ESA Science & Technology - Publication Archive

Scientific editor: A. Gimenez
Editor: A. Wilson The report for the 35th COSPAR Meeting covers, as in previous issues, the missions of the Scientific Programme of ESA in the areas of astronomy, Solar System exploration and fundamental physics. This year's COSPAR Meeting occurs only weeks after the Saturn-orbit insertion of the Cassini spacecraft - carrying Europe's Huygens probe to explore the atmosphere of Titan - and at the same time as the launch of the second satellite of the Double Star project.

This is an edited version of the Final Report of the Europa Low Resource Radar study, performed by the Concurrent Design Facility (CDF) at ESA ESTEC for the Science Directorate (SCI-A), in the frame of the Jupiter Minisat Explorer Technology Reference Study. The objectives of the study were to:

• perform instrument conceptual design and trades
• prepare a preliminary instrument design including budgets and subsystem designs with required performance
• show science requirements compliance
• define critical design issues requiring further analysis
• assess and analyse programme, risk and costs

Further the constraints imposed by the chosen spacecraft platform and orbit were analysed and described where appropriate. This document reports on the analysis performed and conclusions for a Europa Low Resource Radar (ELRR) conceptual design.

Presented at the 4th International Spacecraft Propulsion Conference, Chia Laguna (Cagliari), Sardinia, Italy, 2-4 June 2004

Electric propulsion represents one of the most promising technologies for application in future space missions; Hall Effect Thrusters (HET's) and Gridded Ion Engines (GIE) are particularly interesting for their relatively high thrust capability coupled with a specific impulse which is up to one order of magnitude higher than latest generation chemical systems. The knowledge of the plasma plume evolution in the thrusters surrounding space is of fundamental importance, at system design level, for new generation satellites, in order to integrate the propulsive subsystem with the other vehicle subsystems: as known, the use of electro-magnetic thrusters can create compatibility problems, due to the electrically charged particle flow, which can interfere with telecommunication signals and generate erosion and insulation loss for critical satellite surfaces (e.g. solar panels, optical instruments and sensors etc.) A number of Hall thrusters plume models have been developed, present simulation techniques 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). In this paper the 3D plume simulation model developed by Alta will be described, and applied to model the SNEMA PPS® 1350 thruster used on the SMART-1 satellite.

We discuss the observational properties of a remarkably faint triply imaged galaxy revealed in a deep z'-band Advanced Camera for Surveys observation of the lensing cluster A2218 (z=0.175). A well-constrained mass model for the cluster, which incorporates the outcome of recent Keck spectroscopic campaigns, suggests that the triple system arises via a high-redshift (z>6) source viewed at high magnification (~=25 times). Optical and infrared photometry from Hubble Space Telescope and the Keck Observatory confirms the lensing hypothesis and suggests that a significant discontinuity occurs in the spectral energy distribution within the wavelength interval 9250-9850 Å. If this break is associated with Gunn-Peterson absorption from neutral hydrogen, a redshift of 6.6 < z < 7.1 is inferred. Deep Keck spectroscopy conducted using both optical and infrared spectrographs fails to reveal any prominent emission lines in this region. However, an infrared stellar continuum is detected whose decline below 9800 Å suggests a spectroscopic redshift toward the upper end of the range constrained photometrically, i.e., z~=7. Regardless of the precise redshift, the source is remarkably compact (<~1 h^-1₇₀ kpc) and faint (z_F850LP=28.0) yet is undergoing vigorous star formation at a rate ~=2.6 M_solar yr^-1. An intriguing property is the steep slope of the ultraviolet continuum implied by the photometry, which may suggest that the source is representative of an early population of galaxies responsible for cosmic reionization. Independent verification of these results is highly desirable, but our attempts highlight the difficulty of studying such sources with present facilities and the challenges faced in pushing back the frontiers of the observable universe beyond z~6.5.

Mercury is one of the least explored planets, and has many perplexing features. A conventional sample return mission requires significant launch mass, due to the large ¿v required for the outbound and return trips, and the large mass allocation necessary for a planetary lander and ascent vehicle. Solar sailing can be used to reduce the lander mass allocation by delivering the lander to low, thermally safe orbit close to the terminator. In addition, the ascending node of the solar sail parking orbit plane can be artificially forced to avoid out-of-plane manoeuvres during ascent from the planetary surface.

Contents

4	Solving the puzzles of Saturn and Titan
6	High ambitions for an outstanding planetary mission
8	A long and rich journey
12	What lies beneath?
16	Mysterious Titan
20	Vehicles of discovery

In the 1970s, NASA and ESA took up the idea of a space-based telescope. Funding began to flow in 1977. Later, it was decided to name the telescope after Edwin Hubble. Although the Hubble Space Telescope (HST) was downsized later to a 2.4 m primary mirror diameter from the initial 3 m, the project started to attract significant attention from astronomers.

