Asteroid Size Research Articles

Dynamical evolution of the inner asteroid belt

ABSTRACT A determination of the dynamical evolution of the asteroid belt is difficult because the asteroid belt has evolved since the time of asteroid formation through mechanisms that include: (1) catastrophic collisions, (2) rotational disruption, (3) chaotic orbital evolution, and (4) orbital evolution driven by Yarkovsky radiation forces. The time-scales of these loss mechanisms are uncertain and there is a need for more observational constraints. In the inner main belt (IMB), the mean size of the non-family asteroids increases with increasing inclination. Here, we use that observation to show that all IMB asteroids originate from either the known families or from ghost families, that is, old families with dispersed orbital elements. We estimate that the average age of the asteroids in the ghost families is a factor of 1/3 less than the Yarkovsky orbital evolution time-scale. However, this orbital evolution time-scale is a long-term average that must allow for the collisional evolution of the asteroids and for stochastic changes in their spin directions. By applying these constraints on the orbital evolution time-scales to the evolution of the size-frequency distribution of the Vesta asteroid family, we estimate that the age of this family is greater than 1.3 Gyr and could be comparable with the age of the Solar system. By estimating the number of ghost families, we calculate that the number of asteroids that are the root sources of the meteorites and the near-Earth asteroids that originate from the IMB is about 20.

Impact of neutron energy on asteroid deflection performance

In the future, a hazardous asteroid will find itself on a collision course with Earth. For asteroids of moderate size or larger, a nuclear device is one of humanity’s only technologies capable of mitigating this threat via deflection on a timescale of less than a decade. This work examined how the output neutron energy from a nuclear device standoff detonation affects the deflection of a notional asteroid that is 300 meters in diameter and composed of silicon dioxide at a bulk density of 1.855 g/cm3. 14.1 MeV and 1 MeV neutron energy sources were modeled in MCNP to quantify the energy deposition in the asteroid target. The asteroid’s irradiated region was discretized in angle by tracing the rays emanating from the point of detonation and in depth by considering the neutron mean-free-paths. This high-fidelity approach was shown to deviate from previous analytic approximations commonly used for asteroid energy deposition. 50 kt and 1 Mt neutron yields of the energy deposition mappings were imported into a hydrodynamic asteroid model in ALE3D to simulate the deflective response due to blow-off ejecta. Underexplored in literature, changing the neutron energy was found to have up to a 70% impact on deflection performance due to induced differences in the energy deposition profile and in the energy coupling efficiency. The magnitude of energy deposition accounted for most of the observed variation in the asteroid velocity change, making the coupling efficiency more significant than the spatial profile characteristics. These findings are vital for determining the optimal source neutron energy spectrum for asteroid deflection applications.

The Formation of Bilobate Comet Shapes through Sublimative Torques.

Recent spacecraft and radar observations have found that ~70 percent of short-period comet nuclei, mostly Jupiter-family comets (JFCs), have bilobate shapes (two masses connected by a narrow neck). This is in stark contrast to the shapes of asteroids of similar sizes, of which ~14% are bilobate. This suggests that a process or mechanism unique to comets is producing these shapes. Here we show that the bilobate shapes of JFC nuclei are a natural byproduct of sublimative activity during their dynamical migration from their trans-Neptunian reservoir, through the Centaur population, and into the Jupiter family. We model the torques resulting from volatile sublimation during this dynamical migration and find that they tend to spin up these nuclei to disruption. Once disrupted, the rubble pile-like material properties of comet nuclei (tensile strengths of ~1-10 Pa and internal friction angles of ~35°) cause them to reform as bilobate objects. We find that JFCs likely experienced rotational disruption events prior to entering the Jupiter family, which could explain the prevalence of bilobate shapes. These results suggest that the bilobate shapes of observed comets developed recently in their history (within the past ~1-10 Myr), rather than during solar system formation or collisions during planet migration and residency in the trans-Neptunian population.

