ESA Science & Technology - Publication Archive

In the proceedings of the 6th International Conference on Space Optics

This paper summarizes the results of an ESA feasibility study of a Wide-Field Optical Infrared Imager (WFI) that would search for Type Ia supernovae at low redshift with the aim to measure the changing rate of expansion of the universe. WFI multi-spectral images of the deep universe could also benefit to many other research area in astrophysics. The WFI payload includes a 2 m class telescope, a 1 square degree field of view imaging camera and a low-resolution integral field spectrometer. A mission concept was identified that consists of a 2000 kg spacecraft launched by a Soyuz-Fregat into a L2 halo orbit. The WFI mission could benefit from the technology developed for the ESA Herschel and Gaia missions and for the NIRSpec ESA instrument. A fully European WFI mission would require improvement of existing European detector and on-board processor technology as well as some effort to support the utilization of the 26 GHz Ka band.

A conventional Mercury sample return mission requires significant outbound and return trips, and the large mass of a planetary lander and ascent vehicle. In this paper, it is shown that solar sailing can be used to reduce lander mass allocation by delivering the lander to a low, thermally safe orbit close to the planetary 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. Propellant mass is not an issue for solar sails, so a sample can be returned relatively easily, without resorting to lengthy, multiple gravity assists. A 275 m square solar sail with a sail assembly loading of 5.9 g m-2 is used to deliver a lander, cruise stage and science payload to a forced Sun-synchronous orbit at Mercury in 2.85 years. The lander acquires samples, and conducts limited surface exploration. An ascent vehicle delivers a small cold gas rendezvous vehicle containing the samples for transfer to the solar sail. The solar sail then spirals back to Earth in one year. The total mission launch mass is 2353 kg, on an H2A202-4S class launch vehicle (C3=0). Extensive launch date scans have revealed an optimal launch date in April 2014 with sample return to Earth 4.4 years later. Solar sailing reduces launch mass by 60% and trip time by 40%, relative to conventional mission concepts. In comparison, mission analysis has demonstrated that solar sail Mars and Venus sample return appears to have only modest benefit in terms of reduced launch mass, at the expense of longer mission durations than conventional propulsion systems.

Determining the origin and composition of asteroids is a key step in understanding the nature of the solar system. Believed to be a captured asteroid, Deimos, Mars's moon, is therefore of dual scientific interest. The upper regolith of the moon contains Martian material accreted during the late heavy bombardment period. Retrieving a sample from Deimos would contain both asteroidal and Martian material. The perceived scientific interest in Deimos, and for small body sample return missions, are the key reasons that Deimos Sample Return (DSR) was chosen as one of ESA's Technology Reference Studies.

Technology Reference Studies (TRS) are a technology development tool introduced by ESA's Science Payload and Advanced Concepts Office, whose purpose is to provide a focus for the development of strategically important technologies that are of likely future relevance for scientific missions. This is accomplished through the study of several technologically demanding and scientifically interesting missions, which are currently not part of the ESA science programme.

The goal of the DSR TRS is to study the feasibility and the technologies required to collect a scientifically significant sample of regolith from Deimos' surface and return it to Earth. The DSR mission profile consists of a small spacecraft, launched on a Soyuz-Fregat 2B. After transferring to the Martian system, the spacecraft will enter into a co-orbit with Deimos where it will perform remote sensing observations and ultimately perform a series of sampling manoeuvres. Upon completion of sampling the spacecraft will return to Earth, where the sample canister will perform a direct Earth entry.

The X-Ray Observatory, also known as XEUS (X-Ray Evolving-Universe Spectroscopy), is one of the potential future missions identified in the framework of the ESA Call for Themes issued in April 2004. Preliminary studies on a post XMM-Newton mission assumed a LEO scenario, with two spacecraft in formation flying, 5 m² (at 1 keV) effective area mirror and a focal length of 35 m. The mirror optics was originally based on the same technology used for XMM (replicated nickel mirrors), while the mission scenario was assuming a multiple launch approach and the use the ISS as servicing post for the observatory.

