During the first half of 1996, the Agency will launch a unique flotilla of spacecraft to study the interaction between Sun and Earth in unprecedented detail.
The Cluster mission was first proposed to the Agency in late 1982 and has since evolved into a complex four-spacecraft mission to carry out three-dimensional measurements of the magnetosphere, covering both large- and small-scale phenomena in the sunward and tail regions.
The series of firsts achieved by the mission is impressive, this being:
The assessment study ran from February to August 1983 and was followed by a design study (so-called Phase-A) during 1984/85, which was presented to the scientific community in late 1985. One proposal at that point was for a four-spacecraft mission consisting of a mother spacecraft weighing 270 kg and carrying 46 kg of scientific payload, and three daughter spacecraft each of 217 kg and carrying 26 kg of scientific payload. These four spacecraft were intended to be launched into a polar orbit of 4 x 22 RE (Earth radii).
During the period of mission selection, the SOHO mission had also been studied at Phase-A level, and both missions were originally intended to be an international collaboration with NASA. In the meantime, the Science Programme Committee had approved the Agency's long-term programme 'Space Science: Horizon 2000', and in February 1986 the combined Cluster and SOHO missions were selected as the first 'Cornerstone' of that Horizon 2000 plan, together constituting the Solar Terrestrial Physics (STP) mission.
The mission was initially approved by ESA's Science Programme Committee (SPC) as a European- only mission, but once detailed definition of the STP mission was pursued it became obvious that the costs could not be accommodated within the 400 MAU baseline figure allocated to Horizon 2000 Cornerstones. Thereafter, further collaboration with NASA was rigorously pursued.
As a part of the cost-reduction exercise, two ESA committees of external scientists then embarked on a process of scientific descoping, and with international cooperation from NASA the cost estimates for the STP mission were reduced to 484 MAU, a figure finally accepted by the SPC as a target cost for the mission. This figure, escalated to current prices and adjusted for changes in the over-head rates remains the cost envelope for the two missions and has been contained in all the subsequent years of mission evolution.
At one stage, as part of the collaborative effort with NASA, a proposal was made to use the first Cluster spacecraft in place of the planned NASA Equator spacecraft, and to launch this first spacecraft on a NASA launch vehicle into an equatorial orbit for an initial one-year mission. The remaining three Cluster spacecraft would then have been launched by ESA into their polar orbit, to be joined by the first 'equatorial' spacecraft to form the cluster of four. This concept was subsequently abandoned due to the expected degradation of payload units after the first year in equatorial orbit. The difficulties of intercalibration of the four spacecraft's scientific instruments after the launch of the other three were thought to be insurmountable.
The final baseline mission, renamed the Solar-Terrestrial Science Programme (STSP), consisted of a two-thirds/one-third cooperative endeavour between ESA and NASA, which occurs principally on SOHO with its NASA-supplied launch vehicle and full spacecraft and science operations, from Goddard SpaceFlight Centre. Cluster, on the other hand, was able to benefit from a special concession within the framework of the Ariane-5 Apex development programme, eventually securing a launch on the first test firing of Ariane-5 (V501) .
Figure 1. The four Cluster flight-model spacecraft together for the first time in the cleanroom at IABG, Ottobrunn (D). Two spacecraft are shown complete with thermal hardware and stacked in launch configuration, whilst the other two are open with the payload platform visible
The four Cluster spacecraft will be placed in nearly identical, highly eccentric polar orbits, with anominal apogee of 19.6 R E and a perigee of 4 R E (Fig. 2). This nominal orbit is essentially inertially fixed, so that in the course of the two-year mission it will enable a detailed examination to be made of all significant regions of the magnetosphere. The plane of this orbit bisects the geomagnetic tail at apogee during the northern-hemisphere winter, and passes through the northern cusp region of the magnetosphere six months later. The operational orbit finally selected for each spacecraft will depend upon the actual launch and orbit-transfer scenario.
Figure 2. The Cluster operational orbit. This highly elliptical polar orbit of 4 x 19.6 R E is essentially inertially fixed, so that in the course of a year the apogee moves from the tail region into the solar-wind region, thereby allowing all areas of the magnetosphere to be addressed
The orbit for each spacecraft will be selected so that each is located at a vertex of a pre-determined tetrahedron (Fig. 3) when crossing the regions of interest within the magnetosphere. The relative separations within this constellation of spacecraft will be adjusted during the mission to correspond with the spatial scales of the structures to be studied, and will vary from a few hundred kilometres to a few Earth radii. The separation manoeuvres will be performed at intervals of approximately six months, synchronised with normal orbit-maintenance manoeuvres.
Figure 3. The tetrahedral formation in orbit. The four Cluster spacecraft are placed in slightly different orbits, which ensures that at a specific point in space they form the vertices of a tetrahedron. This will allow true three-dimensional measurements to be made in the regions of primary scientific interest
In the programme baseline, all four spacecraft will be injected into Geostationary Transfer Orbit (GTO) on a single Ariane-5 launch vehicle. The spacecraft will then be transferred one by one to their mission orbits through a series of spacecraft propulsive manoeuvres (Fig. 4).
