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A Proton rocket and Block DM upper stage climbed into space Saturday from the Baikonur Cosmodrome in Kazakhstan with Spektr-RG, an astronomical observatory with dual X-ray telescopes developed by Russian and German scientists on the hunt for the signature of dark energy.

The new observatory will detect X-ray emissions from across the sky, including those from huge clusters of galaxies. By measuring the mass, luminosity and distance of distant galaxies, Spektr-RG could help astronomers better understand dark energy, the mysterious force driving the accelerating expansion of the universe.

During a four-year all-sky survey, the mission will scan the complete sky eight times. By the end of its all-sky survey, Spektr-RG could discover millions of new X-ray sources, building up a statistical catalog of objects and cosmological structures to fill in gaps in the map of the known universe, according to Mikhail Pavlinsky, the mission’s lead scientist from IKI, the Space Research Institute of the Russian Academy of Sciences.

The Spektrum-Röntgen-Gamma mission, also known as Spektr-RG, rode a 187-foot-tall (57-meter) Proton rocket into a clear sky over Baikonur, a historic space base on the steppe of Kazakhstan. Liftoff occurred at 1230:57 GMT (8:30:57 a.m. EDT) to begin a two-hour launch sequence, culminating in the deployment of the Spektr-RG spacecraft after two firings by the Proton’s Block DM upper stage engine.

Roscosmos, the Russian space agency, announced the successful separation of the 5,980-pound (2,712.5-kilogram) Spektr-RG spacecraft, and confirmed the probe extended its power-generating solar panels as it begins a three-month cruise to an observing post nearly a million miles (1.5 million kilometers) from the night side of Earth.

Scientists selected the faraway perch for Spektr-RG to give the observatory an unobstructed view of the entire sky, without the Earth or sun regularly drifting into the spacecraft’s field-of-view. Spektr-RG is heading for a location known as the L2 Lagrange point, a gravitationally-stable location home to several other space telescopes.

Russian and German teams working on the Spektr-RG mission heralded the successful launch Saturday, which came after a delay from June 21 to allow launch crews at Baikonur to replace a battery on the launch vehicle. Russian officials then called off a launch attempt Friday to address a separate technical concern with the Proton launcher.

“The launch was not easy, but all worked perfectly,” tweeted Dmitry Rogozin, head of Roscosmos. “The rocket, the booster and the spacecraft did not let us down.”

Spektr-RG is a Russian-led mission, but its primary instrument comes from Germany.

Astronomers at the Max Planck Institute for Extraterrestrial Physics, or MPE, in Germany head up the eROSITA telescope, an instrument consisting of seven individual mirror modules. Scientists designed eROSITA — the extended ROentgen Survey with an Imaging Telescope Array — as a follow-up to the German ROSAT mission, which launched in 1990 and conducted the first all-sky X-ray imaging survey.

A second X-ray telescope on Spektr-RG, developed by a Russian science team, will be sensitive to higher-energy X-rays than eROSITA. The Russian telescope, named ART-XC, will fly with X-ray mirror modules fabricated at NASA’s Marshall Space Flight Center in Alabama.

Earth’s atmosphere absorbs X-ray radiation, so astronomers must use satellites or high-altitude balloons for X-ray observations, which are useful in observing black holes and large-scale cosmic structures with clouds of super-heated gas.

“Time for that bottle!” tweeted Kirpal Nandra, director of the high-energy astrophysics group at MPE, where astronomers celebrated the launch with champagne.

Ground teams received the first telemetry from Spektr-RG soon after deployment from the Proton/Block DM launcher.

“We built eROSITA to transform the way we see the X-ray sky, and to unravel the mysteries of cosmology and black holes,” said Peter Predehl, principal investigator of the eROSITA telescope at MPE. “This is the moment when the efforts of the team working for more than a decade come to fruition.”

In the coming weeks, engineers will command the spacecraft, built by Russian contractor NPO Lavochkin, to open protective covers shielding the optics of the eROSITA and ART-XC instruments, allowing ground teams to begin calibrating the telescopes.

Spektr-RG’s journey toward the L2 Lagrange point will take more than three months. The mission’s all-sky X-ray survey should begin by early November.

The observatory’s mission is expected to last seven years, with four years dedicated to the all-sky survey, followed by three years of pointed observations to follow up on specific targets.

Up to 80 percent of what Spektr-RG sees will have been previously undetected, Pavlinsky said. A machine analysis of the data collected by the observatory will root out what is new, and what has been catalogued.

“We will discover 80 percent new sources each day,” Pavlinsky said in an interview with Spaceflight Now. “It’s a few hundred sources per day, so that means, within four years, we will receive a lot of new information.

Observations by the eROSITA telescope alone will help astronomers find 100,000 previously-undiscovered X-ray emitting galaxy clusters, several million active black holes in the centers of galaxies, and rare objects such as isolated neutron stars, the collapsed remnants left behind by cataclysmic stellar explosions, according to MPE.

“In its first year, eROSITA will discover more new X-ray sources than have been seen in the entire 50-plus year history of X-ray astronomy,” scientists wrote in a press release.

“These will be sources we can’t find in any catalogs,” Pavlinsky said. “That’s a big challenge for us. We don’t know exactly how it will look.”

