purpose is.
Table 1 lists analogue facilities used to investigate human performance, endurance, and behavior. This list is by no means exhaustive but features sites that have either operated over many years or been the subject of academic articles (see Table 2). Facilities are operated by space agencies (e.g. NASA, ESA), universities (e.g. Florida International University, University of Hawaii), volunteer-driven non-government organisations (e.g. Mars Institute, Mars Society, Austrian Space Forum), or private entities (e.g. D-MARS). Many new analogue sites are being planned as public-private partnerships, citizen-science projects, educational outreach programs, or adventure tourism experiences. The growing interest in Mars analogue sites can be attributed to the emerging plans to undertake human missions to Mars and success of recent robot missions to Mars such as the Phoenix Lander, Curiosity and Perseverance rovers, and Ingenuity helicopter missions.
This review provides an overview of published research since 2010 with a focus on analogue activities to support human health in future Martian missions. The review is preliminary in nature and aims to better understand how different analogue activities address human health and performance related issues.
The bibliometric analysis is based on the results of a Scopus search. Scopus is Elsevier’s abstract and citation database covering more than 22,500 active titles in the life sciences, social sciences, physical sciences, and health sciences. The database can be queried through a web browser interface or via an application programming interface (API). This review used an R package called rscopus to query the database via the API . After reading some background material on human missions to Mars, we crafted three search queries that focused on human performance. Crafting three separate queries made tuning of queries easier. The first query addressed the health risks associated with a human mission to Mars:
Each query considered articles published from 2010 onward and specifically excluded articles focused on geological or astrobiological analogues. Merging the query results created an initial corpus of 1606 articles. The merged query result includes title, abstract, author keywords, source title details, unique document identifier, and information about funding.
Despite crafting specific queries, the number of false positives was high. The term MARS is a popular acronym used in a variety of contexts unrelated to planetary exploration. To fix this, a three-pronged strategy was employed. This strategy involved discarding articles with nonrelevant, subject codes, performing topic modeling to identify articles of primary interest, and eliminating any remaining false positives manually. We employed several rscopus query functions to capture specific information about each article, namely subject codes (as per the All Science Journal Classification Codes (ASJC) schema) and author details (such as current affiliation and publication record). The initial corpus of 1606 articles covered 251 subject codes, of which 190 were deemed relevant after manual scrutiny. Removing articles with irrelevant subject codes reduced the corpus to 1409 articles.
Topic modeling was performed on 1409 abstracts using the topicmodels package . This package uses Latent Dirilecht Allocation (LDA) to identify topics of interest. LDA is a probabilistic topic model and, in our case, treats each abstract as a mixture of topics and each topic as a mixture of words. LDA allows abstracts to overlap each other in terms of content, rather than being separated into discrete groups, in a way that mirrors typical use of natural language . We employed the ldatuning package  to decide the optimal number of topics to model (the optimal value). In our case, the optimal value was 40 (Supplementary Figure S1). Of the 40 topics, only 16 are relevant for this review. Table 3 lists the 40 topics in descending order of dominance, together with the top 10 terms associated with each topic.
Using the three-pronged strategy reduced the initial corpus from 1606 articles to 579 articles (1027 articles were either irrelevant or out of scope for this review). We used the Source-Normalized Impact per Paper (SNIP) metric to assess the potential impact of each article . SNIP scores are the ratio of a source’s average citation count and citation potential (the number of citations that a journal may expect to receive for its subject field). It allows for direct comparison between fields of research with different publication and citation practices. A downside is that SNIP only considers peer-reviewed sources.
Results from the filtered Scopus queries were used to generate co-author networks, tables of the most cited articles, leading researchers, most active research groups, main providers of research funding, and a breakdown of which subjects codes are receiving the most scholarly attention and which countries are producing the most articles.
