ESA Science & Technology - Publication Archive

Definition of the Rosetta ground segment began in 1996 and it soon became clear that a common checkout and mission-control system would be very beneficial for the mission. The chosen approach for achieving this goal was to develop building blocks for the Central Checkout System that can be re-utilised later in the development of the Flight Control System. The Rosetta prime contractor and AIV contractor fully endorsed this approach and the complete system is currently under development. The first delivery of the database system should take place in November 1998, followed by that of the first Central Checkout System in 1999.

The XMM (X-ray Multi-Mirror) spacecraft, a space-borne X-ray observatory to be launched by Ariane-5 in 1999, stands 10 m-high and measures over 4 m in diameter in launch configuration, for a launch mass of just under 4 tonnes. Such a tall spacecraft challenges the capabilities of existing European environmental testing facilities. The spacecraft design does allow for a relatively straightforward splitting into modules. The Structural and Thermal Model (STM) of the XMM spacecraft has successfully undergone mechanical environmental testing at the European Space Research & Technology Centre (ESTEC). This article briefly introduces the XMM project, presents an overview of the XMM configuration constraints, explains the spacecraft-level model philosophy and mechanical test flow, and summarises the present status of the tests performed.

The Infrared Space Observatory (ISO), the world's first true orbiting infrared observatory, was switched off in May 1998, long after the expiry date foreseen in the specifications for the mission. Instead of the required 18 months, the highly-successful in-orbit operations of this excellent satellite continued for more than 28 months leading to an extensive database of observations which will be providing astronomical surprises for years to come. This article looks back at the way operations were conducted.

The Rosetta mission is designed to study in-situ a cometary nucleus' environment and its evolution in the inner Solar System. To be launched in January 2003 by an Ariane-5, Rosetta will rendezvous with Comet P/Wirtanen in 2011, after one Mars- and two Earth-gravity assists, and two asteroid fly-bys. The near-comet operations, which are scheduled to last about 1.5 years, will require a minimum return-link telemetry data rate of 5 kbit/s to meet the scientific goals, with about 14 hours of daily coverage.

The SMART-1 mission, to be launched at the end of 2001, is intended to demonstrate innovative and key technologies for deep-space scientific missions. Its use, for example, of solar electric propulsion as its primary drive mechanism will be a first for Europe and is essential in paving the way for future ESA projects with large velocity requirements, such as the Mercury Cornerstone mission. SMART-1 will also be a test case for a new approach in terms of implementation strategy and spacecraft procurement for the ESA Science Programme. The total life-cost budget allocated to SMART-1 is 50 MECU. This budget constraint imposes use of a cheap launch option, such as an Ariane-5 auxiliary payload launch into a standard GTO or a Rockot escape-trajectory launch. This in turn limits the planetary bodies that can be reached within a given short (1.5 - 2 year) overall mission lifetime, which do, however, include the Moon and Earth-crossing asteroids or comets.

Mars Express, planned to be the first 'flexible mission' in the revised ESA Long-Term Scientific Programme, is based on a fast implementation scenario and will be launched towards Mars in June 2003 by a Soyuz/Fregat launcher. The mission is cost-capped at 150 MECU and will be submitted for approval by ESA's Science Programme Committee in November 1998. Its payload has already been selected and European industry will submit bids for design and development phases (Phases-B/C and D) at the beginning of September 1998.

The XMM satellite will be launched in August 1999 and a simulator has been developed to test and validate the supporting ground segment. Due to the very tight schedule and the high fidelity of the modelling required, this development effort has provided numerous unique challenges. This article details these challenges and the approaches that have been taken in meeting them, as well as the simulator's architecture and current status.

Product Assurance has both a preventative and a corrective role in terms of quality control in a spacecraft project. This article summarises how it was approached within the XMM project, what unforeseen problems were encountered, and what lessons can be learned from our experience.

The XMM (X-ray Multi-Mirror) spacecraft, a spaceborne X-ray observatory to be launched by Ariane-5, stands 10 m high and measures over 4 m in diameter in launch configuration, for a launch mass of just under four tons. Such a tall spacecraft challenges the capabilities of existing European environmental testing facilities. Provisions were made in the design for a split according to geometry into an Upper Module and a Lower Module for environmental test purposes. Optical testing of the X-ray Mirror Modules - the core technological challenge - required the use of several existing and custom-built test facilities. In the face of strict schedule requirements, spacecraft-level test flows were organised around extensively parallel flows and all tests were scrutinised for their potential for early problem identification. This article briefly introduces the XMM configuration and schedule constraints, explains the spacecraft-level model philosophy, discusses the consequences for each category of test in terms of facility and test specimen configurations, and summarises the spacecraft test flows and the results achieved.

Scaling the XMM data-handling system to the specific needs of the mission has resulted in a flexible and relatively uncomplicated On-Board Data-Handling (OBDH) Subsystem. The XMM spacecraft is operated in real-time and is fully compliant with the ESA packet data-handling standards. Packets are of variable length and presented to the OBDH when available. The OBDH telemetry/telecommand cycles are asynchronous to the instrument and subsystem cycles. The telemetry rates can easily be reallocated to cope with unforeseen spacecraft configurations, e.g. lost telemetry sources. The absence of requirements for automatic reconfiguration has contributed to the reduced subsystem complexity.

