Inactive Comets Research Articles

Systematic Characterization of and Search for Activity in Potentially Active Asteroids

Abstract We report on our long-term observational campaign to characterize and monitor a select sample of 75 dynamically selected dormant comet candidates and six near-Sun asteroids. Both asteroid subpopulations can be considered likely to display comet-like activity. Dormant comets are currently inactive comet nuclei that can still harbor volatiles in subsurface layers, whereas near-Sun asteroids have extremely low perihelion distances and are thus prone to catastrophic disruption. As a result of our 4 yr long observing campaign, we find only dormant comet 3552 Don Quixote to show activity during our program. We furthermore find that (51 ± 10)% of dynamically selected dormant comet candidates in near-Earth space have comet-like physical properties, as well as (56 ± 16)% of dynamically selected dormant comet candidates in other parts of the solar system. All of our near-Sun asteroid sample targets are of nonprimitive nature, suggesting that primitive near-Sun asteroids are more likely to disrupt than nonprimitives. We furthermore find a significant fraction of our near-Sun asteroid sample to display extremely blue V–I color indices, potentially hinting at physical alterations of surface material close to the Sun.

Spectral and orbital survey of medium-sized meteoroids

Aims. We investigate the spectra, material properties, and orbital distribution of millimeter- to decimeter-sized meteoroids. Our study aims to distinguish the characteristics of populations of differently sized meteoroids and reveal the heterogeneity of identified meteoroid streams. We verify the surprisingly large ratio of pure iron meteoroids on asteroidal orbits detected among mm-sized bodies. Methods. Emission spectra and multi-station meteor trajectories were collected within the AMOS network observations. The sample is based on 202 meteors of −1 to −14 magnitude, corresponding to meteoroids of mm to dm sizes. Meteoroid composition is studied by spectral classification based on relative intensity ratios of Na, Mg, and Fe and corresponding monochromatic light curves. Heliocentric orbits, trajectory parameters, and material strengths inferred from empirical KB and PE parameters were determined for 146 meteoroids. Results. An overall increase of Na content compared to the population of mm-sized meteoroids was detected, reflecting weaker effects of space weathering processes on larger meteoroids. The preservation of volatiles in larger meteoroids is directly observed. We report a very low ratio of pure iron meteoroids and the discovery of a new spectral group of Fe-rich meteors. The majority of meteoroids on asteroidal orbits were found to be chondritic. Thermal processes causing Na depletion and physical processes resulting in Na-rich spectra are described and linked to characteristically increased material strengths. Numerous major and minor shower meteors were identified in our sample, revealing various degrees of heterogeneity within Halley-type, ecliptical, and sungrazing meteoroid streams. Our results imply a scattered composition of the fragments of comet 2P/Encke and 109P/Swift-Tuttle. The largest disparities were detected within α-Capricornids of the inactive comet 169P/NEAT and δ-Aquarids of the sungrazing 96P/Machholz. We also find a spectral similarity between κ-Cygnids and Taurids, which could imply a similar composition of the parent objects of the two streams.

Thermal infrared and optical photometry of Asteroidal Comet C/2002 CE10

C/2002 CE10 is an object in a retrograde elliptical orbit with Tisserand parameter −0.853 indicating a likely origin in the Oort Cloud. It appears to be a rather inactive comet since no coma and only a very weak tail was detected during the past perihelion passage. We present multi-color optical photometry, lightcurve and thermal mid-IR observations of the asteroidal comet. With the photometric analysis in BVRI, the surface color is found to be redder than asteroids, corresponding to cometary nuclei and TNOs/Centaurs. The time-resolved differential photometry supports a rotation period of 8.19 ± 0.05 h. The effective diameter and the geometric albedo are 17.9 ± 0.9 km and 0.03 ± 0.01, respectively, indicating a very dark reflectance of the surface. The dark and redder surface color of C/2002 CE10 may be attribute to devolatilized material by surface aging suffered from the irradiation by cosmic rays or from impact by dust particles in the Oort Cloud. Alternatively, C/2002 CE10 was formed of very dark refractory material originally like a rocky planetesimal. In both cases, this object lacks ices (on the surface at least). The dynamical and known physical characteristics of C/2002 CE10 are best compatible with those of the Damocloids population in the Solar System, that appear to be exhaust cometary nucleus in Halley-type orbits. The study of physical properties of rocky Oort cloud objects may give us a key for the formation of the Oort cloud and the solar system.