Figures 1–5 depict the top five connected graph components (research clusters) of the co-author network. Each cluster represents a stand-alone sub-graph in the co-author network (clusters are not connected to each other in any way). Single authors are excluded from the network (isolated nodes in a graph). Authors are colored according to which country they currently work in. The intensity of the color is controlled by the author’s h-index (the more intense or saturated the color, the higher their h-index). Authors are sized according to their betweenness centrality (authors with high betweenness centrality connect otherwise disconnected research communities). Edges are weighted by the number of publications co-authored between any two researchers (a measure of collaboration strength).
The top five connected graph components represent the most prominent research clusters in our corpus of 579 articles. Note that the research clusters are ordered in terms of size. Cluster 1 is the biggest, whereas Cluster 5 is much smaller. Authors in Cluster 1 (Figure 1) are primarily concerned with various aspects of mission planning and design. Most of the authors in this cluster are from the USA. What is particularly interesting is that the edges in Cluster 1 show no ongoing collaborations between authors (there are no edges with ). We also note that most authors have a low h-index which suggests either the authors are quite young or are more practically inclined than academic. Mission planning and design is likely to have a strong engineering focus. Authors Carmen Felix, Melanie Grande, and Chel Stromgren stand out as key connectors in Cluster 1. One would expect these authors to have a good handle on various aspects of mission planning and design. The loosely-coupled subclusters within Cluster 1 also reflect the diverse nature of mission planning and design.
Authors in Cluster 2 (Figure 2) are concerned with measuring space radiation and assessing the risk to human spaceflight. Cluster 2 is the most diverse in terms of the nationality of co-authors. The level of collaboration in Cluster 2 is relatively high, judging by the cluster density and heavily weighted edges in two subclusters (some co-authors have nine publications between them). Unlike Cluster 1, many of the authors in Cluster 2 have a high h-index (we see more saturated node colors). This implies the authors in Cluster 2 are have strong academic credentials and that the quality of publications is likely to be high. Authors Carey Zeitlin and Igor Mitrofanov stand out as key connectors in Cluster 2.
Authors in Cluster 3 (Figure 3) are primarily concerned with evaluating human performance in simulated extravehicular activity. This cluster is also quite diverse in terms of the nationality of co-authors. Cluster 3 is noticeably smaller than Clusters 1 and 2. Most of the authors in Cluster 3 have a relatively low h-index (Mario Lassnig is a notable exception). Authors that stand out as key connectors in Cluster 3 include Sheryl Bishop, Gernot Groemer, Norbert Frischauf, Olga Bannova, and Kim Binsted.
Authors in Cluster 4 (Figure 4) are concerned about the physiological and psychological effects of prolonged isolation in space habitats. Authors from Russia, Germany, and Italy dominate Cluster 4. Many of the authors in this cluster have a relatively high h-index, an indicator the research quality in this cluster is high. Vadim Gushin and Elena Feichtinger stand out as central actors in this cluster.
Cluster 5 is quite small (Figure 5). Authors here are particularly interested in how prolonged isolation affects the human immune system. Cluster 5 is dominated by authors from Russia and Italy. Igor Nichiporuk and Galina Vassilieva are the main actors in this cluster.
The top twenty manuscripts by citation count are shown in Table 4. Papers relating to the risks of exposure to ionizing radiation make up ten of these papers. Thirteen of these papers acknowledge grants or funding from space agencies (eight of which are NASA). The top funding organisations acknowledged from the corpus are listed in Supplementary Table S4.
The ten journals with the combined highest number of citations in the corpus were: Acta Astronautica (439 citations, from 35 publications), Science (323, 1), Space Weather (292, 14), Scientific Reports (162, 6), PLos ONE (103, 1), Advances in Space Research (99, 7), Microbiome (47, 2), Experimental Neurology (46, 1), Construction and Building Materials (44, 1), and Astronomy and Astrophysics (41, 2).
The top ten journals in order of the number of submissions were: Acta Astronautica (35 publications), Human Physiology (20), Space Weather (14), Advances in Space Research (7), Journal of the British Interplanetary Society (7), Scientific Reports (6), Aviation Space and Environmental Medicine (5), Advances in the Astronautical Sciences (4), Aerospace Medicine and Human Performance (4), and Journal of Physics: Conference Series (3).
