A numerical simulation of the Oort cloud is used to explain the observed orbital distributions and numbers of Jupiter-family (JF) and Halley-type (HT) short-period (SP) comets. Comets are given initial orbits with perihelion distances between 5 and 36 au, and evolve under planetary, stellar and Galactic perturbations for 4.5 Gyr. This process leads to the formation of an Oort cloud (which we define as the region of semimajor axes a > 1,000 au), and to a flux of cometary bodies from the Oort cloud returning to the planetary region at the present epoch. The results are consistent with the dynamical characteristics of SP comets and other observed cometary populations: the near-parabolic flux, Centaurs, and high-eccentricity trans-Neptunian objects. To achieve this consistency with observations, the model requires that the number of comets versus initial perihelion distance is concentrated towards the outer planetary region. Moreover, the mean physical lifetime of observable comets in the inner planetary region ( q < 2.5 au) at the present epoch should be an increasing function of the comets’ initial perihelion distances. Virtually all observed HT comets and nearly half of observed JF comets come from the Oort cloud, and initially (4.5 Gyr ago) from orbits concentrated near the outer planetary region. Comets that have been in the Oort cloud also return to the Centaur (5 < q < 28 au, a < 1,000 au) and near-Neptune high-eccentricity regions. Such objects with perihelia near Neptune are hard to discover, but Centaurs with characteristics predicted by the model (e.g. large semimajor axes, above 60 au, or high inclinations, above 40°) are increasingly being found by observers. The model provides a unified picture for the origin of JF and HT comets. It predicts that the mean physical lifetime of all comets in the region q < 1.5 au is less than ∼200 revolutions.

A model of the Oort cloud has been developed by accounting for planetary, stellar and Galactic perturbations using numerical symplectic integrations covering 4.5 Gyr. The model is consistent with the broad dynamical characteristics of the observed cometary populations injected from the Oort cloud into different regions of the Solar system. We show that the majority of observed high-eccentricity trans-Neptunian objects, Centaurs and short-period comets have visited the Oort cloud ( a > 1000 au) during their dynamical history. Assuming from observations that the near-parabolic flux from the Oort cloud with absolute magnitudes H-10 < 7, perihelion distances q < 5 au and a > 104 au is approximately 1 comet per year, our calculations imply a present Oort cloud population of similar to 5 x 10(11) comets with H-10 < 10.9. Roughly half this number have a > 104 au. The number of comets reaching the planetary region from the Oort cloud ( a > 1000 au) is more than an order of magnitude higher per unit perihelion distance immediately beyond Neptune than in the observable zone q < 5 au. Similarly, the new-comet flux from the Oort cloud per unit perihelion distance is a few tens of times higher in the near-Neptune region than in the observable zone. The present number of high-eccentricity trans-Neptunian objects ( q > 30 au and 60 < a < 1000 au) originating from the Oort cloud is in the approximate range 1-3 x 10(10), depending on details of the initial model. A substantial fraction of these have a > 200 au and/or q > 40 au, and they are found mostly to originate from initial orbits with 25 < q < 36 au. Similarly, the number of Centaurs produced from the Oort cloud, where we define Centaurs to have 5 < q < 28 au and a < 1000 au, is smaller by a factor of 20-30. About 90 per cent of these Centaurs have a > 60 au. Objects that have visited the Oort cloud represent a substantial fraction of the Jupiter-family comet population, achieving short-period orbits by a process of gradual dynamical transfer, including a Centaur stage, from the outer Solar system to near-Earth space. A similar mechanism produces Halley-type comets, in addition to the well-known diffusion process operating at small perihelion distances.

The Optical, Spectroscopic, and Infrared Remote Imaging System OSIRIS is the scientific camera system onboard the Rosetta spacecraft (Figure 1). The advanced high performance imaging system will be pivotal for the success of the Rosetta mission. OSIRIS will detect 67P/Churyumov-Gerasimenko from a distance of more than 10 6 km, characterise the comet shape and volume, its rotational state and find a suitable landing spot for Philae, the Rosetta lander. OSIRIS will observe the nucleus, its activity and surroundings down to a scale of ~2 cm px −1 . The observations will begin well before the onset of cometary activity and will extend over months until the comet reaches perihelion. During the rendezvous episode of the Rosetta mission, OSIRIS will provide key information about the nature of cometary nuclei and reveal the physics of cometary activity that leads to the gas and dust coma. OSIRIS comprises a high resolution Narrow Angle Camera (NAC) unit and a Wide Angle Camera (WAC) unit accompanied by three electronics boxes. The NAC is designed to obtain high resolution images of the surface of comet 67P/Churyumov-Gerasimenko through 12 discrete filters over the wavelength range 250–1000 nm at an angular resolution of 18.6 μ rad px −1 . The WAC is optimised to provide images of the near-nucleus environment in 14 discrete filters at an angular resolution of 101 μ rad px −1 . The two units use identical shutter, filter wheel, front door, and detector systems. They are operated by a common Data Processing Unit. The OSIRIS instrument has a total mass of 35 kg and is provided by institutes from six European countries.

