Research

We present results from a program of optical light curve observations of near-Earth asteroids (NEAs) with diameters under 1 km, designed to detect, and determine the distribution of, rotation periods shorter than a few hours. We obtain measurements or estimates of rotation period P for approximately one third of the 83 NEAs observed. Most of the measured periods are in the fast-rotating asteroid (FRA) regime (P < 2 h). We assess our detection sensitivity using simulated light curves and a new Monte Carlo algorithm (SALSA), which allows us to de-bias the counts of detected FRAs and determine the fraction of objects that are fast rotators as a function of H. We find that the FRA fraction F rises sharply from zero to a value statistically consistent with unity from H = 21.4 to H = 23.6, a span corresponding to a factor of only 2.8 in nominal diameter. Almost nothing larger than 170 m, and almost everything smaller than 60 m, is a fast rotator, assuming a mean S class albedo of 0.17. The formal 95% confidence limits are F < 13% for 18.5 < H < 21.4 and F > 56% for 23.6 < H < 26.3. Relative to a distribution with the same FRA fraction that is uniform in frequency (as implied by models of evolution following the YORP cycle) up to a size-dependent upper cutoff, the actual spin distribution extends to shorter periods. Approximately two thirds of our sample shows ambiguous light curves from which no period estimate can be obtained. Finite photometric errors account for some of these, but do not explain the steep increase in the ambiguous fraction toward larger objects, which suggests an increase in the actual fraction of very slow rotators, very fast rotators, or nearly axisymmetric objects. With a significantly larger data set, our fully general SALSA procedure will be able to extract the NEA spin rate distribution as a function of absolute magnitude H. Determining this distribution to the accuracy needed to constrain the physical properties of NEAs and their dynamical evolution will require larger samples, and homogeneous, unbiased reporting of the data, including accurate errors, for all objects observed, not just those with measured periods.

Measuring the strengths of asteroidal materials is important for developing mitigation strategies for potential Earth impactors and for understanding properties of in situ materials on asteroids during human and robotic exploration. Studies of asteroid disruption and fragmentation have typically used the strengths determined from terrestrial analog materials, although questions have been raised regarding the suitability of these materials. The few published measurements of meteorite strength are typically significantly greater than those estimated from the stratospheric breakup of meter-sized meteoroids. Given the paucity of relevant strength data, the scale-varying strength properties of meteoritic and asteroidal materials are poorly constrained. Based on our uniaxial failure studies of centimeter-sized cubes of a carbonaceous and ordinary chondrite, we develop the first Weibull failure distribution analysis of meteorites. This Weibull distribution projected to meter scales, overlaps the strengths determined from asteroidal airbursts and can be used to predict properties of to the 100 m scale. In addition, our analysis shows that meter-scale boulders on asteroids are significantly weaker than small pieces of meteorites, while large meteorites surviving on Earth are selected by attrition. Further, the common use of terrestrial analog materials to predict scale-dependent strength properties significantly overestimates the strength of meter-sized asteroidal materials and therefore is unlikely well suited for the modeling of asteroid disruption and fragmentation. Given the strength scale-dependence determined for carbonaceous and ordinary chondrite meteorites, our results suggest that boulders of similar composition on asteroids will have compressive strengths significantly less than typical terrestrial rocks.

We present the first self-consistent simulations of the coupled spin-shape evolution of small gravitational aggregates under the influence of the YORP effect. Because of YORP's sensitivity to surface topography, even small centrifugally driven reconfigurations of aggregates can alter the YORP torque dramatically, resulting in spin evolution that can differ qualitatively from the rigid-body prediction. One-third of our simulations follow a simple evolution described as a modified YORP cycle. Two-thirds exhibit one or more of three distinct behaviors—stochastic YORP, self-governed YORP, and stagnating YORP—which together result in YORP self-limitation. Self-limitation confines rotation rates of evolving aggregates to far narrower ranges than those expected in the classical YORP cycle, greatly prolonging the times over which objects can preserve their sense of rotation. Simulated objects are initially randomly packed, disordered aggregates of identical spheres in rotating equilibrium, with low internal angles of friction. Their shape evolution is characterized by rearrangement of the entire body, including the deep interior. They do not evolve to axisymmetric top shapes with equatorial ridges. Mass loss occurs in one-third of the simulations, typically in small amounts from the ends of a prolate-triaxial body. We conjecture that YORP self-limitation may inhibit formation of top-shapes, binaries, or both, by restricting the amount of angular momentum that can be imparted to a deformable body. Stochastic YORP, in particular, will affect the evolution of collisional families whose orbits drift apart under the influence of Yarkovsky forces, in observable ways.

Aims. From light curve and radar data we know the spin axis of only 43 near-Earth asteroids. In this paper we attempt to constrain the spin axis obliquity distribution of near-Earth asteroids by leveraging the Yarkovsky effect and its dependence on an asteroid’s obliquity.

Methods. By modeling the physical parameters driving the Yarkovsky effect, we solve an inverse problem where we test different simple parametric obliquity distributions. Each distribution results in a predicted Yarkovsky effect distribution that we compare with a χ2 test to a dataset of 125 Yarkovsky estimates.

Results. We find different obliquity distributions that are statistically satisfactory. In particular, among the considered models, the best-fit solution is a quadratic function, which only depends on two parameters, favors extreme obliquities consistent with the expected outcomes from the YORP effect, has a 2:1 ratio between retrograde and direct rotators, which is in agreement with theoretical predictions, and is statistically consistent with the distribution of known spin axes of near-Earth asteroids.