Origin Of Short-period Comets Research Articles

Structure and Dynamics of the Centaur Population: Constraints on the Origin of Short-Period Comets

The origin of Jupiter-family comets is linked to the intermediate stage of evolution through the Centaur region. Thus the structure of the Centaur population provides important constraints on sources of short-period comets. We show that our model of the Oort cloud evolution gives results which are consistent with the orbital distribution of observed Centaurs. In particular, it explains the existence of the large population of Centaurs with semimajor axes greater than 60 AU. The main source for these objects is the inner Oort cloud. Both Jupiter-family and Halley-type comets are produced by Centaurs originating from the Oort cloud. The injection rate for Jupiter-family comets coming from the inner Oort cloud is, at least, not less than that for a model based on the observed sample of high-eccentricity trans-Neptunian objects.

A Brief Summary of Kuiper Belt Research

Ideas about the contents of the Solar System beyond Neptune and Pluto can be traced back to at least Edgeworth (1943, 1949) and Kuiper (1951), who speculated on the existence of pre-planetary small bodies in the outer Solar System beyond the orbit of Neptune - remnants of the accretion process in the primordial Solar Nebula. The basis for the speculation was primarily the argument that the Solar Nebula was unlikely to have been abruptly truncated at the orbit of Neptune, and that in the trans-Neptunian accretion timescales were too long for bodies larger than about ˜ 1000 km in radius to have formed in the 4.5 billion year age of the Solar System. Another important theoretical argument relevant to this region of the Solar System is related to the origin of short period comets. Fernández (1980) suggested that the short period comets may have an origin in a disk of small bodies beyond Neptune, rather than being “captured” from the population of long period comets originating in the Oort Cloud, the latter scenario having considerable difficulty reconciling the observed flux of short period comets with the exceedingly low efficiency of transfer of long period comet orbits to short period ones by means of the gravitational perturbations of the giant planets. The new scenario received further strength in the numerical work of Duncan et al. (1988) and Quinn et al. (1990) which showed that the relatively small orbital inclinations of the Jupiter-family short period comets were not consistent with a source in the isotropic Oort Cloud of comets but could be reproduced with a source in a low-inclination reservoir beyond Neptune’s orbit. Duncan et al. named this hypothetical source the Kuiper Belt, and the name has come into common use in the last decade (although other names are also in use, e.g. Edgeworth-Kuiper Belt, and trans-Neptunian objects). A recent theoretical milestone was the work by Holman and Wisdom (1993) and Levison and Duncan (1993) on the long term stability of test particle orbits in the trans-Neptunian Solar System. This work showed that low-eccentricity, low-inclination orbits with semimajor axes in excess of about 43 AU are stable on billion year timescales, but that in the region between 35 AU and 43 AU orbital stability times range from 107 yr to more than 109 yr [see, for example, figure 1 in Holman (1995)]. Orbital instability in this intermediate region typically leads to a close encounter with Neptune which causes dramatic orbital changes, with the potential for subsequent transfer to the inner Solar System. Thus, this region could in principle serve as the reservoir of short period comets at the present epoch. However, the idea of a kinematically cold — i.e. low-eccentricity, low-inclination — population in this region is at odds with recent observations, and the question of the origin of short period comets remains unsettled at the present time.

Chaotic Diffusion and the Origin of Comets from the 2/3 Resonance in the Kuiper Belt

The dynamical structure of the 2/3 resonance at small inclination is explored using numerical integrations of test particles to compute the evolution of proper elements with time. The basic features are related to the analytically computed geography of secular resonances. This paper focuses on the existence of slowly diffusing chaotic orbits, which escape from the 2/3 resonance after billions of years. The origin of short-period comets may be related to the existence of such orbits. We numerically determine the rate at which 2/3 resonant objects are delivered to close encounters with Neptune. From this result we estimate the number of comet-sized objects that should presently be in the 2/3 resonance to explain the influx rate of observed short period comets; the result of 10 8to 10 9seems to imply that 2/3 resonant bodies should be collisionally evolved.

