Disruptive Collisions Research Articles

Discovery of the triple asteroidal system 87 Sylvia

After decades of speculation, the existence of binary asteroids has been observationally confirmed, with examples in all minor planet populations. However, no triple systems have hitherto been discovered. Here we report the unambiguous detection of a triple asteroidal system in the main belt, composed of a 280-km primary (87 Sylvia) and two small moonlets orbiting at 710 and 1,360 km. We estimate their orbital elements and use them to refine the shape of the primary body. Both orbits are equatorial, circular and prograde, suggesting a common origin. Using the orbital information to estimate its mass and density, 87 Sylvia appears to have a rubble-pile structure with a porosity of 25-60 per cent. The system was most probably formed through the disruptive collision of a parent asteroid, with the new primary resulting from accretion of fragments, while the moonlets are formed from the debris, as has been predicted previously.

Interiors of small bodies: foundations and perspectives

With the surface properties and shapes of solar system small bodies (comets and asteroids) now being routinely revealed by spacecraft and Earth-based radar, understanding their interior structure represents the next frontier in our exploration of these worlds. Principal unknowns include the complex interactions between material strength and gravity in environments that are dominated by collisions and thermal processes. Our purpose for this review is to use our current knowledge of small body interiors as a foundation to define the science questions which motivate their continued study: In which bodies do “planetary” processes occur? Which bodies are “accretion survivors”, i.e., bodies whose current form and internal structure are not substantially altered from the time of formation? At what characteristic sizes are we most likely to find “rubble-piles”, i.e., substantially fractured (but not reorganized) interiors, and intact monolith-like bodies? From afar, precise determinations of newly discovered satellite orbits provide the best prospect for yielding masses from which densities may be inferred for a diverse range of near-Earth, main-belt, Trojan, and Kuiper belt objects. Through digital modeling of collision outcomes, bodies that are the most thoroughly fractured (and weak in the sense of having almost zero tensile strength) may be the strongest in the sense of being able to survive against disruptive collisions. Thoroughly fractured bodies may be found at almost any size, and because of their apparent resistance to disruptive collisions, may be the most commonly found interior state for small bodies in the solar system today. Advances in the precise tracking of spacecraft are giving promise to high-order measurements of the gravity fields determined by rendezvous missions. Solving these gravity fields for uniquely revealing internal structure requires active experiments, a major new direction for technological advancement in the coming decade. We note the motivation for understanding the interior properties of small bodies is both scientific and pragmatic, as such knowledge is also essential for considering impact mitigation.

Moonlet Collisions and the Effects of Tidally Modified Accretion in Saturn's F Ring

We both test and offer an alternative to a meteoroid bombardment model (M. R. Showalter 1998, Science 282, 1099–1102) and suggest that anomalous localized brightenings in the F ring observed by Voyager result from disruptive collisions involving poorly consolidated moonlets, or “rubble piles.” This model can also explain the transient events observed during ring plane crossing. We have developed an evolutionary model that considers both the competing effects of accretion and disruption at the location of the F ring. Our numerical model is a Markov process where probabilities of mass transfer between the states of the system form a “transition matrix.” Successive multiplications of this matrix by the state vector generate expectation values of the distribution after each time step as the system approaches quasi-equilibrium. Competing effects of accretion and disruption in the F ring are found to lead to a bimodal distribution of ring particle sizes. In fact, our simulation predicts the presence of a belt of kilometer-sized moonlets in the F ring. These moonlets may continually disrupt one another and re-accrete on short time scales. We also agree with J. N. Cuzzi and J. A. Burns (1988, Icarus 74, 284–324), who suggest that the classical F ring itself may be the consequence of a relatively recent collision between two of the largest of these yet unseen objects. Cassini observations can confirm the existence of the moonlet belt by directly observing these objects or the waves they create in the rings.