S-Cam 3 is the 3rd generation of a cryogenic camera, based on superconducting tunnel junctions (STJs), for ground-based optical astronomy, deployed at the 4.2m William Herschel Telescope (WHT) at La Palma (Spain). It exploits a 10x12 pixel array of Ta/Al STJs, covering a field of view of ~9°x11° on the sky. The wavelength band extends from 330-750nm, with a wavelength resolving power of ~10 at 500nm. The detectors are operated at ~285mK, achieved with a double stage ⁴He-³He sorption cooler. Each pixel has its own electronic readout chain at room temperature, with a JFET-based charge sensitive preamplifier. The instrument has undergone extensive testing and calibration, followed by the first observation campaign at La Palma in July 2004. This campaign has focused on point sources with time variability, exploiting the instrument's unique combination of spectro-photometry with high time resolution.

To overcome the limited field of view which can be achieved with single STJ arrays, DROIDs (Distributed Read Out Imaging Devices) are being developed. DROIDs consist of a superconducting absorber strip with proximized STJs on either end. The ratio of the two signals from the STJs provides information on the absorption position and the sum signal is a measure for the energy of the absorbed photon. In our devices the absorber is an epitaxial Ta strip that extends underneath the Ta/Al read-out STJs. Thus, the bottom electrode of the STJs is an integral part of the absorber. Due to the proximity effect, the STJs have a lower energy gap than the absorber, causing trapping of quasiparticles in the STJs. The trapping will change with thicker Al layers because the energy gap of the devices will decrease. A series of 50x200µm and 20x200µm absorbers (including 50x50µm STJs) and different Al trapping layer thicknesses, ranging from 65 to 130nm, have been tested. The devices have been illuminated with 6 keV 55Fe photons. The position resolution is found to improve with increasing Al thickness. It is found that the current model needs to be adapted for DROIDs to account for different injection of quasiparticles into the STJ and extra losses to the absorber.

To overcome the limited field of view that can be achieved with single STJ arrays, DROIDS (Distributed Read Out Imaging Devices) are being developed. DROIDs consist of a superconducting absorber strip with proximized STJs on either end. The ratio of the two signals from the STJs provides information on the absorption position and the sum signal is a measure for the energy of the absorbed photon. To produce a large field of view with the least number of connection wires possible, the size of the DROID is an important parameter. A set of devices with different lengths, ranging from 200 to 700µm, has been tested at optical wavelengths. The widths of the DROIDs are 30µm with 30x30µm STJs Ta/Al STJs on either side. With 30nm layer thickness of Al the trapping of quasiparticles in the STJ is not optimal, but the devices can comfortably be operated at 300mK. All devices have been processed on a single wafer and are located on the same chip. Thus the STJs are all identical and any variation in response can be attributed to a difference in geometry. The position resolution is found to be degraded for shorter absorbers due to cross talk between the two STJs. The charge output of the different devices decreases with length due to reduced tunnel probability and losses in the absorber.

In a quest to further improve the performance of superconducting tunnel junctions as photon detectors over the broad spectral range from optical to X-ray wavelengths, a fundamental understanding of their limits is required. Recently fabricated Ta/Al STJs have shown an exceptionally high responsivity (number of collected charge carriers versus absorbed photon energy) and spectral resolution (R = E/Delta_E > 22 at 2.5eV). This high spectral resolution has now revealed some unique features when plotted against photon wavelength. The experimental data indicate the important role of the photon absorption profile. We have shown that vertical inhomogeneity is a fundamental consequence of the quasiparticle generation process in the thin film such that pair breaking phonons emitted in the process of energy down conversion have a chance to escape depending on the absorption depth. This results in an inhomogeneous broadening of the detected signal. We also found that another, previously unknown fundamental noise source exists which is related to statistical fluctuations of the angular distribution of phonons emitted in the down-conversion process. We present the new experimental data and compare them to the predictions of the down-conversion theory. We show that, while the responsivity is rather constant in the optical wavelength range, the intrinsic resolution exhibits a number of features which can be explained by changing statistical variations of the phonon losses as function of absorption depth.

