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NASA NTRS2019

How Much Testing is Needed to Manage Supportability Risks for Beyond-LEO Missions?

Supportability will be a significantly greater driver of cost and risk for future deep-space crewed missions than it has been in the past. Spares requirements and maintenance risk mitigation in particular present an unprecedented challenge for missions beyond Low Earth Orbit (LEO), since, for the first time in human spaceflight history, crews will be weeks or months away from resupply or a safe return to Earth in the event of an abort. Under these conditions, failure rates are a critical parameter that must be well-understood in order to manage logistics and risk effectively. However, failure rates cannot be measured directly, and can only be estimated based on past experience and test results. Previous research has shown that International Space Station (ISS) operational experience has provided significant benefits to future missions by reducing uncertainty and improving accuracy in failure rate estimates, resulting in significant reductions in mass and risk for beyond-LEO missions. This paper updates and expands on that research and quantifies the potential value of continued testing for future mission supportability. Frequentist and Bayesian models for evaluating, validating, and updating failure rate estimates are described, and are combined with supportability models to examine potential impacts of additional operating experience for future missions in terms of logistics mass reduction. The implications of these results for technology development, system design, and program planning are discussed along with lessons learned and recommendations for future system development. In the end, there is no simple answer to the question of how much testing is required, but the models described in this paper provide a way to evaluate the potential impacts of testing in order to inform test planning.

Andrew C Owens↗

NASA NTRS2026

Thermal Considerations for 2039 Opposition Class Nuclear Electric Propulsion/Chemical Propulsion Crewed Mars Mission

The high specific impulse (Isp) of Nuclear Electric Propulsion (NEP) technology offers the potential for advanced space mission capabilities. However, the five critical technology elements of NEP vehicles have yet to prove technical maturity levels for consideration into mission design. In response to the critical reviews by the NASA Engineering and Safety Center (NESC) and the National Academies of Sciences, Engineering, and Medicine (NASEM), NASA’s Space Nuclear Propulsion (SNP) project created an NEP Technology Maturation Plan (TMP) for focused development of NEP technology. The TMP called for a coordinated set of technology development efforts to meet this objective. The Modular Assembled Radiators for NEP VehicLes (MARVL) Early Career Initiative (ECI) project was initiated to develop a portion of the fifth Critical Technology Element (CTE) of the NEP vehicle: the Primary Heat Rejection Subsystem (PHRS). A target application of a 2039 human-rated Mars mission was outlined in the TMP. For the outlined mission, a NEP vehicle will experience several thermal environments which will impact the design and operation of the PHRS. To maintain radiator temperatures within the required effective temperature range, the effect of natural, induced, and NEP internally generated heat loads on the radiator panel must be well understood. Furthermore, this analysis is critical for analyzing the influence of various orientations and positions of the NEP vehicle relative to nearby celestial bodies throughout the mission. This study conducted a complete enveloping analysis of the thermal environments influencing the NEP vehicle throughout the mission. Thermal analysis was conducted for the radiator panels based on the defined mission environments. This thermal analysis concludes with the selection of ideal radiator orientations for the NEP vehicle, and the identification of worst case hot and cold environmental sink temperatures throughout the mission. For the target application, the environmental sink temperature while the reactor is powered OFF or powered ON ranges from 30 K to 353 K and 2.7 K to 243 K respectively. When considering interplanetary space, the minimum environmental sink temperature when the reactor is powered OFF and the radiators are oriented “edge to Sun” is 2.7 K. The environmental thermal models generated in this study will be used for future studies with the full vehicle system model. The environmental sink temperature curves generated will be used for future radiator and component analysis to predict transient performance in the space environment. The environmental sink temperature and heat rejection capability curves will inform the trade between commissioning orbits that are in consideration. The model may also serve as a useful tool as reference for future crewed space missions, missions involving radiators or temperature sensitive equipment, or other missions requiring analysis of natural orbital thermal environments.

