ESA Science & Technology - Publication Archive

The Ground-based Asteroseismology Uniform Database Interface (GAUDI) is the result of the preparatory work performed for the COROT satellite. In the data available in GAUDI we discovered 17 B-type stars that show emission in their Balmer lines and were not known to display such emission before, including at least 16 non-supergiant ones. We thus reclassify those stars as Be stars. These 17 new Be stars increase the number of Be stars in the field of view of COROT by ~25%, which is important for the target selection of the mission. Moreover, ~70% of the discovered Be stars are of late subtypes. Be stars have been mostly found among early subtypes until now, but this could be due to an observational bias. Finally, one of the discovered star is either a slowly rotating shell Be star or a Herbig Be star with a low vsini, which makes this star especially interesting.

The equatorial Medusae Fossae Formation (MFF) is enigmatic and perhaps among the youngest geologic deposits on Mars. They are thought to be composed of volcanic ash, eolian sediments, or an ice-rich material analogous to polar layered deposits. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument aboard the Mars Express Spacecraft has detected nadir echoes offset in time-delay from the surface return in orbits over MFF material. These echoes are interpreted to be from the subsurface interface between the MFF material and the underlying terrain. The delay time between the MFF surface and subsurface echoes is consistent with massive deposits emplaced on generally planar lowlands materials with a real dielectric constant of ~2.9 ± 0.4. The real dielectric constant and the estimated dielectric losses are consistent with a substantial component of water ice. However, an anomalously low-density, ice-poor material cannot be ruled out. If ice-rich, the MFF must have a higher percentage of dust and sand than polar layered deposits. The volume of water in an ice-rich MFF deposit would be comparable to that of the south polar layered deposits.

We present multi-wavelength observations of stellar features in the HI tidal bridge connecting M81 and M82 in the region called Arp's Loop. We identify eight young star-forming regions from Galaxy Evolution Explorer ultraviolet observations. Four of these objects are also detected at HI. We determine the basic star formation history of Arp's Loop using F475W and F814W images obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope. We find both a young (< 10 Myr) and an old (> 1 Gyr) stellar population with a similar spatial distribution and a metallicity Z ~ 0.004. We suggest that the old stellar population was formed in the stellar disk of M82 and/or M81 and ejected into the intergalactic medium during a tidal passage (~ 200-300 Myr ago), whereas the young UV-bright stars have formed in the tidal debris. The UV luminosities of the eight objects are modest and typical of small clusters or OB associations. The tidal bridge between M81-M82 therefore appears to be intermediate between the very low levels of star formation seen in the Magellanic bridge and actively star-forming tidal tails associated with major galaxy mergers.

Mars Express arrived at its destination in December 2003 to probe every facet of the Red Planet, from the interior to the ionosphere, in unprecedented detail. In addition to these global studies, the mission's unifying theme is the search for water in its various forms everywhere on the planet. The mission has been extended into 2009, and could last even longer.

In the course of Gaia's 5-year astronomical survey, the equivalent of around 20 000 DVDs of raw information on our Galaxy will be harvested and transmitted to Earth. Sophisticated processing is needed to distill this flood of complex data into the final Gaia Catalogue of about 1000 million celestial objects. A group of more than 300 European scientists and software developers is rising to the challenge: the Data Processing and Analysis Consortium is already preparing for Gaia's launch in 2011.

