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Statler et al., NASA’s Double Asteroid Redirection Test (DART) is the first full-scale test of an asteroid deflection technology. Results from the hypervelocity kinetic impact and Earth-based observations, coupled with LICIACube and the later Hera mission, will result in measurement of the momentum transfer efficiency accurate to ∼10% and characterization of the Didymos binary system. But DART is a single experiment; how could these results be used in a future planetary defense necessity involving a different asteroid? We examine what aspects of Dimorphos’s response to kinetic impact will be constrained by DART results; how these constraints will help refine knowledge of the physical properties of asteroidal materials and predictive power of impact simulations; what information about a potential Earth impactor could be acquired before a deflection effort; and how design of a deflection mission should be informed by this understanding. We generalize the momentum enhancement factor β, showing that a particular direction-specific β will be directly determined by the DART results, and that a related direction-specific β is a figure of merit for a kinetic impact mission. The DART β determination constrains the ejecta momentum vector, which, with hydrodynamic simulations, constrains the physical properties of Dimorphos’s near-surface. In a hypothetical planetary defense exigency, extrapolating these constraints to a newly discovered asteroid will require Earth-based observations and benefit from in situ reconnaissance. We show representative predictions for momentum transfer based on different levels of reconnaissance and discuss strategic targeting to optimize the deflection and reduce the risk of a counterproductive deflection in the wrong direction., This work was supported in part by the DART mission, NASA contract No. 80MSFC20D0004 to JHU/APL. Some of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). S.C. acknowledges funding through the Crosby Fellowship of the Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology. E.D., S.I., A.L., M.P., A.R., and P.T. are grateful to the Italian Space Agency (ASI) for financial support through agreement No. 2019-31-HH.0 in the context of ASI’s LICIACube mission, and agreement No. 2022-8-HH.0 for ESA’s Hera mission. R.L. appreciates the funding from the European Union’s Horizon 2020 research and innovation program, grant agreement No. 870377 (project NEO-MAPP). P.M. acknowledges funding support from the French space agency CNES, ESA and the European Union’s Horizon 2020 research and innovation program under grant agreement No. 870377 (project NEO-MAPP). R.M. acknowledges support from a NASA Space Technology Graduate Research Opportunities (NSTGRO) Award (80NSSC22K1173). R.N. acknowledges support from NASA/FINESST (NNH20ZDA001N). J.O. and I.H. were supported by the Spanish State Research Agency (AEI) project No. MDM-2017-0737 Unidad de Excelencia “María de Maeztu”—Centro de Astrobiología (CSIC-INTA). They are also grateful for all logistical support provided by Instituto Nacional de Técnica Aeroespacial (INTA). S.R.S. acknowledges support from the DART Participating Scientist Program, grant No. 80NSSC22K0318. J.M.T.R. was funded by FEDER/Ministerio de Ciencia e Innovación—Agencia Estatal de Investigación of Spain (grant No. PGC2018-097374-B-I00).

Funding Information: To achieve these objectives, Milani is carrying two scientific payloads, the ASPECT visual and near-infrared (Vis-NIR) imaging spectrometer and the VISTA thermogravimeter aimed at collecting and characterizing volatiles and dust particles below 10 μm. Additionally, navigation payloads include a visible navigation camera and lidar. The Milani consortium is composed of entities and institutions from Italy, the Czech Republic, and Finland. The consortium Prime is Tyvak International, responsible for the whole program management and platform design, development, integration, testing, and final delivery to the customer. Politecnico di Torino is tasked with defining requirements and performing thermal, radiation, and debris analysis. Politecnico di Milano is responsible for mission analysis and GNC. Altec will support the Ground Segment architecture and interface definition. Centro Italiano per la Ricerca Aerospaziale (CIRA) is responsible for the execution of the vehicle environmental campaign. HULD contributes to developing the mission-specific software. VTT is the main payload (ASPECT hyperspectral imager) provider and is supported by the following entities dealing with ASPECT-related development: University of Helsinki (ASPECT calibration); Reaktor Space Lab (ASPECT Data Processing Unit development), Institute of Geology of the Czech Academy of Sciences (ASPECT scientific algorithms requirements and testing); and Brno University of Technology (ASPECT scientific algorithms development). INAF-IAPS is the secondary Payload (VISTA, dust detector) provider. Funding Information: The Mission PI is appointed by ESA and is the primary interface to ESA. The Hera SMB consists of the ESA Hera Project Scientist (ESA PS), the Mission PI, and the Hera Advisory Board, consisting of four mission advisors. The Mission PI chairs the HIT and is supported by the Hera Advisory Board. The tasks of the Hera SMB are 1. advising the Hera mission project team on all aspects related to the Hera mission objectives; 2. ensuring that the WGs’ activities cover the needs of the Hera mission; 3. providing recommendations to ESA concerning the membership in the HIT; and 4. implementing the Publication Policy. Funding Information: Hera is the ESA contribution to the AIDA collaboration. Hera, Juventas, Milani, and their instruments are developed under ESA contract supported by national agencies. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 870377 (project NEO-MAPP), the CNRS through the MITI interdisciplinary programs, ASI, CNES, JAXA, the Academy of Finland project no. 335595, and was conducted with institutional support RVO 67985831 of the Institute of Geology of the Czech Academy of Sciences. M.L., E.P., P.T .and E.D. are grateful to the Italian Space Agency (ASI) for financial support through Agreement No. 2022-8-HH.0 in the context of ESA’s Hera mission. We are grateful to the whole Hera team, including Working Group core members and other contributors for their continuous efforts and support. Their names can be found on the following website: https:// www.heramission.space/team. Publisher Copyright: © 2022. The Author(s). Published by the American Astronomical Society. Hera is a planetary defense mission under development in the Space Safety and Security Program of the European Space Agency for launch in 2024 October. It will rendezvous in late 2026 December with the binary asteroid (65803) Didymos and in particular its moon, Dimorphos, which will be impacted by NASA’s DART spacecraft on 2022 September 26 as the first asteroid deflection test. The main goals of Hera are the detailed characterization of the physical properties of Didymos and Dimorphos and of the crater made by the DART mission, as well as measurement of the momentum transfer efficiency resulting from DART’s impact. The data from the Hera spacecraft and its two CubeSats will also provide significant insights into asteroid science and the evolutionary history of our solar system. Hera will perform the first rendezvous with a binary asteroid and provide new measurements, such as radar sounding of an asteroid interior, which will allow models in planetary science to be tested. Hera will thus provide a crucial element in the global effort to avert future asteroid impacts at the same time as providing world-leading science.

