Engineering topics

Willis, Emily M.

Publications and source records attributed to Willis, Emily M..

32 records · Page 2

NASA NTRS2014

International Space Station (ISS) Plasma Contactor Unit (PCU) Utilization Plan Assessment Update

The International Space Station (ISS) vehicle undergoes spacecraft charging as it interacts with Earth's ionosphere and magnetic field. The interaction can result in a large potential difference developing between the ISS metal chassis and the local ionosphere plasma environment. If an astronaut conducting extravehicular activities (EVA) is exposed to the potential difference, then a possible electrical shock hazard arises. The control of this hazard was addressed by a number of documents within the ISS Program (ISSP) including Catastrophic Safety Hazard for Astronauts on EVA (ISS-EVA-312-4A_revE). The safety hazard identified the risk for an astronaut to experience an electrical shock in the event an arc was generated on an extravehicular mobility unit (EMU) surface. A catastrophic safety hazard, by the ISS requirements, necessitates mitigation by a two-fault tolerant system of hazard controls. Traditionally, the plasma contactor units (PCUs) on the ISS have been used to limit the charging and serve as a "ground strap" between the ISS structure and the surrounding ionospheric plasma. In 2009, a previous NASA Engineering and Safety Center (NESC) team evaluated the PCU utilization plan (NESC Request #07-054-E) with the objective to assess whether leaving PCUs off during non-EVA time periods presented risk to the ISS through assembly completion. For this study, in situ measurements of ISS charging, covering the installation of three of the four photovoltaic arrays, and laboratory testing results provided key data to underpin the assessment. The conclusion stated, "there appears to be no significant risk of damage to critical equipment nor excessive ISS thermal coating damage as a result of eliminating PCU operations during non- EVA times." In 2013, the ISSP was presented with recommendations from Boeing Space Environments for the "Conditional" Marginalization of Plasma Hazard. These recommendations include a plan that would keep the PCUs off during EVAs when the space environment forecast input to the ISS charging model indicates floating potentials (FP) within specified limits. These recommendations were based on the persistence of conditions in the space environment due to the current low solar cycle and belief in the accuracy and completeness of the ISS charging model. Subsequently, a Noncompliance Report (NCR), ISS-NCR-232G, Lack of Two-fault Tolerance to EVA Crew Shock in the Low Earth Orbit Plasma Environment, was signed in September 2013 specifying new guidelines for the use of shock hazard controls based on a forecast of the space environment from ISS plasma measurements taken prior to the EVA [ISS-EVA-312-AC, 2012]. This NESC assessment re-evaluates EVA charging hazards through a process that is based on over 14 years of ISS operations, charging measurements, laboratory tests, EMU studies and modifications, and safety reports. The assessment seeks an objective review of the plasma charging hazards associated with EVA operations to determine if any of the present hazard controls can safely change the PCU utilization plan to allow more flexibility in ISS operations during EVA preparation and execution.

Hernandez-Pellerano, Amri↗

NASA NTRS2014

Natural Environment Capabilities at MSFC

The Natural Environments Branch at Marshall Space Flight Center is integral in developing, maintaining, and investigating NASA missions such as Space Launch Systems (SLS), currently under development, as well as many NASA and other agency satellite missions. We present the space environment capabilities of the Natural Environments Branch at MSFC. These in-house capabilities include model development, analysis of space and terrestrial related data, spacecraft charging anomaly investigations, surface charging modeling including Nascap-2k, space environment definition and radiation parts assessment. All aspects of space and terrestrial design are implemented with the goal of devising missions to be successful at launch and in the space environment of LEO, polar, GEO, and interplanetary orbits. In this poster, we show examples of recent applications of branch capabilities to NASA missions.