We present results of an all-sky hard X-ray survey based on almost four years of observations with the IBIS telescope onboard the INTEGRAL observatory. The dead time-corrected exposure of the survey is ~33 Ms. Approximately 12% and 80% of the sky has been covered to limiting fluxes lower than 1 and 5 mCrab, respectively. Our catalog of detected sources includes 403 objects, 316 of which exceed a 5-sigma detection threshold on the time-averaged map of the sky, and the rest were detected in various subsamples of exposures. Among the identified sources, 219 are Galactic (90 low-mass X-ray binaries, 76 high-mass X-ray binaries, 21 cataclysmic variables, 6 coronally active stars, and other types) and 137 are extragalactic, including 130 active galactic nuclei (AGNs) and 3 galaxy clusters. We derived number-flux functions of AGNs and Galactic sources. The log N-log S relation of non-blazar AGNs is based on 68 sources located at Galactic latitudes |b| > 5°, where the survey is characterized by high identification completeness, with fluxes higher than S_lim = 1.1 × 10^-11 erg s^-1 cm^-2 (~0.8 mCrab) in the 17-60 keV energy band. The cumulative AGN number-flux function can be described by a power law with a slope of 1.62 ± 0.15 and normalization of (5.7±0.7) × 10^-3 sources per deg² at fluxes > 1.43 × 10^-11 erg s^-1 cm^-2 (>1 mCrab). Those AGNs with fluxes higher than S_lim make up ~1% of the cosmic X-ray background at 17-60 keV. We present evidence of strong inhomogeneity in the spatial distribution of nearby (<70 Mpc) AGNs, which reflects the large-scale structure in the local Universe.

The nearby transiting planet HD 189733b was observed during three transits with the ACS camera of the Hubble Space Telescope in spectroscopic mode. The resulting time series of 675 spectra covers the 550-1050 nm range, with a resolution element of ~ 8 nm, at extremely high accuracy (signal-to-noise ratio up to 10,000 in 50 nm intervals in each individual spectrum). Using these data, we disentangle the effects of limb darkening, measurement systematics, and spots on the surface of the host star, to calculate the wavelength dependence of the effective transit radius to an accuracy of ~ 50 km. This constitutes the "transmission spectrum" of the planetary atmosphere. It indicates at each wavelength at what height the planetary atmosphere becomes opaque to the grazing stellar light during the transit. In this wavelength range, strong features due to sodium, potassium and water are predicted by atmosphere models for a planet like HD 189733b, but they can be hidden by broad absorption from clouds or hazes higher up in the atmosphere.
We observed an almost featureless transmission spectrum between 550 and 1050 nm, with no indication of the expected sodium or potassium atomic absorption features. Comparison of our results with the transit radius observed in the near and mid-infrared (2-8 micron), and the slope of the spectrum, suggest the presence of a haze of sub-micron particles in the upper atmosphere of the planet.

Southward-then-northward magnetic perturbations are often seen in the tail plasma sheet, along with earthward jets, but the generation mechanism of such bipolar B_z (magnetic flux rope created through multiple X-line reconnection, transient reconnection, or else) has been controversial. At ~2313 UT on 13 August 2002, Cluster encountered a bipolar B_z at the leading edge of an earthward jet, with one of the four spacecraft in the middle of the current sheet. Application to this bipolar signature of Grad-Shafranov (GS) reconstruction, the technique for recovery of two-dimensional (2D) magnetohydrostatic structures, suggests that a flux rope with diameter of ~2 R_E was embedded in the jet. To investigate the validity of the GS results, the technique is applied to synthetic data from a three-dimensional (3D) MHD simulation, in which a bipolar B_z can be produced through localized (3D) reconnection in the presence of guide field B_y (Shirataka et al., 2006) without invoking multiple X-lines. A flux rope-type structure, which does not in fact exist in the simulation, is reconstructed but with a shape elongated in the jet direction. Unambiguous identification of a mechanism that leads to an observed bipolar B_z thus seems difficult based on the topological property in the GS maps. We however infer that a flux rope was responsible for the bipolar pulse in this particular Cluster event, because the recovered magnetic structure is roughly circular, suggesting a relaxed and minimum energy state. Our results also indicate that one has to be cautious about interpretation of some (e.g., force-free, or magnetohydrostatic) model-based results.

The internal rotation rates of the giant planets can be estimated by cloud motions, but such an approach is not very precise because absolute wind speeds are not known a priori and depend on latitude: periodicities in the radio emissions, thought to be tied to the internal planetary magnetic field, are used instead. Saturn, despite an apparently axisymmetric magnetic field, emits kilometre-wavelength (radio) photons from auroral sources. This emission is modulated at a period initially identified as 10 h 39 min 24 7 s, and this has been adopted as Saturn's rotation period. Subsequent observations, however, revealed that this period varies by 6 min on a timescale of several months to years. Here we report that the kilometric radiation period varies systematically by 1% with a characteristic timescale of 20-30 days. Here we show that these fluctuations are correlated with solar wind speed at Saturn, meaning that Saturn's radio clock is controlled, at least in part, by conditions external to the planet's magnetosphere. No correlation is found with the solar wind density, dynamic pressure or magnetic field; the solar wind speed therefore has a special function. We also show that the long-term fluctuations are simply an average of the short-term ones, and therefore the long-term variations are probably also driven by changes in the solar wind.

