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The International Ultraviolet Explorer: Origins and Legacy

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The International Ultraviolet Explorer: Origins and Legacy Organizations, People and Strategies in Astronomy 2 (OPSA 2), 395-416 Ed. A. Heck, © 2013 Venngeist. THE INTERNATIONAL ULTRAVIOLET EXPLORER: ORIGINS AND LEGACY ALLAN J. WILLIS Department of Physics and Astronomy University College London Gower Street London WC1E 6B, U.K. [email protected] Abstract. The International Ultraviolet Explorer (IUE) satellite was launched on 26 January 1978, and operated as a Guest Observer space observatory for nearly 19 years. IUE was, without doubt, one of the most successful astronomical facilities ever developed. Used by thousands of as- tronomers worldwide, it yielded over 100,000 UV spectra (in the wavelength range 1150-3250A),˚ covering stars of all types, the interstellar medium in our own galaxy and other local galaxies, the galactic halo, normal and ac- tive galaxies and QSO’s, X-ray binaries, novae and supernovae, and in our solar system comets, planets and their moons. Over 3500 refereed papers have been published based on IUE results, and a similar number of papers in conference proceedings. Over 600 PhD theses have been produced based on IUE data. All IUE data are available in archives maintained by the three agencies involved in the mission: NASA, ESA and the UK Science Research Council. In this paper I will discuss the origins of the IUE mission, the spe- cial design and operation which led to its spectacular success and the legacy it left for UV astronomy. 1. Introduction The wavelength range from 3250-912A˚ is referred to in astrophysics as the vacuum ultraviolet (UV) region. The upper bound reflects the approximate cut-off in the transmission of the Earth’s atmosphere due to absorption by Ozone, whilst the lower bound corresponds to the ionisation energy of the neutral Hydrogen atom – below which interstellar H largely absorbs back- ground radiation from distant sources. With suitable reflecting coatings 396 ALLAN J. WILLIS Figure 1. Sir Robert Wilson (1927-2002). Wilson led the initial LAS and UVAS proposals to ESRO for an Ultraviolet Space Observatory, which led to the development with NASA of the IUE mission. (Courtesy D. Rooks/UCL) the optical elements for UV instrumentation are similar to those which are generally used at visible wavelengths: i.e. normal-incidence telescopes, photometers and spectrographs, with either photographic or electronic de- tectors. The UV region is one of the richest in spectral lines in the electromag- netic spectrum, encompassing a very large number of resonance lines of many common elements, covering many of their ionisation stages, e.g. HI, CI, CII, CIII, CIV, NI, NII, NIII, NV, OI, OII, OIII, OV, OVI, PIV, PV, SiI, SiII, SiIII, SiIV, SI, SII, SIII, SIV, FeI, FeII, MgI, MgII. These lines provide crucial diagnostics for low-excitation plasmas covering a range of temperatures from a few degrees up to about 1 million degrees, found in the interstellar medium, stellar atmospheres, galaxies and AGN and plan- ets and comets. For these reasons, the UV was a natural spectral range for development in the early phases of space science. For nearly 20 years one such UV mission formed a cornerstone of modern astrophysics – the International Ultraviolet Explorer (IUE) satellite. In this paper I will discuss the historical origin of IUE, its unique design and operation which led to its stupendous success and the legacy it has © 2013 Venngeist. THE INTERNATIONAL ULTRAVIOLET EXPLORER 397 left. I was fortunate to become involved with the IUE mission some years before launch through my association at UCL with Robert Wilson – then Project Director of the UK involvement in the satellite (Fig. 1). Although, of course, IUE owed its success to many hundreds of scientist and engineers, there can be little doubt the Wilson played an absolutely seminal role in getting the mission developed – see below – and is generally regarded as the “father of IUE”. My own involvement included pre-launch preparations for the commissioning phase, the UK High Priority Observing programme conducted a few months after launch, as a Guest Observer (GO) with IUE over many years, as a member of the UK TAC and as Chairman of the European IUE Time Allocation Committee. 