Figure 3.1.1: On 28 November 1996, the Science Programme Committee approved the principle of recovering the Cluster mission.
Cluster homepage "http://www.estec.esa.nl/spdwww/cluster/html"
The aim of the Cluster mission is to study small-scale structures of the magnetosphere and environs in three dimensions. Cluster requires four spacecraft orbiting in a tetrahedral configuration, with adjustable separations in key regions of geospace, such as the cusp, magnetopause and plasma sheet. The four Cluster spacecraft were destroyed as a result of the Ariane 501 failure shortly after launch on 4 June 1996.
A significant fraction of the time of the Project Scientist and his associate in the pre-launch period was devoted to three main aspects of science operations. One area of activities centred around the timely delivery of the Cluster Science Data System (CSDS) and the Joint Science Operations Centre (JSOC). The commissioning of the 44 instruments required detailed preparation to ensure that the whole activity, including campaigns to inter- calibrate several instruments and to identify undesired interferences, could be performed in 13 weeks. Another requirement for the mission's successful operation was the establishment of the Master Science Plan, covering at least the first of the 2 years of nominal mission life.
Cluster Science Data System (CSDC) and Joint Science
Operations Centre (JSOC)
The European Space Science: Horizon 2000 report (ESA
SP-1070), describing ESA's long-term science plan, listed with
each of the four Cornerstone missions a topic that was expected
to be a key technical challenge for the mission. In the case of
Cluster, it concerned the operation of the multi-point system and
the handling of the resulting data in such a way that the
necessary inter-comparisons could be made. The need for a
coordinated approach to the distribution and, possibly, the
processing of the data was realised very early. At the first
Science Working Team (SWT) meeting in 1988, the Principal
Investigators (PIs) had already recommended the implementation
of a working group, which defined a distributed data system
within about a year. After release of an Announcement of
Opportunity (AO), the CSDS implementation started in early 1992.
The goal was to implement eight data centres, located in Austria,
China, France, Germany, Hungary, Sweden, United Kingdom and the
United States, and make them look to the scientific user like a
single data centre close to his home institute. This was achieved
by interconnecting the centres with a private Internet-subnetwork
(CSDSnet) and a uniform user interface (CSDS User Interface, CUI)
which lets the user browse, retrieve and manipulate data from any
number of instruments on any of the four spacecraft. The data
centres were designed, implemented and planned to be operated as
national projects, while CSDSnet and CUI were delivered by ESA.
The implementation of CSDS was done by the Implementation Working
Group (IWG), which included representatives from all data
centres, instruments and ESA establishments. Some 30-40 people
met bimonthly to run this project on a day-to-day basis. The IWG
reports to the CSDS Steering Committee, whose members represent
the national funding agencies and ESA. The Steering Committee is
chaired by the Cluster Project Manager, while the Project
Scientist chairs the IWG.
The CSDS implementation culminated, after an extensive test period, in the CSDS Readiness Review early in 1996. Based on a vast amount of test results, it could be shown that CSDS was ready for the distributed processing of an average daily amount of more than 400 scientific parameters of 4 s time resolution, and another set of about 100 parameters of 1 min resolution. All parameters would have been remotely accessible to the user for further analysis.
The operation of four payloads such that their data-taking modes would remain coordinated throughout the mission lifetime was a major challenge. Mission success required that data be taken simultaneously on the four spacecraft and that the instruments operate in modes consistent with the scientific goals defined for a given interval along the orbit.
In view of the number of interfaces, instrument modes and time pressure to react to demands for changes of the inter-spacecraft separation changes, it became clear that the Project Scientist alone could not execute this task and interface at the same time with the Operations Control Centre at ESOC in a three-shift mode throughout the mission life time. Approval was thus received in 1992 for the implementation of JSOC, staffed on average with 4.5 people and tasked to interface with the PIs, Project Scientists and ESOC. JSOC had to collect from the PIs all the relevant commands for periods of three orbits, verify them for consistency with the Master Science Plan and check for consistency with spacecraft resources. In case of incompatibilities, JSOC had to iterate with the PIs until a conflict-free command set emerged. This set was then passed to ESOC for verification against all spacecraft resources and either rejected and returned to JSOC for another iteration with the PIs or accepted and uplinked to the spacecraft. JSOC would also have executed instrument and inter- calibration monitoring and health-check functions on behalf of the PIs. The PI teams delivered data analysis software to JSOC for its embedding into the overall JSOC software.
