Engineering topics
Cooke, W. J.
Publications and source records attributed to Cooke, W. J..
Recent Corrections to Meteoroid Environment Models
The dynamical and physical characteristics of a meteoroid affects its behavior in the atmosphere and the damage it does to spacecraft surfaces. Accurate environment models must therefore correctly describe the speed, size, density, and direction of meteoroids. However, the measurement of dynamical characteristics such as speed is subject to observational biases, and physical properties such as size and density cannot be directly measured. De-biasing techniques and proxies are needed to overcome these challenges. In this presentation, we discuss several recent improvements to the derivation of the meteoroid velocity, directionality, and bulk density distributions. We derive our speed distribution from observations made by the Canadian Meteor Orbit Radar. These observations are de-biased using modern descriptions of the ionization efficiency and sharpened to remove the effects of measurement uncertainty, and the result is a meteoroid speed distribution that is skewed slower than in previous analyses. We also adopt a higher fidelity density distribution than that used by many older models. In our distribution, meteoroids with T(sub J) less than 2 are assigned to a low-density population, while those with T(sub J) greater than 2 have higher densities. This division and the distributions themselves are derived from the densities reported by Kikwaya et al. (2009, 2011). These changes have implications for the environment. For instance, helion and antihelion meteors have lower speeds and higher densities than apex and toroidal meteors. A slower speed distribution therefore corresponds to a sporadic environment that is more completely dominated by the helion and antihelion sources than in previous models. Finally, assigning these meteors high densities further increases their significance from a spacecraft damage perspective.
A Comparison of Results from NASA's Meteoroid Engineering Model to the LDEF Cratering Record
NASA's Long Duration Exposure Facility (LDEF) has provided an extensive record of the meteoroid environment in low Earth orbit. LDEF's combination of fixed orientation, large collecting area, and long lifetime imposes constraints on the absolute flux of potentially hazardous meteoroids. The relative impact rate on each of LDEF's fourteen surfaces arises from the underlying velocity distribution and directionality of the meteoroid environment. For the first time, we model the meteoroid environment encountered by LDEF over its operational lifetime using NASA's Meteoroid Engineering Model Release 2 (MEMR2) and compare the model results with the observed craters of potentially hazardous meteoroids (i.e. crater diameters larger than approximately 0.75 mm). We discuss the extent to which the observations and model agree and how the impact rates across all of the LDEF surfaces may be utilized to help calibrate future versions of MEM.
A Comparison of Results From NASA's Meteoroid Engineering Model to the LDEF Cratering Record
NASA's Long Duration Exposure Facility (LDEF) has provided an extensive record of the meteoroid environment in Low Earth Orbit. LDEF's combination of fixed orientation, large collecting area, and long lifetime imposes constraints on the absolute flux of potentially hazardous meteoroids. The relative impact rate on each of LDEF's fourteen surfaces arises from the underlying velocity distribution and directionality of the meteoroid environment. For the first time, we model the meteoroid environment encountered by LDEF over its operational lifetime using NASA's Meteoroid Engineering Model Release 2 (MEMR2) and compare the model results with the observed craters of potentially hazardous meteoroids (i.e. crater diameters larger than approximately 0.6 mm). We discuss the extent to which the observations and model agree and how the impact rates across all of the LDEF surfaces may suggest improvements to the underlying assumptions that go into future versions of MEM.
The Velocity and Density Distribution of Earth-Intersecting Meteoroids: Implications for Environment Models
No abstract available
Simultaneous Optical and Meteor Head Echo Measurements Using the Middle Atmosphere Alomar Radar System (MAARSY)
We describe a two year campaign of simultaneous automated meteor optical and head echo radar measurements conducted with the Middle Atmosphere Alomar Radar System (MAARSY). This campaign was established with the following goals: Compare trajectories as measured by MAARSY and the two optical stations for a range of meteoroid masses. Compare photometric and dynamic mass measured optically with radar-derived masses (inter-calibration of mass scales). Use the best observed simultaneous events to fuse all metric, photometric and ionization estimates together and apply different ablation models to self-consistently model these highest quality events.
The Velocity and Density Distribution of Earth-Intersecting Meteoroids: Implications for Environment Models
Meteoroids are known to damage spacecraft: they can crater or puncture components, disturb a spacecraft's attitude, and potentially create secondary electrical effects. Because the damage done depends on the speed, size, density, and direction of the impactor, accurate environment models are critical for mitigating meteoroid-related risks. Yet because meteoroid properties are derived from indirect observations such as meteors and impact craters, many characteristics of the meteoroid environment are uncertain. In this work, we present recent improvements to the meteoroid speed and density distributions. Our speed distribution is derived from observations made by the Canadian Meteor Orbit Radar. These observations are de-biased using modern descriptions of the ionization efficiency. Our approach yields a slower meteoroid population than previous analyses (see Fig. 1 for an example) and we compute the uncertainties associated with our derived distribution. We adopt a higher fidelity density distribution than that used by many older models. In our distribution, meteoroids with TJ less than 2 are assigned to a low-density population, while those with TJ greater than 2 have higher densities (see Fig. 2). This division and the distributions themselves are derived from the densities reported by Kikwaya et al. These changes have implications for the environment: for instance, the helion/antihelion sporadic sources have lower speeds than the apex and toroidal sources and originate from high-T(sub J) parent bodies. Our on-average slower and denser distributions thus imply that the helion and antihelion sources dominate the meteoroid environment even more completely than previously thought. Finally, for a given near-Earth meteoroid cratering rate, a slower meteoroid population produces a comparatively higher rate of satellite attitude disturbances.
