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M.E. Bailey, Director

Professor Mark Bailey completed work with the former PDRA Vacheslav Emel'yanenko on the capture of Halley-type comets from the long-period cometary flux. He also completed a paper with Ph.D. student Scott Manley on collision probabilities between small solar system bodies, and together with PDRA David Asher and visiting research fellow Vacheslav Emel'yanenko provided the explanation for the unexpected outburst of Leonid fireballs on the morning of 1998 November 17. M.Phil. student Sandra Jeffers began a study of the size and number distribution of potential projectiles in the inner solar system that may, by physical collision with comets, dominate the process of cometary physical evolution.

The motivation for investigating the dynamical capture of Halley-type comets bears on the long-standing question of the origin of short-period comets, defined to be those with periods $P\!<\!200\,$yr. There are fundamentally two different types of short-period comet: those with periods $P\!<\! 20\,$yr, referred to as Jupiter-family comets; and those with slightly longer periods $20\!<\!P\!<\!200\,$yr, called Halley-types. The Jupiter-family comets, it appears, largely originate from orbits resembling those of a class of objects called Centaurs, a group of objects which move through the outer planetary system (roughly between Saturn and Neptune) in low-inclination orbits, and which in turn are believed to derive largely from a belt or disc of objects beyond Neptune, known as the Edgeworth-Kuiper belt.

Halley-types, on the other hand, are produced as a result of planetary perturbations acting on initially very long-period cometary orbits originating in the Oort cloud, a vast nearly spherical cometary reservoir extending roughly halfway to the nearest star. Given the observed long-period flux, the calculated transfer probability from such an orbit to a Halley-type orbit, and the dynamical lifetime of the latter, it is straightforward to predict the number of observed Halley-type objects.

The calculated result, however, is at least two orders of magnitude too large. Therefore, either the observed Halley-type sample is enormously incomplete (for example, the comets could be covered with a layer of very dark, insulating material, and therefore be not easily found), or the comets must be destroyed by some process during their dynamical evolution, possibly disintegrating to dust.

Either of these possibilities presents new avenues for research. For example, if the Halley-type objects are simply very dark, essentially asteroidal in form, then their absolute numbers in space are sufficiently high as to dominate the terrestrial cratering rate for objects larger than $\sim$5-10km in diameter. On the other hand, if these Halley-type `asteroids' do not exist, the structure of the meteoroid streams produced by the disintegration of Halley-type comets becomes an even more urgent problem to address.

These arguments led to research publications on the nature and origin of the dominant impactors on the Earth, the implications for the resulting impact hazard to civilization, and the collision probabilities of comets and asteroids with respect to the terrestrial planets and other objects. The work carried out with Asher & Emel'yanenko provided an exceptionally interesting confirmation of the complex structure of meteoroid streams produced by Halley-type comets, in this case the Leonid meteoroid stream associated with comet 55P/Tempel-Tuttle.

In addition to astronomical research, M.E. Bailey attended several conferences (e.g. the Thessaloniki International Seminar on Current Issues of Astronomical and Planetary Concern, 1998 April; and the ESO (Garching) conference on Minor Bodies in the Outer Solar System, 1998 November), and presented a large number of seminars and public talks, notably at the Seventh European Astrofest meeting (Kensington, 1998 January), at the Thessaloniki meeting, and to astronomical societies in the UK and Ireland. Many press, radio and television interviews were carried out, including appearances in several television documentaries concerning comets and asteroids.