Nuclear Electric Propulsion↗

NASA NTRS2009

Solar Energetic Particle Spectrometer (SEPS)

An outstanding problem of solar and heliospheric physics is the transport of solar energetic particles. The more energetic particles arriving early in the event can be used to probe the transport processes. The arrival direction distribution of these particles carries information about scattering during their propagation to Earth that can be used to test models of interplanetary transport. Also, of considerable importance to crewed space missions is the level of ionizing radiation in the interplanetary medium, and the dose that the crew experiences during an intense solar particle event, as well as the risk to space systems. A recent study concludes that 90% of the absorbed dose results from particles in the energy range 20-550 MeV. We will describe a new compact instrument concept, SEPS, that can cover the energy range from 50-600 MeV with a single compact detector. This energy range has been difficult to cover. There are only limited data, generally available only in broad energy bins, from a few past and present instruments outside Earth s magnetosphere. The SEPS concept can provide improved measurements for this energy range and its simple light-weight design could be easily accommodated on future missions.

Christl, Mark J.↗

NASA NTRSDate not supplied

Supporting Crew Medical Decisions on Deep Space Missions: A Real-Time Performance Monitoring

Crewed missions into deep space will require astronauts to respond autonomously to safety- and time-critical anomalies. These and other potential issues will require the continuous monitoring and recognition of potentially subtle, but complex, anomalous data patterns. NASA continues to investigate real-time metrics for Extravehicular Operations1 and for providing feedback to improve performance2. Since it is unknown how long-term exposure to deep space will affect crew health, continuous monitoring of clinical and subclinical status is warranted. NASA is characterizing crew medical decision-support needs and identifying the recommended requirements of a Clinical Decision Support System (CDSS) within the Exploration Medical Capability Element (ExMC) of the Human Research Program3. Early detection, and continuous monitoring, of performance may provide an effective prognostic capability for assessing crew health state, and the identified requirements could be provided for an in-vehicle CDSS that acts as an assistant for delivering optimal health and performance and medical care during exploration missions. There is evidence to suggest that automatic, unobtrusive collection of keystroke features, such as alphanumeric key latencies, backspaces or typing rhythm, could be a valuable performance screening tool. Keystroke features show significant differences between control subjects and patients with overt clinical illness, such as multiple sclerosis4 and Parkinson’s disease5, as well as behavioral health and performance issues, such as mild cognitive impairment6 and depression7. This work is exploring a capability for the assessment of early task performance decline in crew as they perform daily activities. Timestamped keyboard entries are continuously collected and stored. Nominal keystroke interactions are compared to those obtained after exposure to spaceflight stressors, e.g., fatigue, to determine the methods sensitivity. This work will pave the way toward an objective detection tool that may be deployed in a spaceflight setting.

clinical↗

NASA NTRSDate not supplied

Enabling Space Biological Knowledge Discovery Through Image and Video Data Sharing

Increased biomedical risks associated with deep space crewed missions (cis-Lunar, Mars transit/surface) require development of health countermeasures, novel ecosystem support, risk modeling, and fundamental space biological knowledge discovery. Molecular-omics, physiological-phenotypic-behavioral, and environmental-radiation telemetry data from space biological and health studies are needed for reuse by scientists to address these tasks. The data as well as space-relevant biospecimens are being made more findable, accessible, interoperable, and reusable through NASA’s Open Science Data Repository (OSDR). This new OSDR umbrella grouping includes NASA GeneLab, the NASA Ames Life Sciences Data Archive (ALSDA), and the NASA Biological Institutional Scientific Collection. The OSDR system design appropriately handles metadata and processed-tabular results from ALSDA studies collected from space experiments. But raw and processed ALSDA bioimage and video datasets require an expansion of OSDR’s data architecture to handle ingestion, curation, and egress. The academic-industry bioimaging field saw a scientific renaissance in the past several years through leveraging open-source software, international collaborations, machine learning, and other open science/programming approaches. As crewed missions and more biological experiments are on the deep space horizon, OSDR is embracing data stewardship through listening to feedback from subject matter experts and designing an expanded architecture which is appropriate for NASA’s goals to enable analysis and reuse of bioimaging and video data for the public science community.Discovery Through Image and Video Data Sharing

space biology↗

NASA NTRS2023

ICARUS: in-Situ Studies of the Solar Corona Beyond Parker Solar Probe and Solar Orbiter