We review the results obtained with the Galactic center campaigns of the BeppoSAX Wide Field X-ray Cameras (WFCs). This pertains to the study of luminous low-mass X-ray binaries (LMXBs). When pointed at the Galactic center, the WFC field of view contains more than half of the Galactic LMXB population. The results exemplify the excellent WFC capability to detect brief X-ray transients. Firstly, the WFCs expanded the known population of Galactic thermonuclear X-ray bursters by 50%. At least half of all LMXBs are now established to burst and, thus, to contain a neutron star as compact accretor rather than a black hole candidate. We provide a complete list of all 76 currently known bursters, including the new case 1RXS J170854.4-321857. Secondly, the WFCs have uncovered a population of weak transients with peak luminosities up to ~10^37 erg/s and durations from days to weeks. One is the first accretion-powered millisecond pulsar SAX J1808.4-3658. Thirdly, the WFCs contributed considerably towards establishing that nearly all (12 out of 13) luminous low-mass X-ray binaries in Galactic globular clusters contain neutron stars rather than black holes. Thus, the neutron star to black hole ratio in clusters differs from that in the Galactic disk at a marginal confidence level of 97%.

We report the first high-energy survey catalog obtained with the IBIS gamma-ray imager on board INTEGRAL. The analysis has been performed on the first-year Core Program ISGRI data comprising both Galactic Plane Scan and Galactic Centre Deep Exposure pointings for a total exposure time exceeding 5 Ms. This initial survey has revealed the presence of ~120 sources detected with the unprecedented sensitivity of ~1 mcrab in the energy range 20-100 keV. Each source is located to an accuracy between 1' and 3', depending on its brightness. The outstanding IBIS capability to locate soft gamma-ray emitters has allowed us to identify most of the detected sources with already known Galactic X-ray binary systems, while 28 of the objects are of unknown nature.

An overview on the current status of the SMART-1 mission as of 13 May 2004.

We report the results of extensive high-energy observations of the X-ray transient and black hole candidate XTE J1720-318 performed with INTEGRAL, XMM-Newton, and RXTE.

On April 18, 2002, the Cluster spacecraft were outbound in the northern hemisphere over the pole and entered the cusp. A cusp-like region was observed consecutively three times from 1620 to 1830 UT by all four Cluster Spacecraft although the solar wind dynamic pressure was small and stable. All three cusp encounters were characterized by turbulent magnetic fields, high density plasma and stagnant plasma flow. The cusp region identifications were further confirmed by the clock angle criterion. All three cusps were found to be associated with clear signatures of energetic ions, high He/H and O/H ratios obtained by the RAPID instrument. The observed triple cusps may be either explained as a funnel-shaped cusp trifurcated or swiveled into a complicated geometry in space or as a cusp which shifted back and forth three times in a two hour interval. Observational evidence shows that the observed triple cusps were a temporal sequence rather than a spatial effect. We suggest further that the solar wind azimuthal flow was the controlling factor of the cusp position and was stronger factor than the IMF BY/BZ components. The importance of the solar wind azimuthal and north/south flow as a dynamic driver of the cusp, and even the whole magnetosphere has been more or less neglected or underestimated.

The first simultaneous measurements of discrete chorus emissions on multiple spacecraft, realized in the context of the Cluster mission, revealed a rather unexpected frequency difference of around 1 kHz between nearly identical discrete elements observed on different spacecraft [Gurnett et al., 2001 ]. This frequency difference is interpreted herein as a natural outcome of the dependence of the whistler-mode refractive index on the wave normal angle between the wave vector k and the static magnetic field B₀ and the rapid motion of highly localized source region(s) of chorus of 400 km to 1700 km in extent along the field line, but only less than 100 km transverse to the magnetic field, and moving at speeds of 20,000 km/s to 25,000 km/s. Wave packets emanating from the localized regions propagate to two spacecraft at different wave normal angles, and are observed at different frequencies due to the differential Doppler shift between the two spacecraft. These differences in frequency, as well as the different times of arrival of the similar emissions at the different spacecraft, provide a unique opportunity to estimate the source characteristics, using a model involving rapidly moving sources traveling at speeds comparable to the parallel resonant velocity of counterstreaming electrons moving along the Earth's magnetic field lines. We report the determination of chorus emission source region motion for two different cases observed during 20002001, where these differences in frequency were readily observable due to the relatively large separation of the Cluster spacecraft. We also report a case in 2002 where the spacecraft separations were smaller, so that these frequency differences were not as evident but nevertheless measurable. In general, our results provide the first experimental evidence that the sources that generate the discrete chorus emissions are in rapid motion.