Most of the papers included in the corpus were published in conference proceedings, however they were not highly cited. The top ten main conferences were Proceedings of the International Astronautical Congress (153 publications, 95 citations), IEEE Aerospace Conference Proceedings (26, 108), 62nd International Astronautical Congress 2011 (12, 6), 61st International Astronautical Congress 2010 (11, 9), Accelerating Space Commerce, Exploration, and New Discovery Conference, ASCEND 2020 (10, 1), AIAA SPACE 2015 Conference and Exposition (10, 42), 2018 AIAA SPACE and Astronautics Forum and Exposition (9, 21), AIAA SPACE and Astronautics Forum and Exposition, SPACE 2017 (9, 13), AIAA Space and Astronautics Forum and Exposition, SPACE 2016 (8, 31), and Earth and Space 2021: Space Exploration, Utilization, Engineering, and Construction in Extreme Environments (8, 0).
An analysis of the main subject areas covered by the articles was also conducted. Engineering is the dominant subject (24%), followed by Earth and Planetary Sciences (19%), and Physics and Astronomy (15%). The minor subjects (e.g. Agriculture and Biological Science, Energy, Material Science, Medicine, and Social Science) are of particular interest as these are more likely to address specific aspects of space habitats.
Table 5 lists the top relational stars; these are authors with high betweenness centrality who connect otherwise disconnected research groups . Betweenness centrality is a widely used graph measure that captures a person’s role in allowing information to pass from one part of the network to the other. The top relational stars in the co-author network are Chel Stormgren, Melanie Grande, and Sheryl Bishop. We can assess the potential influence of authors by their network reach (network path length to the furthermost actor). Cary Zeitlin, Jingnan Guo, and Igor Mitrofanov are potentially the most influential authors in the network. Authors with high betweenness centrality and high h-index can be seen as super stars: authors who are not only highly productive researchers but good connectors as well . Cary Zeitlin, Jingnan Guo, and Igor Mitrofanov may also be considered super stars in this instance.
The 20 most prestigious authors in the corpus according to their h-index (an author-level metric that measures both the productivity and citation impact of a scientist or scholar ) were also extracted. Ten of these authors from the United States, with the remaining nine from Europe, and one from Canada (Supplementary Table S2). Interestingly, none of these authors appear in the list of relational stars in Table 5. The most productive research groups in terms of total number of publications are listed in Table 6.
Figure 6 lists the countries with the highest number of articles published, and highlights the ratio of papers with authors from different countries. The most productive research groups are based in the US, China, and Russia (Table 4). NASA dominates the US groups while the Russian groups are part of the Russian Academy of Sciences. All the privately-funded research groups are based in the US.
The section provides an overview and discussion of the highest impact analogue sites referenced in the corpus (and Table 1).
The Mars-500 missions and the Mars Desert Research Station (MDRS) analogue generated the most peer-reviewed research publications (151 and 123 publications, respectively). Mars-500 was hosted by the Institute for Biological and Medical Problems in Moscow under the auspices of the Russian, European, and Chinese space agencies. Volunteers took part in three extended duration missions lasting between 115 and 520 days in a simulated Mars habitat. The goal was to assess the mental and physical state of volunteers and how this impacted team dynamics as each mission progressed . Compared to the three Mars-500 missions, the Mars Desert Research Station missions are much shorter and more varied. Owned and operated by the Mars Society, the Mars Desert Research Station engages in field studies as well as human factors research. The desert landscape allows volunteers to perform extravehicular activities in a Mars-like setting [11, 12].
