The fall of the St-Robert meteorite

Abstract The St-Robert (Québec, Canada) meteorite shower occurred on 1994 June 15 at 0h02m UT accompanied by detonations audible for >200 km from the fireball endpoint. The fireball was recorded by visual observers in Vermont, New York State, New Hampshire, Québec and Ontario as well as by optical and infrared sensors in Earth-orbit. Penetration to an altitude of 36 km occurred ～60 km to the northeast of Montreal, where the bolide experienced several episodes of fragmentation. A total of 20 fragments of this H5 chondrite, comprising a total mass of 25.4 kg, were recovered in an ellipse measuring 8 × 3.5 km. One fragment of the shower partially penetrated the aluminum roof of a shed. Interpretation of the visual and satellite data suggests that the fireball traveled from south-southwest to north-northeast, with a slope from the horizontal of 55°–61°. A statistical evaluation of the likely heliocentric orbits for the body prior to collision with the Earth, coupled with theoretical modeling of the entry, suggests an entry velocity in the range of 12.7–13.3 km/s; the meteoroid had moved in a low-inclination orbit, with orbital perihelion located extremely close to the Earth's orbit. From satellite optical data, it is found that the photometric mass consumed during the largest detonation is ～1200 kg. Estimation of the amplitude of the acoustic signal detected by the most distant observer yields a source energy near 0.5 kt TNT equivalent energy, which corresponds to a mass of order 10 metric tonnes. This measure is uncertain to approximately one order of magnitude. Theoretical modeling of the entry of the object suggests a mass near 1600 kg. Cosmogenic radionuclide activities constrain the lower initial mass to be ～700 kg with an upper limit near 4000 kg. Seismic data possibly associated with the fireball suggest extremely poor coupling between the airwave and the ground. The total mass estimated to have reached the ground is ～100 kg (in material comprising >55 g fragments), while the preatmospheric mass is found to be most probably in the range of 1200–2000 kg.     

CLAIRE: First light for a gamma-ray lens

The objective of the R&D project CLAIRE was to prove the principle of a gamma-ray lens for nuclear astrophysics. CLAIRE's Laue diffraction lens has a diameter of 45 cm and a focal length of 277 cm; 556 germanium-silicon crystals are tuned to focus 170 keV photons onto a 1.5 cm diameter focal spot. Laboratory measurements of the individual crystals and the entire lens have been used to validate a numerical model that we use to estimate the lens performance for a source at infinity. During a stratospheric balloon flight on 2001 June 14, CLAIRE was directed at the Crab nebula by a pointing system able to stabilize the lens to within a few arcseconds of the target. In 72 min of valid pointing time, 33 photons from the Crab were detected in the 3 keV bandpass of the lens: CLAIRE's first light! The performance of CLAIRE's gamma-ray lens, namely the peak reflectivity for a polychromatic source (9±1%), has been confirmed by ground data obtained on a 205 meter long test range. CLAIRE's measured performance validates the principle of a Laue lens for nuclear astrophysics, opening the way for a space-borne gamma-ray lens telescope that will achieve one to two orders of magnitude improvement in sensitivity over present technologies.     

Realizing the Literate Traveller

A current research project is addressing the problem of finding appropriate geographical information by developing a (geo)information realization resource based upon the concepts of the Literate Traveller. It has as its main goal to provide tools for geographical knowledge building and exploration, by providing conventional spatial information plus geographical interpretation and appreciation artifacts. These artifacts are delivered in an interactive multimedia environment. This research examines the formulation of an initial prototype component to assist in providing geographical knowledge. The prototype is based on a study site in Townsville, Australia, and is being used as a vehicle to explore enhanced spatial information provision through the use of rich media. The research examines new ways to prospect for, discover and disseminate spatial knowledge within an intelligence‐briefing context. The paper reports on the concepts behind the design of the application developed for the test site, and presents initial findings from the early evaluation of the prototype.     

Mapping and analysing virtual outcrops

Laser scanning is a very efficient way to generate realistic, high-resolution digital models of 3-D geological outcrops. This paper discusses the methodologies involved in the creation and analysis of virtual outcrops, based on laser scanner data. The visualisation of the laser scanner data as a photorealistic 3-D object is described. Geological features picked out on the virtual outcrop (e.g. fractures, faults or bedding planes) can be extrapolated outward, into space, and inward, into the subsurface, using tension surfaces.Electronic Supplementary Material Supplementary material is available in the online version of this article at Reviewed by: J.D. Clemens, D. Yuen     

The influence of autocorrelation on the ability to detect trend in hydrological series