We present the results of our ground-based astrometric observation campaign of BepiColombo during its Earth fly-by on 2020 April 10. The observational circumstances induced by the fly-by geometry made this event an excellent proxy for a close fly-by or an imminent impact of a natural body with a diameter of a few meters, with the added benefit of having a ground-truth orbit (from radio tracking) to compare with our purely optical orbit determination., The work of TSR was carried out through grant APOSTD/2019/046 by Generalitat Valenciana (Spain). This work was supported by the MINECO (Spanish Ministry of Economy) through grantRTI2018-095076-B-C21 (MINECO/FEDER, UE).

International audience; NASA's Double Asteroid Redirection Test (DART) is designed to be the first demonstration of a kinetic impactor for planetary defense against a small-body impact hazard. The target is the smaller component of the binary asteroid 65803 Didymos. We have conducted high-fidelity rigid full two-body simulations of the mutual dynamics of this system in a broad benchmarking exercise to find the best simulation methodologies, and to understand the sensitivity of the system to initial conditions. Due to the non-spherical shapes of the components and their close proximity, the components cannot be treated as point masses and so the dynamics differ significantly from a simple Keplerian orbit, necessitating the use of numerical simulations to fully capture the system's dynamics. We find that the orbit phase (angular position or true anomaly) of the secondary is highly sensitive to the initial rotation phase of the primary, making prediction of the secondary's location from numerical simulation challenging. Finally, we show that the DART impact should induce significant free and forced librations on the secondary. If this libration can be measured by ESA's recently approved follow-up spacecraft, Hera, it may be possible to constrain properties of the secondary's interior structure.

In 2018, the near-Earth object (155140) 2005 UD (hereafter UD) experienced a close fly by of the Earth. We present results from an observational campaign involving photometric, spectroscopic, and polarimetric observations carried out across a wide range of phase angles (0.°7–88°). We also analyze archival NEOWISE observations. We report an absolute magnitude of H V = 17.51 ± 0.02 mag and an albedo of p V = 0.10 ± 0.02. UD has been dynamically linked to Phaethon due their similar orbital configurations. Assuming similar surface properties, we derived new estimates for the diameters of Phaethon and UD of D = 5.4 ± 0.5 km and D = 1.3 ± 0.1 km, respectively. Thermophysical modeling of NEOWISE data suggests a surface thermal inertia of and regolith grain size in the range of 0.9–10 mm for UD and grain sizes of 3–30 mm for Phaethon. The light curve of UD displays a symmetric shape with a reduced amplitude of Am(0) = 0.29 mag and increasing at a linear rate of 0.017 mag/° between phase angles of 0° and ∼25°. Little variation in light-curve morphology was observed throughout the apparition. Using light-curve inversion techniques, we obtained a sidereal rotation period P = 5.235 ± 0.005 hr. A search for rotational variation in spectroscopic and polarimetric properties yielded negative results within observational uncertainties of ∼10% μm−1 and ∼16%, respectively. In this work, we present new evidence that Phaethon and UD are similar in composition and surface properties, strengthening the arguments for a genetic relationship between these two objects.

The thermal inertia of an asteroid's surface can provide insight into regolith properties, such as the presence of a layer of fine dust, the density and thermal conductivity of a rocky surface, and, together with other observational data, mineralogy. Knowledge of the surface characteristics of asteroids is important for planetary defense initiatives and the extraction of resources ("asteroid mining"). A simple means of estimating asteroid thermal inertia has been proposed by Harris & Drube, which is suitable for application to large sets of thermal-infrared observational data, such as those obtained by infrared space telescopes. We compare results from the Harris-Drube estimator with recently published values of asteroid thermal inertia from detailed thermophysical modeling, and provide an explanation in terms of reduced surface roughness for some discrepant results. Smooth surfaces covered in fine dust may provide an explanation for the unexpectedly low values of thermal inertia derived from thermophysical modeling for some slowly rotating main-belt asteroids (MBAs). In the case of near-Earth objects (NEOs) we show that results from the estimator are in good agreement with those from thermophysical modeling, with just a few exceptions. We discuss the special cases of the NEOs (101955) Bennu, (162173) Ryugu, and (29075) 1950 DA in the context of results from our estimator. Given the data requirements and complexity of thermophysical modeling, data-analysis tools based on relatively simple concepts can play an important role in allowing "quick-look" assessment of thermal-infrared data of asteroids, especially NEOs., 24 pages, 9 figures, Accepted Manuscript, ApJ

著者人数: 15名, Accepted: 2017-09-11, 資料番号: SA1170184000

We present a regional photometric analysis of asteroid (101955) Bennu, using image data from the MapCam color imager of the OSIRIS-REx Camera Suite (OCAMS). This analysis follows the previously reported global photometric analysis of Bennu, which found that Bennu’s roughness was difficult to photometrically model owing to unresolved surface variation. Here we find that, even with a high-resolution shape model (20 cm per facet) and automatic image registration (

