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The Population of Near Earth Asteroids in Coorbital Motion with Venus

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The Population of Near Earth Asteroids in Coorbital Motion with Venus ARTICLE IN PRESS YICAR:7986 JID:YICAR AID:7986 /FLA [m5+; v 1.65; Prn:10/08/2006; 14:41] P.1 (1-10) Icarus ••• (••••) •••–••• www.elsevier.com/locate/icarus The population of Near Earth Asteroids in coorbital motion with Venus M.H.M. Morais a,∗, A. Morbidelli b a Grupo de Astrofísica da Universidade de Coimbra, Observatório Astronómico de Coimbra, Santa Clara, 3040 Coimbra, Portugal b Observatoire de la Côte d’Azur, BP 4229, Boulevard de l’Observatoire, Nice Cedex 4, France Received 13 January 2006; revised 10 April 2006 Abstract We estimate the size and orbital distributions of Near Earth Asteroids (NEAs) that are expected to be in the 1:1 mean motion resonance with Venus in a steady state scenario. We predict that the number of such objects with absolute magnitudes H<18 and H<22 is 0.14 ± 0.03 and 3.5 ± 0.7, respectively. We also map the distribution in the sky of these Venus coorbital NEAs and we see that these objects, as the Earth coorbital NEAs studied in a previous paper, are more likely to be found by NEAs search programs that do not simply observe around opposition and that scan large areas of the sky. © 2006 Elsevier Inc. All rights reserved. Keywords: Asteroids, dynamics; Resonances 1. Introduction object with a theory based on the restricted three body problem at high eccentricity and inclination. Christou (2000) performed In Morais and Morbidelli (2002), hereafter referred to as a 0.2 Myr integration of the orbits of NEAs in the vicinity of Paper I, we estimated the population of Near Earth Asteroids the terrestrial planets, namely (3362) Khufu, (10563) Izhdubar, (NEAs) that are in the 1:1 mean motion resonance (i.e., are 1994 TF2 and 1989 VA, showing that the first three could be- coorbital1) with the Earth in a steady state scenario where come coorbitals of the Earth while the fourth could become NEAs are constantly being supplied by the main belt sources coorbital of Venus. Michel (1997) had previously performed a (Morbidelli et al., 2002). Here, we will apply the same method- 2 Myr integration of the orbit of (45660) Nereus showing that ology to estimate the population of NEAs that are in the 1:1 this object could be captured in the 1:1 mean motion resonance mean motion resonance with Venus. with Venus. Numerical integrations of the orbits of NEAs showed that Recently, other examples of current Earth and Venus coor- these objects can become temporary coorbitals of the terres- bitals were found. Connors et al. (2002) showed that 2002 trial planets. Wiegert and Innanen (1997) showed that (3753) AA29 is in a horseshoe orbit with the Earth. Wiegertetal. Cruithne has an asymmetric horseshoe orbit2 with the Earth. (2002) showed that 2000 PH5 and 2001 GO2 are also in horse- Namouni (1999) later explained in detail the behavior of this shoe orbits with the Earth. Connors et al. (2004) showed that 2003 YN107 is a quasi-satellite of the Earth. Finally, we know * ◦ 2 examples of current Venus coorbitals: Mikkola et al. (2004) Corresponding author. Rua Padre Himalaia 94, 1 Dt. Frt. 4100-553 Porto, showed that 2002 VE68 is a quasi-satellite of Venus and Brasser Portugal. E-mail addresses: [email protected] (M.H.M. Morais), et al. (2004) showed that 2001 CK32 has an asymmetric horse- [email protected] (A. Morbidelli). shoe orbit with Venus. 1 Coorbital objects can have tadpole orbits around the triangular equilibria L4 In this paper we will test the hypothesis of the existence of or L5, horseshoe orbits, quasi-satellite orbits, compound and transition orbits a population of Venus coorbital NEAs which are constantly be- that include the previous modes (Namouni, 1999; Nesvorný et al., 2002). 2 Asymmetric horseshoe (tadpole) orbits are compound orbits that in- ing supplied by main belt sources. Using as a starting point the volve horseshoe (tadpole) modes and quasi-satellite modes (Namouni, 1999; predictions of the NEA model of Bottke et al. (2000, 2002), Nesvorný et al., 2002). we will estimate the steady state number (according to size) of 0019-1035/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2006.06.009 Please cite this article as: M.H.M. Morais, A. Morbidelli, The population of Near Earth Asteroids in coorbital motion with Venus, Icarus (2006), doi:10.1016/j.icarus.2006.06.009. ARTICLE IN PRESS YICAR:7986 JID:YICAR AID:7986 /FLA [m5+; v 1.65; Prn:10/08/2006; 14:41] P.2 (1-10) 