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25 Years Ago: STS-93, Launch of the Chandra X-Ray Observatory


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On July 23, 1999, space shuttle Columbia took to the skies on its 26th trip into space, to deliver its heaviest payload ever – the Chandra X-ray Observatory. The STS-93 crew included Commander Eileen M. Collins, the first woman to command a space shuttle mission, Pilot Jeffrey S. Ashby, and Mission Specialists Catherine “Cady” G. Coleman, Steven A. Hawley, and Michel A. Tognini of the French Space Agency (CNES). On the mission’s first day, they deployed Chandra, the most powerful X-ray telescope. With a planned five-year lifetime, Chandra continues its observations after a quarter century. For the next four days, the astronauts worked on twenty secondary middeck payloads and conducted Earth observations. The successful five-day mission ended with a night landing.

The STS-93 crew patch Official photo of the STS-93 crew of Eileen M. Collins, left, Steven A. Hawley, Jeffrey S. Ashby, Michel A. Tognini of France, and Catherine “Cady” G. Coleman The patch for the Chandra X-ray Observatory
Left: The STS-93 crew patch. Middle: Official photo of the STS-93 crew of Eileen M. Collins, left, Steven A. Hawley, Jeffrey S. Ashby, Michel A. Tognini of France, and Catherine “Cady” G. Coleman. Right: The patch for the Chandra X-ray Observatory.

Tognini, selected by CNES in 1985 and a member of NASA’s class of 1995, received the first assignment to STS-93 in November 1997. He previously flew aboard Mir as a cosmonaut researcher, spending 14 days aboard the station in 1992. On March 5, 1998, First Lady Hilary R. Clinton announced Collins’ assignment as the first woman space shuttle commander in a ceremony at the White House together with President William J. “Bill” Clinton. NASA announced the rest of the crew the same day. For Collins, selected in the class of 1990, STS-93 represented her third space mission, having previously served as pilot on STS-63 and STS-84. Ashby, a member of the class of 1994, made his first flight aboard STS-93, while Coleman, selected in 1992, made her second flight, having flown before on STS-73. Hawley made his fifth flight, having previously served as a mission specialist on STS-41D, STS-61C, STS-31, and STS-82. He has the distinction of making the last flight by any member of his class of 1978, more than 21 years after his selection.

Schematic of the Chandra X-ray Observatory showing its major components Diagram of the trajectory Chandra took to achieve its final operational 64-hour orbit around the Earth – IUS refers to the two burns of the Inertial Upper Stage and IPS to the five burns of Chandra’s Integral Propulsion System
Left: Schematic of the Chandra X-ray Observatory showing its major components. Right: Diagram of the trajectory Chandra took to achieve its final operational 64-hour orbit around the Earth – IUS refers to the two burns of the Inertial Upper Stage and IPS to the five burns of Chandra’s Integral Propulsion System.

Because the Earth’s atmosphere absorbs X-ray radiation emitted by cosmic sources, scientists first came up with the idea of a space-based X-ray telescope in the 1970s. NASA launched its first X-ray telescope called Einstein in 1978, but scientists needed a more powerful instrument, and they proposed the Advanced X-ray Astrophysics Facility (AXAF). After a major redesign of the telescope in 1992, in 1998 NASA renamed AXAF the Chandra X-ray Observatory after Indian American Nobel Prize-winning theoretical physicist Subrahmanyan Chandrasekhar who made significant contributions to our knowledge about stars, stellar evolution, and black holes. Chandra, the third of NASA’s four Great Observatories, can detect X-ray sources 100 times fainter than any previous X-ray telescope. At 50,162 pounds including the Inertial Upper Stage (IUS) it used to achieve its operational orbit, Chandra remains the heaviest payload ever launched by the space shuttle, and at 57 feet long, it took up nearly the entire length of the payload bay. It has far exceeded its expected five-year lifetime, still returning valuable science after 25 years.

