arXiv subjects

Stephen R. Schwartz

Publications and source records attributed to Stephen R. Schwartz.

At least 19 recordsLinked to original sources

arXiv2026

Direct contact binary planetesimal formation from gravitational collapse

Bilobate contact binaries comprise a significant fraction of the relict Kuiper Belt, which includes the exemplary contact binary (486958) Arrokoth. The surfaces of its lobes contain similar amounts of highly volatile chemical species and few craters, indicating formation in a homogeneous and gentle environment. Arrokoth's bilobate shape was initially hypothesized to have formed via the direct gravitational collapse of a pebble cloud in the solar system's protoplanetary disk. However, alternative hypotheses have proposed that Arrokoth may be the result of binary planetesimal formation and the subsequent dynamical evolution of the binary components into contact through external perturbations over long timescales. Here, we show that contact binary planetesimals like Arrokoth can form directly from the gravitational collapse of pebble clouds. We used a soft-sphere discrete element method (SSDEM) to discover that planetesimals form a wide variety of shapes, including bilobate contact binaries. This method creates planetesimals as particle-aggregates with particles resting upon each other's surfaces via mutual surface penetration. The formation of contact binaries in our simulations strengthens the hypothesis that Arrokoth, and perhaps many other contact binaries in the Kuiper Belt, formed directly as bilobate objects from gravitational collapse, and so their shapes and surfaces record the era of planet formation.

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arXiv2025

Considering contact forces during the formation of planetesimals by gravitational collapse: mutual orbits, spin states, and shapes

In this work, we apply a soft-sphere discrete element method (SSDEM) within the PKDGRAV N-body integrator to investigate the formation of planetesimal systems through the gravitational collapse of clouds of super-particles. Previously published numerical models have demonstrated that the gravitational collapse of pebble clouds is an efficient pathway to produce binary planetesimal systems. However, such investigations were limited by their use of a perfect-merger and inflated-radii super-particle approach, which inhibits any analysis of planetesimal shapes and spin states, precludes the formation of the tightest binary orbits, and produces significantly under-dense planetesimals. The SSDEM enables super-particles to rest upon each other through mutual surface penetration and by simulating contact physics. Super-particles do not need to be inflated and collisions are not treated as perfect mergers; we can thus track the evolution of planetesimal shapes, spins, and tight binary orbits. We demonstrate that the SSDEM is an excellent method to model the collapse process, and is capable of producing many binary planetesimal systems from a single cloud. Our results confirm the findings of previously published perfect-merging models while also producing novel results about planetesimal spin and shape properties. Newly-formed planetesimals exhibit 10-hr rotation periods on average and can be characterized by a wide variety of shapes (spherical, oblate, top-shaped, flattened, egg-shaped, or prolate), with the most-massive planetesimals primarily forming as spheres and oblate-spheroids.

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arXiv2025

The Dynamical State of the Didymos System Before and After the DART Impact

NASA's Double Asteroid Redirection Test (DART) spacecraft impacted Dimorphos, the natural satellite of (65803) Didymos, on 2022 September 26, as a first successful test of kinetic impactor technology for deflecting a potentially hazardous object in space. The experiment resulted in a small change to the dynamical state of the Didymos system consistent with expectations and Level 1 mission requirements. In the pre-encounter paper Richardson (2022), predictions were put forward regarding the pre- and post-impact dynamical state of the Didymos system. Here we assess these predictions, update preliminary findings published after the impact, report on new findings related to dynamics, and provide implications for ESA's Hera mission to Didymos, scheduled for launch in 2024 with arrival in late December 2026. Pre-encounter predictions tested to date are largely in line with observations, despite the unexpected, flattened appearance of Didymos compared to the radar model and the apparent pre-impact oblate shape of Dimorphos (with implications for the origin of the system that remain under investigation). New findings include that Dimorphos likely became prolate due to the impact and may have entered a tumbling rotation state. A possible detection of a post-impact transient secular decrease in the binary orbital period suggests possible dynamical coupling with persistent ejecta. Timescales for damping of any tumbling and clearing of any debris are uncertain. The largest uncertainty in the momentum transfer enhancement factor of the DART impact remains the mass of Dimorphos, which will be resolved by the Hera mission.

