ESA Science & Technology - Publication Archive
Soft gamma-ray repeaters (SGRs) are neutron stars that emit short (<~1 s) and energetic (<~1042 erg s-1) bursts of soft gamma-rays. Only four of them are currently known. Occasionally, SGRs have been observed to emit much more energetic "giant flares'' (~1044-1045 erg s-1). These are exceptional and rare events. We report here on serendipitous observations of the intense gamma-ray flare from SGR 1806-20 that occurred on 2004 December 27. Unique data from the Cluster and Double Star TC-2 satellites, designed to study the Earth's magnetosphere, provide the first observational evidence of three separate timescales within the early (first 100 ms) phases of this class of events. These observations reveal that in addition to the initial very steep (<0.25 ms) X-ray onset, there is first a 4.9 ms exponential rise timescale followed by a continued exponential rise in intensity on a timescale of 70 ms. These three timescales are a prominent feature of current theoretical models, including the timescale (several milliseconds) for fracture propagation in the crust of the neutron star.
Published: 16 July 2005
We have discovered rapid quasi-periodic oscillations (QPOs) in RXTE/PCA measurements of the pulsating tail of the 2004 December 27 giant flare of SGR 1806-20. QPOs at ~92.5 Hz are detected in a 50 s interval starting 170 s after the onset of the giant flare. These QPOs appear to be associated with increased emission by a relatively hard unpulsed component and are seen only over phases of the 7.56 s spin period pulsations away from the main peak. QPOs at ~18 and ~30 Hz are also detected ~200-300 s after the onset of the giant flare. This is the first time that QPOs are unambiguously detected in the flux of a soft gamma-ray repeater or any other magnetar candidate. We interpret the highest QPOs in terms of the coupling of toroidal seismic modes with Alfvén waves propagating along magnetospheric field lines. The lowest frequency QPO might instead provide indirect evidence on the strength of the internal magnetic field of the magnetar.
Published: 16 July 2005
With the rapid development of space science and technology, the possibility for multi-segment missions has lately received substantial focus. This is especially true for smaller spacecrafts, where they together form a formation, in order to collectively perform measurements that often surpass what a single big satellite can achieve. Satellite's formation flying is consequently a very popular but also difficult discipline of the world's space centers. Many new and exciting ideas are emerging in this area every year. This paper provides a survey of operational, as well as planned, space missions involving formation flying aspects, which have been developed by different countries and space agencies. From an analysis of these missions, by making appropriate comparisons between the satellite's constellation and formation flights, a synthesis aimed at reach appropriate definitions of formation flights are performed. Using these analysis the paper then addresses the several guidance, navigation and control problems that are native to formation flying missions. Finally, an example of system configuration (including space and ground segment), mainly emphasis on the satellite's attitude and orbit control subsystem, is given to illustrate one of the formation flying missions presently being under phase B development.
Published: 06 April 2005
We present results from an HST/ACS imaging study of the metal-poor blue compact dwarf galaxy SBS 1415+437. It has been argued previously that this is a very young galaxy that started to form stars only ~100 Myr ago. However, we find that the optical color-magnitude diagram prominently reveals asymptotic giant branch and red giant branch (RGB) stars. The brightness of the RGB tip yields a distance D H 13.6 Mpc. The color of the RGB implies that its stars must be older than <1.3 Gyr, with the exact age depending on the assumed metallicity and dust extinction. The number of RGB stars implies that most of the stellar mass resides in this evolved population. In view of these and other HST results for metal-poor galaxies, it seems that the local universe simply may not contain any galaxies that are currently undergoing their first burst of star formation.
Published: 21 September 2005
The Xeus mission is designed to explore the X-ray emission from objects in the Universe at high redshifts, and these science requirements necessitate a very large effective area. We describe a completely revised mission scenario that mitigates previous concerns about the deployable mass and use of the ISS. New mirror technology with lightweight optics enables a direct launch to a L2 operational orbit, and we describe the outline of the Mirror and Detector Spacecraft that are deployed in formation flying to achieve the 50m focal distance separation.
