The Hubble Space Telescope was launched from the Kennedy Space Center on April 24, 1990. Its initial check-out indicates that all sub-systems of the satellite are working very well, with two key exceptions: The line-of-sight pointing is subject to occasional jitter apparently induced by thermal stresses in the solar arrays; this is expected to be overcome. The telescope mirrors are found to contain approximately 0.5 wave rms of spherical aberration which cannot be overcome by any controls on board the satellite. This defect will limit the scientific performance of the telescope in the short run. However, the aberration can be fully corrected in the optical designs of future replacement instruments, and the delivery schedules of these instruments are being accelerated.

The Cosmic Background Explorer, launched November 18, 1989, has nearly completed its first full mapping of the sky with all three of its instruments: a Far Infrared Absolute Spectrophotometer (FIRAS) covering 0.1 to 10 mm, a set of Differential Microwave Radiometers (DMR) operating at 3.3, 5.7, and 9.6 mm, and a Diffuse Infrared Background Experiment (DIRBE) spanning 1 to 300 µm in ten bands. A preliminary map of the sky derived from DIRBE data is presented. Initial cosmological implications include: a limit on the Comptonization y parameter of 10−3, on the chemical potential μ parameter of 10−2, a strong limit on the existence of a hot smooth intergalactic medium, and a confirmation that the dipole anisotropy has the spectrum expected from a Doppler shift of a blackbody. There are no significant anisotropies in the microwave sky detected, other than from our own galaxy and a cosθ dipole anisotropy whose amplitude and direction agree with previous data. At shorter wavelengths, the sky spectrum and anisotropies are dominated by emission from ‘local’ sources of emission within our Galaxy and Solar System. Preliminary comparison of IRAS and DIRBE sky brightnesses toward the ecliptic poles shows the IRAS values to be significantly higher than found by DIRBE at 100 μm. We suggest the presence of gain and zero-point errors in the IRAS total brightness data. The spacecraft, instrument designs, and data reduction methods are described.

The Diffuse Infrared-Background Experiment (DIRBE) on the Cosmic Background Explorer (COBE) satellite is a 10-band absolute photometer covering the wavelengths 1–300 microns using photovoltaic, photoconductive, and bolometric detectors. The input is via a 19-cm, off-axis, highly-baffled Gregorian telescope, with the detectors located at a pupil plane so they share the same field of view (0.7 × 0.7 degrees). The whole assembly is mounted inside a 1.4 K; super-fluid, liquid-He dewar, which is shared with the Far Infrared Absolute Spectrometer (FIRAS) instrument. Each day half of the sky is surveyed, as the line-of-sight of the DIRBE is canted 30 degrees to the COBE spin axis. The whole sky is fully observed in 6 months, as the spin axis precesses at about 1 degree per day. At present each sky pixel has been observed at least once. The basic findings on the general brightness of the sky - Zodiacal light and galaxy - are provided, as well as a synopsis of the advantages and disadvantages associated with a space-borne observatory. The relationship of our experience and findings with respect to possible future missions and their scientific goals is presented.

The participants in the project are:

1. – USSR, by developing the Granat spacecraft (Scientific and Production Association S.A. Lavochkin, G.N. Babakin Research Center) and the instruments in the scientific equipment packages

2. – ART-P, ART-S, KONUS-V, ‘Podsolnukh’ (Sunflower), KS-18M (IKI of the Academy of Sciences of the USSR, Leningrad Institute of Physics and Technology of the Academy of Sciences of the USSR, Nuclear Physics Science and Research Institute of the Moscow State University);

3. – France (CNES), by developing and delivering the gamma ray telescope SIGMA, the high energy spectrometer Phoebus, a memory device with 128 Mbit capacity, and a system for determining the current orientation of the telescope (star sensor);

4. – Denmark (Danish Institute for Space Research), by developing the wide-angle X-Ray telescope VOTCh;

5. – Bulgaria (TsLKI Bulgarian Academy of Sciences), by developing and delivering certain control instruments for the ‘Podsolnukh’ apparatus.