During asteroid entry, energy is deposited in the atmosphere through thermal ablation and momentum-loss due to aerodynamic drag. Analytic models of asteroid entry and breakup physics are used to compute the energy deposition, which can then be compared against measured light curves and used to estimate ground damage due to airburst events. This work assesses and compares energy deposition results from four existing approaches to asteroid breakup modeling, and presents a new model that combines key elements of those approaches. The existing approaches considered include a liquid drop or “pancake” model where the object is treated as a single deforming body, and a set of discrete fragment models where the object breaks progressively into individual fragments. The new model incorporates both independent fragments and aggregate debris clouds to represent a broader range of fragmentation behaviors and reproduce more detailed light curve features. All five models are used to estimate the energy deposition rate versus altitude for the Chelyabinsk meteor impact, and results are compared with an observationally derived energy deposition curve. Comparisons show that four of the five approaches are able to match the overall observed energy deposition profile, but the features of the combined model are needed to better replicate both the primary and secondary peaks of the Chelyabinsk curve.

In summer 2018, NASA’s chief scientist asked the National Academies of Sciences, Engineering, and Medicine to establish a study to address the issue of the relative advantages and disadvantages of infrared and visible observations of near Earth objects (NEOs). NASA has had an NEO observation program for nearly two decades using ground-based telescopes to search the night sky for NEOs that are large enough to cause major damage if they impact Earth. Since 2005, NASA has been guided in its search by the requirements of the George E. Brown, Jr. Near-Earth Object Survey Act. In recent years, NASA has used a space-based telescope to aid in its NEO search and has studied the possibility of using a dedicated space-based telescope to continue this work. This report of the Committee on Near Earth Object Observations in the Infrared and Visible Wavelengths addresses the space-based telescope subject while acknowledging that there are many larger issues associated with detecting, tracking, and characterizing NEOs. In December 2018, an asteroid exploded in the upper atmosphere over the Bering Sea (western Pacific Ocean) with an explosive force initially estimated to be nearly 200 kilotons, or over 10 times that of the Hiroshima bomb. This event, which was detected by various sensors and spotted by a Japanese weather satellite, demonstrates that Earth is frequently hit by objects, some of which could cause significant damage if they hit a populated area, as happened almost 6 years earlier over the Russian city of Chelyabinsk. Currently, NASA funds a network of ground-based telescopes and a single, soon-to-expire space-based asset to detect and track large asteroids that could cause major damage if they struck Earth. In 2018, the National Academies of Sciences, Engineering, and Medicine established the ad hoc Committee on Near Earth Object Observations in the Infrared and Visible Wavelengths to investigate and make recommendations about a space-based telescope’s capabilities, focusing on the following tasks: - Explore the relative advantages and disadvantages of infrared (IR) and visible observations of near Earth objects (NEOs). - Review and describe the techniques that could be used to obtain NEO sizes from an infrared spectrum and delineate the associated errors in determining the size. - Evaluate the strengths and weaknesses of these techniques and recommend the most valid techniques that give reproducible results with quantifiable errors.

As evidenced by the Chelyabinsk and Tunguska airburst events in Russia, decameter-scale Near-Earth Objects (NEOs) can pose a hazard to human life and infrastructure from the energy they deposit in the atmosphere as they break up. To understand the potential damage these small NEOs can cause on Earth's surface, it is imperative to be able to model their atmospheric entry quickly and accurately. Here we compare three semi-analytical models of asteroid airbursts that differ in their descriptions of fragment separation and spreading. Each model can be calibrated to produce a good fit to the energy deposition curve inferred from Chelyabinsk observations, but in each case the implied initial meteoroid strength is different and when the calibrated models are upscaled to Tunguska, the results diverge. This introduces an inter-model uncertainty that compounds the large range of uncertain physical and model parameters that influence probabilistic hazard assessment. Uncertainty quantification of airburst energy deposition was performed for a theoretical impacting object with H-magnitude 27, assuming no prior knowledge of any other impactor or model parameter. Each of the three models produces a different distribution of airburst outcomes, however, the variation attributable to physical parameter uncertainty is far larger than the inter-model differences. To constrain the initial conditions of the Tunguska event, the same uncertainty quantification was performed for an H-magnitude 24 event. Among the scenarios consistent with Tunguska observations (5–10 km burst altitude, 10–60° trajectory angle, 3–50 MT TNT total energy release) the most likely range of impact conditions was: radius of 25–75 m, mass of 1× 108– 2.5× 109 kg, initial velocity of 11.5–33 km/s, and angle of 25–60°.

We report on our detailed characterization of Earth's second known temporary natural satellite, or minimoon, asteroid 2020 CD3. An artificial origin can be ruled out based on its area-to-mass ratio and broadband photometry, which suggest that it is a silicate asteroid belonging to the S or V complex in asteroid taxonomy. The discovery of 2020 CD3 allows for the first time a comparison between known minimoons and theoretical models of their expected physical and dynamical properties. The estimated diameter of 1.2+0.4-0.2 m and geocentric capture approximately a decade after the first known minimoon, 2006 RH120, are in agreement with theoretical predictions. The capture duration of 2020 CD3 of at least 2.7 yr is unexpectedly long compared to the simulation average, but it is in agreement with simulated minimoons that have close lunar encounters, providing additional support for the orbital models. 2020 CD3's atypical rotation period, significantly longer than theoretical predictions, suggests that our understanding of meter-scale asteroids needs revision. More discoveries and a detailed characterization of the population can be expected with the forthcoming Vera C. Rubin Observatory Legacy Survey of Space and Time., 22 pages, 6 Figures, 5 Tables, to appear in the Astronomical Journal

The current work explores the interactions between asteroid fragments and the associated flow topology to motivate a physically consistent representation of the fragmentation process following a fragmentation event during atmospheric entry. Multibody aerodynamic simulations run with computational fluid dynamics (CFD) solvers were used to generate a lookup table of forces detailing the interactions of two spheres. Trajectory simulations parsing the resulting database to determine the relative motions of any two spherical fragments were then validated with hypersonic wind tunnel experiments. A following series of fragment interaction simulations yielded categorization of the fragments' final relative states and an estimate of the total time of interaction. The fragment interaction model was nondimensionalized to permit study over a wide range of possible asteroid impacts. The interaction parameters are presented with explicit semi-analytic equations, defining the asteroid fragment-flow interaction model and thereby eliminating the need to perform a separate fragment interaction simulation for each fragmentation event in an atmospheric entry model. Finally, a set of illustrative examples demonstrates the efficacy of the model in a variety of fragmentation situations.

