Close Encounters Research Articles

Apophis: may a meteor activity happen on Earth after the 2029 closest approach?

ABSTRACT The potentially hazardous asteroid 99942 Apophis will have a very close approach to the Earth in 2029. The encounter on its own may provide measurements of Earth’s effects on Apophis’ surface and also contribute to the improvement of some physical characteristics of the asteroid. In a previous work, we assumed the existence of a hypothetical disc of particles around Apophis before the 2029 encounter, and identified the particles that would escape from the gravity domain of Apophis due to the Earth's gravitational perturbation during the close encounter. In the current work, we investigate the possibility of a meteor activity originating from this event. We study the orbital evolution of these particles computing the MOIDs of the particles with respect to the Earth for the following 200 yr. Our results are not favourable for a meteor activity on Earth. However, a meteoroid activity on the Moon might happen during the encircling period after 88 yr of the 2029 encounter.

Simulating the tidal disruption of stars by stellar-mass black holes using moving-mesh hydrodynamics

In the centers of dense star clusters, close encounters between stars and compact objects are likely to occur. We studied tidal disruption events of main-sequence (MS) stars by stellar-mass black holes (termed μTDEs), which can shed light on the processes occurring in these clusters, including being an avenue in the mass growth of stellar-mass BHs. Using the moving-mesh hydrodynamics code AREPO, we performed a suite of 58 hydrodynamics simulations of partial μTDEs of realistic, MESA-generated MS stars by varying the initial mass of the star (0.5 M⊙ and 1 M⊙), the age of the star (zero-age, middle-age and terminal-age), the mass of the black hole (10 M⊙ and 40 M⊙), and the impact parameter (yielding almost no mass loss to full disruption). We then examined the dependence of the masses, spins, and orbital parameters of the partially disrupted remnant on the initial encounter parameters. We find that the mass lost from a star decreases roughly exponentially with increasing approach distance and that a 1 M⊙ star loses less mass than a 0.5 M⊙ one. Moreover, a more evolved star is less susceptible to mass loss. Tidal torques at the closest approach spin up the remnant very close to break-up velocity when the impact parameter is low. The remnant star can be bound (eccentric) or unbound (hyperbolic) to the black hole; hyperbolic orbits occur when the star’s central density concentration is relatively low and the black-hole-star mass ratio is high, which is the case for the disruption of a 0.5 M⊙ star. Finally, we provide best-fit analytical formulae for the aforementioned range of parameters that can be incorporated into cluster codes to model star-black-hole interaction more accurately.

Close Encounters

Close Encounters

Hypervelocity star candidates from Gaia DR2 and DR3 proper motions and parallaxes

Context. Hypervelocity stars (HVSs) unbound to the Galaxy can be formed with extreme stellar interactions, for example close encounters with supermassive black holes or in massive star clusters, supernova explosions in binary systems, or the stripping of dwarf galaxies. Observational evidence comes from measurements of radial velocities (RVs) of objects crossing the outer Galactic halo and of tangential velocities based on high proper motions (HPMs) and distances of relatively nearby stars. Aims. I searched for new nearby HVS candidates and reviewed known objects using their Gaia astrometric measurements. Methods. Candidates were selected with significant Gaia parallaxes of >0.1mas, proper motions of >20 mas yr−1, and computed galactocentric tangential velocities vtan_g>500 km s−1. The DR2 and DR3 samples of several thousand HVS candidates were studied with respect to their proper motions, sky distribution, number of observations, location in crowded fields, colour-magnitude diagrams, selection effects with magnitude, and RVs in DR3. The 72 most extreme (vtan_g>700 km s−1) and nearest (within 4kpc) DR3 HVS candidates were investigated with respect to detected close neighbours, flags, and astrometric quality parameters of objects of similar magnitudes in DR3. The quality checks involved HPM objects in a global comparison and all objects in the vicinity of each target. Results. Spurious HPMs in the Galactic centre region led to false HVS interpretations in Gaia DR2 and are still present in DR3, although to a lesser extent. Otherwise there is good agreement between the HPMs of HVS candidates in DR2 and DR3. However, HVS candidates selected from DR2 tend to have larger parallaxes, and hence lower tangential velocities in DR3. Most DR3 RVs are much lower than the tangential velocities, indicating that the DR3 HVS candidates are still affected by underestimated parallaxes. None of the 72 extreme nearby DR3 HVS candidates, including three D6 stars, passed all the quality checks. Their tangential velocities may turn out to be lower, but at least some of them still appear unbound to the Galaxy.

