If everything goes according to plan - and there are no indications to the contrary so far - the second half of this year will see an unprecedented series of events in ESA's Scientific Programme. Three launches are scheduled in the period September to November, sending a total of six scientific spacecraft into orbit around planet Earth to explore its magnetosphere, the Sun and the infrared radiation sources deep in outer space. These missions will demonstrate the high technical standards of ESA and European Space Industry, the desire of European scientists to explore the Universe with their state-of-the-art instrumentation, and the merits of successful cooperation with other international space agencies such as NASA and Japan's Institute of Space and Astronautical Sciences (ISAS).
The first mission 'to go' will be the Infrared Space Observatory (ISO), launch of which is scheduled for September 1995. With its scientific instruments cooled to the temperature of superfluid helium, ISO will observe the heat radiation from very weak celestial sources out to extreme extragalactic distances. Launch will take place from Kourou in French Guiana, on an Ariane 44P rocket.
This launch will be followed by that of the Solar and Heliospheric Observatory (SOHO) at the end of October 1995. SOHO will be placed into a halo orbit around the first Lagrangian point, between the Earth and the Sun, about 1.5 million km from Earth. This special orbit will be ideal for the mission's scientific objective of continuous study of the Sun, including the solar corona, the solar wind and the solar interior structure. Being a cooperative programme with NASA, SOHO will be put into orbit from the Kennedy Space Center in Florida by an Atlas Centaur IIAS launcher.
The year will be rounded off with another major highlight for the Scientific Programme with the launch of the Cluster mission: for the first time in the Agency's history, four identical scientific spacecraft will be put into orbit by a single launcher. It will be the first qualification flight (V501) of Europe's biggest launcher, Ariane-5, that will carry the four Cluster satellites into space. The four spacecraft will fly in formation in a near-polar orbit, in one of the most ambitious space missions attempted so far. They will chart, in three dimensions, the tenuous plasma structures that fill the Earth's immediate space environment.
The first major step in this direction was taken with the Infrared Astronomical Satellite (IRAS), which surveyed almost the entire sky in the 8-120 micron wavelength range in 1983. Compared with IRAS, ISO will have a longer operational time, wider wavelength coverage (2.5-200 microns), better angular resolution, more sophisticated instruments, and a sensitivity gain of several orders of magnitude. Most importantly, ISO will be operated as an observatory, able to point to specific objects in space and observe them continuously.
The ISO mission was approved by ESA's Science Programme Committee (SPC) in March 1983. The design and development phase of the project started in early 1987. The project is currently in the final system-testing phase and is getting ready for the launch from Kourou in September.
The satellite will be operated from ESA's Villafranca ground station, near Madrid, for at least 18 months. A second ground station, at Goldstone in the USA, will be used to relay telecommands and telemetry for some time each day.
Figure 1. Configuration of the ISO satellite
The spacecraft has to provide an extremely cold environment for the telescope and its instruments to detect and measure extremely weak heat sources. In the case of ISO, this is done by placing both the instruments and the telescope in a large, thermally insulated vacuum vessel filled with superfluid helium at a temperature of less than 2 K (about -271 deg C). The helium slowly evaporates to provide the necessary cooling, and is vented to space. The initial volume of helium in the tank therefore determines the satellite's lifetime.
The spacecraft consists of a payload module (the upper cylindrical part in Fig. 1) and the service module (below) with its interface to the Ariane launch vehicle. The main characteristics of the ISO spacecraft are summarised in Table 1. Figure 2 shows the flight-model spacecraft during testing at ESTEC.
The payload module is essentially a large cryostat, with a long toroidal tank filled with superfluid helium. In the centre of the cryostat is mounted a 60 cm-diameter telescope. The scientific instruments are mounted behind the telescope's primary mirror. Some instrument detectors are strapped directly to the liquid-helium tank to keep them at very cold temperatures (2-4 K). Infrared radiation from an object in space passes through the telescope and into the instruments as indicated by the red arrows in Figure 1. The payload-module cryostat is well- insulated with thermal blankets and the sunshield mounted on the side protects it from direct heating by the Sun.