Predicting the accuracy of asteroid size estimation with data from the Rubin Observatory Legacy Survey of Space and Time

Recent work has shown that the correlation between SDSS colors and optical albedo can be used to estimate asteroid sizes from optical data alone. We revisit a correlation between SDSS colors and optical albedo for asteroids, with the albedo derived using WISE-based size estimates. Moeyens, Myhrvold & Ivezić (2020) showed that this correlation can be used to estimate asteroid sizes with optical data alone, with a precision of about 17% relative to WISE-based size estimates. We present here several more sophisticated data-driven models for the variation of optical albedo with colors and estimate the contribution of SDSS photometric errors to the albedo and size estimate uncertainties. We use the results of our analysis to predict that LSST data will enable asteroid size precision of about 15% relative to WISE-based size estimates. Compared to the accuracy of WISE-based size estimates of 15%–20%, the implied accuracy of optical size estimates, in the range 21%–25%, is thus only a factor of 1.3 to 1.4 worse. This size estimation accuracy is significantly better than commonly assumed for optical data and is due to accurate and homogeneous multi-band photometry delivered by modern digital sky surveys.

Observational Completion Limit of Minor Planets from the Asteroid Belt to Jupiter Trojans

Abstract With the growing numbers of asteroids being discovered, identifying an observationally complete sample is essential for statistical analyses and for informing theoretical models of the dynamical evolution of the solar system. We present an easily implemented method of estimating the empirical observational completeness in absolute magnitude, , as a function of semimajor axis. Our method requires fewer assumptions and decisions to be made in its application, making results more transportable and reproducible among studies that implement it, as well as scalable to much larger data sets of asteroids expected in the next decade with the Vera C. Rubin Observatory’s Legacy Survey of Space and Time. Using the values of (a) determined at high resolution in semimajor axis, a, we demonstrate that the observationally complete sample size of the main belt asteroids is larger by more than a factor of 2 compared with using a conservative single value of , an approach often adopted in previous studies. Additionally, by fitting a simple, physically motivated model of (a) to ∼7 × 105 objects in the Minor Planet Database, our model reveals statistically significant deviations between the main belt and the asteroid populations beyond the main belt (Hungarias, Hildas, and Trojans), suggesting potential demographic differences, such as in their size, eccentricity, or inclination distributions.

Spin evolution of Ceres and Vesta due to impacts

AbstractAll asteroid spins evolve due to collisions. Geophysical analysis (Mao and McKinnon 2018b) implies Ceres might have been modestly despun (by ~6.5%) by impacts. Vesta’s postaccretion rotation rate, before the formation of the Veneneia and Rheasilvia basins, has also been proposed to have been higher (by ~6%) than today (Fu et al. 2014). We have designed Monte Carlo simulations to investigate Ceres’ and Vesta’s plausible spin evolution by impacts. We use the main belt asteroid size–frequency distribution, Ceres’ and Vesta’s cratering records, and their present‐day impact velocity probability distributions to quantify possible spin histories. We consider dynamical effects from escaping ejecta, adopt mass‐velocity scaling laws for two endmember surface materials (porous and nonporous), and track potential catastrophic disruptions. Results show that Ceres’ spin period likely changed by a fraction of an hour (up or down) compared with its “initial,” accretional spin, mostly due to large impacts. Different surface materials do not yield statistically distinguishable final spin distributions, but imply opposite crustal evolutions where Ceres loses/accretes mass, depending on surface porosity; as much as several 100 m worth of exogenous material, globally averaged, can be accreted if the surface remains porous. Vesta’s final Monte Carlo spin distribution is more concentrated around its “initial” value, possibly implying a more relict spin state than Ceres. However, explicitly modeling the angular momentum changes wrought by Vesta’s two existing planetary‐scale impact basins shows that their formation alone can account for Vesta’s proposed spindown, although spinup is a much more likely outcome because of the scale and location of the impacts.

