arXiv subjects

Apostolos A. Christou

Publications and source records attributed to Apostolos A. Christou.

At least 19 recordsLinked to original sources

arXiv2025

New asteroid clusters and evidence of collisional fragmentation in the L5 Trojan cloud of Mars

Context. Trojan asteroids of Mars date from an early phase of solar system evolution. Based on a sample of <10 asteroids, MT distribution has been previously shown to be both asymmetric and inhomogeneous with population evolution dominated by thermal radiation forces acting over timescales of Gyr. Remarkably, a single asteroid family associated with (5261) Eureka (H~16) at L5 contains most MTs and all members of the stable population fainter than H=18. Aims. Using the currently available sample of MTs and their orbits, we take a fresh look at this population to re-evaluate these earlier conclusions and to search for additional features diagnostic of the evolutionary history of MTs and the Eureka family in particular. Methods. We perform harmonic analysis on numerical time series of the osculating elements to compile a new proper element catalogue comprising 16 L5 and 1 L4 Mars Trojan asteroids. We then combine sample variance analysis with statistical hypothesis testing to identify clusters in the distribution of orbits and assess their significance. Results. We identify two small clusterings significant at 95% confidence of three H=20-21 asteroids each and investigate their likely origin. One of the clusters is probably the result of rotational breakup of a Eureka family asteroid ~10^8 yr ago. The significantly higher tadpole libration width of asteroids in the other cluster is more consistent with an origin as impact ejecta from Eureka itself and on a timescale comparable to the ~1 Gyr age of its family. We further confirm the previously reported correlations in Eureka family orbital distribution attributed to the long-term action of radiation-driven forces and torques on the asteroids.

astro-ph.EP↗

arXiv2024

Feasibility of meteor surveying from a Venus orbiter

Meteor and bolide phenomena caused by the atmospheric ablation of incoming meteoroids are predicted to occur at the planet Venus. Their systematic observation would allow to measure and compare the sub-mm to m meteoroid flux at different locations in the solar system. Using a physical model of atmospheric ablation, we demonstrate that Venus meteors would be brighter, shorter-lived, and appear higher in the atmosphere than Earth meteors. To investigate the feasibility of meteor detection at Venus from an orbiter, we apply the SWARMS survey simulator tool to sets of plausible meteoroid population parameters, atmospheric models and instrument designs suited to the task, such as the Mini-EUSO camera operational on the ISS since 2019. We find that such instrumentation would detect meteors at Venus with a 1.5x to 2.5x higher rate than at Earth. The estimated Venus-Earth detection ratio remains insensitive to variations in the chosen observation orbit and detector characteristics, implying that a meteor survey from Venus orbit is feasible, though contingent on the availability of suitable algorithms and methods for efficient on-board processing and downlinking of the meteor data to Earth. We further show that a hypothetical camera onboard the upcoming EnVision mission to Venus similar to the ISS instrument should detect many times more meteors than needed for an initial characterisation of the large meteoroid population at 0.7 au from the Sun.

astro-ph.EP↗

arXiv2023

Deconvolving the complex structure of the asteroid belt

The asteroid belt is a unique source of information on some of the most important questions facing solar system science. These questions include the sizes, numbers, types and orbital distributions of the planetesimals that formed the planets, and the identification of those asteroids that are the sources of meteorites and near-Earth asteroids. Answering these questions requires an understanding of the dynamical evolution of the asteroid belt, but this evolution is governed by a complex interplay of mechanisms that include catastrophic disruption, orbital evolution driven by Yarkovsky radiation forces, and chaotic orbital evolution driven by gravitational forces. While the timescales of these loss mechanisms have been calculated using estimates of some critical parameters that include the thermal properties, strengths and mean densities of the asteroids, we argue here that the uncertainties in these parameters are so large that deconvolution of the structure of the asteroid belt must be guided primarily by observational constraints. We argue that observations of the inner asteroid belt indicate that the size-frequency distribution is not close to the equilibrium distribution postulated by Dohnanyi (1969). We also discuss the correlations observed between the sizes and the orbital elements of the asteroids. While some of these correlations are significant and informative, others are spurious and may arise from the limitations of the Hierarchical Clustering Method that is currently used to define family membership.