The Hipparcos astrometry satellite was launched on 8 August 1989, and after spacecraft and payload commissioning, commenced the routine data acquisition phase on 26 November 1989. Having failed to reach its planned geostationary orbit, major revisions in the mission operations were made, and the post-launch expectations of the mission were strongly degraded with respect to the original goals - principally due to the greatly reduced observational efficiency (caused by the lack of ground station coverage) and the anticipated degraded mission lifetime (as a result of the high-energy particle degradation of the solar arrays in the geostationary transfer orbit).

The final astrometric accuracies attainable by the Hipparcos mission will be influenced by the spacecraft and payload performances on the one hand, and by fraction of useful data and mission lifetime on the other. It will be shown that the elemental observational measurements correspond very closely to the predictions, and the data recovery fraction now stands at around 60–70 per cent, so that the ultimate scientific value of the Hipparcos results will be tied directly to the satellite lifetime. A measurement duration of at least 18 months is mandatory if the astrometric parameters (parallaxes and proper motions) are to be decoupled through the data reductions procedures. A somewhat longer lifetime (2.5–3 years) is necessary in order to reduce the errors on the astrometric parameters to the astrophysically-significant accuracies of around 2 milli-arcsec.

It will be shown that the present indications of the satellite performances, and the significant progress already made in the data reductions, indicate that the difficulties of the ‘revised’ Hipparcos mission have been largely overcome, and that these target accuracies could still be achievable.

The International Ultraviolet Explorer (IUE) was launched into a geosynchronous orbit on 26 January 1978. It is equipped with a 45-cm mirror and spectrographs operating in the far-ultraviolet (1150–2000 Å) and the mid-ultraviolet (1900–3200 Å) wavelength regions. In a low-dispersion mode, the spectral resolution is some 6–7 Å. In a high-dispersion echelle mode, the resolution is about 0.1 Åat the shortest wavelength and about 0.3 Åat the longest. It is a collaborative program among NASA, ESA and the British SERC. The IUE is operated in real time 16 hours a day from NASA Goddard Space Flight Center near Washing ton, D.C. and 8 hours daily from ESA’s Villafranca groundstation near Madrid, Spain. By the end of 1989, 1870 papers, using IUE observations, have been published in refereed journals. During the same period, over 1700 different astronomers from all over the world used the IUE for their research.

The X-ray astronomy satellite Ginga carries three scientific instruments, the Large Area proportional Counters (LAC), All Sky X-ray Monitor (ASM) and Gamma-ray Burst Detector (GBD). The LAC is the main instrument with an effective area of 4000 cm2 giving it the highest sensitivity to hard X-rays so far achieved. Ginga observed about 250 targets up to the end of 1989.

The instrumental characteristics, observational capabilities and scientific results of the Voyager 1 and 2 ultraviolet spectrometers are reviewed. These instruments provide current and ongoing access to low resolution spectra for a wide variety of astronomical sources in the 500 to 1700 Å band. Observations of the brightest OB stars and hot subluminous stars as faint as V = 15 mag. are possible. In the EUV, at wavelengths shortward of 900 Å, several new sources have been detected and a host of potential sources ruled out. In the Far UV, particularly at wavelengths between 900 and 1200 Å, Voyager is capable of observing a wide range of stellar and non-stellar sources. Such observations can often provide a valuable complement to IUE and other data sets at longer wavelengths. The Voyager spectrometers have proved remarkably stable photon counting instruments, capable of extremely long integration times. The long integration times, relatively large field of view, and location in the outer solar system also provide an ideal platform for observations of sources of faint diffuse emission, such as nebulae and the general sky background.

The ROSAT launch on June 1, 1990 happened to be after the IAU Colloquium No. 123 before the deadline for submitting manuscripts. I therefore take the liberty to deviate grossly from the manuscript of my talk and give a short up to date mission summary. A more complete description of the mission can be found in References 1 and 2.

