Albedo and color contrasts on asteroid surfaces

Asteroids in general display only small or negligible variations in spectrum or albedo during a rotational cycle. Color variations with rotation are described in the literature but are usually comparable to the noise in the measurements. Twenty-four asteroids have been systematically monitored for such color changes. Only 3 Juno, 4 Vesta, 6 Hebe, 71 Niobe, 349 Dembowska, and 944 Hidalgo display color variations larger than 0.03 mag. In each of these cases the asteroid appears redder near maximum brightness. Of seven asteroids monitored polarimetrically, only 4 Vesta shows a convincing variation, attributed to an albedo change with rotation. The lightcurve can be explained by albedo differences alone; Vesta apparently has a nearly spheroidal shape. Notwithstanding the above results, the degree of uniformity of most asteroid surfaces is remarkable. If asteroids exist with large discrete domains of ferrosilicate, metallic, and/or carbonaceous material together on their surfaces, they have not yet been identified.     

The sizes of color patches on the surface of the asteroid 4 Vesta

The sizes of color patches on the surface of the asteroid 4 Vesta were estimated with the spectral-frequency method. We used the digital records of the asteroid spectra obtained on February 2, 3, 4, and 7, 2002, with the TV system and the slitless afocal spectrograph of the MTM-500 telescope at the Research Institute of the Crimean Astrophysical Observatory. The spectral resolution caused by the image quantity was about 40 Å and the exposure duration was 30 s. The energy calibration was performed both with the artificial light-emitting diode standard and three standard stars. The synthetic color indices B-V and V-R were calculated from the asteroid spectra taken out of the atmosphere. From these data, the changes with an asteroid rotation phase were eliminated. After that, the periods that were significantly smaller than the asteroid rotation period were searched for, and each time the data were whitened for the obtained period. Assuming that the patch sizes are determined by the half-obtained period and that the patches are located in the equatorial region of the asteroid, we estimated the sizes of 20 and 19 patches in the long-and short-wavelength ranges, respectively. The smallest found patch was about 9 km across. The statistical estimates of the reddish patches and the comparison with the statistics of old craters allowed us to suggest that the reddish patches on the asteroid Vesta surface are old formations.     

Observations of asteroid occultations by the trailed-image method

The method of trailed CCD images for observations of asteroid occultations is described. This method was used to observe 9 asteroid occupations at the Lisnyky observational station in 2006 with the telescope AZT-8 (D= 0.7m and F= 2.8m) equipped by the CCD ST-8 XME. In the case of occultation of the star TYC 0587-00209-1U by the asteroid 76 Freia in November 4, 2006, the distance between the asteroid center and the star, as well as the time of asteroid occultation were determined. The size of asteroid 76 Freia is determined assuming that its shape is spherically symmetric. In other cases, the minimal distances between the asteroid center and the star are determined. The method makes it possible to observe asteroid occultations with high time resolution.     

Analytical method for the effects of the asteroid belt on planetary orbits

Analytic expressions are derived for the perturbation of planetary orbits due to a thick constant density asteroid belt. The derivations include extensions and adaptations of Plakhov's analytic expressions for the perturbations in five of the orbital elements for closed orbits around Saturn's rings. The equations of Plakhov are modified to include the effect of ring thickness and additional equations are derived for the perturbations in the sixth orbital element, the mean anomaly. The gravitational potential and orbital perturbations are derived for the asteroid belt with and without thickness, and for a hoop approximation to the belt. The procedures are also applicable to Saturn's rings and the newly discovered rings of Uranus.The effects of the asteroid belt thickness on the gravitational potential coefficients and the orbital motions are demonstrated. Comparisons between the Mars orbital perturbations obtained using the analytic expressions and those obtained using numerical integration are discussed. The effects of the asteroid belt on the Earth based ranging to Mars are also demonstrated.     

Comparison of feature sizes on the surface of the asteroid 4 Vesta determined using ground-based and space observations

Vesta’s crater sizes that are based on images returned from the Dawn probe orbiting the asteroid 4 Vesta are compared with sizes of spots on its surface derived from hydrosilicate equivalent widths and asteroid color indices B-V and V-R observed spectrophotometrically on earth using a spectral-frequency method. The sizes of craters and spots at the asteroid poles prove to correspond to the sizes of spots that are assumed to be located at latitudes of 40°...45° N and S. Comparative results for the crater sizes ranging from 10 to 100 km are tabulated. We conclude that crater sizes on asteroid surfaces can be determined using the spectral-frequency method.     

