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By NASA
5 min read
Preparations for Next Moonwalk Simulations Underway (and Underwater)
In-person participants (L-R) – Back row: Jason Lytle, Stuart Lee, Eric Bershad, Ashot Sargsyan, Aaron Everson, Philip Wells, Sergi Vaquer Araujo, Steven Grover, John A. Heit, Mehdi Shishehbor, Laura Bostick; Middle row: Sarah Childress Taoufik, Stephan Moll, Brandon Macias, Kristin Coffey, Ann-Kathrin Vlacil, Dave Francisco; Front row: James Pavela, Doug Ebert, Kathleen McMonigal, Esther Kim, Emma Hwang; Not pictured: Tyson Brunstetter, J. D. Polk
Online participants: Stephen Alamo, Mark Crowther, Steven Nissen, Mark Rosenberg, Jeffrey Weitz, R. Eugene Zierler, Serena Aunon, Tina Bayuse, Laura Beachy, Becky Brocato, Daniel Buckland, Jackie Charvat, Diana Cruz Topete, Quinn Dufurrena, Robert Haddon, Joanne Kaouk, Kim Lowe, Steve Laurie, Karina Marshall-Goebel, Sara Mason, Shannan Moynihan, James Pattarini, Devan Petersen, Ruth Reitzel, Donna Roberts, Lucia Roccaro, Mike Stenger, Terry Taddeo, Gavin Travers, Mary Van Baalen, Liz WarrenNASA In October 2024, NASA’s Office of the Chief Health and Medical Officer (OCHMO) initiated a working group to review the status and progress of research and clinical activities intended to mitigate the risk of venous thromboembolism (VTE) during spaceflight. The working group took place over two days at NASA’s Johnson Space Center; a second meeting on the topic was held in December 2024 at the European Space Agency (ESA) facility in Cologne, Germany.
Read More about the Risk of VTE The working group was assembled from internal NASA subject matter experts (SMEs), the NASA OCHMO Standards Team, NASA and ESA stakeholders, and external SMEs, including physicians and medical professionals from leading universities and medical centers in the United States and Canada.
Background
Spaceflight Venous Thrombosis (SVT)
Spaceflight Venous Thrombosis (SVT) refers to a phenomenon experienced during spaceflight in which a thrombus (blood clot) forms in the internal jugular vein (and/or associated vasculature) that may be symptomatic (thrombus accompanied by, but not limited to, visible internal jugular vein swelling, facial edema beyond “nominal” spaceflight adaptation, eyelid edema, and/or headache) or asymptomatic. Obstructive thrombi have been identified in a very small number of crewmembers, as shown in the figure below.
Note that the figure below is for illustrative purposes only; locations are approximate, and size is not to scale.
Approximate location of identified thrombi in crewmembers.Source: Modified from Cerebral Sinus Venous Thrombosis – University of Colorado Denver With treatment, crewmembers were able to complete their mission, and anticoagulants were discontinued several days prior to landing to minimize the risk of bleeding in the event of a traumatic injury. Some thromboses completely resolved post landing, and some required additional treatment.
Pathophysiology of Venous Thromboembolism (VTE)
The proposed pathogenesis of VTE is referred to as Virchow’s triad and suggests that VTE occurs as the result of:
Alterations in blood flow (i.e., stasis), Vascular endothelial injury/changes, and/or, Alterations in the constituents of the blood leading to hypercoagulability (i.e., hereditary predisposition or acquired hypercoagulability). Note: pathophysiology are the changes that occur during a disease process; hypercoagulability is the increased tendency to develop blood to clots.
The Virchow’s triad of risk factors for venous thrombosis.Bouchnita, 2017 Blood stasis, or venous stasis, refers to a condition in which the blood flow in the veins slows down which leads to pooling in the veins. This slowing of the blood may be due to vein valves becoming damaged or weak, immobility, and/or the absence of muscular contractions. Associated symptoms include swelling, skin changes, varicose veins, and slow-healing sores or ulcers. In terrestrial medicine, venous thrombosis is typically caused by damaged or weakened vein valves, which can be due to many factors, including aging, blood clots, varicose veins, obesity, pregnancy, sedentary lifestyle, estrogen use, and hereditary predisposition.
