Focus Meeting 1 - Abstracts
Differentiation Signatures among Asteroid Families
Oszkiewicz, Dagmara
The ”missing mantle problem” is a long standing puzzle in planetary science. From one side abundant HED (eg. Bunburra Rockhole, Ibitra, Pasamonte, PCA 91007, NWA 011, A-881394) and iron meteoritic evidence and Solar System formation theories predict that there once existed from 30 to 150 differentiated (into geologically distinct layers: iron core, silicate mantle and crust) planetesimals in the Solar System. On the other hand observationally only one large (4 Vesta and its collisional family) and a few small V-type (parts of crusts and mantles) asteroids in the mid and outer Main Belt (1459 Magnya, 21238 Panarea, 40521 1999 RL95, 10537 1991RY16) have been identified as traces of differentiated planetesimals, thus not summing up to 30-150 different differentiated parent bodies. Several different hypothesis were put forward to explain the problem. Currently our understanding of the problem shifted from "battered to bits” hypothesis to postulates such as partial differentiation (Weiss and Elkins-Tanton 2013, Thomas et al. 2017), surface alternation (though space weathering and shock impact processes) and new formation theories (Bottke et al. 2006). In this talk I will review the current meteoritic and observational evidence of differentiation in the Solar System and the various possible explanations for the mismatch between meteoritic and observational indicators.
Recent Results in Family Identification
Kazantsev, Anatolii
To select the asteroid families more clearly the D(a) distribution of asteroid sizes by their semimajor axes and the N(p) distribution of the number of steroids by their albedo values for individual families were used. The families identified by others scientists using the Hierarchical Clustering Method were analyzed with the use of these distributions, and correctly and incorrectly isolated families were found. A reduction in the mean albedo with increasing semimajor axis is observed for almost all correctly identified families that are not truncated by resonances. This reduction is statistically significant for the majority of these families. Not a single family exhibits a statistically significant increase in albedo. This points on an action of a specific nongravitational effect (NGE) in the asteroid belt results in the spatial separation of asteroids with different albedos. Besides, another convincing argument for reality of the NGE is found. There were calculated differences (da) of semimajor axes of asteroid orbits for 10 years, which are not caused by the gravitational influence of the planets. It was obtained a significant dependence of the da values on the asteroid albedos. Confirmation of the NGE reality also serves as confirmation of decreasing in the mean albedo with increasing semimajor axis for separate families.
Families among the Hildas and Trojans
Vinogradova, Tamara
A search for asteroid families among the Hildas and Trojans is especially difficult because these asteroids move in resonance zones. The Hildas are in the 3:2 and Trojans in the 1:1 mean motion resonances with Jupiter. In resonance regions the calculation of proper elements by analytical methods is complicated. Modern methods calculate proper elements in these zones by means of numerical integration on a very long time scale. On the other hand, there is fundamentally different approach to this problem. We propose the empirical method of proper elements calculation. The method uses distributions of osculating orbital elements, such as the longitude of ascending node - inclination and the longitude of perihelion - eccentricity. These distributions make it possible to obtain long-period perturbations of the inclination and eccentricity. A removal of the long-period terms was done using the coordinate transformation formula. The derived proper elements are accurate enough to search for asteroid families. The empirical method may be used in both non-resonant and resonant regions. It is an advantage of this method. Moreover the procedure of computing proper elements by the empirical method is not time-consuming. All available now multi-opposition asteroids were used for calculation. Increased number of faint asteroids enables us to get new reliable results in such complicated regions as the Hildas and Trojans. As a result two robust asteroid families of (1911) Schubart and (153) Hilda were identified in the Hilda-group. And four families were found among L4-trojans: (3548) Eurybates, (2148) Epeios, (624) Hektor, and (9799) 1996RJ.
The ages of asteroid families
Spoto, Federica
The computation of the ages of asteroid families is a fundamental step in understanding the history of our Solar System. At present we know roughly 120 families, and almost one-third of the asteroids in the main belt are members of an asteroid dynamical family.Despite the fact that asteroid families have been discovered one hundred years ago (Hirayama 1918), the age computation has been developed only in the last decades, due to our increased knowledge ofnon-gravitational perturbations. The most important non-gravitational force is the Yarkovsky effect, a subtle non-gravitational phenomenon related to the anisotropic thermal emission of Solar Systemobjects. The Yarkovsky effect causes a secular semi-major axis drift, which has been sculpting the main belt for millions of years (Vokrouhlický 2000).I present the state of the art and the future perspectives of the computation of the ages of asteroid families, pointing out the main results achieved, but also the principal limitations in our attempted chronology of the solar system, mostly due to the lack of knowledge of asteroid physical properties.
Modeling of Family-Forming Impacts
Jutzi, Martin
The observed asteroid families are composed of bodies that are thought to be a result of energetic collisions which lead to catastrophic disruptions of their larger parent bodies. Asteroid families such as the one associated with asteroid Vesta can also be created by less energetic cratering impacts.As a complement to experimental and theoretical approaches, numerical modeling has become an important component to study asteroid collisions and impact processes. The formation of asteroid families as a result of large-scale disruptions consists two distinct phases: the impact / fragmentation phase, and the gravitational reaccumulation phase. They are characterized by very different time scales and therefore require a hybrid modeling approach, coupling shock-physics code models and gravitational N-body methods.In numerous modeling studies, the effects of target properties and impact conditions on the outcome of family-forming collisions have been investigated. Monolithic, pre-shattered, micro-porous or rubble-pile targets have been studied and a range of target sizes has been explored. The comparison between simulation outcomes for various kinds of parent-body structures and the actual family properties can help to constrain the internal properties of the parent body of the considered families.We will discuss recent modeling approaches and results of studies of catastrophic disruptions and family-forming collisions. Various numerical methods and material models used in the shock-physics codes will be presented and the inherent model uncertainties will be discussed.
Asteroid Fragmentation as revealed by Families
Cotto-Figueroa, Desiree
Determining the strengths of asteroidal materials and understanding how those strengths will scale with size is important for constraining the internal structure of the parent bodies of asteroid families. Modeling studies of asteroid disruption and fragmentation use strength and fracture properties derived from experiments using analog materials such as basalt, even though these are unlikely to be representative of asteroid materials. Meteorites, which derive from asteroids, provide a unique opportunity to study the fundamental physical and mechanical properties of asteroidal materials. But to date, few direct studies of physical properties have been conducted on these materials. Given the paucity of relevant strength data, the scale-varying strength properties of meteoritic and asteroidal materials are poorly constrained. Here, we present the first scale-dependent measurements of meteorite strength. Our goal is to span the spatial scale of the mechanical and physical properties of meteorites, and to amass a comprehensive dataset that enables realistic simulations of asteroid fragmentation.
