arXiv subjects

Peter G. Brown

Publications and source records attributed to Peter G. Brown.

At least 19 recordsLinked to original sources

arXiv2026

From Earth Meteors to Mars: Predicting Where to See the First Martian Meteors

Predictions of optical meteors at Mars have largely relied on classical single-body ablation models, despite high-resolution terrestrial observations showing that mm-sized meteoroids frequently fragment. This study quantifies how fragmentation alters the predicted brightness and peak-luminosity altitudes of sporadic mm-sized meteoroids in the Martian atmosphere and evaluates the single-body approximation as a reference baseline. Physical properties were inferred for 144 sporadic meteoroids observed on Earth using dynamic nested sampling with the erosion-fragmentation model. The resulting best-fit meteoroids were then re-simulated under Martian atmospheric conditions to generate predicted light curves. Because the physical trigger of fragmentation onset remains uncertain, three hypotheses were tested based on atmospheric mass density, dynamic pressure, and total accumulated heat. The results were also compared with predictions from a single-body ablation model. The data-driven simulations predict peak absolute magnitudes of $M_{\rm peak}\sim$ 2 - 7 for Martian meteors spanning diameters of 0.4 - 10 mm and entry speeds of 10 - 56 km/s. We find most events are luminous between $\sim$ 55 and 110 km heights. Relative to the single-body ablation baseline, the fragmentation-based predictions are brighter by $\sim$ 0.8 mag at peak brightness and concentrate luminosity within a narrower vertical range ($\sim$ 17 km instead of 36 km). The modelling accounting for fragmentation also produce shorter luminous trails ($\sim$ 20 km instead of 45 km). The resulting altitude-brightness maps provide observation-ready guidance for future Mars missions, while the fragmentation-based framework supports the interpretation of meteoroid-related ionospheric metal layers and improvements to Mars meteoroid-environment models.

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arXiv2026

Predictions of Imminent Earth Impactors Discovered by LSST

Imminent impactors are natural bodies discovered in space before impacting the Earth. They provide a rare opportunity to characterize individual near-Earth objects (NEOs) in great detail as asteroids in space, meteors in Earth's atmosphere and meteorites on the ground. The Vera C. Rubin Observatory's upcoming Legacy Survey of Space and Time (LSST) is expected to transform our understanding of the NEO population. In this work, we evaluate LSST's expected discovery performance for imminent impactors using $343$ meter-size objects previously recorded in NASA's CNEOS database as fireballs impacting Earth's atmosphere. We simulate pre-impact observations of these CNEOS impactors with the Sorcha survey simulator under LSST's default three-night discovery strategy and a one-night strategy for fast-moving objects that relies on matching aligned streaks in two exposures on the same night. We estimate that LSST will discover $\sim1-2$ meter-size and larger imminent impactors per year, representing $\sim4\%$ of all Earth impactors $\gtrsim1$ m in diameter and almost doubling the current discovery rate of imminent impactors. The median time of discovery and median time of first observation for impactors discovered in our simulations are $\sim1.57$ and $\sim3.06$ days before impact, respectively. The spatial distribution of the 11 previously discovered imminent impactors is biased towards the Northern Hemisphere, where the observatories that discovered them are located. We find a similar trend towards Southern Hemisphere impacts in our simulated LSST detections of the CNEOS impactors, suggesting Rubin will provide a powerful counterpart to existing asteroid surveys primarily located in the Northern Hemisphere.

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arXiv2026

Meteor statistics I: The distribution of instrumental magnitudes

The distribution of meteor magnitudes is known to follow an exponential distribution, where the base of this distribution is called the population index. The distribution of observed magnitudes preserves this behavior, but is truncated by the detection threshold. If both the population index and detection threshold can be determined, observed meteor rates can be converted to fluxes and extrapolated to any desired brightness or size. We argue that the distribution of observed or instrumental meteor magnitudes is best modeled as an exponentially modified Gaussian (exGaussian) distribution. This is for three reasons: first, an exGaussian distribution is the natural result of random variations in detection threshold and/or post-detection measurement errors in magnitude. Second, an exGaussian distribution provides a better fit to the magnitude distribution than all other competing distributions in the literature; we demonstrate this using both a set of faint optical meteor magnitudes and a set of radar meteor echo amplitudes. Finally, the population index, mean detection threshold, and random variation/error terms are easily extracted from the best-fit parameters of an exGaussian distribution.