The service module at the bottom of the spacecraft provides the traditional spacecraft functions, such as power conditioning, data handling, telecommunications and attitude control. The attitude-control subsystem can point the satellite with an accuracy of a few arcseconds. The onboard computer system also provides for safe autonomous satellite operations (in the event of loss of control from the ground) for at least three days. This system ensures that the telescope will never be pointed towards the Sun or the Earth, to avoid damaging the scientific instruments.
The spacecraft design employs very advanced technologies and therefore demanded an extensive development programme, especially for the payload module's cryogenic system and telescope. Special development models of the cryostat and telescope were built and tested. Similarly, the attitude-control system is very complex and its development was also highly demanding. Considerable effort and care were dedicated over several years to the develop-ment of these three critical-technology areas in order to solve all potential problems prior to the assembly and testing of the flight-model satellite. The approach taken was very successful: the final flight-model test programme was a great success and was completed on schedule and without any major problems.
Figure 2. The ISO flight-model satellite after mechanical/environmental testing at ESTEC in Noordvijk (NL)
Table 1. ISO spacecraft characteristics
The ISO instrument complement consists of a camera, an imaging photopolarimeter and two spectrometers (Table 2). Each instrument was developed by an international consortium of institutes using national funding, and then delivered to ESA for integration into the satellite. These instruments are all very sophisticated and have been optimised to form a complete, complementary and versatile common-user facility.
The development of these instruments was also highly challenging and took place over many years, starting before the satellite Phase-B. The result of these efforts is a set of advanced instruments which perform very well in the flight-model satellite, and the scientific com-munity is already eager to exploit them.
Table 2. ISO scientific instruments
ISO will be launched by an Ariane-44P vehicle into transfer orbit and its hydrazine reaction-control system will then be used to attain the operational orbit. This will have a 24 h period, a perigee height of 1000 km, an apogee height of 70 000 km, and an inclination to the equator of 5 degrees. The scientifically useful time in this orbit (Fig. 3) is that spent outside the Earth's radiation belts, namely about 16 h per day. Instrument performance is generally degraded when in the radiation belts, where operations are therefore generally restricted to satellite and instrument setup, etc.
ISO will be operated from ESA's Villafranca ground station, near Madrid, Spain. A second ground station, at Goldstone in California (USA) will be dedicated to ISO under a cooperative agreement with NASA and the Japanese Institute of Space and Astronautical Sciences (ISAS). In return for the support given, both NASA and ISAS may each use ISO for 0.5 h per day. Over one third of ISO's scientifically useful time is guaranteed time for the parties involved in the development of the scientific instruments, and the remaining time is available to the general astronomical community.
The concept for ISO operations is shown in Figure 4. Astronomers submit their proposals, which are screened by peer review groups. Following selection, the full proposals are entered electronically, scheduled taking into account all relevant constraints, and then executed by the satellite, which sends the data that has been gathered to the ground.
The observing programme is currently being established. The guaranteed time, for those scientists involved in the construction and operation of the facility, involves some 13 000 observations. Approximately 1000 proposals for the open time have been received, which have been ranked in order of priority. Some 500 proposals have been recommended, and the average time alloted per proposal is about 6 h; this may entail in the order of several tens of thousands of individual observations. Clearly, there is very high interest in the astronomical community in using ISO.
Figure 3. ISO's orbit
Figure 4. ISO mission operations concept
The flight-model satellite, with its scientific instruments, has successfully concluded all of its system environmental tests. The satellite operations interfaces with the ground segment have also been successfully tested. The total satellite and ground-segment system will be finally tested in April before ISO is packed for shipment to Kourou.
All efforts are geared to a successful launch campaign over the summer months for the intended launch in September 1995.
The Solar and Heliospheric Observatory, or SOHO, is a spacecraft that carries many international experiments to observe the Sun and measure the particles it generates from a special vantage point, the so-called 'Lagrangian first equilibrium point (L1) 'between the Earth and the Sun, 1.5 million km from Earth. It will orbit for at least two years (possibly up to six) in a 'halo 'orbit around L1, which will allow uninterrupted observation of the daylight star for the first time in solar-physics investigations.
