Close Encounters Research Articles

Direct N-body Simulations of Satellite Formation around Small Asteroids: Insights from DART’s Encounter with the Didymos System

We explore binary asteroid formation by spin-up and rotational disruption considering the NASA DART mission's encounter with the Didymos–Dimorphos binary, which was the first small binary visited by a spacecraft. Using a suite of N-body simulations, we follow the gravitational accumulation of a satellite from meter-sized particles following a mass-shedding event from a rapidly rotating primary. The satellite’s formation is chaotic, as it undergoes a series of collisions, mergers, and close gravitational encounters with other moonlets, leading to a wide range of outcomes in terms of the satellite's mass, shape, orbit, and rotation state. We find that a Dimorphos-like satellite can form rapidly, in a matter of days, following a realistic mass-shedding event in which only ∼2%–3% of the primary's mass is shed. Satellites can form in synchronous rotation due to their formation near the Roche limit. There is a strong preference for forming prolate (elongated) satellites, although some simulations result in oblate spheroids like Dimorphos. The distribution of simulated secondary shapes is broadly consistent with other binary systems measured through radar or lightcurves. Unless Dimorphos's shape is an outlier, and considering the observational bias against lightcurve-based determination of secondary elongations for oblate bodies, we suggest there could be a significant population of oblate secondaries. If these satellites initially form with elongated shapes, a yet-unidentified pathway is needed to explain how they become oblate. Finally, we show that this chaotic formation pathway occasionally forms asteroid pairs and stable triples, including coorbital satellites and satellites in mean-motion resonances.

Constraining the Influence of Callisto's Perturbed Electromagnetic Environment on Energetic Particle Observations

AbstractThis study focuses on constraining the role that Callisto's perturbed electromagnetic environment had on energetic charged particle signatures observed during the Galileo mission. To do so, we compare data from the Energetic Particle Detector (EPD) obtained during four close encounters of the moon with a model framework that combines hybrid simulations for low‐energy plasma and test‐particle tracing simulations for high‐energy particles. By comparing model results for energetic particle dynamics in both uniform and perturbed electromagnetic fields, we systematically disentangle the role that geometric effects (i.e., absorption of particles by Callisto's solid surface) have on observed energetic particle signatures compared to those associated with Callisto's perturbed electromagnetic environment (generated by the moon's induced magnetic field and plasma interaction currents). We show that observed flux drop‐outs in the energetic ion pitch angle distributions (PADs) are largely driven by their absorption by Callisto's surface: their large gyroradii exceed the size of the moon, facilitating their impact onto the icy surface and preventing their detection by EPD. However, features observed in the energetic electron PADs can only be explained with an accurate representation of the moon's perturbed environment, since electrons closely follow the orientation of the electromagnetic fields. Our findings therefore illustrate the key role that the moon's induced field and magnetospheric plasma interaction have on the dynamics of energetic electrons, emphasizing the importance of accurately modeling Callisto's locally perturbed electromagnetic environment when attempting to interpret data from past and future encounters, including those anticipated from the upcoming JUICE mission.

B-Plane Resonant Flybys: Theory Extension to Elliptical Orbits and Dynamic Programming Application

A convenient formalism for the design of resonant flyby trajectories is the B-plane, where postencounter orbits with target resonance conditions or semimajor axes can be analytically mapped. This framework has been developed by many authors starting from the classical Öpik’s theory for close encounters, but it still relies on the approximation of circular orbits of the flyby bodies. In this work, the theory is analytically extended to account for elliptical planetary orbits. The classical and the extended formulations are tested and compared on two mission design applications, showing nonnegligible differences when the flyby body has a marked eccentricity. Further validation using a full three-body model confirmed the greater accuracy of the extended formulation. The second part of the paper proposes an efficient dynamic programming approach to the design of unperturbed resonant flybys. This problem has traditionally been addressed within broader optimization frameworks involving generic multiple gravity assists. However, focusing only on the resonant sequence allows the problem to feature a convenient structure for the implementation of dynamic programming. The developed algorithm is tested by reproducing the design of Solar Orbiter’s resonant phase with Venus, used in the actual mission to gradually raise the ecliptic inclination and reduce the distance at perihelion.

