IUE article: first submitted to Novosti Kosmonavtiki, Moskva (don't know if they're going to publish it yet). Based on an article in Space Report, it's oriented more towards aerospace engineers than astronomers, so would need some editing. - Jonathan Obituary - IUE -------------- The International Ultraviolet Explorer (IUE), an ultraviolet astronomical observatory, was shut down at 1842 UT on 1996 Sep 30. Prior to the final orbit raising burn, IUE was in a 1437 min, 29992 x 41616 km orbit inclined 35.9 deg. It ended up in a 1439 min, 30214 x 41474 km orbit inclined 35.6 deg. IUE was probably the most successful scientific satellite ever. The IUE spacecraft was a joint project between NASA Goddard Space Flight Center, the European Space Agency, and the UK Science Research Council. IUE was originally intended to be Small Astronomical Satellite SAS-D, successor to the Scout-class Uhuru, SAS-2 and SAS-3 high energy astrophysics satellites, but it was upgraded to a Delta-class mission. Although it was part of the Explorer scientific satellite series, NASA stopped numbering Explorers in 1975, otherwise IUE would have been Explorer 57. IUE was launched on 26 January 1978 by a Delta 2914 rocket from Cape Canaveral into an elliptical transfer orbit. A Thiokol Star 24 apogee motor was fired to place the satellite in its operational elliptical orbit of 1435.7 min, 25669 x 45888 km x 28.6 deg. In this orbit, the satellite drifted back and forward across the Atlantic Ocean. Operations were divided into three eight-hour shifts. For one shift each day, it was controlled from the ESA ground station at Villafranca in Madrid, and for the remaining two shifts control was handed over to the NASA station at Goddard Space Flight Center - the orbital mechanics reflecting approximately the relative budgetary contributions of the Europeans and Americans to the project. The satellite carried a small 0.45m Ritchey-Chretien telescope and four spectrographs (two prime and two backup) which obtained ultraviolet spectra of astronomical objects. The SWP (Short Wavelength Prime) camera covered the 1100 to 2000 Angstrom range, while the LWP (Long Wavelength Prime) camera covered the 2000 to 3000 Angstrom range. Because of some problems with the LWP, the LWR (Long Wavelength Redundant) camera was also used extensively; the SWR camera was never used after the initial commissioning period. Exceeding its nominal six month mission duration by almost a factor of 40, IUE remained an essential scientific tool right into the 1990s. Even the success of the HST repair mission did not remove the need for IUE, because there are many projects, such as monitoring time variability of bright sources, for which you need a small telescope for a long time instead of a large telescope for a short time (and you can't get HST for a long time!). IUE results have affected every field of astronomy, and in particular much of our current understanding of hot stars in this galaxy and of the inner regions of quasars is due to work with IUE. The last month of IUE's life was devoted to ultraviolet observations of the planet Jupiter. Ultraviolet radiation from objects in space includes ordinary continuum radiation from hot objects. The Sun puts out most of its energy in the visible band, but massive, hotter stars like Vega put out a lot of ultraviolet. The hottest ordinary stars are the O and B stars, which typically have gas flowing away from them in very strong stellar winds. White Dwarf stars are even hotter, and study of their ultraviolet spectra can reveal the strong gravity on their surfaces. Another common site for ultraviolet radiation is in the hot gas of an accretion disk, matter in the process of falling into a deep gravitational well such as a neutron star or black hole, either in a binary star system in our galaxy, or on much larger scales in a quasar. Most of the radiation produced by a quasar comes out in the ultraviolet region. As well as the continuum radiation, ultraviolet spectra include 'spectral lines' which are the fingerprints of the chemical elements in the hot gas producing the radiation. The pattern of the lines can be used to determine the chemical composition of the star or gas cloud, as well as their density, temperature, and velocity. The most important ultraviolet line is hydrogen Lyman alpha, at 1216 Angstroms, which is the `fundamental musical tone' of the most common instrument in the celestial orchestra. Another important line is Carbon IV, from three times ionized carbon at 1549 Angstroms. In the study of quasars, we use temporal variations these lines emitted by gas clouds in the vicinity of the quasar to guess the size of the cloud region. We then use the velocities deduced from the clouds to measure the strength of gravity near the quasar and infer the mass of the central black hole. IUE was the first common user astronomy satellite, where any astronomer with a good idea could apply for observing time. Because its synchronous orbit allowed real time commanding, it remains the only satellite which astronomers were able to use like a ground based telescope, going to the control center and making decisions on the fly - for instance, whether to change the length of the next exposure if the previous one was over or under exposed. Since many ultraviolet stars and quasars vary in brightness, so you don't know how bright they will be on the day you observe, this is really useful - as I can testify from personal experience as an IUE observer. You can't do that with a low orbit satellite like HST where observations must be scheduled weeks in advance. (Besides, having real time authority over a geosynchronous orbit spacecraft is probably the closest I'll ever get to sitting in Captain Picard's chair :-)). IUE was a pleasure to use, and during the 1980s was crucial to the research of many astronomers including myself. IUE will be missed.