Uranus Orbiter and Probe: A Radio Science Investigation to Determine the Planet’s Gravity Field, Depth of the Winds, and Tidal Deformations

The most recent Planetary Science and Astrobiology Decadal Survey has proposed Uranus as the target for NASA’s next large-scale mission. The interior structure and atmosphere of the planet are currently poorly understood, and objectives for investigating Uranus’s deeper regions and composition are highly ranked. Traditionally, gravity science has served as one of the primary means for probing the depths of planetary bodies and inferring their internal density distributions. In this work, we present precise numerical simulations of an onboard radio science experiment designed to determine Uranus’s gravity field and tidal deformations, which would offer a rare view into the planet’s interior. We focus on the mission’s orbital planning, discussing crucial parameters such as the number of pericenter passes, orbital inclination, and periapsis altitude necessary to meet the gravity measurement requirements for a Uranus orbiter. Our findings suggest that eight close encounters may be sufficient to determine the zonal gravity field up to J 8 with a relative accuracy of 10%, if the trajectory is optimized. This would allow for the decoupling of the gravity field components due to interior structure and zonal winds. Additionally, we find that the expected end-of-mission uncertainty on Uranus’s Love number k 22 is of order ∼0.01 (3σ). This level of accuracy may offer crucial information about Uranus’s inner state and allow for discriminating between a liquid and solid core, thus shedding light on crucial aspects of the planet’s formation and evolution.

An informed response to Kloppenburg et al. (2024)—Nagoya Protocol

Comment on Kloppenburg, J, Calderón, CI, Ané, J-M. 2024. The Nagoya Protocol and nitrogen-fixing maize: Close encounters between Indigenous Oaxacans and the men from Mars (Inc.). DOI: https://doi.org/10.1525/elementa.2023.00115

Escape from a rotating barred galaxy

We study the dynamics in the outer regions of a rotating barred galaxy and in particular, we observe the escape of a test particle from the gravitational potential of the galaxy. The acceleration mechanism of the test particle is a close encounter with one of the ends of the bar with the right relative phase of the position of the bar. This is a possible mechanism for the creation of high-velocity stars, i.e. stars with an energy sufficient to escape from the galaxy. Our results suggest that good candidates for high-velocity stars accelerated by this mechanism are the old low-mass high-velocity stars moving close to the disc. We will encounter the asymptotic dynamics which leads naturally to the study of a type of Poincaré map which can be reinterpreted as a scattering map. Thereby an iterated scattering map enters the picture in a quite natural form. The present work is a supplement to previous work on the dynamics in the inner region of the same model.

The formation of transiting circumplanetary debris discs from the disruption of satellite systems during planet–planet scattering

Abstract Several stars show deep transits consistent with discs of roughly 1 R⊙ seen at moderate inclinations, likely surrounding planets on eccentric orbits. We show that this configuration arises naturally as a result of planet–planet scattering when the planets possess satellite systems. Planet–planet scattering explains the orbital eccentricities of the discs’ host bodies, while the close encounters during scattering lead to the exchange of satellites between planets and/or their destabilisation. This leads to collisions between satellites and their tidal disruption close to the planet. Both of these events lead to large quantities of debris being produced, which in time will settle into a disc such as those observed. The mass of debris required is comparable to a Ceres-sized satellite. Through N-body simulations of planets with clones of the Galilean satellite system undergoing scattering, we show that 90 % of planets undergoing scattering will possess debris from satellite destruction. Extrapolating to smaller numbers of satellites suggests that tens of percent of such planets should still possess circumplanetary debris discs. The debris trails arising from these events are often tilted at tens of degrees to the planetary orbit, consistent with the inclinations of the observed discs. Disruption of satellite systems during scattering thus simultaneously explains the existence of debris, the tilt of the discs, and the eccentricity of the planets they orbit.

A Kustaanheimo–Stiefel regularization of the elliptic restricted three-body problem and the detection of close encounters with fast Lyapunov indicators

We present the Kustaanheimo–Stiefel (KS) regularization of the elliptic restricted three-body problem (ER3BP) at the secondary body P2, and discuss its use to study a category of transits through its Hill’s sphere (fast close encounters). Starting from the Hamiltonian representation of the problem using the synodic rotating–pulsating reference frame and the true anomaly of P2 as independent variable, we perform the regularization at the secondary body analogous to the circular case by applying the classical KS transformation and the iso-energetic reduction in an extended 10-dimensional phase-space; this translates into an efficient algorithm that can be readily implemented to numerically integrate the equations of motion. Using such a regularized Hamiltonian we recover a definition of fast close encounters in the ER3BP for small values of the mass parameter μ (while we do not require a smallness condition on the eccentricity of the primaries), and we show that for these encounters the solutions of the variational equations are characterized by an exponential growth during the fast transits through the Hill’s sphere. Thus, for small μ, we justify the effectiveness of the regularized fast Lyapunov indicators (RFLIs) to detect orbits with multiple fast close encounters. Finally, we provide numerical demonstrations and show the benefits of the regularization in terms of the computational cost.