“Tidal Peeling Events”: Low-eccentricity Tidal Disruption of a Star by a Stellar-mass Black Hole

Close encounters between stellar-mass black holes (BHs) and stars occur frequently in dense star clusters and in the disks of active galactic nuclei. Recent studies have shown that in highly eccentric close encounters, the star can be tidally disrupted by the BH in a microtidal disruption event (microTDE), resulting in rapid mass accretion and possibly bright electromagnetic signatures. Here we consider a scenario in which the star might approach the stellar-mass BH in a gradual, nearly circular inspiral, under the influence of dynamical friction in a circum-binary gas disk or three-body interactions in a star cluster. We perform hydrodynamics simulations of this scenario using the smoothed particle hydrodynamics code PHANTOM. We find that under certain circumstances (for initial eccentricity e 0 ≳ 0.4 and penetration factor β = 1, or e 0 < 0.4 and β ≲ 0.67), the mass of the star is slowly stripped away by the BH. We call this gradual tidal disruption a “tidal-peeling event.” Additionally, we discover that some low-eccentricity microTDEs (e 0 < 0.4 and β = 1) are a new form of fast luminous transients similar to parabolic microTDEs. Depending on the initial distance and eccentricity of the encounter, these low-eccentricity microTDEs might exhibit significant accretion rates and orbital evolution distinct from those of a typical (eccentric) microTDE.

The Nagoya Protocol and nitrogen-fixing maize: Close encounters between Indigenous Oaxacans and the men from Mars (Inc.)

In this article, we discuss the Nagoya Protocol and its implications for access and benefit-sharing (ABS) agreements related to genetic resources and biodiversity. We analyze a case study of the appropriation of olotón, a maize variety with an unusual nitrogen-fixing trait grown in many communities throughout Oaxaca and probably beyond that region. Samples of olotón were acquired by scientists affiliated with Mars, Inc., the University of California-Davis, and the University of Wisconsin-Madison. The article unpacks how the olotón ABS agreement with the single Oaxacan community of Totontepec became a topic of controversy despite its approval by the Mexican government under the auspices of the Nagoya Protocol. We pose the question: Is the olotón ABS agreement to be considered “fair and equitable” or could it be an example of the unjust appropriation of genetic resources known as “biopiracy”? To answer that question, we proceed to analyze the “procedural,” “distributive,” “commutative,” and “representational” forms of justice as they apply to the acquisition of olotón. The Nagoya Protocol was created in part to impede biopiracy by providing a framework for acquiring and using biodiversity in a “fair and equitable” way, but the Nagoya Protocol’s provisions are ambiguous and allow for confidentiality clauses that favor the acquisition of genetic materials in ways that we do not consider fair and equitable. We join other researchers and academics who cite the need for an improved approach for ethically accessing, using, and sharing genetic resources and knowledge.

Activity Discovered on Mars-Crossing Jupiter Family Comet 2018 OR by Citizen Scientists

We present the discovery of cometary activity on 2018 OR as part of our Active Asteroids project, a NASA Partner Program fueled by Zooniverse Citizen Scientists. Volunteers found 2018 OR with a long, diffuse tail in archival images from the Dark Energy Camera on the Blanco 4 m telescope at the Cerro Tololo Inter-American Observatory in Chile. Our team identified additional Canada–France–Hawaii–Telescope MegaCam and Zwicky Transient Facility archival data after classification by Citizen Scientists. Activity originating from 2018 OR and directed in the anti-solar and anti-velocity directions was visible in archival images between UT 2018 September 5–18. Our dynamical simulations indicate 2018 OR experiences close encounters with Jupiter over hundred-year timescales. The orbital period and dynamics suggest 2018 OR is a Jupiter Family Comet, and we recommend further observations from the community to reduce observational uncertainties and investigate activity patterns.

Tidal Disruption of Near-Earth Asteroids during Close Encounters with Terrestrial Planets

Numerical modeling has long suggested that gravitationally bound (or so-called rubble-pile) near-Earth asteroids (NEAs) can be destroyed by tidal forces during close and slow encounters with terrestrial planets. However, tidal disruptions of NEAs have never been directly observed nor have they been directly attributed to any families of NEAs. Here we show population-level evidence for the tidal disruption of NEAs during close encounters with Earth and Venus. Debiased model distributions of NEA orbits and absolute magnitudes based on observations by the Catalina Sky Survey during 2005–2012 underpredict the number of NEAs with perihelion distances coinciding with the semimajor axes of Venus and Earth. A detailed analysis of the orbital distributions of the excess NEAs shows that their characteristics agree with the prediction for tidal disruptions, and they cannot be explained by observational selection effects or orbital dynamics. Accounting for tidal disruptions in evolutionary models of the NEA population partly bridges the gap between the predicted rate of impacts by asteroids with diameters of tens of meters and observed statistics of fireballs in the same size range.

