ESA Science & Technology - Publication Archive

This article is based on a talk given by Professor Rudolf von Steiger, Director of the International Space Science Institute, Bern.

The brightness of type Ia supernovae, and their homogeneity as a class, makes them powerful tools in cosmology, yet little is known about the progenitor systems of these explosions. They are thought to arise when a white dwarf accretes matter from a companion star, is compressed and undergoes a thermonuclear explosion. Unless the companion star is another white dwarf (in which case it should be destroyed by the mass-transfer process itself), it should survive and show distinguishing properties. Tycho's supernova is one of only two type Ia supernovae observed in our Galaxy, and so provides an opportunity to address observationally the identification of the surviving companion. Here we report a survey of the central region of its remnant, around the position of the explosion, which excludes red giants as the mass donor of the exploding white dwarf. We found a type G0-G2 star, similar to our Sun in surface temperature and luminosity (but lower surface gravity), moving at more than three times the mean velocity of the stars at that distance, which appears to be the surviving companion of the supernova.

We report the association of the recently discovered hard X-ray source IGR J17475-2822 with the giant molecular cloud Sgr B2 in the Galactic Center region. The broad band (3-200 keV) spectrum of the source constructed from data of different observatories strongly supports the idea that the X-ray emission of Sgr B2 is Compton scattered and reprocessed radiation emitted in the past by the Sgr A* source. We conclude that 300-400 years ago Sgr A* was a low luminosity (L~ 1.5×10³⁹ erg s^-1 at 2-200 keV) AGN with a characteristic hard X-ray spectrum (photon index Gamma ~ 1.8). We estimate the mass and iron abundance of the Sgr B2 scattering gas at 2×10⁶ M_Sun(r/10 pc)² and 1.9 solar, respectively (where r is the radius of the cloud).

Space plasmas present intriguing and challenging puzzles to the space community. Energy accessible to excite instabilities exists in a variety of forms, particularly for the magnetospheric environment prior to substorm expansion onsets. A general consensus of the pre-expansion magnetosphere is the development of a thin current sheet in the near-Earth magnetosphere. This review starts with a short account of the two major substorm paradigms. Highlights of some observations pertaining to the consideration of potential plasma instabilities for substorm expansion are given. Since a common thread of these paradigms is the development of a thin current sheet, several efforts to model analytically a thin current sheet configuration are described. This leads to a review on the instability analyses of several prominent candidates for the physical process responsible for substorm expansion onset. The potential instabilities expounded in this review include the cross-field current, lower-hybrid-drift, drift kink/sausage, current driven Alfvénic, Kelvin-Helmholtz, tearing, and entropy anti-diffusion instabilities. Some recent results from plasma simulations relevant to the investigation of these plasma instabilities are shown. Although some of these instabilities are generally conceived to be excited in spatially localized regions in the magnetosphere, their potentials in yielding global consequences are also explored.

A paper originally presented by EADS Astrium at the 55th IAC in Vancouver in October 2004.

On October 26^th of next year, Venus Express spacecraft will depart from Baikonur on-board the Soyuz/Fregat Launch Vehicle. It will be the very first European mission to the "morning star", two years after the first European trip to Mars. Venus Express will carry 7 science payloads dedicated to global investigation of the Venusian atmosphere.

The Solar Wind Anisotropies (SWAN) instrument is a scanning Lyman-alpha imager on board the Solar and Heliospheric Observatory (SOHO) spacecraft. Since becoming operational in January 1996 SWAN has been producing full sky Lyman-alpha maps which are primarily used to study the interaction between solar wind and the interplanetary neutral hydrogen. In addition to that SWAN images can be used to study the hydrogen coma of comets down to about a visual magnitude of 12. After the retargeting decision of the Rosetta mission the SWAN archive was checked for possible occurrences of 67P/Churyumov-Gerasimenko. Five values were obtained for the 1996 apparition but none for the 2002 apparition because of degraded instrument sensitivity and larger observing distance. The observations suggest a perihelion water production rate of about 8x10^27 per second and possible post perihelion increase of activity.

The objectives of the study were to perform a system conceptual design and trades, prepare a preliminary system design including budgets and subsystem designs with required performance, show science requirements compliance, define critical design issues requiring further analysis and assess and analyse programme, risk and costs. Further the constraints imposed by the chosen design were analysed and described, where appropriate. This document reports on the analysis performed and conclusions drawn for an X-ray Evolving Universe Spectroscopy conceptual design.

