IUE homepage http://www.vilspa.esa.es/iue/iue.html
IUE, launched on 28 January 1978, was a general user facility for UV spectroscopy at 115-320 nm, conducted jointly by NASA, ESA and the UK's Particle Physics & Astronomy Research Council. European operations were controlled from the Villafranca Satellite Tracking Station (VILSPA) near Madrid, normally on an 8 h/day basis. The IUE operational philosophy allowed extremely flexible and interactive science operations. Even though the spacecraft subsystems would have allowed the continuation of IUE, operations were stopped after 18½ years on 30 September 1996.
During the current reporting period, a large effort was made to allow a scientifically meaningful termination of the extremely successful IUE science mission. In the face of the increasing pressure of budgetary constraints, particularly at NASA, a complete review of the operational philosophy was made at VILSPA to define a cost-effective operations mode to ensure continuing operations as NASA reduced its support to IUE. A highly efficient 'hybrid' IUE operations scheme was developed and successfully implemented from 1 October 1995, allowing up to 24 h/day of science operations from VILSPA. This operational scheme was projected to continue for 2 years before operations were terminated. However, the additional budgetary constraints imposed as a consequence of the ESA Ministerial Meeting in October 1995 changed the conditions, so that only 1 year of the 19th and final episode of IUE observing could be accomplished.
This required a revision of the IUE science programme, which was defined by an open call for proposals and included the observation of Earth-passing asteroids as well as high redshift quasars. This revision was further complicated by the failure of gyro #5 in March 1996, requiring the implementation of a 1-gyro attitude control system. The final IUE science programme was especially designed to optimise the spacecraft's unique capabilities, taking into consideration the increased constraints associated with the 1-gyro control mode. The final programme included a detailed study of the satellite interactions in the Jovian magnetospheric system, an exhaustive multi-frequency campaign with complete time sampling of a Seyfert I galaxy (NGC 7469), and special programmes to determine the critical parameters in the winds from hot stars (Wolf-Rayet and O-stars). All of these programmes were successfully completed on 24 September 1996. Final engineering tests and spacecraft decommissioning, particularly hydrazine venting, were then conducted.
Essential and continuing tasks are data distribution and maintenance of the IUE Archive. Archive usage remains extremely high. The support for archive retrieval through the VILSPA- developed Uniform Low Dispersion Archive (ULDA) continues, with 24 national Host Institutes, supplying some 25 000 spectra per year (43% of the total IUE data usage) to a user community of more than 1000 scientists from 28 countries.
With the termination of orbital operations, the main focus is on the completion of the IUE Final Archive (IUEFA). This consists of a complete reprocessing of all 114 000 spectra taken with IUE, using improved data reduction software. The final distribution system is being designed to limit the costs associated with the continued long-term availability of the IUE data, with a heavy emphasis on the easy availability of data in a directly astrophysically applicable form. It will be modelled on the successful distributed archive model, pioneered in 1986 with the ULDA, but adopting up-to-date information technology.
As a final note, the scientific value of the mission may be judged by the more than 3400 papers based on IUE data published in the refereed literature to date.
Figure 3.2.1/1: The revamping of IUE science operations. Top:
the situation as it was for most of the mission. Bottom: the
change to support the 'hybrid' IUE operations. The straight
yellow arrows show the previous data flow, which was completely
redesigned; the red flashes show data and information flow
without human intervention.
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