INTEGRAL is performing very well and we have witnessed a number of key events in the recent months which will be addressed in more detail here. Consequently this issue of the ISOC Newsletter is a bit longer than usual.

Authors: Bernard H. Foing, Chief Scientist
G. Racca, A. Marini and SMART-1 Project team
M. Grande, J. Huovelin, J.L. Josset, H. Keller, A. Nathues, D. Koschny, M. Almeida, J. Zender and SMART-1 Science & Technology team

The status and first results of ESA's SMART-1 mission were presented at the 1st General Assembly of the European Geosciences Union on 26 April 2004. The presentation covered:

• SMART-1 Technology Mission: Solar Electric Propulsion to the Moon
• Payload Technology and Science objectives
• Lunar and planetary science with SMART-1
• Performances, Status and first results data integration
• SMART-1 Contribution to preparing Future Planetary exploration

Contact: Bernard H. Foing, Chief Scientist, ESTEC/SCI-SR, ESA Science Directorate, Bernard.Foingesa.int

We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2 and deep ground-based images of the Red Rectangle (RR), a bipolar proto-planetary nebula associated with the post-AGB binary system HD 44179. The high-resolution HST images reveal complex new structures, many of them unique to this object. The RR nebula is dominated by a discontinuous "bicone,'' whose bright, sharp linear edges give the nebula an overall X-shaped appearance. The edges of the bicone are connected by a remarkable series of linear features elongated perpendicular to the radius vector, giving the object a ladder-like structure. The "rungs'' of the ladder structure show a quasi-periodic spacing, suggesting that they have arisen from discrete episodes of mass loss from the central star, separated by a few hundred years. The total timescale over which mass has been shed into the visible nebula is of order 14 000 yr. Outside the X-shaped bicone, parabolas curl inward, resembling wineglasses, which terminate on the bicone edges in large, limb-brightened vortices. The central object is bisected by a dark band, indicating that the star is not seen directly but is instead obscured by a surrounding opaque dust disk.

We present a search for rings or arcs in the haloes of planetary nebulae (PNe). We discovered such structures in eight PNe, tripling the sample of PNe with known rings. This shows that, contrary to what was believed to date, the occurrence of mass loss fluctuations with timescales of 10²-10³ yrs at the end of the asymptotic giant branch phase (AGB) is common. We estimate a lower limit of the occurrence rate of rings in PN haloes to be ~35%. Using these new detections and the cases previously known, we discuss the statistical properties of ring systems in PNe haloes. We estimate that the mass modulation producing the rings takes place during the last 10 000 or 20 000 yrs of AGB evolution. In PNe, the spacing between rings ranges from <0.01 pc to 0.06 pc, significantly larger than those seen in proto-PNe. This, together with the finding of a possible positive correlation of spacing with the post-AGB age of the nebulae, suggests that the spacing of the rings increases with time. These properties, as well as the modest surface brightness amplitudes of rings, are consistent with the predictions of the dust-driven wind instability model explored by Meijerink et al., but do not immediately exclude other proposed models.

Measurements of a spacecraft floating potential, on the four Cluster spacecraft, are used as a proxy for electron plasma density to study, for the first time, the macroscopic density transition scale at 98crossings of the quasiperpendicular terrestrial bow shock. A timing analysis gives shock speeds and normals; the shock speed is used to convert the temporal measurement to a spatial one. A hyperbolic tangent function is fitted to each density transition, which captures the main shock transition, but not overshoot or undershoot nor foot features. We find that, at a low Mach number M, the density transition is consistent with both ion inertial scales c/ω_pi and convected gyroradii v_sh,n/Ω_ci,2, while at M ≥ 4-5 only the convected gyroradius is the preferred scale for the shock density transition and takes the value L ≈ 0.4v_sh,n/Ω_ci,2.

The Milky Way is known to be an abundant source of gamma-ray photons, now determined to be mainly diffuse in nature and resulting from interstellar processes. In the soft gamma-ray domain, point sources are expected to dominate, but the lack of sensitive high-resolution observations did not allow for a clear estimate of the contribution from such sources. Even the best imaging experiment revealed only a few point sources, accounting for about 50% of the total Galactic flux. Theoretical studies were unable to explain the remaining intense diffuse emission. Investigating the origin of the soft gamma-rays is therefore necessary to determine the dominant particle acceleration processes and to gain insights into the physical and chemical equilibrium of the interstellar medium. Here we report observations in the soft gamma-ray domain that reveal numerous compact sources. We show that these sources account for the entirety of the Milky Way's emission in soft gamma-rays, leaving at most a minor role for diffuse processes.