The Global Shape, Gravity Field, and Libration of Enceladus

AbstractIn order to improve our understanding of the interior structure of Saturn's small moon Enceladus, we reanalyze radiometric tracking and onboard imaging data acquired by the Cassini spacecraft during close encounters with the moon. We compute the global shape, gravity field, and rotational parameters of Enceladus in a reference frame consistent with the International Astronomical Union's definition, where the center of the Salih crater is located at −5° East longitude. We recover a quadrupole gravity field with J3 and a forced libration amplitude of 0.091° ± 0.009° (3‐σ). We also compute a global shape model using a stereo‐photoclinometry technique with a global resolution of 500 m, although some local maps have higher resolutions ranging from 25 to 100 m. While our overall results are generally consistent with previous studies, we infer a thicker 27–33 km mean ice shell, a thinner 21–26 km mean ocean thickness, and a mean core density range of 2,270–2,330 kg/m3.

Close Encounters of the Third Gender

Close Encounters of the Third Gender

ESTIMATING GAMMA-RAY FLUX FROM MILLISECOND PULSARS ORIGINATING IN GLOBULAR CLUSTERS NEAR THE GALACTIC CENTER

In this study, we investigate the contribution of millisecond pulsars (MSPs) to the gamma-ray excess observed in the Galactic Center by analyzing data from high-resolution direct N-body simulations of six globular clusters (GCs) that experience close encounters with the nuclear star cluster. Using the φ-GPU code, we tracked the orbits of individual neutron stars (NSs) formed during the simulations, assuming a fraction of these NSs evolve into MSPs. Our model includes state-of-the-art single stellar evolution code including prescription for neutron star formation. We estimated the gamma-ray flux from these MSPs, considering known values for their gamma-ray emission. Our results show that MSPs originating from the six modeled GCs contribute a small but non-negligible fraction of the observed gamma-ray flux. This finding suggests that the actual gamma-ray flux from MSPs could be much higher when considering the entire population of GCs, potentially significantly contributing to the gamma-ray excess. This study highlights the importance of considering MSPs in the Galactic Center, originating from nearby globular clusters, as a potential source of the observed gamma-ray excess. Future work will involve more sophisticated simulations incorporating binary stellar evolution and comparing the fraction of MSPs in observed GCs to refine our models and improve the accuracy of our estimates.

Decision‐Making and Path Planning for Head‐On Collision Avoidance on Curved Roads

Deviating to the left on two‐way roads can result in fatal head‐on collisions. This article presents an intelligent decision‐making and path‐planning algorithm aimed at avoiding collision with a vehicle that has deviated from the opposing lane. The path‐planning process utilizes the model predictive control (MPC) approach, employing a linear kinematic prediction model with a horizon of 2 seconds. Considering that the deviated vehicle may abruptly return to its original lane at any moment, its motion is associated with significant uncertainty. To address this challenge, the path‐planning algorithm directs the ego vehicle (EV) under specific constraints to ensure that both the left and right sides of the road are symmetrically reachable in future time steps. This enables the decision‐making algorithm to select the safer direction for evasive maneuver at the appropriate moment. The motion prediction of the threat vehicle (TV) is conducted until the potential collision time, taking into account its motion history, and is utilized in the decision‐making process. Once the maneuver direction is determined, the collision‐free path planning continues using the MPC method. To evaluate the algorithm, six simulations are conducted, modeling various distant and close encounter states of the vehicles on roads with left‐ and right‐hand curves. The simulation results indicate the flexibility and appropriate performance of the algorithm in planning safe and maneuverable paths.