Possible temperature variation effects on the interpretation of spatially resolved reflectance observations of asteroid surfaces

The visible and near-IR spectral reflectance of some minerals is a function of temperature. Hence, surface temperature variation must be considered for mineralogical interpretation of spectral reflectance data obtained from a spatially resolved surface which has a temperature distribution. An example is presented to illustrate the errors which can be encountered when interpreting the spectral reflectance of a silicate mineral assemblage, measured at various temperatures that might be expected across an asteroid surface. Independent knowledge of the surface temperature distribution would allow the optimal extraction of mineralogical information in such cases.     

Scaling forces to asteroid surfaces: The role of cohesion

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.     

Binary asteroid systems: Tidal end states and estimates of material properties

The locations of the fully despun, double synchronous end states of tidal evolution, where the rotation rates of both the primary and secondary components in a binary system synchronize with the mean motion about the center of mass, are derived for spherical components. For a given amount of scaled angular momentum J/J′, the tidal end states are over-plotted on a tidal evolution diagram in terms of mass ratio of the system and the component separation (semimajor axis in units of primary radii). Fully synchronous orbits may not exist for every combination of mass ratio and angular momentum; for example, equal-mass binary systems require J/J′ > 0.44. When fully synchronous orbits exist for prograde systems, tidal evolution naturally expands the orbit to the stable outer synchronous solution. The location of the unstable inner synchronous orbit is typically within two primary radii and often within the radius of the primary itself. With the exception of nearly equal-mass binaries, binary asteroid systems are in the midst of lengthy tidal evolutions, far from their fully synchronous tidal end states. Of those systems with unequal-mass components, few have even reached the stability limit that splits the fully synchronous orbit curves into unstable inner and stable outer solutions.Calculations of material strength based on limiting the tidal evolution time to the age of the Solar System indicate that binary asteroids in the main belt with 100-km-scale primary components are consistent with being made of monolithic or fractured rock as expected for binaries likely formed from sub-catastrophic impacts in the early Solar System. To tidally evolve in their dynamical lifetime, near-Earth binaries with km-scale primaries or smaller created via a spin-up mechanism must be much weaker mechanically than their main-belt counterparts even if formed in the main belt prior to injection into the near-Earth region. Small main-belt binaries, those having primary components less than 10 km in diameter, could bridge the gap between the large main-belt binaries and the near-Earth binaries, as, depending on the age of the systems, small main-belt binaries could either be as strong as the large main-belt binaries or as weak as the near-Earth binaries. The inherent uncertainty in the age of a binary system is the leading source of error in calculation of material properties, capable of affecting the product of rigidity μ and tidal dissipation function Q by orders of magnitude. Several other issues affecting the calculation of μQ are considered, though these typically affect the calculation by no more than a factor of two. We also find indirect evidence within all three groups of binary asteroids that the semimajor axis of the mutual orbit in a binary system may evolve via another mechanism (or mechanisms) in addition to tides with the binary YORP effect being a likely candidate.     

Pole coordinates of the asteroid 511 Davida as determined via the Amplitude-Magnitude method

The Amplitude-Magnitude (AM) method is used for the pole determination of the asteroid 511 Davida, using observations from six oppositions. The possible North poles are found to be λ₁ = 92° ± 7°; β₁ = 33° ± 6°, and λ₂ = 303° ± 4°; β₂ = 34° ± 5°, when scattering effect is not taken into account. When scattering is accounted for, solutions not significantly different from (λ₁, β₁) and (λ₂, β₂) are obtained. The moderately eccentric and inclined orbit of 511 Davida does not allow us to distinguish between the two pole solutions. A comparison with other methods is necessary in order to make a definitive choice.     

Curation planning and facilities for asteroid Bennu samples returned by the OSIRIS-REx mission

NASA's OSIRIS-REx spacecraft collected samples from carbonaceous near-Earth asteroid (101955) Bennu on October 20, 2020, and will deliver them to the Earth on September 24, 2023. The samples will be processed at the NASA Johnson Space Center (JSC), where most of the sample collection will be subsequently curated in a new cleanroom suite. The spacecraft collected loose regolith two ways: in a bulk sample chamber capable of holding up to 2 kg, and on industrial Velcro “contact pads” intended to collect small particles at the surface. Included in the JSC collection will be the bulk sample, the contact pads, contamination-monitoring witness plates, and supporting hardware. Planning for the curation of the samples and hardware started at the earliest phase of proposal development and continued in parallel with project development and execution. Because a major mission goal is characterization of organic compounds in the Bennu samples, extra effort was spent in the design stage to ensure a clean curation environment. Here, we describe the preparations to receive the sample, including the design, construction, outfitting, and monitoring of the cleanrooms at JSC; the planned recovery of the sample-containing capsule when it lands on Earth; and the approach to characterizing and cataloging the samples. These curation efforts will result in the distribution of pristine Bennu samples from JSC to the OSIRIS-REx science team, international partners, and the global scientific community for years to come.     