SOHO is one of the two missions - the other being Cluster - making up the Solar Terrestrial Science Programme (STSP), the first 'Corner-stone' of ESA's Horizon 2000 Programme. The SOHO heritage comes from the initial solar missions of OSO and Skylab's Apollo Telescope Mount Mission of the 1970s, which were followed by several European/American proposals (GRIST, DISCO).
The SOHO mission was proposed in 1982 and approved by ESA's Science Programme Committee (SPC) in February 1986. The payload composition was announced, after the selection process, in March 1988; the industrial Phase-B contract was awarded to MMS-F (then Matra Espace) in late 1989; the Phase-C/D contract was kicked-off in May 1991. The launch, planned at mission selection for summer 1995, is foreseen for 30 October this year. The launch vehicle, supplied by NASA, will be an Atlas IIAS, lifting off from Kennedy Space Center in Florida.
ESA retains overall mission responsibility for SOHO and the interfaces with the NASA centres - Goddard Space Flight Center (GSFC), Kennedy Space Center (KSC) and Lewis Research Center (LeRC) - in implementing this responsibility. NASA will in particular implement the operations, SOHO being controlled in this phase from GSFC.
The SOHO spacecraft in launch configuration will weigh about 1875 kg (Fig. 5 and Table 3). It consists of a lower section, the service module (Fig. 6), which houses all the services and its single large propellant tank containing 235 kg of hydrazine at launch. The upper section is the payload module (Fig. 7), around which all instruments are clustered; the large coronal instruments in particular cover the sides of this module in carefully engineered installations.
Three main elements have driven the design of the satellite: pointing stability, both short- and long-term; modularity, to allow maximum parallel processing for the service module and the payload module with its extremely complex and heavy payload; and finally cost and schedule. The uniquely stable thermal environment in the operational halo orbit has allowed aluminium-alloy structures to be used also for the payload module, which supports the large optical instruments. A finely tunable thermal-control system provides an overall platform stability of 1 arcsec over 15 min (short-term stability) including attitude control contributions, and 10 arcsec over 6 months (absolute stability).
The attitude-control system is principally based on a very accurate fine-pointing Sun sensor for pitch and yaw and two starmappers for roll control. Four reaction wheels allow a long period (8 weeks) of undisturbed operations before momentum off-loading. Both house-keeping telemetry and most scientific data are to be stored onboard on a solid-state recorder (a European development) and a NASA-supplied tape recorder and transmitted to the ground station via the steerable high-gain antenna at the rear of the spacecraft, facing Earth. The solar-array wings will deliver 1.5 kW of power.
Special attention has been paid to minimising all disturbance sources that could lead to high-frequency 'blurring' of the pointing performance. Key sources for possible disturbances (reaction wheels, tape recorders, experiment mechanisms, etc.) have been analysed and balanced as much as possible during the spacecraft's development and correlated with specific measurements on available flight elements.
The system development effort for SOHO has been limited to a structural/engineering model (1993) and a flight model, which underwent the only thermal-vacuum/thermal-balance test of the development programme at the end of last year.
Specific attention has also been paid to the extremely stringent cleanliness requirements, dictated in particular by the presence of ultraviolet optics, both during the assembly and testing of the flight-model spacecraft and for in-orbit operations. This has been achieved on the ground without resorting to specially-built facilities, by applying a careful combination of nitrogen purging, protections (doors) on the most sensitive payload elements, and the use of industry- standard clean rooms and laminar-flow tents (at MMS-F, MMS-UK and Intespace).
Table 3. SOHO spacecraft characteristics
Figure 5. The complete SOHO spacecraft just before thermal-balance testing at Ontespace in Toulouse (F) in December 1994
Figure 6. The SOHO service module entering the SIMLES solar-simulation facility at Intespace in Toulouse (F)
Figure 7. The SOHO payload module, without thermal blankets, at the end of its integration and testing at MMS in Portsmouth (UK)
The SOHO payload is by far the most complex and heaviest payload produced so far by a scientific community for an ESA mission, with its 650 kg and 11 state-of-the-art instruments involving 39 international institutes and a total of 33 separate units.