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arXiv2024

A New Database of Giant Impacts over a Wide Range of Masses and with Material Strength: A First Analysis of Outcomes

In the late stage of terrestrial planet formation, planets are predicted to undergo pairwise collisions known as giant impacts. Here we present a high-resolution database of giant impacts for differentiated colliding bodies of iron-silicate composition, with target masses ranging from 10^-4 M_Earth up to super-Earths (5 M_Earth). We vary impactor-to-target mass ratio, core-mantle (iron-silicate) fraction, impact velocity, and impact angle. Strength in the form of friction is included in all simulations. We find that due to strength, collisions with bodies smaller than about 2*10^-3 M_Earth can result in irregular shapes, compound core structures, and captured binaries. We observe that the characteristic escaping velocity of smaller remnants (debris) is approximately half of the impact velocity, significantly faster than currently assumed in N-body simulations of planet formation. Incorporating these results in N-body planet formation studies would provide more realistic debris-debris and debris-planet interactions.

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arXiv2023

Successful Kinetic Impact into an Asteroid for Planetary Defense

While no known asteroid poses a threat to Earth for at least the next century, the catalog of near-Earth asteroids is incomplete for objects whose impacts would produce regional devastation. Several approaches have been proposed to potentially prevent an asteroid impact with Earth by deflecting or disrupting an asteroid. A test of kinetic impact technology was identified as the highest priority space mission related to asteroid mitigation. NASA's Double Asteroid Redirection Test (DART) mission is the first full-scale test of kinetic impact technology. The mission's target asteroid was Dimorphos, the secondary member of the S-type binary near-Earth asteroid (65803) Didymos. This binary asteroid system was chosen to enable ground-based telescopes to quantify the asteroid deflection caused by DART's impact. While past missions have utilized impactors to investigate the properties of small bodies those earlier missions were not intended to deflect their targets and did not achieve measurable deflections. Here we report the DART spacecraft's autonomous kinetic impact into Dimorphos and reconstruct the impact event, including the timeline leading to impact, the location and nature of the DART impact site, and the size and shape of Dimorphos. The successful impact of the DART spacecraft with Dimorphos and the resulting change in Dimorphos's orbit demonstrates that kinetic impactor technology is a viable technique to potentially defend Earth if necessary.

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arXiv2023

Ejecta from the DART-produced active asteroid Dimorphos

Some active asteroids have been proposed to be the result of impact events. Because active asteroids are generally discovered serendipitously only after their tail formation, the process of the impact ejecta evolving into a tail has never been directly observed. NASA's Double Asteroid Redirection Test (DART) mission, apart from having successfully changed the orbital period of Dimorphos, demonstrated the activation process of an asteroid from an impact under precisely known impact conditions. Here we report the observations of the DART impact ejecta with the Hubble Space Telescope (HST) from impact time T+15 minutes to T+18.5 days at spatial resolutions of ~2.1 km per pixel. Our observations reveal a complex evolution of ejecta, which is first dominated by the gravitational interaction between the Didymos binary system and the ejected dust and later by solar radiation pressure. The lowest-speed ejecta dispersed via a sustained tail that displayed a consistent morphology with previously observed asteroid tails thought to be produced by impact. The ejecta evolution following DART's controlled impact experiment thus provides a framework for understanding the fundamental mechanisms acting on asteroids disrupted by natural impact.

