ESA Science & Technology - Publication Archive

Available online 23 August 2012

While landing on Titan, several instruments onboard Huygens acquired measurements that indicate the probe did not immediately come to rest. Detailed knowledge of the probe's motion can provide insight into the nature of Titan's surface. Combining accelerometer data from the Huygens Atmospheric Structure Instrument (HASI) and the Surface Science Package (SSP) with photometry data from the Descent Imager/Spectral Radiometer (DISR) we develop a quantitative model to describe motion of the probe, and its interaction with the surface. The most likely scenario is the following. Upon impact, Huygens created a 12 cm deep hole in the surface of Titan. It bounced back, out of the hole onto the flat surface, after which it commenced a 30-40 cm long slide in the southward direction. The slide ended with the probe out of balance, tilted in the direction of DISR by around 10°. The probe then wobbled back and forth five times in the north-south direction, during which it probably encountered a 1-2 cm sized pebble. The SSP provides evidence for movement up to 10 s after impact. This scenario puts the following constraints on the physical properties of the surface ... [Abstract abbreviated due to character limitations.]

In the last few years Cassini-VIMS, the Visible and Infrared Mapping Spectrometer, returned to us a comprehensive view of the Saturn's icy satellites and rings. After having analyzed the satellites' spectral properties (Filacchione, G., Capaccioni, F., McCord, T.B., Coradini, A., Cerroni, P., Bellucci, G., Tosi, F., D'Aversa, E., Formisano, V., Brown, R.H., Baines, K.H., Bibring, J.P., Buratti, B.J., Clark, R.N., Combes, M., Cruikshank, D.P., Drossart, P., Jaumann, R., Langevin, Y., Matson, D.L., Mennella, V., Nelson, R.M., Nicholson, P.D., Sicardy, B., Sotin, C., Hansen, G., Hibbitts, K., Showalter, M., Newman, S. [2007]. Icarus 186, 259-290, paper I) and their distribution across the satellites' hemispheres (Filacchione, G., Capaccioni, F., Clark, R.N., Cuzzi, J.N., Cruikshank, D.P., Coradini, A., Cerroni, P., Nicholson, P.D., McCord, T.B., Brown, R.H., Buratti, B.J., Tosi, F., Nelson, R.M., Jaumann, R., Stephan, K. [2010]. Icarus 206, 507-523, paper II), we proceed in this paper to investigate the radial variability of icy satellites (principal and minor) and main rings average spectral properties. This analysis is done by using 2264 disk-integrated observations of the satellites and a 12×700 pixels-wide rings radial mosaic acquired with a spatial resolution of about 125 km/pixel. Using different VIS and IR spectral indicators, e.g. spectral slopes and band depths, we perform a comparative analysis of these data aimed to measure the distribution of water ice and red contaminant materials across Saturn's system. The average surface regolith grain sizes are estimated with different indicators through comparison with laboratory and synthetic spectra. [Abstract abbreviated due to character limitations.]

This paper describes Herschel observations of the nearby (8.5 pc) G5V multi-exoplanet host star 61 Vir at 70, 100, 160, 250, 350 and 500 micron carried out as part of the DEBRIS survey. These observations reveal emission that is significantly extended out to a distance of >15 arcsec with a morphology that can be fitted by a nearly edge-on (77° inclination) radially broad (from 30 AU out to at least 100 AU) debris disc of fractional luminosity 2.7 × 10^-5, with two additional (presumably unrelated) sources nearby that become more prominent at longer wavelengths. Chance alignment with a background object seen at 1.4 GHz provides potential for confusion, however, the star's 1.4 arcsec/yr proper motion allows archival Spitzer 70 m images to confirm that what we are interpreting as disc emission really is circumstellar. Although the exact shape of the disc's inner edge is not well constrained, the region inside 30 AU must be significantly depleted in planetesimals. This is readily explained if there are additional planets outside those already known (i.e. in the 0.5-30 AU region), but is also consistent with collisional erosion. We also find tentative evidence that the presence of detectable debris around nearby stars correlates with the presence of the lowest mass planets that are detectable in current radial velocity surveys. Out of an unbiased sample of the nearest 60 G stars, 11 are known to have planets, of which six (including 61 Vir) have planets that are all less massive than Saturn, and four of these have evidence for debris. The debris towards one of these planet hosts (HD 20794) is reported here for the first time ... [Abstract abbreviated due to character limitations.]