The launch of the Delta II rocket was perfect and the orbital parameters reached are very close to nominal: height 580 km, inclination 53°. The predicted satellite’s lifetime in this orbital is at least 10 years. The switch-on of the spacecraft and instrument subsystems could be completed without any losses. All systems are in good health.

The Gamma-Ray Observatory (GRO), a Shuttle-launched free-flying Observatory is currently scheduled for launch in November 1990. The mission provides nearly six orders of magnitude in spectral coverage, from about 50 KeV to about 30 GeV, with a sensitivity over the entire range over an order of magnitude greater than that of previous observations. The 16,000 kilogram Observatory contains four instruments on a stabilized platform. The mission duration is expected to be from six to ten years. A Science Support Center has been established at the Goddard Space Flight Center for the purpose of supporting a vigorous Guest Investigator Program.

The Advanced X-Ray Astrophysics Facility (AXAF) is a remarkable and unique scientific endeavor. It will be a space-based earth orbiting X-ray observatory, designed to address fundamental questions in astronomy and astrophysics. It is remarkable in that it represents enormous advances in observational capability. It is unique in that it will be the best means of obtaining high resolution, spectrally resolved X-ray images of astronomical objects for the rest of this century and beyond.

Astro-D, the fourth X-ray astronomy mission of ISAS following Ginga, is a high-throughput imaging and spectroscopic X-ray observatory, scheduled for launch in early 1993. It utilizes multilayer conical X-ray mirrors which provide a large effective area over a wide range energy range up to 12 keV. The focal plane instruments comprize a set of CCD cameras and Imaging gas scintillation proportional counters. Main features of Astro-D are described, and the important astrophysical objectives are discussed.

The capabilities of the X-ray Timing Explorer (XTE) are described with particular attention paid to current scientific problems it will address from galactic neutron star systems to active galactic nuclei. It features a low-background continuous 2-200 keV response with large apertures (a 0.63-m2 proportional counter array and a 0.16-m2 dual rocking NaI/CsI scintillation array). Rapid response (in hours) to temporal phenomena, e.g. transients, is obtained by virtue of a scanning all-sky monitor and rapid maneuverability. XTE will carry out detailed energy-resolved studies of phenomena close to neutron stars (e.g. QPO’s) because of its sub-millisecond timing (to 10 μs), its high telemetry rates (to 256 kb/s), and the high throughput of its data system (to ≳ 2 × 105 c s−1).

JET-X is one of the main payload elements in the Soviet Spectrum-X Gamma mission scheduled for launch in 1993. It consists of two Wolter I X-ray telescopes feeding cooled arrays of CCD’s. The combination of good angular resolution and long exposures during the four day orbit of Spectrum-X Gamma, will allow the JET-X telescopes to reach a sensitivity level ten times fainter than that of the Einstein Observatory deep survey. The CCD’s will provide good spectral data for many thousands of brighter sources over an energy band ~0.3–10 keV. The JET-X project is an international collaboration involving groups in the United Kingdom, Italy, West Germany and the Soviet Union together with an input from the Space Science Department at ESTEC. A brief description is given of the design and anticipated performance of JET-X together with illustrations of the anticipated scientific results.

SPECTRUM-RÖNTGEN-GAMMA (SRG) is one of a new series of large astronomical missions being planned by the Soviet Union and is scheduled for launch in mid-1993. The Space Research Institute (IKI) of the Academy of Sciences of the USSR and the Babakin Center (BC) are responsible for the scientific supervision and spacecraft construction, respectively. Mission objectives include broad and narrow band imaging spectroscopy over a wide range of energies from EUV through gamma rays with particular emphasis on extragalactic objects. The design of the Soviet Danish Röntgen Telescope (SODART) consists of two thin foil, conical shell approximations to Wolter 1 geometry. The reflectors are rolled aluminum foils which have been dipped in acrylic lacquer and coated with gold resulting in a super smooth surface. Each telescope has an aperture of 60 cm, a focal length of 8 m, a field of view of 1 deg and is designed to have a halfpower width of ≤2 arcmin. The conical geometry contributes 15 arcsec and manufacturing tolerances in the support structure and the quality of the figure of the foil the rest. The contribution from X-ray scattering is insignificant. Focal plane slides can position one of four instruments at the focus of each telescope. Images and spectra will be recorded with position sensitive proportional counters with spectral resolution as good as 13% at 6 keV. Spectral resolution of 2.5% at 6 keV is provided by an array of 19 cooled silicon detectors. A broad band polarimeter will be sensitive to residual polarization as low as 1%. An objective Bragg crystal panel, placed in front of one of the telescopes, will be capable of high resolution spectroscopic studies ((E/ΔE)) ~1000) of point- and extended sources.

