2,211 research outputs found

    Debris about asteroids: Where and how much?

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    We summarize several recent findings on the size and shape of the region within which material can stably orbit an asteroid. If the asteroid (with assumed density 2.38 g/cu cm) circles the Sun at 2.55 AU, co-planar prograde material will remain trapped whenever started on unperturbed circular orbits at less than about 220 R(sub A) (asteroid radii); co-planar retrograde particles are stable out twice as far. Our 3-D stability surface, which encloses several hundred numerically calculated orbits that start with various inclinations, is shaped like a sphere with its top and bottom sliced off; its dimensions scale like the Hill radius =(mu/3)(exp 1/3)R, where mu is the asteroid-to-solar mass ratio and R is the asteroid's orbital radius. If the asteroid moves along an elliptical orbit, a fairly reliable indicator of the dimensions of the hazard zone is the size of its Hill sphere at the orbit's pericenter. Grains with radii less than a few mm will be lost through the action of radiation forces which can induce escape or cause collisions with the asteroid on times scales of a few years; interplanetary micrometeoroids produce collisional break-up of these particles in approximately 10(exp 4) yrs. The effects of Jupiter and of asteroids that pass close to the target asteroid allow particles to diffuse from the system, again shrinking the hazard zone. None of the considered sources-primordial formation, debris spalled off the asteroid during micrometeoroid impact, captured interplanetary particles, feeder satellites, etc., seem capable of densely populating distant orbits from the asteroid. No certain detections of debris clouds or of binary asteroids have been made. Thus, it seems highly unlikely that a spacecraft fly-by targeted at 100 R(sub A) from the asteroid over its orbital pole would encounter any material

    The phenomenology of customer delight: a case study of product evaluation

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    This thesis presents a phenomenological case study of customer delight during product evaluation. The literature presents two existing 'theories' of customer delight. The first, from the field of Consumer Research, presents a cognitive model of post-purchase customer delight as the affective result of expectation disconfirmation. The second, from the Manufacturing literature, proposes that customers are delighted when products contain unexpected features or levels of qualities that exceed expectations. This research was motivated by the fact that our current understanding of this commercially important phenomenon is confined by expectation-based thinking. Furthermore, both streams of research have neglected to study the naturalistic occurrence of delight from the customer's perspective. The aim of this research was to generate an integrated understanding of the affective, behavioural, and cognitive nature of customer delight and its product basis. A case study methodology, incorporating interview, self-report and observational methods, was adopted to generate a triangulated understanding of productbased customer delight. The naturalistic product evaluations of 918 customers were observed and self-reported delight reactions were collected from 66 research participants. In total 414 customer delight reactions were analysed in detail. This approach aimed to generate new theory, rather than test the existing models, and this new integrative understanding of customer delight is the primary contribution of this thesis. A new model of product-based customer delight is presented, and the existing Manufacturing model is extended to incorporate the empirical findings of the case study. Whilst the findings of this research support concepts contained within the existing theories of customer delight, they also demonstrate their limitations. The cognitive and affective diversity of customer delight reactions, previously unaccounted for in the literature, was uncovered and five product-based routes to delight were identified. The emergent theory successfully integrates the two previously separate concepts of delight and builds upon them by identifying the behaviours associated with customer delight resulting from both attribute-based and holistic product appraisals

    Did the HMO Revolution Cause Hospital Consolidation?

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    During the 1990s US healthcare markets underwent a significant transformation. Managed care rose to become the dominant form of insurance in the private sector. Also, a wave of hospital consolidation occurred. In 1990, the mean population-weighted hospital Herfindahl-Hirschman Index (HHI) in a Health Services Area (HSA) was .19. By 2000, the HHI had risen to .26. This paper explores whether the rise in managed care caused the increase in hospital concentration. We use an instrumental variables approach with 10-year differences to identify the relationship between managed care penetration and hospital consolidation. Our results strongly imply that the rise of managed care did not cause the hospital consolidation wave. This finding is robust to a number of different specifications.

    The Welfare Consequences of Hospital Mergers

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    In the 1990s the US hospital industry consolidated. This paper estimates the impact of the wave of hospital mergers on welfare focusing on the impact on consumer surplus for the under-65 population. For the purposes of quantifying the price impact of consolidations, hospitals are modeled as an input to the production of health insurance for the under-65 population. The estimates indicate that the aggregate magnitude of the impact of hospital mergers is modest but not trivial. In 2001, average HMO premiums are estimated to be 3.2% higher than they would have been absent any hospital merger activity during the 1990s. In 2003, we estimate that because of hospital mergers private insurance rolls declined by approximately .3 percentage points or approximately 695,000 lives with the vast majority of those who lost private insurance joining the ranks of the uninsured. Our estimates imply that hospital mergers resulted in a cumulative consumer surplus loss of over 42.2billionbetween1990and2001.Itisestimatedthatallbutamodest42.2 billion between 1990 and 2001. It is estimated that all but a modest 95.4 million of the loss in consumer surplus is transferred from consumers to providers.

    Nuclear Thermal Propulsion Reactor Materials

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    Nuclear thermal propulsion (NTP) systems have been studied in both the USA and the former Soviet Union since the 1950s for use in space science and exploration missions. NTP uses nuclear fission to heat hydrogen to very high temperatures in a short amount of time so that the hydrogen can provide thrust as it accelerates through an engine nozzle. Benefits of NTP systems compared to conventional chemical and solar electric powered propulsion systems include higher fuel efficiency, greater mission range, shorter transit times, and a greater ability to abort missions and return to Earth in the event of system failure. As a result of these benefits, the US National Aeronautics and Space Administration (NASA) is evaluating NTP for use in crewed missions to Mars, and plans for a possible mid-2020s flight demonstration of a NTP engine are under development. The extremely harsh conditions that NTP systems must operate in present a number of significant engine design and operational challenges. The objective of this chapter will be to describe the history of NTP material development, describe current NTP material fabrication and design practices, and discuss possible future advances in space propulsion material technologies

    The association between life events, social support, and antibody status following thymus-dependent and thymus-independent vaccinations in healthy young adults

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    This study determined whether stressful life events and social support were related to antibody status following both thymus-dependent and thymus-independent vaccinations. Life events in the previous year and customary social support were measured in 57 healthy students at baseline. Antibody status was also assessed at baseline and at five weeks and five months following vaccination with the trivalent influenza vaccine and the meningococcal A+C polysaccharide vaccine. Taking into account baseline antibody titre, high life events scores prior to vaccination were associated with lower responses to the B/Shangdong influenza strain at both five weeks and five months and meningococcal C at five weeks. Life events scores were not associated with response to the other two influenza viral strains nor response to meningococcal A. Those with high social support scores had stronger 5-week and 5-month antibody responses to the A/Panama influenza strain, but not to any of the other strains. These associations could not be accounted for by demographic or health behaviour factors, and also emerged from analyses comparing those who exhibited a four-fold increase in antibody titre from baseline with those who did not. Life events and social support were related to antibody status following influenza vaccination in distinctive ways that may be partly determined by vaccine novelty and prior naturalistic exposure. Life events also predicted poor antibody response to meningococcal C polysaccharide vaccination after previous meningococcal C conjugate vaccination. Neither psychosocial factor was associated with response to primary meningococcal A polysaccharide vaccination