The debiased absolute-magnitude and orbit distributions as well as source regions for near-Earth objects (NEOs) provide a fundamental frame of reference for studies of individual NEOs and more complex population-level questions. We present a new four-dimensional model of the NEO population that describes debiased steady-state distributions of semimajor axis, eccentricity, inclination, and absolute magnitude $H$ in the range $17, Comment: 33 pages, 28 figures, accepted for publication in Icarus

We report radar observations (2380-MHz, 13-cm) by the Arecibo Observatory and optical light curves observed from eight different observatories and collected at the Ondřejov Observatory of the triple near-Earth asteroid system (153591) 2001 SN 263 . The radar observations were obtained over the course of ten nights spanning February 12–26, 2008 and the light curve observations were made throughout January 12 - March 31, 2008. Both data sets include observations during the object’s close approach of 0.06558 AU on February 20th, 2008. The delay-Doppler images revealed the asteroid to be comprised of three components, making it the first known triple near-Earth asteroid. Only one other object, (136617) 1994 CC is a confirmed triple near-Earth asteroid. We present physical models of the three components of the asteroid system. We constrain the primary’s pole direction to an ecliptic longitude and latitude of ( 309 ° , - 80 ° ) ± 15 ° . We find that the primary rotates with a period 3.4256 ± 0.0002 h and that the larger satellite has a rotation period of 13.43 ± 0.01 h , considerably shorter than its orbital period of approximately 6 days. We find that the rotation period of the smaller satellite is consistent with a tidally locked state and therefore rotates with a period of 0.686 ± 0.002 days (Fang et al. [2011]. Astron. J. 141, 154–168). The primary, the larger satellite, and the smaller satellite have equivalent diameters of 2.5 ± 0.3 km , 0.77 ± 0.12 km , 0.43 ± 0.14 km and densities of 1.1 ± 0.2 g / cm 3 , 1.0 ± 0.4 g / cm 3 , 2.3 ± 1.3 g / cm 3 , respectively.

The paper presents statistics of the asteroids observed and discovered at the Molėtai Observatory, Lithuania in 2000–2004 within the project for astrometric observations of the near-Earth objects (NEOs), the main belt asteroids and comets. CCD observations of asteroids were obtained with the 35/51 cm Maksutov-type meniscus telescope and the 1.65 m Ritchey-Chretien reflector. In the Minor Planet Circulars and the Minor Planet Electronic Circulars (2000–2004) we published 6629 astrometric positions of 1114 asteroids. Among them 78 were newly discovered asteroids at Molėtai, a few NEOs were found by our team independently. For the 67 asteroids discovered at Molėtai the precise orbits were calculated. Because of small number of observations, a few asteroids have low-precision orbits and some asteroids have been lost. For seven objects we present their ephemerides for 2015.

We collected data on rotations and elongations of 46 secondaries of binary and triple systems among near-Earth, Mars-crossing and small main belt asteroids. 24 were found or are strongly suspected to be synchronous (in 1:1 spin–orbit resonance), and the other 22, generally on more distant and/or eccentric orbits, were found or are suggested to have asynchronous rotations. For 18 of the synchronous secondaries, we constrained their librational angles, finding that their long axes pointed to within 20° of the primary on most epochs. The observed anti-correlation of secondary synchroneity with orbital eccentricity and the limited librational angles agree with the theories by Cuk and Nesvorný (Cuk, M., Nesvorný, D. [2010]. Icarus 207, 732–743) and Naidu and Margot (Naidu, S.P., Margot, J.-L. [2015]. Astron. J. 149, 80). A reason for the asynchronous secondaries being on wider orbits than synchronous ones may be longer tidal circularization time scales at larger semi-major axes. The asynchronous secondaries show relatively fast spins; their rotation periods are typically 10 h. An intriguing observation is a paucity of chaotic secondary rotations; with an exception of (35107) 1991 VH, the secondary rotations are single-periodic with no signs of chaotic rotation and their periods are constant on timescales from weeks to years. The secondary equatorial elongations show an upper limit of a 2 / b 2 ∼ 1.5 . The lack of synchronous secondaries with greater elongations appears consistent, considering uncertainties of the axis ratio estimates, with the theory by Cuk and Nesvorný that predicts large regions of chaotic rotation in the phase space for a 2 / b 2 ≳ 2 . Alternatively, secondaries may not form or stay very elongated in gravitational (tidal) field of the primary. It could be due to the secondary fission mechanism suggested by Jacobson and Scheeres (Jacobson, S.A., Scheeres, D.J. [2011]. Icarus 214, 161–178), as its efficiency is correlated with the secondary elongation. Sharma (Sharma, I. [2014]. Icarus 229, 278–294) found that rubble-pile satellites with a 2 / b 2 ≲ 1.5 are more stable to finite structural perturbations than more elongated ones. It appears that more elongated secondaries, if they originally formed in spin fission of parent asteroid, are less likely to survive intact and they more frequently fail or fission.