On a possible cometary origin of the object 2015TB145

The Earth-crossing asteroid 2015TB145 was discovered on 10 October 2015 and on 31 October 2015 it already approached close to the Earth at the minimal distance. On the base of obtained radio images of the asteroid, the value of an albedo has estimated as p=0.06. Coming from the albedo value and the comet-like orbit, it was suggested, that the object is a dead comet. In order to verify the supposition, the orbital evolution of 2015TB145 was investigated under the perturbing action of major planets for the time interval of 50 kyrs. As a result, it was found that one cycle of variations of the argument of perihelion is equal to nearly 40 kyrs and during this period the object intersects the Earth's orbit eight times, i.e. it is the octuple crosser. Consequently, if the object has a cometary origin, then it can be associated with a meteoroid stream producing eight meteor showers which should be observable on the Earth. Features of the predicted meteor showers, theoretically associated with 2015TB145, were calculated and a search for observable showers identical to predicted ones was realized using all published catalogues. It turned out, that seven of eight predicted showers were identified with the active observable meteor showers. So, comet-like orbit, low value of an albedo and association with the meteoroid stream producing identified showers are strong evidences pointing that 2015TB145 is really inactive comet. A conclusion was made that the potentially hazardous object 2015TB145 is very likely extinct nucleus of a parent comet of the found meteoroid stream.

Characterizing cometary electrons with kappa distributions

AbstractThe Rosetta spacecraft has escorted comet 67P/Churyumov‐Gerasimenko since 6 August 2014 and has offered an unprecedented opportunity to study plasma physics in the coma. We have used this opportunity to make the first characterization of cometary electrons with kappa distributions. Two three‐dimensional kappa functions were fit to the observations, which we interpret as two populations of dense and warm (density = 10 cm−3, temperature = 2 × 105 K, invariant kappa index = 10−>1000), and rarefied and hot (density = 0.005 cm−3, temperature = 5 × 105 K, invariant kappa index = 1–10) electrons. We fit the observations on 30 October 2014 when Rosetta was 20 km from 67P, and 3 AU from the Sun. We repeated the analysis on 15 August 2015 when Rosetta was 300 km from the comet and 1.3 AU from the Sun. Comparing the measurements on both days gives the first comparison of the cometary electron environment between a nearly inactive comet far from the Sun and an active comet near perihelion. We find that the warm population density increased by a factor of 3, while the temperature cooled by a factor of 2, and the invariant kappa index was unaffected. We find that the hot population density increased by a factor of 10, while the temperature and invariant kappa index were unchanged. We conclude that the hot population is likely the solar wind halo electrons in the coma. The warm population is likely of cometary origin, but its mechanism for production is not known.

When comets get old: A synthesis of comet and meteor observations of the low activity comet 209P/LINEAR

It is speculated that some weakly active comets may be transitional objects between active and dormant comets. These objects are at a unique stage of the evolution of cometary nuclei, as they are still identifiable as active comets, in contrast to inactive comets that are observationally indistinguishable from low albedo asteroids. In this paper, we present a synthesis of comet and meteor observations of Jupiter-family Comet 209P/LINEAR, one of the most weakly active comets recorded to-date. Images taken by the Xingming 0.35-m telescope and the Gemini Flamingo-2 camera are modeled by a Monte Carlo dust model, which yields a low dust ejection speed (1/10 of that of moderately active comets), dominance of large dust grains, and a low dust production of 0.4kgs-1 at 19d after the 2014 perihelion passage. We also find a reddish nucleus of 209P/LINEAR that is similar to D-type asteroids and most Trojan asteroids. Meteor observations with the Canadian Meteor Orbit Radar (CMOR), coupled with meteoroid stream modeling, suggest a low dust production of the parent over the past few hundred orbits, although there are hints of a some temporary increase in activity in the 18th century. Dynamical simulations indicate 209P/LINEAR may have resided in a stable near-Earth orbit for ∼104yr, which is significantly longer than typical JFCs. All these lines of evidence imply that 209P/LINEAR as an aging comet quietly exhausting its remaining near surface volatiles. We also compare 209P/LINEAR to other low activity comets, where evidence for a diversity of the origin of low activity is seen.