NASA is sponsoring several analogue activities as part of its human research program. NASA has tested equipment and run extravehicular activity scenarios at the Haughton-Mars Project Research Station on Devon Island in the Canadian Arctic . It uses the underwater Aquarius Reef Facility operated by Florida International University to run the NASA Extreme Environment Mission Operation (NEEMO)  and sponsors the Hawai’i Space Exploration Analog and Simulation (HI-SEAS) on the slopes of the Mauna Loa volcano on Hawai’i Island . It operates the Human Exploration Research Analog (HERA) and Human Exploration Spacecraft Testbed for Integration and Advancement (HESTIA) at the Johnson Space Center outside Houston, Texas . NASA also operates a mobile analogue site as part of the Desert Research and Technology Studies (D-RATS) program . The D-RATS mobile analogue consists of a relocatable habitat and two crewed all-terrain vehicles (multimission space exploration vehicles). NASA is also involved in the Scientific International Research in a Unique Terrestrial Station (SIRIUS) at the Institute for Biological and Medical Problems in Moscow and Cooperative Adventure for Valuing and Exercising human behavior and performance Skills (CAVES) operated by ESA. SIRIUS is a follow-up to Mars-500.
The NASA Space Radiation Laboratory located at the Brookhaven National Laboratory in New York is an important facility for simulating the effects of cosmic radiation on biological material and appears in a number of the highest impact (by citation) publications [17–19].
4.1. Remote Analogue Sites and Facilities
4.1.1. Sites Used by NASA
NASA uses remote facilities operated by third parties for testing communications, evaluating equipment, and practicing vehicular and extravehicular operations. Facilities include the Haughton-Mars Project (HMP) research station in the Canadian Arctic, the Mars Desert Research Station in Utah, the Aquarius habitat off Key Largo in Florida , and the the Hawai’i Space Exploration Analog and Simulation (HI-SEAS) facility on Hawai’i Island.
The Haughton-Mars Project Research Station is owned by the Mars Institute, headquartered at the NASA Ames Research Center. This analogue facility was established in 1997 and has received significant funding from NASA and the Canadian Space Agency. The Haughton-Mars Project encompasses both a science and an exploration program . The science program focuses on the site’s geology and biology and how this relates to other planets and moons in the Solar System. The exploration program uses the site to develop, test, and validate new exploration technologies and strategies for future human and robotic exploration of the Moon and Mars. Exploration systems studied include habitats, spacesuits, ground vehicles, unmanned aerial vehicles, robotic rovers, drills, instruments, tools, life support systems, plant-growth systems, communications, and other information systems. Other research conducted at the Haughton-Mars Project includes human factors and crew management studies [21, 22].
The Aquarius Reef Base Habitat is used by the NASA Extreme Environment Mission Operations (NEEMO). This underwater facility was established in 2001 by NOAA but is now owned and operated by Florida International University. The facility mimics the isolation, constrained habitat, harsh environment, and reduced gravity of space exploration missions. NASA runs an annual mission between two and three weeks to train crew and conduct behavioral, physiological, and psychological experiments. Other mission activities include testing hardware and operational procedures .
The HI-SEAS facility was created as a public-private partnership between NASA and the University of Hawai’i. The facility has hosted six long duration NASA analogue missions that have explored biopsychosocial impacts of isolated and confined living conditions, and to assess space-flight crew dynamics, behaviors, roles, and performance [23, 24] The final eight-month mission was stopped prematurely in 2018 and no further NASA-funded deployments to this facility have occurred. Since 2018, the research station has been operated by the International MoonBase Alliance [25, 26]. Although the University of Hawai’i operation of the facility has ceased, NASA continues to fund the database of research results from the five successful deployments.
The NASA Desert RATS Program has been operational since 1997. The mobile analogue tests roving and extravehicular activity operations in an environment that features extreme temperatures and challenging terrain . The Desert RATS program has involved exploration missions at Black Point Lava Flow, Arizona, Meteor Crater, and private ranch land (all in the general vicinity of Flagstaff, Arizona) as well as one deployment to the NASA Johnson Space Center’s Rock Yard . The Desert RATS Program aims to investigate the most practical combination of rovers, habitats, and robotic systems, optimum crew size, effects of communication delays, autonomous operations, and how to maximize science outcomes.
4.1.2. ESA Sites
ESA launched an astronaut training program in 2011 called CAVES (Cooperative Adventure for Valuing and Exercising human behavior and performance Skills). The program has trained 34 astronauts from six different space agencies (ESA, NASA, JAXA, ROSCOSMOS, CSA, and CNSA). The primary objective of CAVES is to develop individual and team performance through exposure to the challenges of a real mission within an unknown and dangerous environment. The course’s training activities are based around a real scientific and technological program focused on cave science .