The nature and origin of the asteroids orbiting in near-Earth space, including those on a potentially hazardous trajectory, is of both scientific interest and practical importance. We aim here at determining the taxonomy of a large sample of near-Earth (NEA) and Mars-crosser (MC) asteroids and analyze the distribution of these classes with orbit. We use this distribution to identify their source regions and to study the strength of planetary encounters to refresh asteroid surfaces. We measure the photometry of these asteroids over four filters at visible wavelengths on images taken by the SDSS. These colors are used to classify the asteroids into a taxonomy consistent with the widely used Bus-DeMeo taxonomy based on spectroscopy. We report here on the taxonomic classification of 206 NEAs and 776 MCs determined from SDSS photometry, representing an increase of 40% and 663% of known taxonomy classifications in these populations. Using the source region mapper by Greenstreet et al. (2012), we compare the taxonomic distribution among NEAs and main-belt asteroids of similar diameters. Both distributions agree at the few percent level for the inner part of the Main Belt and we confirm this region as a main source of near-Earth objects. The effect of planetary encounters on asteroid surfaces are also studied by developing a simple model of forces acting on a surface grain during planetary encounter, which provides the minimum distance at which a close approach should occur to trigger resurfacing events. By integrating numerically the orbit of the 519 S-type and 46 Q-type asteroids back in time and monitoring their encounter distance with planets, we seek to understand the conditions for resurfacing events. The population of Q-type is found to present statistically more encounters with Venus and the Earth than S-types, although both types present the same amount of encounters with Mars., Accepted for publication in Icarus. 45 pages, 11 figures, 4 tables, 2 tables in appendix (supplementary material)

Besides being of great scientific interest, near-Earth objects represent a well-founded threat to life on our planet. Nonetheless, up to now there has been no concerted international plan on how to deal with the impact threat, and how to prepare and implement mitigation measures. The NEOShield project is funded by the European Commission to address such issues, to investigate the feasibility of techniques to prevent a potentially catastrophic impact on Earth by an asteroid or a comet, and to develop detailed designs of appropriate space missions to test deflection techniques. In this work we present and discuss the scientific and strategic aspects of the asteroid impact threat, highlighting the necessary steps so as to be ready to react to future hazardous objects.

The density measurements of Almahata Sitta ureilites reveal a bulk density of ∼3.1 g/cm 3 . This value, together with the 2008 TC 3 asteroid shape model and albedo, was used to estimate the asteroid’s mass. Based on the study of recovered meteorites and atmospheric entry observations Asteroid 2008 TC 3 is compositionally heterogeneous and of low mechanical strength. Thus we consider the presence of significant macroporosity likely, lowering asteroid’s bulk density compared to that of the Almahata Sitta ureilites. Most realistic albedos lie in a range of 0.09-0.2 and the presence of significant macroporosity leads to mass estimates below 20 × 10 3 kg, which is lower than previously estimated. The presence of a non-ureilitic fraction and space weathering may affect the albedo and also influence the mass estimates. However, from current data it is not possible to quantify this effect.

International audience; In this paper, we present our study of the orbital and thermal evolutions, due to solar radiative heating, of four near-Earth asteroids (NEAs) considered as potential target candidates for sample return space missions to primitive asteroids. We used a dynamical model of the NEA population to estimate the most likely source region and orbital history of these objects. Then, for each asteroid, we integrated numerically over their entire lifetime a set of 14 initially indistinguishable orbit (), obtained by small variations of the nominal initial conditions. Using a thermal model, we then computed surface and sub-surface temperatures of these bodies during their dynamical history. Our aim is to determine whether these bodies are likely to have experienced high temperature level, and whether great temperature changes can be expected due to the orbital changes as well as their maximum and minimum values. Such information is important in the framework of sample return space missions whose goal is to bring back pristine materials. The knowledge of the temperature range of materials at different depth over the orbital evolution of potential targets can help defining sampling strategies that ensure the likelyhood that unaltered material will be brought back. Our results suggest that for all the considered potential targets, the surface has experienced for some time temperatures greater than 400 K and at most 500 K with 50% probability. This probability drops rapidly with increasing temperature. Sub-surface materials at a depth of only 3 cm are much more protected from high temperature and generally do not reach temperatures exceeding 450 K (with 50% probability). They should thus be unaltered at this depth at least from a sun-driven heating point of view. On the other hand, surface material for some of the considered objects can have a range of temperature which can make them less reliable as pristine materials. However, it is assumed here that the same material is constantly exposed to solar heat, while regolith turnover may occur. The latter can be caused by different processes such as seismic shaking and/or impact cratering. This would reduce the total time that materials are exposed to a certain temperature. Thus, it is very likely that a sample collected from any of the four considered targets, or any primitive NEA with similar dynamical properties, will have components that will be thermally unaltered as long as some of it comes from only 3 to 5 cm depth. Such a depth is not considered difficult to reach with some of the current designs of sampling devices.

In this paper we present the observational campaign carried out at ESO NTT and VLT in April and May 2006 to investigate the nature and the structure of the Near Earth Object (144898) 2004 VD17. In spite of a great quantity of dynamical information, according to which it will have a close approach with the Earth in the next century, the physical properties of this asteroid are largely unknown. We performed visible and near--infrared photometry and spectroscopy, as well as polarimetric observations. Polarimetric and spectroscopic data allowed us to classify 2004 VD17 as an E-type asteroid. A good agreement was also found with the spectrum of the aubrite meteorite Mayo Belwa. On the basis of the polarimetric albedo (p_v=0.45) and of photometric data, we estimated a diameter of about 320 m and a rotational period of about 2 hours. The analysis of the results obtained by our complete survey have shown that (144898) 2004 VD17 is a peculiar NEO, since it is close to the breakup limits for fast rotator asteroids, as defined by Pravec and Harris (2000). These results suggest that a more robust structure must be expected, as a fractured monolith or a rubble pile in a "strength regime" (Holsapple 2002)., Comment: 32 pages, 7 figure, paper accepted for publication in Icarus

The Amor-type near-Earth Asteroid (10302) 1989 ML has an “Earth-like” orbit (period 1.44 yr, eccentricity 0.14, inclination 4.4°), therefore the energy required to reach it from the Earth is relatively small making it a very attractive target for rendezvous missions. We have observed 1989 ML in the thermal–infrared using the Spitzer Space Telescope, and compared these data with optical and near-infrared observations. The Spitzer results imply a diameter of 0.28 ± 0.05 km and a geometric albedo of 0.37 ± 0.15 ; together with the reflectance spectrum they are consistent with the relatively rare E classification.