The Effects of a Stellar Encounter on a Planetesimal Disk

We investigate the effects of a passing stellar encounter on a planetesimal disk through analytical calculations and numerical simulations, and derive the boundary radius ( a planet) outside which planet formation is inhibited by disruptive collisions with high relative velocities. S. Ida, J. Larwood, and A. Burkert (2000, Astrophys. J. 528, 1013–1025) suggested that a stellar encounter caused inhibition of planet formation in the outer part of a protoplanetary disk. We study orbital eccentricity ( e) and inclination ( i) of planetesimals pumped up by perturbations of a passing single star. We also study the degree of alignment of longitude of pericenter and ascending node to estimate relative velocities between the planetesimals. We model a protoplanetary system as a disk of massless particles circularly orbiting a host star, following S. Ida, J. Laywood, and A. Burkert (2000, Astrophys. J. 528, 1013–1025). The massless particles represent planetesimals. A single star as massive as the host star encounters the protoplanetary system. Numerical orbital simulations show that in the inner region at semimajor axis a≲0.2 D where D is the pericenter distance of the encounter, e and i have power-law dependence on ( a/ D) as e∝( a/ D) 5/2 and i∝( a/ D) 3/2, and the longitudes are aligned, independent of the encounter parameters. In the outer region a≳0.2 D, and the radial gradient is steeper and is not expressed by a single power law. The longitudes are not aligned. Since planet accretion is inhibited by e as small as 0.01, we focus on the weakly perturbed inner region. We analytically reproduce the power-law dependence and explicitly give numerical factors of the power-law dependence as functions of encounter parameters. We derive the boundar y radius ( a planet) of the planet-forming region as a function of dynamical parameters of a stellar cluster, assuming the protoplanetary system belongs to the stellar cluster. The radial gradient of e is so steep that the boundary is sharply determined. Planetesimal orbits are significantly modified beyond the boundary, while they are almost intact inside the boundary. This tendency is strengthened by reduction of relative velocity due to the longitude alignment in the inner region. We find a planet ∼ 40–60 AU in the case of D ∼ 150–200 AU. D ∼ 200 AU may be likely to occur in a relatively dense cluster. We point out that the size of planetary systems ( a planet) born in a dense cluster may be necessarily restricted to that comparable to the size of a planet region (∼30–40 AU) of our SolarSystem.

Evolution of porosity in small icy bodies

In this paper we consider self-compaction of icy bodies with radii from 60 to 200 km. They could be some of the icy satellites of the giant planets and some of the Kuiper belt objects. It is assumed that the considered globes were formed as porous bodies by the process of homogeneous accretion. They are not products of disruptive collisions. The evolution is considered from an early stage of formation (the embryo stage), through the stage when accretion is completed (present-day mass is settled), until the time when the present state (present-day radius, as well as the moment of inertia, if available) is reached. The model we use to calculate the evolution of the distributions of density (or porosity) and of temperature is based on that presented by Kossacki and Leliwa-Kopystyński (1993. Planet. Space Sci. 41(10), 729–741). The model can be applied with various rheological formulae for pressure- and temperature-dependent compaction of granular, initially porous, icy-mineral medium. The bodies under consideration are assumed to be composed of water ice with an admixture of ammonia and of silicates. The components are uniformly distributed with a mass ratio C of silicates to total being fixed. An abundance x of ammonia relative to water is one of the crucial parameters of the model. The internal sources of energy leading to the evolution are: (i) the gravitational energy of initially porous globes; and (ii) the energy of radioactive decay of radionuclides dispersed within the mineral component. The amount of long lived isotopes is that corresponding to chondritic meteorites. Moreover, an initial presence of short lived Al 26 is not excluded a priori. Its initial abundance is another parameter of the model. The particular examples of calculations concern bodies with the sizes and densities of Mimas, Janus and Epimetheus. The three-dimensional (3-D) presentation of the results has allowed us to estimate physically reasonable ranges for ammonia and Al 26 contents.

Collision risk for high inclination satellite constellations

We assess the collision hazard for a constellation of telecommunication satellites such as IRIDIUM, arising from the possible chance impact break-up of one of the satellites. The resulting swarm of fragments will orbit the Earth at about the same altitude as the surviving satellites, but will gradually spread due to orbital perturbations, so as to make possible impacts with satellites staying on orbital planes different from that of the parent satellite. We find that at intermediate fragment masses of the order of 1 kg, sufficient to trigger subsequent catastrophic impacts, the self-generated collision hazard for the constellation satellites exceeds the background level due to the overall debris population for several years. This is true, in particular, when differential precession of the orbits leads the fragments to encounter satellites revolving around the Earth in the opposite sense, resulting both in higher impact speeds and in enhanced collision probabilities. We estimate that there is about a 10% chance that a first-generation fragment will trigger subsequent disruptive collisions in the constellation within a decade.

