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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.12.013 Byline: Matija Auk (a), Brett J. Gladman (b), Sarah T. Stewart (a) Keywords: Moon; Moon, Surface; Cratering; Asteroids, Dynamics; Planets, Migration Abstract: Multiple impact basins formed on the Moon about 3.8 Gyr ago in what is known as the lunar cataclysm or Late Heavy Bombardment. Many workers currently interpret the lunar cataclysm as an impact spike primarily caused by main-belt asteroids destabilized by delayed planetary migration. We show that morphologically fresh (class 1) craters on the lunar highlands were mostly formed during the brief tail of the cataclysm, as they have absolute crater number density similar to that of the Orientale basin and ejecta blanket. The connection between class 1 craters and the cataclysm is supported by the similarity of their size-frequency distribution to that of stratigraphically-identified Imbrian craters. Majority of lunar craters younger than the Imbrium basin (including class 1 craters) thus record the size-frequency distribution of the lunar cataclysm impactors. This distribution is much steeper than that of main-belt asteroids. We argue that the projectiles bombarding the Moon at the time of the cataclysm could not have been main-belt asteroids ejected by purely gravitational means. Author Affiliation: (a) Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA (b) Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC, V6T 1Z1, Canada Article History: Received 25 June 2009; Revised 5 November 2009; Accepted 6 December 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.10.022 Byline: Matija Auk, Brett J. Gladman Keywords: Celestial mechanics; Moon; Earth; Satellites; formation; Satellites; dynamics; Resonances; orbital Abstract: Multiple large impact basins on the lunar nearside formed in a relatively-short interval around 3.8-3.9 Gyr ago, in what is known as the Lunar Cataclysm (LC; also known as Late Heavy Bombardment). It is widely thought that this impact bombardment has affected the whole Solar System or at least all the inner planets. But with non-lunar evidence for the cataclysm being relatively weak, a geocentric cause of the Lunar Cataclysm cannot yet be completely ruled out [Ryder, G., 1990. Eos 71, 313, 322-323]. In principle, late destabilization of an additional Earth satellite could result in its tidal disruption during a close lunar encounter (cf.). If the lost satellite had D500km, the resulting debris can form multiple impact basins in a relatively short time, possibly explaining the LC. Canup et al. [Canup, R.M., Levison, H.F., Stewart, G.R., 1999. Astron. J. 117, 603-620] have shown that any additional satellites of Earth formed together with (and external to) the Moon would be unable to survive the rapid initial tidally-driven expansion of lunar orbit. Here we explore the fate of objects trapped in the lunar Trojan points, and find that small lunar Trojans can survive the Moon's orbital evolution until they and the Moon reach 38 Earth radii, at which point they are destabilized by a strong solar resonance. However, the dynamics of Trojans containing enough mass to cause the LC (diameters >150 km) is more complex; we find that such objects do not survive the passage through a weaker solar resonance at 27 Earth radii. This distance was very likely reached by the Moon long before the LC, which seems to rule out the disruption of lunar Trojans as a cause of the LC. Author Affiliation: Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Rd., Vancouver, BC V6T 1Z1, Canada Article History: Received 13 May 2008; Revised 22 October 2008; Accepted 31 October 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.08.034 Byline: David NesvornA1/2 (a)(b), Fernando Roig (b), Brett Gladman (c), Daniela Lazzaro (b), Valerio Carruba (d), Thais Mothe-Diniz (b) Keywords: Asteroid Vesta; Asteroids; dynamics; Mineralogy Abstract: Spectroscopic observations of Asteroid (4) Vesta and numerous members of the Vesta family located in the inner asteroid belt have determined that these objects have reflectance properties of basaltic material. A plausible hypothesis is that the surface of Vesta was punctured by large impacts in the past which dispersed fragments of its basaltic crust into space and produced one of the most prominent asteroid families ever created in the belt. Until recently, Vesta was the only known object in the asteroid belt which underwent differentiation and survived to the present epoch. Since 2000, many new small basaltic asteroids have been discovered in the inner and outer parts of the asteroid belt, possibly representing fragments from distinct differentiated bodies. These discoveries may help us to better understand the number and nature of objects in the inner Solar System that underwent geological differentiation. To investigate these issues we performed extensive numerical simulations whose aim was to reproduce, as precisely as possible, the dynamical evolution of Vesta's ejected fragments over timescales comparable to the family's