Access to the UV range is fundamental for astrophysics since thermal phenomena at temperatures T>10,000 K occur in a wide range of astophysical events, with flux emission mostly in the UV. Moreover, UV spectroscopic and imaging capabilities are a fundamental tool to study plasmas at temperatures in the 3,000-300,000 K range. Also, the electronic transitions of the most abundant molecules in the Universe (H₂, CO, OH, CS, CO₂₊, CO₂) are in the UV range. The UV radiation field is also a powerful astrochemical and photoionizing agent.

Optical observations provide most of the standards for the study of stellar populations, and, coupled with UV observations, the rest frame fundamental information for the population synthesis models, and therefore the study of stellar population of high-z objects.

The scientific plans for WSO-UV are very ambitious, and span all of the astronomical research branches. WSO-UV will be operating in the second decade of this century, and it will be a fundamental tool for the development of astronomical knowledge, fully integrated with the many other space and ground-based observatories.

WSO-UV will give a significant contribution to solve the key astronomical problems individuated by Science VisionASTRONET consortium, and which are driving the European Space Agency “Cosmic Vision” program and the NASA “Origins” program.
In particular, the data collected by WSO-UV will be used to answer to the following questions (as formulated by the ASTRONET consortium): 

• Do we understand the extreme of the Universe? Important inputs for this problem will come from WSO-UV observations of: supernovae, gamma ray bursts, interacting binaries (millisecond pulsars, low-mass X-ray binaries, cataclysmic variables, blue stragglers, etc.), as well as observations of active galactic nuclei, their surrounding environment, and the accretion and outflow processes in their central black hole.

• How do galaxies form and evolve? Important inputs for this problem will come from WSO-UV surveys in far-UV and near-UV of galaxy clusters at different redshifts, of the Virgo cluster, and from the extension to the far-UV and near UV of the GOODS ultra deep field survey. Surface brightness fluctuation technique will be used to investigate the unresolved stellar population in distant galaxies, while far-UV and near UV observations of galaxies in the local universe up to z>1 will be used to solve the still open problem of the UV-upturn in early-type galaxies and in bulges. Particular effort will be devoted to the origin and early evolution of our Galaxy, using as fundamental probe, among others, the Galactic globular cluster system.

• What is the origin and evolution of stars and planets? Important inputs for this problem will come from WSO-UV observations of the stellar population in different environments (stellar associations, open and globular clusters, Local Group galaxies, including the dwarf satellites of our Galaxy). WSO-UV observations will provide fundamental inputs in the study of young stellar objects, including the processes of accretion and outflow. Additional information on stellar structure and evolution will come from asteroseismological studies, from the study of stellar magnetic activity, and of stellar variability. Efforts will also be devoted to the study of the interstellar medium. WSO-UV will also provide important data for the study of the extrasolar planet atmospheres (including a number of biomarkers). Also the study of Solar System bodies will benefit from observations with the different instruments onboard of WSO-UV.

Moreover, with the synergy of the HST archive, WSO-UV will allow long term photometric and spectroscopic monitoring of a variety of astronomical objects. Moreover, the extended temporal coverage with high angular resolution UV-optical imaging will allow the measurement of relative and absolute proper motions with unprecedented accuracy, de facto opening a new, completely unexplored research branch. Astronomers have just started to exploit this possibility. If properly equipped with cameras with diffraction limit imaging capabilities, WSO-UV will give the opportunity to fully develop this research activity, allowing proper motion measurements with the same accuracy of GAIA, but in a fully complementary way. WSO-UV+HST proper motions will be measured down to much fainter targets than GAIA, and in much more crowded environments. And all this will be done many years before the final catalog of GAIA will be available.