Russia’s beleaguered space science program is hoping for a rare triumph this month. Spektr-RG, an x-ray satellite to be launched on 21 June from Kazakhstan, aims to map all of the estimated 100,000 galaxy clusters that can be seen across the universe. Containing as many as 1000 galaxies and the mass of 1 million billion suns, the clusters are the largest structures bound by gravity in the universe. Surveying them should shed light on the evolution of the universe and the nature of the dark energy that is accelerating its expansion.
First proposed more than 30 years ago as part of a Soviet plan for a series of ambitious “great observatories” along the lines of NASA’s Hubble Space Telescope, Spektr-RG fell victim to cost cutting in cash-strapped, post-Soviet Russia. But the roughly €500 million satellite, which will carry German and Russian x-ray telescopes, was reborn early last decade with a new mission. Its original goal was to scan the sky for interesting x-ray sources, such as supermassive black holes gorging on infalling material; now, by mapping galaxy clusters, it would find out what makes the universe tick. The new goal meant further delays. “There have been many ups and downs,” says Peter Predehl, leader of the team at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, that built one of the satellite’s two telescopes. “Whenever we thought we were out of the woods, a new one came along.”
Spektr-RG was born in the late 1980s. Glasnost was encouraging Soviet researchers to collaborate with Western colleagues, and studies of SN 1987A, the nearest supernova in modern times, had demonstrated the power of x-rays for tracing such violent events. Rashid Sunyaev of Moscow’s Space Research Institute (IKI) proposed an x-ray observatory to orbit above Earth’s atmosphere, which blocks x-rays. The 6-ton mission soon bristled with five telescopes and involved 20 institutes in 12 countries, including the United States. But after the collapse of the Soviet Union, Roscosmos, Russia’s space agency, struggled to keep its Mir space station aloft and contribute to the growing International Space Station (ISS). “They told us the spacecraft was too large for Russia, too ambitious,” says Sunyaev, now at the Max Planck Institute for Astrophysics in Garching. “It just died.”
Resurrection began in 2003 with plans for a smaller mission that would carry a U.K.-built all-sky x-ray monitor and MPE’s x-ray survey telescope, called ROSITA—which had been destined for the ISS but was grounded by the Challenger space shuttle disaster. The new impetus was cosmology. Studies of distant supernovae in the 1990s had revealed that the expansion of the universe is accelerating. Researchers wanted to know more about dark energy, the mysterious force that was causing it, and whether it varied in space or over time. Galaxy clusters are among the best indicators, says x-ray astronomer Andrew Fabian of the Institute of Astronomy (IoA) in Cambridge, U.K. “Clusters are the most massive objects in the universe, the pinnacle of galaxy formation, and are very sensitive to cosmological models.”
They are best seen in x-rays because the gaps between galaxies are filled with gas that is heated to millions of degrees as the galaxies jostle together in a cluster. By mapping the clusters, Spektr-RG “will study the evolution of the structure of the universe,” says Esra Bulbul, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who recently joined the MPE team.
The challenge was to boost the capabilities of the existing ROSITA telescope, which could only garner up to 10,000 galaxy clusters. Discussions led to a €90 million “extended” eROSITA, paid for by MPE and the German Aerospace Center, DLR. It is an array of seven identical telescopes with five times the effective collecting area of the original instrument. Russia and Germany signed an agreement in 2007 with launch penciled in for 2012.
But mission development was not smooth. The U.K. instrument failed to win funding and was replaced with a Russian telescope that will complement eROSITA by detecting scarcer “hard” x-rays. Though harder to collect, those higher energy photons are particularly useful for seeing the supermassive black holes at galactic centers, because they pierce the clouds of gas and dust that shroud them.
Making the mirrors for eROSITA posed another challenge. Because x-rays would penetrate a flat telescope mirror, focusing them requires cylindrical mirrors that gather the photons in glancing, low-angle reflections off inner surfaces. Each of eROSITA’s seven scopes contains 54 gold-plated cylindrical mirrors, nested inside one another, that must be shaped precisely to bring the photons to a focus. Making them proved so hard that the MPE team had to fire its main contractor part way through. “It almost killed us,” Predehl says.
A decision to site the telescope at a stable, gravitationally balanced point beyond the moon, outside the shelter of Earth’s magnetic field, meant electronics had to be hardened against solar radiation. Incompatibility between the German and Russian electronics delayed the launch, as did problems with the spacecraft’s communications system and a change in launch rocket.
Now that Spektr-RG is finally ready, expectations are high. “It’s going to be revolutionary in terms of numbers,” says IoA astronomer George Lansbury, taking x-ray studies into “the big data regime.”
It may also be a rare high point for Russia’s great observatories program. Previously, only one has made it into orbit: 2011’s Spektr-R, a radio astronomy mission that fell short of expectations and could not be revived after malfunctioning earlier this year (Science, 29 July 2011, p. 512).
Astronomers may face a long wait for Spektr-RG’s successors: the ultraviolet telescope Spektr-UV and Spektr-M, a millimeter-wave radio telescope. Spektr-UV has survived moments of near-death, most recently in 2014 when Russia’s annexation of Ukraine’s Crimean peninsula caused major Ukrainian partners to withdraw. The mission is now slated for a 2025 launch, but, Sunyaev says, some collaborators, including a German team supplying a spectrograph, have dropped out. Spektr-M, which would come next, is not yet fully funded, he says. And in the meantime, rival telescopes launched by other countries may scoop up the science the Russian missions aim to do.
“Russia is doing as much as possible with the budget available,” says Spektr-RG chief Mikhail Pavlinsky of IKI. He notes that Roscosmos’s lean budget, worth $20.5 billion over 10 years, faces multiple demands. Russia is building the landing system for the European ExoMars rover, due to launch next year, and like other countries it hopes to return to the moon, with the Luna 25 lander in 2021. For Russia’s astrophysicists, Pavlinksy says, “It means slow progress.”
Science 14 Jun 2019:
Vol. 364, Issue 6445, pp. 1020-1021