Meter-sized Bodies Research Articles

Supercatastrophic Disruption of Asteroids in the Context of SOHO Comet, Fireball, and Meteor Observations

Abstract Granvik et al. reported an absence of asteroids on orbits with perihelia near the Sun that they attribute to the “supercatastrophic disruption” of these bodies. Here we investigate whether there is evidence for this process among other bodies with similarly low perihelia: near-Earth asteroids, Solar and Heliospheric Observatory (SOHO) comets, and meter- and millimeter-sized meteoroids. We determine that no known near-Earth asteroids have past (last 104 yr) histories residing significantly inside the Granvik et al. limit, indirectly supporting the disruption hypothesis. The exception is asteroid (467372) 2004 LG, which spent 2500 yr within this limit and thus presents a challenge to that theory. Phaethon has a perihelion distance hovering just above the limit and may be undergoing slow disruption, which may be the source of its dust complex. We find that the rate at which ungrouped SOHO comets are observed is consistent with expected rates for the injection of small (25 m) class asteroids into the near-Sun region and suggest that this fraction of the SOHO-observed comet population may in fact be asteroidal in origin. We also find that there is an absence of meter-sized bodies with near-Sun perihelia but an excess of millimeter-sized meteoroids. This implies that if near-Sun asteroids disrupt, they do not simply fragment into meter-sized chunks but ultimately disintegrate into millimeter-sized particles. We propose that the disruption of near-Sun asteroids, as well as the anomalous brightening and destruction processes that affect SOHO comets, occur through meteoroid erosion, that is, the removal of material through impacts by high-speed near-Sun meteoroids.

Triggering the Activation of Main-belt Comets: The Effect of Porosity

Abstract It has been suggested that the comet-like activity of Main-belt comets (MBCs) is due to the sublimation of sub-surface water-ice that is exposed when these objects are impacted by meter-sized bodies. We recently examined this scenario and showed that such impacts can, in fact, excavate ice and present a plausible mechanism for triggering the activation of MBCs. However, because the purpose of that study was to prove the concept and identify the most viable ice-longevity model, the porosity of the object and the loss of ice due to the heat of impact were ignored. In this paper, we extend our impact simulations to porous materials and account for the loss of ice due to an impact. We show that for a porous MBC, impact craters are deeper, reaching to ∼15 m, implying that if the activation of MBCs is due to the sublimation of sub-surface ice, this ice has to be within the top 15 m of the object. Results also indicate that the loss of ice due to the heat of impact is negligible, and the re-accretion of ejected ice is small. The latter suggests that the activities of current MBCs are most probably from multiple impact sites. Our study also indicates that for sublimation from multiple sites to account for the observed activity of the currently known MBCs, the water content of MBCs (and their parent asteroids) needs to be larger than the values traditionally considered in models of terrestrial planet formation.

Magnetic and Ionospheric Effects of a Meteoroid Plume

This paper is concerned with the study of the possibility of products of a meteoroid explosion in the atmosphere (meteoroid plume) to reach ionospheric altitudes. It has been shown that, in the case of meter-sized or larger space bodies entering the atmosphere, the plume is able to reach the lower ionosphere. The plume can be one of the sources of the formation of nacreous and noctilucent clouds. The aerosols ejected by the plume to lower ionospheric altitudes can lead to the formation of dust plasma, significantly changing the electrodynamic properties of the medium. The motion of the plume with a velocity of ~1 km/s is accompanied by the generation of a ballistic shock with a radius of 1–10 km. The relative excess pressure in the shock front can cause relative disturbances in the electron content at the altitudes of D, E, and F1 layers by ~10–100%. The geomagnetic effect of the plume and ballistic shock can reach ~1–10 nT.

