Directed Acyclic Graphs: A Tool for Understanding the NASA Human Spaceflight System Risks - Human System Risk Board

For over a decade, the National Aeronautics and Space Administration (NASA) has tracked
and configuration-managed approximately 30 risks to astronaut health and performance that
occur before, during and after spaceflight. The Human System Risk Board (HSRB), a Health
and Medical Technical Authority (HMTA) Board at NASA Johnson Space Center, is the entity
responsible for identifying, assessing, analyzing, and monitoring the official understanding of the risk or risk posture for each of the Human System Risks and determining – based on evaluation
of the available evidence – when that risk posture changes. The ultimate purpose of tracking
and researching these risks is to find ways to reduce the risk that astronaut crews face during
spaceflight. Historically, research, development and operations relevant to one risk have been
conducted in isolation from other risks; these individual risk ‘silos’ enabled initial characterization
of each specific risk. In spaceflight however, the impact of exposure to risk for astronaut crews
is cumulative, and not independent of exposures or other risks, as all the adverse effects of the
spaceflight environment begin at launch, continue throughout the duration of the mission and in
some cases across the lifetime of the crews. In January of 2020, the HSRB at NASA embarked
on a pilot project designed to assess the potential value of causal diagramming as a tool to
facilitate understanding of these cumulative and interdependent effects as applied within Human
System Risk management. This process uses directed acyclic graphs as a means of formalizing
a shared mental model of the causal flow of risk among Risk Board stakeholders. Initially this
model was to improve communication among those stakeholders, but the potential value
exceeds communication alone. The causal diagrams are formulated as directed acyclic graphs
(DAGs) to function as a type of knowledge graph for reference for the board and its
stakeholders.

This document is a sister document to NASA/TM 20220006812 Directed Acyclic Graph
Guidance Documentation (1). In that document, the basic guidance for creating and
standardizing directed acyclic graphs as tools for cross-risk analysis is provided. This document
contains the initial configuration managed DAGs that were created as a result of applying those
principles. These initial versions were accepted by the HSRB in January of 2022. Each of the
Human System Risks are represented by a DAG that has been reviewed by the larger Human
Health and Performance community at NASA including life scientists, physical scientists,
physicians, nurses, pharmacists, exercise specialists and more. These results show the starting
point for Human System Risk DAGs as shared mental models and communication aids across
the boundaries of the various expertise needed to understand and mitigate the human risks in
spaceflight. Because they are a starting point, each of these DAGs can be expected to change
over time as new or refined evidence becomes available. The process for updating these DAGs
can be found in the JSC-66705 Human System Risk Management Plan (2) that is publicly
available on the NASA Technical Reports Server.