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arXiv2022

After DART: Using the first full-scale test of a kinetic impactor to inform a future planetary defense mission

NASA's Double Asteroid Redirection Test (DART) is the first full-scale test of an asteroid deflection technology. Results from the hypervelocity kinetic impact and Earth-based observations, coupled with LICIACube and the later Hera mission, will result in measurement of the momentum transfer efficiency accurate to ~10% and characterization of the Didymos binary system. But DART is a single experiment; how could these results be used in a future planetary defense necessity involving a different asteroid? We examine what aspects of Dimorphos's response to kinetic impact will be constrained by DART results; how these constraints will help refine knowledge of the physical properties of asteroidal materials and predictive power of impact simulations; what information about a potential Earth impactor could be acquired before a deflection effort; and how design of a deflection mission should be informed by this understanding. We generalize the momentum enhancement factor $β$, showing that a particular direction-specific $β$ will be directly determined by the DART results, and that a related direction-specific $β$ is a figure of merit for a kinetic impact mission. The DART $β$ determination constrains the ejecta momentum vector, which, with hydrodynamic simulations, constrains the physical properties of Dimorphos's near-surface. In a hypothetical planetary defense exigency, extrapolating these constraints to a newly discovered asteroid will require Earth-based observations and benefit from in-situ reconnaissance. We show representative predictions for momentum transfer based on different levels of reconnaissance and discuss strategic targeting to optimize the deflection and reduce the risk of a counterproductive deflection in the wrong direction.

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arXiv2022

Effects of impact and target parameters on the results of a kinetic impactor: predictions for the Double Asteroid Redirection Test (DART) mission

The Double Asteroid Redirection Test (DART) spacecraft will impact into the asteroid Dimorphos on September 26, 2022 as a test of the kinetic impactor technique for planetary defense. The efficiency of the deflection following a kinetic impactor can be represented using the momentum enhancement factor, Beta, which is dependent on factors such as impact geometry and the specific target material properties. Currently, very little is known about Dimorphos and its material properties that introduces uncertainty in the results of the deflection efficiency observables, including crater formation, ejecta distribution, and Beta. The DART Impact Modeling Working Group (IWG) is responsible for using impact simulations to better understand the results of the DART impact. Pre-impact simulation studies also provide considerable insight into how different properties and impact scenarios affect momentum enhancement following a kinetic impact. This insight provides a basis for predicting the effects of the DART impact and the first understanding of how to interpret results following the encounter. Following the DART impact, the knowledge gained from these studies will inform the initial simulations that will recreate the impact conditions, including providing estimates for potential material properties of Dimorphos and Beta resulting from DARTs impact. This paper summarizes, at a high level, what has been learned from the IWG simulations and experiments in preparation for the DART impact. While unknown, estimates for reasonable potential material properties of Dimorphos provide predictions for Beta of 1-5, depending on end-member cases in the strength regime.

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arXiv2022

Predictions for the Dynamical States of the Didymos System before and after the Planned DART Impact

NASA's Double Asteroid Redirection Test (DART) spacecraft is planned to impact the natural satellite of (65803) Didymos, Dimorphos, around 23:14 UTC on 26 September 2022, causing a reduction in its orbital period that will be measurable with ground-based observations. This test of kinetic impactor technology will provide the first estimate of the momentum transfer enhancement factor $β$ at a realistic scale, wherein ejecta from the impact provides an additional deflection to the target. Earth-based observations, the LICIACube spacecraft (to be detached from DART prior to impact), and ESA's follow-up Hera mission to launch in 2024, will provide additional characterization of the deflection test. Together Hera and DART comprise the Asteroid Impact and Deflection Assessment (AIDA) cooperation between NASA and ESA. Here the predicted dynamical states of the binary system upon arrival and after impact are presented. The assumed dynamically relaxed state of the system will be excited by the impact, leading to an increase in eccentricity and slight tilt of the orbit together with enhanced libration of Dimorphos with amplitude dependent on the currently poorly known target shape. Free rotation around the moon's long axis may also be triggered and the orbital period will experience variations from seconds to minutes over timescales of days to months. Shape change of either body due to cratering or mass wasting triggered by crater formation and ejecta may affect $β$ but can be constrained through additional measurements. Both BYORP and gravity tides may cause measurable orbital changes on the timescale of Hera's rendezvous.

