5,955 research outputs found
The inside-out view on neutron-star magnetospheres
No full textWe construct hydromagnetic neutron star equilibria which allow for a non-zero electric current distribution in the exterior. The novelty of our models is that the neutron star's interior field is in equilibrium with its magnetosphere, thus bridging the gap between previous work in this area, which either solves for the interior assuming a vacuum exterior or solves for the magnetosphere without modelling the star itself. We consider only non-rotating stars in this work, so our solutions are most immediately applicable to slowly rotating systems such as magnetars. Nonetheless, we demonstrate that magnetospheres qualitatively resembling those expected for both magnetars and pulsars are possible within our framework. The ‘inside-out’ approach taken in this paper should be more generally applicable to rotating neutron stars, where the interior and exterior regions are again not independent but evolve together
ROSETTA Lander Philae – Touch-down Reconstruction
No full textThe landing of the ROSETTA-mission lander Philae on November 12th 2014 on Comet 67P/Churyumov-Gerasimenko was planned as a descent with passive landing and anchoring by harpoons at touch-down. Actually the lander was not fixed at touch-down to the ground due to failing harpoons. The lander internal damper was actuated at touch-down for 42.6 mm with a speed of 0.08 m/s while the lander touch-down speed was 1 m/s. The kinetic energy before touch-down was 50 J, 45 J were dissipated by the lander internal damper and by ground penetration at touch-down, and 5 J kinetic energy are left after touch-down (0.325 m/s speed). Most kinetic energy was dissipated by ground penetration (41 J) while only 4 J are dissipated by the lander internal damper. Based on these data, a value for a constant compressive soil-strength of less than 1.7 kPa is calculated.
This paper focuses on the reconstruction of the touch-down at Agilkia over a period of around 20 s from first ground contact to lift-off again. After rebound Philae left a strange pattern on ground documented by the OSIRIS Narrow Angle Camera (NAC). The analysis shows, that the touch-down was not just a simple damped reflection on the surface. Instead the lander had repeated contacts with the surface over a period of about 20 s +/-10 s.
This paper discusses scenarios for the reconstruction of the landing sequence based on the data available and on computer simulations. Simulations are performed with a dedicated mechanical multi-body model of the lander, which was validated previously in numerous ground tests. The SIMPACK simulation software was used, including the option to set forces at the feet to the ground. The outgoing velocity vector is mostly influenced by the timing of the ground contact of the different feet. It turns out that ground friction during damping has strong impact on the lander outgoing velocity, on its rotation, and on its nutation. After the end of damping, the attitude of the lander can be strongly changed by the additional ground contacts even with the flywheel still running inside the lander.
The simulation shows that the outbound velocity vector and the lander rotation were formed immediately at touch-down during the first 1.5 s. The outbound velocity vector is found to be formed by the ground slope and the lander damping characteristic, especially the nearly horizontal flight out
NOC Liverpool report for the miniSTABLE benthic lander deployments as part of the UK-SSB research programme
No full textA review of the key features of a series of seabed based scientific lander deployments undertaken by the National Oceanography Centre at Liverpool, UK as part of the UK Shelf Seas Biogeochemistry (UK-SBS) Programme (www.uk-ssb.org) is provided in this document. A bespoke lander design provided a unique platform for a broad range of scientific measurements to facilitate novel benthic or near seabed scientific research. A complex and diverse set of lander based instrumentation included dissolved oxygen flux or ‘eddy correlation’ sensors, sonar based localised seabed distance and contour profiling, high resolution water velocity measurements and measurements of suspended particulate matter in the lower water column. The sensor suite was complimented by an automated, water sampler for collecting and preserving samples with a programmable sample volume and collection time. These seawater samples were suitable for determining dissolved inorganic nutrient levels close to the seabed. Inline filters were used to assess the levels of particulate concentrations at the time of each sample collection. A series of scientific survey cruises, using the research vessel RRS Discovery, occurred from March 2014 to September 2015 as part of the UK-SSB programme. Within this sequence of scientific cruises four key Celtic Sea based sites were surveyed. The lander deployment sites used provided a diverse range of seabed based scientific study conditions
L-DEPP Definition Study for Lunar Lander FINAL REPORT "
No full textA complete set of scientific instrumentation for the L-DEPP dust, plasma and waves package for the ESA Lunar Lander mission has been proposed and fully assessed with respect to the system and scientific requirements derived by ESA and the L-DEPP project team. The preliminary design from the system level down to the individual sensors is presented
Equilibria and oscillations of magnetised neutron stars