Spaceflight Considerations
Altered Venous Blood Flow and Spaceflight Associated Neuro-ocular Syndrome
In addition to the terrestrial risk factors of VTE, there are physiological changes associated with spaceflight that are hypothesized to potentially play a role in the development of VTE in weightlessness. Specifically, researchers have explored the effects of the microgravity environment and subsequent observed headward fluid shifts that occur, and the potential impact on blood flow. Crewmembers onboard the International Space Station (ISS) experience weightlessness due to the microgravity environment and thus experience a sustained redistribution of bodily fluids from the legs toward the head. The prolonged headward fluid shifts during weightlessness results in facial puffiness, decreased leg volume, increased cardiac stroke volume, and decreased plasma volume.
Crewmembers have also experienced altered blood flow during spaceflight, including retrograde venous blood flow (RVBF) (the backflow of venous blood towards the brain) or stasis (a stoppage or slowdown in the flow of blood). While the causes of the observed stasis and retrograde blood flow in spaceflight participants is not well understood, the potential clinical significance of the role it may have in the development of thrombus formation warrants further investigation.
Doppler imaging of a retrograde flow in the left internal jugular vein.Yan & Seow, 2009 Other physiological concerns affected by fluid shifts are being studied to consider if any relation to VTE exists. Chronic weightlessness can cause bodily fluids such as blood and cerebrospinal fluid to move toward the head, which can lead to optic nerve swelling, folds in the retina, flattening of the back of the eye, and swelling in the brain. This collection of eye and brain changes is called “spaceflight associated neuro-ocular syndrome,” or SANS. Some astronauts only experience mild changes in space, while others have clinically significant outcomes. The long-term health outcome from these changes is unknown but actively being investigated. The risk of developing SANS is higher during longer-duration missions and remains a top research priority for scientists ahead of a Mars mission.
Conclusions and Further Work
Based on expert opinion and the assessment of the risk factors for thrombosis, an algorithm was developed to provide guidance for in-mission assessment and treatment of thrombus formation in weightlessness. The algorithm is based on early in-flight ultrasound testing to determine the flow characteristic of the left internal jugular vein and associated vasculature.
NASA Working Group Recommendations
The working group recommended several areas for further investigation to assess feasibility and potential to mitigate the risk of thrombosis in spaceflight:
Improved detection capabilities to identify when a thrombus has formed in-flight, Pathophysiology/factors leading to thrombi formation during spaceflight, Countermeasures and treatment
For more information on the working group meeting and a complete list of references, please see the Risk of Venous Thromboembolism (VTE) During Spaceflight Summary Report.
Risk of Venous Thromboembolism (VTE) During Spaceflight Summary Report Share
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Last Updated Mar 14, 2025 EditorKim Lowe Related Terms
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By NASA
6 min read
Preparations for Next Moonwalk Simulations Underway (and Underwater)
Risks Concept Risk is inherent in human spaceflight. However, specific risks can and should be understood, managed, and mitigated to reduce threats posed to astronauts. Risk management in the context of human spaceflight can be viewed as a trade-based system. The relevant evidence in life sciences, medicine, and engineering is tracked and evaluated to identify ways to minimize overall risk to the astronauts and to ensure mission success. The Human System Risk Board (HSRB) manages the process by which scientific evidence is utilized to establish and reassess the postures of the various risks to the Human System during all of the various types of existing or anticipated crewed missions. The HSRB operates as part of the Health and Medical Technical Authority of the Office of the Chief Health and Medical Officer of NASA via the JSC Chief Medical Officer.
The HSRB approaches to human system risks is analogous to the approach the engineering profession takes with its Failure Mode and Effects Analysis in that a process is utilized to identify and address potential problems, or failures to reduce their likelihood and severity. In the context of risks to the human system, the HSRB considers eight missions which different in their destinations and durations (known as Design Reference Missions [DRM]) to further refine the context of the risks. With each DRM a likelihood and consequence are assigned to each risk which is adjusted scientific evidence is accumulated and understanding of the risk is enhanced, and mitigations become available or are advanced.
Human System Risks This framework enables the principles of Continuous Risk Management and Risk Informed Decision Making (RIDM) to be applied in an ongoing fashion to the challenges posed by Human System Risks. Using this framework consistently across the 29 risks allows management to see where risks need additional research or technology development to be mitigated or monitored and for the identification of new risks and concerns. Further information on the implementation of the risk management process can be found in the following documents:
Human System Risk Management Plan – JSC-66705 NASA Health and Medical Technical Authority (HMTA) Implementation – NPR 7120.11A NASA Space Flight Program and Project Management Requirements – NPR 7120.5 Human System Risk Board Management Office
The HSRB Risk Management Office governs the execution of the Human System Risk management process in support of the HSRB. It is led by the HSRB Chair, who is also referred to as the Risk Manager.