Published: 01 January 2004
The IBIS/ISGRI imager on Integral detected for the first time a hard X-ray source, IGR J17456-2901, located within 1' of Sgr A* over the energy range 20-100 keV. Here we present the results of a detailed analysis of ~7x106s of Integral observations of the Galactic Centre. With an effective exposure of 4.7x106s we have obtained more stringent positional constraints on this high-energy (HE) source and constructed its spectrum in the range 20-400 keV. Furthermore, by combining the Isgri spectrum with the total X-ray spectrum corresponding to the same physical region around SgrA* from XMM data, and collected during part of the Integral observations, we constructed and present the first accurate wide band HE spectrum for the central arcmins of the Galaxy. Our complete analysis of the emission properties of IGR shows that it is faint but persistent with no variability above 3 sigma contrary to what was alluded to in our first paper. This result, in conjunction with the spectral characteristics of the X-ray emission from this region, suggests that the source is most likely not point-like but, rather, that it is a compact, yet diffuse, non-thermal emission region. The centroid of IGR is estimated to be R.A.=17h45m42.5s, decl.=-28°59'28'', offset by 1' from the radio position of Sgr A* and with a positional uncertainty of 1'. Its 20-400 keV luminosity at 8 kpc is L=5.4x1035 erg/sec. Very recently, Hess detected of a source of ~TeV gamma-rays also located within 1' of Sgr A*. We present arguments in favor of an interpretation according to which the photons detected by Integral and Hess arise from the same compact region of diffuse emission near the central black hole and that the supernova remnant Sgr A East could play an important role as a contributor of very HE gamma-rays to the overall spectrum from this region.
Published: 05 August 2005
The Van Allen radiation belts are two regions encircling the Earth in which energetic charged particles are trapped inside the Earth's magnetic field. Their properties vary according to solar activity and they represent a hazard to satellites and humans in space. An important challenge has been to explain how the charged particles within these belts are accelerated to very high energies of several million electron volts. Here we show, on the basis of the analysis of a rare event where the outer radiation belt was depleted and then re-formed closer to the Earth, that the long established theory of acceleration by radial diffusion is inadequate; the electrons are accelerated more effectively by electromagnetic waves at frequencies of a few kilohertz. Wave acceleration can increase the electron flux by more than three orders of magnitude over the observed timescale of one to two days, more than sufficient to explain the new radiation belt. Wave acceleration could also be important for Jupiter, Saturn and other astrophysical objects with magnetic fields.
Published: 09 September 2005
Presented at the 6th International Symposium "Reducing the Costs of Spacecraft Ground Systems and Operations (RCSGSO)", held 14-17 June 2005 at ESA/ESOC, Darmstadt, Germany. Smart-1 is the first of a series of ESA Small Missions for Advance Research and Technology where elements of the platform and the payload technology have been conceived as a demonstration for future cornerstone missions and an early opportunity for science. It was launched on 27th of September 2003 and spiraled out over a 14-month period until being captured by the Moon on 15/11/2004, thus successfully achieving the primary objective set to demonstrate Solar Electric Propulsion.
The paper will show the pros and contras in some of the choices made for Smart-1 together with the developments and the solutions implemented to mitigate the problems found during the mission:
- Impact of on-board problems on operations
- Ground Segment automation
- Keeping the mission control team reduced
- The increased importance of the Mission Planning System
- Fast distribution of spacecraft data through internet for anomaly identification and analysis
- Summary of lessons learnt
Published: 15 July 2005
This paper describes the operational orbit determination of the first Small Mission for Advanced Research and Technology, SMART-1, emphasising the experiences gained navigating a spacecraft with solar electric propulsion (SEP). Since launch, interruptions to planned thrust arcs by unforeseen platform events and both long and short term small variations of the SEP performance have had an impact on the spacecraft navigation. These impacts are discussed and in particular the evolution of the SEP performance throughout the mission and the response of the navigation team is analysed. Finally the operational orbit determination is presented in some detail, including illustrative examples.
