Among all the asteroid dynamical groups, Centaurs have the highest fraction of objects moving in retrograde orbits. The distribution in absolute magnitude,
H
, of known retrograde Centaurs with ...semi-major axes in the range 6–34 AU exhibits a remarkable trend: 10 % have
H
<10 mag, the rest have
H
>12 mag. The largest objects, namely (342842) 2008 YB
3
, 2011 MM
4
and 2013 LU
28
, move in almost polar, very eccentric paths; their nodal points are currently located near perihelion and aphelion. In the group of retrograde Centaurs, they are obvious outliers both in terms of dynamics and size. Here, we show that these objects are also trapped in retrograde resonances that make them unstable. Asteroid 2013 LU
28
, the largest, is a candidate transient co-orbital to Uranus and it may be a recent visitor from the trans-Neptunian region. Asteroids 342842 and 2011 MM
4
are temporarily submitted to various high-order retrograde resonances with the Jovian planets but 342842 may be ejected towards the trans-Neptunian region within the next few hundred kyr. Asteroid 2011 MM
4
is far more stable. Our analysis shows that the large retrograde Centaurs form an heterogeneous group that may include objects from various sources. Asteroid 2011 MM
4
could be a visitor from the Oort cloud but an origin in a relatively stable closer reservoir cannot be ruled out. Minor bodies like 2011 MM
4
may represent the remnants of the primordial planetesimals and signal the size threshold for catastrophic collisions in the early Solar System.
Abstract
The discovery of 1991 VG on 1991 November 6 attracted an unprecedented amount of attention as it was the first near-Earth object (NEO) ever found on an Earth-like orbit. At that time, it was ...considered by some as the first representative of a new dynamical class of asteroids, while others argued that an artificial (terrestrial or extraterrestrial) origin was more likely. Over a quarter of a century later, this peculiar NEO has been recently recovered and the new data may help in confirming or ruling out early theories about its origin. Here, we use the latest data to perform an independent assessment of its current dynamical status and short-term orbital evolution. Extensive N-body simulations show that its orbit is chaotic on time-scales longer than a few decades. We confirm that 1991 VG was briefly captured by Earth's gravity as a minimoon during its previous fly-by in 1991–1992; although it has been a recurrent transient co-orbital of the horseshoe type in the past and it will return as such in the future, it is not a present-day co-orbital companion of the Earth. A realistic NEO orbital model predicts that objects like 1991 VG must exist and, consistently, we have found three other NEOs – 2001 GP2, 2008 UA202 and 2014 WA366 –
which are dynamically similar to 1991 VG. All this evidence confirms that there is no compelling reason to believe that 1991 VG is not natural.
On 2008 October 7, small asteroid
2008
TC
3
turned itself into the parent body of the first meteor ever to be predicted before entering the Earth’s atmosphere. Over five years later, the 2014 AA ...event became the second instance of such an occurrence. The uncertainties associated with the pre-impact orbit of
2008
TC
3
are relatively small because thousands of observations were made during the hours preceding the actual meteor airburst. In sharp contrast, 2014 AA was only observed seven times before impact and consequently its trajectory is somewhat uncertain. Here, we present a recalculation of the impact parameters—location and timing—of this meteor based on infrasound recordings. The new values—
(
λ
impact
,
ϕ
impact
,
t
impact
)
=
(
−
44
∘
,
+
11
∘
,
2456659.618
JD UTC
)
—and their uncertainties together with Monte Carlo and
N
-body techniques, are applied to obtain an independent determination of the pre-impact orbit of 2014 AA:
a
=
1.1623
AU
,
e
=
0.2116
,
i
=
1
.
∘
4156
,
Ω
=
101
.
∘
6086
, and
ω
=
52
.
∘
3393
. Our orbital solution is used to investigate the possible presence of known near-Earth objects (NEOs) moving in similar orbits. Among the objects singled out by this search, the largest is
2013
HO
11
with an absolute magnitude of 23.0 (diameter 75–169 m) and a MOID of 0.006 AU. Prior to impact, 2014 AA was subjected to a web of overlapping secular resonances and it followed a path similar to those of
2011
GJ
3
,
2011
JV
10
,
2012
DJ
54
, and
2013
NJ
4
. NEOs in this transient group have their orbits controlled by close encounters with the Earth–Moon system at perihelion and Mars at aphelion, perhaps constituting a dynamical family. Extensive comparison with other studies is also presented.
Context.
Most flybys in the Galactic disk are distant, beyond 10
4
AU, and have characteristic velocities of ~70 km s
−1
. However, deep and fast encounters also take place, albeit with lower ...probability, particularly if one of the objects involved is a stellar remnant ejected during a supernova. WD 0810-353 might be a high velocity white dwarf, and it was recently identified as heading straight for the Solar System; however, the
Gaia
DR3 data that support its future deep and fast flyby are regarded as suspicious.