Spacecraft Risk Posed by the 2016 Perseid Outburst
The Perseids are one of the more prolific annual showers, known for high rates and for producing bright meteors. Outbursts of this shower have been noted in the 1860s, the early 1990s, 2004, and 2009, with the 1993 outburst being especially active (peak ZHR above 300). The 1993 Perseids also affected the space-faring nations, as the launch of the STS-51 mission was delayed by NASA until after the shower maximum due to an inability to predict the shower intensity, and the ESA telecommunications satellite Olympus suffered a mission-ending anomaly attributed to a static discharge caused by a Perseid impact [1]. Rates were again high (peak ZHR around 200) in 2009, when the NASA/USGS imaging satellite Landsat-5 experienced a gyro anomaly just before the shower peak; however in this case, the satellite was recovered and normal operations resumed one week later [2]. It is interesting to note that both spacecraft anomalies were not what is typically expected from meteoroid strikes, i.e., physical damage or an attitude displacement due to transfer of momentum. It would appear that the very fast Perseids (59 km s(sup -1) have a marked ability to produce plasma upon impact, which can then serve as a conductive path for discharge currents. The shower is expected to outburst again in 2016, and we present the results from the MSFC Meteoroid Stream Model [4], which predicts enhanced activity on a level similar to that of 2009 as the Earth passes through several debris trails on the night of August 11-12 (UT). We then compare our results to those of other modelers.
Optical and Radar Measurements of the Meteor Speed Distribution
The observed meteor speed distribution provides information on the underlying orbital distribution of Earth-intersecting meteoroids. It also affects spacecraft risk assessments; faster meteors do greater damage to spacecraft surfaces. Although radar meteor networks have measured the meteor speed distribution numerous times, the shape of the de-biased speed distribution varies widely from study to study. Optical characterizations of the meteoroid speed distribution are fewer in number, and in some cases the original data is no longer available. Finally, the level of uncertainty in these speed distributions is rarely addressed. In this work, we present the optical meteor speed distribution extracted from the NASA and SOMN allsky networks [1, 2] and from the Canadian Automated Meteor Observatory (CAMO) [3]. We also revisit the radar meteor speed distribution observed by the Canadian Meteor Orbit Radar (CMOR) [4]. Together, these data span the range of meteoroid sizes that can pose a threat to spacecraft. In all cases, we present our bias corrections and incorporate the uncertainty in these corrections into uncertainties in our de-biased speed distribution. Finally, we compare the optical and radar meteor speed distributions and discuss the implications for meteoroid environment models.
Determination of the Meteor Limiting Magnitude
The limiting meteor magnitude of a meteor camera system will depend on the camera hardware and software, sky conditions, and the location of the meteor radiant. Some of these factors are constants for a given meteor camera system, but many change between meteor shower or sporadic source and on both long and short timescales. Since the limiting meteor magnitude ultimately gets used to calculate the limiting meteor mass for a given data set, it is important to have an understanding of these factors and to monitor how they change throughout the night, as a 0.5 magnitude uncertainty in limiting magnitude translates to a uncertainty in limiting mass by a factor of two.
NASA Meteoroid Engineering Model Release 2.0
The Meteoroid Engineering Model release 2.0 (MEMR2) software is NASA's most current and accurate model of the meteoroid environment. It enables the user to generate a trajectory-specific meteoroid environment for spacecraft traveling within the inner solar system. In addition to the total meteoroid flux, MEMR2 provides the user with meteoroid directionality and velocity information. Users have the ability to make a number of analysis and output choices that tailor the resulting environment to their needs. This Technical Memorandum outlines the history of MEMR2, the meteoroid environment it describes, and makes recommendations for the correct use of the software and interpretation of its results.
The 2016 Perseids
The Perseid meteor shower has been observed since ancient times. One of the most prolific annual showers, the Perseids have also been known to outburst. At least two spacecraft have suffered anomalies potentially caused by meteoroid impacts during Perseid outbursts. Olympus, an ESA telecommunications satellite, was likely impacted by a Perseid meteoroid during the 1993 outburst that ultimately led to the termination of the spacecraft's mission. Landsat-5, an imaging satellite jointly managed by NASA and the USGS, lost gyro stability during the peak of the Perseids in 2009. The Perseid meteor shower is expected to outburst again in 2016. Stream model predictions place the peak activity on the night of August 11-12 (UT) as the Earth passes through several old debris trails from parent comet 109P/Swift-Tuttle. Observing geometry favors Europe at the onset, but increased activity for about half a day means that North America is also well-placed for observations. A call for observations to characterize the stream and constrain numerical models is made. Modeling results, observing geometry, and spacecraft risk during the 2016 Perseids will be discussed.