The Hirayama families of asteroids: Various aspects of discovery
Yoshida, Seiko
Japanese astronomy after the Meiji Restoration grew in the cradle of geodesy while joining the network of IAG. Hirayama Kiyotsugu (1874-1943) began his career as a researcher with latitude observation and then exited from this field, and pioneered a new field of dynamical research of asteroids in japan. October in 1918, his paper “Groups of asteroids probably common origin” was published in the Astronomical Journal. He tackled the challenges of celestial mechanics that attracted astronomers’ attention.We have discussed three aspects of the likely background that led him to the discovery of the asteroid families, but they were all classified as scientific activities: 1. mathematical and astronomical training through Tisserand’s book Traité de Mécanique Céleste before he went to the USA; 2. new interests motivated by E. W. Brown (1866-1938); 3. the Kirkwood gaps as a clue to the discovery.It is better for the discovery story to be told not only from those aspects but also from other aspects: 1. social situation when he was promoted to associated professor at the Tokyo Imperial University (1906); 2. Circumstances that he was to engage in computing ephemerides of the moon and others at the Tokyo Astronomical Observatory (1908). With this calendar-making, he got a chance to learn the latest lunar theory under Brown.We have pointed out that his discovery were seen by the majority of Japanese scientific community as being simply statistical. We said that such a view was influenced by his statistical analysis of the Kimura’s Z-term. Finally, we consider the meaning of "statistical" of the time and feed back to the story.S. Nakayama (1928-2014) stated Hirayama’s researches were based on statistics as well as on the known principles of celestial mechanics. Y. Kozai (1928-2018) kept emphasizing Hirayama had used the proper elements instead of the osculating elements to identify families.Ref. Highlighting the History of Astronomy in the Asia-Pacific Region (2011)
Haumea and its icy family
Licandro, Javier
The dynamical identification and physical characterization of numerous collisional families in the asteroid main belt (MB) has had a remarkable impact on the understanding of the formation and evolution of asteroids. It proved the importance that collisions had in the region. Collisions have also had a fundamental role in the evolution of the trans-neptunian objects (TNOs), but the dynamical identification of a collisional family in the trans-neptunian belt (TNb) is considerably more difficult than in the MB.Up to now, only one collisional family has been identified in the TNb, the Haumea family. Haumea is not only one of the fourth largest TNOs, is the largest member of a group of TNOs that present very similar dynamical and spectroscopical properties that appears to be fragments of the ejected ice mantle of Haumea.In this work I will summarize the very unique know properties of Haumea including: a surface covered almost by water ice; a high albedo; a very elongated shape; a rapid rotation; two known moons; and a ring. I will also summarize the properties of its icy family members. I will finally discuss the implications of this asteroid family, the feasibility of identifying new families in the TNb, and the perspective of future studies using Earth-based thirty meter class telescopes (like the ELT and TMT), and the James Webb Space Telescope.
Asteroid Family-Forming Events and Their Connections to Impact Showers
Bottke, William
Asteroid collisional breakups and the dynamical evolution of their fragments have helped shape the main asteroid belt. These processes not only give us insights into planetesimal formation/evolution but they also help us understand terrestrial planet impact histories. We can broadly characterize these processes over the last few Gyr with this sequence of events: (1) A large asteroid in the main belt undergoes a catastrophic collision that creates a swarm of smaller fragments close to the impact site.. (2) Family members with diameter D < 30 km obtain mobility from non-gravitational forces such as the Yarkovsky effect, defined as a thermal radiation force that causes asteroids to drift in semimajor axis via the absorption and reemission of sunlight. Smaller fragments drift faster than bigger ones, such that the family’s orbital distribution can be used “as a clock” to date the time of the break-up. (3) Some fragments migrate into planetary resonances that drive them onto planet-crossing orbits. A few go on to strike the terrestrial planets. The history of asteroid breakups, therefore, is partially constrained by our knowledge of near-Earth objects and craters on inner solar system worlds (and vice versa). Here we will discuss recent advances in our attempts to link changes in the impact history of the Earth and Moon to the evolution of specific asteroid families (e.g., Baptistina, Flora, Gefion). A few large and well-placed families are likely connected to substantial changes in the impact flux of inner solar system worlds, but the precise timing and magnitude of these impact showers, as well as the evidence supporting changes in the impact flux, remain interesting issues for on-going work.
The History of Asteroid Family Identification
Kneževic, Zoran
Asteroid families are discovered a century ago by Kyotsugu Hirayama (1918). In this paper a review of the history of this discovery and of the asteroid family identification in general is presented. Starting from Hirayama’s work and his ingenious interpretation of these findings, we briefly mention several important subsequent developments which brought us to the present day’s understanding of families as key evidence of the solar system collisional evolution.I describe here the two intertwined threads: the development of theories of asteroid motion and the computation of their proper orbital elements that serve as parameters for classification into families, and of the methods of family identification themselves. In the period following Hirayama’s pioneering work and lasting to the mid-fifties of the 20th century, the efforts were mostly devoted to identification of families by using an ever increasing catalog of known asteroids, but also to attempts towards understanding of origins and properties of families; the next few decades were characterized by a gradual increase of interest in the families and by a growing understanding of their role in the big picture of the solar system’s evolution; this eventually resulted in an explosion of work and results in the subsequent period, a contemporary era, that lasts until nowadays.
The system of modified proper elements for the study of very young asteroid families and pairs
Rosaev, Alexey
It is evident that proper elements are not effective in the process of studying very young asteroid families (VYFs) and pairs. Many examples of complex dynamic behavior can be observed in the Datura family [1]. Another noticeable example is the resonant behavior of the node longitude in the Emilkowalski family [2]. Additionally, for a more in-depth study of such young compact asteroid clusters and pairs, the values of orbital elements at the epoch t=0 are very important. All facts above argue in favour of the construction of a specific system of proper elements for VYFs. We suppose the Fourier approximation of backward numeric integration will be sufficient, but with the following modifications: 1) The observed time interval is shorter; no larger than 1.6 Myrs, which may be optimised form case to case. 2) All resonance terms are used. This is important, because the respective periods may be longer than the cluster’s existence so averaging cannot be applied. 3) Similarly, the effect of the most massive asteroids may be expressed in an explicit form. 4) Information concerning the non-gravitation effects (Yarkovsky etc.) is necessary where it is known. In this presentation we show the nonlinear approximation requirement for node longitude and inclination evolution for the (6070) Rheinland and (54827) 2001 NQ 8 pair and some VYFs. We believe that the construction of an established, solid base of modified proper elements can strongly accelerate our understanding of the nature VYFs and pairs. References: Rosaev A., Plavalova E.: (2018) On relative velocity in very young asteroid families., Icarus, accepted. Nesvorny, D., & Vokrouhlicky, D. (2006). New Candidates for Recent Asteroid Breakups. The Astronomical Journal , 132, 1950-1958. doi:10.1086/507989
The method of relative velocity calculation for the study of very young asteroid families and pairs
Plávalová, Eva
Asteroid families are groups of minor planets that have a common origin in a catastrophic breakup event. Very young compact asteroid clusters are a natural laboratory in which to study the impact processes and dynamics of asteroid orbits. In our paper [1], we define the term very young asteroid families (VYFs), that is to say, younger than 1.6 Myrs, and explain why we have defined this group as being separate from young families (younger than 100 Myr), due to specific characteristics, in particularly, non-gravitational forces which have a minimal effect (which could be negligible) on their dynamics and the role of the initial conditions in VYFs as being more significant. We present the method employed to calculate the components of relative velocity using the results of backward numerical integration as a method of studying VYFs. For the most part, the calculation of VYFs' normal component of relative velocity using backward numerical integration, exhibited a clear, deep minimum which was close to the breakup epoch. In this presentation, we demonstrate the above method on two VYFs and prove its relevance for studying young asteroid pairs. We have exemplified that the z-component of relative velocity may prove to be a powerful and useful criterion for VYF age estimations. References: Rosaev A., Plavalova E. (2018). On relative velocity in very young asteroid families, Icarus, accepted.