The Human Orrery is an innovative outdoor exhibit in the grounds of the Armagh Observatory in Northern Ireland. Stainless steel disks mark the orbits of the classical planets, two comets, and the dwarf planet Ceres with a high level of precision. The idea is to provide a large-scale interactive educational facility to promote greater public understanding of astronomy, mathematics, and space science. In the Human Orrery, people play the part of planets moving in their orbits. Thus, visitors can learn, through active involvement, about the motions of the planets and the position of the Earth and the Solar System in space.

A numerical study of an ensemble of orbits based on observed objects in the near-Neptune high-eccentricity (NNHE) region, with perihelion distances q in the range 28 < q < 35.5 au and semimajor axes a in the range 60 < a < 1000 an, is used to predict the orbital distribution of Centaurs (5 < q < 28 au) for comparison with observations after correcting for discovery biases. The majority of Centaurs produced in this way have a less than or similar to 60 au. However, the intrinsic number of observed Centaurs is dominated by longer period objects, the number with a > 60 au being roughly an order of magnitude greater than that for a < 60 au, and therefore inconsistent with a source in the NNHE region, which is broadly similar to the so-called 'Scattered Disc'. The observed distribution of Centaurs with a less than or similar to 60 au is also inconsistent with this source, although it is conceivable that in this region the discrepancies might be explained by factors such as out-gassing, splitting or varying albedo not included in our model. Thus, although Centaurs can be produced from the NNHE region, their numbers and orbital distributions are inconsistent with this region being the dominant source for all Centaurs. We conclude that there must be another source flux, especially for the longer period, more populous group, and suggest that the most likely source for these objects is the Oort cloud. Thus, there are two separate, but overlapping, dynamical classes of Centaurs, one originating from the Oort cloud and the other from the NNHE region. The two source regions produce roughly similar contributions to Centaurs with a less than or similar to 60 an and to the observed Jupiter family of comets.

The terrestrial impact rate appears to be substantially higher than current near-Earth-object population models imply, consistent with a significant unseen cometary contribution to the terrestrial impact hazard.

Stellar occultations by solar-system objects (asteroids and satellites) constitute a method of determining the sizes and shapes of large samples of these bodies using small-aperture instruments to a level only otherwise achievable, in some cases, by a few large-aperture, ground-based, adaptive-optics facilities or the Hubble Space Telescope(1). They are also a means of testing the astrometric precision of their ephemerides(2) and the physical properties of their extant atmospheres(3). During an occultation, which is a time-critical phenomenon, the asteroid-to-star geometry projects a narrow, roughly cylindrical shadow track onto the surface of the Earth, within which the star is briefly occulted. An observing team of twelve staff and students, including summer students, was divided into three groups: John McFarland, Bebe Ishak, Anne O'Leary, Mark Purver.- Apostolos Christou, David Asher, Chia-Hsien Lin, Eleanor Nolan. and Mark Bailey, Jonathan McAuliffe, Sharon McClure, Ciara Quinn. On the evening Of 2004 July 20, they travelled approximately too miles west of Armagh to observing locations west of Sligo, Ireland, each close to the centre of the predicted shadow track, and separated from each other by a few miles. The aim was to observe, using small telescopes and binoculars, the predicted occultation of the magnitude-6 - 5 star 31 Psc (= HD 224995 = HIP 186) by the D-type minor planet (773) Irmintraud (diameter c. 95 km) at approximately 01: 19:22 UT on 2004 July 2 1. Not only was this a rare occultation of a fairly bright star, but it has been suggested(4) that Irmintraud could be a representative of a rare class of meteorite parent body similar to that which produced the extremely primitive Tagish Lake meteorite(5).

Evidence for spiral structure in distant galaxies was first noticed by William Parsons, the Third Earl of Rosse, in April 1845 within a few months of the first trial of his great six-foot reflector the “Leviathan of Parsonstown” on 11 February 1845. Despite the significance of this discovery there are puzzling inconsistencies in the story, and the discovery date — sometime in April — is curiously ...

Large-scale simulations of the Centaur population are carried out. The evolution of 23 328 particles based on the orbits of 32 well-known Centaurs is followed for up to 3 Myr in the forward and backward direction under the influence of the four massive planets. The objects exhibit a rich variety of dynamical behaviour with half-lives ranging from 540 kyr (1996 AR20) to 32 Myr (2000 FZ53). The mean half-life of the entire sample of Centaurs is 2.7 Myr. The data are analysed using a classification scheme based on the controlling planets at perihelion and aphelion, previously given in Horner et al. Transfer probabilities are computed and show the main dynamical pathways of the Centaur population. The total number of Centaurs with diameters larger than 1 km is estimated as similar to44300, assuming an inward flux of one new short-period comet every 200 yr. The flux into the Centaur region from the Edgeworth-Kuiper Belt is estimated to be one new object every 125 yr. Finally, the flux from the Centaur region to Earth-crossing orbits is one new Earth-crosser every 880 yr.

