Origins for the Near-Earth Asteroids

The near-Earth asteroid population from two decades of observations

Dynamical analyses on the transitivity from JFC and Centaur regions to NEAs with Tisserand parameters from 2 to 3

In the absence of dense photometry for a large population of near-Earth objects (NEOs), the best method of obtaining a shape distribution comes from sparse photometry and partial light curves. We have used 867 partial light curves obtained by Spitzer to determine a shape distribution for subkilometer NEOs. From this data we find a best-fit average elongation of . We compare this result with a shape distribution obtained from 1869 NEOs in the same size range observed by Pan-STARRS 1 (PS1) and find the Spitzer-obtained elongation to be in excellent agreement with this PS1 value of . These values are also in agreement with literature values for 1 < D < 10 km objects in the main asteroid belt, however, there is a size discrepancy between the two data sets. Using a smaller sample of NEOs in the size range of 1 < D < 5 km from PS1 data, we obtain an average axis ratio of b/a = 0.70 ± 0.12. This is more elongated than the shape distribution for main belt objects in the same size regime, although the current uncertainties are sizeable and this should be verified using a larger data set. As future large surveys come online it will be possible to observe smaller main belt asteroids to allow for better comparisons of different subkilometer populations.

The population of near-Earth asteroids

<p>We report 491 new near-infrared spectroscopic measurements of 420 Near-Earth Objects (NEOs) collected on NASA’s IRTF in the context of MITHNEOS (PI: DeMeo). The measurements were combined with previously published data (Binzel et al. 2019) and bias-corrected for albedo variations to derive the intrinsic compositional distribution of the overall NEO population. We also investigated individual subpopulations coming from various escape routes (ERs) in the asteroid belt by use of the dynamical model of Granvik et al. (2018). The resulting distributions reflect well the compositional gradient of the asteroid belt, with decreasing fractions of silicate-rich (S- and Q-type) bodies and increasing fractions of carbonaceous (B-, C-, D- and P-type) bodies as a function of increasing ER distance from the Sun. The compositional match between NEOs and their predicted source populations validates dynamical models used to identify ERs and argues against strong composition change in the main belt between approximately 5 km and 100 m. An exception comes from the overabundance of D-type NEOs from the 5:2J and, to a lesser extent, the 3:1J and ν6 ERs, hinting at the presence of a large population of small D-type asteroids in the main belt. Alternatively, this excess may indicate spectral evolution from D-type surfaces to C and P types due to space weathering or point to preferential fragmentation of D-types in the NEO space. No further evidence for the existence of collisional families in the main belt, below the detection limit of current main-belt surveys, was found in this work.</p>

Abstract:This study investigates the physical properties of asteroids using optical data obtained by the Unistellar network of citizen astronomers. Leveraging the extensive observations provided by hundreds of users, we aim to characterize the size, shape, and rotation properties of a diverse sample of asteroids. Our analysis encompasses data collected over multiple observing campaigns conducted between 2021 and 2024, focusing on both near-Earth and main belt asteroids. By combining photometric light curves and stellar occultations, we derive comprehensive physical models for these celestial bodies. Our findings contribute to a deeper understanding of the asteroid population and provide valuable insights into their formation and evolution.Introduction:Asteroids represent remnants of the early solar system and offer valuable insights into its formation and evolution. Studying their physical properties, such as size, shape, and rotation, is crucial for understanding their composition and origins. The Unistellar network of citizen astronomers comprised of over 10,000 telescopes provides a unique opportunity to collect large-scale optical data for a wide range of asteroids, enabling comprehensive studies of their physical characteristics.Data and Methods:We utilized optical data collected by the Unistellar network between 2021 and 2024, focusing on observations of both near-Earth and main belt asteroids. Photometric light curves and stellar occultations were used to derive the sizes, shapes, and rotation properties of the asteroids. We analyzed data from multiple observing campaigns that targeted specific asteroids of scientific interest. We utilized the convex inversion method developed by Mikko Kaasalainen.Results:Our analysis resulted in rotation state properties and detailed 3D models for tens of asteroids. We obtained accurate measurements of size, shape, and rotation period for numerous celestial bodies, shedding light on their individual characteristics and variability within the asteroid population. Additionally, we identified intriguing phenomena, such as concavities and asymmetric light curves, providing insights into the dynamical processes shaping these objects.Conclusions:The optical data obtained by the Unistellar network of citizen astronomers offers valuable insights into the physical properties of asteroids. By leveraging these observations, we have advanced our understanding of the asteroid population, contributing to ongoing efforts to explore the solar system's early history. Our study highlights the importance of citizen science initiatives in expanding our knowledge of celestial objects and their origins.Figure 1. The Unistellar European network comprises citizen astronomers. As of May 2023, approximately 10,000 eVscopes are distributed globally, with over 3,000 of them located in EuropeFigure 2. The plot showcases light curve observations of asteroid (216) Kleopatra revealing variations in brightness over its rotation.Figure 3. The plot showcases light curve observations of near-Earth asteroid (1627) Ivar revealing variations in brightness over its rotation.

