The operational work of the Interplanetary Solar Stereoscopic Observatory (ISSO) may continue for 11 years. The ISSO Project is designed to insert two spacecraft (SC) into Lagrangian triangular libration points of the “Sun + barycenter (the Earth + the Moon)” system. The Project scientific program consists of two sets of experiments related to solar physics and to dynamical astronomy, respectively.

Various options of the astronomical observation program are possible. We discuss the option where the stereoscopic mode is applied to the direct triangulation observations of the solar system objects with respect to the ICRS coordinate frame.

The stereoscopic mode with the baseline equal to appears to be suitable for receiving the three-dimensional (3-D) measurement series for planets during the ISSO’s lifetime. Extended investigation of the planetary aberration based on the observations of major and minor planets, the direct distance determinations for minor planets and Kuiper’s belt objects, and the new set of star parallaxes may be used for constructing a new fundamental ephemeris and establishing an alternative scale of stellar distances. The accuracy of the angular measurements is expected to attain the milliarcsecond level.

The Struve space astrometric project (Yershov et al. 1995) is being developed by a consortium of Russian space instrumentation institutes in order to extend the Hipparcos satellite reference system (the project initially named AIST, owes its current name to the first director of the Pulkovo Observatory).

The Space Astrometry and Photometry Project Struve is being designed at the Pulkovo Observatory in cooperation with other Russian space institutes. It is expected to be launched before 2010 with a duration of mission of at least 36 months.

The main objectives are to extend at milliarcsecond accuracy the Hipparcos satellite reference system to fainter objects including quasars and to get a second epoch for Hipparcos stars. The project Struve, along with the recently suggested project DIVA, will fill the intermediate place between Hipparcos and microarcsecond astrometry. Unlike DIVA, we propose far more extensive astrometric and photometric surveys.

We expect an Output Catalogue of 20 million stars (density of about 500 stars per square degree). A sky survey will be complete down to V = 14 (about 15 million stars), and selected objects down to V = 19.5 will be observed within a special program. The proper motions of the Hipparcos stars will be determined with an accuracy of about 0.1 mas/yr. The mean accuracy of star positions in the output catalogue is expected to be 0.6 mas which could be achieved by proper design of the satellite (symmetry, smooth rotation etc.), optics and the micrometer. A properly designed micrometer (with CCD arrays, special processors for image processing and the compression of the data flux to the ground station) will give the possibility of observing all objects of the sky down to a definite limiting magnitude.

The arguments are given of the choice for the Vilnius seven color medium band photometric system as the most effective one for the AIST space project. A prediction is made of the limiting stellar magnitudes V for every Vilnius passband and the integral band T at σΣ ≤ 30mmag from a 3 year AIST mission.

The researches on the AIST-project are directed to the design of the focal plane assembly. The concept of the chip-mosaic and numeric simulation of the two-pixels mode of the signal sampling are presented.

The results of an initial technical design of the AIST spacecraft and payload are presented. The spacecraft design is subjected to the main requirement: to supply the smooth rotation. Two telescopes are symmetrically placed around central beam combiner unit. The fine adjustment of the light pressure centre position by means of special-purpose sails is proposed. Scientific objectives of the project are the implementation of astrometric survey of the sky and obtaining a series of star position catalogues with milliarcsecond accuracy, including a second epoch of Hipparcos/Tycho catalogues. The survey list will consist of nearly 10–15 million stars brighter than 15th magnitude.

An improved astrometric space mission similar to Hipparcos is proposed for launch ten years after the present Hipparcos. The new mission should provide positions, parallaxes and annual proper motions with an accuracy about one milli-arcsecond for some 400 000 stars. The positions from the two missions would give proper motions for the 120 000 common stars with an accuracy about 0.2 milli-arcsecond per year, ten times less than expected for the first mission. The improvements of the satellite relate e.g. to: larger telescope aperture, smooth attitude motion, enhanced star mapper for four-colour observations (B V U R) of two million stars. The proposal resulted from discussions with members of the Pulkovo Observatory and the Hipparcos Science Team, and it is being considered by the Mission Control Centre, Moscow.

Some aspects of the realisation of a celestial reference frame using a space astrometry facility are considered. An observational program is described, consisting of observing stars up to magnitude 14, radiostars and bright QSO's, planets, asteroids and of laser signals from the Earth. A scheme of an astrometric facility consisting of two telescopes on board the satellite is proposed. The overview strategy with “inita” is estimated, and the estimates of the accuracy of a single observation (0.″02-0.″05) and of the output catalogues (0.″001-0.″007) are made.