Generalization of Szebehely's regions for asteroid stability

Using Hill's modified stability criterion in the three-dimensional restricted three-body problem with the Sun and Jupiter as the primaries, regions of stability for the semi-major axis (a) are determined. These regions are given for a range of eccentricity (e) and inc.l.ination (i), and include the effects of the remaining orbital elements. This generalizes Szebehely's previous results which only examined the effects of e and i on the regions of stability for the semima jor axis. It is shown that if the other orbital elements are also considered, the previous results are sharpened but are not contradicted.     

A fast method for finding bound systems in numerical simulations: Results from the formation of asteroid binaries

We present a new code (companion) that identifies bound systems of particles in O(NlogN) time. Simple binaries consisting of pairs of mutually bound particles and complex hierarchies consisting of collections of mutually bound particles are identifiable with this code. In comparison, brute force binary search methods scale as O(N²) while full hierarchy searches can be as expensive as O(N³), making analysis highly inefficient for multiple data sets with N?¹⁰³. A simple test case is provided to illustrate the method. Timing tests demonstrating O(NlogN) scaling with the new code on real data are presented. We apply our method to data from asteroid satellite simulations [Durda et al., 2004. Icarus 167, 382-396; Erratum: Icarus 170, 242; reprinted article: Icarus 170, 243-257] and note interesting multi-particle configurations. The code is available at http://www.astro.umd.edu/zoe/companion/ and is distributed under the terms and conditions of the GNU Public License.     

Water combinations on the surface of the asteroid 4 Vesta

The spectral-frequency method (Busarev et al., 2007b) allowed us to obtain data on 16 sizes of hydrosilicate spots on the surface of the asteroid 4 Vesta. Large sizes (800 and 750 km) show clusters of small hydrosilicate spots near a well-known crater. Small spots of 50–13 km cover more than 50% of the surface. The predominant number of small-sized spots suggests their recent origin. Our data confirm the presence of water combinations on the surface of 4 Vesta and allow us to draw a conclusion on their recent appearance in collisions of this asteroid with primitive bodies arriving from the zone of Jupiter.     

Collisional evolution of the asteroid belt

A new synthesis of asteroid collisional evolution is motivated by the question of whether most asteroids larger than ∼1 km size are strengthless gravitational aggregates (rubble piles). NEAR found Eros not to be a rubble pile, but a shattered collisional fragment, with a through-going fracture system, and an average of about 20 m regolith cover. Of four asteroids visited by spacecraft, none appears likely to be a rubble pile, except perhaps Mathilde. Nevertheless, current understanding of asteroid collisions and size-dependent strength, and the observed distribution of rotation rates versus size, have led to a theoretical consensus that many or most asteroids larger than 1 km should be rubble piles. Is Eros, the best-observed asteroid, highly unusual because it is not a rubble pile? Is Mathilde, if it is a rubble pile, like most asteroids? What would be expected for the small asteroid Itokawa, the MUSES-C sample return target? An asteroid size distribution is synthesized from the Minor Planet Center listing and results of the Sloan Digital Sky Survey, an Infrared Space Observatory survey, the Small Main-belt Asteroid Spectroscopic Survey and the Infrared Astronomical Satellite survey. A new picture emerges of asteroid collisional evolution, in which the well-known Dohnanyi result, that the size distribution tends toward a self-similar form with a 2.5-index power law, is overturned because of scale-dependent collision physics. Survival of a basaltic crust on Vesta can be accommodated, together with formation of many exposed metal cores. The lifetimes against destruction are estimated as 3 Gyr at the size of Eros, 10 Gyr at ten times that size, and 40 Gyr at the size of Vesta. Eros as a shattered collisional fragment is not highly unusual. The new picture reveals the new possibility of a transition size in the collisional state, where asteroids below 5 km size would be primarily collisional breakup fragments whereas much larger asteroids are mostly eroded or shattered survivors of collisions. In this case, well-defined families would be found in asteroids larger than about 5 km size, but for smaller asteroids, families may no longer be readily separated from a background population. Moreover, the measured boulder size distribution on Eros is re-interpreted as a sample of impactor size distributions in the asteroid belt. The regolith on Eros may result largely from the last giant impact, and the same may be true of Itokawa, in which case about a meter of regolith would be expected there. Even a small asteroid like Itokawa may be a shattered object with regolith cover.     