Risk Custodian Teams
Along with the Human System Risk Manager, a team of risk custodians (a researcher, an operational researcher or physician, and an epidemiologist, who each have specific expertise) works together to understand and synthesize scientific and operational evidence in the context of spaceflight, identify and evaluate metrics for each risk in order to communicate the risk posture to the agency.
Directed Acyclic Graphs
Summary
The HSRB uses Directed Acyclic Graphs (DAG), a type of causal diagramming, as visual tools to create a shared understanding of the risks, improve communication among those stakeholders, and enable the creation of a composite risk network that is vetted by members of the NASA community and configuration managed (Antonsen et al., NASA/TM– 20220006812). The knowledge captured is the Human Health and Performance community’s knowledge about the causal flow of a human system risk, and the relationships that exist between the contributing factors to that risk.
DAGs are:
Intended to improve communication between: Managers and subject matter experts who need to discuss human system risks Subject matter experts in different disciplines where human system risks interact with one another in a potentially cumulative fashion Visual representations of known or suspected relationships Directed – the relationship flows in one direction between any two nodes Acyclic – cycles in the graph are not allowed Example of a Directed Acyclic Graph. This is a simplified illustration of how and the individual, the crew, and the system contribute to the likelihood of successful task performance in a mission. Individual readiness is affected by many of the health and performance-oriented risks followed by the HSRB, but the readiness of any individual crew is complemented by the team and the system that the crew works within. Failures of task performance may lead to loss of mission objectives if severe.NASA View Larger (Example of a Directed Acyclic Graph) Image
Details
At NASA, the Human System Risks have historically been conceptualized as deriving from five Hazards present in the spaceflight environment. These are: altered gravity, isolation and confinement, radiation, a hostile closed environment, and distance from Earth. These Hazards are aspects of the spaceflight environment that are encountered when someone is launched into space and therefore are the starting point for causal diagramming of spaceflight-related risk issues for the HSRB.
These Hazards are often interpreted in relation to physiologic changes that occur in humans as a result of the exposure; however, interaction between human crew (behavioral health and performance), which may be degraded due to the spaceflight environment – and the vehicle and mission systems that the crew must operate – can also be influenced by these Hazards.
Each Human System Risk DAG is intended to show the causal flow of risk from Hazards to Mission Level Outcomes. As such, the structure of each DAG starts with at least one Hazard and ends with at least one of the pre-defined Mission Level Outcomes. In between are the nodes and edges of the causal flow diagrams that are relevant to the Risk under consideration. These are called ‘contributing factors’ in the HSRB terminology, and include countermeasures, medical conditions, and other Human System Risks. A graph data structure is composed of a set of vertices (nodes), and a set of edges (links). Each edge represents a relationship between two nodes. There can be two types of relationships between nodes: directed and undirected. For example, if an edge exists between two nodes A and B and the edge is undirected, it is represented as A–B, (no arrow). If the edge were directed, for example from A to B, then this is represented with an arrow (A->B). Each directed arrow connecting one node to another on a DAG indicates a claim of causality. A directed graph can potentially contain a cycle, meaning that, from a specific node, there exists a path that would eventually return to that node. A directed graph that has no cycles is known as acyclic. Thus, a graph with directed links and no cycles is a DAG. DAGs are a type of network diagram that represent causality in a visual format.
DAGs are updated with the regular Human System Risk updates generally every 1-2 years. Approved DAGs can be found in the NASA/TP 20220015709 below or broken down under each Human System Risk.