On the Formation of Planetesimals: Radial Contraction of the Dust Layer Interacting with the Protoplanetary Disk Gas

Radial contraction of the dust layer in the midplane of a gas–dust protoplanetary disk that consists of large dust aggregates is modeled. Sizes of aggregates vary from centimeters to meters assuming the monodispersion of the layer. The highly nonlinear continuity equation for the solid phase of the dust layer is solved numerically. The purpose of the study is to identify the conditions under which the solid matter is accumulated in the layer, which contributes to the formation of planetesimals as a result of gravitational instability of the dust phase of the layer. We consider the collective interaction of the layer with the surrounding gas of the protoplanetary disk: shear stresses act on the gas in the dust layer that has a higher orbital velocity than the gas outside the layer, this leads to a loss of angular momentum and a radial drift of the layer. The stress magnitude is determined by the turbulent viscosity, which is represented as the sum of the α-viscosity associated with global turbulence in the disk and the viscosity associated with turbulence that is localized in a thin equatorial region comprising the dust layer and is caused by the Kelvin–Helmholtz instability. The evaporation of water ice and the continuity of the mass flux of the nonvolatile component on the ice line is also taken into account. It is shown that the accumulation of solid matter on either side of the ice line and in other regions of the disk is determined primarily by the ratio of the radii of dust aggregates on either side of the ice line. If after the ice evaporation the sizes (or density) of dust aggregates decrease by an order of magnitude or more, the density of the solid phase of the layer’s matter in the annular zone adjacent to the ice line from the inside increases sharply. If, however, the sizes of the aggregates on the inner side of the ice line are only a few times smaller than behind the ice line, then in the same zone there is a deficit of mass at the place of the modern asteroid belt. We have obtained constraints on the parameters at which the layer compaction is possible: the global turbulence viscosity parameter (α < 10−5), the initial radial distribution of the surface density of the dust layer, and the distribution of the gas surface density in the disk. Restrictions on the surface density depend on the size of dust aggregates. It is shown that the timescale of radial contraction of a dust layer consisting of meter-sized bodies is two orders of magnitude and that of decimeter ones, an order of magnitude greater than the timescale of the radial drift of individual particles if there is no dust layer.

The January 7, 2015, superbolide over Romania and structural diversity of meter-sized asteroids

Superbolides, i.e. extremely bright meteors produced by entries of meter-sized bodies into terrestrial atmosphere, are rare events. We present here detailed analysis of a superbolide, which occurred over Romania on January 7, 2015. The trajectory, velocity, and orbit were determined using two all-sky photographs from a station of the European Fireball Network (EN) in Slovakia and five casual video records from Romania, which were carefully calibrated. Bolide light curve was measured by EN radiometers. We found that the entry speed was 27.76±0.19kms−1, significantly lower than reported by US Government sensors. The orbit was asteroidal with low inclination and aphelion inside Jupiter's orbit. The atmospheric behavior was, however, not typical for an asteroidal body. The peak brightness of absolute magnitude of −21 was reached at a quite large height of 42.8km and the brightness then decreased quickly. The bolide almost disappeared at a height of 38.7km, leaving just a stationary luminous trail visible for several seconds. Only one small fragment continued until the height of 36km. Brief meteorite searches were unsuccessful. The modeling of the light curve revealed that the body of initial mass of about 4500kg remained almost intact until the dynamic pressure reached 0.9MPa but it was then quickly disintegrated into many tiny fragments and dust under 1–3MPa. A comparison was made with three other superbolides for which we have radiometric light curves: ordinary chondrite fall Košice, carbonaceous chondrite fall Maribo, and cometary Taurid bolide of October 31, 2015. The Romanian superbolide was not similar to any of these and represents probably a new type of material with intrinsic strength of about 1MPa.

Search for meter-sized bodies in meteoroid streams

We present the results of a ground-based observational effort to detect large meter-sized meteoroids in known meteoroid streams, while they are still moving in space a few hours to days in advance of their approach to Earth. Although no stream objects have been detected in any of the targeted streams, our observations are sufficient to place meaningful constraints to the population of objects in this poorly explored size regime. In particular, for at least two streams (Geminids and Taurids) we give evidence that the lack of objects is limited to the meter to decameter size range, while significant populations exist for both smaller and larger sizes. This information, when combined with other properties of the streams, is useful to better clarify the possible physical mechanisms that may be involved in the formation of at least some streams of possible asteroidal origin.

