scholarly journals Radar Observations of Comet Nuclei and Comae

2005 ◽  
Vol 13 ◽  
pp. 763-763
Author(s):  
Donald B. Campbell ◽  
John K. Harmon ◽  
Micael C. Nolan ◽  
Steven J. Ostro
Keyword(s):  
Cross Sections ◽  
Cometary Nucleus ◽  
Size Distributions ◽  
Radar Systems ◽  
The Earth ◽  
Radar Cross Sections ◽  
Short Period ◽  
Grain Size Distributions ◽  
Resolution Imaging

Nine comets have been detected with either the Arecibo (12.6 cm wavelength) or Goldstone (3.5 cm) radar systems. Included are six nucleus detections and five detections of echoes from coma grains. The radar backscatter cross sections measured for the nuclei correlate well with independent estimates of their sizes and are indicative of surface densities in the range of 0.5 to 1.0 g cm-3. Like most asteroids, comets appear to have surfaces that are very rough at scales much larger than the radar wavelength. Coma echo models can explain the radar cross sections using grain size distributions that include a substantial population of cm-sized grains. A long term goal of the cometary radar program has been the high resolution imaging of a cometary nucleus. Eleven short period comets are potentially detectable over the next two decades a few of which may be suitable for imaging. We are always waiting for the arrival of a new comet with an orbit that brings it within 0.1 AU of the earth.

Organizational Transformation through Strategic Dialogue

1998 ◽  
Vol 3 (1-2) ◽  
pp. 53-76 ◽  
Author(s):  
Annemieke J.M. Roobeek ◽  
Erik U. Mandersloot ◽  
Cristijn J.C. du Marchie Sarvaas
Keyword(s):  
Strategic Management ◽  
Cross Sections ◽  
Integral Transformation ◽  
Organizational Transformation ◽  
Strategy Development ◽  
Short Period ◽  
Business Experience ◽  
Participative Approach ◽  
Rapid Transformation

A well-founded strategy is no longer sufficient for a business. Experience has taught us that it is usually not implemented, even though that is why it was formulated in the first place. We believe that the complex problems that confront business and government demand a different approach to strategy development: one that guarantees that the organization will actually change. We have developed the concept of Strategic Management from Below, and since 1990 we have been gaining experience with its use in business and government. Since that time, the concept has developed into a participative approach that enables an integral transformation to be realized within a very short period of time. The Strategic Dialogue in cross-sections of the organization forms the centerpiece of the concept. This article discusses the application of our approach to the creation of the long-term strategy for Amsterdam Airport Schiphol. How have the principles of the concept of Strategic Management from Below, and the Strategic Dialogue in particular, contributed to a successful, rapid transformation under extremely turbulent conditions?

scholarly journals Characterisation of long-term climate change by dimension estimates of multivariate palaeoclimatic proxy data

2006 ◽  
Vol 13 (5) ◽  
pp. 485-497 ◽  
Author(s):  
R. Donner ◽  
A. Witt
Keyword(s):  
Climate Change ◽  
Trace Elements ◽  
Sediment Cores ◽  
Quantitative Measure ◽  
Relevant Information ◽  
Size Distributions ◽  
Spatially Extended ◽  
Grain Size Distributions ◽  
Variance Decay

Abstract. The problem of extracting climatically relevant information from multivariate geological records is tackled by characterising the eigenvalues of the temporarily varying correlation matrix. From these eigenvalues, a quantitative measure, the linear variance decay (LVD) dimension density, is derived. The LVD dimension density is shown to serve as a suitable estimate of the fractal dimension density. Its performance is evaluated by testing it for (i) systems with independent components and for (ii) subsystems of spatially extended linearly correlated systems. The LVD dimension density is applied to characterise two geological records which contain information about climate variability during the Oligocene and Miocene. These records consist of (a) abundances of different chemical trace elements and (b) grain-size distributions obtained from sediment cores offshore the East Antarctic coast. The presented analysis provides evidence that the major climate change associated with the Oligocene-Miocene transition is reflected in significant changes of the LVD dimension density. This is interpreted as a change of the interrelationships between different trace elements in the sediment and to a change of the provenance area of the deposited sediment.