No full textWe investigate equilibrium configurations and oscillation spectra of neutron stars,modelled as rotating magnetised fluid bodies in Newtonian gravity. We also explorethe idea that these model neutron stars could have dynamics analogous to rigid-bodyfree precession.In axisymmetry, the equations of magnetohydrodynamics reduce to a purelytoroidal-field case and a mixed-field case (with a purely poloidal-field limit). Wesolve these equations using a nonlinear code which finds stationary rotating magnetisedstars by an iterative procedure. We find that despite the general nature ofour approach, the mixed-field configurations we produce are all dominated by theirpoloidal component. We calculate distortions induced both by magnetic fields andby rotation; our results suggest that the relationship between the magnetic energyand the induced ellipticity should be close to linear for all known neutron stars.We then investigate the oscillation spectra of neutron stars, using these stationaryconfigurations as a background on which to study perturbations. This is doneby evolving the perturbations numerically, making the Cowling approximation andspecialising to purely toroidal fields for simplicity. The results of the evolutionsshow a number of magnetically-restored Alfv´en modes. We find that in a rotatingstar pure inertial and pure Alfv´en modes are replaced by hybrid magneto-inertialmodes. We also show that magnetic fields appear to reduce the effect of the r-modeinstability.Finally, we look at precession-like dynamics in magnetised fluid stars, using bothanalytic and numerical methods. Whilst these studies are only preliminary, theyindicate deficiencies in previous research on this topic. We suggest ways in whichthe problem of magnetised-fluid precession could be better understood
Magnetic fields in axisymmetric neutron stars
No full textWe derive general equations for axisymmetric Newtonian magnetohydrodynamics and use these as the basis of a code for calculating equilibrium configurations of rotating magnetized neutron stars in a stationary state. We investigate the field configurations that result from our formalism, which include purely poloidal, purely toroidal and mixed fields. For the mixed-field formalism, the toroidal component appears to be bounded at less than 7 per cent. We calculate distortions induced both by magnetic fields and by rotation. From our non-linear work, we are able to look at the realm of validity of perturbative work: we find for our results that perturbative-regime formulae for magnetic distortions agree to within 10 per cent of the non-linear results if the ellipticity is less than 0.15 or the average field strength is less than 10^17 G. We also consider how magnetized equilibrium structures vary for different polytropic indices
A new ultrasonic rheometer for space exploration in lander missions
No full textViscosity is a fundamental characteristic of liquid and viscoelastic materials. Viscosity measurements can give an insight in the mechanics and chemistry of biotic and geophysical samples; therefore, lander exploration of extra-terrestrial planets will benefit from an integrated viscometer. However, conventional viscometers are unsuitable for space missions. A novel miniaturised ultrasonic sensor is here presented as a rheometer for the in situ analysis of viscoelastic samples in extra-terrestrial lander missions. The measurement accuracy of the ultrasonic sensor was compared against a conventional viscometer for the measurement of aqueous solutions of L- and D-amino acids, with concentrations ranging from 1 to 10% (mass/volume). The instrument measured the viscosity of the aqueous solutions with a precision of 10% in a frequency range of 1.8 to 13.5 MHz. The instrument was further used to measure the viscosity of solutions of D-serine and L-serine 10% in water at sub-zero temperatures. Their viscosity was measured at the freezing point, demonstrating that this new sensor can provide innovative means for the study of ice rheology in situ. The compact dimensions of this sensor and the high precision of the measurements make it an ideal tool for in situ mechanical characterisation of biotic and geological samples in alien worlds
COMPASS Final Report: Low Cost Robotic Lunar Lander
No full textThe COllaborative Modeling for the Parametric Assessment of Space Systems (COMPASS) team designed a robotic lunar Lander to deliver an unspecified payload (greater than zero) to the lunar surface for the lowest cost in this 2006 design study. The purpose of the low cost lunar lander design was to investigate how much payload can an inexpensive chemical or Electric Propulsion (EP) system deliver to the Moon s surface. The spacecraft designed as the baseline out of this study was a solar powered robotic lander, launched on a Minotaur V launch vehicle on a direct injection trajectory to the lunar surface. A Star 27 solid rocket motor does lunar capture and performs 88 percent of the descent burn. The Robotic Lunar Lander soft-lands using a hydrazine propulsion system to perform the last 10% of the landing maneuver, leaving the descent at a near zero, but not exactly zero, terminal velocity. This low-cost robotic lander delivers 10 kg of science payload instruments to the lunar surface
The Beagle 2 stereo camera system
No full textThe stereo camera system (SCS) was designed to provide wide-angle multi-spectral stereo imaging of the Beagle 2 landing site. Based on the Space-X micro-cameras, the primary objective was to construct a digital elevation model of the area in reach of the lander's robot arm. The SCS technical specifications and scientific objectives are described; these included panoramic 3-colour imaging to characterise the landing site; multi-spectral imaging to study the mineralogy of rocks and soils beyond the reach of the arm and solar observations to measure water vapour absorption and the atmospheric dust optical density. Also envisaged were stellar observations to determine the lander location and orientation, multi-spectral observations of Phobos & Deimos and observations of the landing site to monitor temporal changes
Rosetta lander Philae - soil strength analysis
No full textThe landing of Philae, the lander of ESA’s ROSETTA-mission, on November 12th 2014 on Comet 67P/Churyumov-Gerasimenko, was planned as a descent with passive landing activating a damper system and anchoring by harpoons at touch-down. The lander was not fixed to the ground at touch-down due to failing harpoons. The lander damper, however, was actuated for a length of 42.6 mm with a maximal speed of 0.08 m/s, while the lander speed was 1 m/s. Based on the damper data and a detailed mechanical model of Philae, an estimate can be made for the forces acting and the energy dissipated at touch-down inside the lander and the energy dissipated by ground penetration. The forces acting at ground penetration provide constraints on the mechanical strength of the soil. Two different soil models are investigated. Assuming constant compressive strength σ, one obtains σ ≈ 2 kPa. Assuming an increasing σs strength with penetration depth with results in σs = 3kPa/m fits the damper data best
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