A survey for occultation astrometry of main belt: expected astrometric performances

Context.Occultations of stars by asteroids are an efficient method to study the properties of minor bodies, and can be exploited as tools to derive very precise asteroid astrometry relative to the target star. With the availability of stellar astrometry thanks to the ESA missionGaia, the frequency of good predictions and the quality of the astrometry have been strongly enhanced.Aims.Our goal is to evaluate the astrometric performance of a systematic exploitation of stellar occultations, with a homogeneous data set and a given instrument setup. As a reference instrument, we adopt the example of a robotic 50 cm telescope, which is under construction at the Observatoire de la Côte d’Azur. We focus in particular on single-chord occultations.Methods.We created a data set of simulated light curves, that are modelled by a Bayesian approach. To build the final statistics, we considered a list of predicted events over a long time span, and stellar astrometry fromGaiadata release 2.Results.We derive an acceptable range of observability of the events, with clear indications of the expected errors in terms of timing uncertainties. By converting the distribution of such errors to astrometric uncertainties, we show that the precision on a single chord can reach levels equivalent to the performance ofGaia(sub-milli-arcseconds). The errors on the asteroid position are dominated by the uncertainty on the position of the occultation chord with respect to the barycentre of the object.Conclusions.The limiting factor in the use of occultation astrometry is not the light curve uncertainty, but our knowledge of the asteroid's shape and size. This conclusion is valid in a wide range of flux drops and magnitudes of the occulted star. The currently increasing knowledge of the shape, spin properties, and size, must be used to mitigate this source of error.

TOPOGRAPHIC MAPPING AND ANALYSIS BASED ON 3D RECONSTRUCTION MODEL OF SIMULATED ASTEROID

Abstract. Asteroid exploration is of great scientific significance in understanding the evolution of the solar system. Unlike lunar missions, asteroids are far away from the earth, the communication between the spacecraft and the ground station has a large delay. Moreover, the small size, irregular shape and uneven mass distribution of asteroids further increase the difficulty of landing missions. This paper presented a topographic mapping and analysis method based on three-dimensional (3D) reconstruction model of a simulated asteroid. First, 3D reconstruction of a simulated asteroid was achieved using Structure from Motion (SfM) technique. Then Digital Elevation Model (DEM) and Digital Orthophoto Map (DOM) were produced based on the 3D point cloud data. By analysing the DEM data, we finally obtained the obstacles distribution map. In order to verify the proposed method, we used a simulated asteroid to conduct experiments. And the experimental results demonstrated the effectiveness of the developed method. The topographic mapping and analysis method presented in this study can be applied to landing/sampling area selection in further asteroid exploration research.

Interpreting the Cratering Histories of Bennu, Ryugu, and Other Spacecraft-explored Asteroids

Abstract Asteroid crater retention ages have unknown accuracy because projectile–crater scaling laws are difficult to verify. At the same time, our knowledge of asteroid and crater size–frequency distributions has increased substantially over the past few decades. These advances make it possible to empirically derive asteroid crater scaling laws by fitting model asteroid size distributions to crater size distributions from asteroids observed by spacecraft. For D > 10 km diameter asteroids like Ceres, Vesta, Lutetia, Mathilde, Ida, Eros, and Gaspra, the best matches occur when the ratio of crater to projectile sizes is f ∼ 10. The same scaling law applied to 0.3 < D < 2.5 km near-Earth asteroids such as Bennu, Ryugu, Itokawa, and Toutatis yield intriguing yet perplexing results. When applied to the largest craters on these asteroids, we obtain crater retention ages of ∼1 billion years for Bennu, Ryugu, and Itokawa and ∼2.5 billion years for Toutatis. These ages agree with the estimated formation ages of their source families and could suggest that the near-Earth asteroid population is dominated by bodies that avoided disruption during their traverse across the main asteroid belt. An alternative interpretation is that f ≫ 10, which would make their crater retention ages much younger. If true, crater scaling laws need to change in a substantial way between D > 10 km asteroids, where f ∼ 10, and 0.3 < D < 2.5 km asteroids, where f ≫ 10.