Lower hybrid drift waves (LHDW) are commonly observed at plasma boundaries in space and laboratory, often having the strongest measured electric fields within these regions. We use data from two of the Cluster satellites (C3 and C4) located in the Earth's magnetotail and separated by a distance of the order of the electron gyroscale. These conditions allow us, for the first time, to make cross-spacecraft correlations of the LHDW and to determine the phase velocity and wavelength of the LHDW. Our results are in good agreement with the theoretical prediction. We show that the electrostatic potential of LHDW is linearly related to fluctuations in the magnetic field magnitude, which allows us to determine the velocity vector through the relation: Integral_of delta_E dt v = phi_delta_B_par . The electrostatic potential fluctuations corresponds to ~10 per cent of the electron temperature, which suggests that the waves can strongly affect the electron dynamics.

Published online on 31 July 2012.

Coronal radio-sounding experiments were carried out using the S-band (2.3 GHz) and X-band (8.4 GHz) signals of the ESA Mars Express, Venus Express, and Rosetta spacecraft during five superior conjunctions occurring in 2004, 2006 (3×), and 2008/2009. Differential frequency and propagation delay (ranging) observations were recorded during these opportunities over the better part of a solar cycle, yielding information on the large-scale structure of the coronal electron-density distribution and its variations, including fluctuations on time scales from seconds to hours. These results concern primarily regions of slow solar wind because the radio propagation path is generally confined to the low heliolatitude regions by the conjunction. The mean frequency fluctuation and total electron content are determined as a function of heliocentric distance, and, with a few exceptions caused by streamers and CMEs, are found to be consistent with previous results from experiments on Ulysses. Dense coronal streamers and several coronal mass ejection (CME) events were identified in the radio-frequency data, some of which were observed in white light by the LASCO coronagraphs onboard SOHO. For those events with sufficient mutual coverage, good correlations are found between the electron-content fluctuations and structure imaged by the LASCO instrument.

Stars form with gaseous and dusty circumstellar envelopes, which rapidly settle into disks that eventually give rise to planetary systems. Understanding the process by which these disks evolve is paramount in developing an accurate theory of planet formation that can account for the variety of planetary systems discovered so far. The formation of Earth-like planets through collisional accumulation of rocky objects within a disk has mainly been explored in theoretical and computational work in which post-collision ejecta evolution typically is ignored although recent work has considered the fate of such material. Here we report observations of a young, Sun-like star (TYC 8241 2652 1) where infrared flux from post-collisional ejecta has decreased drastically, by a factor of about 30, over a period of less than two years. The star seems to have gone from hosting substantial quantities of dusty ejecta, in a region analogous to where the rocky planets orbit in the Solar System, to retaining at most a meagre amount of cooler dust. Such a phase of rapid ejecta evolution has not been previously predicted or observed, and no currently available physical model satisfactorily explains the observations.