Analysis of the Hungaria asteroid population

There are approximately 5000 known asteroids in the Hungaria orbital space, a region defined by orbits with high inclination (16° < i < 34°), low eccentricities (e < 0.18), and semi-major axes 1.78 < a < 2.0 AU. We argue that this region is populated by a large number of asteroids formed after a catastrophic collision involving (434) Hungaria, the presumptive largest fragment of the Hungaria collisional family. The remaining objects form a background population that share orbital characteristics with the family members. Due to the general dynamic stability of the region, it is likely that most asteroids in Hungaria space (the Hungaria “group”) have been in this region since the formation of the Solar System or at least since the planets assumed their current orbital configuration. Our examination of the Hungaria group included comparing rotation rates, taxonomic classification, and orbital dynamics to determine the characteristics of the family and background populations. We first found there is an excess of slow rotators among the group but, otherwise, the distribution of spin frequencies is essentially uniform, i.e., that a plot of the cumulative number of objects over the range of 1 d⁻¹ < f < 9 d⁻¹ is nearly a straight line or, put another way, if the distribution over the range is binned by equal intervals of f (1-2 d⁻¹, 2-3 d⁻¹, etc.), the number of objects in each bin is statistically the same.There is a distinct family within the Hungaria group, centered at a semi-major axis of 1.940 AU, with a dispersion range that increases with decreasing size of members, as expected of an evolved collisional family. The larger members with well-determined taxonomic class, including (434) Hungaria itself, have flat spectra, mostly likely type E or similar. The degree of spreading versus size of family members is consistent with that expected from Yarkovsky thermal drift in roughly 0.5 Gyr, suggesting that age for the family. The Asteroid (434) Hungaria is displaced in semi-major axis by 0.004 AU from the center of the Hungaria family. The collision event that produced the family should not have left the largest body displaced by more than 0.001 AU from the original orbit, thus we infer that the displacement of (434) Hungaria is mainly due to Yarkovsky drift, and is consistent with the expected drift for that size body in ∼0.5 Gyr. Below ∼1.93 AU heliocentric distance the Hungaria family is perturbed by at least two secular resonances, 2g − g₅ − g₆ and one of the family of 4th or 6th order secular resonances near s ∼ −22.25 ″/year. Their combined effect results in larger inclination dispersion of the family members.     

On the definition of asteroid magnitudes

The problem of a sharp definition of two photometric parameters for asteroids in the presence of empirical phase functions is discussed. In a rational magnitude system advantage should be taken of the nearly linear phase curve.     

A comparison between 2D and 3D wavelet analysis method for asteroid family dynamical study

As found in previous studies (Bendjoyaet al. 1991) the wavelet analysis is a reliable tool for the determination of asteroid families. A comparison between the 2D and a newly developed 3D wavelet analysis is performed on 14 fictitious families numerically generated the members of which area priori known. It clearly appears that the 3D method brings improvements to the 2D approach. Indeed the 3D analysis appears well suited for all cases even for those corresponding to the very dense Flora region and for exotic families such as filamentar ones. With the 2D method, satisfying results are obtained for the simulations which correspond to 70% of the real asteroid families previously found. The aim of this paper is to show quantitatively the improvement of the 3D method versus the 2D one.     

On the possible hazard on the major cities caused by asteroid impact in the Pacific ocean

Semi-quantitative investigation is made of hazard expected from an asteroidal impact in the Pacific. An impact ofd (diameter) = 200 m asteroid has a probability of hitting somewhere in the Pacific once in 15000 y. By carrying out a Monte Carlo simulation, such an impact, on average, is shown to create a tsunami as high as 16, 14, 15, and 21 m at Japan, Taiwan, Shanghai and Hawaii, respectively. Wooden houses, stone and brick houses, and reinforced concrete buildings are likely to be demolished by tsunamis of height 2, 7 and 20 m respectively. Thus, there is a probability of 1% or so that most of the artificial constructions on the coast lines of the Pacific be destroyed in the next century by an asteroidal impact.