Simultaneous estimation of spacecraft position and asteroid diameter during final approach of Hayabusa2 to Ryugu

This paper presents the optical navigation results of the asteroid explorer Hayabusa2 during the final rendezvous approach phase with the asteroid Ryugu. The orbit determination of Hayabusa2 during the cruising phase uses a triangulation-based method that estimates the probe and asteroid orbits using the directions from which they are observed. Conversely, the asteroid size is available as optical information just prior to arrival. The size information allows us to estimate the relative distance between the probe and the asteroid with high accuracy, that is strongly related to the success or failure of the rendezvous. In this study, the relative distance and asteroid size in real space are simultaneously estimated in real time by focusing on the rate of change of the asteroid size observed in sequential images. The real-time estimation results coincided with those of precise analyses performed after arrival.

Asteroid migration due to the Yarkovsky effect and the distribution of the Eos family

ABSTRACT Based on a linearized model of the Yarkovsky effect, we investigate in this paper the dependence of the semimajor axis drift Δa of a celestial body on its size, spinning obliquity, initial orbit, and thermal parameters on its surface. With appropriate simplification and approximation, we obtain the analytical solutions to the perturbation equations for the motion of asteroids influenced by the Yarkovsky effect, and they are then verified by numerical simulations of the full equations of motion. These solutions present explicitly the dependences of Δa on the thermal and dynamical parameters of the asteroid. With these analytical formulae for Δa, we investigate the combined seasonal and diurnal Yarkovsky effects. The critical points where the migration direction reverses are calculated and the consequent selective effects according to the size and rotation state of asteroids are discussed. Finally, we apply the analytical formulae to calculate the migration of Eos family members. The space distribution of asteroids is well reproduced. Our calculations suggest that statistically the orientations of spin axes of family members satisfy a random-obliquity distribution, and the rotation rate ωrot of an asteroid depends on its size R by ωrot ∝ R−1.

An extended parameter space study of the effect of cohesion in gravitational aggregates through spin-up simulations

Asteroids are subject to the YORP effect, a slow change in spin caused by reflection and reemission of solar radiation. There is an upper limit to how fast an asteroid can spin before some form of failure takes place, depending on the strength of the asteroid. The maximum spin rate for a particle to remain at the equator of a rigid body depends only on the overall shape of the asteroid and its bulk density, not on its size. However, observations show that many smaller asteroids are spinning faster than the theoretical limit for a particle to remain on the equator, while most larger asteroids are not. Using the PKDGRAV simulation package, we conducted simulations modeling the spin-up of simulated rubble-pile asteroids, over a broad parameter space. First the effect of cohesion was tested, and we found that the stronger the cohesive force, the faster a simulated asteroid can spin before failing, confirming previous analytical and numerical work, but for a wider range of body shapes that included oblate and prolate bodies. Next, the effect of overall size of asteroids was tested, at low and high resolution (fewer and more particles, respectively). Results show that while cohesion has an effect on simulated asteroids of all sizes, it is less important for larger bodies, for which gravity dominates, also as noted in previous studies, but also confirmed in our study for objects with a size distribution in particle size. Spin-up outcomes for our tested set included reshaping and fission, as well as satellite formation from gravitational reaccumulation of material shed from the progenitor equator.

ATM: An open-source tool for asteroid thermal modeling and its application to NEOWISE data