We present further results from the ongoing XMM-Newton validation follow-up of Planck cluster candidates, detailing X-ray observations of eleven candidates detected at a signal-to-noise ratio of 4.5 < S/N < 5.3 in the same 10-month survey maps used in the construction of the Early SZ sample. The sample was selected in order to test internal SZ quality flags, and the pertinence of these flags is discussed in light of the validation results. Ten of the candidates are found to be bona fide clusters lying below the RASS flux limit. Redshift estimates are available for all confirmed systems via X-ray Fe-line spectroscopy. They lie in the redshift range 0.19 < z < 0.94, demonstrating Planck’s capability to detect clusters up to high z. The X-ray properties of the new clusters appear to be similar to previous new detections by Planck at lower z and higher SZ flux: the majority are X-ray underluminous for their mass, estimated using Y_X as mass proxy, and many have a disturbed morphology. We find tentative indication for Malmquist bias in the Y_SZ–Y_X relation, with a turnover at Y_SZ ~ 4 × 10^-4 arcmin². We present additional new optical redshift determinations with ENO and ESO telescopes of candidates previously confirmed with XMM-Newton. The X-ray and optical redshifts for a total of 20 clusters are found to be in excellent agreement. We also show that useful lower limits can be put on cluster redshifts using X-ray data only via the use of the Y_X vs. Y_SZ and X-ray flux F_X vs. Y_SZ relations.

SOIR is a high-resolution spectrometer flying on board the ESA Venus Express mission. It performs solar occultations of the Venus high atmosphere, and so defines unique vertical profiles of many of the Venus key species. In this paper, we focus on the Venus main constituent, carbon dioxide. We explain how the temperature, the total density, and the total pressure are derived from the observed CO₂ density vertical profiles. A striking permanent temperature minimum at 125 km is observed. The data set is processed in order to obtain a Venus Atmosphere from SOIR measurements at the Terminator (VAST) compilation for different latitude regions and extending from 70 up to 170 km in altitude. The results are compared to many literature results obtained from ground-based observations, previous missions, and the Venus Express mission. The homopause altitude is also determined.

We report a periodicity of ~1 day in the highly elevated X-ray emission from the protostar V1647 Ori during its two recent multiple-year outbursts of mass accretion. This periodicity is indicative of protostellar rotation at near-break-up speed. Modeling of the phased X-ray light curve indicates that the high-temperature (~50 MK), X-ray-emitting plasma, which is most likely heated by accretion-induced magnetic reconnection, resides in dense ( >~5 × 10¹⁰ cm^-3), pancake-shaped magnetic footprints where the accretion stream feeds the newborn star. The sustained X-ray periodicity of V1647 Ori demonstrates that such protostellar magnetospheric accretion configurations can be stable over timescales of years.

We present new constraints on the star formation histories of the ultra-faint dwarf (UFD) galaxies, using deep photometry obtained with the Hubble Space Telescope (HST). A galaxy class recently discovered in the Sloan Digital Sky Survey, the UFDs appear to be an extension of the classical dwarf spheroidals to low luminosities, offering a new front in efforts to understand the missing satellite problem. They are the least luminous, most dark-matter-dominated, and least chemically evolved galaxies known. Our HST survey of six UFDs seeks to determine if these galaxies are true fossils from the early universe. We present here the preliminary analysis of three UFD galaxies: Hercules, Leo IV, and Ursa Major I. Classical dwarf spheroidals of the Local Group exhibit extended star formation histories, but these three Milky Way satellites are at least as old as the ancient globular cluster M92, with no evidence for intermediate-age populations. Their ages also appear to be synchronized to within ~1 Gyr of each other, as might be expected if their star formation was truncated by a global event, such as reionization.

The loss of relativistic electrons from the Earth's radiation belts can be described in terms of the quasi-linear pitch angle diffusion by cyclotron-resonant waves, provided that their frequency spectrum is broad enough. Chorus waves at large wave-normal angles with respect to the magnetic field are often present in CLUSTER and THEMIS measurements in the outer belt at moderate to high latitudes. An approximate analytical formulation of diffusion coefficients has been derived in the low-frequency limit, leading to a simplified analytical expression of diffusion coefficients and lifetimes for energetic trapped electrons. Large values of the wave-normal angles between the Gendrin and resonance angles are shown to induce important increases in diffusion, thereby strongly reducing the particle lifetimes (by almost two orders of magnitude). The analytical diffusion coefficients and lifetimes obtained here are found to be in a good agreement with full numerical calculations based on CLUSTER chorus waves measurements in the outer belt for electron energies ranging from 100 keV to 2 MeV. Such very oblique chorus waves could contribute to a predominantly perpendicular anisotropy of the global equatorial electron population on the dayside and to a relative isotropization at low energy under disturbed conditions. It is also suggested that they might play a significant role in pulsating auroras.