Published: 12 October 2004
The advanced Moon micro-imager experiment (AMIE) is the imaging system on board ESA mission to the Moon SMART-1; it makes use of a miniaturised detector and micro-processor electronics developed by SPACE X in the frame of the ESA technical programme. The AMIE micro-imager will provide high resolution CCD images of selected lunar areas and it will perform colour imaging through three filters at 750, 915 and 960nm with a maximum resolution of 46 m/pixel at the perilune of 500 km. Specific scientific objectives will include (1) imaging of high latitude regions in the southern hemisphere, in particular the South Pole Aitken basin (SPA) and the permanently shadowed regions close to the South Pole, (2) determination of the photometric properties of the lunar surface from observations at different phase angles (physical properties of the regolith), (3) multi-band imaging for constraining the chemical and mineral composition of the surface, (4) detection and characterisation of lunar non-mare volcanic units, (5) study of lithological variations from impact craters and implications for crustal heterogeneity. The AMIE micro-imager will also support a Laser-link experiment to Earth, an On Board Autonomous Navigation investigation and a Lunar libration experiment coordinated with radio science measurements.
Published: 02 August 2005
The X-ray Evolving Universe Spectroscopy (XEUS) mission is under study by ESA and JAXA in preparation for inclusion in the ESA long term Science Programme (the Cosmic Vision 2015-2025 long-term plan). With very demanding science requirements, missions such as XEUS can only be implemented for acceptable costs, if new technologies and concepts are applied. The identification of the key technologies to be developed is one of the drivers for the early mission design studies, and in the case of XEUS this has led to the development of a novel approach to building X-ray optics for ambitious future high-energy astrophysics missions. XEUS is based on a single focal plane formation flying configuration, building on a novel lightweight X-ray mirror technology. With a 50 m focal length and an effective area of 10 m2 at 1 keV this observatory is optimized for studies of the evolution of the X-ray universe at moderate to high redshifts. This paper describes the current status of the XEUS mission design, the accommodation of the large optics, the corresponding deployment sequence and the associated drivers, in particular regarding the thermal design of the system. The main results were obtained in two Concurrent Design Facility (CDF) studies and other internal activities at ESTEC
Published: 31 August 2005
The XEUS (X-ray Evolving Universe Spectroscopy) mission is designed to explore the X-ray emission from objects in the Universe at high red shifts. A core set of instruments has been selected that allows the scientific goals of the mission to be met. It comprises narrow field imaging spectrometers of both Transition Edge Sensor (TES) and Superconducting Tunnel Junction (STJ) designs, and a Wide Field Imager with novel Silicon Active - Pixel sensing elements. We discuss the additional science goals for XEUS such as high time resolution, polarimetry and extensions to high energies >10keV, and the additional instruments with modest resource requirements which may facilitate these goals.
Published: 31 August 2005
The X-ray telescope forms the core of the high energy astrophysics observatory XEUS, currently under study at ESA as a well positioned candidate for its Cosmic Visions 1525 Science Programme, which is presently under formulation. The science requirements of XEUS are particularly demanding, combining a large effective area (10m2 at 1 keV), moderate angular resolution (5 requirement, with a goal of 2), and a low mass for the optics system. The preferred operational orbit for XEUS is a halo orbit around the Lagrangian Point 2 (L2). Background and costing considerations led to the requirement of a single focal plane location, which in combination with the required broad energy response function, in turn requires a focal length of 50m. The mission design is based on formation flying, with the Mirror Spacecraft (MSC) flying inertially, and the Detector Spacecraft (DSC) actively following the focal point. The ambitious XEUS telescope relies on the novel X-ray technology currently under development in Europe. The X-ray optics technology development activities and status as well as the telescope design in general are addressed.
Published: 25 August 2005
Future missions that may be deployed in the European Space Agencys Cosmic Visions 2025 scientific programme may include high energy astrophysics observatories that require focusing optics with unprecedented collection area. We describe scientific drivers for such missions, and discuss various implementations of optics designs that could satisfy the requirements. Options for lightweight reflectors and a possible implementation scenario are described and trade-offs for various coatings are presented.
Published: 25 August 2005
The Advanced Moon micro-Imager Experiment (AMIE), on-board SMART-1, the first European mission to the Moon, is an imaging system with scientific, technical and public outreach objectives. The science objectives are to image the Lunar South Pole, permanent shadow areas (ice deposit), eternal light (crater rims), ancient Lunar Non-mare volcanism, local spectrophotometry and physical state of the lunar surface, and to map high latitudes regions (south) mainly at far side (South Pole Aitken basin). The technical objectives are to perform a laserlink experiment (detection of laser beam emitted by ESA/Tenerife ground station), flight demonstration of new technologies and on-board autonomy navigation. The public outreach and educational objectives are to promote planetary exploration and space. We present here the first results obtained during the cruise phase.
