Tidal disruptions: A continuum theory for solid bodies     

Keith A. Holsapple  Patrick Michel 《Icarus》2006,183(2):331-348

Although the theory of Roche 1847 for the tidal disruption limits of orbiting satellites assumes a fluid body, a length to diameter of exactly 2.07:1, and a particular body orientation, the theory is commonly applied to the satellites of the Solar System and to small asteroids and comets passing nearby a planet. Clearly these bodies are neither fluid nor generally are that elongated, so a more appropriate theory is needed. Here we present exact analytical results for the distortion and disruption limits of solid spinning ellipsoidal bodies subjected to tidal forces, using the Drucker-Prager strength model with zero cohesion. It is the appropriate model for dry granular materials such as sands and rocks, for rubble-pile asteroids and comets, and for larger satellites, asteroids and comets where the cohesion can be ignored. This study uses the same approach as the studies of spin limits for solid ellipsoidal bodies given in [Holsapple, K.A., 2001. Icarus 154, 432-448; Holsapple, K.A., 2004. Icarus 172, 272-303]. It is a static theory that predicts conditions for breakup and predicts the nature of the deformations at the limit state, but does not track the dynamics of the body as it comes apart. The strength is characterized by a single material parameter associated with an angle of friction, which can range from zero to 90°. The case with zero friction angle has no shear strength whatsoever, so it is then the model of a fluid or gas. The case of 90° represents a material that cannot fail in shear, but still has zero tensile strength. Typical dry soils have angles of friction of 30°-40°. Since the static fluid case is included in the theory as a special case, the classical results of Roche [Roche, E.A., 1847. Acad. Sci. Lett. Montpelier. Mem. Section Sci. 1, 243-262] and Jeans [Jeans, J.H., 1917. Mem. R. Astron. Soc. London 62, 1-48] are included and re-derived in their entirety; but the general solid case has much more variety and applicability. We consider both the spin-locked case, appropriate for most satellites of the Solar System; and the zero spin case, a possible case for a passing stray body. Detailed plots of many special cases are presented, in terms of shape, orientation and mass densities. A very typical result gives a closest approach d=1.5(ρ/ρP)^1/3R in terms of the planet radius R, and the satellite and planet mass densities ρ and ρP. We also use the theory to distinguish between conditions allowing global shape changes leading to new equilibrium states, or those leading to complete disruption. We apply the theory to the potentially hazardous Asteroid 99942 Apophis due to pass very near the Earth in 2029, and conclude it is extremely unlikely to experience any tidal readjustments during its passage. The states of many of the satellites of the Solar System are compared to the theory, and we find that all are well within their tidal disruption limits for expected values of the internal friction.  相似文献   

Spin limits of Solar System bodies: From the small fast-rotators to 2003 EL61     

Keith A. Holsapple 《Icarus》2007,187(2):500-509

Holsapple [Holsapple, K.A., 2001. Icarus 154, 432-448; Holsapple, K.A., 2004. Icarus 172, 272-303] determined the spin limits of bodies using a model for solid bodies without tensile or cohesive strength, but with the pressure-induced shear strengths characteristic of dry sands and gravels. That theory included the classical analyses for fluid bodies given by Maclaurin, Jacobi and others as a special case. For the general solid bodies, it was shown that there exists a very wide range of permissible shapes and spin limits; and explicit algebraic results for those limits were given. This paper gives an extension of those analyses to include geological-like materials that also have tensile and cohesive strength. Those strengths are necessary to explain the smaller, fast-rotating asteroids discovered in the last few years. I find that the spin limits for these more general solids have two limiting regimes: a strength regime for bodies with a diameter <3 km, and a gravity regime for the larger bodies with a diameter >10 km (which is the case covered by the earlier papers). I derive explicit algebraic forms for the dependence of the spin limits on shape, mass density and material strength properties. The comparison of the theory to the database for the spins of asteroids and trans-neptunian objects (TNO's) objects shows excellent agreement. For large bodies (diameter D>10 km), the presence of cohesive and/or tensile strength does not permit higher spin rates than would be allowed for rubble pile bodies. Thus, the fact that the spin rates of all large bodies is limited to periods greater than about 2 h does not imply that they are rubble piles. In contrast, for small bodies (D<10 km) the presence of even a very small amount of strength allows much more rapid spins. Small bodies might then be rubble piles but require a small amount of bonding. Finally, I make some remarks about the application of the theory to the TNO's and large asteroids, and question whether a common assumption by researchers that those bodies must take on relaxed fluid shapes is warranted. If not, then the densities and shapes required by that assumption are not valid. I use 2003 EL61 as a prime example.  相似文献   