Mercury is a very difficult planet to observe from the Earth, and space missions that target Mercury are essential for a comprehensive understanding of the planet. At the same time, it is also difficult to orbit because it is deep inside the Sun's gravitational well. Only one mission has visited Mercury; that was Mariner 10 in the 1970s. This paper provides a brief history of Mariner 10 and the numerous imaginative but unsuccessful mission proposals since the 1970s for another Mercury mission. In the late 1990s, two missions - MESSENGER and BepiColombo - received the go-ahead; MESSENGER is on its way to its first encounter with Mercury in January 2008. The history, scientific objectives, mission designs, and payloads of both these missions are described in detail.

Studying the dynamics of the Venus atmosphere, one of the main goals of the Venus Express mission, requires global imaging of the planet. The Venus Monitoring Camera (VMC) meets this goal by having the relatively wide field-of-view of 17.5º. VMC is recording images using four narrowband filters, from UV to near-IR, all sharing one CCD. The spatial resolution is 0.2-45 km per pixel, depending on the distance from the planet. The planet's full disc is captured near the apocentre of the orbit. VMC is complementing the mission's other instruments by tracking cloud motions at ~70 km (cloud tops) and at ~50 km (main cloud layer) altitudes, mapping oxygen night-glow and its variability, mapping the nightside thermal emission from the surface, and studying the lapse rate and water content in the lower 6-10 km. In addition, VMC is providing imaging context for the whole mission, and its movies of the atmosphere are of significant interest for science and the public outreach programme.

The VIRTIS imaging spectrometer built for ESA's Rosetta cometary mission is a versatile instrument that is also well-suited to Venus observations. The discovery of the near-IR windows in the atmosphere of Venus from ground-based observations in the 1980s showed that the surface of the planet can be studied via IR observations over the nightside. Imaging spectroscopy in the visible and near-IR can study the atmosphere from the uppermost layers down to the deepest levels. With its unique combination of mapping capabilities at low spectral resolution (VIRTIS-M) and high spectral resolution slit spectroscopy (VIRTIS-H), the instrument is ideal for making extensive IR and visible spectral images of the planet.

The Venus Express Radio-Science Experiment (VeRa) is using radio signals at X- and S-band (3.5 cm and 13 cm wavelengths, respectively) to probe the Venus surface, neutral atmosphere, ionosphere and gravity field, and the interplanetary medium. An ultrastable oscillator (USO) is providing a high-quality onboard reference frequency source; instrumentation on Earth is sampling amplitude, phase, propagation time and polarisation of the received signals. Simultaneous coherent measurements at the two wavelengths allow separation of dispersive media effects from classical Doppler shift. The execution of a radio-science experiment involves the precise interaction of many complex spaceborne and ground-based systems. The quality of the measurements depend critically not only on the noise performance of the USO, the quality of the radio link and the performance of the ground station, but also on the precision of the timing, ephemeris data, orbit prediction and the attitude-control manoeuvres that are needed to perform the experiments and to extract the data.

SPICAV (SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus) is a suite of three UV-IR spectrometers dedicated to the study of the atmosphere of Venus, from ground level to the outermost hydrogen corona at more than 40 000 km altitude. It is derived from the SPICAM instrument already flying on Mars Express with great success, with the addition of the new Solar Occultation IR (SOIR) high-resolution spectrometer working in the solar occultation mode. In nadir orientation, SPICAV UV (110-310 nm) will analyse the albedo spectrum to retrieve SO₂ and the distribution of the UV-blue absorber (of unknown origin) on the dayside with implications for cloud structure, and atmospheric dynamics. On the nightside, the g and d bands of NO will be studied, as well as emissions produced by electron precipitations. In the stellar occultation mode, the UV sensor will measure the vertical profiles of CO₂, temperature, SO₂, SO, clouds and aerosols. The density/temperature profiles obtained with SPICAV will constrain and aid in the development of dynamical atmospheric models, from cloud top (~60 km) to 160 km in the atmosphere. UV observations of the upper atmosphere will allow studies of the ionosphere through the emissions of CO, CO⁺ and O₂ ⁺, and its direct interaction with the solar wind. It will study the H corona, with its two different scale heights, and it will allow a better understanding of escape mechanisms and estimates of their magnitude, crucial for insight into the long-term evolution of the atmosphere.