2 M.H.M. Morais, A. Morbidelli / Icarus ••• (••••) •••–••• Venus coorbital NEAs, we will obtain their orbital elements’ feeding the IS region. Therefore, the subpopulation of the IS re- distribution, and finally we will investigate their observability. gion that feeds the TR region, Ns , which is initially a fraction We will also compare these results with those obtained in Paper f of the total population of the IS region (i.e., Ns(0) = fNIS), I for the population of Earth coorbital NEAs. starts decaying into the TR region at a rate dN 2. Methodology s =−r N , (2) dt IS s The methodology for estimating the population of Venus where rIS is the fractional decay rate into the TR region which coorbital NEAs is the same as the one described in Paper I. is approximately constant for small t. We followed the evolution of test bodies with e and I cho- On the other hand, the number of bodies in the IS region that sen according to the NEA model of Bottke et al. (2000, 2002) still have not entered the TR region at time t is for 0.8 <a<0.9 AU (our intermediate source region) as they N(t)= N (t) + N − fN , (3) became coorbital with the Venus (our target region) and un- s IS IS til they collide with the Sun or a terrestrial planet, or achieve where NIS is the initial population in the IS region. a>10 AU. The orbital elements of these test bodies (as well We obtain Ns(t) by solving Eq. (2) with initial condition as those of the planets) were outputted every 100 years as a Ns(0) = fNIS. Then we substitute this into Eq. (3) and expand compromise between being able to accurately identify coorbital it in a Taylor series up to first order around t = 0, obtaining motion with Venus (the coorbital period of Venus’ tadpole or- = [− ] bits is about 150 years3) and generating manageable amounts N(t) NIS exp rISft , (4) of data. which is valid for small t. In order to decide if a test body is captured in the 1:1 mean Finally, as the chosen initial conditions are representative of motion resonance with Venus we checked for oscillation of its the steady state orbital distribution in the IS region, the flux of semi-major axis around 0.72 AU. Oscillations with periodic- entrance in the TR region is given by ity larger than 10,000 years (thus much larger than a typical = coorbital period; see above) or that happened for less than 10 F rISfNIS (5) consecutive output times were rejected. and we can estimate rISf by fitting the data from the simula- As in Paper I, we stress that the setting of the intermedi- tions to Eq. (4). ate source (IS) as the NEA region with 0.8 <a<0.9AUis correct as any test bodies coming from the main belt will en- 3. Results and discussion ter this region before becoming Venus coorbitals and are also likely to remain in this region for a time larger than 10,000 years 3.1. The numerical integration scheme (the interval at which the output of the numerical integrations is sampled in the NEA model) with the exception of those rare As in Paper I, all our numerical integrations were made using cases in which they jump over the IS region due to a deep plan- the “swift-rmvs3” integrator (Levison and Duncan, 1994) with etary close encounter. The advantage of this choice of IS region a time-step of 4 days. This is a modification of the symplectic is that, in this way, we increase the likelihood of a test body be- algorithm proposed by Wisdom and Holman (1991) which is ing captured as a Venus coorbital (because we start with a large able to deal with planetary close encounters. population in a region already quite close to the target region) and can therefore obtain better statistics than if we followed the 3.2. Statistics from the numerical integrations test bodies all the way from the main belt sources. Obviously, we could not follow this methodology if we did not have yet a We followed a set of 2000 test bodies initially in the IS re- prediction of the NEAs orbital and size distribution in the re- gion as their orbits evolve subject to gravitational perturbations gion with 0.8 <a<0.9AU. from the 7 planets Venus to Neptune. This set was followed first In a steady state scenario, the population in the target (TR) for 5 Myr, in order to estimate the flux of entrance in the TR re- region is gion. Now, according to Eq. (5), the flux of entrance in the TR re- NTR = F × LTR, (1) gion depends on the rate rISf and NIS (the steady state number where F is the flux of entrance in the TR region and LTR is the of NEAs in the IS region). mean life-time in the TR region. The quantity rISf is, according to Eq.
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