Nature of the small main belt Asteroid 3169 Ostro     

P. Descamps  T. Michalowski  J. Berthier  J.-P. Teng-Chuen-Yu  B. Payet  Y. Léonie  H. Berrouachdi  F. Benard 《Icarus》2007,189(2):362-369

We present a set of rotational lightcurve measurements of the small main belt Asteroid 3169 Ostro. Our observations reveal an unambiguous, double-peaked rotational lightcurve with a peak-to-peak variation up to 1.2±0.05 mag and a synodic period of 6.509±0.001 h. From the large flux variation and the overall shape of the lightcurves, we suggest that 3169 Ostro could be a tightly bound binary or a contact binary, similar to the Trojan Asteroid 624 Hektor. A shape model of this system is proposed on the assumption that 3169 Ostro is a Roche binary described by a pair of homogeneous elongated bodies, with a size ratio of 0.87, in hydrostatic equilibrium and in circular synchronous motion around each other. The direction of the spin axis is determined modulo 180° by its J2000 ecliptic coordinates λ₀=50±10°, β₀=±54±5°. The binary interpretation and the pole solution adequately fit the earlier photometric observations made in 1986 and 1988. However, additional supporting lightcurves are highly desirable especially in the next mutual events occurrence of 2008 and 2009 in order to remove the pole ambiguity and to confirm unambiguously the binary nature of 3169 Ostro.  相似文献   

High- and low-velocity impact experiments on porous sintered glass bead targets of different compressive strengths: Outcome sensitivity and scaling     

M. Setoh  A.M. Nakamura  K. Hiraoka  S. Hasegawa  K. Okudaira 《Icarus》2010,205(2):702-711

Impact experiments on porous targets consisting of sintered glass beads have been performed at different impact velocities in order to investigate the disruption impact energy threshold (also called Q^∗) of these targets, the influence of the target compressive strength on this threshold and a scaling parameter of the degree of fragmentation that takes into account material strength. A large fraction of small bodies of our Solar System are expected to be composed of highly-porous material. Depending on their location and on the period considered during the Solar System history, these bodies collide with each other at velocities which cover a wide range of values from a few m/s to several km/s. Determining the impact response of porous bodies in both high- and low-velocity regimes is thus crucial to understand their collisional evolution over the entire Solar System history, from the early stages of planetary formation through collisional accretion at low impact velocities to the current and future stages during which impact velocities are much higher and lead to their disruption. While these problems at large scale can only be addressed directly by numerical simulations, small scale impact experiments are a necessary step which allows the understanding of the physical process itself and the determination of the small scale behavior of the material used as target. Moreover, they are crucial to validate numerical codes that can then be applied to larger scales.Sintered glass beads targets of different shapes and porosity have been built and their main material properties, in particular their compressive strength and their porosity, have been measured. The outcomes of their disruptions both at low and high impact velocities have then been analyzed.We then found that the value of Q^∗ strongly depends on the target compressive strength. Measuring the particle velocities as a function of their distance to the impact point, we first found that the attenuation rate of the stress wave in our sintered glass bead targets does not depend on the impact velocity regime. Ejecta velocities as a function of the distance from the impact point can thus be well fitted by a power law with an exponent about −2 in both velocity regimes. We then looked for a scaling parameter that can apply to both regimes. We found that the scaling parameter PI, which is related to the initial peak pressure and to the stress wave attenuation can be used to represent the outcome in a general way. Future investigations will be performed to determine whether these results can be generalized to other kinds of porous materials.  相似文献   