The 39th meeting of the Committee on Space Research (COSPAR) was held 14-22 July 2012 in Mysore, India.

This report to COSPAR on the scientific activities of the European Space Agency was written by members of the Directorate of Earth Observation, the Directorate of Human Spaceflight and Operations and the Directorate of Science and Robotic Exploration.

Contents:

• Foreword by Jean-Jacques Dordain, Director General, ESA
• Earth Observation
  1. Introduction
  2. The Living Planet Programme
  3. The Earth Explorer Missions
  4. ERS and Envisat
• Human Spaceflight and Operations
  1. Introduction
  2. Overview: Columbus and ISS Facilities
  3. Funding Europe's ISS Research: ELIPS
  4. Research on the ISS
  5. Ongoing Research Using Other Platforms
  6. Projects under Development
• Science and Robotic Exploration
  1. Introduction
  2. Missions in Operation
  3. Missions in the Post-Operations and Archiving Phases
  4. Projects under Development
  5. Missions under Study

Published online on 28 June 2012

We have detected in Cassini data the signature of the periodic tidal stresses within Titan driven by the eccentricity (e = 0.028) of its 16-day orbit around Saturn. Precise measurements of the acceleration of the Cassini spacecraft during six close flybys between 2006 and 2011 have revealed that Titan responds to the variable tidal field exerted by Saturn with periodic changes of its quadrupole gravity, at about 4% of the static value. Two independent determinations of the corresponding degree-2 Love number yield k₂ = 0.589 ± 0.150 and k₂ = 0.637 ± 0.224 (2-sigma). Such a large response to the tidal field requires that Titan's interior is deformable over time scales of the orbital period, in a way that is consistent with a global ocean at depth.

Reference: SRE-PA/2011-075/TN/PW

This document describes the Payload of STE-QUEST, a mission in the Fundamental Physics domain conceived to test to high accuracy the different aspects of the Einstein Equivalence Principle. This document derives directly from the STE-QUEST Science Requirements Document and STE-QUEST Mission Requirements Document. It also integrates elements included in the STE-QUEST proposal (and associated reference documents), the STE-QUEST CDF Study and the STE-QUEST Instruments Mid-Term Review datapackages.

This document provides the baseline description of the payload, its instruments, supporting units and interfaces, as defined at the beginning of the STE-QUEST assessment study. It shall be consolidated as part of the study and shall be formally updated at the end of it. This is the first revision of the document, following the completion and closure of the Instruments mid-term Review held in May 2011.

Reference: ESA/SPC(2012)20, rev. 1

JUpiter ICy moons Explorer (JUICE) is an ESA-led L-class mission of the ESA's Cosmic Vision 2015-25 Programme. It aims at a comprehensive exploration of the Jovian system with particular emphasis on Jupiter, its environment, and Galilean moons Ganymede, Europa and Callisto by investigating them as planetary bodies and potential habitats.

The Science Management Plan (SMP) describes the approach that will be implemented to ensure the fulfilment of the scientific objectives of the JUICE mission and to optimise its scientific return, with special emphasis on payload procurement, science operation and data management.