Deconstructing Castalia: Evaluating a Postimpact State

We describe the rotational and translational states of the fragments of a disrupted asteroid. All the fragments are found to be in complex rotation and have widely varying rotation periods. Although this is only one example of a possible collision, the numerical results for small fragment rotation rates are consistent with recent observations, including the small asteroid 1998KY26, and they lend insight into the latter stages of impact disruption. In particular, fragment “memory” of parental rotation appears to be negligible. This is the most comprehensive description to date of the state of an asteroid or comet immediately following a disruptive collision.

Semimajor axis mobility of asteroidal fragments

The semimajor axes of asteroids up to about 20 kilometers in diameter drift as a result of the Yarkovsky effect, a subtle nongravitational mechanism related to radiation pressure recoil on spinning objects that orbit the sun. Over the collisional lifetimes of these objects (typically, 10 to 1000 million years), orbital semimajor axes can be moved by a few hundredths of an astronomical unit for bodies between 1 and 10 kilometers in mean radius. This has implications for the delivery of multikilometer near-Earth asteroids, because the Yarkovsky drift drives many small main-belt asteroids into the resonances that transport them to the Mars-crossing state and eventually to near-Earth space. Recent work has shown that, without such a drift, the Mars-crossing population would be depleted over about 100 million years, a time scale much smaller than the age of the solar system. Moreover, the Yarkovsky semimajor axis mobility may spread in an observable way the tight semimajor axis clustering of small asteroids produced as a consequence of disruptive collisions.

The influence of the fragmentation threshold on the long term evolution of the orbital debris environment

By means of the code STAT, developed in the past four years in Pisa under an ESA contract, we have carried out a number of numerical simulations of the long term evolution of the orbital debris population, and monitored the growth in time of the number of objects larger than 1 mg in mass. Previous work had shown that after a phase of gradual, quasi-linear growth due to launches and explosions, collisions in the low-orbit population lead to a kind of a chain reaction, yielding a runaway proliferation of fragments. The epoch at which this behavior takes place as well as the characteristic time constant of the nearly-exponential growth phase are sensitively related to some parameters of the physical model adopted for the collisional outcomes. The most critical of such parameters is the fragmentation threshold Q ∗ , defined as the limiting value of the ratio between the projectile kinetic energy and the target mass which discriminates between small scale “cratering” impacts and disruptive collisions. Here we report on the results of a set of simulations performed by varying Q ∗ by plus or minus a factor two with respect to the nominal value of 4.5 × 10 4 J/kg, inferred from laboratory experiments. The main results of these simulations are the following: first, we confirm that a phase of nearly-exponential runaway growth of the small-size debris population is always observed, but the initial “waiting time” interval is at least ≈ 200 years for our range of Q ∗ values: second, the onset time for the exponential regime as well as its characteristic time constant are growing functions of Q ∗ ; and third, this behavior agrees well with the forecasts of much simpler mathematical models developed earlier (Farinella and Cordelli, Science & Global Security 2, 365–378, 1991). Our results highlight the need for more extensive experimental estimates of the fragmentation threshold parameter and its dependence on the size, shape and structure of the space objects. Such estimates should then be incorporated into a future generation of more realistic simulation models.

