A two-level perturbation method for connecting unstable periodic orbits with low fuel cost and short time of flight: application to a lunar observation mission

The proposed method connects two unstable periodic orbits by employing trajectories of their associated invariant manifolds that are perturbed in two levels. A first level of velocity perturbations is applied on the trajectories of the discretized manifolds at the points where they approach the nominal unstable periodic orbit in order to accelerate them. A second level of structured velocity perturbations is applied to trajectories that have already been subjected to first level perturbations in order to approximately meet the necessary conditions for a low \(\varDelta \text {V}\) transfer. Due to this two-level perturbation approach, the number of the trajectories obtained is significantly larger compared with approaches that employ traditional invariant manifolds. For this reason, the problem of connecting two unstable periodic orbits through perturbed trajectories of their manifolds is transformed into an equivalent discrete optimization problem that is solved with a very low computational complexity algorithm that is proposed in this paper. Finally, the method is applied to a lunar observation mission of practical interest and is found to perform considerably better in terms of \(\varDelta \text {V}\) cost and time of flight when compared with previous techniques applied to the same project.     

A study of the observed shift in the peak position of olivine Raman spectra as a result of shock induced by hypervelocity impacts

Kuebler et al. (2006) identified variations in olivine Raman spectra based on the composition of individual olivine grains, leading to identification of olivine composition from Raman spectra alone. However, shock on a crystal lattice has since been shown to result in a structural change to the original material, which produces a shift in the Raman spectra of olivine grains compared with the original unshocked olivine (Foster et al. 2013). This suggests that the use of the compositional calculations from the Raman spectra, reported in Kuebler et al. (2006), may provide an incorrect compositional value for material that has experienced shock. Here, we have investigated the effect of impact speed (and hence peak shock pressure) on the shift in the Raman spectra for San Carlos olivine (Fo₉₁) impacting Al foil. Powdered San Carlos olivine (grain size 1–10 μm) was fired at a range of impact speeds from 0.6 to 6.1 km s^?1 (peak shock pressures 5–86 GPa) at Al foil to simulate capture over a wide range of peak shock pressures. A permanent change in the Raman spectra was found to be observed only for impact speeds greater than ~5 km s^?1. The process that causes the shift is most likely linked to an increase in the peak pressure produced by the impact, but only after a minimum shock pressure associated with the speed at which the effect is first observed (here 65–86 GPa). At speeds around 6 km s^?1 (peak shock pressures ~86 GPa), the shift in Raman peak positions is in a similar direction (red shift) to that observed by Foster et al. (2013) but of twice the magnitude.     

Effects of Docks on Salt Marsh Vegetation: an Evaluation of Ecological Impacts and the Efficacy of Current Design Standards

Private docks are common in estuaries worldwide. Docks in Massachusetts (northeast USA) cumulatively overlie ~ 6 ha of salt marsh. Although regulations are designed to minimize dock impacts to salt marsh vegetation, few data exist to support the efficacy of these policies. To quantify impacts associated with different dock designs, we compared vegetation characteristics and light levels under docks with different heights, widths, orientations, decking types and spacing, pile spacing, and ages relative to adjacent control areas across the Massachusetts coastline (n = 212). We then evaluated proportional changes in stem density and biomass of the dominant vegetation (Spartina alterniflora and Spartina patens) in relation to dock and environmental (marsh zone and nitrogen loading) characteristics. Relative to adjacent, undeveloped habitat, Spartina spp. under docks had ~ 40% stem density, 60% stem biomass, greater stem height and nitrogen content, and a higher proportion of S. alterniflora. Light availability was greater under taller docks and docks set at a north-south orientation but did not differ between decking types. Dock height best predicted vegetation loss, but orientation, pile spacing, decking type, age, and marsh zone also affected marsh production. We combined our proportional biomass and stem elemental composition estimates to calculate a statewide annual loss of ~ 2200 kg dry weight of Spartina biomass (367 kg per ha of dock coverage). Managers can reduce impacts through design modifications that maximize dock height (> 150 cm) and pile spacing while maintaining a north-south orientation, but dock proliferation must also be addressed to limit cumulative impacts.     

A multi-proxy lake-sediment record of middle through late Holocene hydroclimate change in southern British Columbia,Canada

Journal of Paleolimnology - Analysis of the oxygen isotopic composition (δ18O) of sedimentary carbonates in Turquoise Lake (N50.83°, W121.69°, 807 m), southwestern British...     

