  The orbital observatory Spectrum-Roentgen-Gamma (SRG), equipped with the
grazing-incidence X-ray telescopes Mikhail Pavlinsky ART-XC and eROSITA, was
launched by Roscosmos to the Lagrange L2 point of the Sun-Earth system on July
13, 2019. The launch was carried out from the Baikonur Cosmodrome by a Proton-M
rocket with a DM-03 upper stage. The German telescope eROSITA was installed on
SRG under an agreement between Roskosmos and the DLR, the German Aerospace
Agency. In December 2019, SRG started to perform its main scientific task:
scanning the celestial sphere to obtain X-ray maps of the entire sky in several
energy ranges (from 0.2 to 8 keV with eROSITA, and from 4 to 30 keV with
ART-XC). By mid-June 2021, the third six-month all-sky survey had been
completed. Over a period of four years, it is planned to obtain eight
independent maps of the entire sky in each of the energy ranges. The sum of
these maps will provide high sensitivity and reveal more than three million
quasars and over one hundred thousand massive galaxy clusters and galaxy
groups. The availability of eight sky maps will enable monitoring of long-term
variability (every six months) of a huge number of extragalactic and Galactic
X-ray sources, including hundreds of thousands of stars with hot coronae. The
rotation of the satellite around the axis directed toward the Sun with a period
of four hours enables tracking the faster variability of bright X-ray sources
during one day every half year. The chosen strategy of scanning the sky leads
to the formation of deep survey zones near both ecliptic poles. The paper
presents sky maps obtained by the telescopes on board SRG during the first
survey of the entire sky and a number of results of deep observations performed
during the flight to the L2 point in the frame of the performance verification
program.(Abriged)