astro-ph.EP↗

arXiv2023

Physical and dynamical properties of selected Earth co-orbital asteroids

We present our investigations of the physical and dynamical properties of selected Earth co-orbital asteroids. The photometric optical light curves as well as rotation periods and pole solutions for a sample of four Earth co-orbital asteroids, namely (138175) 2000 EE104 (P=13.9476+/-0.0051 hrs), (418849) 2008 WM64 (P=2.4077+/-0.0001 hrs), 2016 CA138 (P=5.3137+/-0.0016 hrs) and 2017 SL16 (P=0.3188+/-0.0053 hrs), are determined or improved and presented in this work. For this investigation, we combine observations carried out at the Bulgarian National Astronomical Observatory - Rozhen using the FoReRo2 instrument attached to the 2mRCC telescope as well as sparse data from AstDys2 database. Parallel to the rotational properties we did numerical dynamical simulations to investigate the orbital stability of those objects and to find out if there is a relation with their rotational properties. Our results show that the orbit stability is affected by the orbit itself and mainly its eccentricity and inclination, which are responsible for the close encounters with other Solar system planets. We cannot make a definitive conclusion about the relation between orbit stability and the rotational state of the asteroids, so we need further investigations and observations in order to prove or disprove it.

astro-ph.EP↗

arXiv2022

Chaotic diffusion of asteroids in the exterior 1:2 mean motion resonance with Mars

The inner asteroid belt between 2.1 and 2.5 au is of particular dynamical significance because it is the dominant source of both chondritic meteorites and near-Earth asteroids. This inner belt is bounded by an eccentricity-type secular resonance and by the 1:3 mean motion resonance with Jupiter. Unless asteroid perihelia are low enough to allow scattering by Mars, escape requires transport to one of the bounding resonances. In addition Yarkovsky forces are generally ineffective in changing either the eccentricity and/or inclination for asteroids with diameter $\gtrsim$30 km. Thus, large asteroids with pericentres far from Mars may only escape from the inner belt through large changes in their eccentricities. In this paper we study chaotic diffusion of orbits near the 1:2 mean motion resonance with Mars in a systematic way. We show that, while chaotic orbital evolution in both resonant and non-resonant orbits increase the dispersion of the inclinations and eccentricities, it does not significantly change their mean values. We show further that, while the dispersive growth is greatest for resonant orbits, at high $e$ the resonance acts to mitigate asteroid scattering by Mars - making the asteroid lifetime in the belt longer than it would have been for a non-resonant orbit. For asteroids of all sizes in both resonant and non-resonant orbits, the changes in eccentricity needed to account for the observations cannot be achieved by gravitational forces alone. The role of resonant trapping in protecting asteroids from encounters with Mars is also analysed.

astro-ph.EP↗

arXiv2021

Long-term dynamical survival of deep Earth coorbitals

We investigate the long-term dynamical survival of Earth co-orbital asteroids, focusing on near-circular, near-planar orbits which existing studies suggest are the most stable. Through numerical integration of test particles we show that about a quarter of an initial population can survive for at least 50\% of the age of the solar system with horseshoe particles being four to five times more likely to survive than L4/L5 Trojans. From the end state statistics we constrain the existence of planetesimal-sized objects originally in co-orbital libration, finding that typically $5^{-2}_{+7}$ such planetesimals and no more than $27^{-9}_{+30}$ (95\% confidence) could have been present. Our simulations also suggest that episodic variations in the terrestrial orbital eccentricity may have caused bulk escape of co-orbitals, though variations large enough ($>$0.01) to generate such episodes are statistically unlikely. We then consider the orbital evolution of co-orbital asteroids of sizes down to $D = 50$ m under the Yarkovsky effect and find that objects with $D$ $<$ 1 km should escape over 4 Gyr with the smallest asteroids escaping after 200 Myr. Further, we test whether Earth's co-orbital region may be populated by asteroids arriving via outward Yarkovsky drift, as conjectured by Zhou et al.~(A\&A, 622, A97, 2019). We find this is an inefficient process, as planetary close encounters rapidly scatter the orbits far from Earth's and towards the asteroid belt. Finally, we discuss how the destabilising action of Yarkovsky may be mitigated through spin state evolution or late collisional comminution of large parent asteroids.