The STS-93 crew during the Terminal Countdown Demonstration Test The Chandra X-ray Observatory loaded into Columbia’s payload bay Liftoff of Columbia on the STS-93 mission carrying the Chandra X-ray Observatory and the first woman shuttle commander
Left: The STS-93 crew during the Terminal Countdown Demonstration Test. Middle: The Chandra X-ray Observatory loaded into Columbia’s payload bay. Right: Liftoff of Columbia on the STS-93 mission carrying the Chandra X-ray Observatory and the first woman shuttle commander.

Columbia returned to KSC following its previous flight, the STS-90 Neurolab mission, in May 1998. Workers in KSC’s Orbiter Processing Facility (OPF) serviced the orbiter and removed the previous payload. With all four orbiters at KSC at the same time, workers temporarily stowed Columbia in the Vehicle Assembly Building (VAB), returning it to the OPF for final preflight processing on April 15, 1999. Rollover of Columbia from the OPF to the VAB took place on June 2, where workers mated it with an external tank and two solid rocket boosters. Following integrated testing, the stack rolled out to Launch Pad 39B on June 7. The crew participated in the Terminal Countdown Demonstration Test on June 24. Workers placed Chandra in Columbia’s payload bay three days later.

On July 23, 1994, Columbia thundered into the night sky from KSC’s Launch Pad 39B to begin the STS-93 mission. Two previous launch attempts on July 20 and 22 resulted in scrubs due to a faulty sensor and bad weather, respectively. As Columbia rose into the sky, for the first time in shuttle history a woman sat in the commander’s seat. Far below, problems arose that could have led to a catastrophic abort scenario. During the engine ignition sequence, a gold pin in Columbia’s right engine came loose, ejected with great force by the rapid flow of hot gases, and struck the engine’s nozzle, punching holes in three of its hydrogen cooling tubes. Although small, the hydrogen leak caused the engine’s controller to increase the flow of oxidizer, making the engine run hotter than normal. Meanwhile, a short-circuit knocked out the center engine’s digital control unit (DCU) and the right engine’s backup DCU. Both engines continued powered flight without a redundant DCU, with a failure in either causing a catastrophic abort. Although this did not occur, the higher than expected oxidizer usage led to main engine cutoff occurring 1.5 seconds early, leaving Columbia in a lower than planned orbit. The shuttle’s Orbiter Maneuvering System engines made up for the deficit. The harrowing events of the powered flight prompted Ascent Flight Director John P. Shannon to comment, “Yikes! We don’t need any more of these.”

Eileen M. Collins, the first woman shuttle commander, shortly after reaching orbit First time space flyer STS-93 Pilot Jeffrey S. Ashby, shortly after reaching space
Left: Eileen M. Collins, the first woman shuttle commander, shortly after reaching orbit. Right: First time space flyer STS-93 Pilot Jeffrey S. Ashby, shortly after reaching space.

After reaching orbit, the crew opened the payload bay doors and deployed the shuttle’s radiators, and removed their bulky launch and entry suits, stowing them for the remainder of the flight. The astronauts prepared for the mission’s primary objective, deployment of Chandra, and also began activating some of the middeck experiments.

The Chandra X-ray Observatory in Columbia’s payload bay shortly after reaching orbit Chandra raised to the deployment angle Chandra departs Columbia
Left: The Chandra X-ray Observatory in Columbia’s payload bay shortly after reaching orbit. Middle: Chandra raised to the deployment angle. Right: Chandra departs Columbia.

Coleman had prime responsibility for deploying Chandra. After initial checkout of the telescope by ground teams, the astronauts tilted Chandra and the IUS to an angle of 29 degrees. After additional checks, they tilted it up to the release angle of 58 degrees. A little over seven hours after launch, Coleman deployed the Chandra/IUS stack. Collins and Ashby flew Columbia to a safe distance, and about an hour after deployment, the IUS fired its first stage engine for about two minutes, followed by a two-minute burn of the second stage. This placed Chandra in a temporary elliptical Earth orbit with a high point of 37,200 miles. After separation of the IUS, Chandra used its own propulsion system over the next 10 days to raise its altitude to 6,214 miles by 86,992 miles, its operational orbit, circling the Earth every 64 hours. For the next four days of the mission, the astronauts operated about 20 middeck experiments, including a technology demonstration of a treadmill vibration isolation system planned for the International Space Station.