The topographic limits of gravitationally bound, rotating sand piles     

David A. Minton 《Icarus》2008,195(2):698-704

Rubble pile asteroids can attain shapes that are dramatically different from those of rotating, self-gravitating equilibrium fluids. A new numerical technique, called “seed growth,” is demonstrated for calculating three-dimensional bodies that are self-gravitating and rotating, and whose every surface is approximately at a constant angle, ?, with respect to the local horizontal. By altering the configuration of cusps, which are points along a constant longitude path where the surface angle changes sign but not magnitude, multiple solution shapes that satisfy the condition that all surface slopes are at a constant angle are possible. Five different cusp configurations are explored here, three of which yield solutions for 20°???30°. Rotational effects are explored, and it is found that for some solution shapes, the ratios of their shortest to longest dimensions, c/a, can fall outside the limits published in the literature for rotating, cohesionless, spheroidal bodies. Solution shapes show some similarities to observed small bodies, such as the saturnian satellite Atlas, the near-Earth Asteroid 1999 KW4, and some contact binary asteroids.  相似文献   

On YORP-induced spin deformations of asteroids     

Keith A. Holsapple 《Icarus》2010,205(2):430-442

The alteration of the spin states of small Solar-System bodies by the YORP thermal effect has recently become a plausible and, for some, the favorite candidate for the formation of binary asteroids. The idea is that if an asteroid is slowly spun up to a state where some strength measure is exceeded; it can no longer remain rigid and adjusts to a new configuration. Such a process might involve global fission, global shape changes without fission, or gradual surface mass loss with subsequent mass re-accumulations forming a secondary body.Here I analyze the changes in the shape, spin, and state during slowly increasing angular momentum of rubble-pile, self-gravitating, homogeneous ellipsoidal bodies undergoing homogeneous motions. I use, as appropriate for rubble-pile asteroids, the strength models of granular materials with zero tensile strength (cohesionless but arbitrary dilatancy); those are characterized by the “angle of friction” material constant. There are distinct limit spins depending on that angle of friction and the shape, which were previously presented [Holsapple, K.A., 2001. Icarus 154, 432-448; Holsapple, K.A., 2004. Icarus 172, 272-303]. Here the deformations and state changes when the angular momentum is slowly increased from that of a limit spin state are determined, to study the YORP processes. When a body is at its limit spin and the angular momentum increases further, the body deforms in a unique way along definite paths in the ellipsoidal shape space: it evolves as an elongating shape with an increasing rotational inertia, which in most cases produces a decreasing spin. I give exact analytical solutions for those shape and spin histories, as well as the histories of the mass density, angular momentum and energy. Comparison to other approaches is made.  相似文献   

Binary near-Earth asteroid formation: Rubble pile model of tidal disruptions     

Kevin J. Walsh  Derek C. Richardson 《Icarus》2006,180(1):201-216

We present numerical simulations of near-Earth asteroid (NEA) tidal disruption resulting in bound, mutually orbiting systems. Using a rubble pile model we have constrained the relative likelihoods for possible physical and dynamical properties of the binaries created. Overall 110,500 simulations were run, with each body consisting of ∼1000 particles. The encounter parameters of close approach distance and velocity were varied, as were the bodies' spin, elongation and spin axis direction. The binary production rate increases for closer encounters, at lower speeds, for more elongated bodies, and for bodies with greater spin. The semimajor axes for resultant binaries are peaked between 5 to 20 primary radii, and there is an overall trend for high eccentricity, with 97% of binaries having e > 0.1. The secondary-to-primary size ratios of the simulated binaries are peaked between 0.1 and 0.2, similar to trends among observed asteroid binaries. The spin rates of the primary bodies are narrowly distributed between 3.5- and 6-h periods, whereas the secondaries' periods are more evenly distributed and can exceed 15-h periods. The spin axes of the primary bodies are very closely aligned with the angular momenta of the binary orbits, whereas the secondary spin axes are nearly random. The shapes of the primaries show a large distribution of axis ratios, where those with low elongation (ratio of long and short axis) are both oblate and prolate, and nearly all with large elongation are prolate. This work presents results that suggest tidal disruption of gravitational aggregates can make binaries physically similar to those currently observed in the NEA population. As well, tidal disruption may create an equal number of binaries with qualities different from those observed, mostly binaries with large separation and with elongated primaries.  相似文献   

