Author
Max K. Wallis
Other affiliations: Cardiff University
Bio: Max K. Wallis is an academic researcher from University of Buckingham. The author has contributed to research in topics: Comet & Halley's Comet. The author has an hindex of 19, co-authored 95 publications receiving 1709 citations. Previous affiliations of Max K. Wallis include Cardiff University.
Topics: Comet, Halley's Comet, Solar wind, Bow wave, Cosmic dust
Papers published on a yearly basis
Papers
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TL;DR: The Giotto magnetometer experiment at comet Halley has for the first time provided magnetic field measurements in all the important spatial regions characterizing the front-side interaction between the solar-wind magnetoplasma and a cometary atmosphere.
Abstract: The Giotto magnetometer experiment at comet Halley has for the first time provided magnetic field measurements in all the important spatial regions characterizing the front-side interaction between the solar-wind magnetoplasma and a cometary atmosphere. Upstream waves of cometary origin have been observed at distances greater than two million km from the comet, both inbound and outbound. A cometary bow shock has been identified at 1.15 million inbound on the dawn side and a thick quasi-parallel cometary bow shock outbound. A turbulent magnetosheath has been observed further inside. A magnetic pile-up region has been identified inside 135,000 km, inbound, and 263,000 km, outbound, with fields up to 57 and 65 nT, respectively. A cavity region with essentially zero magnetic field has been discovered, with a width of 8500 km along the trajectory around closest approach.
337 citations
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University of Kent1, Baylor University2, Rutherford Appleton Laboratory3, European Space Research and Technology Centre4, Max Planck Society5, Jet Propulsion Laboratory6, University of Sheffield7, Technische Universität München8, Lund University9, Community emergency response team10, Cardiff University11
TL;DR: Data at large masses recently analysed from the DIDSY data set show clear evidence of a decrease in the mass distribution index at these masses within the coma, and it is shown that such a value of the mass index can provide sufficient mass for consistency with the observed deceleration.
Abstract: Analysis of the data from Giotto’s Dust Impact Detection System experiment (DIDSY) is presented These data represent measurement of the size of dust grains incident on the Giotto dust shield along its trajectory through the coma of comet P/Halley on 1986 March 13/14 First detection occurred at some 287000 km distance from the nucleus on the inbound leg; the majority of the DIDSY subsystems remained operational after closest approach (604 km) yielding the last detection at about 202000 km from the nucleus
150 citations
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University of Kent1, Baylor University2, Rutherford Appleton Laboratory3, Science and Engineering Research Council4, Max Planck Society5, Jet Propulsion Laboratory6, University of Sheffield7, Technische Universität München8, Lund University9, Community emergency response team10, University of Bari11, European Space Research and Technology Centre12, Cardiff University13
TL;DR: In this paper, the density and the mass spectrum of the dust near comet Halley have been measured by the Giotto space probe's dust impact detection system, showing depletion in small and intermediate masses; at about 600 km from the nucleus, however, the dust activity rises and the spectrum is dominated by larger masses.
Abstract: The density and the mass spectrum of the dust near comet Halley have been measured by the Giotto space probe's dust impact detection system. The dust spectrum obtained at 291,000 km from the comet nucleus show depletion in small and intermediate masses; at about 600 km from the nucleus, however, the dust activity rises and the spectrum is dominated by larger masses. Most of the mass striking Giotto is noted to reside in the few large particles penetrating the dust shield. Momentum balances and energy considerations applied to an observed deceleration suggest that a large mass of the spacecraft was detached by an impact.
123 citations
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TL;DR: The three-dimensional positive ion analyzer aboard the Giotto spacecraft has been used to study the interaction between protons and alpha-particles in the solar wind and positive ions from comet Halley as discussed by the authors.
Abstract: The three-dimensional positive ion analyzer aboard the Giotto spacecraft has been used to study the interaction between protons and alpha-particles in the solar wind and positive ions from comet Halley Although the first impression of the overall structure is that the plasma flow evolves smoothly as the nucleus is approached, three sharp transitions of relatively small amplitude can be identified on both the inbound and outbound legs of the trajectory The outermost one, at about one million km from the nucleus, appears to be a multiple crossing of a weak bow shock The innermost one, at 80,000 km, is the boundary where the flowing plasma becomes depleted On a microscopic scale, the turbulence created by the interaction between the two ion populations extends to a distance of several million km from the nucleus At Giotto's closest approach to the nucleus, the plasma produced around the spacecraft by dust and gas impacts was much more energetic than had been expected
115 citations
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TL;DR: In this article, a central, low pressure vapour-droplet mixture is described, which is conceived to be retained within an ice shell, and providing a potentially hospitable environment for elementary life forms.