New methodology to determine the terminal height of a fireball

Despite ablation and drag processes associated with atmospheric entry of meteoroids were a subject of intensive study over the last century, little attention was devoted to interpret the observed fireball terminal height. This is a key parameter because it not only depends on the initial mass, but also on the bulk physical properties of the meteoroids and hence on their ability to ablate in the atmosphere. In this work we have developed a new approach that is tested using the fireball terminal heights observed by the Meteorite Observation and Recovery Project operated in Canada between 1970 and 1985 (hereafter referred as MORP). We then compare them to the calculation made. Our results clearly show that the new methodology is able to forecast the degree of deepening of meteoroids in the Earth’s atmosphere. Then, this approach has important applications in predicting the impact hazard from cm- to meter-sized bodies that are represented, in part, in the MORP bolide list.

Radiative forces on macroscopic porous bodies in protoplanetary disks: laboratory experiments

In optically thin parts of protoplanetary disks photophoresis is a significant force not just for dust grains, but also for macroscopic bodies. The absolute strength on the supposedly highly porous objects is not known in detail as yet. We set up a low pressure torsion balance and studied photophoretic forces. We investigated the dependence on plate dimensions and on ambient pressure and considered the influence of channels through the plates. As samples for full (no channel) plates we used tissue with 2mm thickness and circular shape with diameters of 10mm, 30mm and 50mm. The influence of channels was probed on rectangular-shaped circuit boards of 35mm x 35mm area and 1.5mm thickness. The number of channels was 169 and 352. At low pressure, the absolute photophoretic force is proportional to the cross section of the plates. At high pressure, gas flow through the channels enhances the photophoretic force. The pressure dependence of the radiative force can (formally) be calculated by photophoresis on particles with a characteristic length. We derived two characteristic length scales l depending on the plate radius r_1, the channel radius r_2, and the thickness of the plate which equals the length of the channel d as l=r^{0.35} x d^{0.65}. The highest force is found at a pressure p_max = 15 x l^{-1}Pa mm. In total, the photophoretic force on a plate with channels can be well described by a superposition of the two components: photophoresis due to the overall size and cross section of the plate and photophoresis due to the channels, both with their characteristic pressure dependencies. We applied these results to the transport of large solids in protoplanetary disks and found that the influence of porosity on the photophoretic force can reverse the inward drift of large solids, for instance meter-sized bodies, and push them outward within the optically thin parts of the disk.

An Analysis of the Content of Large Bodies in Meteor and Fireball Streams

The issue of the presence of meter- and decameter-sized bodies in meteor and fireball streams is of fundamental importance in the context of the theory of disintegration of parent bodies. Our observations, which have been carried out since 1995, indicate that such bodies are present in the Perseid, α-Capricornid, Leonid, and Coma-Berenicid meteoric streams. A catalogue is presented here for the parameters of the detected bodies. The spatial density of the meter-sized bodies is estimated based on the observational material. Within the errors, our estimates agree with the estimates based on the extrapolation of meteor and fireball data. An analysis of the observations suggests some inferences on the structure of the streams under study.

Sticky Ice Grains Aid Planet Formation: Unusual Properties of Cryogenic Water Ice

There is limited time for the dust in the nebula around a newborn star to form planetesimals: in a few million years or less the star's stellar winds will disperse most of the unagglomerated dust. It has been difficult to explain the efficiency by which dust grains must have agglomerated to form planetesimals in circumstellar disks. A major obstacle is the fragility of aggregates, leading to collisional fragmentation, which makes it difficult for them to grow to, and beyond, meter-sized bodies. The distinct properties of cryogenic (5-100 K) amorphous water ice, which composes or coats the grains in the cooler parts of the nebulae (≳Jovian distances), may be able to account for the rapid agglomeration. Measurements are presented that show that this ice readily acquires persistent macroscopic electric dipoles, strongly enhancing grain-grain adhesion. In addition, measurements were made showing that vapor-deposited amorphous water ice is also highly mechanically inelastic (≈10% rebound). Together these may explain this efficient net sticking and net growth. Similar properties of higher temperature grains may aid agglomeration in the inner regions of the nebulae.