Asteroid families and the next generation surveys
Masiero, Joseph
In the century since the first realization of the existence of asteroid families, our understanding of them has grown through surveys of their astrometric, photometric, spectroscopic, polarimetric, and radiometric properties. In particular, the last twenty years have seen a rapid expansion of our knowledge as surveys for near-Earth objects such as the Catalina Sky Survey and Pan-STARRS have boosted our catalogs of known objects, and astrophysical surveys such as SDSS and WISE have incidentally measured properties of large fractions of the known asteroid population. Thus, we should expect that future large-scale surveys will similarly advance the scientific understanding of asteroid families. I will discuss some of the upcoming surveys that will be particularly beneficial for family studies, as well as longer-term possibilities that may come to pass in the next century of asteroid family science.
Dynamical evolution of asteroid pairs on close orbits
Kuznetsov, Eduard
We apply natural metrics (Kholshevnikov metrics) defined in the space of Keplerian orbits to search for asteroids in close orbits. First, we use as a metric the distance between two orbits in the five-dimensional space of Keplerian orbits. Then, we apply the distance in three-dimensional factor-space of positional orbital elements. We have identified new asteroid pairs with a possible common origin. Once the asteroid pairs candidates are identified, we analyze their dynamical evolution. We consider orbital evolution of two the tightest pairs: (63440) 2001 MD30 and (331933) 2004 TV14, and (355258) 2007 LY4 and (404118) 2013 AF40. To carry out high accuracy numerical simulation is necessary to take the Yarkovsky effect into account. We numerically integrated the orbits of pairs with backward in time (a time span of 20 kyr) with the code known as Orbit9. The numerical integrations were made taking the nominal orbits given by AstDyS database as initial conditions. Showed that the Yarkovsky effect is required to take into account accurately to carry out precise simulation of dynamical evolution of the asteroid pairs. Determination of physical and rotational parameters of asteroids is needed to solve this problem. This research is part of the Kourovka Asteroid Pairs Research (KASPAR) project that started in Kourovka Astronomical Observatory of the Ural Federal University in September 2017. The KASPAR project includes not only the observational side, involving astrometric and photometric observations of asteroid pairs on close orbits, but also its theoretical and computational counterpart involving numerical simulations of the dynamical evolution of such candidate pairs that take the Yarkovsky effect into account to calculate their proper orbital elements. The reported study was funded by RFBR according to the research project no. 18-02-00015.
The ages of young asteroid families
Carruba, Valerio
Young asteroid families are families that formed in timescales of 20 Myr or less. Because of their very young age, chaotic dynamics did not had enough time to erase traces of the event that formed the family. The longitudes of node (and, in some cases, of pericenter) of the family members converge to within a very limited range when integrated backward over the estimated age of the family. The Backward Integration Method (BIM) allows to i) identify family members and ii) estimate the family age with a precision not available for other, older asteroid groups. While this behavior was studied and understood in previous works, discoveries of new asteroids over the last ten years dramatically improved the number of young asteroid families.Here we took advantage of this new larger sample to i) refine the ages of previously studied families, such as Karin, a subfamily of the Koronis asteroid family, Veritas, and Nele, and ii) to obtain preliminary estimates of the age of newly identified asteroid families. Out of 28 asteroid families to our knowledge not previously studied with BIM, we identified 4 groups for which we observe a possible convergence of the angles, and for 3 of them, those of Jones, Kazuya, and 2001 GB11, we obtain an age estimate at 1-sigma probability confidence level.
Cladistics as a tool in Asteroid Taxonomy
Holt, Timothy
Cladistics is traditionally used in the biological sciences to examine the relationships between organisms, commonly referred to as the ‘tree of life’. Recent works in galactic taxonomy, stellar phylogenetics and satellite classification have expanded the technique into Astronomy, collectively called Astrocladistics. The advantage of this method over other analytical techniques is the inclusion of objects with limited information. A full data-set can then be used without truncation.Our aim is to present how cladistics may be used to study asteroid taxonomy. We start by using the Jovian Trojan asteroids as an example population. The Jovian Trojan asteroids are two swarms of captured asteroids, located at the L4 and L5 Lagrange points of Jupiter. The Jovian Trojans provide a test case, as several have well known characteristics, while the majority have limited information available, with the complete population of computationally manageable size.The cladistical method involves the use of algorithms to link possibly related objects in a parsimonious fashion. The results are presented as a dendritic tree, where related objects are closer to one another. Using the cladisical method, we classify the Jovian Trojan swarms, using the inherent characteristics of the asteroids. The resulting taxonomic system can then be compared with existing classifications and identified dynamical families.We present preliminary results from this study, with an indication of how the cladistical technique could be expanded to larger data-sets, and used in the of future asteroid taxonomy.
Color study of asteroid families within the MOVIS catalog
Morate, David
The aim of this work is to study the compositional diversity of asteroid families based on their near-infrared colors, using the data within the MOVIS catalog. As of 2017, this catalog presents data for 53436 asteroids observed in at least two near-infrared filters (Y, J, H, or Ks). Among these, we find information for 6299 asteroids belonging to collisional families with both Y-J and J-Ks colors defined. The work presented here complements the data from SDSS and NEOWISE, allowing detailed description of the overall composition of asteroid families. We derived a near-infrared parameter, the ML*, that allows us to distinguish between four generic compositions: two different primitive groups (P1 and P2), a rocky population, and basaltic asteroids. We conducted statistical tests comparing the families in the MOVIS catalog with the theoretical distributions derived from our ML*, to classify them according to the aforementioned groups. We also studied the background populations, to check how similar they are to their associated families. Finally, we used this parameter in combination with NEOWISE and SDSS to check for possible bimodalities in the data. We found 43 families with ML*err<0.071 and with at least 8 asteroids observed: 5 classified as P1, 10 classified as P2, 19 families associated to the rocky population, and 9 families that were not linked to any of the previous populations. In these cases, we compared our samples with different combinations of these theoretical distributions, to find the one that best fits the family data. We also show, using the data from MOVIS and NEOWISE, that the Bapistina family presents a two-cluster distribution in the near-infrared albedo vs. ML* parameter space, which might be related to a common differentiated parent body. Finally, we show that the backgrounds we defined seem to be linked to their associated families.
