Longest Astronomy Mission to Shut Down After 18 Years
One of the longest and most productive missions in the history of space science is drawing to a close. Budgetary restrictions have forced the European Space Agency (ESA) to terminate the orbital operations of the International Ultraviolet Explorer (IUE) satellite on 30 September 1996. Science operations will end shortly before that date to allow for necessary end-of-life testing of the spacecraft. "It is sad to be shutting down something still producing solid science year in, year out," said Professor David Southwood, former chairman of ESA's science programme committee. "But we just cannot afford to run it any further."
The shutdown of IUE will leave a gap in astronomical observations which will not be filled for some time. "There is nothing up there to replace IUE and nothing coming up in the foreseeable future," said UK project manager for IUE, Dr David Stickland of the Rutherford Appleton Laboratory in Oxfordshire. "The Hubble Space Telescope is able to do some of the things but IUE is in a very high orbit so it can do things that you can't do with the Space Telescope."
When IUE was launched on 26 January 1978, its expected lifetime was three years. However, it has continued to function for more than 18 years, even though spacecraft control has become increasingly difficult. Most recently, a gyro failure in March 1996 left the IUE with only one functional gyro out of the original six.
Despite this handicap, ESA ground controllers were able to track and observe the nucleus of Comet Hyakutake from 23 to 27 March, obtaining exposures of up to 5 hours in duration. As a result of this remarkable achievement, IUE was able to return short- and long-wavelength spectra which revealed important new data on the chemical processes taking place inside the comet. Astronomers were able to calculate the rates of production of carbon monoxide and water were and confirm that the 'break-up' event on 24 March involved only a small piece of the comet.
This is just the latest example in almost two decades of highly productive astronomical observations. In the words of astrophysicist Freeman Dyson from the Institute of Advanced Studies in Princeton, IUE has been "A little half-metre mirror sitting in the sky, unnoticed by the public, pouring out results". Since 1979 IUE has completed more than 100,000 observations of celestial objects. There have been 10 international symposia to discuss IUE results, with more than 3,500 scientific papers based on the spectrographic data.
Over the years, IUE has contributed to many branches of astronomical research, ranging from studies of objects in the solar system to observations of the most distant objects in the universe. The latest planetary studies involving Jupiter and its four Galilean satellites coincide with the Galileo orbiter's mission to the giant planet. All of the other planets, including distant Pluto, have been observed by IUE, in addition to many comets.
Further afield, IUE has made a significant contribution to our knowledge of hot stars by allowing researchers to follow the development of instabilities in the winds of charged particles ejected from them. In the case of supernova 1987A, IUE provided confirmation of the nature of the original precursor star which exploded. For the less dramatic exploding star Nova Cygni 1978, IUE returned a series of ultraviolet observations which revealed the chemical elements present in the star. IUE has also taken place in a major coordinated campaign in x-ray, ultraviolet and optical wavelengths to determine the nature of the mini-quasar at the heart of the Seyfert 1 Galaxy NGC 7469.
Probably the most significant contribution of IUE will be the Final Data Archive, which will be completed by the end of 1997. More than 100,000 observations of spectra are currently being reprocessed using newly developed software. These will eventually be stored for future reference.
IUE was first proposed by Professor Bob Wilson (now Sir Robert) at Culham Laboratory in the 1960s. Culham and Appleton Laboratories in Oxfordshire and University College, London were responsible for much of the design, testing and assembly of the science payload, particularly the spectral detector systems (cameras). IUE eventually became a joint project of NASA, ESA and the Particle Physics and Astronomy Research Council (PPARC - formerly the SRC, then SERC) in the United Kingdom.
Sixteen hours each day of spacecraft control and science operations were allocated to the NASA IUE Observatory at Goddard Space Flight Center in Maryland, with the remaining 8 hours allocated to the ESA IUE Observatory at Villafranca near Madrid. In 1994, financial restructuring plans by NASA threatened the termination of the mission.
Eventually, a review by the three agencies resulted in a compromise solution which resulted in day-to-day operational control being fully transferred to ESA on 1 October 1995. All of IUE's science observations (16 hours per day), data processing and distribution to scientists around the world were performed through Villafranca. NASA continued to provide spacecraft maintenance support, satellite communications and data re- processing for the Final Archive. Budgetary restrictions imposed on ESA finally caused the satellite's demise. In February 1996, the agency's Science Programme Committee agreed to finally close it down. The last six months of active life have been spent carrying out a number of special observational programmes to ensure that the Final Archive will not have omitted any vital astronomical observations.
IUE follows an elliptical orbit rangingin height between 42,000 and 26,000 km. This geosynchronous orbit keeps the satellite over the same spot on the Earth's surface so that it can be monitored continuously from Maryland and for about 12 hours per day from Spain. Apart from constraints caused by the Earth, Sun and Moon, IUE can view the entire sky.
The satellite is 4.2 m long and weighs 671 kg. Power is provided by two solar panels. The scientific payload consists of a 45 cm aperture telescope which feeds ultraviolet radiation to one of two spectrographs, one which operates at 115-195 nanometers and the other at 185-335 nanometers. These have provided astronomers with unprecedented wavelength coverage in the ultraviolet region of the spectrum.
Ultraviolet radiation cannot be studied by ground-based observatories since it is absorbed by Earth's atmosphere. IUE studied 'invisible' radiation just beyond the violet end of the visible spectrum. Many of the fundamental atomic processes taking place in comets, planetary atmospheres, stars and galaxies give rise to the emission of radiation at UV wavelengths.
Dr David Stickland, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX.
Tel: (44) (0)1235-44-6523. Fax: (44) (0)1235-44-5848.
Dr Alan Willis, Dept. of Physics & Astronomy, UCL, Gower Street, London WC1E 6BT.
Professor David Southwood, Physics Dept., Imperial College, Prince Consort Road, London SW7 2BZ.