The dynamical evolution of trans-Neptunian objects (TNOs) to the inner Solar system is investigated. The study is based on the observed sample of high-eccentricity TNOs with perihelia in the near-Neptune region, using a procedure to take account of observational biases. It is shown that observations favour TNOs in high-eccentricity orbits as the main source of Jupiter-family (JF) comets. The relative fraction of objects captured per year from the near-Neptune region to JF comets with perihelion distances q<1.5 au is estimated as 0.2x10(-10). The maximum lifetime of typical JF comets with q<1.5 au is approximately 200 revolutions. Based on the observed population of JF comets, there should be similar to10(10) TNOs of cometary size in high-eccentricity orbits with 28<q<35.5 au. If this population originated 4.5 Gyr ago, the primordial number must have been at least 20 times as large as the present one.

Detailed orbit integrations of clones of five Centaurs - namely, 1996 AR20, 2060 Chiron, 1995 SN55, 2000 FZ53 and 2002 FY36 - for durations of similar to3 Myr are presented. One of our Centaur sample starts with perihelion initially under the control of Jupiter (1996 AR20), two start under the control of Saturn (Chiron and 1995 SN55) and one each starts under the control of Uranus (2000 FZ53) and Neptune (2002 FY36), respectively. A variety of interesting pathways are illustrated with detailed examples including: capture into the Jovian Trojans, repeated bursts of short-period comet behaviour, capture into mean-motion resonances with the giant planets and into Kozai resonances, as well as traversals of the entire Solar system. For each of the Centaurs, we provide statistics on the numbers (i) ejected, (ii) showing short-period comet behaviour and (iii) becoming Earth- and Mars-crossing. For example, Chiron has over 60 per cent of its clones becoming short-period objects, while 1995 SN55 has over 35 per cent. Clones of these two Centaurs typically make numerous close approaches to Jupiter. At the other extreme, 2000 FZ53 has similar to2 per cent of its clones becoming short-period objects. In our simulations, typically 20 per cent of the clones which become short-period comets subsequently evolve into Earth-crossers.

A new classification scheme is introduced for comet-like bodies in the Solar system. It covers the traditional comets as well as the Centaurs and Edgeworth-Kuiper belt objects. At low inclinations, close encounters with planets often result in near-constant perihelion or aphelion distances, or in perihelion-aphelion interchanges, so the minor bodies can be labelled according to the planets predominantly controlling them at perihelion and aphelion. For example, a JN object has a perihelion under the control of Jupiter and aphelion under the control of Neptune, and so on. This provides 20 dynamically distinct categories of outer Solar system objects in the Jovian and trans-Jovian regions. The Tisserand parameter with respect to the planet controlling perihelion is also often roughly constant under orbital evolution. So, each category can be further subdivided according to the Tisserand parameter.The dynamical evolution of comets, however, is dominated not by the planets nearest at perihelion or aphelion, but by the more massive Jupiter. The comets are separated into four categories - Encke-type, short-period, intermediate and long-period - according to aphelion distance. The Tisserand parameter categories now roughly correspond to the well-known Jupiter-family comets, transition types and Halley types. In this way, the nomenclature for the Centaurs and Edgeworth-Kuiper belt objects is based on, and consistent with, that for comets. Given the perihelion and aphelion distances together with the Tisserand parameter, our classification scheme provides a description for any comet-like body in the Solar system. The usefulness of the scheme is illustrated with examples drawn from numerical simulations and from the present-day Solar system.

Background: The aim was to assess the acceptability and safety of day-case laparoscopic fundoplication for gastro-oesophageal reflux disease (GORD).Methods: This prospective study commenced in December 1999 and lasted for 18 months. All patients had proven symptomatic GORD. Inclusion criteria were American Society of Anesthesiologists grade I or 11 with adequate home support. A standard anaesthetic, analgesic and antiemetic protocol was used. Patients were contacted by telephone on the night of discharge and arrangements were made for a nurse to visit the following day. Postoperative pain and nausea were assessed using visual analogue scores (scale 0-10) on a self-completion questionnaire.Results: Twenty patients were included. There were no postoperative complications. All patients were discharged on the day of surgery. Median time to discharge was 6 h 30 min (range 4.5 to 9 h). One patient reattended casualty the following morning but none required readmission. There was no significant difference in median pain or nausea scores the evening after surgery or the next morning. All patients were satisfied with the information given and aftercare provided. All would recommend it to a friend and 19 of 20 would undergo the procedure as a day case again.Conclusion: This study suggests that day-case laparoscopic fundoplication is feasible. Patients find it acceptable and it appears safe.

A symplectic integrator is used to study the evolution of high-eccentricity trans-Neptunian objects (TNOs) over the age of the Solar system. For 26 objects, a few cloned orbits were integrated. We demonstrate the existence of several known bodies that are in relatively stable orbits located far from Neptune for the age of the Solar system. Thus, we provide an indication of the structure of the protoplanetary disc immediately after the period of planet formation. The orbits of these bodies cannot be explained by a model in which a near-Neptune disc of planetesimals is gravitationally scattered by Neptune. In this paper, therefore, we demonstrate the existence of a new, and populous, class of ‘outer’ TNOs which have substantially different dynamical characteristics from those of scattered disc objects.