4.1.3. Mars Society
The Mars Society is a nongovernmental organisation promoting human exploration and settlement of Mars. It operated two analogue sites, namely the Mars Desert Research Station (MDRS) near Hanksville, Utah, and the Flashline Mars Arctic Research Station (FMARS) on Devon Island in the Canadian Arctic.
The MDRS site started operating in 2001 as a volunteer enterprise. Because the site resembles the geology of Mars, it is well-suited for field studies and associated human factors research. Most crews carry out two- to three-week missions under the constraints of a simulated Mars mission 1. Several missions have examined crew function and performance from a variety of perspectives, e.g., psychological support [29, 30], challenging workloads [31, 32], and team composition . These missions aimed to explore different ways to support crew psychological health in prolonged space missions. Other missions have evaluated habitat design with particular emphasis on greenhouse operations [34, 35], considered the difficulty of maintaining critical habitat infrastructure , and measured the range and duration of extravehicular exploration activities .
The FMARS site was established in 2000 but is currently dormant. There have been no documented missions since 2017. Research at FMARS has focused on crew function and performance related to geological and astrobiological fieldwork . Of the two analogues sites operated by the Mars Society, MDRS has generated at least 150 publications over its lifetime, whereas FMARS has only generated about 23 or so publications over its lifetime.
4.1.4. Austrian Space Forum
The Austrian Space Forum is a national network of aerospace specialists and space enthusiasts. Instead of operating permanent analogue sites, it runs missions every two to three years using temporary analogue sites in different countries. Apart from running analogue missions, the forum also builds spacesuit simulators and offers analogue astronaut training programs .
The forum carried out a mission that tested its Aouda.X spacesuit simulator and Phileas rover at the Rio Tinto geological analogue site in southern Spain. Analogue astronauts collected geological samples in a simulated mission that involved communication from a mission control center based in Innsbruck . The mission highlighted the importance of good situational awareness for extravehicular activities. A follow-up mission carried out in Morocco introduced delayed communication and tested responses to emergency scenarios . The most recent analogue mission in the Dhofar region of Oman established an inflatable surface habitat and examined the interaction between humans and robots in fieldwork . The mission also assessed team dynamics . A critical lesson from this mission was not to use technology that was still at an early stage of development . Mission success was measured using a new analogue mission performance tool .
4.1.5. Other Remote Sites and Facilities
Additional isolated, confined, and extreme environments which provide good long duration space analogues include multiple permanent stations located in Antarctica. On the high Antarctic plateau, these stations include the Concordia research station, the Russian Vostok, and the US South Pole Stations [46, 47]. The Australian Antarctic Division has collaborated with NASA since 1993 in the areas of human biology, medical research, and operational medical support .
4.2. Urban-Based Habitats
As the name implies, urban-based habitats are located near or close to urban areas. These are typically used as engineering mock-ups or for isolation and bedrest studies. There are a number of smaller facilities such as the Analogue Astronaut Training Center (AATC) , however the main urban-based centers are located in the United States, Russia, and Germany.
4.2.1. NASA Sites
NASA’s Johnson Space Flight Center in Houston, Texas, operates two analogue facilities, namely the Human Exploration Research Analog (HERA) and the Human Exploration Spacecraft Testbed for Integration and Advancement (HESTIA). HERA is three-story habitat designed to serve as an analogue for isolation, confinement, and remote conditions.
4.2.2. Institute of Biomedical Problems
The Russian Institute of Biomedical Problems (IBMP) operates the Nezemnyy Eksperimental’nyy Kompleks (NEKS) analogue on its premises in Moscow. Built in 1964, NEKS is the longest running analogue facility in the world. NEKS was used for the Mars-500 psychosocial isolation experiment conducted between 2007 and 2011. Mars-500 simulated a 520-day mission to Mars and back . NEKS is now being used for experiments under the Scientific International Research in Unique Terrestrial Station (SIRIUS). SIRIUS experiments are focused on lunar habitats and applications.