Reviving Philosophical Discussion of Close Encounters through the Strieber Letters in advance

Reviving Philosophical Discussion of Close Encounters through the Strieber Letters in advance

Cometary Activity Discovered on Vacationing Centaur 2019 OE31

We have detected cometary activity on minor planet 2019 OE31 through both the Active Asteroids Citizen Science program and an independent archival search. Before 2013, 2019 OE31 was on a Centaur orbit, between the orbits of Jupiter and Neptune. Centaurs are objects in transition from the outer solar system to the inner solar system. They play a vital role in the understanding of the Kuiper Belt and comets. In 2013 October, following a close encounter with Jupiter, 2019 OE31 moved to an orbit entirely interior to that of Jupiter. This reduced orbital distance and, hence, increased temperature is likely the cause of the observed activity. Through a suite of orbital dynamics simulations, we find that 2019 OE31 will experience many more similar encounters and is statistically likely to return to a Centaur orbit, potentially within the next 80 yr, from its current “vacation.”

NEOMOD 2: An updated model of Near-Earth Objects from a decade of Catalina Sky Survey observations

Catalina Sky Survey (CSS) is a major survey of Near-Earth Objects (NEOs). In a recent work, we used CSS observations from 2005–2012 to develop a new population model of NEOs (NEOMOD). CSS’s G96 telescope was upgraded in 2016 and detected over 10,000 unique NEOs since then. Here we characterize the NEO detection efficiency of G96 and use G96’s NEO detections from 2013–2022 to update NEOMOD. This resolves previous model inconsistencies related to the population of large NEOs. We estimate there are 936±29 NEOs with absolute magnitude H<17.75 (diameter D>1 km for the reference albedo pV=0.14) and semimajor axis a<4.2 au. The slope of the NEO size distribution for H=25–28 is found to be relatively shallow (cumulative index ≃2.6) and the number of H<28 NEOs (D>9 m for pV=0.14) is determined to be (1.20±0.04)×107, about 3 times lower than in Harris & Chodas (2021). Small NEOs have a different orbital distribution and higher impact probabilities than large NEOs. We estimate 0.034±0.002 impacts of H<28 NEOs on the Earth per year, which is near the low end of the impact flux range inferred from atmospheric bolide observations. Relative to a model where all NEOs are delivered directly from the main belt, the population of small NEOs detected by G96 shows an excess of low-eccentricity orbits with a≃1–1.6 au that appears to increase with H (≃30% excess for H=28). We suggest that the population of very small NEOs is boosted by tidal disruption of large NEOs during close encounters to the terrestrial planets. When the effect of tidal disruption is (approximately) accounted for in the model, we estimate 0.06±0.01 impacts of H<28 NEOs on the Earth per year, which is more in line with the bolide data. The impact probability of a H<22 (D>140 m for pV=0.14) object on the Earth in this millennium is estimated to be ≃4.5%.

Overview: Regularization and Numerical methods in Celestial Mechanics and Dynamical Astronomy

"The relevant study of the n-body problem (predicting individual motions of a group of celestial objects interacting with each other gravitational) is substantial in space dynamics. Further, the various perturbations effects, the collisions or the close encounters between celestial bodies are trajectory modifiers. In the digital age, the behaviour of non-integrable systems is usually and primarily studied with restricted models or coupled with the help of numerical integrators. Over time, these numerical integrators progressed, and the numerical challenge led to the introduction of many new techniques. The question is, which numerical algorithm to choose for the correct research? Is what we get through numerical integration close to the proper orbit? In other words, how can one be sure that the obtained trajectories reflect reality and that their prediction can be taken into account for a long time? This paper presents some analytical and numerical methods with highly accurate computations, such as regularization methods and symplectic integrators, which can be useful in obtaining the corresponding more accurate results."

LUMINOSITY OUTBURSTS IN INTERACTING PROTOPLANETARY SYSTEMS

FU Orionis type objects (fuors) are characterized by rapid (tens to hundreds years) episodic outbursts, during which the luminosity increases by orders of magnitude. One of the possible causes of such events is a close encounter between stars and protoplanetary disks. Numerical simulations show that the fuor-like outburst ignition requires a very close encounter ranging from a few to a few tens of au. In contrast, the observed stellar objects in fuor binaries are usually hundreds of au apart. Simple mathematical estimates show that if such a close approach took place, the binary stellar components would have an unrealistic relative velocity, at least an order of magnitude greater than the observed velocity dispersion in young stellar clusters. Thus, the bursts are either triggered with a certain delay after passage of the periastron or their ignition does not necessary require a close encounter and hence the outburst is not caused by the primordial gravitational perturbation of the protoplanetary disk. In this work, an encounter of a star surrounded by a protoplanetary disk with a diskless external stellar object was modeled using numerical hydrodynamics simulations. We showed that even fly-bys with a relatively large periastron (at least 500 au) can result in fuor-like outbursts. Moreover, the delay between the periastron passage and the burst ignition can reach several kyr. It was shown for the first time by means of numerical modeling that the perturbation of the disk caused by the external object can trigger a cascade process, which includes the development of the thermal instability in the innermost disk followed by the magneto-rotational instability ignition. Because of the sequential development of these instabilities, the rapid increase in the accretion rate occurs, resulting in the luminosity increase by more than two orders of ma-gnitude.

