1,721,046 research outputs found
Mars encounters cause fresh surfaces on some near-Earth asteroids
No full textAll airless bodies are subject to the space environment, and spectral differences between asteroids and meteorites suggest many asteroids become weathered on very short (<1 Myr) timescales. The spectra of some asteroids, particularly Q-types, indicate surfaces that appear young and fresh, implying they have been recently been exposed. Previous work found that Earth encounters were the dominant freshening mechanism and could be responsible for all near-Earth object (NEO) Q-types. In this work we increase the known NEO Q-type sample of by a factor of three. We present the orbital distributions of 64 Q-type near-Earth asteroids, and seek to determine the dominant mechanisms for refreshing their surfaces. Our sample reveals two important results: (i) the relatively steady fraction of Q-types with increasing semi-major axis and (ii) the existence of Q-type near-Earth asteroids with Minimum Orbit Intersection Distances (MOID) that do not have orbit solutions that cross Earth. Both of these are evidence that Earth-crossing is not the only scenario by which NEO Q-types are freshened. The high Earth-MOID asteroids represent 10% of the Q-type population and all are in Amor orbits. While surface refreshing could also be caused by Main Belt collisions or mass shedding from YORP spinup, all high Earth-MOID Q-types have the possibility of encounters with Mars indicating Mars could be responsible for a significant fraction of NEOs with fresh surfaces.National Science Foundation (U.S.) (Grant 6920422)United States. National Aeronautics and Space Administration (Grant 09-NEOO009-0001)National Science Foundation (U.S.) (Grant 0506716
Global Scale Impacts
Full text linkGlobal scale impacts modify the physical or thermal state of a substantial fraction of a target asteroid. Specific effects include accretion, family formation, reshaping, mixing and layering, shock and frictional heating, fragmentation, material compaction, dilatation, stripping of mantle and crust, and seismic degradation. Deciphering the complicated record of global scale impacts, in asteroids and meteorites, will lead us to understand the original planet-forming process and its resultant populations, and their evolution in time as collisions became faster and fewer. We provide a brief overview of these ideas, and an introduction to models
Modeling asteroid collisions and impact processes
No full textAs a complement to experimental and theoretical approaches, numerical modeling has become an important component to study asteroid collisions and impact processes. In the last decade, there have been significant advances in both computational resources and numerical methods. We discuss the present state-of-the-art numerical methods and material models used in "shock physics codes" to simulate impacts and collisions and give some examples of those codes. Finally, recent modeling studies are presented, focussing on the effects of various material properties and target structures on the outcome of a collision
Collisional Formation and Modeling of Asteroid Families
No full textIn the last decade, thanks to the development of sophisticated numerical codes, major breakthroughs have been achieved in our understanding of the formation of asteroid families by catastrophic disruption of large parent bodies. In this review, we describe numerical simulations of asteroid collisions that reproduced the main properties of families, accounting for both the
fragmentation of an asteroid at the time of impact and the subsequent gravitational interactions of the generated fragments. The simulations demonstrate that the catastrophic disruption of bodies larger than a few hundred meters in diameter leads to the formation of large aggregates
due to gravitational reaccumulation of smaller fragments, which helps explain the presence of large members within asteroid families. Thus, for the first time, numerical simulations successfully reproduced the sizes and ejection velocities of members of representative families. Moreover, the simulations provide constraints on the family dynamical histories and on the possible internal structure of family members and their parent bodies
Rotationally resolved spectroscopy of asteroid pairs: No spectral variation suggests fission is followed by settling of dust