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arXiv2022

Ricochets on Asteroids II: Sensitivity of laboratory experiments of low velocity grazing impacts on substrate grain size

We compare low velocity impacts that ricochet with the same impact velocity and impact angle into granular media with similar bulk density, porosity and friction coefficient but different mean grain size. The ratio of projectile diameter to mean grain length ranges from 4 in our coarsest medium to 50 in our finest sand. Using high speed video and fluorescent markers, we measure the ratio of pre- to post-impact horizontal and vertical velocity components, which we refer to as coefficients of restitution, and the angle of deflection caused by the impact in the horizontal plane. Coefficients of restitution are sensitive to mean grain size with the ratio associated with the horizontal velocity component about twice as large for our coarsest gravel as that for our finest sand. This implies that coefficients for hydro-static-like, drag-like and lift-like forces, used in empirical force laws, are sensitive to mean grain size. The coefficient that is most strongly sensitive to grain size is the lift coefficient which decreases by a factor of 3 between our coarsest and finest media. The deflection angles are largest in the coarser media and their size approximately depends on grain size to the 3/2 power. This scaling is matched with a model where momentum transfer takes place via collisions with individual grains. The dependence of impact mechanics on substrate size distribution should be considered in future models for populations of objects that impact granular asteroid surfaces.

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arXiv2021

Collision Chains among the Terrestrial Planets. II. An Asymmetry between Earth and Venus

During the late stage of terrestrial planet formation, hit-and-run collisions are about as common as accretionary mergers, for expected velocities and angles of giant impacts. Average hit-and-runs leave two major remnants plus debris: the target and impactor, somewhat modified through erosion, escaping at lower relative velocity. Here we continue our study of the dynamical effects of such collisions. We compare the dynamical fates of intact runners that start from hit-and-runs with proto-Venus at 0.7 AU and proto-Earth at 1.0 AU. We follow the orbital evolutions of the runners, including the other terrestrial planets, Jupiter, and Saturn, in an N-body code. We find that the accretion of these runners can take $\gtrsim$10 Myr (depending on the egress velocity of the first collision) and can involve successive collisions with the original target planet or with other planets. We treat successive collisions that the runner experiences using surrogate models from machine learning, as in previous work, and evolve subsequent hit-and-runs in a similar fashion. We identify asymmetries in the capture, loss, and interchange of runners in the growth of Venus and Earth. Hit-and-run is a more probable outcome at proto-Venus, being smaller and faster orbiting than proto-Earth. But Venus acts as a sink, eventually accreting most of its runners, assuming typical events, whereas proto-Earth loses about half, many of those continuing to Venus. This leads to a disparity in the style of late-stage accretion that could have led to significant differences in geology, composition, and satellite formation at Earth and Venus.

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arXiv2021

Collision Chains among the Terrestrial Planets. III. Formation of the Moon

In the canonical model of Moon formation, a Mars-sized protoplanet "Theia" collides with proto-Earth at close to their mutual escape velocity $v_{\rm esc}$ and a common impact angle 45°. The "graze-and-merge" collision strands a fraction of Theia's mantle into orbit, while Earth accretes most of Theia and its momentum. Simulations show that this produces a hot, high angular momentum, silicate-dominated protolunar system, in substantial agreement with lunar geology, geochemistry, and dynamics. However, a Moon that derives mostly from Theia's mantle, as angular momentum dictates, is challenged by the fact that O, Ti, Cr, radiogenic W, and other elements are indistinguishable in Earth and lunar rocks. Moreover, the model requires an improbably low initial velocity. Here we develop a scenario for Moon formation that begins with a somewhat faster collision, when proto-Theia impacts proto-Earth at ~1.2 $v_{\rm esc}$, also around 45°. Instead of merging, the bodies come into violent contact for a half-hour and their major components escape, a "hit-and-run collision." N-body evolutions show that the "runner" often returns ~0.1-1 Myr later for a second giant impact, closer to $v_{\rm esc}$; this produces a postimpact disk of ~2-3 lunar masses in smoothed particle hydrodynamics simulations, with angular momentum comparable to canonical scenarios. The disk ends up substantially inclined, in most cases, because the terminal collision is randomly oriented to the first. Proto-Earth contributions to the silicate disk are enhanced by the compounded mixing and greater energy of a collision chain.