Young asteroid clusters and their relation to asteroid pairs
Pravec, Petr
In our recent work (Icarus 304, 110, 2018), we studied the membership, size ratio and rotational properties of 13 asteroid clusters consisting of between 3 and 19 known members that are on similar heliocentric orbits. By backward integrations of their orbits, we confirmed their cluster membership and estimated times elapsed since separation of the secondaries (the smaller cluster members) from the primary (i.e., cluster age) that are between 105 and a few 106 years. We ran photometric observations for all the cluster primaries and a sample of secondaries and we derived the cluster size ratios and primary spin periods. We found that 11 of the 13 clusters follow the same trend of primary period vs mass ratio as asteroid pairs that was revealed by Pravec et al. (Nature 266, 1085, 2010). We generalized the model of the post-fission system for asteroid pairs by Pravec et al. (2010) to a system of N components formed by rotational fission and we found excellent agreement between the data for the 11 asteroid clusters and the prediction from the theory of their formation by rotational fission. The two exceptions are the high-mass ratio (q > 0.7) clusters of (18777) Hobson and (22280) Mandragora for which a different formation mechanism is needed. Two candidate mechanisms for formation of more than one secondary by rotational fission were published: the secondary fission process proposed by Jacobson and Scheeres (Icarus 214, 161, 2011) and a cratering collision event onto a nearly critically rotating primary proposed by Vokrouhlicky et al. (Astron. Astrophys. 598, A91, 2017). We found also certain further interesting properties and features of the asteroid clusters that place constraints on the theories of their formation, among them the most intriguing being the possibility of a cascade disruption for some of the clusters. I will compare the asteroid clusters with asteroid pairs and active asteroids, as all these asteroid kinds appear to be related to certain degree.
Cratering families: identification and collsional models
Milani comparetti, Andrea
Given the synthetic proper elements catalogs now available for 540,000 asteroids, we are maintaining a periodically updated family classification currently with 118 families and more than 130,000 members. This classification uses only the proper element: by using the absolute magnitude and the available albedo data it has been possible to estimate more than 50 family ages. Because the new families contain many more smaller asteroids, many of the families are found to be of the cratering type, that is with a substantial parent body and many small fragments. For cratering families we propose a quantitative definition. There are many cratering families with two separate ages, thus resulting from two collisional events; this is not a surprise. We list several new cases of cratering families, previously ignored, and include results on cratering families obtained by using resonant proper elements, such as the Astraea family. Our methods for both computation of proper elements and dynamical family classification are rigorous. However, given all the available data, our classification results do provide a reliable history of the collisional evolution of the asteroid main belt? Apart from the only two cases (Vesta, Ceres) for which spacecraft data can be used, the only check about the reliability of our proposed collisional history is to derive a realistic model of the original dispersion of velocities. Such a model for cratering families requires that the original velocity field is not isotropic and that the ejection velocities are not large with respect to the escape velocity from the parent body. Moreover, correlations between the velocities and the spin states of the fragments must be taken into account. We show that an initial velocity field can be reasonably well reconstructed in many cases, by accounting for the post-formation dynamical evolution of family members.
Numerical Simulations of Catastrophic Impacts Resolving Shapes of Remnants
Sugiura, Keisuke
Asteroids in an asteroid family are considered to be fragments formed through collisional destruction of single parent body. Thus, statistical information obtained from an asteroid family such as size or rotation rate distribution may provide some insights into the physics of collisional destruction. Asteroids have another unique statistical information, that is, irregular shapes. Irregular shapes of asteroids are formed through reaccumulation of fragments after catastrophic disruption due to asteroidal impacts and can be important information to investigate the origin of that family. In this study, we perform high resolution simulations of asteroidal collisions, and reproduce statistical information of many asteroids formed through catastrophic disruption. We use Smoothed Particle Hydrodynamics (SPH) method for elastic dynamics (Libersky and Petschek 1990) with the self-gravity, the model of fracture of brittle solid (Benz and Asphaug 1995), and the model of friction of disrupted material (Jutzi 2015). Our simulation code is parallelized using MPI and OpenMP with the aid of Framework for Developing Particle Simulator (FDPS: Iwasawa et al. 2015, 2016). We conduct some simulations of catastrophic impacts. An equal-mass impact of asteroids with radii 50 km and the impact velocity of 350 m/s produces the largest remnant with mass of about 0.1 of that of an initial asteroid, i.e., catastrophic disruption. Exponent of the cumulative size distribution is about -2.4. The rotation rate distribution has the peak at 10 hours, and most remnants have rotation rate between 5 and 20 hours. Most of remnants have spherical shapes, and bilobed shapes are also formed through coalescence of two spherical remnants. We find five bilobed remnants with the ratio between intermediate and major axis length less than 0.6. We also find a trend that smaller remnants have larger axis ratios, i.e., rounder shapes.
Are large main belt asteroid families homogeneus?