Were planetesimals formed by dust accretion in the solar nebula?

The growth of meter-sized bodies in the solar nebula by dust accretion is examined. The meter-sized bodies have velocity about 50 m/s relative to the gas and small dust aggregates. When a small dust aggregate hits a meter-sized body, the aggregate breaks into dust monomers. These monomers accrete onto the body after several bouncing as proposed by Wurm et al., Icarus (2001), if the mean free path of the gas molecules is larger than the radius of the body. On the other hand, the monomers never hit the surface of the body again, if the body is much larger than the mean free path of the molecules. The sizes of bodies would be limited to the order of 10 times the mean free path. Kilometer-sized planetesimals were hardly formed by dust accretion in the region within 5 AU from the sun where the mean free path is less than 1 m. The planetesimals were probably formed by the gravitational instabilities in this region.

Dynamical Evolution of Main Belt Meteoroids: Numerical Simulations Incorporating Planetary Perturbations and Yarkovsky Thermal Forces

In the Yarkovsky effect, the recoil from asymmetric, reradiated thermal energy causes objects to undergo semimajor axis drift as a function of their spin, orbit, and material properties. We consider the role played by this mechanism in delivering meteoroids from parent bodies in the main belt to chaotic resonance zones where they can be transported to Earth-crossing orbits. Previous work has approximated the dynamical evolution of meteoroids via Yarkovsky forces, mostly through the use of the perturbations equation and simplified dynamics (e.g., Monte Carlo codes). In this paper, we calculate more precise solutions by formulating the seasonal and diurnal variants of this radiation force and incorporating them into an efficient N-body integrator capable of tracking test bodies for tens of millions of years with all relevant planetary perturbations included. Tests of our code against published benchmarks and the perturbation equations verify its accuracy. Results from long-term numerical integration of meter-sized bodies started from likely meteoroid parent bodies (e.g., 4 Vesta) indicate that dynamical evolution in the inner main belt can be complex. Chaotic effects produced by weaker planetary resonances allow many meteoroids to reach Mars-crossing orbits well before entering the 3:1 mean-motion resonance with Jupiter or the ν 6 secular resonance. Outward-evolving meteoroids sometimes become captured in these weaker resonances, increasing e and/or i while a stays constant. Conversely, inward-evolving meteoroids frequently jump across mean-motion resonances with Jupiter, bypassing potential “escape hatches” from the main belt. Despite these effects, our simulations indicate that most stony meteoroids reach Earth-crossing orbits via the 3:1 or ν 6 resonance after tens of Myr of evolution in the main belt. These time scales correspond well to the measured cosmic ray exposure ages of chondrites and achondrites. The source of these meteorites, however, is less clear, since Yarkovsky drift allows nearly any body in the main belt to add to the cumulate meteoroid flux. Our results suggest that small parent bodies dominate the meteoroid flux if the main belt size distribution at sub-km sizes is in collisional equilibrium, while big parent bodies dominate if observed population trends for km-sized bodies persist to smaller sizes.

99/03867 Demonstration test results of 71-MWe Wakamautsu PFBC combined power plant: Ikeda, N. et al. Sekitan Riyo Gijutsu Kaigi Koenshu, 1998, 8, 26–38. (In Japanese)

Despite ablation and drag processes associated with atmospheric entry of meteoroids were a subject of intensive study over the last century, little attention was devoted to interpret the observed fireball terminal height. This is a key parameter because it not only depends on the initial mass, but also on the bulk physical properties of the meteoroids and hence on their ability to ablate in the atmosphere. In this work we have developed a new approach that is tested using the fireball terminal heights observed by the Meteorite Observation and Recovery Project operated in Canada between 1970 and 1985 (hereafter referred as MORP). We then compare them to the calculation made. Our results clearly show that the new methodology is able to forecast the degree of deepening of meteoroids in the Earth’s atmosphere. Then, this approach has important applications in predicting the impact hazard from cm- to meter-sized bodies that are represented, in part, in the MORP bolide list.