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arXiv2026

Inferring Meteoroid Properties with Dynamic Nested Sampling: A Case Study of Orionid and Capricornid Shower Meteors

Accurate estimation of meteoroid bulk density is crucial for assessing spacecraft impact hazards from sub-millimeter to millimeter-sized meteoroids. Previous studies often used manual tuning or optimization methods to fit ablation and fragmentation models to optical meteor data, but subjective choices made physical properties and uncertainties difficult to compare. We develop a global, statistically robust method that uses Dynamic Nested Sampling to fit an erosion-fragmentation model to meteor light curves and deceleration measured by the Canadian Automated Meteor Observatory (CAMO) mirror tracking system and Electron-Multiplied CCD (EMCCD) cameras. Applied to 15 shower meteors, the method returns posterior distributions and Bayesian evidences for single- and double-fragmentation scenarios. Tests on four synthetic cases recover the known inputs, with best-guess solutions matching the true parameters. For 9 Orionids and 6 Alpha Capricornids with masses 1e-6 to 1e-5 kg, the median bulk density is 159 (+558/-57) kg/m3 for Orionids and 333 (+1089/-114) kg/m3 for Alpha Capricornids. Orionids are consistent with low-density cometary material, while Alpha Capricornids are systematically denser and show a second density cluster near 1300 kg/m3, consistent with higher-density asteroidal material. This framework enables automated, statistically rigorous characterization of meteoroid properties and will be extended to larger samples of shower and sporadic meteors across orbital classes.

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arXiv2026

Decameter-sized Earth Impactors -- II: A Bayesian Inference Approach to Meteoroid Ablation Modeling

Small asteroids and large meteoroids frequently impact the Earth, though their physical and material properties remain poorly understood. When observed as fireballs in Earth's atmosphere, these properties can be inferred from their ablation and fragmentation behavior. The 2022 release of previously classified United States Government (USG) satellite sensor data has provided hundreds of new fireball light curves, allowing for more detailed analysis. Here we present a new Bayesian inference method based on dynamic nested sampling that can robustly estimate these objects' physical parameters from their observed light curves, starting from relatively uninformative, flat priors. We validate our method against seven USG sensor-observed fireballs with independent ground-based observations and demonstrate that our results are consistent with previous estimates. We then apply our technique to $13$ decameter-size Earth impactors to conduct the most detailed population-level study of their structure and material strength to date. We identify three structurally distinct groups within the decameter impactors. The first group are primarily structurally homogeneous, weak objects which catastrophically disrupt below $\sim1.5$ MPa. The second group are heterogeneous objects which progressively fragment starting from $\sim1$ MPa typically up to $\sim3-8$ MPa. The third group are strong aggregates which remain mostly intact until $9-10$ MPa. Our results also suggest that decameter-size asteroids fragment in two distinct phases: an initial phase at $\sim0.04-0.09$ MPa and a second at $\sim1-4$ MPa. While decimeter- to meter-size objects typically lose most of their mass in the initial phase, larger decameter-size objects instead lose most of their mass in the second phase.