Documents
Directed Acyclic Graph Guidance Documentation – NASA/TM 20220006812 Directed Acyclic Graphs: A Tool for Understanding the NASA Human Spaceflight System Risks – NASA/TP 20220015709 Publications
npj Microgravity – Causal diagramming for assessing human
system risk in spaceflight
Apr 22, 2024
PDF (3.09 MB)
npj Microgravity –
Levels of evidence for human system risk
evaluation
Apr 22, 2024
PDF (2.47 MB)
npj Microgravity –
Updates to the NASA human system risk management process
for space exploration
Apr 22, 2024
PDF (2.24 MB)
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Human System Risks Share
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Last Updated Mar 11, 2025 EditorRobert E. LewisLocationJohnson Space Center Related Terms
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By NASA
1 min read
Preparations for Next Moonwalk Simulations Underway (and Underwater)
NASA astronaut Douglas Hurley is helped out of the SpaceX Crew Dragon Endeavour spacecraft onboard the SpaceX GO Navigator recovery ship after he and NASA astronaut Robert Behnken landed in the Gulf of Mexico off the coast of Pensacola, Florida, Sunday, Aug. 2, 2020. The Demo-2 test flight for NASA’s Commercial Crew Program was the first to deliver astronauts to the International Space Station and return them safely to Earth onboard a commercially built and operated spacecraft. Behnken and Hurley returned after spending 64 days in space. Photo Credit: (NASA/Bill Ingalls)NASA New spacecraft that will transport crews to the Lunar and Martian surfaces and return them to Earth may have diverse landing modalities which will function in different landing environments. Additionally, the crew may be deconditioned on landing, impacting their ability to independently egress the vehicles, perform post-landing tasks in a timely manner, and perform surface EVAs post-landing -including those required for emergencies.
Boeing and NASA teams work around Boeing’s CST-100 Starliner spacecraft after it landed at White Sands Missile Range’s Space Harbor, Wednesday, May 25, 2022, in New Mexico. Boeing’s Orbital Flight Test-2 (OFT-2) is Starliner’s second uncrewed flight test to the International Space Station as part of NASA’s Commercial Crew Program. OFT-2 serves as an end-to-end test of the system’s capabilities. Photo Credit: (NASA/Bill Ingalls) Directed Acyclic Graph Files
+ DAG File Information (HSRB Home Page)
+ Crew Egress Risk DAG and Narrative (PDF)
+ Crew Egress Risk DAG Code (TXT)
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By NASA
1 min read
Preparations for Next Moonwalk Simulations Underway (and Underwater)
The space shuttle Endeavour is seen on launch pad 39a as a storm passes by prior to the rollback of the Rotating Service Structure (RSS), Thursday, April 28, 2011, at Kennedy Space Center in Cape Canaveral, Fla. During the 14-day mission, Endeavour and the STS-134 crew will deliver the Alpha Magnetic Spectrometer (AMS) and spare parts including two S-band communications antennas, a high-pressure gas tank and additional spare parts for Dextre. Launch is targeted for Friday, April 29 at 3:47 p.m. EDT.NASA It is important to protect humans from unintended electrical current flow during spaceflight. The thresholds for contact electrical shock are well established, and standards and requirements exist that minimize the probability of contact electrical shock. Current thresholds were chosen (vs. voltage thresholds) because body impedance varies depending on conditions such as wet/dry, AC/DC, voltage level, large/small contact area, but current thresholds and physiological effects do not change. By addressing electrical thresholds, engineering teams are able to provide the appropriate hazard controls, usually through additional isolation (beyond the body’s impedance), current limiters, and/or modifying the voltage levels. Risk assessment determined that the probability of an event was extremely low, and the most serious consequence is expected to be involuntary muscle contraction.
Lightning strikes the Launch Pad 39B protection system as preparations for launch of NASA’s Space Launch System (SLS) rocket with the Orion spacecraft aboard continue, Saturday, Aug. 27, 2022, at NASA’s Kennedy Space Center in Florida. NASA’s Artemis I flight test is the first integrated test of the agency’s deep space exploration systems: the Orion spacecraft, SLS rocket, and supporting ground systems. Launch of the uncrewed flight test is targeted for no earlier than Aug. 29 at 8:33 a.m. ET. Photo Credit: (NASA/Bill Ingalls) Directed Acyclic Graph Files
+ DAG File Information (HSRB Home Page)
+ Electrical Shock Risk DAG and Narrative (PDF)
+ Electrical Shock Risk DAG Code (TXT)
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By NASA
1 min read
Preparations for Next Moonwalk Simulations Underway (and Underwater)
Astronaut Mark Vande Hei swaps out components on an advanced new toilet installed inside the International Space Station.NASA Exposure to the altered gravity in the spaceflight environment may cause physiological changes. One of these changes is the inability to completely empty the bladder or urinary retention. Causes of urinary retention in the early phases of flight include altered baseline physiology seen with exposure to microgravity, the anticholinergic side effects of medications that are taken to combat space motion sickness, and other factors. Urinary retention may impact health on orbit by causing discomfort and increasing the risk of urinary tract infection. Treatment, including urethral catheterization, has been performed on orbit.
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