Slyusarev, Ivan
Asteroid families were formed during collisional disruptions and their physical properties provide unique information about internal material of the parent bodies. We have performed an analysis of the homogeneity of physical properties of 56 large main belt asteroid families from Nesvorny database. Using data on albedo (p) and color index (a*) from WISE and SDSS databases we found out, that all points on “color (a*) - albedo (p)” plots for all families can be separated in to three subgroups: I (p<0.1; a*<-0.05); II (0.1<p<0.25; a*<0.05)="" and="" iii="" (p="">0.15; a*>0.05). The dark subgroup I and high albedo subgroup III are presented in all bimodal families with some exceptions. Only two families include all three subgroups. Analysis of possible taxonomic interpretation of these three subgroups gives a clear result only for subgroup III which is linked to the S-type asteroids. Subgroup I is a mixture of dark asteroids that belong to F, C, P, D types. Subgroup II may be consistent with the M-type asteroids. Analyzed families are divided into homogeneous: (27 families); bimodal (13 families); and trimodal families (2 families Vesta and Flora). Families Aeolia, Xizang, Aeria and 15477 have not show bimodality in color and albedo distribution, but they contain asteroids that are intermediate between low (I) and middle-albedo (II) subgroups. All these families are located very near 2.7 au. We have analysed the distribution of proper elements for subgroups and V-shape plots for each bimodal family and found that several families consists of two overlapping families as in the case of Nisa-Polyana. Our analysis have shown that the significant fraction (25%) of the considered families are inhomogeneous in terms of albedos and colours. A fraction of the dark subgroup (I) in bimodal families is not negligible (10-30%).</p<0.25;></p>
New spectra for the Eureka family's smaller members
Polishook, David
Seven asteroids form the Eureka orbital family, named after the largest member, which orbits within the fifth Martian Lagrange point. Dynamical calculations (e.g. Cuk et al. 2015) argue that these seven shared a common origin – a progenitor that disintegrated by multiple YORP-induced spin-ups. Spectral observations of the largest three objects suggest similar olivine-rich bodies (e.g. Polishook et al. 2017), supporting the shared origin theory. Since olivine-rich bodies (a.k.a. A-type asteroids) are extremely rare in the main belt (0.4%; DeMeo & Carry 2013) and since Mars' mantle consist of olivine, we suggested that Eureka and its family are impact ejecta excavated from Mars mantle, and thus may be the only known asteroids linked to a terrestrial planet origin. We will present new measurements of additional family members, scheduled to take place in the coming spring, that will support or dispute previous conclusions. In addition, Cuk et al. (2015) noticed that dynamically, asteroid 2011 SC191 is the most distant family member, and suggested it was ejected from Eureka before the other members. Olivine is highly affected by the space weathering process, which causes an increase in spectral slope (Brunetto et al. 2006). Detecting a reflectance spectrum for this asteroid with a higher spectral slope has the potential to provide constraints on the timescale of the space weathering process on olivine.
Spin-axis distribution of members of asteroid families
Durech, Josef
The spin state of members of asteroid collisional families is an important physical property that is affected by the initial conditions (presumably isotropic distribution of spin vectors) and by the secular evolution mainly due to the YORP effect. The rotation period, the direction of the rotation axis, and the global shape model can be derived for individual asteroids from their disc-integrated photometry by the lightcurve inversion method. This way, models of about 1650 asteroids have been derived, out of which ~600 belong to families. For other ~340 family members we have at least information about the ecliptic latitude of their spin axis, which allows us to distinguish between prograde and retrograde rotation. We will present our analysis of the distribution of spin axis of members of the largest asteroid families and discuss the results. In general, the observed distributions are in agreement with the theoretical expectation that the V-shape in the semimajor axis a vs. size D parameter space is caused by the Yarkovsky effect, which is dependent on the spin axis orientation. Therefore, asteroids that have larger semimajor axis than the center of the family are preferentially prograde, while those with smaller a are retrograde. In principle, the information about the spin axis distribution and its dependence on size is another constraint that can be used for family age estimation.
The Near Earth Asteroid associations
Jopek, Tadeusz
Clustering among the small bodies of the Solar System is a longstanding problem. Prominent peaks in histograms of the orbital parameters suggests existence of the groups of objects of potential common origin, just like in case of the meteoroid streams and the main belt asteroid families. These concepts are accepted beyond the doubts.Similarity between the NEAs orbits was discovered ~40 years ago. Drummond, among 708 NEAs found 14 associations of 4-25 members. However, he came to a conclusion that many groups might be attributed to chance alignments. Further studies made by different researchers did not bring conclusive results. Therefore, whether the NEAs associations of a common origin exist is an open question.In this study we made an extensive search for associations amongst ~17000 NEAs. We used a few D- functions and rigorous cluster analysis approach.We have found a dozen groups of 30 or more members. Statistical reliability of this finding is quite high, 95%. However, by the cluster analysis technique one can not decide about a common origin of these associations. One need additional test, e.g. a study of the dynamical past history of each group. We undertaken such study.Nevertheless, irregardless of their common origin, the existence of the NEAs associations is undisputed fact, they were found just like one detects the meteoroid streams. The origin of them is an open concern.Recognition of the NEAs associations is very important because their serious thread to the Earth. Analogously to meteoroid streams, each year the Earth is almost crossing the orbits of each association. We do not know how many members consists each group, perhaps 50, perhaps 500?.To facilitate monitoring of an association we have calculated coordinates of its theoretical radiant and a calendar date of its potential activity. Such information allows the observers better planning of the observation campaigns of these hazardous objects.
Collisions, reaccumulations and asteroid families: tracking shapes and spins
Michel, Patrick
Asteroids that suffer a catastrophic impact can produce a huge array of fragments that may re-accumulate into a family of smaller asteroids owing to their self-gravity. Numerical models of this process can match many properties of observed asteroid families. The shape and spin of the re-accumulated fragments has not typically been modeled, as most previous efforts depend on techniques that merges particles to save computational cost. Here we describe two new modeling techniques that allow for the inclusion of shape and spin during the re-accumulation process and for comparing with actual family members' properties. Simulations of asteroid collisions are typically done in two stages - a hydrodynamics code is used to model the fragmentation of a target asteroid, and a gravitational N-body code is used to model the gravitational re-accumulation of the fragments. The challenges for the N-body side include managing the hand-over from the hydrodynamic code to the N-body code, resolution and time-step issues that arise because of some very high particle velocities. We used multiple techniques to account for shapes and spins, including a resolution reduction routine and bonded-aggregates that freeze shapes together. These efforts, and a handful of very expensive soft-sphere discrete element models, have opened new doors in modeling asteroid re-accumulation and asteroid family formation.The science questions addressed with these new capabilities relate to the full size spectrum of asteroid family members. Results show that more readily elongated objects form especially when disruptions are near but below the catastrophic threshold, but a greater diversity of both small and large bodies is produced during more energetic events. Constraints and tests for these models can be obtained from near-Earth asteroid shapes, lightcurves of small main belt asteroids and the shape and spin of large main belt asteroids with satellites, which typically have rapid spin and elongated shapes.
The common origin of family and non-family asteroids
Dermott, Stanley
A century ago Hirayama discovered the clustering of asteroid orbital elements, revealing that some asteroids are in families originating from the disruption of a few large primordial bodies. However, the asteroids now classified as family members constitute a minority of the asteroids in the main belt. Here we show that the non-family asteroids in the inner belt have orbital inclinations that increase and orbital eccentricities that decrease with increasing asteroid size. These correlations can be accounted for if both the non-family and the family asteroids originate from the disruption of a small number of large asteroids. Separating the non-family asteroids into halo and non-halo asteroids allows us to estimate that 85% of all the asteroids in the inner main belt originate from the Flora, Vesta, Nysa, Polana and Eulalia families with the remaining 15% originating from either the same families or, more likely, a few ghost families. These new results imply that we must seek explanations for the differing characteristics of the various meteorite groups and the near-Earth asteroids in the evolutionary histories of a few, large, precursor bodies. These results also support the model that asteroids formed big through the gravitational collapse of material in a protoplanetary disk.