Parker, Linda Neergaard↗

NASA NTRS2014

An Investigation of Low Earth Orbit Internal Charging

Low Earth orbit is usually considered a relatively benign environment for internal charging threats due to the low flux of penetrating electrons with energies of a few MeV that are encountered over an orbit. There are configurations, however, where insulators and ungrounded conductors used on the outside of a spacecraft hull may charge when exposed to much lower energy electrons of some 100's keV in a process that is better characterized as internal charging than surface charging. For example, the minimal radiation shielding afforded by thin thermal control materials such as metalized polymer sheets (e.g., aluminized Kapton or Mylar) and multilayer insulation may allow electrons of 100's of keV to charge underlying materials. Yet these same thermal control materials protect the underlying insulators and ungrounded conductors from surface charging currents due to electrons and ions at energies less than a few keV as well as suppress the photoemission, secondary electron, and backscattered electron processes associated with surface charging. We investigate the conditions required for this low Earth orbit "internal charging" to occur and evaluate the environments for which the process may be a threat to spacecraft. First, we describe a simple one-dimensional internal charging model that is used to compute the charge accumulation on materials under thin shielding. Only the electron flux that penetrates exposed surface shielding material is considered and we treat the charge balance in underlying insulation as a parallel plate capacitor accumulating charge from the penetrating electron flux and losing charge due to conduction to a ground plane. Charge dissipation due to conduction can be neglected to consider the effects of charging an ungrounded conductor. In both cases, the potential and electric field is computed as a function of time. An additional charge loss process is introduced due to an electrostatic discharge current when the electric field reaches a prescribed breakdown strength. For simplicity, the amount of charge lost in the discharge is treated as a random percentage of the total charge between a set maximum and minimum amount so a user can consider partial discharges of insulating materials (small loss of charge) or arcing from a conductor (large loss of charge). We apply the model to electron flux measurements from the NOAA-19 spacecraft to demonstrate that charging can reach levels where electrostatic discharges occur and estimate the magnitude of the discharge.

NeergaardParker, Linda↗

NASA NTRS2014

Spacecraft Charging Analysis of a CubeSat

Spacecraft charging occurs when charged particles from the surrounding space plasma environment contact a spacecraft and unequal charging currents result in a net charge density accumulation on or in spacecraft materials. Charging becomes a threat when differential potentials between two points on the spacecraft or between the spacecraft and the ambient space environment build to the level that electric fields associated with the potentials exceed the electric breakdown strength of the spacecraft materials and electrostatic discharge arcs are generated. Electrostatic discharges resulting from spacecraft charging can adversely affect telemetry and cause irreparable damage to electronics. Other spacecraft charging effects include damage of solar arrays and thermal protection, enhancement of contamination of surfaces, and degradation of optics. Typically, the large government and commercial space programs include spacecraft charging analysis as part of the design process. CubeSat projects, however, usually do not have the time or funding to include a spacecraft charging analysis due to their low budget and quick-turnaround requirements. CubeSat projects also tend to rely heavily on commercial "off-the-shelf" products, many of which are not qualified for use in space, and are particularly vulnerable to the effects of the space environment. As the demand for longer and more complex CubeSat missions increases, it is becoming more and more important to consider the effects of spacecraft charging in the design process. Results of surface charging analysis using Nascap-2k on a typical CubeSat design for a polar orbit scenario are illustrated. These results show that for a polar orbiting CubeSat, spacecraft charging could be an issue and steps should be taken to mitigate the effects for these small satellites.

Willis, Emily M.↗

NASA NTRS2014

Correlation of ISS Electric Potential Variations with Mission Operations

Spacecraft charging on the International Space Station (ISS) is caused by a complex mix of the low Earth orbit plasma environment, space weather events, operations of the high voltage solar arrays, and changes in the ISS configuration and orbit parameters. Measurements of the ionospheric electron density and temperature along the ISS orbit and variations in the ISS electric potential are obtained from the Floating Potential Measurement Unit (FPMU) suite of four plasma instruments (two Langmuir probes, a Floating Potential Probe, and a Plasma Impedance Probe) on the ISS. These instruments provide a unique capability for monitoring the response of the ISS electric potential to variations in the space environment, changes in vehicle configuration, and operational solar array power manipulation. In particular, rapid variations in ISS potential during solar array operations on time scales of tens of milliseconds can be monitored due to the 128 Hz sample rate of the Floating Potential Probe providing an interesting insight into high voltage solar array interaction with the space plasma environment. Comparing the FPMU data with the ISS operations timeline and solar array data provides a means for correlating some of the more complex and interesting ISS electric potential variations with mission operations. In addition, recent extensions and improvements to the ISS data downlink capabilities have allowed more operating time for the FPMU than ever before. The FPMU was operated for over 200 days in 2013 resulting in the largest data set ever recorded in a single year for the ISS. This presentation will provide examples of a number of the more interesting ISS charging events observed during the 2013 operations including examples of rapid charging events due to solar array power operations, auroral charging events, and other charging behavior related to ISS mission operations.