Published online 16 June 2012, in Online First

The Earth's bow shock is the most studied example of a collisionless shock in the solar system. It is also widely used to model or predict the behaviour at other astrophysical shock systems. Spacecraft observations, theoretical modelling and numerical simulations have led to a detailed understanding of the bow shock structure, the spatial organization of the components making up the shock interaction system, as well as fundamental shock processes such as particle heating and acceleration. In this paper we review the observations of accelerated ions at and upstream of the terrestrial bow shock and discuss the models and theories used to explain them. We describe the global morphology of the quasi-perpendicular and quasi-parallel shock regions and the foreshock. The acceleration processes for field-aligned beams and diffuse ion distribution types are discussed with connection to foreshock morphology and shock structure. The different possible mechanisms for extracting solar wind ions into the acceleration processes are also described. Despite several decades of study, there still remain some unsolved problems concerning ion acceleration at the bow shock, and we summarize these challenges.

Cataloguing the night sky is an essential part of astronomy. Before astronomers can investigate a celestial object, they must know where to find it. Without this knowledge, astronomers would wander helplessly in what Galileo once termed a 'dark labyrinth'.

ESA's Gaia mission will create a detailed map of this labyrinth, finding clues to the origin, structure and evolution of our home galaxy, the Milky Way.

Contents:

• The discovery machine
• Stars as individuals and collectives
• Our Solar System and others
• How does Gaia work?
• Building Gaia
• Launch
• The flood of data

An analytical model of magnetosheath plasma flow is described and compared with a large dataset of magnetosheath ion flow velocity measurements from Cluster and THEMIS spacecraft. The model is based on previous works by Kobel and Flückiger (1994) and Génot et al. (2011) and has been modified to overcome the restrictions of these models on the shape of model magnetopause and bow shock. Our model is compatible with any parabolic bow shock model and arbitrary magnetopause model. The model is relatively simple to implement and computationally inexpensive, and its only inputs are upstream solar wind parameters. Comparison with observed data yields a good correspondence: median error in the direction of flow velocity is comparable with the instrumental error, and flow magnitude is predicted with a reasonable accuracy (relative error in flow speed was less than 25% for 86.5% of observations).

The Milky Way galaxy has several components, such as the bulge, disk and halo. Unravelling the assembly history of these stellar populations is often restricted because of difficulties in measuring accurate ages for low-mass, hydrogen-burning stars. Unlike these progenitors, white dwarf stars, the 'cinders' of stellar evolution, are remarkably simple objects and their fundamental properties can be measured with little ambiguity. Here I report observations of newly formed white dwarf stars in the halo of the Milky Way, and a separate analysis of archival data in the well studied 12.5-billion-year-old globular cluster Messier 4. I measure the mass distribution of the remnant stars and invert the stellar evolution process to develop a mathematical relation that links this final stellar mass to the mass of their immediate progenitors, and therefore to the age of the parent population. By applying this technique to a small sample of four nearby and kinematically confirmed halo white dwarf stars, I calculate the age of local field halo stars to be 11.4 ± 0.7 billion years. The oldest globular clusters formed 13.5 billion years ago. Future observations of newly formed white dwarf stars in the halo could be used to reduce the uncertainty, and to probe relative differences between the formation times of the youngest globular clusters and the inner halo.

Published online on 1 October 2011. To appear in a Special Issue of the journal, in press. We have analyzed Cluster magnetic field and plasma data during a high-altitude cusp crossing in 2003. The Cluster separation was ~5000 km and provided unique measurements of high energy particle properties both inside the DiaMagnetic Cavity (DMC) and surrounding magnetosheath. Most of the high energy electrons and protons had pitch angles of ~90 degrees in the cavity and the high energy particle intensities dropped as a function of distance from the cavity boundary. By assuming conservation of the first adiabatic invariant for the electrons our analysis indicates that most of the high-energy electrons in the diamagnetic cavity cannot directly originate from the magnetosheath or from the magnetosphere. Our test particle simulations in a local 3-D high-resolution MHD cusp model show that particles can gain up to 40 keV and their pitch angles become nearly 90 degrees in the local cusp geometry due to gradients in reconnection 'quasi-potential' agreeing with the Cluster RAPID observations. These results strongly support a local acceleration of particles in the cusp diamagnetic cavities.