Figure of the double Asteroid 90 Antiope from adaptive optics and lightcurve observations     

P. Descamps  T. Michalowski  F. Colas  M. Assafin  M. Polinska  D. Hestroffer  R. Vieira-Martins  M. Birlan  A. Peyrot  J. Dorseuil  T. Dijoux 《Icarus》2007,187(2):482-499

A long-term adaptive optics (AO) campaign of observing the double Asteroid (90) Antiope has been carried out in 2003-2005 using 8-10-m class telescopes, allowing prediction of the circumstances of mutual events occurring during the July 2005 opposition [Marchis, F., Descamps, P., Hestroffer, D., Berthier, J., de Pater, I., 2004. Bull. Am. Astron. Soc. 36, 1180]. This is the first opportunity to use complementary lightcurve and AO observations to extensively study the (90) Antiope system, an interesting visualized binary doublet system located in the main belt. The orbital parameters derived from the AO observations have served as input quantities for the derivation of a whole set of other physical parameters (namely shapes, surface scattering, bulk density, and internal properties) from analysis of collected lightcurves. To completely model the observed lightcurves, we employed Roche figures to construct an overall shape solution. The combination of these complementary observations has enabled us to derive a reliable physical and orbital solution for the system. Our model is consistent with a system of slightly non-spherical components, having a size ratio of 0.95 (with Ravg=42.9±0.5 km, separation=171±1 km), and exhibiting equilibrium figures for homogeneous rotating bodies. A comparison with grazing occultation event lightcurves suggests that the real shapes of the components do not depart from Roche equilibrium figures by more than 10%. The J2000 ecliptic coordinates of the pole of the system are λn=200°±2° and αn=38°±2°. The orbital period was refined to P=16.5051±0.0001 h, and the density is found to be slightly lower than previous determinations, with a value of 1.25±0.05 g/cm³, leading to a significant macro-porosity of 30%. Possible scenarios for the origin of the system are also discussed.  相似文献   

Searching for inner-Earth objects: a possible ground-based approach     

Gianluca Masi 《Icarus》2003,163(2):389-397

The likely existence of bodies orbiting the Sun with aphelia Q < 0.983 AU has been suggested by numerical simulations of the dynamical evolution of the near-Earth objects (NEOs) population. For obvious reasons, these hypothetical minor bodies are called inner-Earth objects (IEOs). While much progresses has been made in learning more about the Amor, Apollo, and Aten population from surveys optimized for their discovery, no large, systematic, and similar observation projects devoted to the search of IEOs have been started. For their own orbital nature, IEOs can be observed only at small solar elongations (<90°), corresponding to regions of the sky currently neglected by the modern, ongoing surveys. This paper discusses a possible ground-based approach to look for IEOs, providing some useful tricks and the results of simulated surveys devoted to their discovery. It will be shown that such a search promises interesting results, the setup of a dedicated project being highly recommended.  相似文献   

A possible YORP effect on C and S Main Belt Asteroids     

A. Carbognani 《Icarus》2011,211(1):519-527

A rotating frequency analysis in a previous paper, showed that two samples of C and S-type asteroids belonging to the Main Belt, but not to any families, present two different values for the transition diameter to a Maxwellian distribution of the rotation frequency, respectively 48 and 33 km. In this paper, after a more detailed statistical analysis, aiming to verify that the result is physically relevant, we found a better estimate for the transition diameter, respectively D_C = 44 ± 2 km and D_S = 30 ± 1 km. The ratio between these estimated transition diameters, D_C/D_S = 1.5 ± 0.1, can be supported with the help of the YORP (Yarkovsky-O’Keefe-Radzievskii-Paddack) effect, although other physical causes cannot be completely ruled out.In this paper we have derived a simple scaling law for YORP which, taking into account the different average heliocentric distance, the bulk density, the albedo and the asteroid “asymmetry surface factor”, has enabled us to reasonably justify the ratio between the diameters transition of C-type and S-type asteroids. The same scaling law can be used to estimate a new ratio between the bulk densities of S and C asteroids samples (giving ρ_S/ρ_C ≈ 2.9 ± 0.3), and can explain why the asteroids near the transition diameter have about the same absolute magnitude. For C-type asteroids, using the found density ratio and other estimates of S-type density, it is also possible to estimate an average bulk density equal to 0.9 ± 0.1 g cm⁻³, a value compatible with icy composition. The suggested explanation for the difference of the transition diameters is a plausible hypothesis, consistent with the data, but it needs to be studied more in depth with further observations.  相似文献   