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arXiv2025

A Near-Earth Object Model Calibrated to Earth Impactors

The population of Earth-impacting meteoroids and its size-dependent orbital elements are key to understanding the origin of meteorites and informing on planetary defence efforts. Outstanding questions include the role of collisions in depleting meteoroids on highly evolved orbits and the relative importance of delivery resonances. Those depend on size, with current dynamical models considering only asteroids larger than 10m in diameter. Based on 1,202 sporadic meteoroids observed by the Global Fireball Observatory, we created a debiased model of the near-Earth meteoroid population in the 10g - 150kg in size (approximately 1cm - 0.5m) as they dynamically evolved from the main asteroid belt onto Earth-crossing orbits. The observed impact population is best matched with a collisional half-life decreasing from 3Myr for meteoroids of 0.6kg (7cm) or higher, to 1Myr below this size, extending to the model lower bound of 10g. Placing our results in context with near-Earth object models for larger sizes, we find that the inner main belt continues to dominate feeding the small 1m to 10m diameter population primarily via the $ν_6$ secular resonance and the 3:1J mean motion resonance. We also evaluated the potential significance of physical processes other than collisions on Earth-impacting meteoroids, such as low-perihelion disruptions from thermal stresses.

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arXiv2025

Model Predictions for the 2025 October Draconid Outburst

The October Draconid meteor shower, produced by comet 21P/Giacobini-Zinner, is notorious for rare but intense outbursts, some exceeding rates of about 10 000 meteors per hour. In 2025, Earth will encounter young trails ejected by the comet in 2005 and 2012, producing a meteor outburst and providing a rare opportunity to probe their structure and benchmark meteoroid stream models. We present predictions from three independent dynamical models (NIMS, MSFC, Sisyphus), calibrated against updated activity profiles including the newly observed 2019 and 2024 outbursts. All simulations predict enhanced activity on 2025 October 8, dominated by faint meteors (m < 0.01 g; +4 mag and fainter) primarily detectable by radar. Our best estimate is a radar outburst near 15:00 - 16:00 UT, driven mainly by the 2012 trail with a possible minor contribution from 2005. The 2025 Draconids may represent one of the strongest radar dominated outbursts of the decade. Coordinated observing campaigns, especially radar measurements across the Northern Hemisphere and optical coverage from Asia, will be essential to validate these forecasts, constrain the dust environment of comet 21P, and improve future predictions of young meteoroid trails.

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arXiv2025

A Statistical Approach to Quantifying Uncertainty in Meteoroid Physical Properties

Meteoroid bulk density is a critical value required for assessing impact risks to spacecraft, informing shielding and mission design. Direct bulk density measurements for sub-millimeter to millimeter-sized meteoroids are difficult, often relying on forward modeling without robust uncertainty estimates. Methods based solely on select observables can overlook noise-induced biases and non-linear relations between physical parameters. This study aims to automate the inversion of meteoroid physical parameters from optical meteor data, focusing on bulk density and its associated uncertainties. We compare an observables-based selection method (PCA) with an RMSD-based approach used to select among millions of ablation model runs using full light and deceleration curves as constraints. After validating both approaches on six synthetic test cases, we apply them to two Perseid meteors recorded by high sensitivity Electron-Multiplied CCD (EMCCD) cameras and high precision mirror-tracked meteors detected by the Canadian Automated Meteor Observatory (CAMO). Our results show that relying only on observables, as in the PCA approach can converge to wrong solutions and can yield unphysical solutions. In contrast, the RMSD-based method offers more reliable density constraints, particularly for bright and strongly decelerating meteor. Small relative measurement precision in brightness and lag relative to the full range of observed lag and luminosity is the key to tight solution. We provide the first objectively derived uncertainty bounds for the physical properties of meteoroids. Our approach solves the solution degeneracy problem inherent in forward modelling of meteors. This strategy can be generalized to other showers, paving the way for improved meteoroid models and enhanced spacecraft safety.

