Publication archive

Publication archive

Huygens is the ESA-provided element of the joint NASA/ESA/ASI Cassini/Huygens mission to Saturn and its largest moon Titan. The spacecraft, delivered to the interface altitude of 1270 km above the surface by NASA/JPL, dived into the dense atmosphere of Titan on 14th January 2005 and landed on the surface after a nominal descent of 2.5 hours. The scientific and housekeeping data was continuously transmitted after the heat shield release to Cassini and relayed back to Earth in a later retransmission through the Deep Space Network. Probably the most challenging activity after launch was the identification and recovery from a design flaw in the communications system that, if not corrected, would have led to major loss of scientific data, accounting up to 80 - 90% of the complete dataset. It was February 2000 and the first in-flight test of the Probe relay link was executed. Although results confirmed the expected carrier level performance, unexpected behavior was observed at data-stream level: in particular, the receiver showed anomalous behavior when working at the mission Doppler.
Published: 02 May 2006
The Huygens probe entered into the dense atmosphere of Titan on 14th January 2005 and landed on the surface after a nominal descent of about 2.5 hours [1]. Huygens is the ESA-provided element of the joint NASA/ESA/ASI Cassini/Huygens mission. The probe was delivered to the interface altitude of 1270 km above the surface by NASA/JPL. The propagation and reconstruction of the trajectory from that point onwards is ESA?s responsibility. An important effort was devoted to the development of an algorithm that aimed at reconstructing the descent trajectory and attitude of Huygens from the scientific instruments and probe sensors measurements ([2], this issue). In order to test this algorithm, the Huygens Synthetic Data Set (HSDS), a simulated mission dataset, was prepared. In this paper we describe the philosophy of the approach for preparing the HSDS, the assumptions made and the limitations of the method. The different tools used for producing the simulated data set are described, mainly a 3 Degree-of-Freedom (DoF) entry and descent trajectory calculation, and 6 DoF entry trajectory and attitude simulator. We report how the scientific and engineering models were used to obtain the most realistic Huygens sensor data, and the latest updates leading to the final v3.1 on the 10th of January, just 4 days prior to the Huygens descent. The different parameters are described, with a special attention to the way the accelerations were generated.
Published: 02 May 2006
Huygens is ESA's main contribution to the joint NASA/ESA/ASI Cassini/Huygens mission to Saturn and its largest moon Titan. The Probe, delivered to the interface altitude of 1270 km above the surface by NASA/JPL Cassini orbiter, entered the dense atmosphere of Titan on 14th January 2005 and landed on the surface after a descent under parachute of slightly less than of 2.5 hours. Huygens continued to function after landing for more than 3 hours. Data was transmitted and successfully recovered by Cassini continuosly Although the Huygens attitude reconstruction based on the flight engineering parameters was not foreseen during the development phase (no gyros were included), a rough descent under parachute and indications of an anomaly in the probe spin direction make the engineering dataset valuable in the frame of the ADRS (Huygens Attitude Determination and Reconstruction Subgroup) as a complement to the scientific measurements. In addition, several scientific teams have a strong interest in understanding the orientation of the probe for interpreting their data, as DISR (Descent Imager and Spectral Radiometer) and HASI-PWA (Huygens Atmospheric Structure Instrument-Permeability, Wave and Altimetry). In this paper we describe the engineering parameters used for the Probe attitude reconstruction, namely (Clausen et al., 2002) the radio link AGC (Automatic Gain Control), RASU and CASU (Radial and Central Accelerometer Sensor Units) and RAU (Radar Altimeter Unit). We explain the methodology applied to indirectly infer from these sensors measurements the attitude information. We also discuss and present the reconstructed information related to attitude: spin rate and azimuthal position (during the atmospheric descent), and landing orientation. Tip and tilt implications are still being worked. Preliminary data on their behaviour is presented.ess clear but hints on their behavior may be inferred.
Published: 02 May 2006
GRB 040223 was observed by INTEGRAL and XMM-Newton. GRB 040223 has a peak flux of (1.6+/-0.13) x10^-8 ergs cm^-2 s^-1, a fluence of (4.4+/-0.4)x10^-7 ergs cm^-2 and a steep photon power law index of -2.3+/-0.2, in the energy range 20-200 keV. The steep spectrum implies it is an x- ray rich GRB with emission up to 200 keV and E_peak < 20 keV. If E_peak is < 10 keV, it would qualify as an x-ray flash with high energy emission. The x-ray data has a spectral index beta_x = -1.7+/-0.2, a temporal decay of t^(-0.75+/-0.25) and a large column density of (1.8 x 10^22) cm^-2. The luminosity-lag relationship was used to obtain a redshift (z = 0.3+/-0.06). The isotropic energy radiated in gamma-rays and x-ray luminosity after 10 hours are factors of 1000 and 100 less than classical GRBs. GRB 040223 is consistent with the extrapolation of the Amati relation into the region that includes XRF 030723 and XRF 020903.