astro-ph.EP↗

arXiv2021

Dynamical evolution of the inner asteroid belt

A determination of the dynamical evolution of the asteroid belt is difficult because the asteroid belt has evolved since the time of asteroid formation through mechanisms that include: (1) catastrophic collisions, (2) rotational disruption, (3) chaotic orbital evolution and (4) orbital evolution driven by Yarkovsky radiation forces. The timescales of these loss mechanisms are uncertain and there is a need for more observational constraints. In the inner main belt, the mean size of the non-family asteroids increases with increasing inclination. Here, we use that observation to show that all inner main belt asteroids originate from either the known families or from ghost families, that is, old families with dispersed orbital elements. We estimate that the average age of the asteroids in the ghost families is a factor of 1/3 less than the Yarkovsky orbital evolution timescale. However, this orbital evolution timescale is a long-term average that must allow for the collisional evolution of the asteroids and for stochastic changes in their spin directions. By applying these constraints on the orbital evolution timescales to the evolution of the size-frequency distribution of the Vesta asteroid family, we estimate that the age of this family is greater than 1.3 $Gyr$ and could be comparable with the age of the solar system. By estimating the number of ghost families, we calculate that the number of asteroids that are the root sources of the meteorites and the near-Earth asteroids that originate from the inner main belt is about 20.

astro-ph.EP↗

arXiv2020

The common origin of family and non-family asteroids

All asteroids are currently classified as either family, originating from the disruption of known bodies, or non-family. An outstanding question is the origin of these non-family asteroids. Were they formed individually, or as members of known families but with chaotically evolving orbits, or are they members of old ghost families, that is, asteroids with a common parent body but with orbits that no longer cluster in orbital element space? Here, we show that the sizes of the non-family asteroids in the inner belt are correlated with their orbital eccentricities and anticorrelated with their inclinations, suggesting that both non-family and family asteroids originate from a small number of large primordial planetesimals. We estimate that ~85% of the asteroids in the inner main belt originate from the Flora, Vesta, Nysa, Polana and Eulalia families, with the remaining ~15% originating from either the same families or, more likely, a few ghost families. These new results imply that we must seek explanations for the differing characteristics of the various meteorite groups in the evolutionary histories of a few, large, precursor bodies. Our findings also support the model that asteroids formed big through the gravitational collapse of material in a protoplanetary disk.

astro-ph.EP↗

arXiv2020

Composition and origin of L5 Trojan asteroids of Mars: Insights from spectroscopy

We investigate the mineralogy of L5 Martian Trojan asteroids via reflectance spectroscopy, in particular (101429) 1998 $\mbox{VF}_{31}$, the only L5 Trojan that does not belong to the Eureka family (Christou, 2013). We find that this asteroid most likely belongs to the Bus-Demeo S-complex, in agreement with Rivkin et al. (2007) and obtain good spectral matches with Sq- or S-type asteroids, the lunar surface and of Martian and lunar meteorites. Mixture fitting to spectral endmembers suggests a surface abundance of Mg-rich orthopyroxene and iron metal or, alternatively, a mixture of plagioclase and metal with a small amount of Mg-poor orthopyroxene. The metallic component may be part of the intrinsic mineral makeup of the asteroid or an indication of extreme space weathering. We discuss several origin scenarios for (101429). The asteroid could be related to iron-rich primitive achondrites (Rivkin et al.), may have originated as impact ejecta from Mars - as proposed recently for the Eureka family asteroids (Polishook et al., 2017) - or could be a relic fragment of the Moon's original solid crust. If, on the other hand, (101429) is a relatively recent addition to the Martian Trojan clouds (Christou et al., 2020), its origin is probably traced to high-inclination asteroid families in the Inner Main Belt. For the olivine-dominated Eureka family, we find that the two smaller asteroids are more spectrally similar to one another than to (5261) Eureka. Spectral profiles of all three asteroids are closely similar shortward of $\sim$0.7$μ$m but diverge at longer wavelengths. For the two smaller asteroids in particular, we find the spectra are virtually identical up to $0.8$$μ$m. We attribute spectral differences in the near-IR region to differences in either: degree of space weathering, olivine chemical composition and/or regolith grain size.