Aims.
Here, we reanalyze the
Gaia
DR3 data set associated with WD 0810-353 to confirm or reject the reality of its Solar System flyby and also to investigate its possible runaway status.
Methods.
We studied the evolution of WD 0810-353 forward in time using
N
-body simulations. We computed the distribution of distances of closest approach and their associated times of perihelion passage. We used a statistical analysis of the kinematics of this object to assess its possible hypervelocity. We compared its mean BP/RP spectrum to those of other well-studied white dwarfs.
Results.
We confirm that WD 0810-353 is headed for the Solar System, but the actual parameters of the encounter depend strongly on its radial velocity. The
Gaia
DR3 value of −373.74 ± 8.18 km s
−1
is strongly disfavored by our analyses. Its mean BP/RP spectrum suggests a value over ten times higher based on the position of its putative H
α
line. However, spectral matching using other white dwarfs with non-
Gaia
data indicate a radial velocity in the interval (−60, −70) km s
−1
.
Conclusions.
These results confirm the future flyby of WD 0810-353 near the Solar System, although the relative velocity could be high enough or the minimum approach distance large enough to preclude any significant perturbation on the Oort cloud.
Among the near-Earth object (NEO) population, there are comets and active asteroids which are sources of fragments that initially move together; in addition, some NEOs follow orbits temporarily ...trapped in a web of secular resonances. These facts contribute to increasing the risk of meteoroid strikes on Earth, making its proper quantification difficult. The identification and subsequent study of groups of small NEOs that appear to move in similar trajectories are necessary steps in improving our understanding of the impact risk associated with meteoroids. Here, we present results of a search for statistically significant dynamical groupings among the NEO population. Our Monte Carlo-based methodology recovers well-documented groupings like the Taurid Complex or the one resulting from the split comet 73P/Schwassmann–Wachmann 3, and new ones that may have been the source of past impacts. Among the most conspicuous are the Mjolnir and Ptah groups, perhaps the source of recent impact events like Almahata Sitta and Chelyabinsk, respectively. Meteoroid 2014 AA, that hit the Earth on 2014 January 2, could have its origin in a marginally significant grouping associated with Bennu. We find that most of the substructure present within the orbital domain of the NEOs is of resonant nature, probably induced by secular resonances and the Kozai mechanism that confine these objects into specific paths with well-defined perihelia.
ABSTRACT Stellar streams are ubiquitous in the Galactic halo and they can be used to improve our understanding of the formation and evolution of the Milky Way as a whole. The so-called Monoceros Ring ...might have been the result of satellite accretion. Guglielmo et al. have used N-body simulations to search for the progenitor of this structure. Their analysis shows that, if the Ring has a dwarf galaxy progenitor, it might be found in the background of one out of eight specific areas in the sky. Here, we use Gaia DR2 data to perform a systematic exploration aimed at confirming or rejecting this remarkable prediction. Focusing on the values of the radial velocity to uncover possible multimodal spreads, we identify a bimodal Gaussian distribution towards Galactic coordinates (l, b) = (271°, +2°) in Vela, which is one of the locations of the progenitor proposed by Guglielmo et al. This prominent feature with central values 60 ± 7 and 97 ± 10 km s−1, may signal the presence of the long sought progenitor of the Monoceros Ring, but the data might also be compatible with the existence of an unrelated, previously unknown, kinematically coherent structure.
Context. Hypervelocity stars move fast enough to leave the gravitational field of their home galaxies and venture into intergalactic space. The most extreme examples known have estimated speeds in ...excess of 1000 km s−1. These can be easily induced at the centres of galaxies via close encounters between binary stars and supermassive black holes; however, a number of other mechanisms operating elsewhere can produce them as well. Aims. Recent studies suggest that hypervelocity stars are ubiquitous in the local Universe. In the Milky Way, the known hypervelocity stars are anisotropically distributed, but it is unclear why. Here, we used Gaia Data Release 2 (DR2) data to perform a systematic exploration aimed at confirming or refuting these findings. Methods. Our basic premise is that the farther the candidate hypervelocity stars are, the more likely they are to be unbound from the Galaxy. We used the statistical analysis of both the spatial distribution and kinematics of these objects to achieve our goals. Monte Carlo sampling techniques were applied to deal with large uncertainties. No global parallax zero-point correction was performed. Results. Focussing on nominal Galactocentric distances greater than 30 kpc, which are the most distant candidates, we isolated a sample with speeds in excess of 500 km s−1 that exhibits a certain degree of anisotropy but remains compatible with possible systematic effects. We find that the effect of the Eddington-Trumpler-Weaver bias is important in our case: over 80% of our sources are probably located further away than implied by their parallaxes; therefore, most of our velocity estimates are lower limits. If this bias is as strong as suggested here, the contamination by disc stars may not significantly affect our overall conclusions. Conclusions. The subsample with the lowest uncertainties shows stronger, but obviously systematic, anisotropies and includes a number of candidates of possible extragalactic origin and young age with speeds of up to 2000 km s−1.