ESA’s X-ray Astronomy Mission, XMM, scheduled for launch in 1998, is the second of four cornerstones of ESA’s long term science program Horizon 2000. Covering the range from about 0.1 to 10 keV, it will provide a high throughput of 5000 cm2 at 7 keV with three independant telescopes, and have a spatial resolution better than 30 arcsec. Broadband spectrophotometry is provided by CCD cameras while reflection gratings provide medium resolution spectroscopy (resolving power of about 400) in the range 0.3–3 keV. Long uninterrupted observations will be made from the 24 hr period, highly eccentric orbit, reaching a sensitivity approaching 10−15 erg cm−2 s−1 in one orbit. A 30 cm UV/optical telescope is bore-sighted with the x-ray telescopes to provide simultaneous optical counterparts to the numerous serendipitous X-ray sources which will be detected during every observation.

The satellite for X-ray astronomy SAX, to be launched at the end of 1993, is devoted to systematic, integrated and comprehensive, studies of galactic and extra-galactic sources in the energy band 0.1–200 keV, and is under joint development by the Italian Space Agency (ASI) and the Netherlands Agency for Aerospace programs (NIVR), with the participation of SRU/SRON and SSD/ESTEC. The basic scientific objectives can be summarized as follows:

1. – Broad band spectroscopy (E/Δ E=12) from 0.1–10 keV with imaging resolution of 1 arcmin.

2. – Continuum and line spectroscopy (E/Δ E=5–20) in the energy range 3–200 keV.

3. – Variability studies of bright source energy spectra on timescales from milliseconds to days and months.

4. – Systematic long term variability studies over the entire sky down to a source intensity of 1 mCrab.

The Extreme Ultraviolet Explorer mission is described. The satellite is now scheduled to be launched in September, 1991. For the first six months, an all sky survey will be carried out covering 90 to 820 Å, or essentially the entire EUV bandpass. This EUV survey will be made in four bands, or colors, λλ90–150 Å; 170–250 Å, 400–600 Å and 550–750 Å. A portion of the sky which is free from the normally intense 304 Å geocoronal helium background will be surveyed at greater sensitivity; the wavelength coverage of this band is from 90 to 400 Å. Following the sky survey portion of the mission, spectroscopy of individual sources will be carried out. Three spectrometers employing novel variable line spaced gratings will provide spectra with ~1 Å resolution over the band from 90 to 800 Å. This spectroscopy will be carried out by guest observers chosen by NASA in a manner roughly analogous to the IUE guest observer program.

The Lyman Far Ultraviolet Spectroscopic Explorer mission will obtain high resolution spectra (R 30,000) in the 912–1200 Å region with sufficient sensitivity to study faint sources throughout the galaxy and at large extragalactic distances. This spectral region provides unique tools for solving problems in cosmology (e.g. deuterium), galactic structure and evolution (e.g. O VI and H2), stellar evolution (e.g. O VI) and planetary science (e.g. H2). Recent advances in optical and detector technology, which make this goal possible with a moderate size experimental package, also enable the spectral coverage to extend down to 100 Å with good sensitivity and only a minimal increase in complexity. Thus a secondary goal is to cover the 100–912 Å region with moderate spectral resolution. In 1989, following a Phase A study of the mission concept, NASA selected the mission for Phase A study in 1989. Both Canada and the United Kingdom are participating in the definition and development of the mission.