Habitable Planet Formation in Binary Star Systems

Assuming current models of terrestrial planet formation in the Solar System, we numerically investigate the conditions under which the secondary star in a binary system will inhibit planet growth in the circumstellar habitable zone. Runaway accretion is assumed to be precluded if the secondary (1) causes the planetesimal orbits to cross within the runaway accretion time scale and (2) if, during crossing, the relative velocities of the planetesimals have been accelerated beyond a certain critical value which results in disruption collisions rather than accretion. For a two solar mass binary with planetesimals in circular orbits about one star at 1 AU, and a typical wide binary eccentricity of 0.5, the minimum binary semimajor axis which would not inhibit planet formation,ac, is 32 AU. If the planetesimals orbit the center of mass of the binary system,ac= 0.10 AU, which is inside the tidal circularization radius. We obtain an empirical formula giving the dependencies ofacon the binary eccentricity, secondary mass, planetesimal location, and critical disruption velocity. Based on the distributions of orbital elements of a bias-corrected sample of nearby G-dwarfs, we find that ≈ 60% of solar-type binaries cannot be excluded from having a habitable planet solely on the basis of the perturbative effect of the secondary star. This conclusion is independent of when the secondary star formed, nebula dissipative mechanisms, and the time scale for runaway planetesimal accretion, and is relatively insensitive to the mass of the secondary star, the critical disruption velocity, and the location of planetesimals within the circumstellar habitable zone. An earlier study of planet formation in binary star systems came to a different conclusion, namely that planet formation, even at Mercury's distance, is unlikely except in widely separated systems (≥50 AU), or when the secondary has a very low mass and near circular orbit as in the Sun–Jupiter system. The discrepancy with the present numerical study is due in part to the different runaway accretion time scales assumed and the neglect in the earlier study of an exact criterion for crossing orbits.

Collisional Evolution of Edgeworth–Kuiper Belt Objects

The Edgeworth–Kuiper Belt contains a population of objects ≈10 3times that of the main asteroid belt, spread over a volume ≈10 3larger and with relative speeds ≈10 times lower. As for the asteroids, the size distribution of Edgeworth–Kuiper Belt objects has been modified by mutual impacts over Solar System history. We have modeled this collisional evolution process using a numerical code developed originally to study asteroid collisional evolution but modified to reflect collision rates in the Edgeworth–Kuiper Belt. Our numerical simulations show that collisional evolution is substantial in the inner part of the Edgeworth–Kuiper Belt, but its intensity decreases with increasing distance from the Sun. In the inner belt, objects with diameters D> 50–100 km are not depleted by disruptive collisions; hence they reflect the original (formative) population (many of them, however, may have been converted into “rubble piles”). On the other hand, smaller objects are mostly multigenerational fragments, although the original population must have contained a significant number of bodies down to at least a few tens of kilometers in size in order to initiate a collisional cascade. About 10 fragments, 1–10 km in size, are produced per year in the inner Edgeworth–Kuiper Belt, with a few percent of them inserted into chaotic resonant orbits. This is in rough agreement with the required influx rate of Jupiter-family comets. Both collisions and dynamical instabilities associated with resonances are processes that can inject comets into the “escape hatches,” but our results indicate that most comets coming from the Edgeworth–Kuiper Belt would be fragments from larger parent bodies, rather than primitive planetesimals. However, this does not apply to Chiron-sized ( D> 100 km) objects, which must be primordial and delivered to the outer Solar System by either dynamical processes or nondisruptive collisions.

The age of the Veritas asteroid family deduced by chaotic chronology

ASTEROID families are groups of objects produced in disruptive collisions of a parent body. Although family members are widely dispersed in real space, they cluster in the parameter space defined by their so-called proper elements, and can thus be distinguished from the background asteroid population1–3. For most asteroids, these parameters are very close to being invariants of motion and families are still apparent billions of years after their formation4'5. But these parameters undergo chaotic diffusion, and in some cases the rate of diffusion might be large enough that a family member exits from the region of proper-element space occupied by the family after a characteristic time which is shorter than the lifetime of the Solar System. In this case, the characteristic time should provide an approximate upper bound to the age of the family. Here we use this 'chaotic chronology' method to estimate the lifetime of the unusually compact Veritas family. Calculations of the evolu-tion of the proper elements of the family show that two members (including the largest, 490 Veritas) wander outside the borders of the family on a timescale of about 50 Myr, indicating that the family has an age of less than this.

Coherent photon-photon processes in disruptive and non-disruptive relativistic heavy-ion collisions

Using an impact parameter formulation, differential probability distributions and cross sections for the production of lepton pairs via the photon-photon mechanism are calculated for relativistic heavy-ion collisions. The characteristic features of lepton pair production in disruptive as well as in non-disruptive A-A collisions are studied. Cross sections can be large, the very low k ⊥-values of the pairs will help to distinguish these pairs from the ones originating from other sources like Drell-Yan or thermal production.