The primary scientific goal of ICARUS (Investigation of Coronal AcceleRation and heating of solar wind Up to the Sun), a mother-daughter satellite mission, proposed in response to the ESA “Voyage 2050” Call, will be to determine how the magnetic field and plasma dynamics in the outer solar atmosphere give rise to the corona, the solar wind, and the entire heliosphere. Reaching this goal will be a Rosetta Stone step, with results that are broadly applicable within the fields of space plasma physics and astrophysics. Within ESA’s Cosmic Vision roadmap, these science goals address Theme 2: “How does the Solar System work?” by investigating basic processes occurring “From the Sun to the edge of the Solar System”. ICARUS will not only advance our understanding of the plasma environment around our Sun, but also of the numerous magnetically active stars with hot plasma coronae. ICARUS I will perform the first direct in situ measurements of electromagnetic fields, particle acceleration, wave activity, energy distribution, and flows directly in the regions in which the solar wind emerges from the coronal plasma. ICARUS I will have a perihelion altitude of 1 solar radius and will cross the region where the major energy deposition occurs. The polar orbit of ICARUS I will enable crossing the regions where both the fast and slow winds are generated. It will probe the local characteristics of the plasma and provide unique information about the physical processes involved in the creation of the solar wind. ICARUS II will observe this region using remote-sensing instruments, providing simultaneous, contextual information about regions crossed by ICARUS I and the solar atmosphere below as observed by solar telescopes. It will thus provide bridges for understanding the magnetic links between the heliosphere and the solar atmosphere. Such information is crucial to our understanding of the plasma physics and electrodynamics of the solar atmosphere. ICARUS II will also play a very important relay role, enabling the radio-link with ICARUS I. It will receive, collect, and store information transmitted from ICARUS I during its closest approach to the Sun. It will also perform preliminary data processing before transmitting it to Earth. Performing such unique in situ observations in the area where presumably hazardous solar energetic particles are energized, ICARUS will provide fundamental advances in our capabilities to monitor and forecast the space radiation environment. Therefore, the results from the ICARUS mission will be extremely crucial for future space explorations, especially for long-term crewed space missions.

Solar wind↗

NASA NTRS2025

NASA Planetary Protection Handbook

This handbook provides guidance to mission providers and Planetary Protection (PP) practitioners on implementing PP measures for both robotic and crewed space missions. This handbook represents major updates to PP practices since a previously drafted handbook from 2010. In 2017, NASA reorganized the Office of Planetary Protection (OPP) from the Science Mission Directorate (SMD) to a Technical Authority (TA) within the Office of Safety and Mission Assurance (OSMA). With this organizational change came a complete overhaul of NASA’s PP policy and technical standards. This handbook reflects the latest NASA PP policy updates in NPR 8715.24, Planetary Protection Provisions for Robotic Extraterrestrial Missions and technical requirements of NASA-STD-8719.27, Implementing Planetary Protection Requirements for Space Flight. This handbook is a companion document to the NPR and technical standard and provides guidance, best practices, background information, and advice for practitioners to consider when implementing PP on NASA missions and NASA-partnered missions.

Nick Benardini↗

NASA NTRS2011

NASA/MOD Operations Impacts from Shuttle Program

Operations plays a pivotal role in the success of any human spaceflight program. This paper will highlight some of the core tenets of spaceflight operations from a systems perspective and use several examples from the Space Shuttle Program to highlight where the success and safety of a mission can hinge upon the preparedness and competency of the operations team. Further, awareness of the types of operations scenarios and impacts that can arise during human crewed space missions can help inform design and mission planning decisions long before a vehicle gets into orbit. A strong operations team is crucial to the development of future programs; capturing the lessons learned from the successes and failures of a past program will allow for safer, more efficient, and better designed programs in the future. No matter how well a vehicle is designed and constructed, there are always unexpected events or failures that occur during space flight missions. Preparation, training, real-time execution, and troubleshooting are skills and values of the Mission Operations Directorate (MOD) flight controller; these operational standards have proven invaluable to the Space Shuttle Program. Understanding and mastery of these same skills will be required of any operations team as technology advances and new vehicles are developed. This paper will focus on individual Space Shuttle mission case studies where specific operational skills, techniques, and preparedness allowed for mission safety and success. It will detail the events leading up to the scenario or failure, how the operations team identified and dealt with the failure and its downstream impacts. The various options for real-time troubleshooting will be discussed along with the operations team final recommendation, execution, and outcome. Finally, the lessons learned will be summarized along with an explanation of how these lessons were used to improve the operational preparedness of future flight control teams.