Willis, Emily M.↗

NASA NTRS2014

Characteristics of Extreme Auroral Charging Events

The highest level spacecraft charging observed in low Earth orbit (LEO) occurs when spacecraft are exposed to energetic auroral electrons. Since auroral charging has been identified as a mechanism responsible for on-orbit anomalies and even possible satellite failures it is important to consider extreme auroral charging events as design and test environments for spacecraft to be used in high inclination LEO orbits. This paper will report on studies of extreme auroral charging events using data from the SSJ/4 and SSJ/5 precipitating electron and ion sensors on the Defense Meteorology Satellite Program (DMSP) satellites. Early studies of DMSP charging to negative potentials ≥100 V focused on statistics of the electron environment responsible for charging. Later statistical studies of auroral charging have generally focused on solar cycle dependence of charging behavior and magnitude of the maximum potential and duration of the charging events. We extend these studies to focus on more detailed investigations of extreme charging event characteristics that are required to evaluate potential threats to spacecraft systems. A collection of example auroral charging events is assembled from the DMSP data set using the criteria that "extreme auroral charging" is defined as periods with spacecraft negative potentials ≥400 V. Specific characteristics to be treated include (but are not limited to) maximum and mean potentials, time history of spacecraft potentials through the events, total charging duration and the time potentials exceed voltage thresholds, frame charging/discharging rates, and information on geographic and geomagnetic latitudes at which the events are observed. Finally, we will comment on the implications of these studies for potential auroral charging risks to the International Space Station.

Minow, Joseph I.↗

NASA NTRS2014

Correlation of ISS Electric Potential Variations with Mission Operations

Spacecraft charging on the International Space Station (ISS) is caused by a complex combination of the low Earth orbit plasma environment, space weather events, operations of the high voltage solar arrays, and changes in the ISS configuration and orbit parameters. Measurements of the ionospheric electron density and temperature along the ISS orbit and variations in the ISS electric potential are obtained from the Floating Potential Measurement Unit (FPMU) suite of four plasma instruments (two Langmuir probes, a Floating Potential Probe, and a Plasma Impedance Probe) on the ISS. These instruments provide a unique capability for monitoring the response of the ISS electric potential to variations in the space environment, changes in vehicle configuration, and operational solar array power manipulation. In particular, rapid variations in ISS potential during solar array operations on time scales of tens of milliseconds can be monitored due to the 128 Hz sample rate of the Floating Potential Probe providing an interesting insight into high voltage solar array interaction with the space plasma environment. Comparing the FPMU data with the ISS operations timeline and solar array data provides a means for correlating some of the more complex and interesting ISS electric potential variations with mission operations. In addition, recent extensions and improvements to the ISS data downlink capabilities have allowed more operating time for the FPMU than ever before. The FPMU was operated for over 200 days in 2013 resulting in the largest data set ever recorded in a single year for the ISS. In this paper we provide examples of a number of the more interesting ISS charging events observed during the 2013 operations including examples of rapid charging events due to solar array power operations, auroral charging events, and other charging behavior related to ISS mission operations.

Willis, Emily M.↗

NASA NTRS2014

Correlation of ISS Electric Potential Variations with Mission Operations

Orbiting approximately 400 km above the Earth, the International Space Station (ISS) is a unique research laboratory used to conduct ground-breaking science experiments in space. The ISS has eight Solar Array Wings (SAW), and each wing is 11.7 meters wide and 35.1 meters long. The SAWs are controlled individually to maximize power output, minimize stress to the ISS structure, and minimize interference with other ISS operations such as vehicle dockings and Extra-Vehicular Activities (EVA). The Solar Arrays are designed to operate at 160 Volts. These large, high power solar arrays are negatively grounded to the ISS and collect charged particles (predominately electrons) as they travel through the space plasma in the Earth's ionosphere. If not controlled, this collected charge causes floating potential variations which can result in arcing, causing injury to the crew during an EVA or damage to hardware [1]. The environmental catalysts for ISS floating potential variations include plasma density and temperature fluctuations and magnetic induction from the Earth's magnetic field. These alone are not enough to cause concern for ISS, but when they are coupled with the large positive potential on the solar arrays, floating potentials up to negative 95 Volts have been observed. Our goal is to differentiate the operationally induced fluctuations in floating potentials from the environmental causes. Differentiating will help to determine what charging can be controlled, and we can then design the proper operations controls for charge collection mitigation. Additionally, the knowledge of how high power solar arrays interact with the environment and what regulations or design techniques can be employed to minimize charging impacts can be applied to future programs.