A reappraisal of ignimbrite emplacement: progressive aggradation and changes from particulate to non-particulate flow during emplacement of high-grade ignimbrite

We propose a mechanism by which massive ignimbrite and layered ignimbrite sequences — the latter liable to have been previously interpreted as multiple flow units-form by progressive aggradation during sustained passage of a single particulate flow. In the case of high-temperature eruptive products the mechanism simplifies interpretation of problematic deposits that exhibit pronounced vertical and lateral variations in texture, including between non-welded, eutaxitic, rheomorphic (lineated) and lava-like. Agglutination can occur within the basal part of a hot density-stratified flow. During initial incursion of the flow, agglutinate chills and freezes against the ground. During sustained passage of the flow, agglutination continues so that the non-particulate (agglutinate) layer thickens (aggrades) and becomes mobile, susceptible to both gravity-induced motion and traction-shear imparted by the overriding particulate part of the flow. The particulate to non-particulate (P-NP) transition occurs in and just beneath a depositional boundary layer, where disruptive collisions of hot viscous droplets give way, via sticky grain interactions, to fluidal behavior following adhesion. Because they have different rheologies, the particulate and non-particulate flow components travel at different velocities and respond to topography in different ways. This may cause detachment and formation of two independent flows. The P-NP transition is controlled by factors that influence the rheological properties of individual erupted particles (strain rate, temperature, and composition including volatiles), by cooling and volatile exsolution during transport, and by the particle-size population and concentration characteristics of the depositional boundary layer. At any one location along the flow path one or more of these can change through time (unsteady flow). Thus the P-NP transition can develop momentarily or repeatedly during the passage of an unsteady flow, or it can occur continuously during the passage of a quasi-steady flow supplied by a sustained explosive eruption. Vertical facies successions developed in the deposit (high-grade ignimbrite) reflect temporal changes in flow steadiness and in material supplied at source. The P-NP transition is also influenced by factors that affect flow behaviour, such as topography. It may occur at any location laterally between a proximal site of deflation (e.g. a fountain-fed lava) and a flow's distal limit, but it most commonly occurs throughout a considerable length of the flow path. Up-sequence variations in welding-deformation fabric (between oblate uniaxial to triaxial and prolate) reflect evolving characteristics of the depositional boundary layer (e.g. fluctuations from direct suspension-sedimentation to deposition via traction carpets or traction plugs), as well as possible modifications resulting from subsequent, post-depositional hot loading and slumping. Similar processes can also account for lateral lithofacies gradations in conduits and vents filled with welded tuff. Our consideration of high-grade ignimbrites has implications for ignimbrite emplacement in general, and draws attention to the limitations of the widely accepted models of emplacement involving mainly high-concentration non-turbulent transport and en masse ‘freezing’ of high-yield-strength plug flows.

Coherent particle production in relativistic heavy ion collisions

Coherent particle production via the γ-γ mechanism occurs in disruptive as well as non-disruptive relativistic heavy ion collisions. Due to the enhancement factor of Z 4, relativistic heavy ion colliders may compete with e +e − colliders in the study of γ-γ physics. To reduce the strong backgroun, non-disruptive collisions must be selected. In disruptive collisions, the dilepton production via coherent γ-γ processes is calculated, as a function of the impact parameter. Cross-sections are large, comparable to those expected from thermal and Drell-Yan mechanisms. However, the very low p ⊥-values will allow a discrimination.

The lifetime of binary asteroids vs gravitational encounters and collisions

We quantitatively assess the long-term stability of binary asteroids with respect to (1) gravitational encounters with passing asteroids and (2) physical collisions. The effects of gravitational encounters are estimated by a simple physical model, which is based on the distinction between penetrating, close, and far encounters, and on the assumption of a random-walk accumulation of the corresponding energy changes. Explicit computations of lifetimes vs disruption by the above-mentioned processes are then made as a function of the main parameters of the systems (separation, size of the components), and estimates are given for a few specific asteroids for which a binary structure has been inferred from different types of observations. The main conclusion is that, especially when bodies of comparatively small sizes are concerned, the lifetimes of asteroidal binaries are limited by the occurrence of disruptive collisions, while gravitational encounters are generally less important. The frequency of catastrophic impacts, however, is quite uncertain, owing to our poor knowledge of asteroid impact strengths and of the projectile flux in the 1-km size range. In general, for systems whose separation does not exceed a few tens of the primary's radius—a limit also set by tidal evolution and Jovian perturbations—typical lifetimes are comparable to or larger than the age of the Solar System.