Phobos and Mars orbit as a base for asteroid exploration and mining

We perform a search for dormant comets, asteroidal objects of cometary origin, in the near-Earth asteroid (NEA) population based on dynamical and physical considerations. Our study is based on albedos derived within the ExploreNEOs program and is extended by adding data from NEOWISE and the Akari asteroid catalog. We use a statistical approach to identify asteroids on orbits that resemble those of short-period near-Earth comets using the Tisserand parameter with respect to Jupiter, the aphelion distance, and the minimum orbital intersection distance with respect to Jupiter. From the sample of NEAs on comet-like orbits, we select those with a geometric albedo $p_V \leq 0.064$ as dormant comet candidates, and find that only $\sim$50% of NEAs on comet-like orbits also have comet-like albedos. We identify a total of 23 NEAs from our sample that are likely to be dormant short-period near-Earth comets and, based on a de-biasing procedure applied to the cryogenic NEOWISE survey, estimate both magnitude-limited and size-limited fractions of the NEA population that are dormant short-period comets. We find that 0.3-3.3% of the NEA population with $H \leq 21$, and $9^{+2}_{-5}$% of the population with diameters $d \geq 1$ km, are dormant short-period near-Earth comets.

In this work we study formation and evolution of meteoroid streams originating from the collisions of near Earth asteroids (NEA) with objects of the Main Asteroid belt (MAB). Such a collision scenario is considered more probable compared to collisions between NEAs, since many NEAs, by virtue of their origin, cross the MAB region, where the number density of objects is significant compared to the inner regions of the Solar System. Meteoroid streams originating from collisions have a number of differences from the streams of cometary origin, both in terms of the formation and the evolution. In this paper, estimates are obtained for the meteoroid formation rate as a result of NEA collisions with MAB asteroids. On the basis of high-velocity collisions models and the DART experiment data, possible particle size and velocity distributions are obtained. We made numerical simulation taking into account gravitational perturbations and radiation forces and the influence of the initial velocity on the evolution was studied. An analysis was made of the rate of dust and meteoroids production, taking into account the distribution of the current population of the NEA, and it was concluded that the rates of influx of meteoroids of asteroid and cometary origin (in terms of mass) can be quite comparable.

Since 2006, the European Near Earth Asteroids Research (EURONEAR) project has been contributing to the research of near-Earth asteroids (NEAs) within a European network. One of the main aims is the amelioration of the orbits of NEAs, and starting in 2014 February we focus on the recovery of one-opposition NEAs using the Isaac Newton Telescope (INT) in La Palma in override mode. Part of this NEA recovery project, since 2014 June EURONEAR serendipitously started to discover and secure thefirst NEAs from La Palma and using the INT, thanks to the teamwork including amateurs and students who promptly reduce the data, report discoveries and secure new objects recovered with the INT and few other telescopes from the EURONEAR network. Five NEAs were discovered with the INT, including 2014 LU14, 2014 NL52 (one very fast rotator), 2014 OL339 (the fourth known Earth quasi-satellite), 2014 SG143 (a quite large NEA), and 2014 VP. Another very fast moving NEA was discovered but was unfortunately lost due to lack of follow-up time. Additionally, another 14 NEA candidates were identified based on two models, all being rapidly followed-up using the INT and another 11 telescopes within the EURONEAR network. They include one object discovered by Pan-STARRS, two Mars crossers, two Hungarias, one Jupiter trojan, and other few inner main belt asteroids (MBAs). Using the INT and Sierra Nevada 1.5 m for photometry, then the Gran Telescopio de Canarias for spectroscopy, we derived the very rapid rotation of 2014 NL52, then its albedo, magnitude, size, and its spectral class. Based on the total sky coverage in dark conditions, we evaluate the actual survey discovery rate using 2-m class telescopes. One NEA is possible to be discovered randomly within minimum 2.8 deg 2 and maximum 5.5 deg 2 . These findings update our past statistics, being based on double sky coverage and taking into account the recent increase in discovery.

