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12 items found page 1 of 1
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| Stacking of silicon pore optics for IXO |
Silicon pore optics is a technology developed to enable future large area X-ray telescopes, such as the International Xray Observatory (IXO), a candidate mission in the ESA Space Science Programme 'Cosmic Visions 2015-2025'. IXO uses nested mirrors in Wolter-I configuration to focus grazing incidence X-ray photons on a detector plane. The IXO mirrors will have to meet stringent performance requirements including an effective area of ~3 m2 at 1.25 keV and ~1 m2 at 6 keV and angular resolution better than 5 arc seconds. To achieve the collecting area requires a total polished mirror surface area of ~1300 m2 with a surface roughness better than 0.5 nm rms. By using commercial high-quality 12" silicon wafers which are diced, structured, wedged, coated, bent and stacked the stringent performance requirements of IXO can be attained without any costly polishing steps. Two of these stacks are then assembled into a co-aligned mirror module, which is a complete X-ray imaging system. Included in the mirror module are the isostatic mounting points, providing a reliable interface to the telescope. Hundreds of such mirror modules are finally integrated into petals, and mounted onto the spacecraft to form an X-ray optic of four meters in diameter. In this paper we will present the silicon pore optics assembly process and latest X-ray results. The required metrology is described in detail and experimental methods are shown, which allow to assess the quality of the HPOs during production and to predict the performance when measured in synchrotron radiation facilities. This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference. |
| Publication date: 31 Aug 2009 |
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| Pupil Alignment Reference (PAR) for the Mid-Infrared Instrument (MIRI) for Optical Alignment and Verification on the Integrated Science Instrument Module (ISIM) in James Webb Space Telescope (JWST) |
| In "Optical System Alignment, Tolerancing, and Verification III", edited by José Sasian, Richard N. Youngworth, Proc. of SPIE Vol. 7433, 74330P, (2009), doi: 10.1117/12.826286
The Mid Infrared Instrument (MIRI), one of the four instruments on the Integrated Science Instrument Module (ISIM) of the James Webb Space Telescope (JWST), supports all of the science objectives of the observatory. MIRI optical alignment is an important step in the verification process, directly affecting mission success. The MIRI optical alignment is verified on the ground at the integrated ISIM level using an element in the MIRI Filter Wheel, the pupil alignment reference (PAR), developed by NASA GSFC and provided to MIRI. It is a ~2.3g aluminum piece that has a flat, specularly reflective, 3mm diameter surface in its center, with laser-etched fiducials within its aperture. The PAR is illuminated via an optical stimulus (ground support equipment) and imaged using a pupil imaging camera, during the ISIM test program in order to determine absolute and relative changes in the alignment that impact pupil shear and roll. Here we describe the MIRI PAR; its physical properties and challenges during its design, manufacturing, and testing. |
| Publication date: 28 Aug 2009 |
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| JWST near infrared detectors: latest test results |
| Infrared Systems and Photoelectronic Technology IV. Edited by Dereniak, Eustace L.; Hartke, John P.; Levan, Paul D.; Longshore, Randolph E.; Sood, Ashok K. Proceedings of the SPIE, Volume 7419, pp. 741907-741907-10 (2009)
The James Webb Space Telescope, an infrared-optimized space telescope being developed by NASA for launch in 2014, will utilize cutting-edge detector technology in its investigation of fundamental questions in astrophysics. JWST's near infrared spectrograph, NIRSpec utilizes two 2048 × 2048 HdCdTe arrays with Sidecar ASIC readout electronics developed by Teledyne to provide spectral coverage from 0.6 microns to 5 microns. We present recent test and calibration results for the "pathfinder NIRSpec detector subsystem" as well as data processing routines for noise reduction and cosmic ray rejection. |
| Publication date: 27 Aug 2009 |
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| Phobos control point network, rotation, and shape |
| A new independent control point network for Phobos was computed from image data obtained by the SRC (Super Resolution Channel) on board the European Mars Express Mission. The network solution includes 3D coordinates of 665 surface control points and was used to observe the forced libration amplitude of Phobos. Based on the network control points a spherical harmonic function model to degree and order 17 was derived, from which volume, bulk density and moments of inertia were computed. The modeled forced libration amplitude agrees to our observation within the error bands, indicating a homogeneous mass distribution for Phobos. To bring both values into exact agreement with the observations, different mass distribution models were applied. It appears that the amplitude is relatively insensitive to a simple two-layer density model. |
| Publication date: 26 Aug 2009 |
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| The Earth's Plasmasphere - A Cluster and Image Perspective |
Darrouzet, F., De Keyser, J., Pierrard, V. (Eds.)