We publicly release ATM, a Python package designed to model asteroid flux measurements to estimate an asteroid's size, surface temperature distribution, and emissivity. A number of the most popular static asteroid thermal models (NEATM, STM, FRM) are implemented with the reflected solar light contribution and Kirchhoff's law accounted for. Priors for fitted parameters can be easily specified and the solution, including the full multi-dimensional posterior probability density function, is found using Markov Chain Monte Carlo (MCMC). We describe the package's architecture and discuss model and fitting validation. Data files with ~10 million WISE flux measurements for ~150,000 unique asteroids and additional Minor Planet Center data are also included with the package, as well as Python Jupyter Notebooks with examples of how to select subsamples of objects, filter and process data, and use ATM to fit for the desired model parameters. The entirety of the analysis presented here, including all the figures, tables, and catalogs, can be easily reproduced with these publicly released Notebooks. We show that ATM can match the best-fit size estimates for well-observed asteroids published in 2016 by the NEOWISE team (Mainzer et al., 2016) with a sub-percent bias and a scatter of only 6%. Our analysis of various sources of random and systematic size uncertainties shows that for the majority of over 100,000 objects with WISE-based size estimates random uncertainties (precision) are about 10%; systematic uncertainties within the adopted model framework, such as NEATM, and with assumed emissivity for WISE W3 and W4 bands, are likely in the range of 10–20%. Hence, the accuracy of WISE-based asteroid size estimates is approximately in the range of 15–20% for most objects, except for unknown errors due to a possibly over-simplified modeling framework (e.g., spherical asteroid approximation). We compare model families to data in WISE color-color diagrams and derive a simple method to estimate size that only uses WISE W3 band flux; we show that it matches estimates based on all four WISE bands to within 10%. We also study optical data collected by the Sloan Digital Sky Survey (SDSS) and show that correlations of optical colors and WISE-based best-fit model parameters indicate robustness of the latter. Our analysis gives support to the claim by Harris & Drube (2014) that candidate metallic asteroids can be selected using the best-fit temperature parameter and infrared albedo. We investigate a correlation between SDSS colors and optical albedo derived using WISE-based size estimates and show that this correlation can be used to estimate asteroid sizes with optical data alone, with a precision of about 17% relative to WISE-based size estimates. After accounting for systematic errors, the difference in accuracy between infrared and optical color-based size estimates becomes less than a factor of two.

Monitoring Jovian Orbital Resonances of a Spacecraft: Classical and Relativistic Effects

Orbital resonances continue to be one of the most difficult problems in celestial mechanics. They have been studied in connection with the so-called Kirkwood gaps in the asteroid belt for many years. On the other hand, resonant trans-Neptunian objects are also an active area of research in Solar System dynamics, as are the recently discovered resonances in extrasolar planetary systems. A careful monitoring of the trajectories of these objects is hindered by the small size of asteroids or the large distances of the trans-Neptunian bodies. In this paper, we propose a mission concept, called CHRONOS (after the greek god of time), in which a spacecraft could be sent to with the initial condition of resonance with Jupiter in order to study the future evolution of its trajectory. We show that radio monitoring of these trajectories could allow for a better understanding of the initial stages of the evolution of resonant trajectories and the associated relativistic effects.

The shape and structure of small asteroids as a result of sub-catastrophic collisions

The overall shape, internal structure and surface morphology of small bodies such as asteroids and comets are determined to a large degree by the last global-scale impact or disruption event. Depending on the specific energy, impacts lead to a large spectrum of outcomes. Sub-catastrophic disruptions take place in an energy range between cratering impacts and catastrophic disruptions. Although less energetic than catastrophic events, they can still significantly alter the overall shape and structure of the target body. This has been demonstrated recently in the case of bi-lobe cometary nuclei (Jutzi and Benz, 2017). Here we present results of a subsequent study on the shapes of asteroids resulting from such collisions. Sizes ranging from a few hundred meters to a few kilometers are considered.We show that impacts on elongated rotating asteroids frequently lead to the formation of contact binaries. Our results confirm that this mechanism is robust and works for a large range of asteroid sizes and impact velocities. Scaling-laws for the prediction of the size and velocity dependent specific energies required for successful bi-lobe formation are presented. Based on these scaling laws, the expected frequency of such sub-catastrophic impacts is calculated and is compared to the one of catastrophic disruptions, which require much higher specific energies and are more rare.Our analysis suggest that the shapes and structures of a large fraction of small asteroids as observed today may be the result of the last major sub-catastrophic impact.