Published: 02 May 2006
The determination of the metal abundances in the neutral interstellar medium (ISM) of dwarf star-forming galaxies is a key step in understanding their physical and chemical evolution. This type of investigation has been possible in the last 5 years thanks to FUSE. We will give a flavor of the issues involved by presenting the work that we are performing in this astrophysical field.
Published: 02 May 2006
The Cluster and Double Star satellites recently observed plasma density holes upstream of Earth's collisionless bow shock to apogee distances of ~19 and 13 Earth radii, respectively. A survey of 147 isolated density holes using 4 s time resolution data shows they have a mean duration of ~17.9±10.4 s, but holes as short as 4 s are observed. The average fractional density depletion (delta n/n) inside the holes is ~0.68±0.14. The upstream edge of density holes can have enhanced densities that are five or more times the solar wind density. Particle distributions show the steepened edge can behave like a shock. Multispacecraft analyses show the density holes move with the solar wind, can have an ion gyroradius scale, and could be expanding. A small normal electric field points outward. Similarly shaped magnetic holes accompany the density holes indicating strong coupling between fields and particles. The density holes are only observed with upstream particles, suggesting that backstreaming particles interacting with the solar wind are important.
Published: 23 May 2006
The Cluster and Double Star satellites recently observed plasma density holes upstream of Earth's collisionless bow shock to apogee distances of ~19 and 13 Earth radii, respectively. A survey of 147 isolated density holes using 4 s time resolution data shows they have a mean duration of ~17.9±10.4 s, but holes as short as 4 s are observed. The average fractional density depletion (delta n/n) inside the holes is ~0.68±0.14. The upstream edge of density holes can have enhanced densities that are five or more times the solar wind density. Particle distributions show the steepened edge can behave like a shock. Multispacecraft analyses show the density holes move with the solar wind, can have an ion gyroradius scale, and could be expanding. A small normal electric field points outward. Similarly shaped magnetic holes accompany the density holes indicating strong coupling between fields and particles. The density holes are only observed with upstream particles, suggesting that backstreaming particles interacting with the solar wind are important.
Published: 23 May 2006
2005 was a crucial year in the Agencys long-term development, with a great deal of time and effort inevitably devoted to preparing for the political and programmatic decisions to be taken at the ESA Council Meeting at Ministerial Level ... Those decisions by the Ministers put ESA on the correct trajectory, providing a long-term vision based on strategic guidelines, allowing us to prepare for the future, and addressing a consistent set of policies and programmes, aiming at increased efficiency and coherence within the Agency and in the European space sector.
Jean-Jacques Dordain, Director General
Published: 16 September 2006
Since its launch on 2 December 1995, SOHO has revolutionised our understanding of the Sun. It has provided the first images of structures and flows below the Sun's surface and of activity on the far side. SOHO has revealed the Sun's extremely dynamic atmosphere, provided evidence for the transfer of magnetic energy from the surface to the outer solar atmosphere, the corona, through a 'magnetic carpet', and identified the source regions of the fast solar wind. It has revolutionised our understanding of solarterrestrial relations and dramatically improved our space weather-forecasting by its continuous stream of images covering the atmosphere, extended corona and far side. The findings are documented in an impressive number of scientific publications: over 2500 papers in refereed journals since launch, representing the work of over 2300 individual scientists. At the same time, SOHO's easily accessible, spectacular data and fundamental scientific results have captured the imagination of the space science community and the general public alike. As a byproduct of the efforts to provide real-time data to the public, amateurs now dominate SOHO's discovery of over 1100 Sungrazing comets.
Published: 16 May 2006
At 07:17 UT (09:17 CEST) on 11 April 2006, the Venus Express spacecraft fired its main engine to enter orbit around Earth's sister planet, making ESA the first space agency to have vehicles orbiting the Moon, Mars and Venus at the same time. With this latest success, the Agency hasadded another celestial body to its range of targets in the Solar System. ESA is operating Mars Express around Mars and SMART-1 around the Moon, and is a partner on the Cassini orbiter circling Saturn. ESA also has the Rosetta spacecraft en route to Comet 67P/Churyumov-Gerasimenko.