The XMM Electrical Power Subsystem (EPS) is 'conventional' in design in that it follows the ESA Power Standard. This article describes the subsystem's main features and performances, and the steps that have been taken in implementing the lessons learnt from the highly successful SOHO spacecraft recovery efforts, which were described in detail in ESA Bulletin No. 97.

The pointing and alignment performance of the XMM spacecraft will have a very strong influence on the quality of the scientific results obtainable. The pre-launch unit and subsystem tests and subsequent analyses have shown that the scientific requirements will indeed be met with comfortable margins and the performance goals will be met for more than 90% of all anticipated observations.

Attitude and orbit control of the XMM spacecraft relies on two subsystems on-board: the Attitude and Orbit Control Subsystem (AOCS) and the Reaction Control Subsystem (RCS). Together they must provide:

• stable Sun-pointing after the spacecraft's separation from the launcher and during solar-array deployment
• increase the spacecraft's perigee altitude by means of a chemical propulsion delta-V of about 210 ms^-1
• provide undisturbed, high-accuracy, three-axis pointing during scientific observations lasting up to 40 h
• slew the spacecraft between observations, and before and after perigee ensure that the Sun remains more than 15 deg away from the telescope at all times.

The XMM spacecraft has a conventional structure and thermal design. Due to the long focal length of the telescopes (7.5 m), the mirrors are far removed from the instruments. On the ground and during the launch, the structure has to maintain the integrity of the whole spacecraft. The thermal control does not make use of onboard software. In orbit, the functions of the structure and the thermal control are mixed. Their global common requirement is to relate and align the set of mirrors at one end of the spacecraft with the set of instruments at the other. Some parts of the instruments will be kept at cryogenic temperatures, but most of the spacecraft will be kept at about room temperature. It is the task of the thermal control to maintain these diverse requirements by passive and active means. Under inevitably varying thermal conditions, the structure has to stay very 'straight'.

We recently predicted the formation of a highly non-equilibrium quasiparticle (qp) distribution in low TC multiple tunnelling superconducting tunnel junctions (STJs) [1]. The situation arises through qp energy gain in cycles of successive forward and back tunnelling events in the absence of relaxation via sub-gap phonon emission. The qps can acquire sufficient energy to emit phonons, which break more Cooper pairs and release additional qps. In this paper we report theoretical and experimental studies of the effect of this process on photon detection by such an STJ. We derived a set of energy-dependent balance equations [2], which describe the kinetics of the qps and phonons, including the qp multiplication process described above. Solution of the balance equations gives the non-equilibrium distribution of the qps as a function of time and energy, and hence the responsivity of the STJ as a function of bias voltage. We compared the theoretical results with experiments on high quality, multiple-tunnelling Al STJs cooled to 35mK in an adiabatic demagnetisation refrigerator, and illuminated with monochromatic photons with wavelengths between 250 and 1000 nm. It was found that in the larger junctions with the longest qp loss time, both responsivity and signal decay time increased rapidly with bias voltage. Excellent agreement was obtained between the observed effects and theoretical modelling.

This article is based on a talk given by Professor Gerhard Beutler of the Astronomical Institute, University of Berne, to the Pro ISSI.

The XMM observatory has, at its heart, three large X-ray telescopes, which will provide a large collecting area (1430 cm² each at 1.5 keV, and 610 cm² each at 8.0 keV) with a spatial resolution of around 14-15 arcsec. At the end of 1998, three months ahead of schedule, the three flight and the two spare models of the X-ray telescope were handed over to the XMM Prime Contractor Daimler Chrysler Aerospace (D). The three flight models were integrated onto the spacecraft's optical platform at ESTEC at the end of March 1999.

The payload carried by the X-Ray Multi-Mirror Mission (XMM), the second Cornerstone of the ESA Horizon 2000 Science Programme, consists of three scientific instruments: the Reflection Grating Spectrometer (RGS), the European Photon Imaging Camera (EPIC), and the Optical Monitor (OM). This article provides a general overview of the main characteristics of all three instruments.

The X-Ray Multi-Mirror Mission (XMM) is an X-ray astrophysics observatory scheduled for launch in December 1999. With a projected lifetime of 10 years, it will enable astronomers to conduct sensitive spectroscopic observations of a wide variety of cosmic sources.

Since its launch on 2 December 1995, the Solar and Heliospheric Observatory (SOHO) has provided an unparalleled breadth and depth of information about the Sun, from its interior, through the hot and dynamic atmosphere, out to the solar wind. Analysis of the helioseismology data from SOHO 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, sub-convection-zone mixing, a possible circumpolar jet, and very slow polar rotation. The ultraviolet imagers and spectrometers have revealed an extremely dynamic solar atmosphere in which plasma flows play an important role. Evidence for an upward transfer of magnetic energy from the Sun's surface toward the corona has been found. Electrons in coronal holes have been found to be relatively 'cool', whereas heavy ions are extremely hot and have highly anisotropic velocity distributions. The source region for the high-speed solar wind has been identified and the acceleration profiles of both the slow and fast solar wind have been measured.