Reconstructing the Chelyabinsk event: pre-impact orbital evolution

The Chelyabinsk superbolide was the largest known natural object to enter the Earth's atmosphere since the Tunguska event in 1908 and it has become a template to understand, manage and mitigate future impacts. Although the event has been documented in great detail, the actual pre-impact orbit of the parent body is still controversial. Here, we revisit this topic using an improved Monte Carlo approach that includes the coordinates of the impact point to compute the most probable solution for the pre-impact orbit (a = 1.62 au, e = 0.53, i = 3.97 degrees, \Omega; = 326.45 and \omega; = 109.71). We also check all the published solutions using a simple yet robust statistical test to show that many of them have problems to cause an impact at the right time. We use the improved orbit and N-body simulations to revisit the dynamical status of a putative Chelyabinsk asteroid family and confirm that it could be linked to resonant asteroids 2007 BD7 and 2011 EO40. In addition, and as the classification of Chelyabinsk meteorites is well established, a search for meteorite falls of the same chondrite group and petrologic type gives some evidence for the existence of an associated LL5 chondrite cluster.Comment: 5+4 pages, 5+1 figures, 1+4 tables, accepted for publication in MNRAS Letters. Revised to reflect final version published in MNRA

Transient Co-orbitals of Venus: An Update

Venus has no known satellites, but has four known co-orbitals: (322756) 2001 CK32, 2002 VE68, 2012 XE133, and 2013 ND15. Here, we present numerical evidence suggesting that 2015 WZ12 is a possible Venus co-orbital; it might have been until recently a transient Trojan. Follow-up observations of this target in the near future will be difficult, though.Comment: 4 pages, 1 figur

Evidence for a possible bimodal distribution of the nodal distances of the extreme trans-Neptunian objects: avoiding a trans-Plutonian planet or just plain bias?

It is a well-known fact that the presence of a massive perturber interacting with a population of minor bodies following very eccentric orbits can strongly affect the distribution of their nodal distances. The details of this process have been explored numerically and its outcome confirmed observationally in the case of Jupiter, where a bimodal distribution of nodal distances of comets has been found. Here, we show evidence for a possible bimodal distribution of the nodal distances of the extreme trans-Neptunian objects (ETNOs) in the form of a previously unnoticed correlation between nodal distance and orbital inclination. This proposed correlation is unlikely to be the result of observational bias as data for both large semimajor axis Centaurs and comets fit well into the pattern found for the ETNOs, and all these populations are subjected to similar background perturbations when moving well away from the influence of the giant planets. The correlation found is better understood if these objects tend to avoid a putative planet with semimajor axis in the range 300-400 au.Comment: 5 pages, 5 figures, 2 tables. Revised to match version published in MNRAS: Letters --MNRAS 471, L61-L65 (2017

Asteroid 2017 FZ2 et al.: signs of recent mass-shedding from YORP?

The first direct detection of the asteroidal YORP effect, a phenomenon that changes the spin states of small bodies due to thermal reemission of sunlight from their surfaces, was obtained for (54509) YORP 2000 PH5. Such an alteration can slowly increase the rotation rate of asteroids, driving them to reach their fission limit and causing their disruption. This process can produce binaries and unbound asteroid pairs. Secondary fission opens the door to the eventual formation of transient but genetically-related groupings. Here, we show that the small near-Earth asteroid (NEA) 2017 FZ2 was a co-orbital of our planet of the quasi-satellite type prior to their close encounter on 2017 March 23. Because of this flyby with the Earth, 2017 FZ2 has become a non-resonant NEA. Our N-body simulations indicate that this object may have experienced quasi-satellite engagements with our planet in the past and it may return as a co-orbital in the future. We identify a number of NEAs that follow similar paths, the largest named being YORP, which is also an Earth's co-orbital. An apparent excess of NEAs moving in these peculiar orbits is studied within the framework of two orbit population models. A possibility that emerges from this analysis is that such an excess, if real, could be the result of mass shedding from YORP itself or a putative larger object that produced YORP. Future spectroscopic observations of 2017 FZ2 during its next visit in 2018 (and of related objects when feasible) may be able to confirm or reject this interpretation.Comment: 20 pages, 23 figures, 2 tables. Accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journa

Colliding with G2 near the Galactic Centre: a geometrical approach

The object G2 will pass within nearly 100 au from Sgr A* in 2014. Due to its very short periapse, the study of the dynamical evolution of this object in the short-term future may offer some insight into the region surrounding the supermassive black hole at the centre of the Galaxy. With this scenario in mind, it has recently been proposed by Bartos et al. (arXiv:1302.3220) that, prior to its perinigricon, G2 will likely experience multiple encounters with members of the black hole and neutron-star populations believed to orbit near the Galactic Centre. Here, we further explore this possibility and study the general case for collisions with the G2 object using the latest orbital solutions provided by Phifer et al. (arXiv:1304.5280) and Gillessen et al., (arXiv:1306.1374) and a Monte Carlo approach to estimate the minimum orbit intersection distance (MOID) with G2 as a function of the orbital parameters of the incoming body. Our results indicate that encounters at distances closer than 100 au started to become statistically significant only during the last few years or so. MOIDs under 100 au are statistically more probable for the most dynamically cold orbits. If there is a population of objects moving in low-inclination, low-eccentricity orbits around the central black hole, the highest probability for a close encounter with G2 is found to be in the period 2014 January-March but enhanced activity due to encounters may start as early as 2013 July-August.Comment: 5 pages, 4 figures. Revised to reflect final version published in MNRAS Letter

Recent multi-kiloton impact events: are they truly random?