astro-ph.EP↗

arXiv2020

Capture of satellites during planetary encounters A case study of the Neptunian moons Triton and Nereid

Single-binary scattering may lead to an exchange where the single object captures a component of the binary, forming a new binary. This has been well studied in encounters between a star--planet pair and a single star. Here we explore the application of the exchange mechanism to a planet--satellite pair and another planet in the gravitational potential of a central star. As a case study, we focus on encounters between a satellite-bearing object and Neptune. We investigate whether Neptune can capture satellites from that object and if the captured satellites have orbits analogous to the Neptunian moons Triton and Nereid. Using $N$-body simulations, we study the capture probability at different encounter distances. Post-capture, we use a simple analytical argument to estimate how the captured orbits evolve under collisional and tidal effects. We find that the average capture probability reaches $\sim$$10\%$ if Neptune penetrates the donor planet's satellite system. Most moons grabbed by Neptune acquire highly eccentric orbits. Post-capture, around half of those captured, especially those on tight orbits, can be circularised, either by tides only or by collisions+tides, turning into Triton-like objects. Captures further out, on the other hand, stay on wide and eccentric orbits like that of Nereid. Both moon types can be captured in the same encounter and they have wide distributions in orbital inclination. Therefore, Triton naturally has a $\sim$50\% chance of being retrograde. A similar process potentially applies to an exoplanetary system, and our model predicts that exomoons can jump from one planet to another during planetary scattering. Specifically, there should be two distinct populations of captured moons: one on close-in circular orbits and the other on far-out eccentric orbits. The two populations may have highly inclined prograde or retrograde orbits.

astro-ph.EP↗

arXiv2020

The origin of Neptune's unusual satellites from a planetary encounter

The Neptunian satellite system is unusual, comprising Triton, a large ($\sim2700$ km) moon on a close-in, circular, yet retrograde orbit, flanked by Nereid, the largest irregular satellite ($\sim$300 km) on a highly eccentric orbit. Capture origins have been previously suggested for both moons. Here we explore an alternative in-situ formation model where the two satellites accreted in the circum-Neptunian disk and are imparted irregular and eccentric orbits by a deep planetary encounter with an ice giant (IG), like that predicted in the Nice scenario of early solar system development. We use $N$-body simulations of an IG approaching Neptune to 20 Neptunian radi ($R_\mathrm{Nep}$), through a belt of circular prograde regular satellites at 10-30 $R_\mathrm{Nep}$. We find that half of these primordial satellites remain bound to Neptune and that 0.4-3\% are scattered directly onto wide and eccentric orbits resembling that of Nereid. With better matches to the observed orbit, our model has a success rate comparable to or higher than capture of large Nereid-sized irregular satellites from heliocentric orbit. At the same time, the IG encounter injects a large primordial moon onto a retrograde orbit with specific angular momentum similar to Triton's in 0.3-3\% of our runs. While less efficient than capture scenarios (Agnor & Hamilton 2006), our model does indicate that an in-situ origin for Triton is dynamically possible. We also simulate the post-encounter collisional and tidal orbital evolution of Triton analogue satellites and find they are decoupled from Nereid on timescales of $\sim$$10^4$ years, in agreement with Cuk & Gladman (2005).

astro-ph.EP↗

arXiv2019

Population control of Mars Trojans by the Yarkovsky & YORP effects

We explore the hypothesis that the population of Martian Trojans is the result of a balance between the production of new asteroids ("YORPlets") through the YORP effect and their eventual escape from the Trojan clouds through Yarkovsky-driven orbital evolution. Our principal observables are: (5261) Eureka, its family of 8 asteroids and the family-less Trojans (101429) 1998 VF31 & (121514) 1999 UJ7. We model the population evolution as a birth-death random process and assume it is in a steady state. We then simulate the discovery of Trojans to-date and find that family members of 101429 and 121514, if they exist, are intrinsically more difficult to detect than Eureka's. Their non-discovery can be used as evidence of their non-existence only under the assumption that their brightness relative to the parent asteroid is similar to that in the Eureka family. To find out how efficiently Mars Trojans are lost from the Trojan clouds due to the Yarkovsky effect, we carry out dynamical simulations of test particles originating from these parent bodies. We find that objects originating from Eureka and 121514 begin escaping after ~1 Gyr, but that those from 101429 are already lost by that time, probably due to that asteroid's proximity to an eccentricity-type secular resonance within Mars's co-orbital region. This is the likely cause behind the absence of Trojans in the orbital vicinity of 101429. In contrast, the solitary status of 121514 points to an intrinsic inability of the asteroid to produce YORPlets during the most recent ~20% of the solar system's history, a finding potentially related to 121514's present, low angular momentum rotational state, unless the Eureka family formed rapidly during a single fission event.