Nonaxisymmetric instabilities of self-gravitating disks. I toroids

We perform an extensive linear investigation of the nonaxisymmetric disk modes referred to in the literature as P, I, and J modes in self-gravitating polytropic toroids with power law angular velocity distributions. For selected models, we also follow the development of instability from the linear regime through the quasi-linear regime to deep into the nonlinear regime. We consider modes with azimuthal dependence e ^imφ , where m is an integer and φ is the azimuthal angle. We find that instability sets in through m=2 barlike I modes at T/|W|∼0.16–0.18 depending upon the chosen angular velocity law where T is the rotational kinetic energy and W is the gravitational energy of the toroid. Instability in the barlike I mode peaks in strength around T/|W|=0.22–0.23 after which it weakens, eventually stabilizing around T/|W|∼0.25–0.26. One-armed modes (m=1 modes) become unstable just after instability in the m=2I modes sets in; instability in m=1 modes sets in at T/|W|∼0.19. They dominate the barlike I modes in toroids with T/|W|≳0.25. However, almost immediately after the m=1 mode overtakes the barlike I mode, higher-m J modes appear. J modes with m=2, 3, and 4 become unstable for T/|W|≳0.25–0.26, 0.23–0.25, and 0.25–0.26, respectively. m≥3J modes dominate the m=1 mode in toroids with T/|W|≳0.27. As T/|W| increases further, nonaxisymmetric instability sets in through higher and higher m modes. We find quantitative agreement between the early nonlinear behavior of the tested unstable toroids and our linear results. Quasi-linear modeling suggests that a gravitational self-interaction torque which arises early in the nonlinear regime saturates growth of the mode and leads to significant transport of mass and angular momentum. Neither I mode nor J mode instabilities produce prompt fission in toroids.     

The effect of orbital evolution on the Haumea (2003 EL61) collisional family

The Haumea family is currently the only identified collisional family in the Kuiper belt. We numerically simulate the long-term dynamical evolution of the family to estimate a lower limit of the family’s age and to assess how the population of the family and its dynamical clustering are preserved over Gyr timescales. We find that the family is not younger than 100 Myr, and its age is at least 1 Gyr with 95% confidence. We find that for initial velocity dispersions of 50–400 m s^?1, approximately 20–45% of the family members are lost to close encounters with Neptune after 3.5 Gyr of orbital evolution. We apply these loss rates to two proposed models for the formation of the Haumea family, a graze-and-merge type collision between two similarly sized, differentiated KBOs or the collisional disruption of a satellite orbiting Haumea. For the graze-and-merge collision model, we calculate that >85% of the expected mass in surviving family members within 150 m s^?1 of the collision has been identified, but that one to two times the mass of the known family members remains to be identified at larger velocities. For the satellite-break-up model, we estimate that the currently identified family members account for ～50% of the expected mass of the family. Taking observational incompleteness into account, the observed number of Haumea family members is consistent with either formation scenario at the 1σ level, however both models predict more objects at larger relative velocities (>150 m s^?1) than have been identified.     

Climate drying and associated forest decline in the lowlands of northern Guatemala during the late Holocene

Palynological studies document forest disappearance during the late Holocene in the tropical Maya lowlands of northern Guatemala. The question remains as to whether this vegetation change was driven exclusively by anthropogenic deforestation, as previously suggested, or whether it was partly attributable to climate changes. We report multiple palaeoclimate and palaeoenvironment proxies (pollen, geochemical, sedimentological) from sediment cores collected in Lake Petén Itzá, northern Guatemala. Our data indicate that the earliest phase of late Holocene tropical forest reduction in this area started at ∼ 4500 cal yr BP, simultaneous with the onset of a circum-Caribbean drying trend that lasted for ∼ 1500 yr. This forest decline preceded the appearance of anthropogenically associated Zea mays pollen. We conclude that vegetation changes in Petén during the period from ∼ 4500 to ∼ 3000 cal yr BP were largely a consequence of dry climate conditions. Furthermore, palaeoclimate data from low latitudes in North Africa point to teleconnective linkages of this drying trend on both sides of the Atlantic Ocean.     

OSL dating of southeast Australian quartz: A preliminary assessment of luminescence characteristics and behaviour

Australian quartz has been widely considered particularly suitable for OSL dating of sediments. We present the results of a preliminary investigation into the luminescence characteristics and behaviour of quartz from various regions and depositional contexts across southeastern Australia. In doing so, we find that quartz from this region is generally well suited to the SAR protocol, although individual sample behaviour should nevertheless be assessed to confirm this. Furthermore, luminescence sensitivity appears to relate to depositional context, with aeolian samples exhibiting the highest sensitivity values, and lacustrine and periglacial sediments the lowest. The extent of a sample's sedimentary history, which in many cases is characteristic of the region from which it is derived, may also influence quartz sensitisation through repeated cycles of burial and reworking.     

Quadrupole ICP‐MS: Introduction to Instrumentation,Measurement Techniques and Analytical Capabilities

The low detection limits and multi‐element capability of inductively coupled plasma‐mass spectrometry (ICP‐MS) makes it an attractive option in a wide range of environmental, medical, biological, industrial and archaeological applications. Quadrupole ICP‐MS is used to determine element concentrations in a diverse range of sample types, often very different from the geological applications for which ICP‐MS was originally developed. Whilst modern instruments are robust and capable of a high degree of automation, it is essential that users of both instrumentation and data are aware of the strengths and limitations of the technique. Many people who are now involved with the operation and application of ICP‐MS instruments are not specialists in the field, as was usually the case amongst early operators. This back‐to‐basics review is aimed at the novice user and includes a guide to ICP‐MS instrumentation and performance. Whilst solids, liquids and gases can all be measured by ICP‐MS, discussion of sample introduction is limited to liquids. Requirements for producing good quality data, including aspects of sample preparation, calibration, and methods of interference limitation are also discussed.