Assessing the physical nature of near-Earth asteroids through their dynamical histories

We analyze a sample of 139 near-Earth asteroids (NEAs), defined as those that reach perihelion distances q<1.3au, and that also fulfill the conditions of approaching or crossing Jupiter’s orbit (aphelion distances Q>4.8au), having Tisserand parameters 2<T<3 and orbital periods P<20yr. In order to compare the dynamics, we also analyze a sample of 42 Jupiter family comets (JFCs) in near-Earth orbits, i.e. with q<1.3au. We integrated the orbits of these two samples for 104yr in the past and in the future. We find that the great majority of the NEAs move on stable orbits during the considered period, and that a large proportion of them are in one of the main mean motion resonances with Jupiter, in particular the 2:1. We find a strong coupling between the perihelion distance and the inclination in the motion of most NEAs, due to Kozai mechanism, that generates many sungrazers. On the other hand, most JFCs are found to move on very unstable orbits, showing large variations in their perihelion distances in the last few 102–103yr, which suggests a rather recent capture in their current near-Earth orbits. Even though most NEAs of our sample move in typical ‘asteroidal’ orbits, we detect a small group of NEAs whose orbits are highly unstable, resembling those of the JFCs. These are: 1997 SE5, 2000 DN1, 2001 XQ, 2002 GJ8, 2002 RN38, 2003 CC11, 2003 WY25, 2009 CR2, and 2011 OL51. These objects might be inactive comets, and indeed 2003 WY25 has been associated with comet Blanpain, and it is now designed as Comet 289P/Blanpain. Under the assumption that these objects are inactive comets, we can set an upper limit of ∼0.17 to the fraction of NEAs with Q>4.8au of cometary origin, but it could be even lower if the NEAs in unstable orbits listed before turn out to be bona fide asteroids from the main belt. This study strengthens the idea that NEAs and comets essentially are two distinct populations, and that periods of dormancy in comets must be rare. Most likely, active comets in near-Earth orbits go through a continuous erosion process in successive perihelion passages until disintegration into meteoritic dust and fragments of different sizes. In this scenario, 289P/Blanpain might be a near-devolatized fragment from a by now disintegrated parent comet.

William Herschel Telescope observations of distant comets

We present results from broad-band V- and R-filter observations obtained at the 4.2-m William Herschel Telescope on La Palma on 2002 July 12–14. A total of six comets were imaged, and their heliocentric distances ranged from 2.8 to 6.1 au. The comets observed were 43P/Wolf–Harrington, 129P/Shoemaker–Levy 3, 133P/Elst–Pizarro, 143P/Kowal–Mrkos, P/1998 U4 (Spahr) and P/2001 H5 (NEAT). A detailed surface brightness profile analysis indicates that three of the targeted comets (43P/Wolf–Harrington, 129P/Shoemaker–Levy 3 and P/1998 U4) were visibly active, and the remaining three comets were stellar in appearance. Further analysis shows that for the three ‘stellar-like’ comets the possible coma contribution to the observed flux does not exceed 12.2 per cent, and in the case of comet 143P/Kowal–Mrkos the coma contribution is expected to be as low as 1 per cent, and so the resulting photometry most likely represents that of the projected nucleus surface. Effective radii for the inactive comets range from 1.02 to 4.56 km, and the effective radius upper limits for the active comets range from 1.94 to 4.15 km. We assume an albedo and phase coefficient of 0.04 and 0.035 mag deg−1, respectively, with the exception of comets 133P/Elst–Pizarro and 143P/Kowal–Mrkos for which phase coefficients were previously measured. These values are compared with previous measurements, and for comet 43P/Wolf–Harrington we find that the nucleus axial ratio a/b could be as large as 2.44. For the active comets we measured dust production levels in terms of the Afρ quantity. Spectral gradients were extracted for two of the inactive comets from their measured broad-band colour indices, and compared with the rest of the comet population for which (V − R) colour and spectral gradient values exist. We find a spectral gradient for 143P/Kowal–Mrkos of 9.9 ± 8.1 per cent/100 nm, which is very typical of Jupiter-family comets, the majority of which have reflectivity gradients in the range 0–13 per cent (100 nm)−1. The spectral gradient for comet 133P/Elst–Pizarro is amongst the bluest yet measured. We measure a (V − R) colour index value of 0.14 ± 0.11 for the nucleus of 133P/Elst–Pizarro which is considerably lower than previous measurements. A possible explanation for this difference is considered.

NEO sizes, shapes and surface physical properties

The sizes, shapes, and surface or subsurface properties of Near Earth Objects (mostly asteroids, also inactive comet nuclei) are still poorly known, since any accurate determination requires in situ missions. It may nevertheless be estimated that the sizes of these irregular bodies range from a few tens of kilometres to a few tens of meters, with possibly about 1000 NEOs with sizes greater than 1 km. Their surfaces are estimated to be quite rough, and at least partially covered with regolith. To cite this article: A.C. Levasseur-Regourd et al., C. R. Physique 6 (2005).