Since its launch on 2 December 1995, the joint ESA/NASA SOHO mission has provided a wealth of information about the Sun, from its interior, through the hot and dynamic atmosphere, to the solar wind and its interaction with the interstellar medium. Analysis of the helioseismology data from SOHO has provided the first images of structures and flows below the Sun's surface and has shed new light on a number of structural and dynamic phenomena in the solar interior, such as the absence of differential rotation in the radiative zone, subsurface zonal and meridional flows, and sub-convection-zone mixing. Evidence for an upward transfer of magnetic energy from the Sun's surface toward the corona has been established. The ultraviolet imagers and spectrometers have revealed an extremely dynamic solar atmosphere where plasma flows play an important role. Electrons in coronal holes were found to be relatively ``cool', whereas heavy ions are extremely hot and have highly anisotropic velocity distributions. The source regions for the high speed solar wind have been identified and the acceleration profiles of both the slow and fast solar wind have been measured. SOHO has also revolutionized our space weather forecasting capabilities by providing a continuous stream of images of the dynamic atmosphere, extended corona, and activity on the far side of the Sun. At the same time, SOHO's easily accessible images and movies have captured the imagination of the science community and the general public alike. This article summarizes some of the key findings from 8 years of SOHO.

We present X-ray spectra of the nucleus of the nearby radio galaxy Centaurus A from observations with the XMM-Newton EPIC CCD cameras (two exposures separated by 12 months) and the Chandra HETGS. For the first time in an FR I type galaxy, we resolve fluorescent K alpha emission from cold, neutral, or near-neutral iron at 6.4 keV, with an rms line width of ~20 eV. The Fe line parameters observed are consistent with fluorescent emission from material at a large distance from the active galactic nucleus, either in the form of an absorber that nearly completely surrounds the central engine or a torus that lies predominantly out of the line of sight. Unresolved emission lines from neutral Si K alpha at 1.74 keV and neutral S K alpha at 2.30 keV are also detected. We find no evidence in the data for a previously reported 6.8 keV broadened Fe line. The continuum spectrum is well fitted with a combination of a heavily absorbed power-law component that we relate, using Bondi theory, to accretion phenomena in the form of a standard, geometrically thin, optically thick disk, and a second, less absorbed, power-law component that we associate with emission from the subparsec VLBI radio jet.

ISOC is completing the preparations for the forthcoming Announcement of Opportunity (AO-3) which will be released on 13 September 2004 calling for new INTEGRAL observing proposals for the period February 2005 until August 2006.

Editor: Andrew Wilson
Scientific Coordination: Agustin Chicarro

This ESA Special Publication focuses on the Mars Express scientific instrumentation and its state about a year after launch in order to include some initial scientific discoveries. In spite of the Beagle 2 failure, the lander's payload is also thoroughly described here because it is of the highest scientific value. Furthermore, the orbiter instruments are looking specifically for possible evidence of past or present life. No other mission to Mars since NASA's Viking missions in the 1970s has made exobiology so central to its scientific goals.

We present here the results of astrometric, photometric and spectroscopic observations leading to the determination of the orbit and dynamical masses of the binary L dwarf 2MASSW J0746425+2000321. High angular resolution observations spread over almost 4 years and obtained with the Hubble Space Telescope (HST), the ESO Very Large Telescope (VLT), and a the W. M. Keck Observatory (Keck) allow us to cover ~36% of the period, corresponding to 60% of the orbit, and, for the first time, to derive a precise estimate of the total and individual masses of such a late-type object. We find an orbital period of 3850.9⁺⁹⁰⁴_-767 days. The corresponding total mass is 0.146^+0.016_-0.006 M_Sun with uncertainties depending on the distance. Spatially resolved low resolution optical (550-1025 nm) spectra have been obtained with HST/STIS, allowing us to measure the spectral types of the two components (L0± 0.5 for the primary and L1.5 ±0.5 for the secondary). We also present precise photometry of the individual components measured on the high angular resolution images obtained with HST/ACS and WFPC2 (visible), VLT/NACO (J, H and KS bands) and Keck I (K_S band). These spectral and photometric measurements enable us to estimate their effective temperatures and mass ratio, and to place the object accurately in a H-R diagram. The binary system is most likely formed by a primary with a mass of 0.085±0.010 M_Sun and a secondary with a mass of 0.066±0.006 M_Sun, thus clearly substellar, for an age of approximately 300±150 Myr. H-alpha variability indicates chromospheric and/or magnetic activity.