PRIMitive Asteroids Spectroscopic Survey - PRIMASS: Current Status
de Leon, Julia
Primitive asteroids are considered transitional objects between rocky and icy bodies. They are characterized by their low albedo in the visible and their abundance in carbon and organic compounds. They are also expected to be volatile-rich, with a certain amount of hydrated minerals on their surfaces. The study of these bodies is crucial to understand the nature of volatile and organic materials in the early Solar System.At present, two primitive near-Earth asteroids (NEAs) are targets of sample-return missions: (101955) Bennu, target of NASA OSIRIS-REx and (162173) Ryugu, target of JAXA Hayabusa2. Characterizing the populations from which these NEAs might originate in the main belt will enhance the scientific return of the missions.In 2010, we started a spectroscopic survey in the visible and near-infrared to characterize primitive asteroids. Our PRIMitive Asteroids Spectroscopic Survey (PRIMASS) uses a variety of ground-based facilities. Most of the visible spectra have been obtained using unique capabilities of the 10.4m Gran Telescopio Canarias (GTC), while near-infrared spectra are obtained with the 3.6m Telescopio Nazionale Galileo (TNG), both located at the El Roque de los Muchachos Observatory (La Palma, Spain). We also use the 3.0m NASA Infrared Telescope Facility (IRTF) on Mauna Kea (Hawaii, USA) and the Southern Astrophysical Research Telescope (SOAR) at Cerro Pachon (Chile).Up to now, PRIMASS contains more than 500 spectra. Results on the Polana-Eulalia complex and the Erigone, Sulamitis and Clarissa inner belt families have been already published, and spectra have been obtained from members of the Klio, Chaldaea, Svea, and Chimaera families. In the outer main belt, we have obtained spectra of asteroids in five different but related groups of primitive bodies: the Hygea, Themis, and Veritas families, and also the Cybeles and Hildas dynamical groups. We present here the current status of our on-going survey.
Asteroid classification in the Bus-DeMeo taxonomy with limited wavelength range
Penttilä, Antti
The taxonomic classification of asteroids is based on the reflectance properties of the asteroid’s surface regolith, giving information on the composition of the surface. The asteroid families are based on the orbital elements with the assumption of a common parent body. Without the exception of the differentiated parent body of the Vesta family, the family members should share the composition and therefore also the taxonomic class.Bus-DeMeo taxonomy is based on the principal component analysis (PCA) transform of the asteroid spectrum in the wavelength range of 450 (or 850) – 2450 nm. After the PCA transform there is a flowchart for the actual classification of an object. However, there are many telescopes with spectrographs capable to observe only a limited range of the required wavelengths. One example is the Gaia space telescope, which observes asteroid spectra from 330 to 1050 nm.While the Gaia mission aims to create its own taxonomic classification system using the abovementioned wavelength range, the ability to assign a Bus-DeMeo taxonomic class for objects that are observed a with limited (compared to 450–2450 nm) wavelength range would be useful. For this reason, I evaluate the possibilities of automatic Bus-DeMeo classification of observations with limited wavelengths that is based on the linear discriminant analysis transform of the spectrum and the naïve Bayes classifier. The fully automated classification algorithm can be derived on the fly for specific wavelengths. I will review the performance of this approach.
Julia asteroid family versus adaptive-optics observations of (89) Julia
Broz, Miroslav
There is a small family in the vicinity of (89) Julia. Our preliminary analysis assured the family can be associtated with this 140-km asteroid, given its large escape velocity of about 100 m/s. We then constructed an N-body orbital model, Monte-Carlo collisional model, and an SPH simulation of the respective event. We fully explain the distribution in the space of proper elements, and determine the dynamical age to be 10 to 100 Myr. The size-frequency distribution clearly corresponds to a cratering event, with the (transient) crater size reaching 60 km according to the SPH.This is spectacularly confirmed by direct observations of (89) Julia. ESO VLT/SPHERE/ZIMPOL adaptive-optics instrument allowed us to acquire 40 images, which were deconvolved using a stellar PSF, and Mistral algorithm. The pixel scale is 3.6 mas which correponds to 3 km at the distance of Julia. For the inversion and shape reconstruction, 38 additional light curves, and 2 occultations were used. The resulting shape indicates there are three craters, with rim-to-rim sizes ranging from 40 to 70 km. We can even prefer one of them, because it is located on the southern hemisphere which seems to be more suitably oriented with respect to the family. For the first time, we were able to suggest such a link solely on the basis of ground-based observations. This may be considered a beginning of a new era of asteroid-family studies, which shall include families -> craters identifications.
Families in the Outer Solar System
Ragozzine, Darin
Though Hirayama's discovery of families in the asteroid belt is 100 years old, the discovery of the first collisional family in the outer solar system was only ~10 years ago; in both cases about 1000 objects were known. Unlike asteroid families, Brown et al. 2007's discovery of a collisional family around the dwarf planet Haumea relied heavily on unique spectral features, but Ragozzine & Brown 2007 showed that the family was dynamically clustered. In fact, the family was too dynamically clustered: all objects with the unique spectral features of the Haumea family lie within dv < 150 m/s, which is an order of magnitude tighter than would be expected based on a standard collision on such a large body (escape velocity of ~900 m/s). This mystery persists to this day with several hypotheses proposed (e.g., Schlichting & Sari 2009, Leinhardt et al. 2010, Marcus et al. 2011, Ortiz et al. 2012, Campo Bagatin et al. 2016). With a new calculation of proper elements tripling the number of family members from 8 to ~25 (Maggard & Ragozzine, in prep.), we use a sophisticated model of the three-dimensional distribution of family members to rule out some of the leading hypotheses (Proudfoot & Ragozzine, in prep.). These calculations show that Haumea family members can now be identified dynamically. We also investigate the possibility of using direct backwards integration to determine the age of the Haumea family and to identify new outer solar system families that formed in the last ~GYr (Marcus et al. 2011). Such advanced modeling is possible in the Kuiper Belt because the Yarkovsky effect is unimportant, but many of our statistical techniques could be adapted to great use in studying asteroid families.