Extreme albedo comets and the impact hazard

Dynamical balance arguments that involve the capture of long-period comets from the Oort cloud imply that there should be ∼ 1000 times more Halley-type objects than are actually observed. If the active comets rapidly become dormant, with albedos comparable to those of known cometary surfaces, hundreds of such bodies should by now have been detected whereas in fact only a few have been found. If, on the other hand, they disintegrate to dust, we show here that the debris would create a bright, near-spherical zodiacal cloud and ∼ 15‐30 strong annual meteor showers, also contrary to observation. Here we demonstrate that the surfaces of inactive comets, if composed of loose, fluffy organic material like cometary meteoroids, develop reflectivities that are vanishingly small in visible light. The near-Earth objects may therefore be dominated by a population of fast, multi-kilometre bodies too dark to be seen with current near-Earth object surveys. Deflection strategies that assume decades or centuries of warning before impact are inapplicable to this hazard.

Are There Many Inactive Jupiter-Family Comets among the Near-Earth Asteroid Population?

We analyze the dynamical evolution of Jupiter-family (JF) comets and near-Earth asteroids (NEAs) with aphelion distances Q>3.5 AU, paying special attention to the problem of mixing of both populations, such that inactive comets may be disguised as NEAs. From numerical integrations for 2×10 6 years we find that the half lifetime (where the lifetime is defined against hyperbolic ejection or collision with the Sun or the planets) of near-Earth JF comets (perihelion distances q<1.3 AU) is about 1.5×10 5 years but that they spend only a small fraction of this time (∼ a few 10 3 years) with q<1.3 AU. From numerical integrations for 5×10 6 years we find that the half lifetime of NEAs in “cometary” orbits (defined as those with aphelion distances Q>4.5 AU, i.e., that approach or cross Jupiter's orbit) is 4.2×10 5 years, i.e., about three times longer than that for near-Earth JF comets. We also analyze the problem of decoupling JF comets from Jupiter to produce Encke-type comets. To this end we simulate the dynamical evolution of the sample of observed JF comets with the inclusion of nongravitational forces. While decoupling occurs very seldom when a purely gravitational motion is considered, the action of nongravitational forces (as strong as or greater than those acting on Encke) can produce a few Enckes. Furthermore, a few JF comets are transferred to low-eccentricity orbits entirely within the main asteroid belt ( Q<4 AU and q>2 AU). The population of NEAs in cometary orbits is found to be adequately replenished with NEAs of smaller Q's diffusing outward, from which we can set an upper limit of ∼20% for the putative component of deactivated JF comets needed to maintain such a population in steady state. From this analysis, the upper limit for the average time that a JF comet in near-Earth orbit can spend as a dormant, asteroid-looking body can be estimated to be about 40% of the time spent as an active comet. More likely, JF comets in near-Earth orbits will disintegrate once (or shortly after) they end their active phases.

The Lightcurve and Colors of Unusual Minor Planet 1998 WU 24

Minor planet 1998 WU 24 is unusual in having the orbital characteristics of a Halley family comet but showing no sign of cometary activity. We present optical data that reveal a double-peaked light-curve with a period of 7.283±0.003 h and a peak-to-peak amplitude of ∼0.54 mag. Infrared spectroscopy and quasi-simultaneous BVRIJHK photometry reveal a featureless K-band spectrum and colors typical of D-type asteroids, and suspected “bare” comet nuclei (B-V=0.78±0.034, V-R=0.53±0.037, V-I=0.99=0.035, V-J=1.67±0.043, V-H=2.10±0.076 and V-K=2.34±0.102). Image profiles from co-added frames in the R band indicate no apparent cometary activity, with an implied upper limit to the dust production rate of 150 g s −1. Assuming a D-type albedo of 0.04 we derive a spherical equivalent diameter of 5.28±0.07 km although the lightcurve amplitude implies an irregular body with an axial ratio of 1.64 : 1. We conclude that 1998 WU 24 is probably an inactive comet nucleus.