The SOHO payload covers three solar-physics disciplines simultaneously for the first time: investigations of the solar atmosphere, helioseismology and solar-wind in-situ measurements (see Table 4). Through its payload complement, SOHO will be able to offer solar scientists a unique opportunity to examine and understand the structure and dynamics of the interior of the Sun (helioseismology), the chromosphere, transition zone and corona (coronal instruments) and the solar wind which will stream around SOHO a few days after originating at the Sun.
Each experiment has been developed by an international consortium of scientists, funded by their own national bodies, and led by Principal Investigators (PI), three of whom lead the NASA contribution to the scientific payload.
Table 4. SOHO scientific instruments
SOHO will be launched by an Atlas IIAS, the most powerful version of the Atlas-Centaur family, and will be injected initially into a low Earth orbit. After a short coast phase of at most 110 min, the Centaur stage will be restarted and will inject SOHO into a transfer orbit towards the Lagrangian point, 1.5 million km from Earth (Figs. 8 & 9). After a four-month transfer phase, the halo orbit will be reached with a further series of manoeuvres before the nominal mission can begin.
In general, one pass of 8 h and two passes of 1.3 h are foreseen with the Deep Space Network (DSN) each day for real-time solar observations and the dumping of data stored on board. For two months each year the DSN coverage will be continuous. An Experiment Operations Facility, headed by the ESA Project Scientist, has been set up at GSFC to host all of the scientific teams involved. There they will operate their experiments in orbit in close coordination with the Flight Operations Team, which will operate the spacecraft.
If the spacecraft's health permits and resources are sufficient at the end of the nominal two-year mission, further mission extensions to up to six years may be considered.
Figure 8. The SOHO mission phases
Figure 9. The SOHO launch and early orbit phase
Earlier space missions to explore the Earth's magnetosphere have shown how dramatic the interaction can be between near-Earth space and the continuous stream of plasma ejected by the Sun, known as the 'solar wind' (Fig. 10). An incredible range of phenomena have been observed - such as the shorting out of satellite components in orbit, power surges in long transmission lines, and disturbances in short-wave radio broadcasting. Auroras are probably the most spectacular visible manifestation of plasma processes in space.
Cluster will address the structure of electro-magnetic fields and the distribution of particles in the solar wind and in the Earth's magnetic field in unprecedented detail. For the first time ever, it will provide the means to investigate the full extent of interesting phenomena in three dimensions. A minimum of four identical spacecraft in carefully designed orbits are required to achieve this, as they must fly in scientifically meaningful configurations. The relative distances between the four spacecraft will be varied between 200 and 18 000 km during the course of the mission (Fig. 11).
The Cluster project was approved by ESA's Science Programme Committee in February 1986, within the framwork of the Solar Terrestrial Science Programme, together with the SOHO project. The design and develop-ment phase started in October 1989. All four spacecraft have now been manufactured and are undergoing final testing, ready to be shipped to the Kourou launch site in July for their November 1995 launch.
Figure 10. The Cluster mission concept
Figure 11.Orbital configuration of the four Cluster spacecraft
Each of the four Cluster spacecraft will carry a full complement of state-of-the-art instruments to measure electromagnetic fields and particles. Each payload consists of eleven instruments and includes six booms, four of which are 50 m-long wire booms and the remaining two are rigid 5 m booms (Fig. 12). Active sensors mounted at the tips of the 50 m booms will detect the ambient electric fields. The short booms carry the sensors for the magnetic-field instruments. The remaining instruments are mounted inside the spacecraft's cylindrical body, with their sensors protruding through the outer skin.
Each payload will constitute about 72 kg of the spacecraft's 550 kg dry mass, and some 650 kg of onboard fuel will be needed to meet the scientific requirements in terms of orbits and orbital separation manoeuvres. The launcher requirement to carry one satellite on top of another in a dual-launch configuration, and the accommodation of the onboard fuel, represented major design drivers.