2009, IV, 296 pages, 100 illus., 60 in colour, Hardcover.
ISBN: 978-1-4419-1322-7 (Print), 978-1-4419-1323-4 (Online)
Reprinted from Space Science Reviews journal, Vol. 145/1-2, 2009.
This book reviews the state of the art in plasmaspheric science based on the modern observations provided by ESA's Cluster and NASA's IMAGE spacecraft. The plasmasphere, discovered at the beginning of the space age, has remained largely unexplored territory. Now, with innovative observational techniques, new light is being shed on this key region of the magnetosphere. This book sketches the emerging overall picture of a highly structured plasma, sculpted by the ever-changing electromagnetic fields that result from the interaction of the solar wind with the magnetosphere.
The Earth's Plasmasphere, written by an international group of scientists representative of the world-wide community, is aimed at researchers and graduate students with an interest in magnetospheric physics, space plasma physics and geophysics.
Table of Contents
Preface (J. L. Burch & C. P. Escoubet)
Foreword (F. Darrouzet, J. De Keyser, & V. Pierrard)
CLUSTER and IMAGE: New Ways to Study the Earth's Plasmasphere (J. De Keyser, et al.)
Plasmaspheric Density Structures and Dynamics: Properties Observed by the CLUSTER and IMAGE Missions
(F. Darrouzet, et al.)
Electric Fields and Magnetic Fields in the Plasmasphere: A Perspective from CLUSTER and IMAGE (H. Matsui, et al.)
Advances in Plasmaspheric Wave Research with CLUSTER and IMAGE Observations (A. Masson, et al.)
Recent Progress in Physics-Based Models of the Plasmasphere (V. Pierrard, et al.)
Augmented Empirical Models of Plasmaspheric Density and Electric Field Using IMAGE and CLUSTER Data (B. W. Reinisch, et al.)
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| Publication date: 21 Aug 2009 |
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| COSPAR report on Planetary Protection |
| The prospects for continued exploration and discovery in the Outer Planets of the Solar
System have never been better. Among others, the Rosetta, Dawn, and New Horizons
missions are enroute, the Cassini mission continues its comprehensive exploration of
the saturnian system, and the Juno mission is preparing for a 2011 launch to Jupiter.
Elsewhere, NASA and ESA are focusing on a flagship-class mission opportunity to
Europa and the rest of the Jovian system, followed by a mission back to Titan, and a
New Frontiers announcement of opportunity has been released by NASA to solicit
missions across a wide spectrum, including missions to the South Pole of the Earth's
Moon, Venus, the surface of a comet (and back again), Mars, Mercury, the Trojan or
Centaur asteroids in Jupiter's orbit, other asteroids (and back again), Io, or Ganymede.
New missions are also under way at ESA. |
| Publication date: 15 Aug 2009 |
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| How we map the first light - the science ground segment of Planck |
| For any space mission, the 'ground segment' is vital for operating a spacecraft and processing data received from its instruments. Planck is no different, with hardware software, telecommunications and other operations reaching from Spain to Australia. |
| Publication date: 15 Aug 2009 |
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| Global Scale-Invariant Dissipation in Collisionless Plasma Turbulence |
| A higher-order multiscale analysis of the dissipation range of collisionless plasma turbulence is presented using in situ high-frequency magnetic field measurements from the Cluster spacecraft in a stationary interval of fast ambient solar wind. The observations, spanning five decades in temporal scales, show a crossover from multifractal intermittent turbulence in the inertial range to non-Gaussian monoscaling in the dissipation range. This presents a strong observational constraint on theories of dissipation mechanisms in turbulent collisionless plasmas. |
| Publication date: 14 Aug 2009 |
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| Altimetry of the Venus cloud tops from the Venus Express observations |
| Simultaneous observations of Venus by Visible and Infrared Thermal Imaging Spectrometer and Venus Monitoring Camera onboard the Venus Express spacecraft are used to map the cloud top altitude and to relate it to the ultraviolet (UV) markings. The cloud top altitude is retrieved from the depth of CO2 absorption band at 1.6 microns. In low and middle latitudes the cloud top is located at 74 ± 1 km. It decreases poleward of ±50° and reaches 63-69 km in the polar regions. This depression coincides with the eye of the planetary vortex. At the same latitude and hour angle, cloud top can experience fast variations of about 1 km in tens of hours, while larger long-term variations of several kilometers have been observed only at high latitudes. UV markings correlate with the cloud altimetry, however, the difference between adjacent UV dark and bright regions does not exceed several hundred meters. Surprisingly, CO2 absorption bands are often weaker in the dark UV features, indicating that these clouds may be a few hundred meters higher or have a larger scale height than neighboring clouds. Ultraviolet dark spiral arms, which are often seen at about ~70°, correspond to higher altitudes or to the regions with strong latitudinal gradient of the cloud top altitude. Cloud altimetry in the polar region reveals the structure that correlates with the thermal emission maps but is invisible in UV images. This implies that the UV optically thick polar hood is transparent in the near IR. |