CCD photometry of distant comets II

R band CCD photometric observations of short period Jupiter family comets in the heliocentric region of 2.11 AU Rh 5:63 AU were performed using the 4.2 m William Herschel Telescope on La Palma in December 1998. 22 comets were targeted, including the comet-asteroid transition object 49P/Arend{Rigaux. Out of a total of ten detected comets, six were seen to display substantial outgassing (48P, 65P, 74P, 103P, 128P, and 139P), with the remaining four comets (7P, 9P, 22P, and 49P) being stellar in appearance. Nuclear radius measurements and relative dust production rates in terms of Af were measured for these comets, along with upper limits for the remaining twelve undetected comets (6P, 44P, 51P, 54P, 57P, 63P, 71P, 73P, 79P, 86P, 87P, and 100P). The inactive comets had nuclear radii in the range 1.8 km rN 4:4 km, while upper limits for the active and undetected comets (assuming they all lay within the eld of view) were between 0.6 km and 12.7 km, for an assumed albedo of 0.04. Even if one applies the previously measured maximum axis ratio of 2.6:1 and the minimum measured albedo of 0.02 to the undetected comets, their projected semi-major axes are all constrained to below 8 km. For the active comets, photometric proles of their dust comae were measured and are consistent with those of steady state coma models.

The inner dust coma of Comet 26P/Grigg-Skjellerup: multiple jets and nucleus fragments?

On 1992 July 10 the European Space Agency's spaceprobe Giotto passed the nucleus of the comparatively inactive comet 26P/Grigg-Skjellerup at a relative velocity of 14 km s(-1). This Giotto Extended Mission (GEM) followed a highly successful encounter in 1986 with Comet 1P/Halley. We present results returned from the Optical Probe Experiment (OPE) and in particular consider data gathered by the channels sensitive to the scattering of solar light by cometary dust grains, in emission-free continuum bands. Owing to the demise of the Halley Multicolour Camera (HMC) during the Halley encounter, and the low number of impacts registered by the Dust Impact Detection System (DIDSY), OPE data offer the best indication of the actual encounter geometry. We find that it is likely that Giotto was on the sunward side of the shadow terminator plane at closest approach, with our modelling results suggesting that Giotto passed similar to 100 km from the nucleus (although distances of up to 300 km cannot be ruled out). We investigate possible causes of the striking 'spike' features, or 'events', in the OPE data. While scattering of sunlight from ejecta particles as a result of dust impacts on the spacecraft body cannot be ruled out, considerations of the hypervelocity impact mechanisms and impact geometry show that this explanation is not without problems, and more investigation is needed before it can be conclusively accepted. As an alternative solution, we find that the complex data profiles can be fitted by jet activity in the innermost coma (which was not resolvable by ground-based observers). One particular event occurring at least 1000 km from the nucleus can be fitted if the OPE line of sight passes close to a nucleus fragment of radius 10-100 m which is situated around 50 km from the spacecraft and which is producing a small dust coma.

Comet Shoemaker–Levy 9 Dust

Comet Shoemaker–Levy 9 (S–L 9) was imaged with the Hubble Space Telescope from January 1994, through solar opposition on April 29, and until impact in July 1994. As noted by several observers, no anti-sunward dust tails were detected east of the fragments after opposition (D. Jewitt and J. Chen 1994,Periodic Comet Shoemaker–Levy 9(1993e), IAU Circular 5924; Z. Sekanina, P. W. Chodas, and D. K. Yeomans 1994,Astron. Astrophys.289, 607–636; H. A. Weaveret al.1994,Science263, 787–791; G. P. Chernova, N. N. Kiselev, and K. Jockers 1995, inEuropean SL-9/Jupiter Workshop(R. West and H. Böhnhardt, Eds.), pp. 11–16, ESO Conference and Workshop Proceedings No. 52), a fact from which some have concluded that the S–L 9 fragments were inactive (e.g., Sekaninaet al.1994, Weaveret al.1994, Chernovaet al.1995). However surface brightness profiles of the brighter fragments G1, H, K, L, and S comae suggest the opposite. Images acquired during January and March 1994 show the brighter S–L 9 fragments' azimuthally averaged brightness profiles varied as ρ−1.1±0.1in the inner ρ ≲ 1″ comae and ρ−1.5±0.2in the outer ρ ≳ 1″ comae, where ρ is the projected distance from a coma photocenter. These profiles are consistent with active comet fragments surrounded by dust comae disturbed by radiation pressure. However inactive comet models, which assume the dust was created during a burst of activity immediately following breakup, produce comae that remain highly elongated along the fragment axis throughout their orbit. This type of coma morphology was not observed.Simulations of an active, dust-producing comet in S–L 9's orbit are presented. Comparisons of models with observations show that the dominant light-reflecting grains in the S–L 9 comae and tails had radii ranging between about 5 and 500 μm emitted from the fragments at outflow velocities of order 1 m/sec. The models show radiation pressure swept the dust into tails which appeared west of the fragments both before and after solar opposition. As the fragments neared Jupiter in late June 1994, the combined effects of the jovian tide and radiation pressure distorted the dust tails into broad fans that consisted of grains larger thanR≳ 100 μm and were oriented along the Jupiter–comet direction. The smallerR≲ 100 μm grains were confined to narrower tails whose projected orientation near the nucleus were approximately in the direction of Jupiter. The additional light contributed by the smaller grains caused the S–L 9 dust to appear brighter on the jovian side than the trailing side of the dust fans.