Published: 16 May 2006
The Mission Summary of the Cornerstone Study Results: LISA.

The primary goal of the Laser Interferometer Space Antenna (LISA) mission is to detect and observe gravitational waves from massive black holes and galactic binaries in the frequency range 10-4 to 10-1 Hz. This low-frequency range is inaccessible to groundbased interferometers because of the unshieldable background of local gravitational noise, and because ground-based interferometers are limited in length to a few kilometres.

Published: 16 August 2000
The Cluster mission allows the study of the plasmasphere with four-point measurements, including its overall density distribution, plasmaspheric plumes close to the plasmapause, and density irregularities inside the plasmasphere. The purpose of this letter is to examine the geometry and orientation of the overall density structure and of the magnetic field. We present a typical Cluster plasmasphere crossing for which we compute the four-point spatial gradient of the electron density and the magnetic field strength, and we compare the direction of both gradients with the local field vector. We discuss the role of the gradient components along and transverse to field lines; transverse density gradients, in particular, are found to suggest the presence of azimuthal density variations.
Published: 27 April 2006
Global mineralogical mapping of Mars by the Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activité (OMEGA) instrument on the European Space Agency's Mars Express spacecraft provides new information on Mars' geological and climatic history. Phyllosilicates formed by aqueous alteration very early in the planet's history (the "phyllocian" era) are found in the oldest terrains; sulfates were formed in a second era (the "theiikian" era) in an acidic environment. Beginning about 3.5 billion years ago, the last era (the "siderikian") is dominated by the formation of anhydrous ferric oxides in a slow superficial weathering, without liquid water playing a major role across the planet.
Published: 22 April 2006
The X-Ray Observatory, also known as XEUS (X-Ray Evolving-Universe Spectroscopy), is one of the potential future missions identified in the framework of the ESA Call for Themes issued in April 2004. Preliminary studies on a post XMM-Newton mission assumed a LEO scenario, with two spacecraft in formation flying, 5 m² (at 1 keV) effective area mirror and a focal length of 35 m. The mirror optics was originally based on the same technology used for XMM (replicated nickel mirrors), while the mission scenario was assuming a multiple launch approach and the use the ISS as servicing post for the observatory.
Published: 01 April 2006
S-Cam 3 is the 3rd generation of a cryogenic camera, based on superconducting tunnel junctions (STJs), for ground-based optical astronomy, deployed at the 4.2m William Herschel Telescope (WHT) at La Palma (Spain). It exploits a 10x12 pixel array of Ta/Al STJs, covering a field of view of ~9°x11° on the sky. The wavelength band extends from 330-750nm, with a wavelength resolving power of ~10 at 500nm. The detectors are operated at ~285mK, achieved with a double stage 4He-³He sorption cooler. Each pixel has its own electronic readout chain at room temperature, with a JFET-based charge sensitive preamplifier. The instrument has undergone extensive testing and calibration, followed by the first observation campaign at La Palma in July 2004. This campaign has focused on point sources with time variability, exploiting the instrument's unique combination of spectro-photometry with high time resolution.
Published: 16 April 2006
To overcome the limited field of view which can be achieved with single STJ arrays, DROIDs (Distributed Read Out Imaging Devices) are being developed. DROIDs consist of a superconducting absorber strip with proximized STJs on either end. The ratio of the two signals from the STJs provides information on the absorption position and the sum signal is a measure for the energy of the absorbed photon. In our devices the absorber is an epitaxial Ta strip that extends underneath the Ta/Al read-out STJs. Thus, the bottom electrode of the STJs is an integral part of the absorber. Due to the proximity effect, the STJs have a lower energy gap than the absorber, causing trapping of quasiparticles in the STJs. The trapping will change with thicker Al layers because the energy gap of the devices will decrease. A series of 50x200µm and 20x200µm absorbers (including 50x50µm STJs) and different Al trapping layer thicknesses, ranging from 65 to 130nm, have been tested. The devices have been illuminated with 6 keV 55Fe photons. The position resolution is found to improve with increasing Al thickness. It is found that the current model needs to be adapted for DROIDs to account for different injection of quasiparticles into the STJ and extra losses to the absorber.