Figure 4. The orbit-manoeuvre scenario. The spacecraft will be injected into a standard GTO orbit by Ariane-5. The change in orbit from equatorial will be accomplished by a series of intermediate manoeuvres
In orbit, the spacecraft will be spin-stabilised at all times. Their attitudes will be selected to ensure a solar-aspect angle of approximately 90 degrees, which optimises the performance of the spacecraft's solar-power generator and thermal-control subsystem throughout the mission. This attitude will be maintained during manoeuvres in the operational phase of the mission, but it will be necessary to reorient the spacecraft temporarily during the orbit-transfer phase in order to permit specific manoeuvres to be carried out. Eclipses will occur during all phases of the mission. Their durations will depend on the final launch date, transfer scenario and mission orbit. Current mission plans suggest that the longest eclipses will last approximately 240 min.
In order to measure the undisturbed local DC magnetic field to the required sensitivity, the DC magnetometer (FGM) sensors had to be mounted away from the main body of the spacecraft, on a deployable rigid boom. The AC magnetometer (STAFF) experiment is sensitive to AC magnetic fields, and also had to be mounted in a position remote from the spacecraft body, on a second rigid boom.
The total mass of each Cluster payload is 72 kg. It produces data at different rates, depending on its operating mode. In its nominal mode, it will deliver data at the moderate rate of 17 kbit/s, but at certain locations within the orbit it will be operated in a 'burst mode' with the significantly higher data rate of 105 kbit/s. In addition, the Wide Band Data (WBD) experiment produces data at the very high rate of some 220 kbit/s, but this experiment will only be but this experiment will only be operational for approximately 30 min during each orbital revolution. The power required by the payload is specified as 47 W and is nominally constant.
Figure 5. ESA's Redu (B) ground-station complex has been equipped with a new 15 m antenna (on extreme left of photo) and associated equipment to support Cluster operations in S-band
A ground station belonging to NASA's Deep-Space Network (DSN) will support the mission during certain specific scientific operations, when data from the WBD experiment is being transmitted at high bit rate. Further NASA/DSN support will be used to back-up the ESA network, including ranging, telemetry acquisition and telecommand back-up functions during the critical transfer-orbit phase.
Orbit determination for all mission phases will also be performed by ESOC. The Cluster mission requires that the relative separations of the four satellites be determined to within 1% or 10 km, whichever is the smaller. This will be achieved by determining the orbit of each spacecraft individually from the ground.
Simultaneous acquisition of two or more spacecraft from one ground station is not planned. However, the OCC is able to monitor and control two spacecraft simultaneously by using two ground stations. All communications with the spacecraft will be in S-band, and the up- and down-links of each individual satellite will be assigned different frequencies.
In the Launch and Early Orbit Phase (LEOP), following injection into Geostationary Transfer Orbit (GTO) by Ariane, ground contact will be established with all four spacecraft at the earliest possible opportunity. This will permit a quick-look status verification of essential parameters, which will be followed by less time-critical checkout and orbit-determination activities.
The Transfer-Orbit Phase (TOP) is characterised by a number of large orbit-profile and inclination-change manoeuvres, to target the spacecraft into the desired mission orbits. The spacecraft attitude will be temporarily adjusted for each of these manoeuvres to align the single main engine with the desired thrust direction. Throughout the TOP, the four spacecraft will be treated as two pairs, and injected with approximately 40 h between spacecraft pairs.
Once in mission orbit, a Commissioning and Verification Phase (CVP) will commence, which will be devoted mainly to payload operations. These include the deployment of the two rigid radial booms, followed by the deployment of the flexible wire booms, the experiment check-out, and the calibration activities.
During the main Mission Operations Phase (MOP), the primary objective is to maximise the scientific data return from the payload. The areas of scientific interest within the orbit will vary seasonally, and do not generally coincide with real-time ground contacts. To accommodate these, in the nominal operating mode the scientific data will be stored on on-board solid-state recorders, which will then play them back, together with the real-time telemetry, during subsequent ground-contact periods.
Phase-B, the design phase, then commenced in October 1989 with a core team of contractors already selected with the Dornier proposal. The full industrial team (Fig. 6) was built up during this phase, with lower-tier subcontractors being selected on a technical-competence and price basis to achieve the requisite geographical distribution targets set for the project.
Figure 6. The Cluster industrial structure
The achievement of those geographical-return targets was made easier by an agreement with the ESA Industrial Policy Committee (IPC) that the targets could be met for the STSP Cornerstone as a whole, rather than at individual project (SOHO and Cluster) level. Several pieces of equipment (power-distribution units and transponders) were in fact procured ad common items between the two projects as a pragmatic way of achieving the required targets.
The main Cluster development programme (Phase-C/D) commenced in April 1991 with delivery of the four spacecraft to the Agency in April 1995 as a target. In reality, despite a number of critical setbacks during the programme, delivery by Dornier was completed in July 1995, still on schedule for the originally foreseen 1 December 1995 launch date.
The system-level environmental test programme was conducted entirely at IABG in Ottobrunn (D) and lasted for two years. During this period, all four Cluster spacecraft successfully underwent sine-vibration, acoustic noise, thermal-balance/vacuum and DC-magnetic testing, in the most extensive test programme of this nature ever undertaken for any ESA project.
The Cluster launch campaign started in late August to meet a by then declared, slightly delayed launch date of 17 January 1996 for Ariane-5's first flight (V501). A natural break point occurs in the programme during November, at the completion of all electrical checks and prior to spacecraft fuelling. At the time of writing (end-September), the launch campaign is expected to be suspended at this break point, pending declaration of a new firm launch date for V501, which is currently expected to take place in late Spring/early Summer 1996.
ESA Bulletin Nr. 84.