Fitzpatrick, Michael↗

NASA NTRS2006

Converging Indicators for Assessing Individual Differences in Adaptation to Extreme Environments: Preliminary Report

This paper describes the development and validation of a new methodology for assessing the deleterious effects of spaceflight on crew health and performance. It is well known that microgravity results in various physiological alterations, e.g., headward fluid shifts which can impede physiological adaptation. Other factors that may affect crew operational efficiency include disruption of sleep-wake cycles, high workload, isolation, confinement, stress and fatigue. From an operational perspective, it is difficult to predict which individuals will be most or least affected in this unique environment given that most astronauts are first-time flyers. During future lunar and Mars missions space crews will include both men and women of multi-national origins, different professional backgrounds, and various states of physical condition. Therefore, new methods or technologies are needed to monitor and predict astronaut performance and health, and to evaluate the effects of various countermeasures on crew during long duration missions. This paper reviews several studies conducted in both laboratory and operational environments with men and women ranging in age between 18 to 50 years. The studies included the following: soldiers performing command and control functions during mobile operations in enclosed armored vehicles; subjects participating in laboratory tests of an anti-motion sickness medication; subjects exposed to chronic hypergravity aboard a centrifuge, and subject responses to 36-hours of sleep deprivation. Physiological measurements, performance metrics, and subjective self-reports were collected in each study. The results demonstrate that multivariate converging indicators provide a significantly more reliable method for assessing environmental effects on performance and health than any single indicator.

Cowings, Patricia S.↗

NASA NTRSDate not supplied

Artificial Neural Networks to Predict Cognitive Impairment of Rodents Subjected to Space Radiation

INTRODUCTION We use artificial neural networks (ANNs) as an example machine learning (ML) tool to predict the cognitive performance impairment of rats induced by irradiation. The experimental data in the analyses is attentional set-shifting (ATSET) test scores from a rodent model exposed to ≤15 cGy of individual galactic cosmic radiation (GCR) ions: 4He, 28Si, or 56Fe, expected for a Lunar or Mars mission [1]. This work investigates rats at a subject-based level and uses applied dose and performance scores taken before irradiation to predict whether a rat will be impaired when irradiated. The results of this study are significant to crewed space missions as they support the potential of predicting an astronaut’s impairment in a specific task before spaceflight through the implementation of appropriately trained ML tools. METHODS Data used in this work are scores from the ATSET, a multi-stage constrained cognitive flexibility test [2]. Our computational model utilizes the number of attempts to reach the criterion to pass a stage as a behavioral performance measure for rats. We use the post-irradiation scores, generate thresholds from cumulative distribution plots of non-irradiated rats, and calculate the percent of irradiated rats whose scores fall below the threshold to infer how each radiation type/dose affects a population. Rats scoring above the threshold are labeled impaired while the others are non-impaired. We then employ ANNs as a typical ML technique, and use each subject’s individual scores taken before radiation along with the applied dose, to predict their personal susceptibility to cognitive impairment due to space radiation exposure. RESULTS AND CONCLUSION A significant finding is the exhibition of a dose-dependent increasing probability of impairment for 1 to 10 cGy of 28Si or 56Fe in the simple discrimination (SD) stage of the ATSET, and for 1 to 10 cGy of 56Fe in the compound discrimination (CD) stage. On a subject-based level, implementing ML classifiers such as ANNs identifies rats that have a higher tendency for impairment after GCR exposure [1]. The receiver operating characteristic (ROC) and the precision-recall (PR) curves of the ML models show a better prediction of impairment when 56Fe is the ion in question in both SD (Figure 1) and CD stages. They, however, do not depict impairment due to 4He in SD (Figure 1) and 28Si in CD, suggesting no dose-dependent impairment response in these cases. In this work, “good” prediction pertains to “better-than-random-chance”, due to the limited sample size and the high inter- and intra-individual variabilities in response to brain stimulation paradigms, as applicable to both animals and humans. More behavioral tests and biomarkers should be investigated on the same subjects, to be fed to the ML models to capture the agents responsible for performance alterations of some individuals versus others.

