The World Space Observatory Ultraviolet (WSO-UV), as a bridge to future UV astronomy B. Shustov1 • A.I. G´omez de Castro 2 • M. Sachkov1 • J.C. Vallejo2 • P. Marcos-Arenal 2 • E. Kanev1 • I. Savanov1 • A. Shugarov1 • S. Sichevskii1 Abstract The ultraviolet (UV) astronomy is a very described in detail in previous publications, and this demanded branch of space astronomy. Many dozens of paper updates the main characteristics of its instru- short-term UV-experiments in space, as well as long- ments and the current state of the whole project. It term observatories, have brought a very important also addresses the major science topics that have been knowledge on the physics and chemistry of the Uni- included in the core program of the WSO-UV, making verse during the last decades. Unfortunately, no large this core program very relevant to the current state of UV-observatories are planned to be launched by most the UV-astronomy. Finally, we also present here the of space agencies in the coming 10 – 15 years. Con- ground segment architecture that will implement this versely, the large UVOIR observatories of the future program. will appear not earlier than in 2030s. This paper briefly describes the projects that have been proposed by var- Keywords space vehicles, space vehicles: instruments, ious groups. We conclude that the World Space Obser- instrumentation: spectrographs, ultraviolet: general, vatory – Ultraviolet (WSO-UV) will be the only 2-m ultraviolet: stars, ultraviolet: planetary systems, ul- class UV telescope with capabilities similar to those of traviolet: ISM, ultraviolet: galaxies the HST for the next decade. The WSO-UV has been B. Shustov 1 Introduction Institute of Astronomy of the RAS, Russia A.I. G´omez de Castro During almost half a century the astronomers have en- AEGORA Research Group, Universidad Complutense, Spain joyed a continuous access to the ultraviolet (UV) do- M. Sachkov main, 91.2 – 320nm, where the resonance transitions Institute of Astronomy of the Russia of the most abundant atoms and ions at temperatures J.C. Vallejo between 3000 and 300000K reside. Because this UV- arXiv:1802.06849v1 [astro-ph.IM] 19 Feb 2018 AEGORA Research Group, Universidad Complutense, Spain range is not accessible from ground-based facilities, P. Marcos-Arenal many short-term UV-experiments in space were carried AEGORA Research Group, Universidad Complutense, Spain out and a number of long-term space UV-observatories E. Kanev were put into orbit. They have brought a very im- Institute of Astronomy of the Russia portant knowledge on the physics and chemistry of the I. Savanov Universe during the last decades. Institute of Astronomy of the Russia Major achievements of UV-astronomy in this half a A. Shugarov century are the following: Institute of Astronomy of the Russia • Direct detection of H2 molecules in space with S. Sichevskii “Aerobee-150” (Carruthers (1970)). Institute of Astronomy of the Russia • Discovery of the hot phase of the interstellar medium 1Institute of Astronomy, Russian Academy of Sciences, with “Copernicus” (Jenkins and Meloy (1974)). Pyatnitskaya 48, 119017 Moscow, Russia • Measurements of the D/H ratio with ”Copernicus” 2AEGORA Research Group, Fac. CC. Matematicas, Universidad (Rogerson and York (1973)). Complutense, Plaza de Ciencias 3, 28040 Madrid, Spain 2 • Massive accurate determination of the chemical com- There is only one large UVOIR observatory currently position of stars and detailed studies of stellar mass flying in space, the Hubble Space Telescope (HST). In loss phenomena across the HR diagram with the IUE addition, there are three UV instruments on-board as- mission (1978 – 1996). trophysical observatories. The first instrument to con- • Identification of Warm-Hot Intergalactic Medium as sider is the Ultraviolet and Optical Telescope (UVOT), the reservoir of missing baryons with HST and FUSE. on-board the SWIFT satellite. UVOT has a 30 cm • Giant progress in understanding physics and chem- aperture that provides a 17′ × 17′ field of view with a istry of comets, planetary atmospheres and exo- spatial resolution of 0.5’/pixel in the optical/UV band spheres of exoplanets with HST. (Roming et al. (2005)). The UVOT is based on the XMM-Newton mission’s Optical Monitor instrument All UV observations carried out from space, rang- (Mason et al. (2001)), with improved optics and up- ing from short-lived rocket and balloon-borned exper- graded onboard processing computers. The third in- iments, to small-aperture cameras and spectrographs strument is the Ultra Violet Imaging Telescope (UVIT) on astrophysical and planetary missions, up to long- on-board ASTROSAT satellite. UVIT consists of twin lifetimes UV-observatories with powerful instruments 38-cm telescopes – one for the FUV region and the other on-board, have shown that UV-instruments on-board for the NUV and visible (VIS) regions. UVIT is primar- satellites bring great scientific