Coherent catastrophism

We review the theoretical and observational evidence that, on timescales relevant to mankind, the prime collision hazard is posed by temporally correlated impacts (coherent catastrophism, Δt ∼ 10 2–10 4 yr) rather than random ones (stochastic catastrophism, Δt ∼ 10 5–10 8 yr). The mechanism whereby coherent incursions into and through the terrestrial atmosphere occur is described as being the result of giant cometary bodies arriving in orbits with perihelia in the inner solar system. Hierarchical fragmentation of such large (100 km-plus) bodies — due to thermal stresses near perihelion, collisions in the asteroid belt, or passages through the Jovian Roche radius — results in numerous ∼kilometre-sized objects being left in short-period orbits, and appearing in telescopic searches as Apollo-type asteroids. Many more smaller objects, in the 10–100 metre size range and only recently observed, by the Spacewatch team, are expected to be in replenished clusters in particular orbits as a result of continuing disintegrations of large, differentiated, cometary objects. Gravitational perturbations by Jupiter bring these clusters around to have a node at 1 au in a cyclic fashion, leading to impacts at certain times of year every few years during active periods lasting a few centuries, such periods being separated by intervals of a few millennia. Furthermore, fragmentations within the hierarchy result in significant bombardment commensurabilities ( Δt ∼ 10–10 2 yr) during active periods occurring at random intervals ( Δt ∼ 10 2–10 3 yr). It appears that the Earth has been subject to such impacts since the break-up of such a comet ∼2×10 4 years ago; currently we are not passing through a high-risk epoch, although some phenomena originating in the products of this break-up have been observed in the 20th century. This most recent hierarchical disintegration, associated with four well-known meteor showers and termed the Taurid Complex, is now recognized as resulting in a dozen apparently asteroidal objects — almost 10% of the discovered Apollo population — as well as Comet P/Encke. A substantial asteroidal remnant of the Taurid progenitor may still be present in resonance with Jupiter. The implications of the existence of any asteroidal or meteoroidal cluster due to the progenitor are briefly discussed. We emphasize finally the relative importance of active and inactive comets in the cratering record, pointing out the potentially significant contribution by disintegrating large comets to the population of inactive Earth-crossers.

Giotto reawakens

Giotto, a space probe of the European Space Agency, has been brought back to life after being “in hibernation” since July 1990. The probe revealed the nucleus of Halley's Comet in 1986 and is now on its way to a second comet, according to ESA, whose engineers confirmed on May 7 that Giotto was reactivated.The probe is aimed for an encounter with the comet Grigg‐Skejellerup on July 10. Giotto will pass as closely as possible to the comet's nucleus—within 1000 meters. Observations of this largely inactive comet will be compared to those from young and vigorous Halley.

Where do the Apollo objects come from?

Monte Carlo calculations have been carried out to trace the orbital evolution of ∼1- to 45-km-diameter bodies produced by collisions in the asteroid belt interior to 2.6 AU. It is found that about 3% of the asteroidal fragments ejected at 100 m/sec in this size range are perturbed into Earth-crossing orbits as a consequence of the 3:1 Jovian commensurability resonance at 2.5 AU and the ν dot 6 secular resonance in the innermost asteroid belt. For ejection velocities of 100 m/sec the calculated steady-state number of Apollo objects (454) and Amor objects (1600) brighter than absolute V magnitude 18 is in the general range of the estimated number based on observations. Considering the uncertainties in both the theoretical results and the observational data, this should be considered to be good agreement. Even though the initial Earth-crossing orbits of these bodies are concentrated near 1 AU, their steady-state orbital distribution is widely dispersed. There is strongly suggestive, but not yet compelling, evidence for an additional component of Apollo-Amor objects that are inactive comets. It is found that a modest production rate (∼1 in 6 × 10 4 years) of extinct comets with V(1,0) < 18 in Encke-like orbits is required to maintain a steady-state population of cometary Apollos equal in number to the asteroidal Apollos.