International audience; Itokawa is an S-type near-Earth asteroid, numbered 25143. This irregular object has been the first asteroidal target for a sample return mission: the JAXA Hayabusa-1 mission. It has established that Itokawa is very small (530 × 290 × 210 m) and has a density of about 1,900 kg/m−3, leading to a rather high porosity. It presents rough terrains scattered with boulders and smooth featureless areas, resulting from fine dust accumulated in local gravitational lows. Its shape suggests that it is a contact binary (as the nucleus of comet Hartley 2). Its density could indicate that it is built from loose-packed rocks, held together by their gravity. Samples of the asteroid have been brought back to Earth and are under analysis.

In this paper we present the results on the polarimetric and spectroscopic observations of the potentially hazardous Asteroid (214869) 2007 PA8 obtained during its favorable apparition in October–November 2012, when it approached the Earth at the minimal distance of 0.043 AU. Polarimetry was carried out at the NOT in the B, V, R, and I bands covering both low (12–23°) and large phase angles (88–99°). Spectroscopy in the visible and near infrared range was obtained at the TNG telescope. The spectrum of 2007 PA8 shows silicates absorption features and a behavior consistent with a Q-type classification. The olivine and pyroxene BI band is centered at 0.9578 ± 0.0042 μm, with a band depth of 16.5%, the BII band is centered at 1.95 ± 0.01 μm, and it has a band depth of about 3.9%. The 2007 PA8 spectral parameters are consistent with those of L chondrites. Also the spectral comparison with meteorites gives the L-type chondrites, and L6 in particular, as best match. The NEA (214869) 2007 PA8 is the forth moderate albedo asteroid and the first Q-type asteroid for which the value of the polarization maximum is determined. The inversion angle of the polarization curve in the V filter is 19.0 ± 1.1°, the corresponding slope parameter (h) is of 0.078 ± 0.010%/°, the maximum value of polarization is 5.99 ± 0.16%, and the extreme value of negative polarization is estimated to be lower than −0.52%. Using the polarimetric slope we derive a geometric albedo of 0.29 ± 0.08 in the V band, that gives an estimated diameter of 1.4 ± 0.2 km, assuming an absolute H v magnitude of 16.2 mag. We find a strong dependence of the polarization in the B, V, R, and I bands with wavelength, and the polarimetric albedo in the four bands is strongly correlated with the asteroid’s spectrum. The 2007 PA8 polarimetric properties resemble those of other 2 NEAs, 1566 Icarus and 25143 Itokawa, which are both S(IV)/Q type. Our spectral and polarimetric analysis indicate that 2007 PA8 has a young and fresh surface almost unweathered, similar to L-type chondrites. These results, together with dynamical simulations made by Nedelcu et al. (Nedelcu, D. A., Birlan, M., Popescu, M., Badescu, O., Pricopi, D. [2014]. Astron. Astrophys. 567, L7, 5pp.) and Nesvorny et al. (Nesvorny, D., Vokrouhlicky, D., Morbidelli, A., Bottke, W. F. [2009]. Icarus 200, 698–701), indicate that 2007 PA8 may be a member of the Gefion family recently ejected from the 5:2 resonance and a potential source of L chondrites.

Using our photometric observations taken between 1996 and 2013 and other published data, we derived properties of the binary near-Earth asteroid (175706) 1996 FG3 including new measurements constraining evolution of the mutual orbit with potential consequences for the entire binary asteroid population. We also refined previously determined values of parameters of both components, making 1996 FG3 one of the most well understood binary asteroid systems. We determined the orbital vector with a substantially greater accuracy than before and we also placed constraints on a stability of the orbit. Specifically, the ecliptic longitude and latitude of the orbital pole are 266{\deg} and -83{\deg}, respectively, with the mean radius of the uncertainty area of 4{\deg}, and the orbital period is 16.1508 +/- 0.0002 h (all quoted uncertainties correspond to 3sigma). We looked for a quadratic drift of the mean anomaly of the satellite and obtained a value of 0.04 +/- 0.20 deg/yr^2, i.e., consistent with zero. The drift is substantially lower than predicted by the pure binary YORP (BYORP) theory of McMahon and Scheeres (McMahon, J., Scheeres, D. [2010]. Icarus 209, 494-509) and it is consistent with the theory of an equilibrium between BYORP and tidal torques for synchronous binary asteroids as proposed by Jacobson and Scheeres (Jacobson, S.A., Scheeres, D. [2011]. ApJ Letters, 736, L19). Based on the assumption of equilibrium, we derived a ratio of the quality factor and tidal Love number of Q/k = 2.4 x 10^5 uncertain by a factor of five. We also derived a product of the rigidity and quality factor of mu Q = 1.3 x 10^7 Pa using the theory that assumes an elastic response of the asteroid material to the tidal forces. This very low value indicates that the primary of 1996 FG3 is a 'rubble pile', and it also calls for a re-thinking of the tidal energy dissipation in close asteroid binary systems., Comment: Many changes based on referees comments

