When its spectrometer-and-telescope ORFEUS instrument had been attached, the total payload had a mass of 3,570 kg and, like the Wake Shield Facility, would be deployed and retrieved with the assistance of the Shuttle's RMS arm. ORFEUS itself was devised by several German and US research institutions, with funding provided by NASA and DARA, and its centrepiece was a 1-m-diameter, 2.4-m-long telescope with built-in far-ultraviolet (dubbed 'Echelle') and extreme-ultraviolet spectro-graphs. An iridium coating on the telescope's primary mirror served as a 'reflection enhancement' to give it ultraviolet sensitivity.
During orbital operations, the two spectrographs were operated alternately, by 'flipping' a mirror into the beam reflected off the instrument's primary mirror. Echelle covered a wavelength range from 90 to 125 nanometres, while its extreme-ultraviolet counterpart encompassed the 40-115 nanometre span. Two reflection gratings then dispersed the incoming light from celestial sources into a spectrum, which was projected onto a two-dimensional microchannel plate detector.
The scientific objectives of the second ORFEUS-SPAS mission were to closely examine the evolution of stars, the structure of distant galaxies and the nature of the 'interstellar medium' - the almost-empty region between stars - in two rarely explored, very-short-wavelength areas of the electromagnetic spectrum. These particular regions are obscured by Earth's atmosphere, thus precluding ground-based observations; nor are they within the Hubble Space Telescope's capabilities. During its maiden flight in the autumn of 1993, ORFEUS-SPAS provided invaluable data on the structure and dynamics of interstellar gas clouds, as well as insights into how molecular hydrogen was created in interstellar space.
It also observed the spectra of a diverse group of important astrophysical objects, including a compact interacting binary star with an enormous magnetic field, three hot white dwarfs and the distant active galaxy PKS2155-304. Despite enormous advances during the Space Age, the formation of stars is still imperfectly understood; nonetheless, the process is known to arise in dense clouds of gas and dust. Under gravitational contraction, these clouds can then become dense enough to trigger starbirth. It was hoped that ORFEUS-SPAS-2 data might help to measure the size, distance, density and temperature of such clouds.
This, in turn, was expected to aid astrophysicists' understanding of the circumstances in which interstellar clouds collapse and precisely how new stars are born. When stars form, their evolutionary paths are predominantly governed by their masses: high-mass stars burn their energy reserves more than 100,000 times faster than our Sun, through processes which produce 'bright' ultraviolet emissions
ORFEUS-SPAS-2: a free-flying observatory 281
shockwaves. ORFEUS-SPAS-2 measured the ultraviolet spectra of such layers in comparatively cool stars to yield insights into the physics of such processes. Many stars ultimately end their lives as 'white dwarfs' which, because they are compact, are very hot and cool very slowly. During this time, they emit much of their radiation at ultraviolet wavelengths, making them among the brightest-known extreme-ultraviolet sources.
Larger stars, on the other hand, often meet their demise by exploding as 'supernovae', thus returning their mass back to the interstellar medium. Another of ORFEUS-SPAS-2's aims was to carefully trace the progress of such stellar remnants. It also explored the 'exchange' of stellar material between the members of binary star systems to investigate how quickly the matter is transferred and discern other characteristics of the hot 'accretion' disks thus formed. Many astrophysicists are certain that similar matter-exchange processes also occur, albeit on a far larger scale, in active galactic nuclei. A large number of these nuclei are suspected to contain massive, hidden black holes, surrounded by accretion disks. For each of these astrophysical objectives, the new data provided by ORFEUS-SPAS-2 was expected to lead to important new insights. Moreover, the second mission of the spectrometer-and-telescope package provided an added bonus: half of its observing time was set aside for the general astronomical community.
Four other scientific payloads were attached to the ASTRO-SPAS satellite in addition to ORFEUS. The Interstellar Medium Absorption Profile Spectrograph (IMAPS) operated for two days during the course of the two-week-long period of free flight, observing numerous extremely bright galactic objects at high resolution. Previously flown on high-atmosphere sounding rockets and on the ORFEUS-SPAS-1 mission, IMAPS was capable of very precisely measuring the motions of interstellar gas clouds to an accuracy of 1.6 km/s.
Meanwhile, the Surface Effects Sample Monitor (SESAM) provided a passive carrier device for evaluating state-of-the-art optics and potential future detectors and strong winds of hot, ionised material. One of ORFEUS-SPAS-2's main tasks was to observe the surfaces and winds of such objects.
Low-mass stars, like our Sun, burn their energy reserves fairly slowly and do not emit large amounts of ultraviolet radiation. Their surfaces can still become very hot, however, due to turbulent convection which creates during various phases of a Shuttle mission. Another experiment helped to develop and validate ground-based simulation facilities for ESA's Automated Transfer Vehicle (ATV) - an unmanned cargo carrier for the International Space Station - as well as evaluating the performance of its onboard software, GPS receivers and optical-rendezvous sensors. Lastly, high-school students from Ottobrunn in Germany provided an electrolysis experiment to investigate various salt solutions.
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