Igneous Rock Associations 24. Near-Earth Asteroid Resources: A Review

The extraction of natural resources located beyond Earth to create products can be described as space resource utilization (SRU). SRU is under active investigation in both the public and private sectors. Near-Earth asteroids (NEAs) are particularly promising early SRU targets due to their relative proximity and enrichments in two key resources: water and platinum group elements (PGEs). Water can be used to create rocket propellant, making it the only resource with significant demand given the current nascent state of the space market. Platinum group elements are valuable enough that their import to the Earth market is potentially economical, making them the other prospective resource in the current embryonic state of SRU. While it is possible to retrieve material from a NEA, doing so on an economical scale will require significant developments in areas such as autonomous robotics and propulsion technology. A parameterization accounting for asteroid size, resource concentration, and accessibility yields just seven and three potentially viable NEA targets in the known population for water and PGEs, respectively. A greater emphasis on spectral observation of asteroids is required to better inform target selection for early prospecting spacecraft. A further complication is the lack of a legal precedent for the sale of extraterrestrial resources. The Outer Space Treaty prohibits the appropriation of celestial bodies but makes no explicit reference to their resources while the U.S.A. and Luxembourg have passed legislation entitling their citizens to own and sell space resources. Whether these laws are a matter of clarification or contradiction is the matter of some debate.
 RÉSUMÉL'extraction de ressources naturelles situées au-delà de la Terre pour créer des produits peut être décrite comme une utilisation des ressources spatiales (URS). L’URS est actuellement examinée à la fois dans les secteurs public et privé. Les astéroïdes proches de la Terre (NEA) sont des cibles URS particulièrement prometteuses en raison de leur proximité relative et de leur enrichissement en deux ressources clés : l’eau et les éléments du groupe du platine (EGP). L'eau peut être utilisée pour créer des agents de propulsion pour vaisseaux spatiaux, ce qui en fait la seule ressource pour laquelle la demande est importante compte tenu de l’émergence du marché spatial actuel. Les EGP sont suffisamment précieux pour que leur importation sur le marché terrestre soit potentiellement économique, ce qui en fait l’autre ressource potentielle étant donné l’état embryonnaire actuel de l’URS. Bien qu'il soit possible de récupérer des matériaux sur un NEA, le faire à une échelle économique nécessitera des développements importants dans des domaines tels que la robotique autonome et la technologie de propulsion. Un paramétrage tenant compte de la taille des astéroïdes, de la concentration des ressources et de l'accessibilité conduit à seulement sept et trois cibles NEA parmi la population connue, potentiellement exploitables pour l'eau et les EGP, respectivement. Il est nécessaire de mettre davantage l'accent sur l'observation spectrale des astéroïdes afin de mieux documenter la sélection des cibles pour les premiers vaisseaux prospecteurs. L'absence de précédent juridique pour la vente de ressources extraterrestres est une complication supplémentaire. Le Traité sur l’espace interdit l’appropriation des corps célestes mais ne fait aucune référence explicite à leurs ressources, tandis que les États-Unis et le Luxembourg ont adopté une législation autorisant leurs citoyens à posséder et à vendre des ressources spatiales. Que ces lois fassent l’objet de clarification ou de contradiction est sujet à débat.

Reprint of “Effects of asteroid property distributions on expected impact rates”

Assessments of the risk posed by asteroid impacts depend implicitly upon estimates of how frequently asteroids of different sizes and properties are expected to strike Earth. These impact rates are generally based upon populations of near-Earth objects as a function of absolute magnitude, which are then translated to equivalent size frequencies by assuming an average albedo. However, this simplified approach neglects the broad distribution of sizes and properties among objects of a given absolute magnitude. In this study, we use a probabilistic asteroid impact risk model to more rigorously investigate the effects of asteroid property distributions on estimated impact rates as a function of diameter, energy, mass, and blast damage radius. We also use impact damage modeling results to compare the effects of the new distribution-based results on expected casualty rates for two sample risk assessment cases. The current study focuses on mid-sized objects with diameters of 20–300m, which could cause substantial local ground damage but are unlikely to produce larger-scale regional or global effects. Particular emphasis is placed on sizes similar to the 1908 Tunguska event, which provides a unique source of evidence for such impacts and is often used as a reference point for damage potential and impact rates in the literature. Results demonstrate that accounting for realistic distributions of asteroid properties can significantly reduce expected impact intervals for these size ranges compared to estimates based on mean albedo, density, and velocity assumptions. These lower impact intervals, in turn, result in higher expected casualty estimates in impact risk assessment applications.