Although the contract for the implementation was placed only at a very late stage, the Rutherford Appleton Laboratory as host of the JSOC performed an outstanding task in delivering JSOC on time and within cost.
Commissioning plan
The Cluster commissioning plan
was a challenge because it involved the commissioning and
verification of four spacecraft with two ground stations, obeying
the man-power constraints of the instrument teams. The maximum
duration of the commissioning phase was restricted to 13 weeks.
Further spacecraft constraints stated that the particle
instruments could not operate when the spacecraft was not
spinning at the nominal rate of 15 rpm, which, for instance,
would happen during the deployment of the electric field wire
booms. It was therefore decided to split the instruments into two
groups, one comprising the particle instruments and the other
holding all the waves/field instruments (including all the boom
deployments). Each group was assigned to a ground station. The
commissioning plan was then divided into three main phases:
The commissioning and gradual start-up of JSOC had to be exercised as part of these three steps, starting with one instrument for 4 h, then half of the payload on one orbit and then the full payload on three orbits. One of the expected difficulties in executing this plan for a spring/summer launch date was its extended duration (around 3 months) with working hours primarily during the night (due to the ground station visibility).
Master Science Plan
The Science Operations Working
Group, comprising the PIs and the Project Scientists with the
support of the Cluster project, ESOC and JSOC, undertook the task
of defining a long-term plan regarding the science operations of
Cluster. The plan defined the data acquisition along the orbit
together with the operational mode of the instruments. All orbits
during the first 6 months of operations (77 orbits in total) were
defined. To make this plan independent of any launch date, each
acquisition interval was fixed with respect to a scientific
region of interest (e.g. the magnetopause) or an orbital event
(e.g. perigee). The transfer into absolute time would have been
done by JSOC once the orbital parameters were known. The first
12 orbits were successfully simulated during a system commanding
test. In addition, ESOC ran a simulation to verify that the
amount of data gathered onboard did not exceed the capacity of
the ground segment. The second period of 6 months of operations
was defined in terms of data acquisition but not in terms of
instrument modes. This task would have been undertaken post-
launch.
Preparation for data analysis
The four Cluster
spacecraft would have allowed the study of the small-scale
structures of the magnetosphere and its environment in three
dimensions. This new step in magnetospheric physics required the
development of new analysis tools. The preparation work began
with the European network for the numerical simulation of space
plasmas, which developed theoretical concepts and simulations of
the prime regions of interest. The status of these tools was
presented during the previous reporting period at two workshops
on 'Data Analysis Tools' and on 'Physical Measurements and
Mission-oriented Theory'. To pursue this work further, a
dedicated working group has been set up at the International
Space Science Institute (ISSI) in Bern (CH). The goal of this
working group is to make available to the scientific users
various tools for multi-spacecraft data analysis. This 'cook-
book' should be available in 1997.
In addition, as part of a scientific collaboration with the Institute of Geophysics and Planetary Physics (IGPP) group at UCLA, the results of a magnetohydrodynamic model of the magnetosphere have been made available. This model allows the derivation of macroscopic physical parameters along the Cluster orbit for any time during the year and various interplanetary conditions. The results available consist of simulation runs for various tilt angles, interplanetary magnetic field orientations and solar wind dynamic pressures. On the experiment side, a description of the instruments was published in a special edition of Space Science Review in January 1997 to aid user understanding of the data.
Post-launch activities
Immediately after the loss
of the mission on 4 June 1996, a series of meetings of the SWT
and all the relevant ESA committees started. The SWT recommended
two options: rebuild four Cluster spacecraft of the existing
design (option 1), or procure four small satellites for a
recovery mission (option 2). In view of the danger that most of
the scientific and technical expertise would be lost without an
immediate start of the recovery programme, the SWT recommended
that one spacecraft (Phoenix) be launched as early as possible.
On 2 July 1996, the SPC decided that the Phoenix space-craft should be assembled from the spare subsystem units and instruments left from the original mission. During its meeting on 28 November 1996, the SPC approved the principle of recovering the Cluster mission, subject to a number of conditions that needed to be fulfilled at the February 1997 meeting. If this was the case, a full Cluster recovery mission could be launched towards the end of 2000.
Figure 3.1.1: On 28 November 1996, the Science Programme
Committee approved the principle of recovering the Cluster
mission.
Reference
Escoubet, C.P., Russell, C.T. & Schmidt,
R. (Eds.) (1997). The Cluster and Phoenix Missions. Space Sci.
Rev. 79, (1-2).
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