Willis, Emily M.↗

NASA NTRS2014

MSFC/EV44 Natural Environment Capabilities

The Natural Environments Branch at Marshall Space Flight Center is an integral part of many NASA satellite and launch vehicle programs, providing analyses of the space and terrestrial environments that are used for program development efforts, operational support, and anomaly investigations. These capabilities include model development, instrument build and testing, analysis of space and terrestrial related data, spacecraft charging anomaly investigations, surface and internal charging modeling, space environment definition, and radiation assessments for electronic parts. All aspects of space and terrestrial design are implemented with the goal of devising missions that are successful from launch to operations in the space environment of LEO, polar, GEO, and interplanetary orbits.

NeergaardParker, Linda↗

NASA NTRS2014

CubeSat Susceptibility to Auroral Space Weather Events

No abstract available

Willis, Emily M.↗

NASA NTRS2013

Near Real Time Tools for ISS Plasma Science and Engineering Applications

The International Space Station (ISS) program utilizes a plasma environment forecast for estimating electrical charging hazards for crews during extravehicular activity (EVA). The process uses ionospheric electron density and temperature measurements from the ISS Floating Potential Measurement Unit (FPMU) instrument suite with the assumption that the plasma conditions will remain constant for one to fourteen days with a low probability for a space weather event which would significantly change the environment before an EVA. FPMU data is typically not available during EVA's, therefore, the most recent FPMU data available for characterizing the state of the ionosphere during EVA is typically a day or two before the start of an EVA or after the EVA has been completed. In addition to EVA support, information on ionospheric plasma densities is often needed for support of ISS science payloads and anomaly investigations during periods when the FPMU is not operating. This presentation describes the application of space weather tools developed by MSFC using data from near real time satellite radio occultation and ground based ionosonde measurements of ionospheric electron density and a first principle ionosphere model providing electron density and temperature run in a real time mode by GSFC. These applications are used to characterize the space environment during EVA periods when FPMU data is not available, monitor for large charges in ionosphere density that could render the ionosphere forecast and plasma hazard assessment invalid, and validate the assumption of "persistence of conditions" used in deriving the hazard forecast. In addition, the tools are used to provide space environment input to science payloads on ISS and anomaly investigations during periods the FPMU is not operating.

Minow, Joseph I.↗

NASA NTRS2013

Near Real Time Tools for ISS Plasma Science and Engineering Applications

The International Space Station (ISS) program utilizes a plasma environment forecast for estimating electrical charging hazards for crews during extravehicular activity (EVA). The process uses ionospheric electron density (Ne) and temperature (Te) measurements from the ISS Floating Potential Measurement Unit (FPMU) instrument suite with the assumption that the plasma conditions will remain constant for one to fourteen days with a low probability for a space weather event which would significantly change the environment before an EVA. FPMU data is typically not available during EVA's, therefore, the most recent FPMU data available for characterizing the state of the ionosphere during EVA is typically a day or two before the start of an EVA or after the EVA has been completed. Three near real time space weather tools under development for ISS applications are described here including: (a) Ne from ground based ionosonde measurements of foF2 (b) Ne from near real time satellite radio occultation measurements of electron density profiles (c) Ne, Te from a physics based ionosphere model These applications are used to characterize the ISS space plasma environment during EVA periods when FPMU data is not available, monitor for large changes in ionosphere density that could render the ionosphere forecast and plasma hazard assessment invalid, and validate the "persistence of conditions" forecast assumption. In addition, the tools are useful for providing space environment input to science payloads on ISS and anomaly investigations during periods the FPMU is not operating.

Minow, Joseph I.↗

NASA NTRS2013

Recent Applications of Space Weather Research to NASA Space Missions

Marshall Space Flight Center s Space Environments Team is committed to applying the latest research in space weather to NASA programs. We analyze data from an extensive set of space weather satellites in order to define the space environments for some of NASA s highest profile programs. Our goal is to ensure that spacecraft are designed to be successful in all environments encountered during their missions. We also collaborate with universities, industry, and other federal agencies to provide analysis of anomalies and operational impacts to current missions. This presentation is a summary of some of our most recent applications of space weather data, including the definition of the space environments for the initial phases of the Space Launch System (SLS), acquisition of International Space Station (ISS) frame potential variations during geomagnetic storms, and Nascap-2K charging analyses.

Willis, Emily M.↗

NASA NTRS2013

Recent Applications of Space Weather Research to NASA Space Missions

Willis, Emily M.↗