| Publication date: 13 Aug 2009 |
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| Storms in the tropics of Titan |
| Methane clouds, lakes and most fluvial features on Saturn's moon Titan have been observed in the moist high latitudes, while the tropics have been nearly devoid of convective clouds and have shown an abundance of wind-carved surface features like dunes. The presence of small-scale channels and dry riverbeds near the equator observed by the Huygens probe at latitudes thought incapable of supporting convection (and thus strong rain) has been suggested to be due to geological seepage or other mechanisms not related to precipitation. Here we report the presence of bright, transient, tropospheric clouds in tropical latitudes. We find that the initial pulse of cloud activity generated planetary waves that instigated cloud activity at other latitudes across Titan that had been cloud-free for at least several years. These observations show that convective pulses at one latitude can trigger short-term convection at other latitudes, even those not generally considered capable of supporting convection, and may also explain the presence of methane-carved rivers and channels near the Huygens landing site. |
| Publication date: 13 Aug 2009 |
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| A possible generation mechanism of interplanetary rotational discontinuities |
| In the present paper, we first examine some interplanetary directional discontinuities with very small Bn /B (<0.1) using intraspacecraft timing method. It is found that the velocity and magnetic field fluctuations of these directional discontinuities satisfy the Walén relation. We suggest that these directional discontinuities are rotational discontinuities. In addition, we investigate the stability of interplanetary rotational discontinuities using one-dimensional hybrid simulations and found that rotational discontinuities with all values of Bn /B can stably exist in the solar wind. In one simulation run, we find that the rotational discontinuity (RD) is still stable when the ratio, Bn /B, equals 0.0001. Finally, from one-dimensional hybrid simulation, we further find that the ratio is significantly reduced after interaction with interplanetary fast shocks. There are a few mechanisms for generation of RDs. Among them, two mechanisms are well accepted. One is nonlinear evolution of Alfvén waves in the solar wind, and another is magnetic reconnection near the solar surface. For magnetic reconnection, the reconnection rate, V1n /VA1(= Bn /B), in the magnetosphere and solar wind, is usually <0.2. Therefore the generated RDs also have Bn /B < 0.2. On the other hand, the nonlinear evolution of Alfvén waves in the solar wind can generate RDs at all values of Bn /B, which contradicts to the Cluster results. We suggest that interplanetary RDs with small Bn /B are likely been generated through magnetic reconnection. |
| Publication date: 11 Aug 2009 |
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| Observed variations of methane on Mars unexplained by known atmospheric chemistry and physics |
| The detection of methane on Mars has revived the possibility of past or extant life on this planet, despite the fact that an abiogenic origin is thought to be equally plausible. An intriguing aspect of the recent observations of methane on Mars is that methane concentrations appear to be locally enhanced and change with the seasons. However, methane has a photochemical lifetime of several centuries, and is therefore expected to have a spatially uniform distribution on the planet. Here we use a global climate model of Mars with coupled chemistry to examine the implications of the recently observed variations of Martian methane for our understanding of the chemistry of methane. We find that photochemistry as currently understood does not produce measurable variations in methane concentrations, even in the case of a current, local and episodic methane release. In contrast, we find that the condensation-sublimation cycle of Mars' carbon dioxide atmosphere can generate large-scale methane variations differing from those observed. In order to reproduce local methane enhancements similar to those recently reported,
we show that an atmospheric lifetime of less than 200 days is necessary, even if a local source of methane is only active around
the time of the observation itself. This implies an unidentified methane loss process that is 600 times faster than predicted by
standard photochemistry. The existence of such a fast loss in the Martian atmosphere is difficult to reconcile with the observed
distribution of other trace gas species. In the case of a destruction mechanism only active at the surface of Mars, destruction of
methane must occur with an even shorter timescale of the order of ~1 hour to explain the observations. If recent observations of spatial and temporal variations of methane are confirmed, this would suggest an extraordinarily harsh environment for the survival of organics on the planet. |
| Publication date: 06 Aug 2009 |
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