A Russian-built satellite hosting an array of X-ray telescopes is awaiting launch Friday from the Baikonur Cosmodrome in Kazakhstan on a mission to measure the mass and distance of colossal clusters of galaxies throughout the universe.
The X-ray observatory, designated Spektr-RG, is scheduled to blast into space on top of a Russian Proton rocket at 1217:14 GMT (8:17:14 a.m. EDT; 5:17:14 p.m. Baikonur time) Friday to kick off Russia’s most prominent space science mission in seven years.
Once in space, Spektr-RG will observe X-ray emissions from throughout the universe, searching for distant clusters of galaxies to help scientists unravel the mysteries of dark energy, the unseen force causing the universe to expand at faster speeds.
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, according to Mikhail Pavlinsky, the mission’s lead scientist from IKI, the Space Research Institute of the Russian Academy of Sciences.
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.
“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.”
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.
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.
“If you have an unbiased look at the whole sky, you have a potential for detections,” said Peter Predehl, head of the eROSITA science team at MPE. “We don’t know what we will see in the end. On the other hand, we designed the instrument for a specific reason, and this is in order to detect 100,000 clusters of galaxies, and that goes into the direction of (studying) dark energy.”
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, also versus time, 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.
With its observations of the mass, luminosity and distance of faraway galaxies, 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 eROSITA instrument is 20 to 25 times more sensitive than ROSAT, according to German scientists. Spektr-RG’s X-ray detectors are also sensitive to higher-energy X-rays than ROSAT.
In an interview with Spaceflight Now before Friday’s launch, Predehl said he is eager to start analyzing the mission’s scientific discoveries.
“To be honest, I’m nervous,” he said earlier this week, before traveling to Baikonur for the launch. “We have been working for many years. We are (days) before the launch. This is very exciting. In our business, risk is not zero.”
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.
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.
The Spektr-RG team will also collaborate with U.S. scientists at Marshall to process the mission’s science data, according to Pavlinsky.
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.
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.
The 5,980-pound (2,712-kilogram) Spektr-RG spacecraft is heading for a distant observation post in orbit around the L2 Lagrange point, a gravitational balance site about a million miles (1.5 million kilometers) from the night side of Earth. The L2 location is a popular destination for space-based telescopes — Europe’s Gaia mission is stationed there, as were the Herschel and Planck observatories, and the James Webb Space Telescope will observe the universe from an orbit around L2 after its 2021 launch.
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.
The Proton rocket and Block DM upper stage assigned to launch Spektr-RG rolled out to the launch pad at the Baikonur Cosmodrome on June 14 for final pre-flight checks.
The launch of Spektr-RG will mark the first flight of a Block DM upper stage with a Proton rocket since September 2015. Recent Proton missions have typically launched with the newer-design Breeze M upper stage to place their payloads into high-altitude orbits.
The three-stage Proton booster features a six-engine first stage producing 2.5 million pounds of thrust, and a four-engine second stage that generates 540,000 pounds of thrust. The Proton’s third stage is powered by a single main engine delivering 131,000 pounds of thrust.
The reignitable Block DM upper stage will separate from the Proton’s third stage less than 10 minutes after liftoff, followed by two burns of the Block DM main engine before deployment of the 5,980-pound Spektr-RG spacecraft two hours into the mission.
All of the engines on the Proton booster and the Block DM upper stage consume a toxic mixture of hydrazine and nitrogen tetroxide propellants.
Within a few hours of Friday’s launch, Spektr-RG should radio its status of ground controllers in Russia and unfurl its power-generating solar array wings. In July, 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 mid-October, Pavlinsky said.
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.
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