Abstract: Comets accreted soon after the initial collapse and cooling of the solar nebula, and containing a plausible fraction of 26Al, would have been significantly heated as this radionuclide decayed. Snow-and-dust balls as described by integrals of the heat conduction equation would melt in the centre if larger than 3–6 km radius. A central, low pressure vapour-droplet mixture is described here, which is conceived to be retained within an ice shell, and providing a potentially hospitable environment for elementary life forms. Refreezing after some million years produces a partially-hollow core.
94 citations
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TL;DR: In this article, the authors presented a self-consistent, interactive simulation of the formation of the giant planets, in which for the first time both the gas and planetesimal accretion rates were calculated in a selfconsistent and interactive fashion.
2,931 citations
01 Jun 1995
1,859 citations
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TL;DR: Major steps in the evolution of African hominids and other vertebrates are coincident with shifts to more arid, open conditions near 2.8 Ma, suggesting that some Pliocene (Plio)-Pleistocene speciation events may have been climatically mediated.
Abstract: Marine records of African climate variability document a shift toward more arid conditions after 2.8 million years ago (Ma), evidently resulting from remote forcing by cold North Atlantic sea-surface temperatures associated with the onset of Northern Hemisphere glacial cycles. African climate before 2.8 Ma was regulated by low-latitude insolation forcing of monsoonal climate due to Earth orbital precession. Major steps in the evolution of African hominids and other vertebrates are coincident with shifts to more arid, open conditions near 2.8 Ma, 1.7 Ma, and 1.0 Ma, suggesting that some Pliocene (Plio)-Pleistocene speciation events may have been climatically mediated.
1,118 citations
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TL;DR: The Giotto space probe's ion mass spectrometer has obtained data on the composition and velocity distributions of cometary ions at distances of between 7.5 million and 1300 km from the comet Halley nucleus as mentioned in this paper.
Abstract: The Giotto space probe's ion mass spectrometer has obtained data on the composition and velocity distributions of cometary ions at distances of between 7.5 million and 1300 km from the comet Halley nucleus. Solar wind He(2+) was found throughout the coma, as close as 5000 km, with the He(+) produced by charge exchange being within about 200,000 km. A pile-up of heavy cometary ions was found at about 10,000 km from the nucleus. Inside the contact surface, which was found at about 4600 km, ion temperatures as low as about 340 K and outflow velocities of about 1 km/sec were found.
391 citations
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TL;DR: Two different proxies were used for prenatal PBDE exposure, and levels in breast milk, but not in placenta, showed an association with congenital cryptorchidism, and these observations are of concern because human exposure to PBDEs is high in some geographic areas.
Abstract: Polybrominated diphenyl ethers (PBDEs) are widely used as flame retardants, and the general population is exposed through products such as upholstery, building materials, insulation, electronic equipment, and contaminated food. PBDEs are added to polymers without being chemically bound and can leach into the environment, where they settle with air particles and sludge. They are persistent, and some—BDE-47, BDE-99, and BDE-153—can accumulate in lipid-rich tissues (Agency for Toxic Substances and Disease Registry 2004; Sjodin et al. 2003).
Concentrations of PBDE in human European breast milk samples are generally low compared with those in the United States, and considered to be well below the estimated lowest observed adverse effect level (LOAEL) of 1 mg/kg/day (Darnerud et al. 2001). Two technical mixtures, penta- and octa-mixtures of PBDEs, have been banned from use in Europe since 2003 (Darnerud et al. 2001), and Swedish studies indicated a decrease in breast milk levels since the middle of the 1990s (Meironyte et al.1999; Sjodin et al. 2003). However, annual production rates of some PBDEs are still considerable in some areas (Alaee et al. 2006; Betts 2002; Law et al. 2006). Animal studies show that some PBDEs exhibit endocrine-disrupting activity, which has been studied predominantly for thyroid hormone transport and metabolism (Legler and Brouwer 2003), but data on adverse effects on reproductive outcome after gestational exposure are also emerging (Lilienthal et al. 2006).
The prevalence of cryptorchidism in newborn boys appears to have increased in some areas, such as Great Britain and Denmark, over the past decades, and its current prevalence is considerably higher in Denmark than in Finland (Anonymous 1986; Boisen et al. 2004). Although the reason for this is as yet unknown, the rapid increase in prevalence suggests that environmental factors are involved (Sharpe 2006; Skakkebaek et al. 2001). Adverse effects of fetal exposure to environmental chemicals on testicular descent and hormonal function may be detectable during the short physiologic activation of the pituitary–gonadal axis at approximately 3 months of age (Andersson et al. 1998; Main et al. 2000, 2006b; Suomi et al. 2006).
In this study we aimed to evaluate the association between exposure to 14 PBDEs (BDEs 28, 47, 66, 71, 75, 77, 85, 99, 100,119, 138, 153, 154, 183) in newborn boys and the position and function of the testes.
375 citations