Free-floating binary planets from ejections during close stellar encounters

Free-floating binary planets from ejections during close stellar encounters

Multiple time-step reversible N-body integrators for close encounters in planetary systems

ABSTRACT We present new ‘almost’ time-reversible integrators for solution of planetary systems consisting of ‘planets’ and a dominant mass (‘star’). The algorithms can be considered adaptive generalizations of the Wisdom–Holman method, in which all pairs of planets can be assigned time-steps. These time-steps, along with the global time-step, can be adapted time-reversibly, often at no appreciable additional compute cost, without sacrificing any of the long-term error benefits of the Wisdom–Holman method. The method can also be considered a simpler and more flexible version of the symba symplectic code. We perform tests on several challenging problems with close encounters and find the reversible algorithms are up to 2.6 times faster than a code based on symba. The codes presented here are available on Github. We also find adapting a global time-step reversibly and discretely must be done in block-synchronized manner or similar.

Long-term dynamics around the Didymos–Dimorphos binary asteroid of boulders ejected after the DART impact

Context. In 2022, the DART mission spacecraft impacted the asteroid Dimorphos, the secondary body of the binary Didymos system, ejecting a large number of dust particles, rocks and boulders. The ESA Hera mission will reach the system in 2026 for post-impact studies and a possible detection of orbiting fragments. Aims. We aim to investigate the long-term dynamics of the large boulders ejected by DART to test if any of these objects survive in orbit until the arrival of the Hera mission. Methods. To model the dynamics of the boulders, we used a numerical model that includes the gravity of non-spherical Didymos and Dimorphos, the solar gravity, and the radiation pressure. The SPICE kernels are used to define the correct reference frame for the integrations. Results. The dynamics of the boulders is highly chaotic, and 1% of the initial boulders survive at least for four years on quasi-stable orbits. These orbits are characterised by wide oscillations in eccentricity in antiphase with those in inclination (including spin flips), a mechanism similar to the Kozai one. This behaviour may protect these bodies from close encounters with both asteroids. We also computed the distribution on the surfaces of the asteroids of sesquinary impacts, which may influence the dust emission (after the initial DART impact) and the surface composition of the asteroids. Conclusions. The probability of observing boulders by the mission Hera is small but non-negligible, and an almost constant flux of escaping boulders is expected in the coming years since their lifetime after the DART impact covers a large time interval. Most re-impacts on Dimorphos occur in the hemisphere opposite the impact site, preferentially close to the equatorial plane.

Gravitational bremsstrahlung in plasmas and clusters

We study the gravitational bremsstrahlung owing to collisions mediated by a 1/r potential. We combine classical and first order Born approximation results in order to construct an approximate gravitational “Gaunt factor” for the total emitted energy. We also obtain the cross section with an angular momentum cutoff and hence the cross section for emission via close hyperbolic encounters in a gravitating cluster. These effects are the dominant source of very high frequency gravitational noise in the Solar System. The total gravitational wave power of the Sun is 76±20 MW. Published by the American Physical Society 2024

Exploring the influence of seafarers’ individual characteristics on the perceived risk in Maritime emergencies: A simulator study

The perceived risk experienced during the operation of a ship varies among seafarers, likely due to individual differences and variations in task requirements. Hence, this study aims to investigate the impacts of seafarers’ individual characteristics on their perceived risk under various emergencies by conducting a high-fidelity ship-handling simulator experiment. A linear mixed effects model is built to assess the impact of individual characteristics (e.g. age, navigational experience, profession, professional level) on the perceived risk of seafarers. Results show that the perceived risk of maritime pilots is significantly higher than deck officers under three emergency situations. The lower perceived risk is found from seafarers with higher navigational experience or at a higher professional level in situations involving poor visibility or an emergency incident. Additionally, the seafarer’s age could affect the perceived risk only in the circumstance of an emergency incident. Another finding is that seafarers at a high professional level (i.e. first mate, second-class pilot) exhibit higher perceived risk than those at a low professional level (i.e. second mate, third-class pilot) under the emergency of close encounters. Seafarers’ individual characteristics have different impacts in various emergency situations, with the influence of their profession being particularly significant. Maritime administrations were suggested to develop diverse training programs for various seafarers in order to reduce the occurrence likelihood of human errors in maritime accidents.