age. Specifically, we numerically integrated the orbital evolution of 6600 test bodies with orbits that started within the Vesta family and dynamically evolved over 2 Gy. Our model included gravitational perturbation of all planets (except Mercury) and the Yarkovsky effect. The results show that a relatively large fraction of the original Vesta family members may have evolved out of the family borders defined by clustering algorithms and are now dispersed over the inner asteroid belt. We compared the orbital distribution of our model fragments with the orbital locations of known basaltic asteroids in various parts of the inner main belt to find that: (i) Most basaltic asteroids with semimajor axis a Author Affiliation: (a) Department of Space Studies, Southwest Research Institute, 1050 Walnut St., Suite 400, Boulder, CO 80302, USA (b) Observatorio Nacional, Rua Gal. Jose Cristino 77, 20921-400 Rio de Janeiro, RJ, Brazil (c) Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC, V6T 1Z1, Canada (d) Departamento de Fisica e Astronomia, Universidade do Vale do Paraiba, Av. Shishima Hifumi 2911, CEP 12244-000, Sao Jose dos Campos, SP, Brazil Article History: Received 2 July 2007; Revised 9 August 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.11.021 Byline: David NesvornA1/2 (a), David VokrouhlickA1/2 (a), William F. Bottke (a), Brett Gladman (b), Therese Haggstrom (c) Keywords: Meteorites; Asteroids; Asteroids; dynamics Abstract: Our understanding of planet formation depends in fundamental ways on what we learn by analyzing the composition, mineralogy, and petrology of meteorites. Yet, it is difficult to deduce the compositional and thermal gradients that existed in the solar nebula from the meteoritic record because, in most cases, we do not know where meteorites with different chemical and isotopic signatures originated. Here we developed a model that tracks the orbits of meteoroid-sized objects as they evolve from the [nu].sub.6 secular resonance to Earth-crossing orbits. We apply this model to determining the number of meteorites accreted on the Earth immediately after a collisional disruption of a D[approximately equal to]200-km-diameter inner-main-belt asteroid in the Flora family region. We show that this event could produce fossil chondrite meteorites found in an [approximately equal to]470 Myr old marine limestone quarry in southern Sweden, the L-chondrite meteorites with shock ages [approximately equal to]470 Myr falling on the Earth today, as well as asteroid-sized fragments in the Flora family. To explain the measured short cosmic-ray exposure ages of fossil meteorites our model requires that the meteoroid-sized fragments were launched at speeds >500 mas.sup.-1 and/or the collisional lifetimes of these objects were much shorter immediately after the breakup event than they are today. Author Affiliation: (a) Department of Space Studies, Southwest Research Institute, 1050 Walnut St., Suite 400, Boulder, CO 80302, USA (b) Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC, V6T 1Z1, Canada (c) Department of Earth Sciences, Marine Geology, Goteborg University, P.O. Box 460, SE-405 30 Goteborg, Sweden Article History: Received 26 August 2006; Revised 23 October 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.05.008 Byline: Harold F. Levison (a), Martin J. Duncan (b), Luke Dones (a), Brett J. Gladman (c) Keywords: Comets; Origin; Solar System; Kuiper belt Abstract: We investigate a new dynamical mechanism for producing Halley-type comets from the scattered disk of comets. Levison and Duncan [Levison, H., Duncan, M., 1997. Icarus 127, 13-32] and Duncan and Levison [Duncan, M., Levison, H., 1997. Science 276, 1670-1672] showed that a significant number of objects leave the scattered disk by evolving to semi-major axes greater than 1000 AU. We find that once these objects reach semi-major axes on the order of 10.sup.4AU, a significant fraction immediately have their perihelia driven inward by the galactic tides. Approximately 0.01% of the objects that reach 10.sup.4AU then evolve onto orbits similar to the observed Halley-like comets due to gravitational interactions with the giant planets. The orbital element distribution resulting from this process is statistically consistent with observations. We discuss the implications of this model for the number of objects in the scattered disk in the text. The model predicts a temporal variation in the influx of HTCs with a period of [approximately equal to]118 Myr. At the peak, the model predicts that there should be roughly 10 times as many HTCs as currently observed (i.e., there should be weak HTC showers). However, the model may inflate the importance of these showers because it does not include the effects of passing stars and giant molecular clouds. Author Affiliation: (a) Department of Space Studies, Southwest Research Institute, Boulder, CO 80302, USA (b) Department of Physics, Queen's University, Kingston, Ontario, Canada K7L 3N6 (c) Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1 Article History: Received 27 April 2004; Revised 17 April 2006

