Russia’s Proton-M rocket – after a lengthy standdown – finally launched the Spektr-RG observatory on Saturday at 12:30 UTC. Proton was expected to lift off from the Baikonur Cosmodrome last month before a delay was required to replace some faulty batteries on the spacecraft. With the aid of a Blok DM-03 upper stage, this mission deployed its payload two hours after launch.
The Spektr-RG satellite is an international collaboration led by the Russian Federal Space Agency, Roskosmos in partnership with the German Aerospace Centre, DLR, and universities and research institutes in both Russia and Germany. It is the second of three satellites in Roskosmos’ Spektr series, which aims to survey the cosmos across a wide range of electromagnetic frequencies. Spektr-RG covers the x-ray region of the spectrum.
Spektr-RG, whose name means Spectrum – Roentgen Gamma, follows the Spektr-R satellite that was launched in July 2011. Specializing in radio astronomy, the Spektr-R satellite exceeded its five-year design life but ceased operations in January 2019 after ground controllers unexpectedly lost contact with the satellite.
Attempts to restore communications continued until the end of May, at which point Spektr-R was declared lost. A third satellite, Spektr-UF, is slated to launch in 2025 and will be equipped for visible-light and ultraviolet observations.
The Spektr-RG mission will build on the research conducted by previous x-ray astronomy satellites, including the Granat observatory operated by the Soviet Union and later Russia from 1989 to 1998 and the German-led ROSAT mission that operated between 1990 and 1999.
Spektr-RG is designed to conduct a series of eight whole-sky x-ray surveys, each lasting six months, in the first four years of operation before beginning more detailed observations of specific targets of interest at high energies. The mission is expected to last at least six and three-quarter years from launch, including six and a half years of scientific observations.
Using its two x-ray telescopes, Spektr-RG’s primary mission is to find and map all massive galaxy clusters in the observable universe at x-ray wavelengths. By observing the universe at X-ray wavelengths, Spektr-RG will be able to see details that may be obscured at lower energies.
It will also look for active galactic nuclei – supermassive black holes at the heart of other galaxies – as well as regions of star formation, accreting white dwarf stars and stars with active coronas, and study x-ray emissions from the interstellar and intergalactic mediums.
Scientists hope that the satellite will help to give an increased understanding of the roles of dark matter and dark energy in establishing the structure of the universe, and of the lifecycles of supermassive black holes.
Spektr-RG was originally formulated as a Russian-led multinational research mission, slated to launch as early as 1995 with participation from the United States, Israel, Turkey and the European Space Agency. The program was canceled in 2002, but several of its instruments had already been built, including the JET-X telescope which now hangs in London’s Science Museum. Spektr-RG was reinstated in 2005 with the involvement of Germany, including a complete redesign of the satellite.
The redesigned Spektr-RG had been manifested to fly aboard a Zenit-3F rocket, however it was later moved to the Proton rocket that will carry it to orbit – the Zenit that had originally been built for the Spektr-RG mission was launched at the end of 2017 with Angola’s Angosat-1 satellite aboard instead.
The Spektr-RG space observatory will be sent to the Baikonur Cosmodrome today, April 24, to continue preparations for its launch on a Proton, which is scheduled for June 21, 2019. #SpektrRG #Proton #launchservices pic.twitter.com/hA7goX9Vri
— ILS (@ILSLaunch) April 24, 2019
NPO Lavochkin constructed the Spektr-RG spacecraft around its Navigator platform, which has previously been used for the Elektro-L series of weather satellites and the Spektr-R satellite. The bus incorporates fuel and propulsion systems and provides three-axis stabilization and control. It can generate 1,805 watts of power through twin deployable solar arrays, while the satellite communicates with the ground through an X-band antenna. Spektr-RG has a total mass at launch of 2,712.5 kilograms (5980.0 lb) including propellant.
Following launch, it will take Spektr-RG about three months to reach its operational orbit and complete initial testing and calibration. The satellite will be operated in a halo orbit around the L2 Lagrangian point between the Earth and the Sun, located 1.5 million kilometers (0.93 million miles, 0.81 million nautical miles) from the Earth in the opposite direction to the Sun.
At this point in space the interaction of the gravitational pull of the Sun and the Earth keeps a spacecraft at the same position relative to the latter. Spektr-RG will take about six months to complete an orbit around this point in space, and one year to complete a revolution around the Sun, along with the Earth.
Because it puts the Earth between the satellite and the Sun, the L2 point is ideal for spacecraft carrying sensitive instruments that must be kept at cold temperatures. For Spektr-RG this simplifies the cooling requirements for its X-ray detectors.
Spektr-RG carries two astronomy payloads: Astronomical Roentgen Telescope – X-ray Concentrator (ART-XC) and Extended Roentgen Survey with an Imaging Telescope Array (eROSITA).
