Comets, Asteroids and Small-Body Interrelationships

This recently reintroduced area of research at the Observatory continues a tradition established at the Observatory by E.J. Opik, focusing on problems involving the origin and evolution of comets, their dynamical transfer from the Oort cloud and other reservoirs into short-period orbits, the interrelationship between comets and asteroids (particularly objects in Earth-crossing orbits), and the detailed processes by which a comet nucleus forms: through hierarchical aggregation, starting with the accretion of ice-covered interstellar grains in dense clouds. This aspect of solar system science has links with many other areas of observational astronomy and astrophysics and the new Director (MEB) is involved with and developing projects in a number of interrelated fields, as described below.

Oort Cloud

The first concerns the origin and evolution of the Oort cloud in the Galaxy. This has links with the origin of the solar system (and hence to theories of star formation and circumstellar material around young stars), and also with questions of galactic dynamics: the nature of the dark matter in the galactic disc, and the mass and structure of molecular clouds and their interaction with the solar system. Survival of the outer Oort cloud for the age of the solar system sets a lower limit on the mass of the inner core which replenishes the dynamically unstable outer regions, whereas the necessity for the inner core not to produce too many short-period comets places an upper limit on the same quantity. The principal objective of this research is to refine these dynamical arguments in order to place firm constraints on different models of the Oort cloud, and hence provide new constraints on theories of the origin of the solar system. It is interesting to note that the dynamical evolution of comets in the loosely bound Oort cloud under the influence of external perturbations has many similarities to that of a newly formed star cluster or of an ensemble of wide binaries in the galactic disc; a detailed study of the cometary cloud therefore links closely to other work carried out at the Observatory.

Short-Period Comets

The second broad area of study concerns the origin of short-period comets, whether from the observed near-parabolic flux or elsewhere. If they principally come from the observed near-parabolic flux in the neighbourhood of Jupiter, then it follows that the observed short-period comets in the Jupiter family are temporarily enhanced in number by a factor of about 10 compared to the steady-state value. This could arise through the break-up of one or more large comets, or by strong time-dependence in the parabolic flux a capture-time ago (e.g. due to a comet shower). Alternatively, short-period comets might originate from a source flux with initial perihelia located in the Uranus-Neptune zone: either an extended (~5000 AU) inner core of the Oort cloud or a compact (~50 AU) comet belt beyond Pluto (to give two extremes). A solution to the problem of the origin of short-period comets requires dynamical studies of a wide range of cometary orbits, and realistic simulations of the capture process to mimic the behaviour of cometary orbits over very long timescales. Research in this area encompasses both high-inclination Halley-type comets and low-inclination Chiron-types, of which the latter appear in the first instance to represent immediate possible source orbits for the Jupiter family.

Physical Evolution

Just as theories in other areas of astrophysics are only as good as the link made between the theoretical tokens and the real stars and galaxies whose compositions, positions and velocities are being assessed, so it is with comets. The comparison of theories of small bodies with observations should include an allowance for selection effects and the physical fading (possibly complete decay) expected to occur during the capture process. It is therefore necessary to develop models of cometary evolution that connect changes of the orbit to those of the nucleus: fading, physical decay and splitting of a parent nucleus into multiple fragments. This has implications for the total number of observable short-period comets, the interrelationship between comets and asteroids, and whether a significant fraction of asteroids are merely dormant or extinct cometary nuclei; it also has a strong link with theories of the origin of the cometary nucleus, whether as a by-product of the formation of solar systems or formed elsewhere, presumably in dense clouds in the interstellar medium.

Interrelationships

Other lines of research may be divided into broadly astronomical and interdisciplinary research topics respectively. The first, for example, includes the hierarchical growth of interstellar grains and interplanetary dust aggregates at various stages of their evolution en route to planet formation, and the implications of such models for direct observations of the cometary nucleus (e.g. sampling of the nucleus by the Rosetta lander) and for theories of cometary evolution which include the dynamical effects of cometary break-up, interplanetary collisions and mass loss as a result of solar heating. The second, extends this understanding of the physical and dynamical evolution of the cometary nucleus to a discussion of how cometary nuclei, asteroids and their respective disintegration products might in principle collide and interact with the Earth. These interrelationships are represented by research programmes aimed at understanding the link between comets and near-Earth asteroids, the origin (and number) of Earth-crossing asteroids, and the ways in which comets and their decay products ('boulders' and dust) may interact with the Earth. The astronomical aspects of these studies involve investigations into the detailed dynamical processes and mechanisms of orbital evolution (e.g. VVE and NWH). This and other recent research has shown that secular perturbations can drive Jupiter-family comets into sub-Jovian orbits similar to those of near-Earth asteroids, whilst on the other hand the asteroid (5335) Damocles has an original orbit similar to that of a Halley-type comet. Sungrazing comets also originate from high-inclination Halley-type short-period orbits, and a close approach to the Sun provides a plausible mechanism, namely tidal disruption, to break up even the largest bodies in the cometary mass distribution, possibly producing a 'shower' of fragments in near-Earth orbits. It is important to understand the Earth's near-space astronomical environment, a point most recently emphasized by the collision of comet D1993 F2 (Shoemaker-Levy 9) with Jupiter. The effects of cometary and asteroid impacts on the Earth, and whether the dust and small bodies with sizes in the 10-100m range are predominantly distributed in streams in the inner solar system, or more uniformly in space, has implications for a correct understanding of the long-term evolution of life on Earth and, on much shorter timescales, possibly also for the evolution of humankind and the development of civilization.

Last Revised: 2009 November 5th