We investigate a new dynamical mechanism for producing Halley-type comets from the scattered disk of comets. Levison and Duncan [Levison, H., Duncan, M., 1997. Icarus 127, 13-32] and Duncan and Levison [Duncan, M., Levison, H., 1997. Science 276, 1670-1672] showed that a significant number of objects leave the scattered disk by evolving to semi-major axes greater than 1000 AU. We find that once these objects reach semi-major axes on the order of [10.sub.4] AU, a significant fraction immediately have their perihelia driven inward by the galactic rides. Approximately 0.01% of the objects that reach [10.sub.4] AU then evolve onto orbits similar to the observed Halley-like comets due to gravitational interactions with the giant planets. The orbital element distribution resulting from this process is statistically consistent with observations. We discuss the implications of this model for the number of objects in the scattered disk in the text. The model predicts a temporal variation in the influx of HTCs with a period of ~ 118 Myr. At the peak, the model predicts that there should be roughly 10 rimes as many HTCs as currently observed (i.e., there should be weak HTC showers). However, the model may inflate the importance of these showers because it does not include the effects of passing stars and giant molecular clouds. Keywords: Comets; Origin, Solar System; Kuiper belt

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.03.005 Byline: Matija Auk, Brett J. Gladman Keywords: Origin; Solar System; Celestial mechanics; Resonances; Satellites of Saturn; Satellites of Uranus Abstract: The passage of Jupiter and Saturn through mutual 1:2 mean-motion resonance has recently been put forward as explanation for their relatively high eccentricities [Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F., 2005. Nature 435, 459-461] and the origin of Jupiter's Trojans [Morbidelli, A., Levison, H.F., Tsiganis, K., Gomes, R., 2005. Nature 435, 462-465]. Additional constraints on this event based on other small-body populations would be highly desirable. Since some outer satellite orbits are known to be strongly affected by the near-resonance of Jupiter and Saturn ('the Great Inequality'; Auk, M., Burns, J.A., 2004b. Astron. J. 128, 2518-2541), the irregular satellites are natural candidates for such a connection. In order to explore this scenario, we have integrated 9200 test particles around both Jupiter and Saturn while they went through a resonance-crossing event similar to that described by Tsiganis et al. [Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F., 2005. Nature 435, 459-461]. The test particles were positioned on a grid in semimajor axes and inclinations, while their initial pericenters were put at just 0.01 AU from their parent planets. The goal of the experiment was to find out if short-lived bodies, spiraling into the planet due to gas drag (or alternatively on orbits crossing those of the regular satellites), could have their pericenters raised by the resonant perturbations. We found that about 3% of the particles had their pericenters raised above 0.03 AU (i.e. beyond Iapetus) at Saturn, but the same happened for only 0.1% of the particles at Jupiter. The distribution of surviving particles at Saturn has strong similarities to that of the known irregular satellites. If saturnian irregular satellites had their origin during the 1:2 resonance crossing, they present an excellent probe into the early Solar System's evolution. We also explore the applicability of this mechanism for Uranus, and find that only some of the uranian irregular satellites have orbits consistent with resonant pericenter lifting. In particular, the more distant and eccentric satellites like Sycorax could be stabilized by this process, while closer-in moons with lower eccentricity orbits like Caliban probably did not evolve by this process alone. Author Affiliation: Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Rd., Vancouver, B.C. V6T 1Z1, Canada Article History: Received 6 December 2005; Revised 28 February 2006