Gasdynamics and star formation in interacting and merging galaxies

view Abstract Citations (60) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Gasdynamics and Star Formation in Interacting and Merging Galaxies Olson, Kevin M. ; Kwan, John Abstract In this paper we describe several additional simulations to those described in Olson and Kwan. We consider the effects various parameters have on the interaction of two galaxies and on the gas cloud collisions which are induced to occur. Increasing the angle of inclination between the spin axis of the galaxy which contains gas clouds and the orbital angular momentum vector, decreasing the mass of the perturbing galaxy, and increasing the kinetic energy of the interaction all have the effect of reducing the perturbation. A case where both galaxies contain gas clouds and the galaxies merge was also studied. As in Olson and Kwan, most of the cloud-cloud collisions which are induced by the interaction are disruptive and a large number of massive clouds is usually not produced. Based on these calculations and those described in Olson and Kwan, we suggest that if the burst of star formation triggered by the interaction of two galaxies occurs near closest approach, it may be related to the large number of disruptive cloud-cloud collisions which are induced. Monitoring the amount of gas mass involved in these collisions allows us to estimate the star-formation rate and the luminosity of the galaxy. We find that the parameters considered here can, to a large degree, account for the broad range of observed ratios of the far-infrared luminosity to molecular gas mass in interacting and merging galaxies. We also show that the radial location of the enhanced star formation depends on these parameters and that, while a large fraction of interacting galaxies which possess strong morphological peculiarities have elevated rates of star formation in or near their nuclei, a certain fraction also have elevated rates of star formation in their disks with no significant star-formation activity being induced in their nuclei. Publication: The Astrophysical Journal Pub Date: October 1990 DOI: 10.1086/169208 Bibcode: 1990ApJ...361..426O Keywords: Gas Dynamics; Interacting Galaxies; Interstellar Gas; Star Formation; Data Simulation; Molecular Clouds; Astrophysics; GALAXIES: INTERACTIONS; STARS: FORMATION full text sources ADS | data products SIMBAD (2)

Gasdynamics in interacting and merging galaxies

A three-dimensional model is developed for gas clouds orbiting in the gravitational potential of a galaxy which, at some later time, is perturbed by the gravitational influence of another galaxy. As the strength of the interaction increases, the interstellar medium becomes more disturbed. The clouds collide at an increased rate and with larger velocities, so that the fraction of collisions which disrupt the clouds rises rapidly as the strength of the interaction increases. On the other hand, no large increases in the rate at which massive clouds are built up are found. Since interacting galaxies are observed to have star formation efficiencies which are about five times higher than those in isolated galaxies, it is concluded that the star formation induced by the interaction of two galaxies is related to the high-velocity, disruptive cloud-cloud collisions. 46 refs.

Cumberland Falls chondritic inclusions—II. Trace element contents of forsterite chondrites and meteorites of similar redox state

Mineralogic study of black inclusions in the Cumberland Falls enstatite achondrite revealed that they constitute a highly unequilibrated chondritic suite distinct from other chondrite groups. This highly shocked suite, the forsterite (F) chondrites, exhibits mineralogic trends apparently produced during primary nebular condensation and accretion over a broad redox range. We analyzed these samples and possibly related meteorites for Ag, As, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U and Zn, trace elements known to yield important genetic information. The results demonstrate the compositional coherence and distinctiveness of the F chondrite suite relative to other chondrites. The Antarctic aubrite, ALH A78113, may include more F chondrite material. Trace element contents do not vary with mineral compositions hence do not reflect redox variations during formation of F chondrite parental matter. Trace element mobilization—during secondary heating episodes in the F chondrite parent or during its disruptive collision with the enstatite meteorite parent body—is not detectable. Chemical trends in F chondrites apparently reflect primary nebular processes. Cosmochemical fractionation of lithophiles from siderophiles and chalcophiles occurred at moderately high temperatures, certainly higher than those existing during formation of primitive carbonaceous, enstatite and ordinary chondrites of petrologic type ≤3.