ConservationBots: Autonomous aerial robot for fast robust wildlife tracking in complex terrains

AbstractRadio tagging and tracking are fundamental to understanding the movements and habitats of wildlife in their natural environments. Today, the most widespread, widely applicable technology for gathering data relies on experienced scientists armed with handheld radio‐telemetry equipment to locate low‐power radio transmitters attached to wildlife from the ground. Although aerial robots can transform labor‐intensive conservation tasks, the realization of autonomous systems for tackling task complexities under real‐world conditions remains a challenge. We developed ConservationBots—small aerial robots for tracking multiple, dynamic, radio‐tagged wildlife. The aerial robot achieves robust localization performance and fast task completion times—significant for energy‐limited aerial systems while avoiding close encounters with potential, counterproductive disturbances to wildlife. Our approach overcomes the technical and practical problems posed by combining a lightweight sensor with new concepts: (i) planning to determine both trajectory and measurement actions guided by an information‐theoretic objective, which allows the robot to strategically select near‐instantaneous range‐only measurements to achieve faster localization, and time‐consuming sensor rotation actions to acquire bearing measurements and achieve robust tracking performance; (ii) a bearing detector more robust to noise; and (iii) a tracking algorithm formulation robust to missed and false detections experienced in real‐world conditions. We conducted extensive studies: simulations built upon complex signal propagation over high‐resolution elevation data on diverse geographical terrains; field testing; studies with wombats (Lasiorhinus latifrons; nocturnal, vulnerable species dwelling in underground warrens) and tracking comparisons with a highly experienced biologist to validate the effectiveness of our aerial robot and demonstrate the significant advantages over the manual method.

Bias Cuts and Data Dumps

Bias Cuts and Data Dumps

Canary in a Coal Mine: From Mine Safety Technique to Animal Metaphor (Animals Special Issue)

ABSTRACT In the early twentieth-century coal miners used canaries to detect the presence of poisonous gases underground. Miners treasured canaries for their cheer, partnership, and assistance. This safety practice had largely ended in the United States by the middle of the twentieth century, but the phrase ‘canary in a coal mine’ took on an extended life as a metaphor, signaling impending disaster in a variety of situations. The metaphor’s malleability encouraged the inclusion of caged birds as a plot device. Horror and science fiction films like The Birds, Close Encounters of the Third Kind, Arrival, and Bird Box use birds to signal invisible, hard-to-explain risks. The coal mine canary therefore became a sentinel for the most troubling social and political anxieties of each era. It signaled people’s worries that they could no longer identify and manage dangers in the modern world, whether those had to do with warfare, the environment, federal surveillance, social media, or epidemics. The birds’ use as a warning symbol, though, took miners’ beloved, sentient animal and turned it instead into a simple, but powerful tool. The coal mine canary’s continued prescence in popular culture also suggests that Americans remain concerned about the viability of new technologies in a rapidly changing world.

Binary planet formation through tides

ABSTRACT The search for satellites around exoplanets represents one of the greatest challenges in advancing the characterization of planetary systems. Currently, we can only detect massive satellites, which resemble additional planetary companions rather than rocky moons. It is not yet well understood whether such substellar pairs, known as binary planets, are common or how they form. In this study, we investigated the formation scenario for binary planets resulting from tidal dissipation during close encounters in the gravitational instability scenario. We conducted seven sets of simulations, varying the number of initial planets injected into the system from two to five, as well as the amount of energy lost due to tides. Our results demonstrate that this formation mechanism is quite efficient in producing binary planets, with an average occurrence rate for the simulated systems of 14.3 per cent. Additionally, we present the distribution of relevant physical parameters (semimajor axis, eccentricity, mass ratios, and formation time) for planet–planet pairs. We also provide comprehensive statistics for single planets and planet–planet pairs.