No full textThe fission of an asteroid due to fast rotation can expose sub-surface material that was never previously exposed to any space weathering process. We examine the spectral properties of asteroid pairs that were disrupted in the last 2 million years to examine whether the site of the fission can be revealed. We studied the possibility that the sub-surface material, perhaps on one hemisphere, has spectral characteristics differing from the original weathered surface. This was achieved by performing rotationally-resolved spectroscopic observations to look for local variations as the asteroid rotates. We spectrally observed 11 asteroids in pairs in the near-IR and visible wavelength range. Photometric observations were also conducted to derive the asteroid lightcurves and to determine the rotational phases of the spectral observations. We do not detect any rotational spectral variations within the signal-to-noise of our measurements, which allows us to tightly constrain the extent of any existing surface heterogeneity. For each observed spectrum of a longitudinal segment of our measured asteroids, we estimate the maximal size of an un-detected “spot” with a spectral signature different than the average. For five asteroids the maximal diameter of such a “spot” is smaller by a factor of two than the diameter of the secondary member of the asteroid. Therefore, the site of the fission is larger than any area with a unique spectral parameters. This means the site of the fission does not have a unique spectrum. In the case of an ordinary chondrite asteroid (S-complex), where the site of fission is expected to present non-weathered spectra, a lack of a fission “spot” (detectable spectroscopically) can be explained if the rotational-fission process is followed by the spread of dust that re-accumulates on the primary asteroid and covers it homogeneously. This is demonstrated for the young Asteroid 6070 that presents an Sq-type spectrum while its inner material, that is presumably revealed on the surface of its secondary member, 54827, has a non-weathered, Q-type spectrum. The spread of dust observed in the disintegration event of the Asteroid P/2013 R3, might be an example of such a process and an indication that P/2013 R3 was indeed formed in a rotational-fission event.AXA Research Fund (Postdoctoral Fellowship)Carnegie Institution of WashingtonNational Science Foundation (U.S.) Astronomy and Astrophysics Postdoctoral ProgramUnited States. National Aeronautics and Space Administration. Planetary Astronomy Program (Grant NNX12AL26G)Space Telescope Science Institute (U.S.) (Hubble Fellowship grant HST-HF-51319.01-A
Spectral and spin measurement of two small and fast-rotating near-Earth asteroids
No full textIn May 2012 two asteroids made near-miss “grazing” passes at distances of a few Earth-radii: 2012 KP24 passed at 9 Earth-radii and 2012 KT42 at only 3 Earth-radii. The latter passed inside the orbital distance of geosynchronous satellites. From spectral and imaging measurements using NASA’s 3-m Infrared Telescope Facility (IRTF), we deduce taxonomic, rotational, and physical properties. Their spectral characteristics are somewhat atypical among near-Earth asteroids: C-complex for 2012 KP24 and B-type for 2012 KT42, from which we interpret the albedos of both asteroids to be between 0.10 and 0.15 and maximum effective diameters of 20 ± 6 and 6 ± 1 m, respectively. Among B-type asteroids, the spectrum of 2012 KT42 is most similar to 3200 Phaethon and 4015 Wilson–Harrington. Not only are these among the smallest asteroids spectrally measured, we also find they are among the fastest-spinning: 2012 KP24 completes a rotation in 2.5008 ± 0.0006 min and 2012 KT42 rotates in 3.634 ± 0.001 min.United States. National Aeronautics and Space Administration (NASA near-Earth object observation Program, Grant NNX10AG27G
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Compositional Diversity and Space Weathering of Near-Earth Objects
No full textAsteroids, as remnants of the early solar system, hold crucial clues about the processes that led to the formation of planets. These small bodies are composed of materials that have remained relatively unchanged since the solar system's formation, making them valuable targets for scientific study. Near-Earth objects (NEOs), which include both asteroid-like and comet-like bodies whose orbits bring them close to Earth, are particularly important. These objects not onlyoffer insights into planetary formation but also pose potential threats to Earth due to their proximity and frequent interactions with terrestrial planets. Understanding their composition, distribution, and behavior is essential for both scientific research and planetary defense. Data Collection Fig. 1: Orbital distribution of Near-Earth Objects by type, showing semimajor axis vs. eccentricity (left) and perihelion vs. inclination (right). Objective and Methodology We gather visible colors of NEOs from several astronomical surveys, including the Sloan Digital Sky Survey (SDSS) (Sergeyev & Carry, 2021), SkyMapper (Sergeyev et al., 2022), Gaia mission (Galluccio et al., 2022), and ground-based observations (Mahlke et al., 2022). These datasets are merged into a single catalog, creating a comprehensive resource for analyzing the compositional properties of NEOs. The orbital distribution of NEOs in our study is shown in Fig. 1. Each data source offers unique contributions: - SDSS: Provides multi-filter observations in u, g, r, i, z bands, allowing for detailed photometric analysis.