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arXiv2021

The Effect of Inefficient Accretion on Planetary Differentiation

Pairwise collisions between terrestrial embryos are the dominant means of accretion during the last stage of planet formation. Hence, their realistic treatment in N-body studies is critical to accurately model the formation of terrestrial planets and to develop interpretations of telescopic and spacecraft observations. In this work, we compare the effects of two collision prescriptions on the core-mantle differentiation of terrestrial planets: a model in which collisions are always completely accretionary (``perfect merging'') and a more realistic model based on neural networks that has been trained on hydrodynamical simulations of giant impacts. The latter model is able to predict the loss of mass due to imperfect accretion and the evolution of non-accreted projectiles in hit-and-run collisions. We find that the results of the neural-network model feature a wider range of final core mass fractions and metal-silicate equilibration pressures, temperatures, and oxygen fugacities than the assumption of perfect merging. When used to model collisions in N-body studies of terrestrial planet formation, the two models provide similar answers for planets more massive than 0.1 Earth's masses. For less massive final bodies, however, the inefficient-accretion model predicts a higher degree of compositional diversity. This phenomenon is not reflected in planet formation models of the solar system that use perfect merging to determine collisional outcomes. Our findings confirm the role of giant impacts as important drivers of planetary diversity and encourage a realistic implementation of inefficient accretion in future accretion studies.

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arXiv2020

Validating N-body code Chrono for granular DEM simulations in reduced-gravity environments

The Discrete Element Method (DEM) is frequently used to model complex granular systems and to augment the knowledge that we obtain through theory, experimentation, and real-world observations. Numerical simulations are a particularly powerful tool for studying the regolith-covered surfaces of asteroids, comets, and small moons, where reduced-gravity environments produce ill-defined flow behaviors. In this work, we present a method for validating soft-sphere DEM codes for both terrestrial and small-body granular environments. The open-source code Chrono is modified and evaluated first with a series of simple two-body-collision tests, and then, with a set of piling and tumbler tests. In the piling tests, we vary the coefficient of rolling friction to calibrate the simulations against experiments with 1 mm glass beads. Then, we use the friction coefficient to model the flow of 1 mm glass beads in a rotating drum, using a drum configuration from a previous experimental study. We measure the dynamic angle of repose, the flowing layer thickness, and the flowing layer velocity for tests with different particle sizes, contact force models, coefficients of rolling friction, cohesion levels, drum rotation speeds and gravity levels. The tests show that the same flow patterns can be observed at Earth and reduced-gravity levels if the drum rotation speed and the gravity-level are set according to the dimensionless parameter known as the Froude number. Chrono is successfully validated against known flow behaviors at different gravity and cohesion levels, and will be used to study small-body regolith dynamics in future works.

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arXiv2020

Ricochets on Asteroids: Experimental study of low velocity grazing impacts into granular media

Spin off events and impacts can eject boulders from an asteroid surface and rubble pile asteroids can accumulate from debris following a collision between large asteroids. These processes produce a population of gravitational bound objects in orbit that can impact an asteroid surface at low velocity and with a distribution of impact angles. We present laboratory experiments of low velocity spherical projectiles into a fine granular medium, sand. We delineate velocity and impact angles giving ricochets, those giving projectiles that roll-out from the impact crater and those that stop within their impact crater. With high speed camera images and fluorescent markers on the projectiles we track spin and projectile trajectories during impact. We find that the projectile only reaches a rolling without slipping condition well after the marble has reached peak penetration depth. The required friction coefficient during the penetration phase of impact is 4-5 times lower than that of the sand suggesting that the sand is fluidized near the projectile surface during penetration. We find that the critical grazing impact critical angle dividing ricochets from roll-outs, increases with increasing impact velocity. The critical angles for ricochet and for roll-out as a function of velocity can be matched by an empirical model during the rebound phase that balances a lift force against gravity. We estimate constraints on projectile radius, velocity and impact angle that would allow projectiles on asteroids to ricochet or roll away from impact, finally coming to rest distant from their initial impact sites.