The passage of Jupiter and Saturn through mutual 1:2 mean-motion resonance has recently been put forward as explanation for their relatively high eccentricities [Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F., 2005. Nature 435, 459-461] and the origin of Jupiter's Trojans [Morbidelli, A., Levison, H.F., Tsiganis, K., Gomes, R., 2005. Nature 435,462-465]. Additional constraints on this event based on other smallbody populations would be highly desirable. Since some outer satellite orbits are known to be strongly affected by the near-resonance of Jupiter and Saturn ('the Great Inequality'; (Cuk, M., Burns, J.A., 2004b. Astron. J. 128, 2518-2541), the irregular satellites are natural candidates for such a connection. In order to explore this scenario, we have integrated 9200 test particles around both Jupiter and Saturn while they went through a resonance-crossing event similar to that described by Tsiganis et al. [Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F., 2005. Nature 435, 459-461]. The test particles were positioned on a grid in semimajor axes and inclinations, while their initial pericenters were put at just 0.01 AU from their parent planets. The goal of the experiment was to find out if short-lived bodies, spiraling into the planet due to gas drag (or alternatively on orbits crossing those of the regular satellites), could have their pericenters raised by the resonant perturbations. We found that about 3% of the particles had their pericenters raised above 0.03 AU (i.e. beyond Iapetus) at Saturn, but the same happened for only 0.1% of the particles at Jupiter. The distribution of surviving particles at Saturn has strong similarities to that of the known irregular satellites. If saturnian irregular satellites had their origin during the 1:2 resonance crossing, they present an excellent probe into the early Solar System's evolution. We also explore the applicability of this mechanism for Uranus, and find that only some of the uranian irregular satellites have orbits consistent with resonant pericenter lifting. In particular, the more distant and eccentric satellites like Sycorax could be stabilized by this process, while closer-in moons with lower eccentricity orbits like Caliban probably did not evolve by this process alone. Keywords: Origin, Solar System; Celestial mechanics: Resonances; Satellites of Saturn; Satellites of Uranus

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.10.032 Byline: Philip D. Nicholson (a), Brett J. Gladman (b) Keywords: Pluto; Mars; Satellites of Mars; Satellites; general; Trans-neptunian objects Abstract: We report the results of searches for outer satellites of Pluto and Mars, carried out with the Hale 5-m telescope in 1999 and 2001, respectively. No new satellites were found down to limiting magnitudes of m.sub.R=25.0 at Pluto and m.sub.V[approximately equal to]22 at Mars, corresponding to diameters of 35 and 1 km, respectively, for an assumed geometric albedo of 0.07. A faint trans-neptunian object, 1999 LB.sub.37, was discovered in the Pluto fields; given the depth of our survey, discovery of one such object in the background Kuiper belt is in reasonable agreement with expectations. Author Affiliation: (a) Department of Astronomy, Cornell University, Ithaca, NY 14853, USA (b) Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada Article History: Received 3 May 2005; Revised 21 October 2005

We report the results of searches for outer satellites of Pluto and Mars, carried out with the Hale 5-m telescope in 1999 and 2001, respectively. No new satellites were found down to limiting magnitudes of [m.sub.R] = 25.0 at Pluto and [m.sub.R] ~ 22 at Mars, corresponding to diameters of 35 and 1 km, respectively, for an assumed geometric albedo of 0.07. A faint trans-neptunian object, 1999 L[B.sub.37], was discovered in the Pluto fields; given the depth of our survey, discovery of one such object in the background Kuiper belt is in reasonable agreement with expectations. Keywords: Pluto; Mars; Satellites of Mars; Satellites, general; Trans-neptunian objects