Establishing the mechanisms by which the solar wind enters Earth's magnetosphere is one of the biggest goals of magnetospheric physics, as it forms the basis of space weather phenomena such as magnetic storms and aurorae. It is generally believed that magnetic reconnection is the dominant process, especially during southward solar-wind magnetic field conditions when the solar-wind and geomagnetic fields are antiparallel at the low-latitude magnetopause. But the plasma content in the outer magnetosphere increases during northward solar-wind magnetic field conditions, contrary to expectation if reconnection is dominant. Here we show that during northward solar-wind magnetic field conditions - in the absence of active reconnection at low latitudes - there is a solar-wind transport mechanism associated with the nonlinear phase of the Kelvin-Helmholtz instability. This can supply plasma sources for various space weather phenomena.

Isolated electrostatic structures are observed throughout much of the 4R_E by 19.6R_E Cluster orbit. These structures are observed in the Wideband plasma wave instrument's waveform data as bipolar pulses (one positive and one negative peak in the electric field amplitude) and tripolar pulses (two positive and one negative peak, or vice versa). These structures are observed at all of the boundary layers, in the solar wind and magnetosheath, and along auroral field lines at 4.5-6.5R_E. Using the Wideband waveform data from the various Cluster spacecraft we have carried out a survey of the amplitudes and time durations of these structures and how these quantities vary with the local magnetic field strength. Such a survey has not been carried out before, and it reveals certain characteristics of solitary structures in a finite magnetic field, a topic still inadequately addressed by theories. We find that there is a broad range of electric field amplitudes at any specific magnetic field strength, and there is a general trend for the electric field amplitudes to increase as the strength of the magnetic field increases over a range of 5 to 500nT. We provide a possible explanation for this trend that relates to the structures being Bernstein-Greene-Kruskal mode solitary waves. There is no corresponding dependence of the duration of the structures on the magnetic field strength, although a plot of these two quantities reveals the unexpected result that with the exception of the magnetosheath, all of the time durations for all of the other regions are comparable, whereas the magnetosheath time durations clearly are in a different category of much smaller time duration. We speculate that this implies that the structures are much smaller in size. The distinctly different pulse durations for the magnetosheath pulses indicate the possibility that the pulses are generated by a mechanism which is different from the mechanism operating in other regions.

The electron density profiles derived from the EFW and WHISPER instruments on board the four Cluster spacecraft reveal density structures inside the plasmasphere and at its outer boundary, the plasmapause. We have conducted a statistical study to characterize these density structures. We focus on the plasmasphere crossing on 11 April 2002, during which Cluster observed several density irregularities inside the plasmasphere, as well as a plasmaspheric plume. We derive the density gradient vectors from simultaneous density measurements by the four spacecraft. We also determine the normal velocity of the boundaries of the plume and of the irregularities from the time delays between those boundaries in the four individual density profiles, assuming they are planar. These new observations yield novel insights about the occurrence of density irregularities, their geometry and their dynamics. These in-situ measurements are compared with global images of the plasmasphere from the EUV imager on board the IMAGE satellite.

In this case study we investigate the source region of whistler-mode chorus located close to the geomagnetic equator at a radial distance of 4.4 Earth radii. We use measurements from the four Cluster spacecraft at separations of less than a few hundreds of km, recorded during the geomagnetic storm of 18 April 2002. The waveforms of the electric field fluctuations were obtained by the WBD instruments in the frequency range 50Hz-9.5kHz. Using these data, we calculate linear and rank correlation coefficients of the frequency averaged power-spectral density measured by the different spacecraft. Those coefficients have been recently shown to decrease with spacecraft separation distance perpendicular to the static magnetic field with a characteristic scale length of 100km. We find this characteristic scale varying between 60 and 200km for different data intervals inside the source region. We examine possible explanations for the observed large scatter of the correlation coefficients, and we suggest a simultaneously acting effect of random positions of locations at which the individual chorus wave packets are generated. The statistical properties of the observations are approximately reproduced by a simple 2-D model of the source region, assuming a perpendicular half-width of 35km (approximately one wavelength of the whistler-mode waves) for the distribution of power radiated from individual active areas.