Harry Clive Minnett 1917 - 2003

2005 ◽  
Vol 16 (2) ◽  
pp. 199 ◽  
Author(s):  
Bruce MacA. Thomas ◽  
Brian J. Robinson
Keyword(s):  
Radio Telescope ◽  
Optical Telescope ◽  
Radar Systems ◽  
South Wales ◽  
Short Period ◽  
Scientific Principles ◽  
Considerable Impact ◽  
Australian Science ◽  
Long Term Impact

In April 1940, Harry Minnett joined the Council for Scientific and Industrial Research (CSIR, renamed CSIRO in May 1949), soon after the establishment of the Radiophysics Laboratory for research into advanced radar systems. He remained with the organization until his retirement in 1981. Harry's ability as a meticulous engineer with a thorough understanding of the related underlying scientific principles was recognized by his being appointed to leading roles in a number of significant projects. These were to have a long-term impact on Australian science and technology. Among the most important was his role in guiding the design of the Parkes 64-m radio telescope to successful completion, and the establishment of a world-recognized research group on antenna design to support radio telescope design and upgrades. He also had considerable impact on the construction of the 3.9-m Anglo-Australian optical telescope at Siding Spring Mountain, New South Wales, and the 'Interscan' aircraft Microwave Landing System being developed by the Division of Radiophysics in collaboration with industry. He was Chief of the Division from 1978 to 1981. Harry died on 20 December 2003 after a short period of illness.

scholarly journals On the orbital evolution of 2020 AV2, the first asteroid ever observed to go around the Sun inside the orbit of Venus

2020 ◽  
Vol 494 (1) ◽  
pp. L6-L10 ◽  
Author(s):  
C de la Fuente Marcos ◽  
R de la Fuente Marcos
Keyword(s):  
Orbital Evolution ◽  
Gravitational Perturbation ◽  
The Sun ◽  
Moon System ◽  
Resonant Orbit ◽  
The Earth ◽  
Short Period ◽  
Orbital Periods ◽  
Near Earth Objects

ABSTRACT The innermost section of the Solar system has not been extensively studied because minor bodies moving inside Earth’s orbit tend to spend most of their sidereal orbital periods at very low solar elongation, well away from the areas more frequently observed by programs searching for near-Earth objects. The survey carried out from the Zwicky Transient Facility (ZTF) is the first one that has been able to detect multiple asteroids well detached from the direct gravitational perturbation of the Earth–Moon system. ZTF discoveries include 2019 AQ3 and 2019 LF6, two Atiras with the shortest periods among known asteroids. Here, we perform an assessment of the orbital evolution of 2020 AV2, an Atira found by ZTF with a similarly short period but following a path contained entirely within the orbit of Venus. This property makes it the first known member of the elusive Vatira population. Genuine Vatiras, those long-term dynamically stable, are thought to be subjected to the so-called von Zeipel–Lidov–Kozai oscillation that protects them against close encounters with both Mercury and Venus. However, 2020 AV2 appears to be a former Atira that entered the Vatira orbital domain relatively recently. It displays an anticoupled oscillation of the values of eccentricity and inclination, but the value of the argument of perihelion may circulate. Simulations show that 2020 AV2 might reach a 3:2 resonant orbit with Venus in the future, activating the von Zeipel–Lidov–Kozai mechanism, which in turn opens the possibility to the existence of a long-term stable population of Vatiras trapped in this configuration.

scholarly journals How do the grain size characteristics of a tephra deposit change over time?

2021 ◽  
Vol 83 (7) ◽  
Author(s):  
Nick A. Cutler ◽  
R. T. Streeter ◽  
A. J. Dugmore ◽  
E. R. Sear
Keyword(s):  
Grain Size ◽  
Size Distributions ◽  
Tephra Layer ◽  
Distinctive Features ◽  
Tephra Deposit ◽  
Grain Size Distributions ◽  
Tephra Layers ◽  
Over Time ◽  
Mount St Helens

AbstractVolcanologists frequently use grain size distributions (GSDs) in tephra layers to infer eruption parameters. However, for long-past eruptions, the accuracy of the reconstruction depends upon the correspondence between the initial tephra deposit and preserved tephra layer on which inferences are based. We ask: how closely does the GSD of a decades-old tephra layer resemble the deposit from which it originated? We addressed this question with a study of the tephra layer produced by the eruption of Mount St Helens, USA, in May 1980. We compared grain size distributions from the fresh, undisturbed tephra with grain size measurements from the surviving tephra layer. We found that the overall grain size characteristics of the tephra layer were similar to the original deposit, and that distinctive features identified by earlier authors had been preserved. However, detailed analysis of our samples showed qualitative differences, specifically a loss of fine material (which we attributed to ‘winnowing’). Understanding how tephra deposits are transformed over time is critical to efforts to reconstruct past eruptions, but inherently difficult to study. We propose long-term, tephra application experiments as a potential way forward.