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arXiv2025

Decameter-sized Earth impactors -- I: Orbital properties

Numerous decameter-sized asteroids have been observed impacting Earth as fireballs. These objects can have impact energies equivalent to hundreds of kilotons of TNT, posing a hazard if they impact populated areas. Previous estimates of meteoroid flux using fireball observations have shown an Earth impact rate for decameter-size objects of about once every $2$-$3$ years. In contrast, telescopic estimates of the near-Earth asteroid population predict the impact rate of such objects to be of order $20$-$40$ years, an order-of-magnitude difference. While the cause of this discrepancy remains unclear, tidal disruption of a larger near-Earth body has been proposed as an explanation for these excess decameter-sized impactors. The release in 2022 of previously classified United States Government (USG) satellite sensor data for fireball events has provided a wealth of new information on many of these impacts. Using this newly available USG sensor data, we present the first population-level study characterizing the orbital and dynamical properties of 14 decameter-sized Earth impactors detected by USG sensors since 1994, with a particular focus on searching for evidence of tidal disruption as the cause of the impact rate discrepancy. We find there is no evidence for recent ($\lesssim 10^4$ years) tidal disruption and weak evidence for longer-term tidal disruption in the decameter impactor population, but that the latter conclusion is limited by small number statistics. We also investigate the origins of both the impactor and near-Earth asteroid populations of decameter-sized objects from the main asteroid belt. We find that both populations generally originate from the same source regions: primarily from the $ν_6$ secular resonance ($\sim70$%) with small contributions from the Hungaria group ($\sim20$%) and the 3:1 Jupiter mean-motion resonance ($\sim10$%).

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arXiv2024

Telescope-to-Fireball Characterization of Earth Impactor 2022 WJ1

Comparing how an asteroid appears in space to its ablation behavior during atmospheric passage and finally to the properties of associated meteorites represents the ultimate probe of small near-Earth objects. We present observations from the Lowell Discovery Telescope and from multiple meteor camera networks of 2022 WJ1, an Earth impactor which was disrupted over the North American Great Lakes on 19 November 2022. As far as we are aware, this is only the second time an Earth impactor has been specifically observed in multiple passbands prior to impact to characterize its composition. The orbits derived from telescopic observations submitted to the Minor Planet Center (MPC) and ground-based meteor cameras result in impact trajectories that agree to within 40 meters, but no meteorites have been found as of yet. The telescopic observations suggest a silicate-rich surface, and thus a moderate-to-high albedo, which results in an estimated size for the object of just D = 40 - 60 cm. Modeling the fragmentation of 2022 WJ1 during its fireball phase also suggests an approximate half-meter original size for the object as well as an ordinary chondrite-like strength. These two lines of evidence both support that 2022 WJ1 was likely an S-type condritic object and the smallest asteroid compositionally characterized in space. We discuss how best to combine telescopic and meteor camera datasets, how well these techniques agree, and what can be learned from studies of ultra-small asteroids.

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arXiv2024

The threshold at which a meteor shower becomes hazardous to spacecraft

Although the risk posed to spacecraft due to meteoroid impacts is dominated by sporadic meteoroids, meteor showers can raise this risk for short periods of time. NASA's Meteoroid Environment Office issues meteor shower forecasts that describe these periods of elevated risk, primarily for the purpose of helping plan extravehicular activities. These forecasts are constructed using a list of meteor shower parameters that has evolved over time to include newly discovered showers and incorporate improved measurements of their characteristics. However, at this point in time, more than a thousand meteor showers have been reported by researchers, many of which are extremely minor, are unconfirmed, or lack measurements of critical parameters. Thus, a comprehensive approach is no longer feasible. In this report we present a quantitative criterion for a potentially hazardous meteor shower and apply this criterion to the list of established meteor showers in order to determine which showers should be included in our annual forecasts.

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arXiv2024

A reference meteor magnitude for intercomparable fluxes

The rate at which meteors pass through Earth's atmosphere has been measured or estimated many times over; existing flux measurements span at least 12 astronomical magnitudes, or roughly five decades in mass. Unfortunately, the common practice of scaling flux to a universal reference magnitude of +6.5 tends to collapse the magnitude or mass dimension. Furthermore, results from different observation networks can appear discrepant due solely to the use of different assumed population indices, and readers cannot resolve this discrepancy without access to magnitude data. We present an alternate choice of reference magnitude that is representative of the observed meteors and minimizes the dependence of flux on population index. We apply this choice to measurements of recent Orionid meteor shower fluxes to illustrate its usefulness for synthesizing independent flux measurements.

