The energetics and dynamics of contact binary asteroids as they approach and pass the rotational fission limit is studied. We presume that the asteroids are subject to an external torque, such as ...from the YORP effect, that increases their angular momentum. Furthermore, we assume the asteroids can be described by a fairly minimal model comprised of a sphere and ellipsoid resting on each other. The minimum energy configurations for contact binary asteroids at different levels of angular momentum are computed and discussed. We find distinct transitions between different configurations as the angular momentum of the system is increased. These indicate that rapidly rotating contact binary asteroids may seek out clearly different relative configurations than slowly rotating systems. We find a single end state of the systems prior to rotational fission, and distinct dynamical outcomes as a function of mass distribution and shape when the rotational fission limit is exceeded. Our theoretical results agree qualitatively with observed properties of near-Earth asteroids, and can be used to help explain the spin-rate barrier, contact binaries, and the observed morphology of most NEO binaries.
•The geopotential on a rapidly spinning spheroidal body is studied.•Estimates for the rate of shedding surface material are given for rapidly spinning spheroids.•Predictions are made on the shapes ...and exposed sub-surface regions on rapidly spinning spheroidal asteroids.
Conditions for regolith landslides to occur on spinning, gravitating spheroidal asteroids and their aftermath are studied. These conditions are developed by application of classical granular mechanics stability analysis to the asteroid environment. As part of our study we determine how slopes evolve across the surface of these bodies as a function of spin rate, the dynamical fate of material that exceeds the angle of repose, and an analysis of how the shape of the body may be modified based on these results. We find specific characteristics for body surfaces and shapes when spun near the surface disruption limit and develop what their observable implications are. The small, oblate and rapidly spinning asteroids such as 1999 KW4 Alpha and 2008 EV5 exhibit some of these observable traits. The detailed mechanisms outlined here can also provide insight and constraints on the recently observed active asteroids such as P/2013 P5, and the creation of asteroidal meteor streams.
We explore the hypothesis that, due to small van der Waals forces between constituent grains, small rubble pile asteroids have a small but nonzero cohesive strength. The nature of this model predicts ...that the cohesive strength should be constant independent of asteroid size, which creates a scale dependence with relative strength increasing as size decreases. This model counters classical theory that rubble pile asteroids should behave as scale‐independent cohesionless collections of rocks. We explore a simple model for asteroid strength that is based on these weak forces, validate it through granular mechanics simulations and comparisons with properties of lunar regolith, and then explore its implications and ability to explain and predict observed properties of small asteroids in the NEA and Main Belt populations, and in particular of asteroid 2008 TC3. One conclusion is that the population of rapidly rotating asteroids could consist of both distributions of smaller grains (i.e., rubble piles) and of monolithic boulders.
A detailed derivation is given of the effect of solar radiation on the rotational dynamics of asteroids, commonly called the YORP effect. The current derivation goes beyond previous discussions ...published in the literature and provides a comprehensive secular dynamical analysis of the effect of solar radiation torques acting on a uniformly rotating body, and the evolution of its rotation state over time. Our predicted model has the global radiation properties of the asteroid as explicit parameters, and hence can be specified independent of these parameters. The resulting secular equations for the rotation rate and rotation pole are characterized by three parameters of the body's shape and explicitly includes the effect of thermal inertia on the evolution of these rotation state parameters. With this detailed model, in conjunction with estimated asteroid shapes and poles, we compute the expected YORP torques and dynamic response of several asteroids and the change in rotation rate for specific shapes as a function of obliquity. Finally, we define a convenient dimensionless parameter that is only a function of the body geometry and that can be used to characterize the effects of YORP.
The scaling of physical forces to the extremely low ambient gravitational acceleration regimes found on the surfaces of small asteroids is performed. Resulting from this, it is found that van der ...Waals cohesive forces between regolith grains on asteroid surfaces should be a dominant force and compete with particle weights and be greater, in general, than electrostatic and solar radiation pressure forces. Based on this scaling, we interpret previous experiments performed on cohesive powders in the terrestrial environment as being relevant for the understanding of processes on asteroid surfaces. The implications of these terrestrial experiments for interpreting observations of asteroid surfaces and macro-porosity are considered, and yield interpretations that differ from previously assumed processes for these environments. Based on this understanding, we propose a new model for the end state of small, rapidly rotating asteroids which allows them to be comprised of relatively fine regolith grains held together by van der Waals cohesive forces.
Asteroid crater retention ages have unknown accuracy because projectile-crater scaling laws are difficult to verify. At the same time, our knowledge of asteroid and crater size-frequency ...distributions has increased substantially over the past few decades. These advances make it possible to empirically derive asteroid crater scaling laws by fitting model asteroid size distributions to crater size distributions from asteroids observed by spacecraft. For D > 10 km diameter asteroids like Ceres, Vesta, Lutetia, Mathilde, Ida, Eros, and Gaspra, the best matches occur when the ratio of crater to projectile sizes is f ∼ 10. The same scaling law applied to 0.3 < D < 2.5 km near-Earth asteroids such as Bennu, Ryugu, Itokawa, and Toutatis yield intriguing yet perplexing results. When applied to the largest craters on these asteroids, we obtain crater retention ages of ∼1 billion years for Bennu, Ryugu, and Itokawa and ∼2.5 billion years for Toutatis. These ages agree with the estimated formation ages of their source families and could suggest that the near-Earth asteroid population is dominated by bodies that avoided disruption during their traverse across the main asteroid belt. An alternative interpretation is that f > 10, which would make their crater retention ages much younger. If true, crater scaling laws need to change in a substantial way between D > 10 km asteroids, where f ∼ 10, and 0.3 < D < 2.5 km asteroids, where f > 10.