- SkyMapper: Offers a combination of shallow and deep sequences in multiple filters, enhancing the dataset's depth and breadth.- Gaia: Contains low-resolution reflectance spectra covering a wide wavelength range, providing crucial spectral data.- Ground-based Observations: High-resolution spectra from various surveys add to the robustness of the dataset. Data ProcessingData from these diverse sources were cross-matched and compared to ensure consistency. Systematic biases were identified and corrected to create a homogeneous dataset. Given the fast-moving nature of NEOs, the study re-measured photometry for these objects in the SDSS to address potential biases related to their rapid motion. This step was crucial for ensuring accurate photometric measurements, which are foundational for subsequent analysis. Taxonomy and Classification The taxonomy of NEOs was determined using photometric colors, which were converted from reflectance spectra. The classification followed a probabilistic approach, assigning each NEO to one of ten taxonomic classes (A, B, C, D, K, L, Q, S, V, X) based on the highest probability. This methodology allows for a comprehensive classification scheme that accommodates the inherent uncertainties in photometric data see Fig. 2.- Multi-color Classification: Utilized combinations of g, r, i, z colors to classify NEOs with high accuracy.- Single-color Classification: Applied when only g-r color was available, providing a broader classification into "red" or "blue" objects. This approach, while less precise, ensures that all available data can be utilized. Fig. 2: Taxonomic classification of NEOs in the SDSS color space. Results The study produced several key findings:- Photometric and Taxonomic Data: The catalog includes updated photometry for 470 NEOs and taxonomic classifications for 7,401 NEOs (Sergeyev et al., 2023) see Table 1. This extensive dataset forms a solid foundation for further analysis.- Spectral Slope and Perihelion Dependence: Confirmed the relationship between spectral slope and perihelion among S-type NEOs, suggesting a rejuvenation mechanism linked to thermal fatigue. This finding supports existing theories about the effects of solar radiation on asteroid surfaces (Graves et al., 2019). Analysis of Space Weathering Space weathering, which alters the surface properties of asteroids through exposure to solar wind and micrometeorite impacts, was analyzed using spectral slope and taxonomic distribution. This analysis provides insights into the aging processes of asteroid surfaces.- S-type Asteroids: Showed a constant spectral slope for smaller diameters and an increase for larger ones, consistent with previous studies. This trend indicates that space weathering affects asteroids differently based on their size. - Q/S Ratio: Indicated a higher fraction of Q types (fresh surfaces) among smaller NEOs, suggesting a size-dependent space weathering process see Fig 3. This ratio is an important indicator of the relative age of asteroid surfaces. Fig. 3: Running mean of the ratio between the number of Q and S asteroids as a function of perihelion, inclination, and diameter. Shaded areas correspond to the uncertainties considering the Poisson statistic for the Q/S ratio. Distribution of A-type AsteroidsA-types, characterized by olivine-rich compositions, are rare in the main belt but more common among NEOs. The study found a higher fraction of A-types near the orbit of Mars, possibly linked to the Hungaria asteroid family (Devogèle et al., 2019). This distribution pattern provides clues about the dynamical processes that bring these objects into near-Earth space. Source Regions of NEOs The study predicted the taxonomic distribution of small asteroids in various source regions, such as the ν6 secular resonance, 3:1 and 2:1 mean-motion resonances with Jupiter, Phocaea, and Hungaria regions, and Jupiter Family Comets (JFC). The results align with existing models, showing the dominance of mafic-silicate-rich asteroids in inner regions and opaque-rich asteroids in outer regions. This distribution reflects the compositional gradients in the asteroid belt and the dynamical processes that transport these object
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