“That goes into the direction of (studying) dark energy,” Predehl said in a pre-launch interview with Spaceflight Now.

Dark energy is the term ascribed by cosmologists for the hidden force that drives the accelerating expansion of the universe. Scientists believe dark energy represents about 70 percent of the energy density of the universe, with dark matter — matter that exerts a gravitational attraction but emits no light — making up about 25 percent of the universe, according to NASA.

Scientists say ordinary matter — stuff we can see — makes up only about 5 percent of the universe.

Gravitational bonds bring together galaxies into groups and clusters along gigantic filaments of hot gas. The filamentary web is composed of ordinary matter and dark matter.

“You may have seen a simulation of the filamentary structure of the universe, and at the crossing points of these filaments, clusters form,” Predehl said. “The growth of a cluster is dominated by dark matter, and the expansion of the universe, which can be measured by the time varying specific density, is driven by the dark energy.”

“What the clusters are doing … is they are growing because they are collecting more and more mass from the outside, from all the filaments,” Predehl said.

Scientists are not sure what constitutes dark energy, if it has been constant throughout the history of the universe, or if its influence will fade with time.

“Both dark components are contributing to the cosmology model,” Predehl said.

Space missions tuned to measure microwave signals from the ancient universe have mapped the distribution of matter in the first 380,000 years after the Big Bang some 13.8 billion years ago.

Spektr-RG could “constrain” parameters in the cosmological model that attempts to explain what is driving the universe’s expansion, according to Predehl.

“We know how the universe looked like 13 billion years ago, and we know how it looks today, but in between, there are many (unknowns), and we hope to fill some of those with eROSITA,” Predehl said.

Observing so many galactic clusters will allow astrophysicists to build up a large enough sample to gauge how they are distributed throughout the universe.

“We need statistics of clusters because clusters are the biggest gravitationally-bound entities in the universe, and counting them and measuring the mass of the clusters give you the specific density of the universe versus time,” Predehl said. “So the evolution of the universe can be studied by measuring that.”

“The main scientific goal of eROSITA is to reveal the large scale structure of the universe and how that structure grows over cosmic time,” said Andrea Merloni, the eROSITA project scientist. “This might help reveal the properties of the mysterious ‘dark energy’ pulling the universe apart.

“The clusters of galaxies that mark out that structure are filled with gas at temperatures of a million degrees or more,” Merloni said. “To see that directly, you have to use an X-ray telescope. With eROSITA covering the whole sky, we can see enough of them to reconstruct their growth history extremely accurately. That, in turn, tells us something about the amount, and perhaps the nature, of dark energy and dark matter.”

The eROSITA instrument is 20 to 25 times more sensitive than ROSAT, its predecessor, according to German scientists. Spektr-RG’s X-ray detectors are also sensitive to higher-energy X-rays than ROSAT.

Astronomers say eROSITA is complementary to other X-ray telescopes, such as NASA’s Chandra observatory, which are more sensitive but designed for pointed imaging of individual X-ray sources. Data from eROSITA could act as a roadmap for Chandra and future X-ray missions to pursue targeted observations.

“We have a virtually unlimited field-of-view, so we can detect large diffuse structures in the sky,” Predehl said.

The telescope should be able to see some galaxies as old as about 13 billion years, Pavlinsky said. Other phenomena observable with Spektr-RG include pulsars and gamma ray bursts, the most violent explosions the universe.

“We will see some of the first supermassive black holes in our universe,” he said.

“Earlier this year, we saw the first image of a supermassive black hole sitting in the centre of a galaxy,” Nandra said in a statement. “eROSITA will now tell us when and where this monster and a million others like it grew over cosmic time.

“It’s staggering to think how much our understanding of the universe has advanced, and most of that is down to new instruments using cutting-edge technologies,” Nandra said. “eROSITA is the pinnacle of that for our group and I’m incredibly proud of the team who have made this a reality.”

Researchers will compare the mission’s X-ray detections with data from optical, infrared and radio telescopes to search for counterparts to Spektr-RG’s discoveries in other light bands.

With roots in the Soviet space program, Spektr-RG was sidelined in the 1990s during a Russian economic downturn, then revived in 2005 on a smaller scale with critical contributions from international partners.

“We had an ambitious plan for the project which didn’t correspond to the power of the country of that moment,” Pavlinsky told Spaceflight Now. “We decided to restart it with a smaller version.”

The Russian and German space agencies signed an agreement in 2009 to jointly develop the Spektr-RG mission, but the project faced additional schedule delays due to technical problems and a decision to switch the observatory from a Zenit launcher to a Proton rocket.

Designers also changed Spektr-RG’s observing location from an orbit around Earth to a looping trajectory around the L2 Lagrange point.

Spektr-RG is the largest Russian astronomy satellite to launch since the Spektr-R radio observatory in 2011. Spektr-R stopped responding to commands from the ground in January after exceeding its planned five-year mission lifetime, and Russian officials declared the mission over in April.

The total cost of the Spektr-RG project is roughly equivalent to a medium-class European Space Agency science mission, according to Predehl and Pavlinsky. That puts Spektr-RG’s cost at approximately $600 million.

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Follow Stephen Clark on Twitter: @StephenClark1.