Migration of Jupiter-family comets and resonant asteroids to near-Earth space.

The orbital evolution of about 26,000 Jupiter-crossing objects and 1,500 resonant asteroids under the gravitational influence of planets was investigated. The rate of their collisions with terrestrial planets was estimated by computing the probabilities of collisions based on random-phase approximations and the orbital elements sampled with a 500 yr step. The Bulirsh-Stoer and a symplectic orbit integrator gave similar results for orbital evolution, but sometimes give different collision probabilities with the Sun. For orbits close to that of Comet 2P, the mean collision probabilities of Jupiter-crossing objects with terrestrial planets were greater by two orders of magnitude than for some other comets. For initial orbital elements close to those of Comets 2P, 10P, 44P, and 113P, a few objects (about 0.1%) entered Earth-crossing orbits with semi-major axes a < 2 AU and aphelion distances Q < 4.2 AU, and moved in such orbits for more than 1 Myr (up to tens or even hundreds of Myr). Some of them even got inner-Earth orbits (Q < 0.983 AU) and Aten orbits. A few comets achieve typical asteroidal orbits for hundreds of Myr. Most former trans-Neptunian objects that have typical near-Earth object orbits moved in such orbits for millions of years; thus, during most of this time they were extinct comet, if they did not disintegrate.

Migration of interplanetary dust.

We numerically investigate the migration of dust particles with initial orbits close to those of the numbered asteroids, observed trans-Neptunian objects, and comet Encke. The fraction of silicate asteroidal particles that collided with the Earth during their lifetime varied from 11% for 100 micron particles to 0.008% for 1 micron particles. Almost all asteroidal particles with diameter d >/= 4 microns collided with the Sun. For migrating asteroidal dust particles, the peaks in semimajor axis distribution at the n:(n + 1) resonances with Earth and Venus, and the gaps associated with the 1:1 resonances with these planets are more pronounced for larger particles. The probability of collisions of cometary particles with the Earth is smaller than for asteroidal particles, and this difference is greater for larger particles.

Smoking patterns in adolescent orthodontic patients and school children.

Smoking is known to be widely practised by young adults and teenagers, a fact that is of immediate importance to the orthodontic profession as it is now well accepted that smokers have poorer periodontal health than non-smokers. The aim of the present study was to determine the level of smoking in teenage orthodontic patients and compare it with that in the general teenage population and, further, to examine how these smoking habits are influenced by the subjects' attitudes. Patients were surveyed as to their smoking habits and attitudes. All were between 10 and 18 years of age and undergoing active fixed-appliance therapy. The same survey was distributed to a local primary (elementary) school and local secondary (high) schools. Of the 729 school respondents, 29 per cent were classified as smokers; of the 333 respondents in the orthodontic group, 11 per cent were classified as smokers. Although the level of smoking amongst orthodontic patients appears to be lower than that for the general adolescent population, significant numbers of patients smoke on a regular basis, and a small proportion of these is under 13 years of age. Because they have regular contact over a number of years with their adolescent patients, orthodontists are in a favourable position to reinforce non-smoking behavior.

Bolometer noise: nonequilibrium theory.

New theoretical results for noise in cryogenic bolometers are derived. Johnson noise is reduced by as much as 60% by electrothermal feedback from the bias supply. Phonon noise in the thermal link is reduced by as much as 30% relative to the usual equilibrium formula. Photon noise in the Rayleigh-Jeans limit is computed with attention to the attenuation of the photon correlations in the light beam. Basic results on bolometer responsivity, time constant, and thermal properties are presented in a new and convenient form. Excess 1/f and contact shot noise are also discussed.

Sky input horn for a far-infrared interferometer.

A unique optical design consisting of a compound parabolic concentrator and an elliptical concentrator joined at their throats is discussed. The optic has a number of advantages over conventional infrared telescopes for applications in interferometry and photometry. Ray-tracing results are presented.

Scientific results from the Cosmic Background Explorer (COBE).

The National Aeronautics and Space Administration (NASA) has flown the COBE satellite to observe the Big Bang and the subsequent formation of galaxies and large-scale structure. Data from the Far-Infrared Absolute Spectrophotometer (FIRAS) show that the spectrum of the cosmic microwave background is that of a black body of temperature T = 2.73 +/- 0.06 K, with no deviation from a black-body spectrum greater than 0.25% of the peak brightness. The data from the Differential Microwave Radiometers (DMR) show statistically significant cosmic microwave background anisotropy, consistent with a scale-invariant primordial density fluctuation spectrum. Measurements from the Diffuse Infrared Background Experiment (DIRBE) provide new conservative upper limits to the cosmic infrared background. Extensive modeling of solar system and galactic infrared foregrounds is required for further improvement in the cosmic infrared background limits.