Asteroid fragments in Earth-crossing orbits

The dynamics of small bodies in the Solar System are ruled not only by gravitational forces but also by non-gravitational forces such as the Poynting Robertson effect and the Yarkovsky effect. Poynting-Robertson is a dissipative effect, thus making a particle subject to it spiral towards the Sun. For meter-sized bodies, Poynting-Robertson is too weak to cause any observable effect. Another transverse force, also induced by radiation pressure, is dominant for this size range: the Yarkovsky effect. The idea behind the Yarkovsky effect is that re-emission of radiation from a rotating body is higher in the dusk hemisphere than it is in the dawn hemisphere, thus creating a force in the opposite direction to the hotter hemisphere. The Yarkovsky force can be dissipative or antidissipative making the body spiral, either converging to or diverging from the Sun, respectively. The purpose of this work is to better understand the dynamics of meter-sized bodies (for instance asteroid fragments) subject to the Yarkovsky effect. We first solve the time-dependent heat conduction equation to offer a new deduction of the Yarkovsky effect. Then we present some examples of the dynamical evolution of meter-sized bodies suffering dissipative and antidissipative forces. The dissipative force may confirm that many meteorites found on the surface of the Earth have their origin in the combined effect of the Yarkovsky force and resonances with Jupiter. With the antidissipative force some good examples of trappings in resonances with Jupiter can be found, because in these cases the orbits are converging and capture is more natural.

The role of collisions with interplanetary particles in the physical evolution of comets

Effects of collisions with interplanetary particles are investigated. To this purpose, collision probabilities for comets with different orbital elements are computed. It is found that collisions may have a non-negligible effect on the physical evolution of comets. In this connection, it is shown that under certain conditions collisional lifetimes may be shorter than dynamical or vaporization lifetimes. In particular, collisional lifetimes are on average shorter for comets in retrograde orbits than those for direct ones. It is further suggested that catastrophic collisions may contribute to prevent long-period comets in retrograde orbits from reaching short-period orbits by orbital diffusion. Collisions may also produce irregularities of the nucleus brightness by leaving exposed regions of fresh volatile material and may in this way lead to a ‘rejuvenation’ of old dusty short-period comets. Catastrophic collision probabilities are too low to account for the observed comet splittings, so other trigger mechanisms should be at work. However, it is shown that collisional mini-bursts (increases in brightness of one magnitude or so) caused by decimeter-sized bodies may occur rather frequently on short-period comets when they pass through the asteroid belt. The burst observed in comet Tempel-2 at ∼3 AU in December, 1978 could be an example of such collisional mini-bursts. The systematic observation of periodic comets when they pass through the asteroid belt could give valuable information about the spatial density of decimeter and meter-sized bodies. In particular, collisional effects for comet Halley, for which a continuous surveillance is planned, are evaluated.

Formation of the parent bodies of the carbonaceous chondrites

We have carried out extensive particle track studies for several C2 chondrites. On the basis of these and the available data on spallogenic stable and radioactive nuclides in several C1 and C2 chondrites, we have constructed a scenario for the precompaction irradiation of these meteorites. We discuss the rather severe constraints which these data place on the events leading to the formation of the parent bodies of the carbonaceous chondrites. Our analyses suggest that the precompaction solar flare and solar wind irradiation of the individual components most probably occurred primarily while the matter had accreted to form swarms of centimeter- to meter-sized bodies. This irradiation occured very early, within a few hundred my of the birth of the solar system; the pressure in the solar system had then dropped below 10 −9 atm. Further, the model assumes that soon after the irradiation of carbonaceous matter as swarms, the small bodies coalesced to form kilometer-sized objects, in time scales of 10 5±1 years, a constraint defined by the low cosmogenic exposure ages of these meteorites. Collisions among these objects led to the formation of much-larger-sized parent bodies of the carbonaceous chondrites. Implicit in this model is the existence of “irradiated” components at all depths in the parent bodies, which formed out of the irradiated swarm material.