V-type asteroids in the middle main belt     

F. Roig  D. Nesvorný  R. Gil-Hutton 《Icarus》2008,194(1):125-136

V-type asteroids are bodies whose surfaces are constituted of basalt. In the Main Asteroid Belt, most of these asteroids are assumed to come from the basaltic crust of Asteroid (4) Vesta. This idea is mainly supported by (i) the fact that almost all the known V-type asteroids are in the same region of the belt as (4) Vesta, i.e., the inner belt (semi-major axis 2.1<a<2.5 AU), (ii) the existence of a dynamical asteroid family associated to (4) Vesta, and (iii) the observational evidence of at least one large craterization event on Vesta's surface. One V-type asteroid that is difficult to fit in this scenario is (1459) Magnya, located in the outer asteroid belt, i.e., too far away from (4) Vesta as to have a real possibility of coming from it. The recent discovery of the first V-type asteroid in the middle belt (2.5<a<2.8 AU), (21238) 1995WV7 [Binzel, R.P., Masi, G., Foglia, S., 2006. Bull. Am. Astron. Soc. 38, 627; Hammergren, M., Gyuk, G., Puckett, A., 2006. ArXiv e-print, astro-ph/0609420], located at ∼2.54 AU, raises the question of whether it came from (4) Vesta or not. In this paper, we present spectroscopic observations indicating the existence of another V-type asteroid at ∼2.53 AU, (40521) 1999RL95, and we investigate the possibility that these two asteroids evolved from the Vesta family to their present orbits by a semi-major axis drift due to the Yarkovsky effect. The main problem with this scenario is that the asteroids need to cross the 3/1 mean motion resonance with Jupiter, which is highly unstable. Combining N-body numerical simulations of the orbital evolution, that include the Yarkovsky effect, with Monte Carlo models, we compute the probability that an asteroid of a given diameter D evolves from the Vesta family and crosses over the 3/1 resonance, reaching a stable orbit in the middle belt. Our results indicate that an asteroid like (21238) 1995WV7 has a low probability (∼1%) of having evolved through this mechanism due to its large size (D∼5 km), because the Yarkovsky effect is not sufficiently efficient for such large asteroids. However, the mechanism might explain the orbits of smaller bodies like (40521) 1999RL95 (D∼3 km) with ∼70-100% probability, provided that we assume that the Vesta family formed ?3.5 Gy ago. We estimate the debiased population of V-type asteroids that might exist in the same region as (21238) and (40521) (2.5<a?2.62 AU) and conclude that about 10 to 30% of the V-type bodies with D>1 km may come from the Vesta family by crossing over the 3/1 resonance. The remaining 70-90% must have a different origin.  相似文献   