Inference of meteoroid characteristics using a genetic algorithm

A methodology is introduced to optimize and extend the inference of pre-entry size, density, strength, and mass of asteroids based on observed light curves. In this development study, a genetic algorithm (GA) approach is coupled with the fragment-cloud model (FCM) to efficiently evaluate entry and breakup for numerous potential asteroid property combinations and determine which case best matches the observed data. FCM produces energy deposition curves based on assumed pre-entry conditions, and the GA finds values for these inputs that minimize an objective function characterizing the difference between the FCM curve and a target curve. We present an overview of the GA approach, and then demonstrate its capability to infer pre-entry properties for three well-characterized events: Chelyabinsk, Lost City, and Benešov. In all cases, our initial mass and size estimates were within the range of published values.

Ancient and primordial collisional families as the main sources of X-type asteroids of the inner main belt

Aims. The near-Earth asteroid population suggests the existence of an inner main belt source of asteroids that belongs to the spectroscopic X complex and has moderate albedos. The identification of such a source has been lacking so far. We argue that the most probable source is one or more collisional asteroid families that have escaped discovery up to now. Methods. We apply a novel method to search for asteroid families in the inner main-belt population of asteroids belonging to the X complex with moderate albedo. Instead of searching for asteroid clusters in orbital element space, which could be severely dispersed when older than some billions of years, our method looks for correlations between the orbital semimajor axis and the inverse size of asteroids. This correlation is the signature of members of collisional families that have drifted from a common centre under the effect of the Yarkovsky thermal effect. Results. We identify two previously unknown families in the inner main belt among the moderate-albedo X-complex asteroids. One of them, whose lowest numbered asteroid is (161) Athor, is ~3 Gyr old, whereas the second one, whose lowest numbered object is (689) Zita, could be as old as the solar system. Members of this latter family have orbital eccentricities and inclinations that spread them over the entire inner main belt, which is an indication that this family could be primordial, that is, it formed before the giant planet orbital instability. Conclusions. The vast majority of moderate-albedo X-complex asteroids of the inner main belt are genetically related, as they can be included into a few asteroid families. Only nine X-complex asteroids with moderate albedo of the inner main belt cannot be included in asteroid families. We suggest that these bodies formed by direct accretion of the solids in the protoplanetary disc, and are thus surviving planetesimals.

Effects of asteroid property distributions on expected impact rates

Assessments of the risk posed by asteroid impacts depend implicitly upon estimates of how frequently asteroids of different sizes and properties are expected to strike Earth. These impact rates are generally based upon populations of near-Earth objects as a function of absolute magnitude, which are then translated to equivalent size frequencies by assuming an average albedo. However, this simplified approach neglects the broad distribution of sizes and properties among objects of a given absolute magnitude. In this study, we use a probabilistic asteroid impact risk model to more rigorously investigate the effects of asteroid property distributions on estimated impact rates as a function of diameter, energy, mass, and blast damage radius. We also use impact damage modeling results to compare the effects of the new distribution-based results on expected casualty rates for two sample risk assessment cases. The current study focuses on mid-sized objects with diameters of 20–300 m, which could cause substantial local ground damage but are unlikely to produce larger-scale regional or global effects. Particular emphasis is placed on sizes similar to the 1908 Tunguska event, which provides a unique source of evidence for such impacts and is often used as a reference point for damage potential and impact rates in the literature. Results demonstrate that accounting for realistic distributions of asteroid properties can significantly reduce expected impact intervals for these size ranges compared to estimates based on mean albedo, density, and velocity assumptions. These lower impact intervals, in turn, result in higher expected casualty estimates in impact risk assessment applications.