Of the European components the first is ROSITA (seven X-ray mirror telescopes), and the second, Lobster (a wide-field X-ray monitor). They were supposed to be delivered to the International Space Station as part of the Columbus module aboard a U.S. shuttle, but the Columbia disaster intervened.

SRG features an ideal equatorial orbit. It has an altitude of 580 to 600 kilometers and a zero inclination. It is placed below the Earth's radiation belts, which otherwise would have created observation noise. Such an orbit, lower than the background by almost one order of magnitude, will result in a significant sensitivity gain, making it possible to produce detailed studies of extended astrophysical objects with low surface brightness (such as clusters of galaxies), providing record spectral sensitivity and angular resolution. About three million cores of active galaxies (super massive black holes) and up to 100,000 galactic clusters are expected to be discovered and help explain the nature of dark matter and dark energy. Current theory has it that they are responsible for the accelerating expansion of the Universe.

Observations of remote galaxies and of the relict radiation from the early stages of evolution confirm that our universe is not stationary, but expands with time. In the past, all models predicted that the expansion was to slow down gradually. Now it has turned out the Universe has been ballooning for the past two billion years with an acceleration, or exploding for the second time after the Big Bang. The physical cause behind the growing expansion is absolutely unclear. Some scientists believe gravitation laws do not closely follow Einstein's general theory of relativity on this scale.

Others do not consider the space vacuum to be absolutely empty, but think it is occupied by a field with some non-zero energy density. Such a field would have remarkable properties, for example, a negative effective pressure, and would "push" the Universe apart. But the forces involved are so small that the vacuum energy can be "seen" only from its effect on the Universe as a whole.

Generally accepted theory does not deny nor postulate the existence of vacuum energy with such properties, and the phenomenon has been dubbed dark energy. It is now known to contribute mostly to the total mass of the Universe - about 73%. This compares with only 4% of matter present in all visible objects (so-called baryon matter) - interstellar gas, the stars, planets, etc. Another 23% is the so-called dark matter.

Once it was established that 95% of the universe consists "of no one knows what," room appeared for new theories, some of them very daring. Many experts in the physics of fundamental interactions and cosmology believe the explanation for dark energy will bring about a new revolution in science, like at the turn of the 19th/20th centuries, when Einstein formulated his theory of relativity. Today one of the key problems addressed by all astronomers and astrophysicists throughout the world is to find how dark energy changes with time. The reasons are clear: it is not too often that astronomical measurements lead to fundamental changes in understanding the physical picture of the world.

The SRG observatory may become the world's first global project to determine hidden mass in the Universe. In its observational capability the project is far ahead of all the other experiments in the world concerned with dark energy studies.

The required orbit will be provided by a launch from the Courou space center in French Guiana of a Russian Soyuz-2 vehicle boosted by a Fregat unit. But the biggest hopes are still pinned on the Baikonur space center, although the inclination of 29o in background terms will not favor observation. However, in neither case will it be possible to control the observatory and pick up information from Russia. Three alternative control centers are currently being considered: South Africa, Australia and India.

Navigator, a development of the Lavochkin Research and Production Association, will provide the basic space platform for the observatory. Its first launch is scheduled for 2007 in the Elekro mission, to be later used in the Radioastron and Ultrafiolet astrophysical projects.

On the scale accepted in the space industry, the launching of SRG in 2011 is "near at hand." But problems, if any, could only occur with the space platform, because the scientific payload is as good as finished. In the first four years, observatory instruments will carry out global surveys of the sky and only observe individual sources in the following three to six years.

The SRG project will mark a major scientific and technological step in the switch from giant observatories - Chandra (U.S.) and XMM-Newton (European Space Agency) - to the next XEVS/Con-X generation of astrophysical observatories scheduled to come online in 2015-2025.

The project was made feasible by a decision of the Russian Space Agency and the European Space Agency on closer coordination of their space programs as regards launch facilities and studies.

Yury Zaitsev is an expert with the Russian Academy of Sciences' Institute of Space Research

The opinions expressed in this article are those of the author and may not necessarily represent the opinions of the editorial board.

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