The ART-XC telescope contains an optical bench with seven mirror modules, focusing incident radiation onto seven detectors at the other end of its carbon fiber structure. It was developed by the Russian Academy of Sciences’ Space Research Institute in cooperation with the Russian Federal Nuclear Centre.
ART-XC’s detectors, which use a Cadmium telluride semiconductor, measure 30 by 30 millimeters (1.2 by 1.2 inches) and are 1 mm (39 thou) thick. The mirror assemblies, each of which contains 28 individual mirror shells, were constructed by NASA’s Marshall Space Flight Center. The mirrors are comprised of a nickel/cobalt alloy with an iridium coating and arranged in a nested structure with increasing diameters to maximize the number of photons focused onto the detectors. The overall instrument has a mass of 350 kilograms (771 lb) and draws 300 watts of power from Spektr-RG.
A radiator is incorporated into the ART-XC instrument to keep the telescope’s detectors close to their design operating temperature of -20 degrees Celsius (-4 degrees Fahrenheit, 253 Kelvin), while a sunshield protects the optics. With a focal length of 2.7 meters (8.9 feet) ART-XC can observe high-energy x-rays between 5 and 30 kiloelectronvolts (wavelengths between 0.25 and 0.041 nanometres) with a 0.3-square-degree field of view and an angular resolution of 45 arcseconds.
As well as contributing to Spektr-RG’s sky survey, ART-XC will record transient x-ray events and attempt to identify their sources. It will also characterize the mass and physical properties of accreting white dwarf stars – binary systems where a white dwarf strips material from its companion star.
The eROSITA telescope is a German contribution to the mission, with the Max Planck Institute for Extraterrestrial Physics leading its investigation. Whereas ART-XC is designed to study high-energy or “hard” X-rays, eROSITA will observe lower-energy “soft” x-rays between 0.3 and 11 keV (4.1 to 0.12 nm wavelength).
Like the Russian instrument, eROSITA consists of seven detectors with independent mirror assemblies, separated by x-ray baffles. Each module contains 54 individual gold-coated nickel mirrors, while the square detectors are made of silicon with sides of 28.8 mm (1.13 in) and are passively cooled to -95 degrees Celsius (-139 degrees Fahrenheit, 178 Kelvin) by a radiator and a system of heat pipes.
eROSITA has a focal length of 1.6 meters (5.2 feet) and can capture a field of view of 0.81 square degrees at an angular resolution of 18 arcseconds. It has a mass of 815 kilograms (1797 lb) and draws 405 watts of power from the parent spacecraft.
eROSITA will specialize in detecting accreting black holes and binary star systems and studying X-ray emissions from other phenomena such as pre-main-sequence stars and supernova remnants. It will also map the intergalactic medium and filaments between clusters of galaxies.
The different energy ranges of the two instruments complement each other, allowing Spektr-RG to cover almost the entire X-ray spectrum. The overlap at energies between 5 and 11 keV allow the telescopes to be calibrated against each other and to verify each other’s observations.
Spektr-RG’s ride into orbit was aboard a Russian Proton-M rocket with a Blok DM-03 upper stage.
Built by the Khrunichev State Research and Production Space Centre, Proton has for many years been the most powerful fully-operational rocket in Russia’s fleet, and has been used to deploy geostationary satellites, interplanetary missions and heavier payloads destined for lower orbits. Blok DM-03 is an evolution of the Blok-D series of upper stages and will carry Spektr-RG to its deployment orbit after the three-stage core Proton vehicle has completed its role in the mission.
Proton’s first three stages burn storable hypergolic propellant: unsymmetrical dimethylhydrazine which is oxidized by dinitrogen tetroxide. The Blok DM-03 burns RG-1 kerosene oxidized by liquid oxygen.
Proton evolved from Vladimir Chelomei’s Universalnaya Raketa – Universal Rocket – concept, a series of rocket and missile designs sharing common technology that were proposed to the Soviet Union during the 1960s. The UR-500 was designed as a two-stage missile to deliver the heaviest nuclear weapons over intercontinental distances, however instead it found its niche launching satellites.
Four UR-500s were launched in 1965 and 1966 with high-energy physics research payloads named Proton. After these initial launches the rocket was adapted for orbital missions, including the addition of a third stage, and the resulting UR-500K vehicle quickly became known as the Proton-K.
Early applications for the Proton-K were in support of the Soviet Union’s space exploration programs, launching test flights of a circumlunar version of the Soyuz spacecraft – the 7K-L1 – and probes bound for the Moon, Mars and later Venus. The Salyut space stations were launched by Proton-K vehicles, as were all modules of the later Mir space station excluding a docking compartment that was delivered by the US Space Shuttle, and the Zarya and Zvezda modules of the International Space Station.
Proton-K remained in service until 2012, the end of its service overlapping with the introduction of its replacement, Proton-M. Proton-M first flew in 2001 as a modernized version of the Proton-K, incorporating digital flight control systems and more powerful first-stage engines.