Extreme sensitivity of the YORP effect to small-scale topography     

Thomas S. Statler 《Icarus》2009,202(2):502-513

Radiation recoil (YORP) torques are shown to be extremely sensitive to small-scale surface topography, using numerical simulations. Starting from a set of “base objects” representative of the near-Earth object population, random realizations of three types of small-scale topography are added: Gaussian surface fluctuations, craters, and boulders. For each, the expected relative errors in the spin and obliquity components of the YORP torque caused by the observationally unresolved small-scale topography are computed. Gaussian power, at angular scales below an observational limit, produces expected errors of order 100% if observations constrain the surface to a spherical harmonic order l?10. For errors under 10%, the surface must be constrained to at least l=20. A single crater with diameter roughly half the object's mean radius, placed at random locations, results in expected errors of several tens of percent. The errors scale with crater diameter D as D² for D>0.3 and as D³ for D<0.3 mean radii. Objects that are identical except for the location of a single large crater can differ by factors of several in YORP torque, while being photometrically indistinguishable at the level of hundredths of a magnitude. Boulders placed randomly on identical base objects create torque errors roughly 3 times larger than do craters of the same diameter, with errors scaling as the square of the boulder diameter. A single boulder comparable to Yoshinodai on 25143 Itokawa, moved by as little as twice its own diameter, can alter the magnitude of the torque by factors of several, and change the sign of its spin component at all obliquities. Most of the total torque error produced by multiple unresolved craters is contributed by the handful of largest craters; but both large and small boulders contribute comparably to the total boulder-induced error. A YORP torque prediction derived from groundbased data can be expected to be in error by of order 100% due to unresolved topography. Small surface changes caused by slow spin-up or spin-down may have significant stochastic effects on the spin evolution of small bodies. For rotation periods between roughly 2 and 10 h, these unpredictable changes may reverse the sign of the YORP torque. Objects in this spin regime may random-walk up and down in spin rate before the rubble-pile limit is exceeded and fissioning or loss of surface objects occurs. Similar behavior may be expected at rotation rates approaching the limiting values for tensile-strength dominated objects.  相似文献   

New V-type asteroids in near-Earth space     

S. Marchi  M. Lazzarin  S. Magrin 《Icarus》2005,175(1):170-174

We present new visible and near-infrared spectroscopic observations of 4 small, previously unclassified, near-Earth objects (NEOs). They appear to have basaltic surfaces, and hence they can be classified as V-types. Their visible spectra exhibit a closer spectral match with the Main-Belt (MB) Asteroid (4) Vesta than the other, presently known, V-type NEOs and MB asteroids. The near-infrared spectrum of Asteroid 2003 FT3 shows—for the first time among NEOs—a peculiar shape of the 1 μm band, maybe suggesting an overabundance of olivine compared to the other V-types and to (4) Vesta. The presence of V-type objects among NEOs may be a consequence of the delivery processes connecting the inner MB to the near-Earth region. On the basis of the orbital parameters of the NEOs presented here, both the resonances (3:1 and ν₆), usually considered as the most relevant gateways for the production of near-Earth asteroids, should have been active to transfer the bodies from the MB region.  相似文献   

Physical investigation of the potentially hazardous Asteroid (144898) 2004 VD17     

F. De Luise  D. Perna  E. Dotto  I.N. Belskaya  G.B. Valsecchi  A. Rossi  P. Paolicchi 《Icarus》2007,191(2):628-635

In this paper we present the observational campaign carried out at ESO NTT and VLT in April and May 2006 to investigate the nature and the structure of the near-Earth object (144898) 2004 VD17. In spite of a great quantity of dynamical information, according to which it will have a close approach with the Earth in the next century, the physical properties of this asteroid are largely unknown. We performed visible and near-infrared photometry and spectroscopy, as well as polarimetric observations. Polarimetric and spectroscopic data allowed us to classify 2004 VD17 as an E-type asteroid. A good agreement was also found with the spectrum of the aubrite meteorite Mayo Belwa. On the basis of the polarimetric albedo (pv=0.45) and of photometric data, we estimated a diameter of about 320 m and a rotational period of about 2 h. The analysis of the results obtained by our complete survey have shown that (144898) 2004 VD17 is a peculiar NEO, since it is close to the breakup limits for fast rotator asteroids, as defined by Pravec and Harris [Pravec, P., Harris, A.W., 2000. Icarus 148, 12-20]. These results suggest that a more robust structure must be expected, as a fractured monolith or a rubble pile in a “strength regime” [Holsapple, K.A., 2002. Speed limits of rubble pile asteroids: Even fast rotators can be rubble piles. In: Workshop on Scientific Requirements for Mitigation of Hazardous Comets and Asteroids, Washington, September, 2002].  相似文献   

Do planetary encounters reset surfaces of near Earth asteroids?     