Shark Dive and Hologram Zoo: Two Case Studies of Virtual Animal Encounters as Possible Models for Sustainable Wildlife Tourism.

Wildlife tourism is commonly argued to positively contribute to nature conservation by providing humans with educational and meaningful first-hand experiences with animals. However, concern for the sustainability of nature due to the negative impacts of wildlife tourism on animals and the environment invites alternative virtual opportunities for close encounters. Participant Observation framed by an Animal Studies perspective is employed to interpret two contemporary Australian family entertainments, Shark Dive and Hologram Zoo, centred on providing people with wild animal interactions but without real animals present. Shark Dive is a theatrical puppetry production replicating a real shark dive, and Hologram Zoo is an augmented reality experience that displays a variety of animals in 3D. The content of both is examined to understand how the animals are imagined, experienced, and given significance to determine whether they offer viable alternatives or complementary additions to in situ wildlife tourism. Findings reveal both hold potential for virtual wildlife tourism. Shark Dive provides a positive representation of marine life, generating awareness about conservation and encouraging self-reflection. Hologram Zoo's depiction of wild animals is impressive, but to more closely align with contemporary wildlife tourism ideals, the experience could enhance its conservation messaging and positive human-animal interactions.

Dynamical study of Geminid formation assuming a rotational instability scenario

Context. Various ideas have been proposed to explain the formation of the Geminid meteoroid stream from the asteroid (3200) Phaethon. However, little has been studied regarding whether the Geminid formation could be the result of mass ejection via rotational instability of this asteroid. Aims. In this work, we present the first dynamical study of the Geminid formation taking into account low-velocity mass ejection caused by Phaethon’s rotational instability. Methods. We conducted numerical simulations for 1-mm and 1-cm particles ejected in a wide range of ejection epochs (103–105 yr ago). We computed the minimum orbital intersecting distance (MOID) of the dust particles as the realistic condition, that is, the Earth’s radius and the Earth–Moon distance to be observed as the Geminid meteoroid stream. Results. We found that the low-velocity ejection model produced the Geminid-like meteoroid stream when the dust particles were ejected more than ~2000 yr ago. In this case, close encounters with terrestrial planets would transport some dust particles from the Phaethon orbit (the current MOID is as large as ~460 Earth radii) to the Earth-intersecting orbits. The optimal ejection epoch and the estimated mass were 18 000 yr ago and ~1010–1014 g (<0.1% of the Phaethon mass). Conclusions. Our results suggest that the JAXA DESTINY+ mission has the potential to find evidence of recent rotational instability recorded on the surface of Phaethon.

Interacting galaxies in the IllustrisTNG simulations – VI: Reconstructed orbits, close encounters, and mergers

ABSTRACT Cosmological simulations have been used to study interacting galaxies as a function of galaxy pair separation, enabling comparisons with observational studies of galaxy pairs. The study of interacting galaxies as a function of time (i.e. merger stage) has mostly been limited to high-resolution merger simulations, due to the poor time sampling available in cosmological simulations. Building on an earlier study of galaxy pairs in the IllustrisTNG cosmological simulations, we reconstruct the orbits of galaxy pairs involving massive galaxies (M* > 1010 M⊙) at redshifts of 0 ≤ z < 1, using a novel kinematic interpolation scheme to model the orbits in between the IllustrisTNG snapshots (which are separated by 162 Myr, on average). We assess the accuracy of these interpolations using a pre-existing suite of merger simulations, and find that kinematic interpolations provide a remarkable improvement in accuracy compared with interpolations that use only radial separations or 3D positions. We find that nearly 90 per cent of the closest pairs (r < 25 kpc) have had a pericentre encounter within the past Gyr. Many of these close pairs are found on rapidly shrinking orbits, and roughly 85 per cent of these pairs will merge within 1 Gyr. However, approximately 3 per cent of these close pairs appear to be flyby systems that will never merge. These reconstructed orbits will be used in future studies to investigate how and when galaxy properties change during close encounters and mergers between galaxies in IllustrisTNG.