4.2.3. German Aerospace Center
The German Aerospace Center (DLR) in Cologne, Germany, hosts the :envihab facility, which enables long-term bedrest studies, in addition to a short-arm centrifuge to simulate artificial gravity countermeasures. Studies have included the NASA/ESA/DLR Artificial Gravity Bed Rest–European Space Agency (AGBRESA) [46, 51]. The center hosts a number of additional capabilities, including confinement simulation .
4.3. Directions for Analogue Research
The first human missions to Mars are currently planned for the early to mid 2030s. The mission duration may be constrained due to the health risks involved . Establishing long-stay surface habitats on Mars may not be possible for some time after the initial missions . Burying a surface habitat under tons of regolith or establishing a habitat in an underground void will be logistically challenging and extremely expensive. Robot missions appear likely to continue to be the preferred way to explore Mars, given crew safety and logistical challenges. Mars terrain analogues are likely to prioritize the testing of robot explorers. Transit habitat analogues appear likely to focus on maintaining crew health. Particular attention may be given to food intake, psychological well-being, and responding to medical emergencies.
4.3.1. Mission Duration
Mission duration is largely controlled by the orbital alignment of Mars and Earth. With current propulsion technologies, it will take at least 18 months to travel to Mars and back using the conventional Hohmann transfer orbit maneuver . The outbound journey will take nine months. Astronauts will then have three months to orbit and explore the surface of Mars before embarking on a six month return trip to Earth .
Planners have long considered 900-day missions to justify the effort and time required to get to Mars and back . NASA is currently developing a concept called the Fast Mars Transfer (FMT) to get humans to Mars and back in a year long mission. The FMT concept involves prepositioning booster rockets in both Earth and Mars orbits to help a transit vehicle achieve higher orbital escape velocities from either planet . Essentially, the outbound leg will take approximately six months, followed by one to two weeks orbiting the planet preparing for a one week sojourn on the surface of Mars, before returning to Earth. A surface habitat is likely to be predeployed to Mars . Transit and surface habitats need to be designed within well-defined mission constraints. These include propulsion and fuel requirements, payload limitations, and maintenance of crew health.
Spending less than two weeks on the surface of Mars has significant implications for terrestrial analogues. Many of the existing analogues assume humans will spend a long time on the surface of Mars. However the emphasis of terrestrial analogues might be better focused on transit habitats and extravehicular activities rather than the establishment of long-term Martian outposts.
4.3.2. Purpose of Analogues
The Mars-500 analogue stands out for the quality and breadth of data collected across the three missions. Much of this can be attributed to its specific focus, staged approach, and analytical depth. Each mission explored the psychological effects of confined isolation, team dynamics and cognitive ability, and the physical impacts of a restricted diet. The Mars-500 analogue generated numerous publications by high-caliber scientists, as evidenced by the greater than 79700 cumulative citation count of co-authors on Mars-500 related papers. Contrast this number to MDRS-related papers, where the cumulative citation count is around 33350. Other analogue missions investigating the effects of human isolation in a confined environment have been much shorter and limited in scope, which has resulted in fewer publications. The Mars-500 analogue is a compelling example of how terrestrial analogues could be designed and operated for maximum scientific impact.
There are several advantages to hosting analogue facilities in suburban settings (such as HERA), and these generally appear to produce the highest quality research. The majority of risks and gaps identified from the NASA Human Research Roadmap 2, which can be studied terrestrially, appear to be best suited to a high-fidelity suburban environment (such as exploring effects and countermeasures for isolation, closed environments, sleep disturbances, temperature extremes, stress, and microgravity through neutral buoyancy and bedrest). There may be benefits to desert analogue locations for simulated missions which combine robotic surface operations, human systems integration, geological exploration, and EVA activities. Although focused on lunar rather than Martian missions, the ESA-DLR LUNA is an example of a suburban habitat design that will enable high-quality science by providing high-fidelity simulations for crew and ground personnel training, technology and systems integration testing.