Origin of short period comets

Reasons for interest in the origin of short-period comets and the difficulties of computing their long-term dynamcal evolution are reviewed. The relative advantages of a source region in an extended ‘inner core’ of the Oort cloud or a compact ‘comet belt’ just beyond the planetary system are finely balanced, and it is premature to consider the problem solved. A complication is that some comets belonging to the Jupiter family may be part of a time-dependent system, possibly the remains of a giant comet such as Chiron which could have been part of the system ≈ 104 yr ago. The origin of short-period comets plays a pivotal role in many areas of solar system science: planet formation, the source of water (possibly life) on the terrestrial planets, the cratering record on the terrestrial planets and satellites of the outer planets, and the environmental impact posed by massive bodies and their decay products in the Earth's near-space environment.

The origin of short-period comets

Recent work has shown that if short-period comets predominantly occur as a result of planetary “captures” of long-period comets, the relevant source flux of nearly parabolic orbits must involve comets with perihelia in the range 5–30 AU, significantly beyond the region in which the flux is observationally constrained. The numbers and orbital parameters of short-period comets therefore in principle provide strong constraints on those theories of cometary origin which predict the existence of a massive inner core to the Oort cloud. Short-period comets now play a role in relation to the inner core as important as that formerly played by the observed near-parabolic flux in relation to the Oort cloud beyond r ⋍ 2 × 10 4 AU .

Occultation Astronomy

The era of occultation astronomy, studying dark objects by observing occultations of bright ones, has begun. The paper of Duncan, Quinn and Tremarne, “The Origin of Short-Period Comets”, Astrophys. J. Letters, 328, 69-73 (1988), greatly improved the prospects for occultation astronomy by demonstrating the existence of a second comet reservoir, the Kuiper Belt, much closer to us than the Oort Cloud and concentrated toward the ecliptic plane. Charles Alcock at Livermore (private communication) has begun work on a practical system of small telescopes to observe occultations of stars by comets. I here propose a system similar in concept of Alcock’s but using different hardware. We need to try various systems on a small scale to find out which are most cost-effective. My proposal is based on the Multiple Telescope Robotic Observatory (MTRO) developed by Boyd and Genet at Fairborn Observatory in Arizona. See Russell M. Genet, “Multiple-Telescope Robotic Observatories in Space”, submitted to P.A.S.P. (1990).

The near-parabolic flux and the origin of short-period comets

The origin of short-period comets, usually believed to arise by planetary perturbations acting on nearly parabolic orbits, has been unclear for many years1. Given the observed near-parabolic flux, the predicted number of such comets is too small by a factor of ∼100 (refs 2–15). It has been suggested, therefore, that the adopted near-parabolic flux is in error16, that a physical effect such as fragmentation3,17,18 could operate, or that short-period comets do not come from the outer Oort cloud at all9,10,13,19. It has been proposed13–15,20,21 that the problem could be resolved by postulating a comet belt or massive inner core to the Oort Cloud. Here I recalculate the flux of nearly parabolic orbits from the Oort Cloud, including the recently discovered dominant injection of comets by the galactic tide22–25. The observed number of short-period comets is consistent with the Oort Cloud having a massive inner core26.

Nature and Origin of Comets

IN so many fields of astrophysics the textbooks and the ordinary review articles have a tendency to present only the ‘generally accepted’ point of view, often not mentioning alternative theories, and even sweeping under the rug those observational facts which are adverse to the dominant view. This also applies to the theory of comets. As the arrival of Comet Kohoutek, and also the space missions to comets which are now being planned, is focusing the interest on comets, we may have unique opportunities in the near future to clarify which views are correct. This calls for a survey of the controversial points. A partial list of these, with which we will be concerning ourselves here, is the following: (1) What is the physical nature of the cometary nucleus? (a) Does it invariably consist of a single monolith ? or (b) could it, in some instances at least, constitute a bunch of particles having no physical coherence ? (2) What is the genetic relationship between comets and the meteor streams with which they are associated ? (a) Do the stream meteoroids invariably derive from the associated comets ? or (b) is the reverse process also possible ? (3) Is there a net dissipation or accretion during the lifetime of a comet ? (4) What is the origin of short-period comets ? (5) What is the origin of long-period comets ? Several other questions with regard to the chemical composition of the nucleus, the mechanisms for the production of the observed radicals and ions, and the nature of the comet solarwind interaction merit discussion but are outside the scope of this letter.