Nulling interferometry, a powerful technique for high-resolution imaging of the close neighbourhood of bright astrophysical objets, is currently considered for future space missions such as Darwin or the Terrestrial Planet Finder Interferometer (TPF-I), both aiming at Earth-like planet detection and characterization. Ground-based nulling interferometers are being studied for both technology demonstration and scientific preparation of the Darwin/TPF-I missions through a systematic survey of circumstellar dust disks around nearby stars. In this paper, we investigate the influence of atmospheric turbulence on the performance of ground-based nulling instruments, and deduce the major design guidelines for such instruments. End-to-end numerical simulations allow us to estimate the performance of the main subsystems and thereby the actual sensitivity of the nuller to faint exozodiacal disks. Particular attention is also given to the important question of stellar leakage calibration. This study is illustrated in the context of GENIE, the Ground-based European Nulling Interferometer Experiment, to be installed at the VLTI and working in the L' band. We estimate that this instrument will detect exozodiacal clouds as faint as about 50 times the Solar zodiacal cloud, thereby placing strong constraints on the acceptable targets for Darwin/TPF-I.

Reference: GIPF-TN-730-VHS-0

This document gives a summary of the work performed within ESA Contract No. 16854/03/NL/HB "Development of a compact Geochemistry Instrument Package Facility (GIPF)". Main objective of this study was the development and assembly of a compact instrument facility for in-situ geochemistry sample analysis in planetary research. Several work packages listed in chapter 2 below have been performed, from the assessment of different geochemistry methods to final testing of the hardware and steps described to achieve a flight model for a given mission. The design was driven by the proposed BepiColombo mission to be used on a lander on planet Mercury. In other words BepiColombo was the "reference mission" for the presented study. The study was lead by von Hoerner & Sulger GmbH, Schwetzingen. Subcontracts to built the spectrometers and the camera have been placed at University of Mainz, Max-Planck-Institut für Chemie, Mainz and DLR, Deutsches Zentrum für Luft- und Raumfahrt e.V., Berlin.

Some examples of space-borne applications that require improvements in detector technology compared with conventional Si and Ge designs are described. Properties of compound semiconductors are noted, and a range of different detector developments are briefly reviewed. Material fabrication improvements for several compound semiconductors have resulted in near Fano-limited performance.

We report on a puzzling event occurred during a long BeppoSAX observation of the slow-rotating binary pulsar GX1+4 . During this event, lasting about 1 day, the source X-ray flux was over a factor 10 lower than normal. The low-energy pulsations disappeared while at higher energies they were shifted in phase by 0.25 . The con- tinuum spectrum taken outside this low-intensity event was well fitted by an absorbed cut-o power law, and exhibited a broad iron line at 6.5 keV probably due to the blending of the neutral (6.4 keV) and ionised (6.7 keV) K iron lines. The spectrum during the event was Compton reflection dominated and it showed two narrow iron lines at 6.4 keV and 7.0 keV, the latter never revealed before in this source. We also present a possible model for this event in which a variation of the accretion rate thickens a torus-like accretion disc which hides for a while the direct neutron star emission from our line of sight. In this scenario the Compton reflected emission observed during the event is well explained in terms of emission reflected by the side of the torus facing our line of sight.

The European Space Agency together with industrial partners has studied a concept for low-cost in-situ exploration of the atmosphere of Venus: the Venus Entry Probe. The Venus Entry Probe is one of ESA's Technology Reference Studies (TRS). TRSs are model science-driven missions that are, although not part of the ESA science programme, able to provide focus to future technology requirements. This is accomplished through the study of several technologically demanding and scientifically meaningful mission concepts, which are strategically chosen to address diverse technological issues. The TRSs complement ESA's current mission specific development programme and allow the ESA Science Directorate to strategically plan the development of technologies that will enable potential future scientific missions. Venus has been targeted because in-situ exploration of the atmosphere of Venus is both scientifically interesting and technologically challenging.

The Venus Entry Probe study is one of the European Space Agency's (ESA) technology reference studies. It aims to identify; the technologies required to develop a low-cost, science-driven mission for in-situ exploration of the atmosphere of Venus, and the philosophy that can be adopted. The mission includes a science gathering spacecraft in an elliptical polar Venus orbit, a relay satellite in highly elliptical Venus orbit, and an atmospheric entry probe delivering a long duration aerobot (aerial robot) which will drop several microprobes during its operational phase.