AIAA 2004-3632: Presented at the 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit 11 - 14 July 2004, Fort Lauderdale, Florida

Electric propulsion represents one of the most promising technologies for application in future space missions. The knowledge of the plasma plume evolution in the thruster surrounding space is still of fundamental importance, at system design level, for new generation satellites, in order to integrate the propulsive subsystem with the other vehicle subsystems. Furthermore, the necessity to simulate realistic configurations leads to the need of powerful and flexible 3-D tools. Alta S.p.A. and Consorzio Pisa Ricerche developed a three-dimensional particle-in-cell code capable to simulate conditions found both in space and in ground vacuum facilities, for realistic satellite configurations for Hall Effect Thrusters and Gridded Ion Engines. The present article will present a brief description of the PICPluS 3D code, including the various physical models that can be used and the code validation. Numerical results related to the ESA's SMART-1 satellite, launched on 27 September 2003, will then be compared with flight data. Finally, an analysis of the influence of the simulation paramaters on the results will follow.

AIAA-2004-3977: Presented at the 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Florida, 11-14 July 2004

Onboard the ESA SMART-1 spacecraft, (Small Mission for Advanced Research in Technology), the Xenon feeding system operates since the September 30th 2003. EPS Contractor, ESTEC, and EPS manufacturer, SNECMA MOTEURS, present in detail the major performances of the Pressure Regulation System, with a comparison to the ground tests results. The PPS® -1350 Hall Effect plasma Thruster needs a regulated xenon pressure as input of the flow controller. Such pressure is delivered and controlled by two pieces of hardware, the "Bang-Bang Pressure Regulation Unit" and the "Pressure Regulation Electronic Card". The concept is described as well as its main features: the robustness by design that cannot allow a direct communication between the high-pressure parts (the xenon tank) and the low-pressure parts (the thruster input). The paper highlights the possibility for various parameters to be tuned by telecommands in order to reach different performance levels of the pressure regulation. The real flexibility of the concept allows smoothing the pressure regulation. This paper describes the performances results of the pressure regulation in space environment compared to the ground tests results. It discusses also the advantage of the regulation tuning capability during the first flight phase. This new features of primary electric propulsion subsystem demonstrates its robustness and flexibility toward thruster initial requested tuning to keep the thruster loop fine pressure regulation in an adequate range.

AIAA-2004-3435: Presented at the 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Florida, 11-14 July 2004

Onboard the ESA SMART-1 spacecraft, (Small Mission for Advanced Research in Technology), the primary Electric Propulsion Subsystem (EPS) operates since the 30th September 2003. EPS Contractor, ESTEC, and EPS manufacturer, SNECMA MOTEURS, present in detail the major performances of the complete electric propulsion system, with a comparison to the ground tests results. The PPS®-1350-G Hall Effect plasma Thruster and its Power processing unit, developed in the frame of the CNES Stentor Program, was tested at Snecma facilities. The main feature of the Smart-1 system is its variable power supply. Integrated into the whole spacecraft the electric propulsion system was tested at ESTEC before the in-flight first firing after the successful Ariane V launch. Results of these main tests demonstrate a good prediction of the in flight EPS behavior including the robust bang-bang xenon pressure regulation for the input pressure and variable electrical power supply. This paper describes the performance results of the PPS®-1350-G firing in space environment. It discusses also the consequences of the Van Allen radiation belt crossing during the first flight phase, particularly the behavior of the floating potential of the thruster with respect to the satellite electrical ground. The initial successful results obtained supports the first technological experience objective of the SMART-1 mission. These new features of primary electric propulsion subsystem and especially the low-power start-up and variable power features can be also a significant added value for any commercial application using electric propulsion for station-keeping and/or orbit transfer.

AIAA 2004-2357: Presented at the 35th AIAA Plasmadynamics and Lasers Conference 28 June - 1 July 2004, Portland, Oregon

Present simulation techniques for plasma thrusters plume simulations usually implement a Particle In Cell / Monte Carlo approach to a plasma flow considered in a quasi-neutral state, with the possibility of a residual atmosphere (typical of a vacuum chamber test facility). Nonetheless it is difficult to compare directly results, even with measurements taken in very similar laboratory configurations, because it's not yet achieved the possibility to simulate at the same time realistic chamber geometry, pumping system performance and effect of the sputtering caused by the ion beam impinging the chamber walls. The present article will show the results of a series of PIC/DSMC simulations executed with CPR/Alta codes on HET plumes, considering a wide range of realistic laboratory configurations, and considering also the effect of different physical models; results will be also compared with experimental ones from literature and Alta testing facilities and flight data from the European SMART-1 mission.