Further upgrades since its introduction have increased the rocket’s payload capacity to enable it to compete for commercial launch contracts from international customers. From the late 1990s through most of the 2000s Proton became a workhorse for commercial satellite operators, however increased competition and a spate of launch failures dented Proton’s share of the market, resulting in a reduced launch rate over the last few years.
When flying to higher orbits, Proton requires a fourth stage to transfer the payload from an initial low Earth orbit or suborbital trajectory to its final destination.
Blok-D was introduced alongside Proton-K for this purpose, followed by a series of upgraded versions including the Blok-DM, DM-2, D-1, D-2 and DM-2M. The Blok DM-03 that will be used for this launch is an enlarged version of the Blok DM-2 with stretched propellant tanks which first flew in 2010. It was manufactured by RKK Energia.
Spektr-RG’s launch marks the fourth flight of Blok DM-03, whose maiden flight in December 2010 ended in failure after engineers loaded too much propellant into the new upper stage, leaving it too heavy for Proton to reach its planned parking orbit.
The stage’s second launch, in July 2013, also failed after the Proton went out of control seconds after liftoff, the result of an incorrectly-installed sensor in the booster’s first stage. At its third attempt, Blok DM-03 successfully inserted the Ekspress AM-8 communications satellite into orbit in September 2015.
Proton’s flight began with ignition of the six RD-275 first-stage engines and liftoff of the rocket from its launch complex at Site 81/24 of the Baikonur Cosmodrome.
One of four Proton launch pads that were built at Baikonur – of which two remain active, Pad 24 is used primarily for Russian Government launches while Pad 39 at nearby Site 200 has been used for more commercial launches in recent years, although neither pad is exclusively tied to either role.
Pad 24 is expected to be decommissioned by the end of next year, at which point Pad 39 will take over all Proton launch duties until the rocket’s planned retirement at the end of the next decade.
The first stage of Proton consists of six radial fuel tanks clustered around a center oxidizer tank – a design chosen to facilitate rail transport of the rocket, keeping each component within the maximum width that could be carried to the launch site by train. The fuel tanks, which each incorporates an engine at its base, can be removed and reinstalled once Proton arrives in Baikonur. Proton’s first stage will burn for 123.8 seconds during the launch.
LAUNCH! Roscosmos Proton-M launches with Spektr-RG from Site 81/24 at the Baikonur Cosmodrome.
— Chris Bergin – NSF (@NASASpaceflight) July 13, 2019
As the first stage approached burnout, the rocket began its stage separation sequence. The second stage ignites before separation occurs, with exhaust gases escaping through the lattice interstage structure that connects the two stages. Once the four second stage engines are burning, the spent first stage is jettisoned. This “fire in the hole” staging technique keeps second-stage propellants settled eliminating the need for additional ullage motors to assist in its startup and is common on Russian rockets.
Proton’s second stage is powered by three RD-0210 engines and a single RD-0211. These are essentially identical, the only difference being the presence of additional hardware on the RD-0211 that is used to keep the propellant tanks pressurized. The second stage burned for three minutes and 27.6 seconds before giving way to the third stage and separating.
An RD-0212 propulsion module – consisting of an RD-0213 main engine and an RD-0214 vernier engine with four steerable chambers for attitude control – powers Proton’s third stage. Just under thirteen seconds into its four-minute, 12.5-second burn Proton’s payload fairing separated from around Spektr-RG at the nose of the vehicle, exposing the satellite to space for the first time. The fairing – which protects the satellite during its climb through Earth’s atmosphere – is no longer needed once the rocket reaches space and is discarded to reduce weight.
After completing its burn, Proton’s third stage was also be jettisoned. Six minutes later Blok DM-03’s RD-58M engine ignited for the first of its two planned burns. The first burn, lasting around two minutes, established the upper stage and payload in an initial parking orbit, where they can coast until in the proper position for the next burn. This coast phase lasted about 78 minute, with the second burn taking nine minutes.
Fourteen minutes after the second burn concludes, Spektr-RG separated from Blok DM-03. Spacecraft separation marked the end of Proton’s role in the satellite’s mission, and the beginning of a series of maneuvers that will take the satellite to its planned station. The spacecraft is expected to make first contact with its ground stations about eight minutes after separating from the carrier rocket, before deploying its solar arrays, establishing a lock on the Sun and powering on the eROSITA instrument within two hours.
The launch – before the delay – was to be the second in less than a month for Proton, following a five-month hiatus between the rocket’s two previous missions. It is the first time since September 2015 that Proton has flown with the Blok DM-03 upper stage, with the last thirteen Protons all using the storable-propellant Briz-M upper stage. Well-suited to the complex long-duration multiple-burn ascents required to deploy geostationary communications satellites from Baikonur, the Briz-M is used for the majority of Proton’s launches.
A Proton-M/Briz-M vehicle will next be in action when it lofts a Blagovest communications satellite for the Russian military.