The atmospheric entry sequence is initiated at 120 km altitude and an entry velocity of 9.8 kms^-1. Once the velocity has reduced to 15 ms^-1 the aerobot is deployed. This consists of a gondola and balloon and has a floating mass of 32 kg (which includes 8 kg of science instruments and microprobes). To avoid Venus' crushing surface pressure and high temperature an equilibrium float altitude of around 55 km has been baselined. The aerobot will circumnavigate Venus several times over a 15-22 lifetime analysing the Venusian middle cloud layer. Science data will be returned at 2.5 kbps over the mission duration. At scientifically interesting locations 15 drop-sondes will be released.

This paper focuses on the final mission design with particular emphasis on system level trade-offs including the balloon and pressurisation system, communications architecture, power system, design for mission lifetime in a hostile and acidic environment. It discusses the system design, design drivers and presents an overview of the innovative mission-enabling and mission-enhancing technologies.

The concept of Technology Reference Studies (TRS), set up by ESA's Science Payload and Advanced Concepts Office (SCI-A) to focus the development of strategically important technologies of likely relevance to future science missions, has already been introduced in 2004 at the 55^th IAC in Vancouver^[1].

Significant progress in the definition of the mission concepts and related technology requirements has been achieved since then. At the present time the Planetary Exploration Studies Section of SCI-A has finished the study of the first four TRSs, the Venus Entry Probe (VEP), the Jupiter Minisat Explorer (JME), the Deimos Sample Return (DSR) and the Interstellar Heliopause Probe (IHP). Current study activities are now focusing on the extension of the Jovian Explorer scenario towards magnetospheric and atmospheric investigations by means of additional orbiter(s) and entry probes. New introduced concepts deal with cross-scale constellation (CSM) of up to 12 spacecrafts to further explore the Earth magnetosphere and a Near Earth Asteroid Sample Return (ASR).

All TRS mission profiles are based on small spacecraft, with miniaturized highly integrated payload suites (HIPS) and launched on Soyuz Fregat-2B (SF-2B) as baseline. TRSs are set up to provide thematic context for technology development based on feasible mission concepts, which may be also used by the scientific community as embryonic building blocks for future mission proposals. This paper describes the current status of the new concepts under study (CSM, JEP, ASR) and the final results of the first four TRSs (JME, DSR, VEP and IHP) in further detail.

We discuss the potential benefits of using compound semiconductors for the detection of X- and gamma-ray radiation. While Si and Ge have become detection standards for energy dispersive spectroscopy in the laboratory, their use for an increasing range of applications is becoming marginalized by one or more of their physical limitations; namely the need for ancillary cooling systems or bulky cryogenics, their modest stopping powers and radiation intolerance. Compound semiconductors encompass such a wide range of physical properties that it is technically feasible to engineer a material to any application. Wide band-gap compounds offer the ability to operate in a wide range of thermal and radiation environments, whilst still maintaining sub-keV spectral resolution at hard X-ray wavelengths. Narrow band-gap materials, on the other hand, offer the potential of exceeding the spectral resolution of both Si and Ge, by as much as a factor of 3. Assuming that the total system noise can be reduced to a level commensurate with Fano noise, spectroscopic detectors could work in the XUV, effectively bridging the gap between the ultraviolet and soft X-ray wavebands. Thus, in principle, compound semiconductor detectors can provide continuous spectroscopic coverage from the far infrared through to gamma-ray wavelengths. However, while they are routinely used at infrared and optical wavelengths, in other bands, their development has been plagued by material and fabrication problems. This is particularly true at hard X- and gamma-ray wavelengths, where only a few compounds (e.g., GaAs, CdZnTe and HgI₂) have evolved sufficiently to produce working detection systems. In this paper, we examine the current status of research in compound semiconductors and by a careful examination of material properties and future requirements, recommend a number of compounds for further development.

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.

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.

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

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.