This document is the final Phase A Science Report of the Australian LYMAN Science Working Group, and describes in detail the scientific objectives, technical feasibility, and engineering implementation of the LYMAN mission as developed in the Australian studies.

The LYMAN project continues the distinguished US and European tradition of the exploration of the Universe at Ultraviolet Wavelengths accessible only from space. These earlier missions, which include OAO, TD-1, ANS, Copernicus and the International Ultraviolet Explorer (IUE), each demonstrated the existence of new and hitherto unsuspected physical phenomena and processes, and have collectively enhanced our understanding of the evolution of stars, galaxies and the Universe.

The importance of both the UV-spectral region as a cornerstone of astronomical research, and of an observatory mode of operation is apparent when it is noted that observations from the IUE satellite are now cited more often than observations from any earth- or sky- based observatory. However, IUE is not eternal, and LYMAN is the next step forward, pushing the sensitivity limit down by a factor of more than 100, and opening up new fields and wavelengths of study. LYMAN is planned as a grazing incidence Wolter-Schwartzschild Type II telescope with an aperture of 80cm, feeding an array of sensitive far-UV and EUV spectrographs equipped with Photon Counting Array (PCA) detectors.

The primary objective of the LYMAN Mission is to provide an Observatory for the study of the waveband between 900 and 1250Å. Only one previous mission, Copernicus, has given us a glimpse into the very rich astrophysical return offered in this far-ultraviolet part of the electromagnetic spectrum. However, LYMAN will provide a sensitivity some ten thousand times greater than Copernicus.

It is the enormous richness of this part of the electromagnetic spectrum which is the prime justification of the mission. No other region offers such a density and variety of atomic, ionic and molecular absorption lines. Collectively, these represent a tool of great diagnostic power in determining physical conditions and chemical compositions of astrophysical plasmas. These plasmas may be cool, and seen in absorption against a hot continuum source, or else they may be hot and emitting in their own right. In either case, LYMAN is the ideal vehicle for their study, be they in solar system objects, near newly-forming stars, in photospheres, transition layers, or coronae of stars, in interstellar or intergalactic space, or close to the active cores of galaxies. For this reason, LYMAN will be a uniquely powerful tool for use in all branches of modern day astrophysics.

The LYMAN Observatory payload is mounted on a service module which which offers pointing, power and telemetry and which has substantial commonality with the SOHO concept. The payload consists of a Wolter-Schwartzschild Type II Grazing Incidence telescope with monolithic primary and secondary elements feeding far-UV and extreme-UV spectrographs. It is designed to offer an effective collecting area of greater than 10 cm2 over a limited field of view with a spectral resolution on astronomical targets of 30000 in the prime ( λ900 - 1250 Å ) spectral range. This will allow high-resolution observations on sources as faint as 15 mag. LYMAN will also be capable of high resolution observations up to 1800Å, and will offer low-resolution spectroscopy in the extreme-UV down to about 100Å.

The LYMAN project has been the subject of industrial Phase A studies in both Australia, and within ESA. These studies have demonstrated an extremely useful international collaboration, as well as a very successful collaboration between University and Industry. Working together, we have identified all critical problems of feasibility, and have developed at least one, usually more than one, method of overcoming these while remaining within the cost and complexity envelope of the overall mission.

The LYMAN mission will be the subject of a competitive selection process within ESA at the end of November 1988. One of five projects will be selected to go forward into later phases. However, the Australian LSWG are confident that, with the enormous support that the LYMAN project commands on both sides of the Atlantic, with the tremendous scientific merit of the proposed mission, and with the demonstration of feasibility of the mission in two separate Phase A studies, that LYMAN will be the successful candidate.