It is customarily assumed that Earth-striking meteoroids are completely random, and that all the impacts must be interpreted as uncorrelated events distributed according to Poisson statistics. If this is correct, their impact dates must be uniformly spread throughout the year and their impact coordinates must be evenly scattered on the surface of our planet. Here, we use a time- and yield-limited sample of Earth-impacting superbolides detected since 2000 to explore statistically this critical though frequently overlooked topic. We show that the cadence of these multi-kiloton impact events is incompatible with a random fall pattern at the 0.05 significance level or better. This result is statistically robust and consistent with the observed distribution of the longitudes of the ascending nodes of near-Earth objects (NEOs). This lack of randomness is induced by planetary perturbations, in particular Jupiter's, and suggests that some of the recent, most powerful Earth impacts may be associated with resonant groups of NEOs and/or very young meteoroid streams. An intriguing consequence of this scenario is that the impact hazard of Chelyabinsk-like objects should peak at certain times in the year.Comment: 5+1 pages, 2 figures, 1 table. Thrice revised to reflect final version published in MNRAS: Letter

Asteroid 2014 OL339: yet another Earth quasi-satellite

Our planet has one permanently bound satellite -the Moon-, a likely large number of mini-moons or transient irregular natural satellites, and three temporary natural retrograde satellites or quasi-satellites. These quasi-moons -(164207) 2004 GU9, (277810) 2006 FV35 and 2013 LX28- are unbound companions to the Earth. The orbital evolution of quasi-satellites may transform them into temporarily bound satellites of our planet. Here, we study the dynamical evolution of the recently discovered Aten asteroid 2014 OL339 to show that it is currently following a quasi-satellite orbit with respect to the Earth. This episode started at least about 775 yr ago and it will end 165 yr from now. The orbit of this object is quite chaotic and together with 164207 are the most unstable of the known Earth quasi-satellites. This group of minor bodies is, dynamically speaking, very heterogeneous but three of them exhibit Kozai-like dynamics: the argument of perihelion of 164207 oscillates around -90 degrees, the one of 277810 librates around 180 degrees and that of 2013 LX28 remains around 0 degrees. Asteroid 2014 OL339 is not currently engaged in any Kozai-like dynamics.Comment: 10 pages, 7 figures, 1 table. Revised to reflect final version published in MNRAS. arXiv admin note: text overlap with arXiv:1401.501

Finding Planet Nine: apsidal anti-alignment Monte Carlo results

The distribution of the orbital elements of the known extreme trans-Neptunian objects or ETNOs has been found to be statistically incompatible with that of an unperturbed asteroid population following heliocentric or, better, barycentric orbits. Such trends, if confirmed by future discoveries of ETNOs, strongly suggest that one or more massive perturbers could be located well beyond Pluto. Within the trans-Plutonian planets paradigm, the Planet Nine hypothesis has received much attention as a robust scenario to explain the observed clustering in physical space of the perihelia of seven ETNOs which also exhibit clustering in orbital pole position. Here, we revisit the subject of clustering in perihelia and poles of the known ETNOs using barycentric orbits, and study the visibility of the latest incarnation of the orbit of Planet Nine applying Monte Carlo techniques and focusing on the effects of the apsidal anti-alignment constraint. We provide visibility maps indicating the most likely location of this putative planet if it is near aphelion. We also show that the available data suggest that at least two massive perturbers are present beyond Pluto.Comment: 7 pages, 4 figures, 3 tables. Revised to match version published in MNRA

A trio of horseshoes: past, present and future dynamical evolution of Earth co-orbital asteroids 2015 XX169, 2015 YA and 2015 YQ1

It is widely accepted that a quasi-steady-state flux of minor bodies moving in and out of the co-orbital state with the Earth may exist. Some of these objects are very good candidates for future in situ study due to their favourable dynamical properties. In this paper, we show that the recently discovered near-Earth asteroids 2015 XX169, 2015 YA and 2015 YQ1 are small transient Earth co-orbitals. These new findings increase the tally of known Earth co-orbitals to 17. The three of them currently exhibit asymmetric horseshoe behaviour subjected to a Kozai resonance and their short-term orbital evolution is rather unstable. Both 2015 YA and 2015 YQ1 may leave Earth's co-orbital zone in the near future as they experience close encounters with Venus, the Earth-Moon system and Mars. Asteroid 2015 XX169 may have remained in the vicinity of, or trapped inside, the 1:1 mean motion resonance with our planet for many thousands of years and may continue in that region for a significant amount of time into the future.Comment: 15 pages, 9 figures, 1 table, accepted for publication in Astrophysics and Space Science. arXiv admin note: text overlap with arXiv:1511.0836

The analemma criterion: accidental quasi-satellites are indeed true quasi-satellites

In the Solar system, a quasi-satellite is an object that follows a heliocentric path with an orbital period that matches almost exactly with that of a host body (planetary or not). The trajectory is of such nature that, without being gravitationally attached, the value of the angular separation between host and quasi-satellite as seen from the Sun remains confined within relatively narrow limits for time-spans that exceed the length of the host's sidereal orbital period. Here, we show that under these conditions, a quasi-satellite traces an analemma in the sky as observed from the host in a manner similar to that found for geosynchronous orbits. The analemmatic curve (figure-eight-, teardrop-, ellipse-shaped) results from the interplay between the tilt of the rotational axis of the host and the properties of the orbit of the quasi-satellite. The analemma criterion can be applied to identify true quasi-satellite dynamical behaviour using observational or synthetic astrometry and it is tested for several well-documented quasi-satellites. For the particular case of 15810 (1994 JR1), a putative accidental quasi-satellite of dwarf planet Pluto, we show explicitly that this object describes a complex analemmatic curve for several Plutonian sidereal periods, confirming its transient quasi-satellite status.Comment: 7 pages, 2 figures, 1 table. Revised to match version published in MNRA