benefits to the astronom- ily an imaging instrument, simultaneously generating ical community. This makes natural a keen demand on images in the FUV, NUV and VIS channels over a field new UV observatories. of diameter 28 arcmin. Detailed description and review The structure of this paper is as follows. In Section of the first results are presented in Subramaniam et al. 2 we briefly review the current state of the art in UV (2016). astronomy, and discuss the ideas and concepts for fu- It is also of interest to report that the first China’s ture UV-observatories and instruments. In Section 3 lunar rover, launched in December 2013, is equipped we present current status of the World Space Observa- with a 150 mm Ritchey-Chr´etien telescope, called LUT tory – Ultraviolet (WSO-UV) mission, which seems to (Lunar Ultraviolet Telescope). This telescope is being be the only 2-m class UV telescope to fly in space in the used to observe galaxies, active galactic nuclei, variable next decade. Section 4 addresses the key science issues stars, binaries, novae, quasars and blazars in the near of this mission. The following section, Section 5, de- UV band (Wang et al. (2015)). It is capable of detect- scribes how the science program will be managed, and ing objects at a brightness as low as magnitude 13, and describes the current status of the WSO-UV ground the thin exosphere and slow rotation of the Moon allow segment. The last section, Section 6, is devoted to the extremely long, uninterrupted observations of the ob- concluding remarks. served targets. Hence, the LUT can be considered the first long term lunar-based astronomical observatory. Some other planetary missions are equipped with 2 On UV-instruments for astrophysics for UV-instruments, which are in use in Target of Opportu- today and tomorrow nity (ToO) mode. But we do not discuss here these mis- sions, neither we will discuss the existing solar physics The telescopes for ultraviolet, optical and near-infrared missions, confining ourselves to astrophysical projects. ranges are commonly combined into the so-called As to the future a lot of ideas and projects are UVOIR telescopes. This is because their optical design, under discussion. Recently, Scowen et al. (2017) pre- detectors and general operational conditions (e.g. cryo- sented the science cases and technological discus- genic temeratures are not required) are fundamentally sions that came from the workshop titled “Finding similar. In this section discuss the current and future the Ultraviolet-Visible Path Forward”, held at NASA UVOIR facilities that are fully or partially intended for GSFC on June 25 – 26, 2015. The material presented astrophysical observations in the near (170 – 320nm ) there outlined the compelling science that can be en- and far (91 – 180 nm) UV wavelength range. Regard- abled by a next generation of space-based observatories ing the extreme UV, the effectiveness of instruments dedicated to UV-visible science, the technologies that for astrophysical observations in the range λ <91 nm are available for the design of those observatories, and is strongly limited by the high opacity of the interstel- the possible launch approaches to enhance the returned lar medium, and after the EUVE (Extreme Ultraviolet UV-science. A number of very large (apertures >4 m) Explorer) mission (Bowyer (1991)) there is no future UVOIR space telescopes have been proposed and inten- significant project for observations beyond the Solar sively discussed during the last two decades. Any fu- System in the extreme UV. ture UV/Optical telescope will require these large aper- tures to complement the very large IR and X-ray space 3 telescopes and the 30m-class ground-based telescopes series of space missions. Besides the “Spektr-UF” the that will arrive in near future. Such large projects series includes the “Spektr-R”, or “Radioastron” mis- were considered at the Kavli IAU Workshop on “Global sion, launched in 2011, and the “Spektr-RG” mission, Coordination: Future Space-Based Ultraviolet-Optical- planned for launch in 2019. According to the Federal Infrared Telescopes”, held in Leiden, the Netherlands, Space Program the launch of the WSO-UV is scheduled on July 17 – 19, 2017. The workshop sessions centered for 2023. We are to reserve up to six months for tests of on 4 themes: (1) Setting the stage – astronomy in the the S/C systems and in-flight calibrations of scientific 2030s, (2) Science drivers – why do we need a large instruments. UVOIR mission?, (3) Visions for large missions – what The observatory will be put in a circular geosyn- capabilities do we need?, and (4) How large missions fit chronous orbit with 51.6 degrees inclination; Earth oc- 1 into long-term space mission plans . cultation will be small and the orbital period will allow New technologies required for future large UV tele- to time-track targets and to have rapid access to tar- scopes are not still all mature.