David Nesvorný  William F. Bottke  Clark R. Chapman 《Icarus》2010,209(2):510-519

Processes such as the solar wind sputtering and micrometeorite impacts can modify optical properties of surfaces of airless bodies. This explains why spectra of the main belt asteroids, exposed to these ‘space weathering’ processes over eons, do not match the laboratory spectra of ordinary chondrite (OC) meteorites. In contrast, an important fraction of Near Earth Asteroids (NEAs), defined as Q-types in the asteroid taxonomy, display spectral attributes that are a good match to OCs. Here we study the possibility that the Q-type NEAs underwent recent encounters with the terrestrial planets and that the tidal gravity (or other effects) during these encounters exposed fresh OC material on the surface (thus giving it the Q-type spectral properties). We used numerical integrations to determine the statistics of encounters of NEAs to planets. The results were used to calculate the fraction and orbital distribution of Q-type asteroids expected in the model as a function of the space weathering timescale, t_sw (see main text for definition), and maximum distance, r^∗, at which planetary encounters can reset the surface. We found that t_sw ∼ 10⁶ yr (at 1 AU) and r^∗ ∼ 5R_pl, where R_pl is the planetary radius, best fit the data. Values t_sw < 10⁵ yr would require that r^∗ > 20R_pl, which is probably implausible because these very distant encounters should be irrelevant. Also, the fraction of Q-type NEAs would be probably much larger than the one observed if t_sw > 10⁷ yr. We found that t_sw ∝ q², where q is the perihelion distance, expected if the solar wind sputtering controls t_sw, provides a better match to the orbital distribution of Q-type NEAs than models with fixed t_sw. We also discuss how the Earth magnetosphere and radiation effects such as YORP can influence the spectral properties of NEAs.  相似文献   

Impact cratering and break up of the small bodies of the Solar System     

J. Leliwa-Kopystyński  M.J. Burchell  D. Lowen 《Icarus》2008,195(2):817-826

We consider the largest impact craters observed on small satellites and asteroids and the impact disruption of such bodies. Observational data are considered from 21 impact-like structures on 13 satellites and 8 asteroids (target body radii in the range 0.7-265 km). If the radius of the target body is R and the diameter of the largest crater observed on this body D, the ratio D/R is then the main observational parameter of interest. This is found on the observed bodies and compared to data obtained in the laboratory. Taking the largest observed value for D/R as a proxy for the ratio Dc/R (where Dc is the diameter of the largest crater that can be formed on a body without shattering it) it was found that for the observed icy satellites Dc,icy≈1.2R and for the asteroids and the rocky satellites Dc,rocky≈1.6R. In laboratory experiments with ice targets at impactor speeds of 1 to 3 km s⁻¹ we obtained Dc,icy≈1.64R.  相似文献   

Express delivery of fossil meteorites from the inner asteroid belt to Sweden     

David Nesvorný  David Vokrouhlický  Brett Gladman 《Icarus》2007,188(2):400-413

Our understanding of planet formation depends in fundamental ways on what we learn by analyzing the composition, mineralogy, and petrology of meteorites. Yet, it is difficult to deduce the compositional and thermal gradients that existed in the solar nebula from the meteoritic record because, in most cases, we do not know where meteorites with different chemical and isotopic signatures originated. Here we developed a model that tracks the orbits of meteoroid-sized objects as they evolve from the ν₆ secular resonance to Earth-crossing orbits. We apply this model to determining the number of meteorites accreted on the Earth immediately after a collisional disruption of a D∼200-km-diameter inner-main-belt asteroid in the Flora family region. We show that this event could produce fossil chondrite meteorites found in an ≈470 Myr old marine limestone quarry in southern Sweden, the L-chondrite meteorites with shock ages ≈470 Myr falling on the Earth today, as well as asteroid-sized fragments in the Flora family. To explain the measured short cosmic-ray exposure ages of fossil meteorites our model requires that the meteoroid-sized fragments were launched at speeds >500 m s⁻¹ and/or the collisional lifetimes of these objects were much shorter immediately after the breakup event than they are today.  相似文献   

