1,721,598 research outputs found
LISA Pathfinder: First steps to observing gravitational waves from space
No full textLISA Pathfinder, the European Space Agency's technology demonstrator mission for future spaceborne gravitational wave observatories, was launched on 3 December 2015, from the European space port of Kourou, French Guiana. After a short duration transfer to the final science orbit, the mission has been gathering science data since. This data has allowed the science community to validate the critical technologies and measurement principle for low frequency gravitational wave detection and thereby confirming the readiness to start the next generation gravitational wave observatories, such as LISA.This paper will briefly describe the mission, followed by a description of the science operations highlighting the performance achieved.Details of the various experiments performed during the nominal science operations phase can be found in accompanying papers in this volume
Automatic laser beam alignment for a Thomson scattering system
No full textA system for the automatic alignment of a pulsed ruby laser beam is under development for the Thomson scattering diagnostics of RFX, a large reversed-field pinch machine now under construction. In this experiment the laser will be 11 m from the machine and the beam alignment at the 750×4-mm scattering volume will be actively maintained to within 0.5 mm. The beam direction in space is measured in two reference planes fixed to the collection optics frame by means of two 80-mm2 fast quadrant photodiodes. A double-channel preamplifier is used for each quadrant in order to measure both the 30-ns FWHM ruby pulse and a low-power cw He-Ne beam propagating through the same optical path. Every time the main laser is fired, the relative direction of the two beams is determined so that the position of the He-Ne beam can be used for feedback control of the steering optics between plasma shots
LISA pathfinder: first step toward a gravitational wave space observatory
No full textWe briefly review the concept of a space-based gravitational wave interferome- ter, and the science it can explore in the milliHertz frequency region. Then we discuss the LISA Pathfinder technology demostrator mission that is currently flying and will soon deliver the first results
Precessional effects on the LISA “constellation”
No full textWe investigate with a simple analytical approach some effects on the orbits of LISA due to the corrections of the gravitational potential of the Sun. The general relativistic correction in the weak field limit, responsible for the classical relativistic perihelion advance, can be obtained by adding to the potential a term scaling as r3 and parametrized by the Sun Schwarzschild radius. Another interesting correction term is provided by the intrinsic quadrupole moment of the Sun. With the current values of the parameters, the effects produced by both these terms are probably within the detection capabilities of LISA. With the same approach can also be evaluated the much smaller effect of a global stationary field, associated to sources like the interplanetary dust or a local dark matter component
Apparatus for multipoint Thomson scattering measurements in the ETA BETA II Reverse Field Pinch experiment
No full textA multipoint Thomson scattering apparatus under development for the measurement of spatial Te and ne profiles in the ETA-BETA II reverse field pinch experiment is described. The apparatus is based upon the two-channel analysis method already adopted in the TFR tokamak. The 90°scattered light is collected simultaneously from seven spatial positions along a plasma diameter by means of high-efficiency, quartz, fiber-optic light guides. The detection system consists of a set of high aperture (f/2.4), plane grating, Littrow spectrometers developed to match the high throughput of the collection optics. These instruments employ photomultipliers with a GaAs photocathode (quantum efficiency approximately-equal-to 20%, λ=700 nm) and have a stray light rejection ratio of 3×10-4. Laboratory tests and preliminary measurements in ETA-BETA II have shown good sensitivity and a S/N ratio sufficient to diagnose plasmas with n e≥1013 cm-3
Gravity gradiometers for planetary geodesy: requirements and concept for a space instrument
Full text linkThe measurement of the gravitational field of Solar System bodies is becoming ever and ever crucial in the physical description of their composition, state and evolution. Indeed, many planetary processes at large scale are ruled by their internal structure, where surface
and tectonic features are mainly the result of heat exchanges from the interior to the surface. Gravity field measurements are one of the observational methods to investigate those processes and to place constraints on the structure of the planetary interiors and on the
formation and geologic evolution of a planet. The retrieval of the spherical harmonic coefficients used to describe the gravitational field of a body gives insights into e.g. its polar oblateness, moment of inertia and deviations from hydrostatic equilibrium. With geologic assumptions and other remote sensing data, significant geophysical parameters, related e.g. to crust and mantle density and thickness, core size and structure, mantle/core coupling can be obtained. These parameters are used in planetary models to address topics such as planets differentiation, thermal evolution, characteristics and composition of the interiors. Moreover, the internal structure can be further investigated (wherever possible)
through seismometers on the surface, exploiting the analysis of seismic waves travelling through the interior (as performed by Apollo missions EASEP and ALSEP packages and currently by Mars Insight).
Until now, the Radio-Tracking technique (RT), part of the Radio Science (RS) observations, jointly with POD (Precise Orbit Determination), has been de-facto the main technique for gathering this type of information. It has been implemented in several deep-space missions, such as Magellan (Venus), MRO (Mars), Cassini (Saturn), Messenger (Mercury), Juno (Jupiter), and, in the forthcoming future, BepiColombo (Mercury) and JUICE (Jupiter and its moons).