Each of the 2.9 m-diameter and 1.3 m-high cylindrical spacecraft will be spin-stabilised. They will be powered by solar arrays covering most of their cylindrical surface. Five batteries will ensure survival during eclipses by powering heaters for the most critical components.
The Cluster spacecraft have been designed and built by a European consortium of space industries, led by Dornier GmbH (D) as Prime Contractor (Fig. 13).
Figure 12. Cluster flight-model spacecraft number 3
Figure 13. The four flight-model Cluster spacecraft photographed together at IABG, Munich (D)
Table 5. Cluster spacecraft characteristics
All four spacecraft will be launched on Ariane-5's maiden flight and delivered into a standard Geostationary Transfer Orbit (GTO). Immediately after their separation from the launch vehicle, the four spacecraft will be controlled from ESA's European Space Operations Centre (ESOC) in Darmstadt, Germany (Fig. 14). Over the following three-week period, the spacecraft will be transferred from their geostationary transfer orbit into their routine mission orbit.
This transfer will require a total of twenty manoeuvres to bring the satellites from the original 8degree Ariane-5 delivery inclination to a 90 degrees inclination, and then finally to the routine elliptical polar orbit (4 x 19.6 R E ). A strategy has been developed to manoeuvre and control the four satellites in pairs during this initial phase, and to reach the mission- phase orbit in the desired tetrahedral configuration with predefined separation distances between the spacecraft when crossing the scientific regions of interest. This spatial configuration will be adjusted for different phases of the mission.
Eleven Principal-Investigator teams will be located at ESOC for a two-month period following launch, in order to commission their experiments, a total of 44 instruments. The combined operation of four spacecraft throughout their in-orbit lifetime (a minimum of 24 months for science product generation) will be performed by ESOC via two ground stations at Redu (B) and Odenwald (D), in conjunction with a Joint Science Operations Centre at RAL (UK) providing scientific mission-planning inputs and command requests originating from the Principal Investigators.
The control concept for the Cluster mission foresees all payload operations being pre- planned and executed from the onboard master schedule, whereby the onboard execution may occur up to 30 h later, because periods with no ground-station coverage can be encountered due to the characteristics of the orbit. For the same reason, data generated onboard are only occasionally directly transmitted to ground (when visibility permits), but will primarily be recorded by the onboard solid-state recorders for dumping to ground at a later time. Upon receipt at the ground station(s), the real-time telemetry data will be sent directly to ESOC, permitting access in near-real-time, whilst the transmission of playback data will occur within 24 h of reception at the ground station.
The scientific data will be distributed on CD-ROM, although the Principal Investigators will also have access to quick-look data via an electronic network.
Figure 14. The cluster mission-control system
Table 6. Cluster scientific instruments
Scientific operation of the Cluster spacecraft will be co-ordinated through the Joint Science Operations Centre in the United Kingdom. The commanding of the four payloads and the scientific analysis of the data downlinked from them will be supported by the Cluster Science Data System (Fig. 15). Major national data centres installed in Austria, France, Germany, Hungary, Sweden, the United Kingdom and the United States will be computer-networked to ensure fast and reliable data exchange. A data centre in China will also participate in this data system.
Figure 15. The Cluster Science Data System
The Cluster mission will contribute significantly to our understanding of the complex interaction between the solar wind and our environment here on Earth. In the future, 'space weather' predictions will be important to warn, for instance, operators of telecommunications satellites and power grids of potential problems ensuing from disturbances at Earth which have their origin in explosive releases of energy at the surface of the Sun. Even airlines could be forewarned to avoid exposing their passengers to excessive doses of radiation from space when flying at high geographic latitudes.
All four flight-model spacecraft, with their scientific instruments, have successfully concluded all of their system environmental tests. They are now in the process of final refurbishment with the flight sensors for the instruments and will be ready for packing and shipping to Kourou in July. The satellite-operations interfaces with the ground segment have also been successfully tested.
The long launch campaign starts in the summer with final checkout of the payload and subsystems, followed by the filling with more than 2 tons of propellant. Final assembly of the four spacecraft with the launch vehicle will then take place, with launch scheduled for late November 1995.