Guidelines for Future UV Observatories

Ultra-violet image sensors and UV optics have been developed for a variety of space borne UV astronomy missions. Technology progress has to be made to improve the performance of future UV space missions. Throughput is the most important technology driver for the future. Required developments for different UV detector types – detectors are one of the most problematic and critical parts of a space born mission – and for optical components of the instruments are given in these guidelines. For near future missions we need high throughput optics with UV sensors of large formats, which show simultaneously high quantum efficiency and low noise performance.     

OBSERVATION OF THE UV SOLAR SPECTRAL IRRADIANCE BETWEEN 200 AND 350 nm DURING THE ATLAS I MISSION BY THE SOLSPEC SPECTROMETER

The SOLSPEC instrument has been built to carry out solar spectral irradiance measurements from space. It consists of three spectrometers designed to measure the solar spectral irradiance from 180 to 3000 nm. It flew for the first time in December 1983 with the SpaceLab 1 mission (SL1) and later with the ATLAS missions after significant improvement of the instrument optics and calibration procedures. For the ATLAS 1 mission in March 1992, the thermal conditions encountered during the measurements were better than those of SL1, leading to better data quality. Furthermore, other Sun spectrometers, two on the same platform and two others on board the Upper Atmosphere Research Satellite, have also carried out UV absolute spectral measurements at the same time. These opportunities allowed comparisons of solar irradiance determinations. The UV part of the measurements made during that mission is presented here as well as its calibration and accuracy analysis.     

Massive stars in the UV

We emphasize in this paper the importance of the UV range for our knowledge of massive stars and the fundamental role played by past and present space-based UV capabilities (IUE, HST, FUSE and others). Based on a review of the work developed in the last years and the state of the art situation for quantitative spectroscopy of massive stars, we present crucial advances which could be addressed by hypothetical future space-based UV missions. Advantages and unique data that these missions could provide are explained in the context of our present knowledge and theories on massive stars in the Milky Way and nearby galaxies. It is argued that these studies are our key to a correct interpretation of observations of more distant objects.     

Wide-field ultraviolet imager for astronomical transient studies

Though the ultraviolet (UV) domain plays a vital role in the studies of astronomical transient events, the UV time-domain sky remains largely unexplored. We have designed a wide-field UV imager that can be flown on a range of available platforms, such as high-altitude balloons, CubeSats, and larger space missions. The major scientific goals are the variability of astronomical sources, detection of transients such as supernovae, novae, tidal disruption events, and characterizing active galactic nuclei variability. The instrument has a 80 mm aperture with a circular field of view of 10.8 degrees, an angular resolution of ～22 arcsec, and a 240 - 390 nm spectral observation window. The detector for the instrument is a Microchannel Plate (MCP)-based image intensifier with both photon counting and integration capabilities. An FPGA-based detector readout mechanism and real time data processing have been implemented. The imager is designed in such a way that its lightweight and compact nature are well fitted for the CubeSat dimensions. Here we present various design and developmental aspects of this UV wide-field transient explorer.     

A View to the Future: Ultraviolet Studies of the Solar System

We discuss the status of ultraviolet knowledge of Solar System objects. We begin with a short historical survey, followed by a review of knowledge gathered so far and of existing observational assets. The survey indicates that UV observations, along with data collected in other spectral bands, are necessary and in some cases essential to understand the nature of our neighbors in the Solar System. By extension, similar observations are needed to explore the nature of extrasolar planets, to support or reject astro-biology arguments, and to compose and test scenarios for the formation and evolution of planetary systems.We propose a set of observations, describing first the necessary instrumental capabilitites to collect these and outlining what would be the expected scientific return. We identify two immediate programmatic requirements: the establishment of a mineralogic database in the ultraviolet for the characterization of planetary, ring, satellite, and minor planet surfaces, and the development and deployment of small orbital solar radiation monitors. The first would extend the methods of characterizing surfaces of atmosphere-less bodies by adding the UV segment. The latter are needed to establish a baseline against which contemporaneous UV observations of Solar System objects must be compared.We identify two types of UV missions, one appropriate for a two-meter-class telescope using almost off-the-shelf technology that could be launched in the next few years, and another for a much larger (5–20 meter class) instrument that would provide the logical follow-up after a decade of utilizing the smaller facility.Michel Festou, our co-author and a very important contributor to this paper, passed away while this paper was being completed. We dedicate it to his memory.Deceased 11 May 2005     