We present BVRI colors of 13 jovian and 8 saturnian irregular satellites obtained with the 2.56 m Nordic Optical Telescope on La Palma, the 6.5 m Magellan Baade Telescope on La Campanas, and the 6.5 m MMT on Mt. Hopkins. The observations were performed from December 2001 to March 2002. The colors of the irregular satellites vary from grey to light red. We have arbitrarily divided the known irregular satellites into two classes based on their colors. One, the grey color class, has similar colors to the C-type asteroids, and the other, the light red color class, has colors similar to P/D-type asteroids. We also find at least one object, the jovian irregular J XXIII Kalyke, that has colors similar to the red colored Centaurs/TNOs, although its classification is insecure. We find that there is a correlation between the physical properties and dynamical properties of the irregular satellites. Most of the dynamical clusters have homogeneous colors, which points to single homogeneous progenitors being cratered or fragmented as the source of each individual cluster. The heterogeneously colored clusters are most easily explained by assuming that there are several dynamical clusters in the area, rather than just one, or that the parent body was a differentiated, heterogeneous body. By analyzing simple cratering/fragmentation scenarios, we show that the heterogeneous colored S IX Phoebe cluster, is most likely two different clusters, a grey colored cluster centered on S IX Phoebe and a light red colored cluster centered on S/2000 S 1. To which of these two clusters the remaining saturnian irregulars with inclinations close to 174[degrees] belong is not clear from our analysis, but determination of their colors should help constrain this. We also show through analysis of possible fragmentation and dispersion of the six known uranian irregulars that they most likely make up two clusters, one centered on U XVI Caliban and another centered on U XVII Sycorax. We further show that, although the two objects have similar colors, a catastrophic fragmentation event creating one cluster containing both U XVI Caliban and U XVII Sycorax would have involved a progenitor with a diameter of ~ 395 km. While such an event is not impossible it seems rather improbable, and we further show that such an event would leave 5-6 fragments with sizes comparable to or larger than U XVI Caliban. The stable region around Uranus has been extensively searched to limiting magnitudes far beyond that of U XVI Caliban. The fact that only U XVI Caliban, the larger U XVII Sycorax and four much smaller objects have been found leaves us with a distribution not compatible with a catastrophic event with such a large progenitor. The most likely solution is therefore two separate events creating two uranian dynamical clusters. Keywords: Photometry; Satellite of Jupiter; Satellite of Saturn; Satellite of Uranus; Surfaces, satellite

Irregular satellites--moons that occupy large orbits of significant eccentricity e and/or inclination I--circle each of the giant planets. The irregulars often extend close to the orbital stability limit, about 1/3-1/2 of the way to the edge of their planet's Hill sphere. The distant, elongated, and inclined orbits suggest capture, which presumably would give a random distribution of inclinations. Yet, no known irregulars have inclinations (relative to the ecliptic) between 47 and 141[degrees]. This paper shows that many high-I orbits are unstable due to secular solar perturbations. High-inclination orbits suffer appreciable periodic changes in eccentricity; large eccentricities can either drive particles with ~70[degrees] < I < 110[degrees] deep into the realm of the regular satellites (where collisions and scatterings are likely to remove them from planetocentric orbits on a timescale of [10.sup.7]-[10.sup.9] years) or expel them from the Hill sphere of the planet. By carrying out long-term ([10.sup.9] years) orbital integrations for a variety of hypothetical satellites, we demonstrate that solar and planetary perturbations, by causing particles to strike (or to escape) their planet, considerably broaden this zone of avoidance. It grows to at least 55[degrees] < I < 130[degrees] for orbits whose pericenters freely oscillate from 0 to 360[degrees], while particles whose pericenters are locked at [+ or -] 90[degrees] (Kozai mechanism) can remain for longer times. We estimate that the stable phase space (over 10 Myr) for satellites trapped in the Kozai resonance contains ~10% of all stable orbits, suggesting the possible existence of a family of undiscovered objects at higher inclinations than those currently known. Key Words: satellites of Jupiter; celestial mechanics; orbits; origins.