Concerning scientific targets of interest, it needs to be highlighted that gravity field models are available (section 2), besides the Earth and the Moon, just for few planetary bodies such as the terrestrial planets Mercury, Venus and Mars. However, often such models are restricted only to large spatial resolutions, about one or more hundreds of kilometers, not enough to understand the geophysical processes that have driven formation and evolution of those bodies. The accuracy of these models is good enough as well but just for the lower part of the gravity field spectra, where a sufficient signal-to-noise ratio is achieved.
Moreover, there is much more lack of data for the external planets, where only few gravity field parameters have been derived for some of the gaseous planets and their main moons.
Any improvement on those targets, with a special attention to Venus, Mars and Galilean moons, would be very helpful in understanding their interior and the geophysical and geological processes that operated on them.
To answer the need for higher space resolution and accuracy in planetary gravity fields, two different approaches can be pursued:
1. to improve the measurement performance of the instrumentation used for RS; in these experiments the gravity field to be studied is inferred by the orbit of a spacecraft (that can be considered a ‘proof mass’ falling in the overall external gravity field) and an accelerometer is used to measure the Non-Gravitational Perturbations (NGP) perturbing the spacecraft free-fall, i.e. its motion from a pure (in principle) geodesic of space-time. An improvement of the accelerometer performance and its integration within an enhanced tracking system used to measure the spacecraft position and velocity, are needed conditions to improve the performance of gravity field reconstruction.
2. to introduce innovative measurement concepts, allowing to overcome some of the bottlenecks of the current methods (non-continuous monitoring, field attenuation with the altitude, disturbances mitigation, etc. In a roadmap definition, one of the more promising is the gravity gradiometry technique, which would allow to directly sense the gravity field by measuring the gravity gradients, and not just indirectly, as for RT, through monitoring the spacecraft gravitational perturbations. Unlike the radio-tracking, spacebased gravity gradiometry has still to unfold its potentialities; indeed, the ESA’s GOCE mission is the first and unique till now that has flown a gravity gradiometer to explore Earth’s gravity in 2009-13. The planetary gradiometry still awaits achievements outside the Earth System.
Satellite gradiometry refers to the measurement of acceleration differences, ideally in all three spatial directions, between the test-masses of an ensemble of accelerometers inside one satellite. The differentiation of gravity accelerations allows to highlight small-scale surface and sub-surface features, making such a technique, differently wrt RT, inherently sensitive to medium and large degrees (i.e. high resolutions) of the spherical harmonic representation of the gravity field. Therefore, the use of gradiometry would allow to improve the gravity field knowledge by measuring medium and large degrees, filling the gap above depicted and fostering the investigation on the structure and evolution of the planets.
The activity of this PhD Thesis starts from the definition of the planetary gravity field state of the art and the identification of the needs of the scientific community to improve the planetary bodies knowledge. Based on this result, a selection of targets of interest will be operated. A
review of the gravity field measurement techniques will be carried out, identifying advantages and drawbacks, pointing out innovative techniques such as gradiometry. On the basis of these activities, a series of numerical simulations will be implemented to produce the time series of gradiometric signals foreseen in a set of case studies. The choice of the case studies will be based on the preliminary studies about the science needs. The main outcome will be a set of requirements to be matched by a typical gradiometric instrument/mission, aiming at fulfilling the scientific needs. An important requirement would be, for instance, the typical instrument sensitivity and spectral band, as well as the expected
acceleration or gravity gradient amplitude of a signal sensed with a reasonable signal-tonoise ratio. Different scenarios will be simulated on the basis of the science needs.
In chapter 2 the gravity field is faced from the theoretical point of view and a snapshot of the current understanding of gravity field of planetary bodies is carried out. At last, science needs are identified and planetary bodies of interest are selected.
In chapter 3 measurement techniques of the gravity field are described, focusing the attention on the gravitational gradiometry. Advantages and drawbacks are considered. Moreover, spaceborne, airborne and groundborne gradiometric instruments have been identified and analysed to identify the current state of the art.
In chapter 4 gravity mission needs are identified in terms of science and mission requirements. Afterwards, a matlab code developed to compute the gravity gradient signal expected in some case studies is described and evaluated. At last, analysis of ways to increase the sensitivity of gradiometers is carried out.
In chapter 5, based on analysis of previous chapters, an instrument concept is introduced and analysed to match the requirements identified. The basic performance are derived, discussed and compared to the signal that is expected to be measured according to the computation carried out with the matlab code. Future work foresees to further develop the concept and to further deep the analysis of the identified gradiometer configurations
Gravitational interferometers in Italy 1976: a first timid attempt. And a missed opportunity
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