Context. The discovery and tracking of 2018 LA marks only the third instance in history that the parent body of a fireball has been identified before its eventual disintegration in our atmosphere. ...The subsequent recovery of meteorites from 2018 LA was only the second time materials from outer space that reached the ground could be linked with certitude to a particular minor body. However, meteoroids like 2018 LA and its forerunners, 2008 TC3 and 2014 AA, are perhaps fragments of larger members of the near-Earth object (NEO) population. As the processes leading to the production of such fragments are unlikely to spawn just one meteoroid per event, it is important to identify putative siblings and plausible candidates from which the observed meteoroids might have originated. Aims. Here, we study the pre-impact orbital evolution of 2018 LA to place this meteoroid within the dynamical context of other NEOs that follow similar trajectories. Methods. Our statistical analyses are based on the results of direct N-body calculations that use the latest orbit determinations and include perturbations by the eight major planets, the Moon, the barycentre of the Pluto–Charon system, and the three largest asteroids. A state-of-the-art NEO orbit model was used to interpret our findings and a randomization test was applied to estimate their statistical significance. Results. We find a statistically significant excess of NEOs in 2018 LA-like orbits; among these objects, we find one impactor, 2018 LA, and the fourth closest known passer-by, 2018 UA. A possible connection with the χ-Scorpiids meteor shower is also discussed. The largest known NEO with an orbit similar to that of 2018 LA is the potentially hazardous asteroid (454100) 2013 BO73 and we speculate that they both originate from a common precursor via a collisional cascade. Conclusions. Future spectroscopic observations of 454100 and other NEOs in similar orbits may confirm or deny a possible physical relationship with 2018 LA.
Venus has three known co-orbitals: (322756) 2001 CK32, 2002 VE68 and 2012 XE133. The first two have absolute magnitudes 18 < H < 21. The third one, significantly smaller at H = 23.4 mag, is a recent ...discovery that signals the probable presence of many other similar objects: small transient companions to Venus that are also potentially hazardous asteroids (PHAs). Here, we study the dynamical evolution of the recently discovered asteroid 2013 ND15. At H = 24.1 mag, this minor body is yet another small Venus co-orbital and PHA, currently close to the Lagrangian point L4 and following the most eccentric path found so far for objects in this group. This transient Trojan will leave the 1:1 mean motion resonance within a few hundred years although it could be a recurrent librator. Due to its high eccentricity (0.6), its dynamics is different from that of the other three known Venus co-orbitals even if they all are near-Earth objects (NEOs). A Monte Carlo simulation that uses the orbital data and discovery circumstances of the four objects as proxies to estimate the current size of this population, indicates that the number of high-eccentricity, low-inclination Venus co-orbital NEOs may have been greatly underestimated by current models. Three out of four known objects were discovered with solar elongation at perigee greater than 135° even if visibility estimates show that less than 4 per cent of these objects are expected to reach perigee at such large elongations. Our calculations suggest that the number of minor bodies with sizes above 150 m currently engaged in co-orbital motion with Venus could be at least one order of magnitude larger than usually thought; the number of smaller bodies could easily be in many thousands. These figures have strong implications on the fraction of existing PHAs that can barely be detected by current surveys. Nearly 70 per cent of the objects discussed here have elongation at perigee <90° and 65 per cent are prospective PHAs.
Many asteroids in the main and trans-Neptunian belts are trapped in mean motion resonances with Jupiter and Neptune, respectively. As a side effect, they experience accidental commensurabilities ...among themselves. These commensurabilities define characteristic patterns that can be used to trace the source of the observed resonant behaviour. Here, we explore systematically the existence of commensurabilities between the known ETNOs using their heliocentric and barycentric semimajor axes, their uncertainties, and Monte Carlo techniques. We find that the commensurability patterns present in the known ETNO population resemble those found in the main and trans-Neptunian belts. Although based on small number statistics, such patterns can only be properly explained if most, if not all, of the known ETNOs are subjected to the resonant gravitational perturbations of yet undetected trans-Plutonian planets. We show explicitly that some of the statistically significant commensurabilities are compatible with the Planet Nine hypothesis; in particular, a number of objects may be trapped in the 5:3 and 3:1 mean motion resonances with a putative Planet Nine with semimajor axis ∼700 au.
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