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arXiv2024

A Physical Survey of Meteoroid Streams: Comparing Cometary Reservoirs

In this work, we present an optical survey of mm-sized meteoroids using the Canadian Automated Meteor Observatory's (CAMO) mirror tracking system. The system tracks meteors to magnitude +7.5 through an image-intensified telescopic system which has a spatial accuracy of $\sim$1 m and a temporal resolution of 10 ms. We analyze 41 meteors from 13 showers with known parent bodies, recorded between 2016 and 2022. We fit a numerical ablation and fragmentation model to our data which models meteoroid fragmentation as erosion into 10 - 500 $μ$m constituent grains and uses the observed wake as a hard constraint on the model parameters. We measure average bulk meteoroid densities which are consistent with in situ measurements: 602 $\pm$ 155 kg m$^{-3}$ for Jupiter-family and 345 $\pm$ 48 kg m$^{-3}$ for Halley-type showers. The Geminids had the highest measured bulk density of 1387 $\pm$ 240 kg m$^{-3}$, consistent with carbonaceous material. We fail to reproduce the high bulk density ($>3000$ kg m$^{-3}$) for Jupiter-family meteoroids previously reported in the literature derived using fragmentation models on data sets with fewer observational constraints. We also provide estimates of the meteoroid grain sizes, grain mass distributions, and energy necessary to trigger the erosion for meteoroids in the analyzed showers.

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arXiv2023

First holistic modelling of meteoroid ablation and fragmentation: A case study of the Orionids recorded by the Canadian Automated Meteor Observatory

18 mm-sized Orionid meteoroids were captured in 2019 and 2020 by the Canadian Automated Observatory's mirror tracking system. Meteor position measurements were made to an accuracy of $\sim1$ m and the meteors were tracked to a limiting magnitude of about $+7.5$ at the faintest point. The trajectory estimation shows the intrinsic physical dispersion of the Orionid radiant is $0.400^{\circ} \pm 0.062^{\circ}$. An erosion-based entry model was fit to the observations to reproduce ablation and fragmentation for each meteor, simultaneously reproducing the light curve, the dynamics, and the wake. Wake observations were found to directly inform the grain mass distribution released in the modelled erosion. A new luminous efficiency model was derived from simultaneous radar and optical observations and applied in the modelling to improve its accuracy. The results show that the apparent strength of Orionids varies with radiant location and time of appearance during the period of shower activity. The average differential grain mass distribution index was 2.15, higher than found from in-situ estimates, possibly due to the evolution of the physical properties of meteoroids since ejection. All Orionids showed leading fragment morphology which was best explained by stopping the erosion at the peak of the light curve, leaving a non-fragmenting meteoroid with $\sim10\%$ of the original mass. The inverted Orionid meteoroid average bulk density of $\sim300$ kg m$^{-3}$, corresponding to porosities of $\sim90\%$, is consistent with in-situ measurements of larger dust particles by Vega-2 at 1P/Halley and Rosetta at 67P.

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arXiv2023

On the Proposed Interstellar Origin of the USG 20140108 Fireball

A critical review of the evidence for the interstellar origin for the USG 20140108 fireball is presented. Examining USG fireball velocities where independent data are available shows the former to have significant (10-15 km/s) uncertainties at large speeds and highly variable radiant accuracy, with average errors in excess of ten degrees. Ablation model fits to the observed lightcurve are possible for normal chondritic impactors only assuming low speeds. To match the high speed and low fragmentation height of the USG 20140108 fireball would require a high density/strength object with low drag and highly aerodynamic shape not made of iron. We suggest the simpliest explanation for the unusual characteristics of USG 20140108 is that the speed, in particular, is substantially overestimated.

