Dynamical evolution of near-Earth objects

<p><span>An apparent discrepancy between the number of observed near-Earth objects (NEOs) with small perihelion distances (q) and the number of objects that models <br />predict, has led to the conclusion that asteroids get destroyed at non-trivial distances from the Sun. Consequently, there must be a, possibly thermal, <br />mechanism at play, responsible for breaking up asteroids asteroids in such orbits.<br /><br />We studied the dynamical evolution of ficticious NEOs whose perihelion distance reaches below the average disruption distance q_dis=0.076 au, as suggested by <br />Granvik et al. (2016). To that end, we used the orbital integrations of objects that escaped from the main asteroid belt (Granvik et al. 2017), and entered the <br />near-Earth region (Granvik et al. 2018). First, we investigated a variety of mechanisms that can lower the perihelion distance of an object to a small-enough <br />value. In particular, we considered mean-motion resonances with Jupiter, secular resonances with Jupiter and Saturn (v_5 and v_6) and also the Kozai resonance.<br /><br />We developed a code that calculates the evolution of the critical argument of all the relevant resonances and identifies librations during the last stages of <br />an object's orbital evolution, namely, just before q=q_dis. Any subsequent evolution of the object was disregarded, since we considered it disrupted. The <br />accuracy of our model is ~96%.<br /><br />In addition, we measured the dynamical 'lifetimes' of NEOs when they orbit the innermost parts of the inner Solar System. More precisely, we calculated the <br />total time it takes for the q of each object to go from 0.4 au to q_dis (τ_lq). The outer limit of this range was chosen such because it is a) the approximate <br />semimajor axis of Mercury, and b) an absence of sub-meter-sized boulders with q smaller than this distance has been proposed by Wiegert et al (2020). Combining <br />this measure with the recorded resonances, we can get a sense of the timescale of each q-lowering mechanism.<br /><br />Next, for a more rigorous study of the evolution of the NEOs with q<0.4 au, we divided this region in bins and measured the relevant time they spend at <br />different distances from the Sun. Together with the total time spent in each bin, we kept track of the number of times that q entered one of the bins. <br />Finally, we computed the actual time each object spends in each bin during its evolution, i.e., the total time it spends in a specific range in radial <br />heliocentric distance.<br /><br />By following this approach, we derived categories of typical evolutions of NEOs that reach the average disruption distance. In addition, since we have the <br />information concerning the escape route from the main asteroid belt followed by each NEO, we linked the q-lowering mechanism and the associated orbital <br />evolutions in the range below the orbit of Mercury, to their source regions and thus were able to draw conclusions abour their physical properties.</span></p>

Hyperbolic Orbits in the Solar System: Interstellar Origin or Perturbed Oort Cloud Comets?

We study the dynamical properties of objects in hyperbolic orbits passing through the inner Solar system in the context of two different potential sources: interstellar space and the Oort cloud. We analytically derive the probability distributions of eccentricity, e, and perihelion distance, q, for each source and estimate the numbers of objects produced per unit of time as a function of these quantities. By comparing the numbers from the two sources, we assess which origin is more likely for a hyperbolic object having a given eccentricity and perihelion distance. We find that the likelihood that a given hyperbolic object is of interstellar origin increases with decreasing eccentricity and perihelion. Conversely, the likelihood that a hyperbolic object has been scattered from the Oort cloud by a passing star increases with decreasing eccentricity and increasing perihelion. By carefully considering their orbital elements, we conclude that both 1I/2017 U1 ’Oumuamua (e ≃ 1.2 and q ≃ 0.26 au) and 2I/2019 Q4 Borisov (e ≃ 3.3 and q ≃ 2 au) are most likely of interstellar origin, not scattered from the Oort cloud. However, we also find that Oort cloud objects can be scattered into hyperbolic orbits like those of the two known examples, by sub-stellar and even sub-Jovian mass perturbers. This highlights the need for better characterization of the low mass end of the free-floating brown dwarf and planet population.

Comets of the Marsden and Kracht groups

Perihelion distances of Marsden and Kracht group comets fall into the range 6RS < q < 12RS (the Meyer group comets also share the same perihelion interval). It is by several folds larger than the perihelion distance of the Kreutz group comets (q < 2RS). Average circulation period for comets of the Marsden group is P = 5.5 years and for the Kracht group is P =5.3 years. The Marsden and Kracht group comets share the same origin; as well as 96P (Machholz), object 196256 (2003 EH1), meteor showers the Daytime Arietids, Northern and Southern δ Aquariids, Quandrantids forming the Machholz interplanetary complex. This work offers computational movement simulation for comet-progenitor fragments. It is shown that the orbits of the representatives of the complex can be explained if the decay of the cometprogenitor for objects 96P and 196256 occurred ~9500 years ago. The following evolution direction has been demonstrated for the complex objects: progenitor comet - comet 96P - the Marsden group comets - the Kracht group comets - the Southern δ Aquariids. However, not all the complex objects will necessarily pass through every stage of the above as it can be preceded by the total disintegration of the object.

Dynamical evolution of C/2017 K2 PANSTARRS

Context. The comet C/2017 K2 PANSTARRS drew attention to its activity at the time of its discovery in May 2017 when it was about 16 au from the Sun. This Oort spike comet will approach its perihelion in December 2022, and the question about its dynamical past is an important issue to explore. Aims. In order to answer the question of whether C/2017 K2 is a dynamically old or new comet it is necessary to obtain its precise osculating orbit, its original orbit, and propagate its motion backwards in time to the previous perihelion. Knowledge of the previous perihelion distance is necessary to distinguish between these two groups of the Oort spike comets. We have studied the dynamical evolution of C/2017 K2 to the previous perihelion (backward calculations for about 3–4 Myr) as well as to the future (forward calculations for about 0.033 Myr) using the swarm of virtual comets (VCs) constructed from a nominal osculating orbit of this comet which we determined here using all positional measurements available at the moment. Outside the planetary system both Galactic and stellar perturbations were taken into account. Results. We derive that C/2017 K2 is a dynamically old Oort spike comet (1/aprev = (48.7 ± 7.9) × 10−6 au−1) with the previous perihelion distance below 10 au for 97% of VCs (nominal qprev = 3.77 au). According to the present data this comet will be perturbed into a more tightly bound orbit after passing the planetary zone (1/afut = (1140.4 ± 8.0) × 10−6 au−1, qfut = 1.79336 ± 0.00006 au) provided that non-gravitational effects will not change the orbit significantly. Conclusions. C/2017 K2 has already visited our planetary zone during its previous perihelion passage. Thus, it is almost certainly a dynamically old Oort spike comet. The future orbital solution of this comet is formally very precise, however, it is much less definitive since the presented analysis is based on pre-perihelion data taken at very large heliocentric distances (23.7–14.6 au from the Sun), and this comet can experience a significant non-gravitational perturbation during the upcoming perihelion passage in 2022.