As technology advances at an ever-increasing rate, there is merit in setting up and maintaining permanent analogue sites to allow continuous testing of new space technology. Following the Mars-500 example, prospective analogue sites should be properly staged with well-defined research objectives. Any testing of new technology should adhere to the same conditions used to test older technology. Not only does this allow comparison of test results but it also facilitates rigorous learning.
4.4. Review Limitations
While Scopus covers more than 22,500 active titles, this does not mean its coverage is complete. About 99.11% and 96.61% of the journals indexed in Web of Science™ are also indexed in Scopus and Dimensions™, respectively . Dimensions is a new bibliographic database launched in 2018 that seeks to rival Web of Science and Scopus. Scopus has 96.42% of its indexed journals also covered by Dimensions. The Dimensions database has the most exhaustive journal coverage, with 82.22% more journal titles than Web of Science and 48.17% more journal titles than Scopus . Even though Dimensions has more journal titles, its API is new and not as well supported by software packages such as R. While Scopus does not have the same coverage as Dimensions, it provides sufficient coverage for this review.
Bibliographic databases do not include peer-reviewed agency reports and likely to miss many book chapters. Although Scopus includes a large body of research material, there may be equally valuable sources of research that are available from agencies and institutes they fund. These reports are typically put through through an internal review process, comparable to a peer review, prior to release. Readers may consider using the results of the various rankings in this paper as guidance for an expanded search of publicly available reports and other documentation.
An additional limitation in the citation based approach in this review, is that more highly cited papers will tend to be the oldest papers in the corpus (those with a publication date closer to 2010 in this review).
Any human mission to Mars will encounter significant physical and psychological health risks. Dominating the scientific literature are articles addressing the health risks associated with space radiation. Microgravity is another critical concern, as is the impact of prolonged space travel on the human microbiome. Research into the psychological impact of prolonged isolation is receiving ongoing attention. A return trip to Mars is a real test of physical and mental endurance. The physical and mental health risks are such that early human missions may be kept short. Due to these risks, there appears to be a strong need for analogue research into ground-based and aerial robots to explore Mars. There also appear to be additional opportunities for the development and validation of aspects of human systems integration architecture , real-time sensor data integration and analytics, and systematic learning from analogue missions (for example ).
Papers published in peer-reviewed journals, with acknowledgment of grants from space agencies are more likely to have higher citations, and thus are more likely to align with space agency strategies.
Long-stay missions to Mars are unlikely to happen for at least a decade, so terrestrial analogues are likely to focus on evaluating new robot technology and, to a lesser extent, on maintaining crew well-being in space transit habitats. Robot missions to Mars will probably expand in scope and ambition as technology advances. Practicing extravehicular surface activities is likely to be a lower priority. Terrain analogues should also allow technology bench-marking. Ideally, any testing of new technology or approaches should use the same criteria as previous tests. Comparing and contrasting the performance of different technologies will ensure greater scientific rigor and more publishable reporting. Habitat analogue opportunities exist in bioregenerative life support systems and enhanced medical and psychological support mechanisms.
The findings of this review support the development of an international registry of analogue sites focusing on the support of human performance on Mars. Ideally, such a registry would include the primary analogue scientific focus, technological capability, previous publications, alignment with space agency requirements (such as the NASA Human Research Roadmap and the International Space Exploration Coordination Group Global Exploration Roadmap), availability, specific requirements, costs, and location. An international certification process would be beneficial to ensure high-quality reproducible research building on previous work. From the frequency of analogue missions and the publication count in Table 2, many analogue experiments appear unreported, which increases the risk of unknowingly reproducing previous work or missing the benefits gained from previous missions. In addition, many published analogue results may not be achieving impact in the scientific literature (47% of papers in the corpus had zero citations).
R scripts for reproducing the results in this article and a comma-delimited ASCII file detailing the 579 articles can be found at https://github.com/aterhorst/mars_bibliometrics. Other data used to support the findings of this study are included within the article.
This research was performed as part of employment of the authors at the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia.
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this article.
A. Terhorst conducted the literature review and analysis. J. Dowling conceived the idea and contributed to analysis. All authors contributed equally to the writing of the manuscript.
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