machine learning↗

NASA NTRSDate not supplied

Machine Learning Models to Predict Cognitive Impairment of Rodents Subjected to Space Radiation

machine learning↗

NASA NTRS2016

Evidence Report: Risk of Radiation Carcinogenesis

As noted by Durante and Cucinotta (2008), cancer risk caused by exposure to space radiation is now generally considered a main hindrance to interplanetary travel for the following reasons: large uncertainties are associated with the projected cancer risk estimates; no simple and effective countermeasures are available, and significant uncertainties prevent scientists from determining the effectiveness of countermeasures. Optimizing operational parameters such as the length of space missions, crew selection for age and sex, or applying mitigation measures such as radiation shielding or use of biological countermeasures can be used to reduce risk, but these procedures have inherent limitations and are clouded by uncertainties. Space radiation is comprised of high energy protons, neutrons and high charge (Z) and energy (E) nuclei (HZE). The ionization patterns and resulting biological insults of these particles in molecules, cells, and tissues are distinct from typical terrestrial radiation, which is largely X-rays and gamma-rays, and generally characterized as low linear energy transfer (LET) radiation. Galactic cosmic rays (GCR) are comprised mostly of highly energetic protons with a small component of high charge and energy (HZE) nuclei. Prominent HZE nuclei include He, C, O, Ne, Mg, Si, and Fe. GCR ions have median energies near 1 GeV/n, and energies as high as 10 GeV/n make important contributions to the total exposure. Ionizing radiation is a well known carcinogen on Earth (BEIR 2006). The risks of cancer from X-rays and gamma-rays have been established at doses above 50 mSv (5 rem), although there are important uncertainties and on-going scientific debate about cancer risk at lower doses and at low dose rates (<50 mSv/h). The relationship between the early biological effects of HZE nuclei and the probability of cancer in humans is poorly understood, and it is this missing knowledge that leads to significant uncertainties in projecting cancer risks during space exploration (Cucinotta and Durante 2006; Durante and Cucinotta 2008).

Huff, Janice↗

NASA NTRS2004

Performance of Off-the-Shelf Technologies for Spacecraft Cabin Atmospheric Major Constituent Monitoring

Monitoring the atmospheric composition of a crewed spacecraft cabin is central to successfully expanding the breadth and depth of first-hand human knowledge and understanding of space. Highly reliable technologies must be identified and developed to monitor atmospheric composition. This will enable crewed space missions that last weeks, months, and eventually years. Atmospheric composition monitoring is a primary component of any environmental control and life support system. Instrumentation employed to monitor atmospheric composition must be inexpensive, simple, and lightweight and provide robust performance. Such a system will ensure an environment that promotes human safety and health, and that the environment can be maintained with a high degree of confidence. Key to this confidence is the capability for any technology to operate autonomously, with little intervention from the crew or mission control personnel. A study has been conducted using technologies that, with further development, may reach these goals.

Tatara, J. D.↗

NASA NTRS2021

Machine Learning Models to Predict Cognitive Impairment of Rodents Subjected to Space Radiation

This research uses machine-learned computational analyses to predict the cognitive performance impairment of rats induced by irradiation. The experimental data in the analyses is from a rodent model exposed to ≤ 15 cGy of individual Galactic Cosmic Radiation (GCR) ions: 4He, 16O, 28Si, 48Ti, or 56Fe, expected for a Lunar or Mars mission. This work investigates rats at a subject-based level and uses performance scores taken before irradiation to predict impairment in Attentional Set-shifting (ATSET) data post-irradiation. Here, the worst performing rats of the control group define the impairment thresholds based on population analyses via cumulative distribution functions, leading to the labeling of impairment for each subject. A significant finding is the exhibition of a dose-dependent increasing probability of impairment for 1 to 10 cGy of 28Si or 56Fe in the Simple Discrimination (SD) stage of the ATSET, and for 1 to 10 cGy of 56Fe in the Compound Discrimination (CD) stage. On a subject-based level, implementing Machine Learning (ML) classifiers such as the Gaussian Naïve Bayes, Support Vector Machine, and Artificial Neural Networks identifies rats that have a higher tendency for impairment after GCR exposure. The algorithms employ the experimental prescreenperformance scores as multidimensional input features to predict each rodent’s susceptibility to cognitive impairment due to space radiation exposure. The receiver operating characteristic and the precision-recall curves of the ML models show a better prediction of impairment when 56Feis the ion in question in both SD and CD stages. They, however, do not depict impairment due to 4Hein SD and 28Siin CD, suggesting no dose-dependent impairment response in these cases. One key finding of our study is that prescreen performance scores can be used to predict the ATSET performance impairments. This result is significant to crewed space missions as it supports the potential of predicting an astronaut’s impairment in a specific task before spaceflight through the implementation of appropriately trained ML tools. Future research can focus on constructing ML ensemble methods to integrate the findings from the methodologies implemented in this study for morerobust predictionsof cognitive decrements due to space radiation exposure.