Effect of duplex grain size distributions on long term stress-rupture life at 600 °C/450 MPa of Nimonic 80A

2015 ◽  
Vol 65 ◽  
pp. 840-846 ◽  
Author(s):  
Yulai Xu ◽  
Qinghuan Liu ◽  
Feng Wang ◽  
Jun Li ◽  
Xueshan Xiao ◽  
...  
Keyword(s):  
Grain Size ◽  
Rupture Life ◽  
Stress Rupture ◽  
Size Distributions ◽  
Stress Rupture Life ◽  
Grain Size Distributions ◽  
Nimonic 80A

The Origin of the Moon and its Relationship to the Origin of the Solar System

1962 ◽  
Vol 14 ◽  
pp. 133-148 ◽  
Author(s):  
Harold C. Urey
Keyword(s):  
Solar System ◽  
The Moon ◽  
The Past ◽  
The Earth ◽  
Short Period ◽  
Origin Of The Moon

During the last 10 years, the writer has presented evidence indicating that the Moon was captured by the Earth and that the large collisions with its surface occurred within a surprisingly short period of time. These observations have been a continuous preoccupation during the past years and some explanation that seemed physically possible and reasonably probable has been sought.

Models of comets with strongly variable nongravitational effects

1999 ◽  
Vol 173 ◽  
pp. 381-387
Author(s):  
M. Królikowska ◽  
G. Sitarski ◽  
S. Szutowicz
Keyword(s):  
Cometary Nucleus ◽  
Spin Axis ◽  
Reactive Force ◽  
Orbital Elements ◽  
Cometary Nuclei ◽  
Short Period ◽  
Basic Parameters ◽  
Polar Radius ◽  
Cometary Activity ◽  
Nongravitational Effects

AbstractThe nongravitational motion of five “erratic” short-period comets is studied on the basis of published astrometric observations. We present the precession models which successfully link all the observed apparitions of the comets: 21P/Giacobini-Zinner, 31P/Schwassmann-Wachmann 2, 32P/Comas Solá, 37P/Forbes, and 43P/Wolf-Harrington. We used the Sekanina's forced precession model of the rotating cometary nucleus to include the nongravitational terms into equations of the comet's motion. Values of six basic parameters (four connected with the rotating comet nucleus and two describing the precession of spin-axis of the nucleus) have been determined along the orbital elements from positional observations of the comets. The solutions were derived with additional assumptions which introduce instantaneous changes of modulus of reactive force,Aand of maximum of cometary activity with respect to perihelion time. The present precession models impose some contraints on sizes and rotational periods of cometary nuclei. According to our solutions the nucleus of 21P/Giacobini-Zinner with oblateness along the spin-axis of about 0.32 (equatorial to polar radius of 1.46) is the most oblate among five investigated comets.

Covering of cometary nucleus by refractory crust and its evolution into asteroid-like body

1999 ◽  
Vol 173 ◽  
pp. 365-370
Author(s):  
Kh.I. Ibadinov
Keyword(s):  
Laboratory Experiments ◽  
Cometary Nucleus ◽  
Sublimation Rate ◽  
Cometary Nuclei ◽  
Short Period Comet ◽  
Short Period ◽  
Brightness Decrease ◽  
Crust Thickness ◽  
Ice Sublimation ◽  
Period Comet

AbstractFrom the established dependence of the brightness decrease of a short-period comet dependence on the perihelion distance of its orbit it follows that part of the surface of these cometary nuclei gradually covers by a refractory crust. The results of cometary nucleus simulation show that at constant insolation energy the crust thickness is proportional to the square root of the insolation time and the ice sublimation rate is inversely proportional to the crust thickness. From laboratory experiments resulted the thermal regime, the gas productivity of the nucleus, covering of the nucleus by the crust, and the tempo of evolution of a short-period comet into the asteroid-like body studied.

Preparation And elemental analysis of ancient fibers

1987 ◽  
Vol 45 ◽  
pp. 410-411
Author(s):  
Allen Angel ◽  
Kathryn A. Jakes
Keyword(s):  
Cross Sections ◽  
Physical Structure ◽  
Energy Dispersive ◽  
Archaeological Sites ◽  
X Ray ◽  
Long Term Storage ◽  
Backscattered Electron ◽  
Electron Imaging

Fabrics recovered from archaeological sites often are so badly degraded that fiber identification based on physical morphology is difficult. Although diagenetic changes may be viewed as destructive to factors necessary for the discernment of fiber information, changes occurring during any stage of a fiber's lifetime leave a record within the fiber's chemical and physical structure. These alterations may offer valuable clues to understanding the conditions of the fiber's growth, fiber preparation and fabric processing technology and conditions of burial or long term storage (1).Energy dispersive spectrometry has been reported to be suitable for determination of mordant treatment on historic fibers (2,3) and has been used to characterize metal wrapping of combination yarns (4,5). In this study, a technique is developed which provides fractured cross sections of fibers for x-ray analysis and elemental mapping. In addition, backscattered electron imaging (BSI) and energy dispersive x-ray microanalysis (EDS) are utilized to correlate elements to their distribution in fibers.

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