Dynamical Evolution Of Asteroids Research Articles

Analytic Model for Tangential YORP

Abstract The tangential YORP effect (TYORP) plays a significant role in the dynamical evolution of asteroids, and up to now has only been studied numerically. This paper describes the first analytic model of the TYORP effect. Although the model rests on numerous physical and mathematical simplifications, the final analytic expression for TYORP is found to be in agreement with the results of rigorous numeric simulations to the accuracy of several tens of percent. The analytic expression obtained is used to estimate the TYORP produced by the non-flat surface of regolith—a contribution to TYORP that has never been considered. It is found that the contribution to TYORP arising from regolith can be comparable to the conventional TYORP produced by boulders. Then, the analytic expression is fitted with a log-normal function and used to integrate TYORP over all boulder sizes. The general trend of TYORP for multiple boulders appears qualitatively similar to the trend of one boulder, and it also demonstrates a maximal TYORP at some particular rotation rate. The expression obtained for integrated TYORP may be instrumental for simulations of the evolution of asteroids subject to TYORP. The physical origin of TYORP is discussed in light of the constructed analytic model.

Numerical simulations of asteroids modelled as gravitational aggregates with cohesion

Evidence is mounting that asteroids larger than a few hundred metres in diameter are gravitational aggregates of smaller, cohesive pieces. For example, images of 25143 Itokawa show a boulder-strewn surface reminiscent of what might be expected following gravitational reaccumulation of material ejected from a catastrophic impact into a larger body. We have developed a new numerical approach to modelling gravitational aggregates that includes for the first time several prescriptions for variable material strength/cohesion while preserving the desirable features of fast and accurate computation from our prior methods. The new model can be used to construct non-idealized rubble piles made up of irregular, competent pieces, or to preserve shape and spin information of reaccumulated bodies in high-resolution simulations of asteroid family formation, by allowing fragments to stick on contact (and optionally bounce or cause further fragmentation, depending on user-selectable parameters). We detail the numerical method, which involves solving the rigid-body equations of motion and handling non-central/off-axis impacts, and present simulations of collisional and rotational disruption of asteroids as illustrative examples. This work is part of an ongoing effort to improve the realism and applicability of numerical simulations to the collisional and dynamical evolution of asteroids and other small solar system bodies.

Acceleration of the rotation of asteroid 1862 Apollo by radiation torques

The anisotropic reflection and thermal re-emission of sunlight from an asteroid's surface acts as a propulsion engine. The net propulsion force (Yarkovsky effect) changes the orbital dynamics of the body at a rate that depends on its physical properties; for irregularly shaped bodies, the propulsion causes a net torque (the Yarkovsky-O'Keefe-Radzievskii-Paddack or YORP effect) that can change the object's rotation period and the direction of its rotation axis. The Yarkovsky effect has been observed directly, and there is also indirect evidence of its role in the orbital evolution of asteroids over long time intervals. So far, however, only indirect evidence exists for the YORP effect through the clustering of the directions of rotation axes in asteroid families. Here we report a change in the rotation rate of the asteroid 1862 Apollo, which is best explained by the YORP mechanism. The change is fairly large and clearly visible in photometric lightcurves, amounting to one extra rotation cycle in just 40 years even though Apollo's size is well over one kilometre. This confirms the prediction that the YORP effect plays a significant part in the dynamical evolution of asteroids.

Identifying near-Earth object families

The study of asteroid families has provided tremendous insight into the forces that sculpted the main belt and continue to drive the collisional and dynamical evolution of asteroids. The identification of asteroid families within the NEO population could provide a similar boon to studies of their formation and interiors. In this study we examine the purported identification of NEO families by Drummond [Drummond, J.D., 2000. Icarus 146, 453–475] and conclude that it is unlikely that they are anything more than random fluctuations in the distribution of NEO osculating orbital elements. We arrive at this conclusion after examining the expected formation rate of NEO families, the identification of NEO groups in synthetic populations that contain no genetically related NEOs, the orbital evolution of the largest association identified by Drummond [Drummond, J.D., 2000. Icarus 146, 453–475], and the decoherence of synthetic NEO families intended to reproduce the observed members of the same association. These studies allowed us to identify a new criterion that can be used to select real NEO families for further study in future analyses, based on the ratio of the number of pairs and the size of strings to the number of objects in an identified association.

Detection of the Yarkovsky effect for main-belt asteroids

The Yarkovsky effect, a non-gravitational acceleration produced by the anisotropic emission of thermal energy (Öpik, 1951, Proc. Roy. Irish Acad. 54, 165–199), plays an important role in the dynamical evolution of asteroids. Current theoretical models of the Yarkovsky effect, however, rely on a number of poorly known parameters that can only approximate how real asteroids respond to solar heating. To improve this situation, we investigated whether the orbital distribution of the Karin cluster, a 5.8±0.2 Myr old S-type asteroid family (Nesvorný et al., 2002a, Nature 417, 720–722), could be used to determine the rate at which multikilometer main-belt asteroids spread in semimajor axis due to the Yarkovsky effect. Our results indicate that the orbital histories of individual Karin cluster members bear clear signatures of having drifted in semimajor axis drift since their formation. Using numerical methods, we determined the drift speed of ≈70 Karin cluster members (asteroids 1–6 km in diameter). This is the first time the speed that main-belt asteroids evolve in the semimajor axis due to the non-gravitational effects have been measured. The magnitude of measured speeds is similar to those predicted by theoretical models of the Yarkovsky force. Taken together, our results represent the first direct detection of the Yarkovsky effect for main-belt asteroids, and they validate in significant ways the asteroid thermal models described in the recent literature (e.g., Vokrouhlický, 1999, Astron. Astrophys. 344, 362–366). By comparing the measured drift speeds to those calculated from theoretical models of the Yarkovsky effect, we determined that Karin cluster members do not have surface thermal conductivities K in excess of ∼0.1 W m −1 K −1 . Instead, their derived K values are consistent with the presence of regolith over most/all of their ∼5.8 Myr lifetimes. This low-conductive regolith layer may be thin because the penetration depth of the diurnal thermal wave is ≲5 cm. The regolith material may have been deposited in the immediate aftermath of the Karin cluster formation event or was produced over time by impacts. Our method also allows us to estimate spin obliquity values for Karin cluster members. We find that members with diameters ≳3.5-km are predominantly retrograde rotators, while those <3.5-km have obliquities more equally distributed between 0° and 180°. These data may be used to study the spin states of asteroids produced by catastrophic disruption events. Interestingly, we find that a few Karin members have drifted further than predicted by our standard Yarkovsky model. We hypothesize these objects may have: (i) faster drift speeds than predicted by theoretical models, (ii) high albedos (≳0.3), and/or (iii) densities ≲2 g cm −3 .

Statistical results on discovered near-Earth asteroids

AbstractThe first part of this paper gives a recent overview (until July 1st, 1998) of the Near-Earth Asteroids (NEAs) database stored at Minor Planet Center. Some statistical interpretations point out strong observational biases in the population of discovered NEAs, due to the preferential discoveries, depending on the objects’ distances and sizes. It is known that many newly discovered NEAs have no accurately determinated orbits because of the lack of observations. Consequently, it is hard to speak about future encounters and collisions with the Earth in terms of mutual distances between bodies. Because the dynamical evolution of asteroids’ orbits is less sensitive to the improvement of their orbital elements, we introduced a new subclass of NEAs named Earth-encounter asteroids in order to describe more reliably the potentially dangerous bodies as impactors with the Earth. So, we pay attention at those asteroids having an encounter between their orbits and that of the Earth within 100 years, trying to classify these encounters.