space radiation↗

NASA NTRS1975

Trade study for water and waste management concepts. Task 7: Support special analysis

Cost analyses and tradeoff studies are given for waste management in the Space Station, Lunar Surface Bases, and interplanetary space missions. Crew drinking water requirements are discussed and various systems to recycle water are examined. The systems were evaluated for efficiency and weight savings. The systems considered effective for urine water recovery were vapor compression, flash evaporation, and air evaporation with electrolytic pretreatment. For wash water recovery, the system of multifiltration was selected. A wet oxidation system, which can process many kinds of wastes, is also considered.

Source record↗

NASA NTRS1988

Training for 21st century space missions

Although specific recommendations are difficult to make about spacecraft and missions not yet designed, several general guidelines are presently formulated concerning the training of future, long-duration space mission crews. Training systems should be embedded in the normal controls and displays of the spacecraft used, so that critical maneuvers can be practiced as often as possible. Some system for computer-based training should be available onboard, in order to maintain the spacecraft system-knowledge of the crew at a high level and deepen understanding of malfunction responses.

Hughes, Frank E.↗

NASA NTRS2002

Experimental Methods in Reduced-gravity Soldering Research

The National Center for Microgravity Research, NASA Glenn Research Center, and NASA Johnson Space Center are conducting an experimental program to explore the influence of reduced gravity environments on the soldering process. An improved understanding of the effects of the acceleration environment is important to application of soldering during current and future human space missions. Solder joint characteristics that are being considered include solder fillet geometry, porosity, and microstructural features. Both through-hole and surface mounted devices are being investigated. This paper focuses on the experimental methodology employed in this project and the results of macroscopic sample examination. The specific soldering process, sample configurations, materials, and equipment were selected to be consistent with those currently on-orbit. Other apparatus was incorporated to meet requirements imposed by operation onboard NASA's KC-135 research aircraft and instrumentation was provided to monitor both the atmospheric and acceleration environments. The contingent of test operators was selected to include both highly skilled technicians and less skilled individuals to provide a population cross-section that would be representative of the skill mix that might be encountered in space mission crews.

Pettegrew, Richard D.↗

NASA NTRS2003

Treatment of Spacecraft Wastewater Using a Hollow Fiber Membrane Biofilm Redox Control Reactor

The purpose of this project was to develop and evaluate design concepts for biological treatment reactors for the purification of spacecraft wastewater prior to reverse osmosis treatment. The motivating factor is that wastewater recovery represents the greatest single potential reduction in the resupply requirements for crewed space missions. Spacecraft wastewater composition was estimated from the characteristics of the three major component streams: urine/flush water, hygiene water, and atmospheric condensate. The key characteristics of composite spacecraft wastewater are a theoretical oxygen demand of 4519 mg/L, of which 65% is nitrogenous oxygen demand, in a volume of 11.5 liter/crew-day. The organic carbon to nitrogen ratio of composite wastewater is 0.86. Urine represents 93% of nitrogen and 49% of the organic carbon in the composite wastestream. Various bioreaction scenarios were evaluated to project stoichiometric oxygen demands and the ability of wastewater carbon to support denitrification. Ammonia nitrification to the nitrite oxidation state reduced the oxygen requirement and enabled wastewater carbon to provide nearly complete denitrification. A conceptual bioreactor design was established using hollow fiber membranes for bubbleless oxygen transfer in a gravity-free environment, in close spatial juxtaposition to a second interspaced hollow fiber array for supplying molecular hydrogen. Highly versatile redox control and an enhanced ability to engineer syntrophic associations are stated advantages. A prototype reactor was constructed using a microporous hollow fiber membrane module for aeration. Maintaining inlet gas pressure within 0.25 psi of the external water pressure resulted in bubble free operation with no water ingress into hollow fiber lumens. Recommendations include the design and operational testing of hollow fiber bioreactors using: 1) Partial nitrification/nitrite predenitrification; 2) Limited aeration for simultaneous nitrification/denitrification or for nitrite reduction/ammonia oxidation; 3) Hydrogenotrophic denitrification.

Smith, Daniel P.↗