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arXiv2020

Realistic On-the-fly Outcomes of Planetary Collisions II: Bringing Machine Learning to N-body Simulations

Terrestrial planet formation theory is at a bottleneck, with the growing realization that pairwise collisions are treated far too simply. Here, and in our companion paper (Cambioni et al. 2019) that introduces the training methodology, we demonstrate the first application of machine learning to more realistically model the late stage of planet formation by giant impacts. We present surrogate models that give fast, reliable answers for the masses and velocities of the two largest remnants of a giant impact, as a function of the colliding masses and their impact velocity and angle, with the caveat that our training data do not yet include pre-impact rotation or variable thermal conditions. We compare canonical N-body scenarios of terrestrial planet formation assuming perfect merger (Chambers 2001) with our more realistic treatment that includes inefficient accretions and hit-and-run collisions. The result is a protracted tail of final events lasting ~200 Myr, and the conversion of about half the mass of the initial population to debris. We obtain profoundly different solar system architectures, featuring a much wider range of terrestrial planet masses and enhanced compositional diversity.

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arXiv2019

Investigating Asteroid Surface Geophysics with an Ultra-Low-Gravity Centrifuge in Low-Earth Orbit

Near-Earth small-body mission targets 162173 Ryugu, 101955 Bennu, and 25143 Itokawa produce gravity fields around 4 orders of magnitude below that of Earth and their irregular shapes, combined with rotational effects produce varying surface potentials. Still, we observe familiar geologic textures and landforms that are the result of the granular physical processes that take place on their surfaces. The nature of these landforms, however, their origins, and how these surfaces react to interrogation by probes, landers, rovers, and penetrators, remain largely unknown, and therefore landing on an asteroid and manipulating its surface material remains a daunting challenge. The AOSAT+ design is a 12U CubeSat that will be in Low-Earth Orbit (LEO) and that will operate as a spinning on-orbit centrifuge. Part of this 12U CubeSat will contain a laboratory that will recreate asteroid surface conditions using crushed meteorite as a regolith proxy. The spinning of the laboratory will simulate the surface gravity of asteroids 2 km and smaller. The result is a bed of realistic regolith, the environment that landers and diggers and maybe astronauts will interact with. A crucial component of this mission involves the reproduction of the experimental results in numerical simulation in order to extract the material parameters of the regolith and its behavior in a sustained, very low but nonzero-gravity environment.

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arXiv2019

The Western Bulge of 162173 Ryugu Formed as a Result of a Rotationally Driven Deformation Process

162173 Ryugu, the target of Hayabusa2, has a round shape with an equatorial ridge, which is known as a spinning top-shape. A strong centrifugal force is a likely contributor to Ryugu's top-shaped features. Observations by Optical Navigation Camera onboard Hayabusa2 show a unique longitudinal variation in geomorphology; the western side of this asteroid, later called the western bulge, has a smooth surface and a sharp equatorial ridge, compared to the other side. Here, we propose a structural deformation process that generated the western bulge. Applying the mission-derived shape model, we employ a finite element model technique to analyze the locations that experience structural failure within the present shape. Assuming that materials are uniformly distributed, our model shows the longitudinal variation in structurally failed regions when the spin period is shorter than ~3.75 h. Ryugu is structurally intact in the subsurface region of the western bulge while other regions are sensitive to structural failure. We infer that this variation is indicative of the deformation process that occurred in the past, and the western bulge is more relaxed structurally than the other region. Our analysis also shows that this deformation process might occur at a spin period between ~3.5 h and ~3.0 h, providing the cohesive strength ranging between ~4 Pa and ~10 Pa.

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