The dynamical lifetimes of small bodies against ejection from the Solar System or collision with the Sun or a planet are often estimated by Monte Carlo codes based on the equations of Opik and using a method implemented by Arnold. Such algorithms assume that orbital changes are dominated by close encounters, and that successive encounters are uncorrelated. We have compared the results of an Opik code (H. J. Melosh and W. B. Tonks, Meteoritics 28, 398 (1993)) and a fast integrator (H. E Levison and M. J. Duncan, Icarus 108, 18 (1994)) to investigate the regimes of validity of the Opik-Arnold approach. We investigate the transfer of ecliptic comets from Neptune-crossing orbits to observable Jupiter-family comets, the dynamics of Halley-type comets, and the transport of meteorites among the terrestrial planets. In all cases, the Opik code overestimates the median lifetime of the small bodies, although both codes show a rapid initial loss of objects followed by a slow decay. For martian impact ejecta, some of which find their way to Earth as the SNC meteorites, the Opik code substantially overestimates lifetimes because of its neglect of secular resonances, which rapidly pump eccentricities (B. J. Gladman et al., Science 271, 1387 (1996)). Key Words: asteroids; dynamics; celestial mechanics; comets, dynamics; meteoroids; orbits.

The delivery dynamics of martian meteorites are examined by means of a directnumerical simulation of their orbital evolution. The dynamics in the martian regionare dominated by secular resonant effects, not by close encounters with Mars. Thesesecular effects rapidly ([approximately] 1 Myr) transport martian ejecta toEarth-crossing orbits. The measured cosmic-ray exposure ages of the martianmeteorites are consistent with their being directly launched as small bodies fromthe martian surface by impacts over the last [approximately equal to]15 Myr.Collisional effects and being driven into the Sun efficiently destroy martianmeteoroids in space on time scales of order 10 Myr. The implications of theseresults for the launch mechanism and microorganism transport are discussed. Key Words: meteorites; meteoroids; celestial mechanics; Mars.

Satellite spin states evolve under the action of solid body torques and tidal forces. The tidal effects result in a damping of fast spin rates and, ultimately, in the locking of the spin into what are known as Cassini states. The dynamical equations for the satellite spin vector are derived, non-dimensionalized, and discussed. The non-dimensional parameters that determine the ultimate fate of the system are identified. A review of Cassini states is given, and then the effect of a permanent triaxial deformation on the evolution of the spin vector is explored. We find that for the parameter ranges of most real Solar System satellites which have been despun to synchronous rotation, occupation of Cassini state 1 is the only possible endpoint. The existence of a non-axially symmetric deformation destabilizes the higher obliquity Cassini state 2. We discuss the possibility of tumbling occurring during the spin-down and argue that this does not effect our basic conclusions. The non-occupancy of Cassini state 2 (except for the Moon, for which state 1 does not exist) is not a function of initial conditions or spin configuration occupied at synchronous lock, as previously hypothesized.

The study of protoplanetary embryos in the solar system helps investigate the motion of a pair of interacting planets orbiting a star. The initially circular planetary orbits exhibit hill stability when fractional orbital separation is related to mass ratios of two planets to the Sun. An analysis of the topological stability of the three-body problem aids the determination of hill stability. Numerical integrations favor the presence of stability conditions.

We examine the dynamics of a sample of 117 near-Earth objects (NEOs) over a time scale of 60 Myr. We find that while 10-20% end their lifetimes by striking a terrestrial planet (usually Venus or Earth), more than half end their lives in a Sun-grazing state, and about 15% are ejected from the Solar System. The median lifetime of our (biased) sample is about 10 Myr. We discuss the exchange of these objects between the various orbital classes and observe the creation of orbits entirely interior to that of Earth. A variety of resonant processes operating in the inner Solar System, while not dominant in determining the dynamical lifetimes, are crucial for understanding the orbital distribution. Several dynamical mechanisms exist which are capable of significantly increasing orbital eccentricities and inclinations. In particular, we exhibit important new routes to the Sun-grazing end-state, provided by the [v.sub.5] and [v.sub.2] secular resonances at high eccentricity between a = 1.3 and 1.9 AU. We find no dynamical reason to demand that any significant component of the NEO population must come from a cometary source, although such a contribution cannot be ruled out by this work. Key Words: asteroids; comets; impacts; near-Earth objects.