Summary of the First NUVA Conference Space Astronomy: the UV Window to the Universe

In this summary of the conference Space Astronomy: the UV Window to the Universe, held in El?Escorial, Spain, May 28 to June 1, 2007, I identify the important scientific questions posed by the speakers and the corresponding discoveries that future ultraviolet space instruments should enable. The science objectives described by the various speakers naturally fall into groups according to the needed instrumental requirements: wavelength coverage, spectral resolution, sensitivity, rapid access to targets, monitoring, and signal/noise. Although most of the science objectives presented during the conference require UV spectra in the 1,170–3,200 Å range, there are important science objectives that require spectra in the 912–1,170 Å range and at shorter wavelengths. I identify the limitations of present instruments for meeting these requirements. To avoid the upcoming UV dark age, important work must be done to properly build the World Space Observatory (WSO) and to plan future space missions.     

FAUST observations near the North Galactic Pole

We analyse a UV observation with FAUST in the direction of the North Galactic Pole. The region includes a cirrus cloud (G251.2+73.3) and a dark globule, and the FAUST image contains 75 UV sources. We discuss the UV source detection and their identification with optical counterparts. We use, for the first time, low-resolution spectral information as the primary means of identifying possible optical counterparts. This is complemented, and sometimes modified, by optical information available from existing data bases. The results are interpreted with the help of maps of the distribution of far-infrared emission and of the neutral hydrogen gas. We discuss the types of objects found, the degree of matching with the predictions of our UV Galaxy model, and the general behaviour of the Galactic UV extinction in this Milky Way part. We compare the UV results for this region with similar observations in the same neighbourhood, which are less affected by dust, and attempt to explain the peculiar distribution of UV magnitudes as a result of a peculiar distribution of foreground dust, which does not follow the accepted dust-to-gas relation.     

Preparing the way to space borne Fresnel imagers

The Fresnel Diffractive Array Imager (FDAI) relies on diffraction focusing to potentially ouput very high wavefront quality particularly in the Ultraviolet. After Chesnokov (Russ Space Bull 1(2), 1993) or Barton (Appl Opt 40(4):447?C451, 2001), we intend to develop tangible optical designs for space missions at the horizon 2025. This paper refers to the phase 0 study completed at CNES. We canvass here different optical scenarios adapted to space formation flying, discussing the technologies involved, their level of maturity and criticity. Large spectral domains were investigated from Lyman-?? to Infra-Red, with competitive aperture size and ambitious objectives. We conclude by a 4-m class UV space mission scenario that could be the first launched imager of this kind.     

UVMag: stellar formation,evolution, structure and environment with space UV and visible spectropolarimetry

Important insights into the formation, structure, evolution and environment of all types of stars can be obtained through the measurement of their winds and possible magnetospheres. However, this has hardly been done up to now mainly because of the lack of UV instrumentation available for long periods of time. To reach this aim, we have designed UVMag, an M-size space mission equipped with a high-resolution spectropolarimeter working in the UV and visible spectral range. The UV domain is crucial in stellar physics as it is very rich in atomic and molecular lines and contains most of the flux of hot stars. Moreover, covering the UV and visible spectral domains at the same time will allow us to study the star and its environment simultaneously. Adding polarimetric power to the spectrograph will multiply tenfold the capabilities of extracting information on stellar magnetospheres, winds, disks, and magnetic fields. Examples of science objectives that can be reached with UVMag are presented for pre-main sequence, main sequence and evolved stars. They will cast new light onto stellar physics by addressing many exciting and important questions. UVMag is currently undergoing a Research & Technology study and will be proposed at the forthcoming ESA call for M-size missions. This spectropolarimeter could also be installed on a large UV and visible observatory (e.g. NASA’s LUVOIR project) within a suite of instruments.     

The space debris environment of the Earth

37 years of space activities have led to a large number of anthropogenic objects orbiting the Earth. Ground-based observations with radar and optical facilities reveal the existence of about 7500 objects in space, which do not represent an immediate excessive danger. However, adequate actions are required to keep the long-term debris hazard for manned and unmanned missions within acceptable tolerances. In this paper the space debris environment of the Earth and its future evolution are described. New developments which could have a major impact on the space environment, are the the planned multi-satellite constellations for communications purposes or solar power stations in Earth orbit. Finally, methods for debris reduction are outlined. Space debris is a global problem which can only be effectively solved by international cooperation.     