astro-ph.EP↗

arXiv2017

Is the Eureka cluster a collisional family of Mars Trojan asteroids?

We explore the hypothesis that the Eureka family of sub-km asteroids in the L5 region of Mars could have formed in a collision. We estimate the size distribution index from available information on family members; model the orbital dispersion of collisional fragments; and carry out a formal calculation of the collisional lifetime as a function of size. We find that, as initially conjectured by Rivkin et al (2003), the collisional lifetime of objects the size of (5261) Eureka is at least a few Gyr, significantly longer than for similar-sized Main Belt asteroids. In contrast, the observed degree of orbital compactness is inconsistent with all but the least energetic family-forming collisions. Therefore, the family asteroids may be ejecta from a cratering event sometime in the past ~1 Gyr if the orbits are gradually dispersed by gravitational diffusion and the Yarkovsky effect (Cuk et al, 2015). The comparable sizes of the largest family members require either negligible target strength or a particular impact geometry under this scenario (Durda et al, 2007; Benavidez et al, 2012). Alternatively, the family may have formed by a series of YORP-induced fission events (Pravec.et.al, 2010). The shallow size distribution of the family is similar to that of small MBAs (Gladman et al, 2009) interpreted as due to the dominance of this mechanism for Eureka-family-sized asteroids (Jacobson et al, 2014). However, our population index estimate is likely a lower limit due to the small available number of family asteroids and observational incompleteness. Future searches for fainter family members, further observational characterisation of the known Trojans' physical properties as well as orbital and rotational evolution modelling will help distinguish between different formation models.

astro-ph.EP↗

arXiv2017

Secular resonances between bodies on close orbits II: prograde and retrograde orbits for irregular satellites

In extending the analysis of the four secular resonances between close orbits in Li and Christou (Celest Mech Dyn Astron 125:133-160, 2016) (Paper I), we generalise the semianalytical model so that it applies to both prograde and retrograde orbits with a one to one map between the resonances in the two regimes. We propose the general form of the critical angle to be a linear combination of apsidal and nodal differences between the two orbits $ b_1 Δ\varpi + b_2 ΔΩ$, forming a collection of secular resonances in which the ones studied in Paper I are among the strongest. Test of the model in the orbital vicinity of massive satellites with physical and orbital parameters similar to those of the irregular satellites Himalia at Jupiter and Phoebe at Saturn shows that $> 20\%$ and $> 40\%$ of phase space is affected by these resonances, respectively. The survivability of the resonances is confirmed using numerical integration of the full Newtonian equations of motion. We observe that the lowest order resonances with $b_1+|b_2|\le 3$ persist, while even higher order resonances, up to $b_1+|b_2|\ge 7$, survive. Depending on the mass, between $10\%-60\%$ of the integrated test particles are captured in these secular resonances, in agreement with the phase space analysis in the semianalytical model.

astro-ph.EP↗

arXiv2016

Secular resonances between bodies on close orbits: a case study of the Himalia prograde group of jovian irregular satellites

The gravitational interaction between two objects on similar orbits can effect noticeable changes in the orbital evolution even if the ratio of their masses to that of the central body is vanishingly small. Christou (2005) observed an occasional resonant lock in the differential node $ΔΩ$ between two members in the Himalia irregular satellite group of Jupiter in the $N$-body simulations (corresponding mass ratio $\sim 10^{-9}$). Using a semianalytical approach, we have reproduced this phenomenon. We also demonstrate the existence of two additional types of resonance, involving angle differences $Δω$ and $Δ(Ω+\varpi)$ between two group members. These resonances cause secular oscillations in eccentricity and/or inclination on timescales $\sim$ 1 Myr. We locate these resonances in $(a,e,i)$ space and analyse their topological structure. In subsequent $N$-body simulations, we confirm these three resonances and find a fourth one involving $Δ\varpi$. In addition, we study the occurrence rates and the stability of the four resonances from a statistical perspective by integrating 1000 test particles for 100 Myr. We find $\sim 10-30$ librators for each of the resonances. Particularly, the nodal resonance found by Christou is the most stable: 2 particles are observed to stay in libration for the entire integration.