Co-moving groups around massive stars in the nuclear stellar disk

Context. Over the last ∼30 Myr, the nuclear stellar disk in the Galactic center has been the most prolific star-forming region of the Milky Way when averaged by volume. Remarkably, the combined mass of the only three clusters present today in the nuclear stellar disk adds up to only ∼10% of the total expected mass of young stars formed in this period. Several causes could explain this apparent absence of clusters and stellar associations. The stellar density in the area is so high that only the most massive clusters would be detectable against the dense background of stars. The extreme tidal forces reigning in the Galactic center could dissolve even the most massive of the clusters in just a few megayears. Close encounters with one of the massive molecular clouds, which are abundant in the nuclear stellar disk, can also rapidly make any massive cluster or stellar association dissolve beyond recognition. However, traces of some dissolving young clusters and associations could still be detectable as co-moving groups. Aims. It is our aim to identify so far unknown clusters or groups of young stars in the Galactic center. We focus our search on known, spectroscopically identified massive young stars to see whether their presence can pinpoint such structures. Methods. We created an algorithm to detect over-densities in the 5D space spanned by proper motions, positions on the plane of the sky, and line-of-sight distances, using reddening as a proxy for the distances. Since co-moving groups must be young in this environment, proper motions provide a good means to search for young stars in the Galactic center. As such, we combined publicly available data from three different surveys of the Galactic center, covering an area of ∼160 arcmin2 on the nuclear stellar disk. Results. We find four co-moving groups around massive stars, two of which are very close in position and velocity to the Arches’ most likely orbit. Conclusions. These co-moving groups are strong candidates to be clusters or associations of recently formed stars, showing that not all the apparently isolated massive stars are run-away former members of any of the three known clusters in the Galactic center or simply isolated massive stars. Our simulations show that these groups or clusters may dissolve beyond our limits of detection in less than ∼6 Myr.

Rendez-vous with massive interstellar objects, as triggers of destabilization

ABSTRACT We study how close passages of interstellar objects of planetary and substellar masses may affect the immediate and long-term dynamics of the Solar system. We consider two nominal approach orbits, namely the orbits of actual interstellar objects 1I/’Oumuamua and 2I/Borisov, assuming them to be typical or representative for interstellar swarms of matter. Thus, the nominal orbits of the interloper in our models cross the inner part of the Solar system. Series of massive numerical experiments are performed, in which the interloper’s mass is varied with a small step over a broad range. We find that, even if a Jovian-mass interloper does not experience close encounters with the Solar system planets (and this holds for our nominal orbits), our planetary system can be destabilized on time-scales as short as several million years. In what concerns substellar-mass interlopers (free-floating brown dwarfs), an immediate (on a time-scale of ∼10–100 yr) consequence of such a massive interstellar object (MISO) flyby is a sharp increase in the orbital eccentricities and inclinations of the outer planets. On an intermediate time-scale (∼103 to 105 yr after the MISO flyby), Uranus or Neptune can be ejected from the system, as a result of their mutual close encounters and encounters with Saturn. On a secular time-scale (∼106 to 107 yr after the MISO flyby), the perturbation wave formed by secular planetary interactions propagates from the outer Solar system to its inner zone.

Goodbye to Chi by Eye: A Bayesian Analysis of Photometric Binaries in Six Open Clusters

We present a robust methodology for identifying photometric binaries in star clusters. Using Gaia DR3, Pan-STARRS, and Two Micron All Sky Survey data, we self-consistently define the cluster parameters and binary demographics for the open clusters (OCs) NGC 2168 (M35), NGC 7789, NGC 6819, NGC 2682 (M67), NGC 188, and NGC 6791. These clusters span in age from ∼200 Myr (NGC 2168) to more than ∼8 Gyr (NGC 6791) and have all been extensively studied in the literature. We use the Bayesian Analysis of Stellar Evolution software suite to derive the age, distance, reddening, metallicity, binary fraction, and binary mass-ratio posterior distributions for each cluster. We perform a careful analysis of our completeness and also compare our results to previous spectroscopic surveys. For our sample of main-sequence stars with masses between 0.6 and 1 M ⊙, we find that these OCs have similar binary fractions that are also broadly consistent with the field multiplicity fraction. Within the clusters, the binary fraction increases dramatically toward the cluster centers, likely a result of mass segregation. Furthermore nearly all clusters show evidence of mass segregation within the single and binary populations. The OC binary fraction increases significantly with cluster age in our sample, possibly due to a combination of mass-segregation and cluster-dissolution processes. We also find a hint of an anticorrelation between binary fraction and cluster central density as well as total cluster mass, possibly due to an increasing frequency of higher-energy close stellar encounters that inhibit long-period binary survival and/or formation.