This study investigated using Monte Carlo simulation the interaction between a linear trend and a lag‐one autoregressive (AR(1)) process when both exist in a time series. Simulation experiments demonstrated that the existence of serial correlation alters the variance of the estimate of the Mann–Kendall (MK) statistic; and the presence of a trend alters the estimate of the magnitude of serial correlation. Furthermore, it was shown that removal of a positive serial correlation component from time series by pre‐whitening resulted in a reduction in the magnitude of the existing trend; and the removal of a trend component from a time series as a first step prior to pre‐whitening eliminates the influence of the trend on the serial correlation and does not seriously affect the estimate of the true AR(1). These results indicate that the commonly used pre‐whitening procedure for eliminating the effect of serial correlation on the MK test leads to potentially inaccurate assessments of the significance of a trend; and certain procedures will be more appropriate for eliminating the impact of serial correlation on the MK test. In essence, it was advocated that a trend first be removed in a series prior to ascertaining the magnitude of serial correlation. This alternative approach and the previously existing approaches were employed to assess the significance of a trend in serially correlated annual mean and annual minimum streamflow data of some pristine river basins in Ontario, Canada. Results indicate that, with the previously existing procedures, researchers and practitioners may have incorrectly identified the possibility of significant trends. Copyright © Environment Canada. Published by John Wiley & Sons, Ltd.     

Book reviews

VANISHING RAIN FORESTS: the ecological transition of Malaysia (Oxford Monographs on Biogeography No.5) by S.R. Aiken and C.H. Leigh. 19 × 25 cm, xvii and 194 pages. Clarendon Press, Oxford 1992. (ISBN 0 19 854242 9) $130.00 (hard)

ECOGEOGRAPHY AND RURAL MANAGEMENT: a contribution to the International Geosphere‐Biosphere Programme by J. Tricart and C. Kiewiet de Jonge. 267 pages. Longman/Wiley, New York 1992. (ISBN 0 582 06273 X) $51.00.

THE NEW SOCIAL ECONOMY: reworking the division of labor by A. Sayer and R. Walker. 15 × 23 cm, viii and 306 pages. Blackwell, Cambridge MA, 1992. (ISBN 1 55786 280 X) $49.95 (soft)

TROPICAL AFRICAN DEVELOPMENT: geographical perspectives edited by M.B. Gleave. 366 pages. Longmans/Wiley, New York 1992. (ISBN 0 582 30147 5)

MINING AND INDIGENOUS PEOPLES IN AUSTRALASIA edited by J. Connell and R. Howitt. 13 × 21 cm, × and 205 pages. Sydney University Press, Sydney 1991. (ISBN 0 424 00177 2) $22.95 (soft)

GOVERNMENT SCHOOLS OF NEW SOUTH 1848 TO 1993 (Fourth edition) edited by M. McPherson. 21 × 30 cm, 242 pp. NSW Department of School Education, Sydney, 1993. (ISBN 0 7310 0288 1). $22.00 (soft)(inc. postage).     

The use of geometric and gamma-related distributions for frequency analysis of water deficit

This paper presents an approach to perform statistical frequency analysis of water deficit duration and severity using respectively the geometric and exponential distributions. Monthly mean water discharges are compared to a given threshold and classified in two mutually exclusive ways. This leads to a two state random variable such that: a success represents the absence of a water deficit event (mean monthly discharge exceeds threshold), and a failure, a water deficit event (mean monthly discharge is below threshold). If we suppose that this random variable gives rise to a Markov process of order 1, then the duration of a water deficit event X (consecutive months in deficit) will have a geometric distribution. In turn, the summation of discharges in deficit will give the severity of a water deficit event which can be represented by a one-parameter exponential distribution. The threshold or base level is taken as a percentile of the observed mean discharges of a given month. This base level, which varies from month to month, can be viewed as the limit of an acceptable deficit (or energetic failure) associated to a given empirical probability of being in deficit. The second step of the approach is to estimate the value of the parameter for each distribution using the maximum likelihood method. Expressions for the estimator of a given percentile, , as well as its variance are deduced. Finally, the presented models are applied to observed data.     

Distribution and chemistry of manganese,iron, and suspended particulates in Orca Basin

The intense halocline and redoxcline in the Orca Basin, northwest Gulf of Mexico, induce dramatic water column profiles for manganese, iron, and suspended particulates. Within a 17 m interval, the salinity of the basin water increases from 66 to 260 & permil and dissolved oxygen decreases to zero. Midway through this transition zone, concentrations of suspended matter peak at 900 g/liter. Dissolved iron and manganese concentrations in the anoxic brine increase from oceanic values to maxima of 1.6 and 22 mg/liter, respectively. Upward migration of dissolved manganese from the brine leads to production of manganese-rich particles in the slightly oxygenated overlying water.