astro-ph.EP↗

arXiv2014

Modelling circumplanetary ejecta clouds at low altitudes: a probabilistic approach

A model is presented of a ballistic, collisionless, steady state population of ejecta launched at randomly distributed times and velocities and moving under constant gravity above the surface of an airless planetary body. Within a probabilistic framework, closed form solutions are derived for the probability density functions of the altitude distribution of particles, the distribution of their speeds in a rest frame both at the surface and at altitude and with respect to a moving platform such as an orbiting spacecraft. These expressions are validated against numerically-generated synthetic populations of ejecta under lunar surface gravity. The model is applied to the cases where the ejection speed distribution is (a) uniform (b) a power law. For the latter law, it is found that the effective scale height of the ejecta envelope directly depends on the exponent of the power law and increases with altitude. The same holds for the speed distribution of particles near the surface. Ejection model parameters can, therefore, be constrained through orbital and surface measurements. The scope of the model is then extended to include size-dependency of the ejection speed and an example worked through for a deterministic power law relation. The result suggests that the height distribution of ejecta is a sensitive proxy for this dependency.

astro-ph.EP↗

arXiv2014

Yarkovsky-Driven Spreading of the Eureka Family of Mars Trojans

Out of nine known stable Mars Trojans, seven appear to be members of an orbital grouping including the largest Trojan, Eureka. In order to test if this could be a genetic family, we simulated the long term evolution of a tight orbital cluster centered on Eureka. We explored two cases: cluster dispersal through planetary gravity alone over 1 Gyr, and a 1 Gyr evolution due to both gravity and the Yarkovsky effect. We find that the dispersal of the cluster in eccentricity is primarily due to dynamical chaos, while the inclinations and libration amplitudes are primarily changed by the Yarkovsky effect. Current distribution of the cluster members orbits are indicative of an initially tight orbital grouping that was affected by a negative acceleration (i.e. one against the orbital motion) consistent with the thermal Yarkovsky effect. We conclude that the cluster is a genetic family formed either in a collision or through multiple rotational fissions. The cluster's age is on the order of 1 Gyr, and its long-term orbital evolution is likely dominated by the seasonal, rather than diurnal, Yarkovsky effect. If confirmed, Gyr-scale dominance of the seasonal Yarkovsky effect may indicate suppression of the diurnal Yarkovsky drift by the related YORP effect. Further study of Mars Trojans is essential for understanding the long-term orbital and rotational dynamics of small bodies in the absence of frequent collisions.

astro-ph.EP↗

arXiv2013

Orbital clustering of Martian Trojans: An asteroid family in the inner solar system?

We report on the discovery of new Martian Trojans within the Minor Planet Center list of asteroids. Their orbital evolution over 10^8 yr shows characteristic signatures of dynamical longevity (Scholl et al, 2005) while their average orbits resemble that of the largest known Martian Trojan, 5261 Eureka. The group forms a cluster within the region where the most stable Trojans should reside. Based on a combinatorial analysis and a comparison with the Jovian Trojan population, we argue that both this feature and the apparent paucity of km-sized Martian Trojans (Trilling et al, 2006) as compared to expectations from earlier work (Tabachnik and Evans, 1999) is not due to observational bias but instead a natural end result of the collisional comminution (Jutzi et al, 2010) or, alternatively, the rotational fission (Pravec et al, 2010) of a progenitor L5 Trojan of Mars. Under the collisional scenario in particular, the new Martian Trojans are dynamically young, in agreement with our age estimate of this "cluster" of < 2 Gyr based on the earlier work of Scholl et al. This work highlights the Trojan regions of the Terrestrial planets as natural laboratories to study processes important for small body evolution in the solar system and provides the first direct evidence for an orbital cluster of asteroids close to the Earth.

astro-ph.EP↗