In anticipation of the Dawn Mission to 4 Vesta, we conducted a ground-based campaign of Bessel BVRI filter photometry of five V-type near-Earth asteroids over a wide range of solar phase angles. We also obtained medium-resolution optical spectroscopy (0.38 mu m < lambda < 0.92 mu m; R similar to 500) of sixteen near-Earth and main-belt V-type asteroids in order to investigate their spectral diversity and to draw connections between spacecraft data of Vesta and V-type asteroids. Our disk-integrated photometry extended the excursion in solar phase angle beyond the maximum of 24 degrees available from Earth for Vesta to 87 degrees, which is more typical of the geometry during the Dawn approach and mapping phases. The majority of our broad-band observations were obtained at the JPL 0.6-m Table Mountain Observatory but multiple nights were also contributed by the Calar Alto 1.2-m and 2.2-m telescopes, as well as by the Purple Mountain 1-m Schmidt. Our results include a determination of rotation periods for 4 asteroids, identification of a binary candidate and four new V-type asteroids, including a confirmation of two main-belt V-type asteroids beyond the Jupiter 1:3 resonance (Cruikshank, D.P., Tholen, DJ., Bell, J.F., Hartmann, W.K., Brown, R.H. [1991]. Icarus 89,1-13; Lazzaro, D. et al. [2000]. Science 288, 2033-2035; Roig, F., Gil-Hutton, R.[20061. Icarus 183(2), 411-419; Moskovitz, N.A., Jedicke, R., Gaidos, E., Willman, M., Nesvorny, D., Fevig, R., Ivezic, Z. [2008]. Icarus 198,77-90). This latter finding supports the hypothesis that some vestoids may be crustal fragments of a disrupted basaltic parent body compositionally similar to 4 Vesta. We also obtained rotationally resolved medium resolution spectra of Vesta during the Dawn orbit insertion phase, which will be valuable for calibration and comparison of spacecraft data. Modeling of a composite V-type asteroid phase curve yielded a generic photometric model for V asteroids. We also find that a significant amount of the spectral diversity in the V class comes from changes in solar phase angle. A fit of a composite solar phase curve containing our vestoid observations, previously published groundbased observations of Vesta, and early disk-integrated Dawn observations show important differences with other asteroids. The macroscopic surface roughness of V-type asteroids is significantly larger than that of C-type or S-types (Helfenstein, P., Veverka, J. [1989]. Physical characterization of asteroid surfaces from photometric analysis. In: Binzel, R., Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. University of Arizona Press, Tucson, pp. 557-593). This result is consistent with radar studies showing that igneous rocky asteroids - the E and V types - exhibit the largest surface roughness (Benner, L et al. [2008]. Icarus 198,294-304). The effects of what appears to be space weathering can be largely explained by phase reddening in our collection of V-type NEOs, but our finding that smaller vestoids, which have shorter lifetimes, are more similar to Vesta suggests that some type of alteration of the surface through time occurs. Our observations confirm that the south polar region of Vesta has a more diogenitic composition than its equatorial regions. The south pole, which is dominated by a large impact feature, thus may offer a view into the interior of Vesta. We derive a visible phase integral of 0.44 +/- 0.02 and a corresponding Bond albedo of 0.15 +/- 0.03 from our composite V-type asteroid solar phase curve. (C) 2014 Published by Elsevier Inc.

The target asteroid of the OSIRIS-REx asteroid sample return mission, (101955) Bennu (formerly 1999 RQ$_{36}$), is a half-kilometer near-Earth asteroid with an extraordinarily well constrained orbit. An extensive data set of optical astrometry from 1999--2013 and high-quality radar delay measurements to Bennu in 1999, 2005, and 2011 reveal the action of the Yarkovsky effect, with a mean semimajor axis drift rate $da/dt = (-19.0 \pm 0.1)\times 10^{-4}$ au/Myr or $284\pm 1.5\;\rm{m/yr}$. The accuracy of this result depends critically on the fidelity of the observational and dynamical model. As an example, neglecting the relativistic perturbations of the Earth during close approaches affects the orbit with $3\sigma$ significance in $da/dt$. The orbital deviations from purely gravitational dynamics allow us to deduce the acceleration of the Yarkovsky effect, while the known physical characterization of Bennu allows us to independently model the force due to thermal emissions. The combination of these two analyses yields a bulk density of $\rho = 1260\pm70\,\rm{kg/m^3}$, which indicates a macroporosity in the range $40\pm10$% for the bulk densities of likely analog meteorites, suggesting a rubble-pile internal structure. The associated mass estimate is $(7.8\pm0.9)\times 10^{10}\, \rm{kg}$ and $GM = 5.2\pm0.6\,\rm{m^3/s^2}$. Bennu's Earth close approaches are deterministic over the interval 1654--2135, beyond which the predictions are statistical in nature. In particular, the 2135 close approach is likely within the lunar distance and leads to strong scattering and therefore numerous potential impacts in subsequent years, from 2175--2196. The highest individual impact probability is $9.5\times 10^{-5}$ in 2196, and the cumulative impact probability is $3.7\times 10^{-4}$, leading to a cumulative Palermo Scale of -1.70.