Published: 16 April 2006
To overcome the limited field of view that can be achieved with single STJ arrays, DROIDS (Distributed Read Out Imaging Devices) are being developed. DROIDs consist of a superconducting absorber strip with proximized STJs on either end. The ratio of the two signals from the STJs provides information on the absorption position and the sum signal is a measure for the energy of the absorbed photon. To produce a large field of view with the least number of connection wires possible, the size of the DROID is an important parameter. A set of devices with different lengths, ranging from 200 to 700µm, has been tested at optical wavelengths. The widths of the DROIDs are 30µm with 30x30µm STJs Ta/Al STJs on either side. With 30nm layer thickness of Al the trapping of quasiparticles in the STJ is not optimal, but the devices can comfortably be operated at 300mK. All devices have been processed on a single wafer and are located on the same chip. Thus the STJs are all identical and any variation in response can be attributed to a difference in geometry. The position resolution is found to be degraded for shorter absorbers due to cross talk between the two STJs. The charge output of the different devices decreases with length due to reduced tunnel probability and losses in the absorber.
Published: 16 April 2006
In a quest to further improve the performance of superconducting tunnel junctions as photon detectors over the broad spectral range from optical to X-ray wavelengths, a fundamental understanding of their limits is required. Recently fabricated Ta/Al STJs have shown an exceptionally high responsivity (number of collected charge carriers versus absorbed photon energy) and spectral resolution (R = E/Delta_E > 22 at 2.5eV). This high spectral resolution has now revealed some unique features when plotted against photon wavelength. The experimental data indicate the important role of the photon absorption profile. We have shown that vertical inhomogeneity is a fundamental consequence of the quasiparticle generation process in the thin film such that pair breaking phonons emitted in the process of energy down conversion have a chance to escape depending on the absorption depth. This results in an inhomogeneous broadening of the detected signal. We also found that another, previously unknown fundamental noise source exists which is related to statistical fluctuations of the angular distribution of phonons emitted in the down-conversion process. We present the new experimental data and compare them to the predictions of the down-conversion theory. We show that, while the responsivity is rather constant in the optical wavelength range, the intrinsic resolution exhibits a number of features which can be explained by changing statistical variations of the phonon losses as function of absorption depth.
Published: 16 April 2006
We present here the results of a deep (130 ks) XMM-Newton observation of the cluster of galaxies 2A 0335+096. The deep exposure allows us to study in detail its temperature structure and its elemental abundances. We fit three different thermal models and find that the multi-temperature wdem model fits our data best. We find that the abundance structure of the cluster is consistent with a scenario where the relative number of type Ia supernovae contributing to the enrichment of the intra-cluster medium is ~25%, while the relative number of core collapse supernovae is ~75%. Comparison of the observed abundances to the supernova yields does not allow us to put any constrains on the contribution of Pop III stars to the enrichment of the ICM. Radial abundance profiles show a strong central peak of both type Ia and core collapse supernova products. Both the temperature and iron abundance maps show an asymmetry in the direction of the elongated morphology of the surface brightness. In particular the temperature map shows a sharp change over a brightness edge on the southern side of the core, which was identified as a cold front in the Chandra data. This suggests that the cluster is in the process of a merger with a subcluster. Moreover, we find that the blobs or filaments discovered in the core of the cluster by Chandra are, contrary to the previous results, colder than the ambient gas and they appear to be in pressure equilibrium with their environment.
Published: 16 April 2006
As part of the first far-IR line survey toward Orion KL, we present the detection of seven new rotationally excited OH Lambda-doublets (at ~48, ~65, ~71, ~79, ~98, and ~115 mum). Observations were performed with the Long Wavelength Spectrometer Fabry-Pérot on board the Infrared Space Observatory. In total, more than 20 resolved OH rotational lines, with upper energy levels up to ~620 K, have been detected at angular and velocity resolutions of ~80" and ~33 km/s, respectively. The OH line profiles show a complex behavior evolving from pure absorption, P Cygni type, to pure emission. We also present a large-scale, 6' declination raster in the OH ²Pi3/2 J=5/2+-3/2- and ²Pi3/2 J=7/2--5/2+ lines (at 119.441 and 84.597 mum) revealing a decrease of excitation outside the core of the cloud. From the observed profiles, mean intrinsic line widths, and velocity offsets between emission and absorption line peaks, we conclude that most of the excited OH arises from Orion outflow(s), that is, the "plateau" spectral component. We determine an averaged OH abundance relative to H2of chi(OH)=(0.5-1.0)×10-6, a kinetic temperature of more than ~100 K, and a density of n(H2)~=5×105 cm-3. Even with these conditions, the OH excitation is heavily coupled with the strong dust continuum emission from the inner "hot core" regions and from the expanding flow itself. Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands, and the UK) and with the participation of ISAS and NASA.
Published: 16 April 2006
23-Sep-2020 07:30 UT

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