Optical Meteor Systems Used by the NASA Meteoroid Environment Office
The NASA Meteoroid Environment Office (MEO) uses two main meteor camera networks to characterize the meteoroid environment: an all sky system and a wide field system to study cm and mm size meteors respectively. The NASA All Sky Fireball Network consists of fifteen meteor video cameras in the United States, with plans to expand to eighteen cameras by the end of 2015. The camera design and All-Sky Guided and Real-time Detection (ASGARD) meteor detection software [1, 2] were adopted from the University of Western Ontario's Southern Ontario Meteor Network (SOMN). After seven years of operation, the network has detected over 12,000 multi-station meteors, including meteors from at least 53 different meteor showers. The network is used for speed distribution determination, characterization of meteor showers and sporadic sources, and for informing the public on bright meteor events. The NASA Wide Field Meteor Network was established in December of 2012 with two cameras and expanded to eight cameras in December of 2014. The two camera configuration saw 5470 meteors over two years of operation with two cameras, and has detected 3423 meteors in the first five months of operation (Dec 12, 2014 - May 12, 2015) with eight cameras. We expect to see over 10,000 meteors per year with the expanded system. The cameras have a 20 degree field of view and an approximate limiting meteor magnitude of +5. The network's primary goal is determining the nightly shower and sporadic meteor fluxes. Both camera networks function almost fully autonomously with little human interaction required for upkeep and analysis. The cameras send their data to a central server for storage and automatic analysis. Every morning the servers automatically generates an e-mail and web page containing an analysis of the previous night's events. The current status of the networks will be described, alongside with preliminary results. In addition, future projects, CCD photometry and broadband meteor color camera system, will be discussed.
When the Sky Falls NASA's Response to Bright Bolide Events Over Continental USA
Being the only U.S. Government entity charged with monitoring the meteor environment, the Meteoroid Environment Office (MEO) has deployed a network of allsky and wide field meteor cameras, along with the appropriate software tools to quickly analyze data from these systems. However, the coverage of this network is still quite limited, forcing the incorporation of data from other cameras posted to the internet in analyzing many of the fireballs reported by the public and media. Information on these bright events often needs to be reported to NASA Headquarters by noon the following day; thus a procedure has been developed that determines the analysis process for a given fireball event based on the types and amount of data available. The differences between these analysis processes are shown by looking at four meteor events that the MEO responded to, all of which were large enough to produce meteorites.
The Flux of Large Meteoroids Observed with Lunar Impact Monitoring
The flux of large meteoroids is not well determined due to relatively low number statistics, due mainly to the lack of collecting area available to meteor camera systems (10(2)-10(5) km2). Larger collecting areas are needed to provide reasonable statistics for flux calculations. The Moon, with millions of square kilometers of lunar surface, can be used as a detector for observing the population of large meteoroids in the tens of grams to kilogram mass range. This is accomplished by observing the flash of light produced when a meteoroid impacts the lunar surface, converting a portion of its kinetic energy to visible light detectable from Earth. A routine monitoring program at NASA's Marshall Space Flight Center has recorded over 300 impact flashes since early 2006. The program utilizes multiple 0.35 m (14 inch) Schmidt-Cassegrain telescopes, outfitted with video cameras using the 1/2 inch Sony EXview HAD CCDTM chip, to perform simultaneous observations of the earthshine hemisphere of the Moon when the lunar phase is between 0.1 and 0.5. This optical arrangement permits monitoring of approximately 3.8x10(6) km2 of lunar surface. A selection of 126 flashes recorded in 266.88 hours of photometric skies was analyzed, creating the largest and most homogeneous dataset of lunar impact flashes to date. Standard CCD photometric techniques outlined in [1] were applied to the video to determine the luminous energy, kinetic energy, and mass for each impactor, considering a range of luminous efficiencies. The flux to a limiting energy of 2.5x10(-6) kT TNT or 1.05×10(7) J is 1.03×10(-7) km(-2) hr(-1) and the flux to a limiting mass of 30 g is 6.14×10(-10) m(-2) yr(-1). Comparisons made with measurements and models of the meteoroid population indicate that the flux of objects in this size range is slightly lower (but within the error bars) than the power law distribution determined for the near Earth object population by [2].
The 2012 Lyrids from Non-traditional Observing Platforms
The NASA Meteoroid Environment Office (MEO) observed meteors during the Lyrid meteor shower peak on 22 April 2012 from three different observing platforms: the ground, a helium-filled balloon, and from the International Space Station (ISS). Even though the Lyrids are not noted for spectacular rates, the combination of New Moon and a favorable viewing geometry from ISS presented a unique opportunity to simultaneously image shower meteors from above the atmosphere and below it. In the end, however, no meteors were observed simultaneously, and it was impossible to identify Lyrids with 100% confidence among the 155 meteors observed from ISS and the 31 observed from the balloon. Still, this exercise proved successful in that meteors could be observed from a simple and inexpensive balloon-based payload and from less-than-optimal cameras on ISS.