2. Ultraviolet Astronomy in the pre-IUE Years The development of UV astronomy during the 1960’s followed a similar pattern in both the USA and (to a smaller extent) in Europe, with groups at Universities and Government establishments pioneering, ever-more so- phisticated experiments using rocket-borne instrumentation, followed by longer-lived and more massive satellite payloads. In the US the Aerobee rockets could deliver payloads of up to 100Kg to altitudes of around 180km. Groups at the Goddard Space Flight Center (GSFC), Universities of Wis- consin, Princeton and Johns Hopkins, together with the Naval Research laboratory, produced a range of spectrometers to yield the first detailed UV spectra of early-type OB stars at sufficient spectral resolution to detect and measure individual spectral lines, particularly with the development of a 3-axis stabilisation system for the Aerobee rockets. These programmes led, inter alia, to the first measurements of UV P-Cygni profiles of Zeta Puppis and other OB stars, yielding estimates of wind velocities and mass- loss rates (Morton 1967), and analyses of interstellar lines in atomic species and molecular Hydrogen (e.g. Caruthers 1970). In addition the first mea- surements of the UV interstellar extinction curve, including the discovery of the broad 2200A˚ band, were secured. In Europe, and particularly in the UK, university-based teams were also developing UV spectrometers. The UK group at the Culham Laboratory led by Robert Wilson, used Skylark rockets to launch 3-axis stabilised pay- loads from Woomera, South Australia, to secure high-resolution spectra of the Sun, to identify and study lines formed in the solar chromosphere tran- sition region and corona at both UV and soft-X-ray wavelengths (Burton et al. 1967). These programmes included an echelle-spectrum of the solar limb during a total eclipse in 1970 (Gabriel et al. 1971) and high-resolution spectra of Zeta Puppis and the Wolf-Rayet star Gamma Velorum covering the wavelength range 900-3200A˚ at 0.3A˚ resolution. Since the duration of a © 2013 Venngeist. 398 ALLAN J. WILLIS typical rocket flight was limited to about 5 minutes or less, each flight could only yield data on one or at best 2-3 objects. Over time successive flights built up data for many of the bright OB stars, some late-type stars, and planets. They had demonstrated the importance of the UV spectral region in furthering astrophysics in the solar, stellar and interstellar fields, and pointed to the potential for further advances if fainter and more diverse ob- jects could be observed with larger instruments and longer exposure times, which required the use of stabilised satellite platforms. This recognition led NASA to embark on a long-range programme of de- velopment of ultraviolet satellites, which was to culminate with the Hubble Space Telescope. Starting in the mid-1960’s this programme centred around the NASA Orbiting Astronomical Observatories (OAO-series). OAO-1 was launched in 1966, but due to a power failure was terminated after only 3 days. Its spare parts were used to develop OAO-2, launched in 1968, with a spectrometer developed by the Goddard Space Flight Center yielding medium-resolution spectrophotometry, at about 10A˚ resolution, covering the wavelength range 1100-3200A.˚ This instrument was highly successful, obtaining data on hundreds of early-type stars and measurements of UV interstellar extinction curves, as well as photometry of some galaxies (Code et al. 1970; Davies et al. 1972). OAO-3 (re-named Copernicus) was launched in 1972, with an 80cm telescope feeding a high-resolution UV spectrograph provided by Prince- ton University (Rogerson et al. 1973). Scanning photomultiplier detectors were used to record spectra at either 0.05A˚ or 0.2A˚ resolution, yielding line strength and profile measurements of the plethora of atomic and molecular species in the wavelength range of 900-3200A.˚ Rather long exposure times were needed to record the spectra, such that the higher resolution mode was restricted to observations of OB stars with V<∼2, and about 6th mag for the lower resolution mode. The mission was a huge success, providing data on stellar mass-loss rates in the upper HR diagram, the effective tem- perature scale for OB stars, the discovery of OVI in the galactic halo, and the distribution of molecular hydrogen and chemical composition of the atomic gas in the ISM. Copernicus was also what one might term the first “long-lived” space observatory, continuing operations until about 1980. In Europe the fledgling European Space Research Organisation (ESRO) launched its first 3-axis stabilised satellite – TD-1 – in 1972, which car- ried two UV instruments. The Dutch/Utrecht S59 instrument comprised a low-resolution spectrometer covering 3 bands in the 2160-2870A˚ range (Hoekstra et al. 1973). The second instrument – S2/68 – was developed by a joint UK/Belgium team, from Culham-UCL/ROE/Li`ege, and comprised a 27cm telescope feeding a scanning spectrophotometer. This covered the wavelength range 1350-2740A˚ at about 35A˚ spectral resolution, and per- © 2013 Venngeist. THE INTERNATIONAL ULTRAVIOLET EXPLORER 399 formed an all-sky UV survey yielding data on stars of all types down to about 9th magnitude (Boksenberg et al. 1973). The data from S2/68 were put in the public domain through its Bright Star and Faint Star Catalogues (Thompson et al. 1978), and the high photometric integrity of the S2/68 calibraton formed the basis of the calibrations for succeeding UV missions, including IUE and the HST. The ANS satellite (Astronomical Netherlands Satellite) launched in 1974, provided broad-band UV photometry in 5 bands covering 1540-3340A˚ of a range of sources including white dwarfs, planetary nebulae, stars, clusters and galaxies.
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