Equilibrium figures of inhomogeneous synchronous binary asteroids     

P. Descamps 《Icarus》2010,207(2):758-768

The present paper deals with the application of the classical theory of equilibrium figures of two rotating liquid masses to the case where bodies exhibit a radially stratified internal density distribution so that they can be considered as inhomogeneous bodies. The derived ellipsoidal shape solutions are applied to five real systems of equal-sized synchronous asteroids. Furthermore, internal inhomogeneity puts strong constraints on the surface grain density. A satisfactory model fit is achieved with internal densities of asteroids steadily increasing outwards. In particular, from such an approach we derived grain densities of the considered systems in agreement with their mineralogical composition inferred from reflectance spectroscopy. According to this new approach, 4492 Debussy, presently of unknown spectral type, is predicted to appear as a C-type object with a grain density on the order of 2 g/cm³.  相似文献   

Taxonomy of asteroids in the Cybele region from the analysis of the Sloan Digital Sky Survey colors     

R. Gil-Hutton  J. Licandro 《Icarus》2010,206(2):729-734

In this paper we search for photometric data of asteroids in the outer region of the Hecuba gap in the Moving Object Catalogue of the Sloan Digital Sky Survey to find the spectrophotometric characteristics of small members of this group. We found that the correlation between size and spectral slope previously identified for Cybele asteroids is correct only for large objects (H_V<12) but it is not supported by data obtained for the small ones. This result argues against the scenario suggesting that D-type objects are more fragile than P-types, favoring disruptive collisions of precursors of the first type and resulting in a larger fraction of the smaller body population being collisional fragments from a few large D-type precursors. A statistical comparison of the spectral slope histograms of Cybeles and Hildas showed that it is not possible to reject the hypothesis that both samples were obtained from the same population at a confident limit of 90%. This result could be indicative of certain homogeneity in the taxonomic distributions of the outer belt populations due to a similar original composition and/or a similar resurfacing processes of these distant bodies. Despite the intrinsic limitations of the five band photometry of the Sloan Digital Sky Survey, the analysis presented is based mainly in the detection of spectral slopes thus providing sufficient indication of the taxonomic type of these asteroids and making us confident about our conclusions.  相似文献   

Equilibrium shapes of rubble-pile binaries: The Darwin ellipsoids for gravitationally held granular aggregates     

Ishan Sharma 《Icarus》2010,205(2):638-657

Binaries are in vogue; many minor-planets like asteroids are being found to be binary or contact-binary systems. Even ternaries like 87 Sylvia have been discovered. The densities of these binaries are often estimated to be very low, and this, along with suspected accretionary origins, hints at a rubble interior. As in the case of fluid objects, a rubble-pile is unable to sustain all manners of spin, self-gravitation, and tidal interactions. This motivates the present study of the possible ellipsoidal shapes and mutual separations that members of a rubble-pile binary system may achieve. Conversely, knowledge of a granular binary’s shape and separation will constrain its internal structure - the ability of the binary’s members to sustain elongated shapes and/or maintain contact will hint at appreciable internal frictional strength. Because the binary’s members are allowed to be of comparable mass, the present investigation constitutes an extension of the second classical Darwin problem to granular aggregates.General equations defining the ellipsoidal rubble-pile binary system’s equilibrium are developed. These are then specialized to a pair of spin-locked, possibly unequal, prolate ellipsoidal granular aggregates aligned along their long axes. We observe that contact rubble-pile binaries can indeed exist. Further, depending on the binary’s geometry, an equilibrium contact binary’s members may, in fact, disrupt if separated. These results are applied to four suspected or known binaries: 216 Kleopatra, 25143 Itokawa, 624 Hektor and 90 Antiope. This exercise helps to bound the shapes and/or provide information about the interiors of these binaries.The binary’s interior will be modeled as a rigid-plastic, cohesionless material with a Drucker-Prager yield criterion. This rheology is a reasonable first model for rubble piles. We employ an approximate volume-averaging procedure that is based on the classical method of moments, and is an extension of the virial method (Chandrasekhar, S., 1969. Ellipsoidal Figures of Equilibrium. Yale University Press, New Haven, CT) to granular solid bodies. The present approach also helps us present an incrementally consistent approach to investigate the equilibrium shapes of fluid binaries, while highlighting the inconsistencies and errors inherent in the popular “Roche binary approximation”.  相似文献