WSO/UV spectrographs: the expected performance of HIRDES

The World Space Observatory Ultraviolet (WSO/UV) is a multi-national project grown out of the needs of the astronomical community to have future access to the UV range. WSO/UV consists of a single UV telescope with a primary mirror of 1.7 m diameter feeding the UV spectrometer and UV imagers. The spectrometer comprises three different spectrographs, two high-resolution echelle spectrographs (the High-Resolution Double-Echelle Spectrograph, HIRDES) and a low-dispersion long-slit instrument. Within HIRDES the 102–310 nm spectral band is split to feed two echelle spectrographs covering the UV range 174–310 nm and the vacuum-UV range 102–176 nm with high spectral resolution (R>55000). The technical concept is based on the heritage of two previous ORFEUS SPAS missions. The phase-B1 development activities are described in this paper considering the performance of the instruments.     

Rotational excitation and damping as probes of interior structures of asteroids and comets

Abstract— Rotational excitation and damping are discussed in the context of inferring structural properties of asteroids and comets. Opportunities for carrying out deterministic experiments are outlined and basic concepts involving space missions are discussed. Spacecraft carrying an impactor or explosives together with an orbiter are suggested as effective probes of the interiors of asteroid and comets. The feasibility of such missions, especially to near‐Earth objects (NEOs), is highlighted as NEOs provide an appropriate cost‐effective path to explore interiors of asteroids and comets.     

All-sky ultraviolet surveys: the needs and the means

This is a crucial time in the history of astronomy with major all-sky surveying work being carried out in all spectral bands, as well as in astrometry. The results of this activity are advancing all fields of astrophysical research, from the investigation of exo-planetary systems to the study of the chemical evolution of the Universe. Full sky surveys are available from the radio domain to X-ray wavelengths but not in the ultraviolet range (UV). While large UV missions are currently under discussion within the astrophysical community and at the major Space Agencies, the efficient use of resources requires preparatory work that can fill the UV surveying gap. This article summarizes the research and on-going activities in this field.     

Sample Return Missions from Minor Bodies: Achievements, Future Plan and Observational Support

We are entering in a new era of space exploration signed by sample return missions. Since the Apollo and Luna Program, the study of extraterrestrial samples in laboratory is gathering an increased interest of the scientific community so that nowadays exploration program of the Solar System is characterized by swelling sample return missions. Beside lunar samples, the NASA Stardust mission was the first successful space mission that on 15 January 2006 brought to Earth solid extraterrestrial samples collected from comet 81P/Wild 2 coma. Grains were collected during cometary fly-by into aerogel and once on Earth have been extracted for laboratory analyses. In the coming two decades many space missions on going or under study will harvest samples from minor bodies. Measurements required for detailed analysis that cannot be performed from a robotic spacecraft, will be carried out on Earth laboratories with the highest analytical accuracy attainable so far. An intriguing objective for the next sample return missions is to understand the nature of organic compounds. Organic compounds found in Stardust grains even if processed to large extend during aerogel capturing are here reported. Major objectives of Marco Polo mission are reported. Various ground-based observational programs within the framework of general characterizations of families and classes, cometary–asteroid transition objects and NEOs with cometary albedo are discussed and linked to sample return mission.     

Round-trip mission requirements for asteroids 1976 AA and 1973 EC

The recent discovery of 1976 AA has renewed interest in the possibility of modest asteroid sample-return missions. Such ventures may be logical precursors to more complex round-trip planetary missions. Both manned and unmanned mission requirements are assessed for two Apollo-Amor objects: 1976 AA and 1973 EC. It is shown that the propulsion requirements of 1-yr manned missions to either target are excessive, i.e., more than 20 Shuttle launches would be required. However, a low-energy 3-yr unmanned round-trip mission to 1973 EC has also been found, which requires only one Shuttle launch with a recoverable upperstage. It is apparent from these results that the discovery of a short-period low-obliquity object could have a profound impact on man's initial ventures beyond Earth-Moon space.     