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arXiv2023

Preatmospheric detection of a meter-sized Earth impactor

On 2020 September 18 US Government sensors detected a bolide with peak bolometric magnitude of -19 over the western Pacific. The impact was also detected by the Geostationary Lightning Mapper (GLM) instrument on the GOES-17 satellite and infrasound sensors in Hawaii. The USG measurements reported a steep entry angle of $67^{\circ}$ from horizontal from a radiant $13^{\circ}$ E of N and an impact speed of 11.7 km s$^{-1}$. Interpretation of all energy yields produces a preferred energy estimate of 0.4 kt TNT, corresponding to a $23000$ kilogram $3$ meter diameter meteoroid. A post-impact search of telescopic images found that the ATLAS survey captured the object just 10 minutes prior to impact at an Earth-centred distance of nearly $11900$ kilometers with apparent magnitude $m\text{=}12.5$. The object appears as a $0.44^{\circ}$ streak originating on the eastern edge of the image extending one-third of the USG state vector-based prediction of $1.26^{\circ}$ over the 30 second exposure. The streak shows brightness variability consistent with small asteroid rotation. The position of Earth's shadow, the object's size, and its consistency with the reported USG state vector confirm the object is likely natural. This is the eighth preatmospheric detection of a Near-Earth Asteroid (NEA) impactor and the closest initial telescopic detection prior to impact. The high altitude of peak fireball brightness suggest it was a weak object comparable in many respects with 2008 TC3 (Almahata Sitta meteorite), with absolute magnitude $H=32.5$ and likely low albedo. Therefore we suggest the NEA was a C-complex asteroid.

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arXiv2023

Modelling the 2022 τ-Herculid outburst

The $τ$-Herculids (IAU shower number #61 TAH) is a minor meteor shower associated with comet 73P/Schwassmann-Wachmann 3, a Jupiter-Family comet that disintegrated into several fragments in 1995. As a consequence of the nucleus break-up, possible increased meteor rates were predicted for 2022. On May 30-31, observation networks around the world reported two distinct peaks of TAH activity, around solar longitudes 69.02° and 69.42°. This work examines the encounter conditions of the Earth with meteoroids ejected from 73P during the splitting event and on previous perihelion passages. Numerical simulations suggest that the main peak observed in 2022 was caused by meteoroids ejected from the splitting nucleus with four times the typical cometary gas expansion speed. High-resolution measurements performed with the Canadian Automated Meteor Observatory indicate that these meteoroids are fragile, with estimated bulk densities of 250 kg/m$^3$. In contrast with the main peak, the first TAH activity peak in 2022 is best modelled with trails ejected prior to 1960. We find that ordinary cometary activity could have produced other TAH apparitions observed in the past, including in 1930 and 2017. The extension of our model to future years predicts significant returns of the shower in 2033 and 2049.

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arXiv2022

Direct measurement of decimeter-sized rocky material in the Oort cloud

The Oort cloud is thought to be a reservoir of icy planetesimals and the source of long-period comets (LPCs) implanted from the outer Solar System during the time of giant planet formation. The abundance of rocky ice-free bodies is a key diagnostic of Solar System formation models as it can distinguish between ``massive" and ``depleted" proto-asteroid belt scenarios and thus disentangle competing planet formation models. Here we report a direct observation of a decimeter-sized ($\sim2$ kg) rocky meteoroid on a retrograde LPC orbit ($e \approx 1.0$, i = $121^{\circ}$). During its flight, it fragmented at dynamic pressures similar to fireballs dropping ordinary chondrite meteorites. A numerical ablation model fit produces bulk density and ablation properties also consistent with asteroidal meteoroids. We estimate the flux of rocky objects impacting Earth from the Oort cloud to be $1.08^{+2.81}_{-0.95} \mathrm{meteoroids/10^6 km^2/yr}$ to a mass limit of 10 g. This corresponds to an abundance of rocky meteoroids of $\sim6^{+13}_{-5}$\% of all objects originating in the Oort cloud and impacting Earth to these masses. Our result gives support to migration-based dynamical models of the formation of the Solar System which predict that significant rocky material is implanted in the Oort cloud, a result not explained by traditional Solar System formation models.

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