The ability to separate multiple stellar and interstellar objects and the ability to study plasma dynamics in space are just a few of the scientific successes from the Hubble Space Telescope that clearly show the importance of the far-UV spectral range to modern astronomy. Practical limitations of the Hubble Telescope, however, such as restrictions on UV observation time and the phasing out of the UV spectrographs, make it necessary to plan for a high-resolution replacement for UV astronomy. "When the Hubble finally fails, access to one of the most important parts of the spectrum will end for the foreseeable future," explains Professor Martin Barstow of the University of Leicester (Leicester, UK). "The World Space Observatory is a completely new approach to carrying out space science, and the only potential replacement for Hubble in the ultraviolet at the moment."
The World Space Observatory would include a 1.7-m telescope based on the Spectrum-UV T-170 telescope, which was originally planned for launch in the mid-1990s, and is under development at Russia's Lavochkin Association.
The importance of the World Space Observatory's (WSO) capabilities is echoed by Isabella Pagano of the Osservatorio Astrofisico di Catania (Catania, Italy). "UV light carries a wealth of information precious in every field of modern astrophysics, from solar system objects to stars, supernovas, galaxies, and more," Pagano says. WSO objectives include a broad range of scientific investigations: gaseous atmospheres of the giant planets in our solar system; ice in the outer solar system; and the life cycles of stars, which can be further evaluated along with more exotic topics such as fundamental physical processes in the atmospheres of white dwarfs, their photospheric composition and structure, and the pattern of circumstellar material surrounding a white dwarf. Supermassive black holes that act as galactic nuclei have primary emissions in the far and extreme UV regions. UV is also a key spectral band used to measure mass outflows in quasars and star-forming galaxies. Ultimately, even the structure of the universe can be explored using UV absorption features in intervening galactic materials.
UV light is absorbed in the Earth's atmosphere and, therefore, is best studied from space-based observatories. The WSO approach would put a UV-only telescope at the second Lagrangian point of the Earth-Sun system, with an optical design of the Ritchey-Chretien type and a 1.7-m hyperbolic primary mirror. The observatory would offer three UV spectrometers, two high-resolution Echelle spectrographs and one low-dispersion long-slit spectrometer. A high-dispersion spectrometer would cover the range from 103 to 180 nm and the other would cover from 178 to 320 nm. The instruments would be used in a sequential mode using the satellite's motion-pointing stability of 0.1 arcsec with instrument-based pointing servos. Although the WSO has a smaller aperture than the Hubble, WSO designers hope that improved instrument efficiencies will make the WSO's instruments five to 10 times more sensitive than Hubble's payload.
The WSO also represents a new approach to astronomy by involving a broad and unprecedented cooperative international effort, thus spreading costs and control across a large number of countries. The Russian Federal Space Agency has recently taken leadership, and the implementation committee has representatives from 19 countries including Germany, France, Italy, China, Sweden, Norway, the Ukraine, and the Baltic states. The European Space Agency and the United Nations are advising on the WSO to help ensure both project management and technical success. Although a much larger number of countries is expected to share in the cost compared to past space-borne observatories, construction funding for WSO has not been allocated. Representatives from WSO countries are studying the instruments and other telescope systems; a report on the overall mission is due by the end of the 2004, with a potential launch date set for sometime between 2008 and 2009.