Key problems in cool-star astrophysics

Selected key problems in cool-star astrophysics are reviewed, with emphasis on the importance of new ultraviolet missions to tackle the unresolved issues.UV spectral signatures are an essential probe of critical physical processes related to the production and transport of magnetic energy in astrophysical plasmas ranging, for example, from stellar coronae, to the magnetospheres of magnetars, and the accretion disks of protostars and Active Galactic Nuclei. From an historical point of view, our comprehension of such processes has been closely tied to our understanding of solar/stellar magnetic activity, which has its origins in a poorly understood convection-powered internal magnetic dynamo. The evolution of the Sun's dynamo, and associated magnetic activity, affected the development of planetary atmospheres in the early solar system, and the conditions in which life arose on the primitive Earth. The gradual fading of magnetic activity as the Sun grows old likewise will have profound consequences for the future heliospheric environment. Beyond the Sun, the magnetic activity of stars can influence their close-in companions, and vice versa.Cool star outer atmospheres thus represent an important laboratory in which magnetic activity phenomena can be studied under a wide variety of conditions, allowing us to gain insight into the fundamental processes involved. The UV range is especially useful for such studies because it contains powerful diagnostics extending from warm (∼ 10⁴ K) chromospheres out to hot (1–10 MK) coronae, and very high-resolution spectroscopy in the UV has been demonstrated by the GHRS and STIS instruments on HST but has not yet been demonstrated in the higher energy EUV and X-ray bands. A recent example is the use of the hydrogen Lyα resonance line—at 110 000 resolution with HST STIS—study, for the first time, coronal winds from cool stars through their interaction with the interstellar gas. These winds cannot be detected from the ground, for lack of suitable diagnostics; or in the X-rays, because the outflowing gas is too thin.A 2m class UV space telescope with high resolution spectroscopy and monitoring capabilities would enable important new discoveries in cool-star astronomy among the stars of the solar neighborhood out to about 150 pc. A larger aperture facility (4–6 m) would reach beyond the 150 pc horizon to fainter objects including young brown dwarfs and pre-main sequence stars in star-forming regions like Orion, and magnetic active stars in distant clusters beyond the Pleiades and α Persei. This would be essential, as well, to characterize the outer atmospheres of stars with planets, that will be discovered by future space missions like COROT, Kepler, and Darwin.Deceased October 23, 2005     

Spacecraft exploration of asteroids

The past, current, and planned space missions for asteroid exploration are reviewed. The main results based on observations performed with satellites in near-Earth orbits (OAO-2, IUE, FIRSSE, IRAS, HST, Hipparcos, ISO, MSX) and space probes sent to particular objects (Galileo, NEAR, DS1, Stardust) are reported. Future space missions (MUSES-C, Rosetta, DOWN, etc.) and their main goals in asteroid study are considered. The feasibility of using spacecraft for minor-body exploration is discussed.Translated from Astronomicheskii Vestnik, Vol. 39, No. 1, 2005, pp. 81–90. Original Russian Text Copyright © 2005 by Shevchenko, Mohamed.     

Lunar pure anorthosite as a spectral analog for Mercury

Abstract— Plans are underway for spacecraft missions to the planet Mercury beginning in the latter part of this decade (NASA's MESSENGER (MErcury, Surface, Space ENvironment, GEochemistry, Ranging) and ESA's BepiColombo). Mercury is an airless body whose surface is apparently very low in ferrous iron. Much of the mercurian surface material is expected to be optically mature, a state produced by the “space weathering” process from direct exposure to the space environment. If appropriate analog terrains can be identified on the Moon, then study of their reflectance spectra and composition will improve our understanding of space weathering of low‐Fe surfaces and aid in the interpretation of data returned from Mercury by the spacecraft. We have conducted a search for areas of the lunar surface that are optically mature and have very low ferrous iron content using Clementine ultraviolet‐visible (UV‐vis) image products. Several regions with these properties have been identified on the farside. These areas, representing mature pure anorthosites (>90% plagioclase feldspar), are of interest because only relatively immature pure anorthosites have previously been studied with Earth‐based spectrometry. A comparison of Mercury with the lunar analogs reveals similarities in spectral characteristics, and there are hints that the mercurian surface may be even lower in FeO content than the lunar pure anorthosites. We also investigate the potential for use of spectral features other than the commonly studied “1 μm” mafic mineral absorption band as tools for compositional assessment when spacecraft spectral measurements of Mercury become available. Most low‐Fe minerals plausibly present on Mercury lack absorption bands, but plagioclase possesses an iron impurity absorption at 1.25 μm. Detection of this diagnostic band may be possible in fresh crater deposits.