Although the Venus Express and Mars Express spacecraft are very similar, key modifications were made to meet the requirements of a Venus mission. This paper provides an overview of the main mission drivers that led to the design changes, and describes the main spacecraft functions.

The Planetary Fourier Spectrometer (PFS) is an infrared spectrometer optimised for atmospheric studies, with a short-wavelength (SW) channel covering the spectral range 1800-11400 cm^-1 (0.9-5.5 mm) and a long-wavelength (LW) channel covering 250-1800 cm^-1 (5.5-45 mm). Both channels have a uniform spectral resolution of 1.3 cm^-1. It is the first Fourier spectrometer at Venus covering the 1-5 mm range. The SW field of view is about 1.6º FWHM, and 2.8º FWHM for the LW, which corresponds to spatial resolutions of 7 km and 12 km, respectively, when Venus is observed from a height of 250 km. PFS can provide unique data for improving our knowledge not only of the atmosphere properties but also the surface properties (temperature) and surface-atmosphere interaction (volcanic activity).

The SW channel uses a PbSe detector cooled to 200-220K, while the LW channel is based on a pyroelectric (LiTaO3) detector working at room temperature. The intensity of the interferogram is measured at every 112 nm displacement of the mirrors (corresponding to 450 nm optical path difference), by using a laser diode monochromatic light interferogram (a sine wave), whose zero crossings control the double pendulum motion. PFS works primarily around the pericentre of the orbit, only occasionally observing Venus from large distances. Each measurement takes 4 s, with a repetition time of 11.5 s. By working for about 1.5 h around pericentre, a total of 460 measurements per orbit can be acquired, plus 60 for calibrations. PFS can take measurements at all local times, facilitating the retrieval of surface temperatures and atmospheric vertical temperature profiles on both the day and night sides.

The MAG (Magnetometer) instrument of Venus Express is investigating the plasma environment of Venus. Although Venus has no intrinsic magnetic moment, its magnetic field plays an important role in the interaction of the solar wind with the planet. The hardware, with heritage from the ROMAP Rosetta Lander magnetometer, consists of two sensors, an electronics box and a carbon fibre boom. One sensor is located on the tip of the boom, while the other is mounted on the spacecraft body; this configuration allows the magnetic effects of spacecraft origin to be separated from the ambient space magnetic field.

The Venus Express Science Operations Centre (VSOC) has the task of defining and performing, under the direct responsibility of the Project Scientist, the science operations for the mission. VSOC ensures that all the science objectives can be fulfilled within the operational constraints. This paper focuses on the planning and commanding activities, and provides an overview of the VSOC activities, outlines its architecture down to the hardware level, and summarises the science planning process. The data-handling and archiving are dealt with in a companion paper.

Following launch and its 5-month interplanetary cruise, Venus Express was injected into a 24 h orbit around Venus. It is now conducting science observations for two Venusian days (486 Earth-days). This paper describes the ground system infrastructure, the orbital requirements and mission control parameters, the flight operations concept, principles and implementation for the various mission phases and payload operations, and the mission products to be made available to the scientific community.

The success of a scientific mission is determined by the quality of the scientific results. The prompt delivery of instrument and ancillary raw data to the instrument teams and the delivery of reduced and calibrated data to the scientific community are therefore key elements in the mission design. This paper describes the data flow from the Venus Express spacecraft through the ground segment via the instrument teams to the final scientific archive. Several software tools and standards are used to support the data dissemination. The functionality of the individual tools is explained, the interfaces to the individual groups are discussed and examples of the graphical user interfaces are shown.