We observed near-Earth asteroid (162421) 2000 ET70 using the Arecibo and Goldstone radar systems over a period of 12 days during its close approach to the Earth in February 2012. We obtained continuous wave spectra and range-Doppler images with range resolutions as fine as 15 m. Inversion of the radar images yields a detailed shape model with an effective spatial resolution of 100 m. The asteroid has overall dimensions of 2.6 km X 2.2 km X 2.1 km (5% uncertainties) and a surface rich with kilometer-scale ridges and concavities. This size, combined with absolute magnitude measurements, implies an extremely low albedo (~2%). It is a principal axis rotator and spins in a retrograde manner with a sidereal spin period of 8.96 +/- 0.01 hours. In terms of gravitational slopes evaluated at scales of 100 m, the surface seems mostly relaxed with over 99% of the surface having slopes less than 30 degrees, but there are some outcrops at the north pole that may have steeper slopes. Our precise measurements of the range and velocity of the asteroid, combined with optical astrometry, enables reliable trajectory predictions for this potentially hazardous asteroid in the interval 460-2813., 17 pages, 11 figures, accepted to Icarus

On 15 February 2013, an asteroid 17 metres in size and weighing some 10,000 tons entered Earth’s atmosphere at around 64,000 kilometres per hour, exploding over Chelyabinsk in Russia. The energy released was equivalent to 500 kilotons of TNT, about 30 times that of the Hiroshima atomic bomb. The resulting blast wave damaged buildings and caused injuries to around 1200 people through flying debris and glass splinters. On the very same day, asteroid 2012 DA14, 45 metres in diameter, passed only 28,000 kilometres from Earth, beneath the orbits of TV and communications satellites. Despite the dramatic combination of the two unrelated events, the collision of a large asteroid or comet with Earth is actually a very rare event. But if such an impact were to occur it could trigger the worst natural catastrophe our civilisation has ever experienced. It is worth repeating to be absolutely clear: what we are talking about here is a minuscule risk, but one linked to potentially devastating consequences. Ignoring this risk is socially, politically and also economically very dangerous; it is a subject in which global thinking and acting is vital.

Gaia is an astrometric mission that will be launched in spring 2013. There are many scientific outcomes from this mission and as far as our Solar System is concerned, the satellite will be able to map thousands of main belt asteroids (MBAs) and near-Earth objects (NEOs) down to magnitude < 20. The high precision astrometry (0.3-5 mas of accuracy) will allow orbital improvement, mass determination, and a better accuracy in the prediction and ephemerides of potentially hazardous asteroids (PHAs). We give in this paper some simulation tests to analyse the impact of Gaia data on known asteroids' orbit, and their value for the analysis of NEOs through the example of asteroid (99942) Apophis. We then present the need for a follow-up network for newly discovered asteroids by Gaia, insisting on the synergy of ground and space data for the orbital improvement., Accepted for publication in Planetary and Space Science in Dec. 2012

We have for the first time calculated the population characteristics of the Earth's irregular natural satellites (NES) that are temporarily captured from the near-Earth-object (NEO) population. The steady-state NES size-frequency and residence-time distributions were determined under the dynamical influence of all the massive bodies in the solar system (but mainly the Sun, Earth, and Moon) for NEOs of negligible mass. To this end, we compute the NES capture probability from the NEO population as a function of the latter's heliocentric orbital elements and combine those results with the current best estimates for the NEO size-frequency and orbital distribution. At any given time there should be at least one NES of 1-meter diameter orbiting the Earth. The average temporarily-captured orbiter (TCO; an object that makes at least one revolution around the Earth in a co-rotating coordinate system) completes $(2.88\pm0.82)\rev$ around the Earth during a capture event that lasts $(286\pm18)\days$. We find a small preference for capture events starting in either January or July. Our results are consistent with the single known natural TCO, 2006 RH$_{120}$, a few-meter diameter object that was captured for about a year starting in June 2006. We estimate that about 0.1% of all meteors impacting the Earth were TCOs., 63 pages, 29 figures. Accepted for publication in Icarus (December 13, 2011)

Examining the albedo distribution of the near-Earth object (NEO) population allows for a better understanding of the relationship between absolute (H) magnitude and size, which impacts calculations of the size frequency distribution and impact hazards. Examining NEO albedos also sheds light on the differences between the NEO and Main Belt populations. We combine albedo results from the ExploreNEOs Warm Spitzer Exploration Science program with taxonomic classifications from the literature, publicly available data sets, and new observations from our concurrent spectral survey to derive the average albedos for C-, D-, Q-, S-, V-, and X-complex NEOs. Using a sample size of 118 NEOs, we calculate average albedos of 0.29+0.05 –0.04, 0.26+0.04 –0.03, and 0.42+0.13 –0.11 for the Q-, S-, and V-complexes, respectively. The averages for the C- and D-complexes are 0.13+0.06 –0.05 and 0.02+0.02 –0.01, but these averages are based on a small number of objects (five and two, respectively) and will improve with additional observations. We use albedos to assign X-complex asteroids to one of the E-, M-, or P-types. Our results demonstrate that the average albedos for the C-, S-, V-, and X-complexes are higher for NEOs than the corresponding averages observed in the Main Belt.

We report on results of observations of near-Earth objects (NEOs) performed with the NASA Spitzer Space Telescope as part of our ongoing (2009-2011) Warm Spitzer NEO survey ("ExploreNEOs"), the primary aim of which is to provide sizes and albedos of some 700 NEOs. The emphasis of the work described here is an assessment of the overall accuracy of our survey results, which are based on a semi-empirical generalized model of asteroid thermal emission. The NASA Spitzer Space Telescope has been operated in the so-called Warm Spitzer mission phase since the cryogen was depleted in 2009 May, with the two shortest-wavelength channels, centered at 3.6 μm and 4.5 μm, of the Infrared Array Camera continuing to provide valuable data. The set of some 170 NEOs in our current Warm Spitzer results catalog contains 28 for which published taxonomic classifications are available, and 14 for which relatively reliable published diameters and albedos are available. A comparison of the Warm Spitzer results with previously published results ("ground truth"), complemented by a Monte Carlo error analysis, indicates that the rms Warm Spitzer diameter and albedo errors are ±20% and ±50%, respectively. Cases in which agreement with results from the literature is worse than expected are highlighted and discussed; these include the potential spacecraft target 138911 2001 AE_2. We confirm that 1.4 appears to be an appropriate overall default value for the relative reflectance between the V band and the Warm Spitzer wavelengths, for use in correction of the Warm Spitzer fluxes for reflected solar radiation.