KEEP-North: Photometric observations of the Baptistina asteroid family
Kim, Myung-Jin
Study of rotational properties of an asteroid family based on time-series photometric observations offer a unique opportunity to have an insight into collisional process and into the dynamical evolution of the family members. There are a number of observational studies on the rotational properties of old-type asteroid families. Slivan (2002) and Slivan et al. (2003) discovered the spin vector alignment in the Koronis family. Kim et al. (2013) showed that the distribution of rotational periods in the Maria asteroid family is non-Maxwellian. At the same time, a number of young asteroid families have been studied: Yoshida et al. (2016) analyzed photometric data of the Karin family, which is considered to have undergone less collisional and orbital evolution, however they concluded that more lightcurve data are needed to confirm the results.The Baptistina family is one of the typical young asteroid families with an estimated age of about 140-320 Myrs (Masiero et al. 2012); considered to do not have enough time to experience significant collisional and dynamical evolution since it formed. Therefore, it may offer a unique insight into spin rate distribution of relatively fresh fragments and physical mechanism of a family break-up event. Photometric observations of the Baptistina family asteroids were conducted using 0.5 m- to 2 m- class optical telescopes at 6 observatories as part of KEEP-North (Kirkwood Excitation and Exile Patrol of the Northern sky) project. We were awarded a total of 130 full nights every year at BOAO, LOAO, and SOAO for five years beginning in 2016. For this study, we used the SOAO 0.6 m telescope, the BOAO 1.8 m telescope in Korea, the LOAO 1.0 m telescope, the McDonald Observatory 2.1 m Otto Struve Telescope in the United States, the TUG 1.0 m telescope in Turkey, and the NARIT 2.4 m telescope in Thailand. Here, we present our preliminary results from lightcurve analyses of Baptistina family members.
Mapping of the asteroid families
Todorovic, Natasa
In a recent study, we produced clear dynamical maps of some regions in the main belt,which enabled us to observe efficient dynamical pathways along which particles can driftaway in short times.In a similar fashion, we map the regions of two asteroid families: Themis and Hungaria.Clear dynamical maps, computed with a good choice of parameters, will enable us to reexamine the role of the neighboring resonances in the distribution of asteroids inside a family, to identify their erosion routesand to get a refined global picture on their dynamics.
The Martian Trojans and the nearest asteroid family to the Sun
Christou, Apostolos
The Main Asteroid Belt (MB) is replete with families formed by collisions over the lifetime of the solar system. Interior to the MB and at the edge of the terrestrial planet region lie the Martian Trojans, which have likely survived there for ~4 Gyr. Among the ten known Trojans with 16<h<20 exists="" a="" tight="" orbital="" grouping="" composed="" of="" 8="" members,="" which="" the="" largest="" is="" 2-km="" asteroid,="" (5261)="" eureka="" (christou,="" icarus,="" 2013).="" this="" group="" was="" recently="" found="" to="" share="" common,="" olivine-rich="" composition="" (borisov="" et="" al,="" mnras,="" 2017)="" and="" may="" represent="" material="" excavated="" from="" martian="" mantle="" (polishook="" nat.="" astron.,="" 2017).="" “eureka="" family”="" probably="" formed="" not="" by="" collisions,="" like="" most="" asteroid="" families,="" but="" rotational="" spinup="" \&="" fission="" (christou="" 2017;="" polishook="" it="" has,="" therefore,="" more="" in="" common="" with="" so-called="" pairs="" small="" clusters="" similar="" way="" mb="" (pravec="" nature,="" 2010;="" pravec="" there="" is,="" however,="" an="" important="" difference:="" family="" likely="" ~100="" times="" older="" (cuk="" 2015)="" -="" unlike="" where="" identification="" among="" background="" unrelated="" asteroids="" possible="" beyond="" ~107="" yr="" effectively="" empty="" trojan="" clouds="" allow="" identify="" gyr-old="" grouping.="" this,="" proximity="" sun="" and ="" absence="" planetary="" close="" encounters="" make="" these="" bodies="" natural="" laboratory="" for="" studying="" long-term="" consequences="" yarkovsky="" &="" yorp="" effects="" implications="" range="" topics:="" calibrating="" models,="" existence="" primordial="" earth="" trojans="" evolution="" systems="" around="" other="" stars.="" during="" presentation="" i="" will="" review="" recent="" findings="" on="" mars="" family,="" currently="" open="" problems="" what="" we="" stand="" learn="" when="" facilities="" such="" as="" large="" synoptic="" survey="" telescope="" come="" online="" near="" future.<="" p="">
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YORP equilibria: pathways out of YORP cycles
Golubov, Oleksiy
The evolution of small asteroids is governed by the YORP effect, which can include several different varieties: normal YORP (NYORP), tangential YORP (TYORP), and binary YORP (BYORP). The conventional wisdom says that asteroids evolve according to YORP cycles: an asteroid is accelerated by the YORP effect to the disruption limit, forms a binary, then the binary decays, and the asteroid starts a new YORP cycle, with a decreased mass, an altered shape and a small angular momentum. In our talk, we will briefly review this and similar concepts of YORP cycles, describe the general framework in which YORP cycles can be classified, and discuss their generic features. The major part of the talk will be devoted to the ways, in which YORP cycles can be interrupted and YORP evolution of asteroids stopped. Four different mechanisms of such stable equilibria will be considered:1) Equilibrium in NYORP, which can vanish in certain rotation states2) Equilibrium between TYORP and NYORP, which can cancel each other at a certain rotation rate3) For a singly synchronous binary, equilibrium between TYORP, NYORP and tides acting on the primary, and between BYORP and tides acting on the secondary4) For a doubly synchronous binary, equilibrium between NYORP and BYORPFor all the four kinds of equilibrium, we describe their physics and estimate their probabilities. The total of the probabilities is of the order of 10 percent, implying that each asteroid should be caught into an equilibrium after several YORP cycles. Stable asteroids survive, while unstable ones are disrupted and recreated in YORP cycles, until they perhaps become stable, in a process similar to natural selection. The effect of these equilibria should be to slow down the disruption of asteroids in YORP cycles.Ultimately, we see that the YORP effect can deposit asteroids into such stable equilibria, stopping their net YORP evolution. Thus the YORP effect can check itself and minimize its own importance for asteroids evolution.
Multi-band photometry of young family asteroids at Maidanak Observatory
Yoshida, Fumi
We have conducted the multi-band photometry of young family asteroids (Karin, Veritas, Iannini) at Maidanak Observatory, Uzbekistan in 2014 - 2016. Since the members of young asteroid families have little time to undergo significant collisional/dynamical evolutions till now, they probably still preserve properties of the disruption event which they were formed. And they probably still keep the “fresh” (not weathered) surfaces, which is the surface that the space weathering process has not completely altered. With these motivation, we investigated the colors of 17 young family asteroids.