Published: 19 August 2005
The second ESA Bulletin was devoted to a report on the Cos-B mission and was published just a few weeks after the launch of the spacecraft. it contains a detailed review of the project through design and development and on to launch and the first detection.
Published: 25 August 1975
In High Resolution Stereo Camera (HRSC) images of the Mars Express Mission a 130 km long interior channel is identified within a 400 km long valley network system located in the Lybia Montes. Ages of the valley floor and the surroundings as derived from crater counts define a period of ~350 Myrs during which the valley might have been formed. Based on HRSC stereo measurements the discharge of the interior channel is estimated at ~4800 m³/s, corresponding to a runoff production rate of ~1 cm/day. Mass balances indicate erosion rates of a few cm/year implying the erosion activity in the valley to a few thousand years for continuous flow, or one or more orders of magnitude longer time spans for more intermittent flows. Therefore, during the Hesperian, relatively brief but recurring episodes of erosion intervals are more likely than sustained flow.
Published: 17 August 2005
Rosetta, the first planetary cornerstone mission of the ESA Scientific Programme, was launched on 2 March 2004 on its ten year journey to rendezvous with comet 67P/Churyumov-Gerasimenko. In summer 2014, Rosetta will go into orbit around the comet's nucleus, approaching to within a few kilometres of its surface, will deliver a Lander called 'Philae' onto its surface to make in-situ measurements, and will then accompany the comet on its onward journey for about 1.5 years. The launch and the first 1.5 years of flight operations have been very smooth, with the spacecraft, its payload and the ground segment performing almost perfectly, with no major anomalies and all parameters well within specification. All planned mission activities have gone according to schedule, and additional 'bonus' scientific and technological operations were even added to the intense operations schedule of the first few months. Among the mission events to date were the observations of the NASA Deep Impact probe's encounter in July 2005 with comet 9P/Tempel-1, from a 'privileged' position in space just 80 million kilometres away.
Published: 16 August 2005
We have identified three multiply imaged galaxies in Hubble Space Telescope images of the redshift z=0.68 cluster responsible for the large-separation quadruply lensed quasar, SDSS J1004+4112. Spectroscopic redshifts have been secured for two of these systems using the Keck I 10 m telescope. The most distant lensed galaxy, at z=3.332, forms at least four images, and an Einstein ring encompassing 3.1 times more area than the Einstein ring of the lensed QSO images at z=1.74, due to the greater source distance. For a second multiply imaged galaxy, we identify Lyalpha emission at a redshift of z=2.74. The cluster mass profile can be constrained from near the center of the brightest cluster galaxy, where we observe both a radial arc and the fifth image of the lensed quasar, to the Einstein radius of the highest redshift galaxy, ~110 kpc. Our preliminary modeling indicates that the mass approximates an elliptical body, with an average projected logarithmic gradient of ~=-0.5. The system is potentially useful for a direct measurement of world models in a previously untested redshift range. Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
Published: 16 August 2005
The next generation astronomical X-ray telescopes (e.g. XEUS) require extremely large collecting area (10 m²) in combination with good angular resolution (5 arcsec). The existing technologies such as polished glass, nickel electroforming and foil optics would lead to excessively heavy and expensive optics, and/or are not able to produce the required large area or resolution. We have developed an entirely novel technology for producing X-ray optics which results in very light, stiff and modular optics which can be assembled into almost arbitrarily large apertures, and which are perfectly suited for XEUS. The technology makes use of commercially available silicon wafers from the semiconductor industry. The latest generation silicon wafers have a surface roughness that is sufficiently low for X-ray reflection, are planparallel to better than a micrometer, have almost perfect mechanical properties and are considerably cheaper than other high-quality optical materials. The wafers are bent into an accurate cone and assembled to form a light and stiff pore structure with pores of the order of a millimeter. The resulting modules form a small segment of a Wolter-I optic, and are easily assembled into an optic with large collecting area. We present the production principle of these silicon pore optics, the facilities that have been set up to produce these modules and experimental results showing the excellent performance of the first modules that have been produced. With further improvement we expect to be able to match the XEUS requirements for imaging resolution and mass.
Published: 01 January 2004
— 20 Items per Page