739 observed NEAs and new 2–4 m survey statistics within the EURONEAR network

Collisions among near-Earth asteroids (NEAs) are less frequent than those involving pairs of main belt (MB) objects; however, the orbits of many NEAs are partially within the MB. Thus the collisional evolution of NEAs is often dominated by collisions with MB asteroids and, thus, these collisions are not less intense than collisions among MB bodies. In particular, the relative velocity of collisions is usually larger. In this Letter we discuss the dependence of collisional processes of NEAs on their orbital parameters, showing that the collisions are more frequent for low inclinations and for semimajor axes and eccentricities entailing an aphelic distance of about 2.75 au. The collisions may also affect the space-weathering processes: the surface involved in an impact and, eventually, in the re-accumulation of the related ejecta, is somehow refreshed, and the spectral reddening – in particular of the S-complex asteroids – is slowed down. This process may complement the already known refreshing due to close planetary passes. If the process is relevant, an anticorrelation between the visible spectral slope of S-complex NEAs and the estimated intensity of collisional processes should be found. An analysis of the available data confirms the suggested anticorrelation.

Constraining the formation processes of small solar system bodies is crucial for gaining insights into planetesimal formation. Their bulk densities, determined by their compressive strengths, offer valuable information about their formation history. In this paper, we utilize a formulation of the compressive strength of dust aggregates obtained from dust N-body simulations to establish the relation between the bulk density and diameter. We find that this relation can be effectively approximated by a polytrope with an index of 0.5, coupled with a formulation of the compressive strength of dust aggregates. The lowest-density trans-Neptunian objects (TNOs) and main-belt asteroids (MBAs) are well reproduced by dust aggregates composed of 0.1 μm sized grains. However, most TNOs, MBAs, comets, and near-Earth asteroids (NEAs) exhibit higher densities, suggesting the influence of compaction mechanisms such as collision, dust grain disruption, sintering, or melting, leading to further growth. We speculate that there are two potential formation paths for small solar system bodies. One involves the direct coagulation of primordial dust grains, resulting in the formation of first-generation planetesimals, including the lowest-density TNOs, MBAs, and the parent bodies of comets and NEAs. In this case, comets and NEAs are fragments or rubble piles of first-generation planetesimals, and the objects themselves or the rubble are composed of 0.1 μm sized grains. The other path involves the further potential fragmentation of first-generation planetesimals into the compact dust aggregates observed in protoplanetary disks, resulting in the formation of second-generation planetesimals composed of compact dust aggregates, which may contribute to explaining another formation process of comets and NEAs.

The Near-Earth Asteroid Size–Frequency Distribution: A Snapshot of the Lunar Impactor Size–Frequency Distribution

Near-Earth Objects (NEOs) are a transient population of small bodies with orbits near or in the terrestrial planet region. They represent a mid-stage in the dynamical cycle of asteroids and comets, which starts with their removal from the respective source regions—the main belt and trans-Neptunian scattered disk—and ends as bodies impact planets, disintegrate near the Sun, or are ejected from the solar system. Here we develop a new orbital model of NEOs by numerically integrating asteroid orbits from main-belt sources and calibrating the results on observations of the Catalina Sky Survey. The results imply a size-dependent sampling of the main belt with the ν 6 and 3:1 resonances producing ≃30% of NEOs with absolute magnitudes H = 15 and ≃80% of NEOs with H = 25. Hence, the large and small NEOs have different orbital distributions. The inferred flux of H < 18 bodies into the 3:1 resonance can be sustained only if the main-belt asteroids near the resonance drift toward the resonance at the maximal Yarkovsky rate (≃2 × 10−4 au Myr−1 for diameter D = 1 km and semimajor axis a = 2.5 au). This implies obliquities θ ≃ 0° for a < 2.5 au and θ ≃ 180° for a > 2.5 au, both in the immediate neighborhood of the resonance (the same applies to other resonances as well). We confirm the size-dependent disruption of asteroids near the Sun found in previous studies. An interested researcher can use the publicly available NEOMOD Simulator to generate user-defined samples of NEOs from our model.