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July 13th (2 months ago) was the 5th anniversary of the launch of the Spectrum-Rentgen-Gamma Orbital Observatory on the space track to the 2nd Lagrangian point of the Sun-Earth system. But, like almost all space observatories, the path to launching the SRG was not simple and direct. It began in 1987 at an international meeting held in Moscow in honor of the 30th anniversary of the launch of the first Earth satellite into orbit. It was a time of great change for the Soviet Union - the “perestroika” proclaimed by Mikhail Gorbachev after coming to power in 1985. INTERCOSMOS, the organization responsible for international cooperation in space in the USSR, brought together Soviet and foreign specialists and scientists at the Institute for Space Research in Moscow to select the main directions in planetary and astrophysical research on Soviet spacecraft with broad international participation. Among the projects discussed was the Spektr-RG X-ray Observatory project, to which scientists and representatives of space agencies from the United States, Germany, Great Britain, Italy, France, Denmark, and several other countries were invited to participate. This meeting recommended three astrophysics projects: Radioastron (with a 10-meter radio telescope on board) for radio interferometric observations in conjunction with the largest ground-based radio telescopes, Spectrum UF with an ultraviolet telescope with a diameter of 1. 7 m and SRG with two large grazing incidence X-ray telescopes with position-sensitive detectors, an X-ray polarimeter, a hard X-ray telescope with a coding aperture, and two all-sky instruments. The history of the choice was not easy: behind each project were famous scientists from both the USSR and Western countries.
These three projects were supported and then included in the plans of Intercosmos and the USSR Academy of Sciences. NASA, space agencies, institutes, and universities in Germany, Great Britain, Italy, Denmark, Finland, and Switzerland joined the project. The Lavochkin factory proposed a high-apogee orbit for the project and launch by a PROTON rocket.
By the early 1990s, the economic situation in the USSR had noticeably deteriorated, and work on the project slowed down. At the end of December 1991, the USSR fragmented into 15 independent states. Nevertheless, Russia continued working on the project, but at a slow pace, accelerating it in 1997. The work was completely stopped only in 2002 due to a big delay in the readiness of part of the instruments and the satellite.
Nevertheless, it was possible to keep the project in the plans of the Academy of Sciences and Roscosmos. In the spring of 2005 it was allowed to start negotiations with foreign partners on the continuation of the project, but with much more modest parameters: a Soyuz rocket instead of Proton and a smaller number and mass of X-ray instruments.
World X-ray astronomy and observational cosmology did not stand still (18 years passed), new directions and new technologies of detectors and X-ray telescopes appeared. The advantages of a halo orbit around the 2nd Lagrangian point became obvious. In Russia, such an orbit was proposed for the RELICT 2 project and immediately aroused great interest. Later NASA decided to launch a WMAP satellite into it. Theoretical works of the 70s on the SZ effect, acoustic peaks in the power spectrum of angular fluctuations of CMB radiation, and baryonic acoustic oscillations aroused the interest of theorists and radio astronomers in the possibility of observing “all” massive galaxy clusters in the observable Universe and using them to determine its parameters. It was obvious that X-ray observations had a chance to compete with ground-based observations in the millimeter range.
The observations of the millions of quasars and AGNs and the search for the tidal disruption events was another important task.
Joachim Trümper's group in MPE was well known to the scientists of IKI from the joint work with the German hard X-ray telescope HEXE on the KVANT module of the MIR space station and the project of the JET-X X-ray telescope for the first variant of the SRG (a copy of this telescope successfully operates on the NASA SWIFT satellite). Of course, the great success of the ROSAT satellite also played its role. As a result of long negotiations (during the conference in Japan), Günther Hasinger and Peter Predehl were persuaded to switch from their proposal to install on SRG a copy of the small telescope for the ABRIXAS satellite to an instrument much larger in size and sensitivity, which eventually became eRosita. The sensitivity of this telescope immediately made it a leader among other proposals for installation on the “new” SRG. In 2006, all necessary decisions and agreements with MPE and DLR were made and the installation of eRosita and the Russian ART XC (now named after Mikhail Pavlinsky) on the SRG satellite was approved.
eRosita has performed well during the two and a half years on board the SRG. After March 2022, ART XC became the primary tool of the SRG. In more than two years it has scanned the plane of our Galaxy in detail, observed dozens of transient X-ray sources, and now continues to scan the entire sky with an interesting change in scanning mode.