Extremely dark asteroids linked to asteroid families
Belskaya, Irina
Several asteroid families which contain low albedo asteroids were identified in the inner part of the main asteroid belt [e.g. Delbo et al. Science 357, 1026, 2017]. Such families are considered to be primordial ones which constrain planetesimals population. Typically all asteroids with geometric albedo <0.12 are considered as belonging to the low albedo C-complex. The diversity inside the C-complex was not taken into account. However, for the long time it was known a relative abundance of the low albedo F-type asteroids in the inner asteroid belt. A large fraction of the F-asteroids were associated with the Nysa-Polana family [Cellino et al. Icarus 152, 225, 2001]. Recent classifications are not able to distinguish the F-type asteroids from B or C-types. The spectroscopic study of low-albedo asteroids from the Polana-Eulalia family complex was not able to reveal spectral diversity of these objects [De León et al. Icarus 266, 57, 2016; Pinilla-Alonso et al. Icarus 274, 231, 2016]. We will show that polarimetric and photometric measurements give a way to separate low-albedo types. The F-type asteroids revealed particular polarimetric and photometric properties different from those of the B and C-type asteroids. The only plausible explanation of the observed particularity of the F-type asteroids is an assumption of their extremely dark surfaces. Among 24 known F-type asteroids, 13 asteroids have similar proper elements within the Polana-Eulalia complex but they are not considered as family members. Moreover, four F-asteroids were classified as belonging to the Hertha family. We need to take into account a diversity of low-albedo asteroid classes to confirm reliability of low albedo family membership.
Identification of asteroid families older than 2 billion years
Bolin, Bryce
Asteroid families are the remnant fragments of asteroids broken apart by collisions. There are only a few known Main Belt (MB) asteroid families with ages greater than 2 Gyr (Brož et al., 2013; Spoto et al., 2015). Estimates based on the family producing collision rate suggest that the lack of > 2 Gyr-old families may be due to a selection bias in classic techniques used to identify families. Family fragments disperse in their orbital elements, semimajor axis, a, eccentricity, e, and inclination, i, due to secular resonances and the non-gravitational Yarkovsky force. This causes the family fragments to be more difficult to identify with the hierarchical clustering method (HCM), which attempts to find cluster in orbital element space, when applied to family fragments’ elements as the fragments age. We have developed a new technique that is insensitive to the spreading of fragments in e and i by searching for V-shaped correlations of family members in a and asteroid diameter, D. A group of asteroids is identified as a collisional family if its boundary in the a vs. 1/D plane has a characteristic V-shape which is due to the size dependent Yarkovsky effect. The V-shape technique is demonstrated on the known families and families difficult to identify by HCM, and used to discover a 4 Gyr-old family linking most dark asteroids in the inner MB not included in any known family (Delbo' et al., 2017). The 4 Gyr-old family reveals asteroids with D > 35 km that do not belong to any asteroid family implying that they originally accreted from the protoplanetary disk and support recent theories on the formation of asteroids (Morbidelli et al., 2009).
Review of asteroid family and meteorite type links
Jenniskens, Peter
There are now 33 meteorite falls (as of February 1, 2018) that have a documented trajectory and pre-atmospheric orbit. Although there are more different meteorite types than this, the more common types are represented in a handfull of cases each. From that, patterns are emerging that suggest that LL type chondrites originated from a source at the inner edge of the inner asteroid belt, while CM type chondrites originated from a source close to the 3:1 mean-motion resonance. H and L type chondrites originated from more than one source. Those sources are past collisions on large asteroids or members of asteroid families, which offer the highest probability for impacts, that liberated enough material to be sampled here at Earth. Following the collision, the meteoroid orbit evolved for 0.9-100 Ma, but model calculations show that they are expected to impact Earth still with an inclination similar to that of the source asteroid orbit, and a semi-major axis distribution that points to the delivery mean-motion or secular resonance responsible for changing their orbital eccentricity. We will review our current understanding of what asteroid families might be responsible for meteorite types documented so far, present preliminary results of recent falls, and discuss new approaches to increasing the number of documented meteorite falls dramatically by using high-resolution fireball tracking and A.I.-equipped drones to help find meteorites from smaller falls.
Asteroid families formed by tidal destruction of near-Earth asteroids?
Granvik, Mikael
Numerous asteroid families have been identified among main-belt asteroids (MBAs) but none have so far been found among near-Earth asteroids (NEAs; see Schunova et al. 2012, Icarus 220, and references therein). Since a collisional origin for NEA families is unlikely, Schunova et al. (2014, Icarus 238) postulated that tidal disruption during a close planetary encounter (Richardson et al. 1998, Icarus 134) is the primary mechanism that produce NEA families. Schunova et al. (2014) also showed that the decoherence time of tidal disruptions is too short to allow identification of NEA families among the currently known NEAs. However, the increase in the rate of NEA discovery in the future can eventually make it possible to identify NEA families formed in tidal-disruption events. The problem is that we have so far only had very limited and indirect evidence of tidal disruptions actually happening. By comparing predicted distributions of NEA orbits and absolute magnitudes with observations by the CSS during 2005–2012 Figure 1 in Granvik et al. (2016, Nature 530) shows that the model underpredicts the number of NEAs with perihelion distance coinciding with the semimajor axes of Venus and the Earth. A detailed analysis of the orbital and absolute-magnitude distributions of the excess NEAs shows that their characteristics agree with the prediction for tidal disruptions and cannot be explained by selection effects or orbital dynamics (Granvik and Walsh, in preparation). We have thus found the population-level evidence for tidal disruptions that we have lacked so far. It is likely that detailed physical modeling of tidal disruption will allow us to place useful constraints on asteroid interior structure using data from, primarily, ongoing and planned NEA surveys.
Formation of families of small asteroids in light of latest research on the active asteroid P/2012 F5
Drahus, Michal
Since their discovery by Hirayama (1918, AJ 31, 185), asteroid families have been traditionally considered the products of collisions between asteroids. However, this long-standing view proves to be highly incomplete in light of the latest research conducted by our team. Using the Keck II telescope on Mauna Kea (Hawaii, USA), we have discovered several ultra-young fragments of the active asteroid P/2012 F5 and determined its rotation rate (Drahus et al. 2015, ApJL 802, L8). Thus, P/2012 F5 has become the first object in the Solar System with both of the properties robustly measured. Our research shows that the rotation rate of P/2012 F5 is the highest known among active asteroids (P = 3.24 hr) and high enough to explain the fragmentation of the object through rotational instability. Subsequent observations made with the Hubble Space Telescope show that P/2012 F5 has just formed a new asteroid family, and that it is likely the first identified asteroid family to originate from rotational fragmentation, rather than a collision with another body (Drahus & Waniak 2016, 48th DPS meeting, 522.03). The obtained results allow asteroid families to be divided into two separate categories: the century-old class of collisional families, and the new class of rotational families. The latter category generalizes the recently identified class of asteroid pairs – also formed by rotational break-up (Pravec et al. 2010, Nature 466, 1085), which we recognize as rotational families with two known components. Strengthless asteroids that are, at the same time, small enough to be susceptible to YORP spin-up, may undergo rotational fragmentation on a massive scale, thus constituting a significant source of families of small asteroids. The number of such families is likely to rise with an increase in the sensitivity of large-scale sky surveys.