•It is shown that small levels of cohesion within a rubble pile asteroid will, when combined with the YORP effect, cause rubble pile asteroids to disaggregate into their fundamental constituents ...within relatively short time spans, on the order of a million years or potentially much less. The size at which disaggregation phase starts and the time for disaggregation are functions of the body density, strength, and other geophysical parameters.•The theory is applied to well characterized binary asteroids to find upper and lower limits of strength as a function of density and spectral type. Lower bounds are at a few Pascals while upper bounds are generally less than a few hundred Pascals, in some cases less than a few tens of Pascals.•The implication of the theory is that rubble pile structure may be relatively rare amongst the smallest asteroids.•This effect may alter the size distribution of asteroids less than a few hundred meters.
The implication of small amounts of cohesion within relatively small rubble pile asteroids is investigated with regard to their evolution under the persistent presence of the YORP effect. We find that below a characteristic size, which is a function of cohesive strength, density and other properties, rubble pile asteroids can enter a “disaggregation phase” in which they are subject to repeated fissions after which the formation of a stabilizing binary system is not possible. Once this threshold is passed rubble pile asteroids may be disaggregated into their constituent components within a finite time span. These constituent components will have their own spin limits – albeit potentially at a much higher spin rate due to the greater strength of a monolithic body. The implications of this prediction are discussed and include modification of size distributions, prevalence of monolithic bodies among meteoroids and the lifetime of small rubble pile bodies in the solar system. The theory is then used to place constraints on the strength of binary asteroids characterized as a function of their type.
Rubble-Pile Asteroid Itokawa as Observed by Hayabusa Fujiwara, A; Kawaguchi, J; Yeomans, D.K ...
Science (American Association for the Advancement of Science),
06/2006, Letnik:
312, Številka:
5778
Journal Article
Recenzirano
During the interval from September through early December 2005, the Hayabusa spacecraft was in close proximity to near-Earth asteroid 25143 Itokawa, and a variety of data were taken on its shape, ...mass, and surface topography as well as its mineralogic and elemental abundances. The asteroid's orthogonal axes are 535, 294, and 209 meters, the mass is 3.51 x 10¹⁰ kilograms, and the estimated bulk density is 1.9 ± 0.13 grams per cubic centimeter. The correspondence between the smooth areas on the surface (Muses Sea and Sagamihara) and the gravitationally low regions suggests mass movement and an effective resurfacing process by impact jolting. Itokawa is considered to be a rubble-pile body because of its low bulk density, high porosity, boulder-rich appearance, and shape. The existence of very large boulders and pillars suggests an early collisional breakup of a preexisting parent asteroid followed by a re-agglomeration into a rubble-pile object.
Small solar system bodies such as asteroids and comets are of significant interest for both scientific and human exploration missions. However, their orbital environments are among the most highly ...perturbed and extreme environments found in the solar system. Uncontrolled trajectories are highly unstable in general and may either impact or escape in timespans of hours to days. Even with active control, the chaotic nature of motion about these bodies can effectively randomize a trajectory within a few orbits, creating fundamental difficulties for the navigation of spacecraft in these environments. In response to these challenges our research has identified robust and stable orbit solutions and mission designs across the whole range of small body sizes and spin states that are of interest for scientific and human exploration. This talk will describe the challenges of exploring small bodies and present the practical solutions that have been discovered which enable their exploration across the range of small body types and sizes.
► The small body orbital environment is the most extreme found in the solar system. ► The challenges of small body exploration missions are described and motivated. ► Robust and stable orbit solutions are presented for small body orbiter missions. ► Mission design analysis of the Rosetta spacecraft at its target comet is presented.
ABSTRACT
The OSIRIS-REx mission collected a sample from the surface of the asteroid (101955) Bennu in 2020 October. Here, we study the impact of the OSIRIS-REx Touch-and-Go Sampling Acquisition ...Mechanism (TAGSAM) interacting with the surface of an asteroid in the framework of granular physics. Traditional approaches to estimating the penetration depth of a projectile into a granular medium include force laws and scaling relationships formulated from laboratory experiments in terrestrial-gravity conditions. However, it is unclear that these formulations extend to the OSIRIS-REx scenario of a 1300-kg spacecraft interacting with regolith in a microgravity environment. We studied the TAGSAM interaction with Bennu through numerical simulations using two collisional codes, pkdgrav and gdc-i. We validated their accuracy by reproducing the results of laboratory impact experiments in terrestrial gravity. We then performed TAGSAM penetration simulations varying the following geotechnical properties of the regolith: packing fraction (P), bulk density, inter-particle cohesion (σc), and angle of friction (ϕ). We find that the outcome of a spacecraft-regolith impact has a non-linear dependence on packing fraction. Closely packed regolith (P ≳ 0.6) can effectively resist the penetration of TAGSAM if ϕ ≳ 28° and/or σc ≳ 50 Pa. For loosely packed regolith (P ≲ 0.5), the penetration depth is governed by a drag force that scales with impact velocity to the 4/3 power, consistent with energy conservation. We discuss the importance of low-speed impact studies for predicting and interpreting spacecraft–surface interactions. We show that these low-energy events also provide a framework for interpreting the burial depths of large boulders in asteroidal regolith.
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