Michel A. Tognini works with the Commercial Generic Bioprocessing Apparatus Jeffrey S. Ashby checks the status of the Space Tissue Lab experiment Catherine G. Coleman harvests plants from the Plant Growth in Microgravity experiment
Left: Michel A. Tognini works with the Commercial Generic Bioprocessing Apparatus. Middle: Jeffrey S. Ashby checks the status of the Space Tissue Lab experiment. Right: Catherine G. Coleman harvests plants from the Plant Growth in Microgravity experiment.

Catherine G. Coleman, left, and Michel A. Tognini pose near the Lightweight Flexible Solar Array Hinge technology demonstration experiment Stephen A. Hawley checks the status of the Micro Electromechanical Systems experiment Tognini places samples of the Biological Research in Canisters experiment into a gaseous nitrogen freezer
Left: Catherine G. Coleman, left, and Michel A. Tognini pose near the Lightweight Flexible Solar Array Hinge technology demonstration experiment. Middle: Stephen A. Hawley checks the status of the Micro Electromechanical Systems experiment. Right: Tognini places samples of the Biological Research in Canisters experiment into a gaseous nitrogen freezer.

Eileen M. Collins runs on the Treadmill Vibration Isolation System Stephen A. Hawley, left, and Michel A. Tognini operate the Southwest Ultraviolet Imaging System instrument Inflight photograph of the STS-93 crew
Left: Eileen M. Collins runs on the Treadmill Vibration Isolation System. Middle: Stephen A. Hawley, left, and Michel A. Tognini operate the Southwest Ultraviolet Imaging System instrument. Right: Inflight photograph of the STS-93 crew.

Laguna Verde in Chile Sunrise over the Mozambique Channel Darling River and lakes in Australia The Society Islands of Bora Bora, Tahaa, and Raiatea
A selection of the STS-93 crew Earth observation photographs. Left: Laguna Verde in Chile. Middle left: Sunrise over the Mozambique Channel. Middle right: Darling River and lakes in Australia. Right: The Society Islands of Bora Bora, Tahaa, and Raiatea.

Eileen M. Collins prepares to bring Columbia home Columbia streaks through the skies over NASA’s Johnson Space Center in Houston during reentry Collins guides Columbia to a smooth touchdown on the Shuttle Landing Facility at NASA’s Kennedy Space Center in Florida
Left: Eileen M. Collins prepares to bring Columbia home. Middle: Columbia streaks through the skies over NASA’s Johnson Space Center in Houston during reentry. Right: Collins guides Columbia to a smooth touchdown on the Shuttle Landing Facility at NASA’s Kennedy Space Center in Florida.

Three holes visible in the hydrogen cooling tubes of Columbia’s right main engine, seen after landing The STS-93 crew pose in front of Columbia on the Shuttle Landing Facility at NASA’s Kennedy Space Center in Florida Eileen M. Collins addresses the crowd at Houston’s Ellington Field during the welcome home ceremony for the STS-93 crew, as Vice President Albert “Al” A. Gore and other dignitaries listen
Left: Three holes visible in the hydrogen cooling tubes of Columbia’s right main engine, seen after landing. Middle: The STS-93 crew pose in front of Columbia on the Shuttle Landing Facility at NASA’s Kennedy Space Center in Florida. Right: Eileen M. Collins addresses the crowd at Houston’s Ellington Field during the welcome home ceremony for the STS-93 crew, as Vice President Albert “Al” A. Gore and other dignitaries listen.

At the end of five days, the astronauts finished the last of the experiments and prepared for the return to Earth. On July 28, they closed Columbia’s payload bay doors, donned their launch and entry suits, and strapped themselves into their seats for entry and landing. Collins piloted Columbia to a smooth landing on KSC’s Shuttle Landing Facility, completing the 12th night landing of the shuttle program. The crew had flown 80 orbits around the Earth in 4 days, 22 hours, and 50 minutes. Columbia wouldn’t fly again until March 2002, the STS-109 Hubble Servicing Mission-3B. A postflight investigation into the cause of the short on ascent that led to two DCUs failing revealed a wire with frayed insulation, likely caused by workers inadvertently stepping on it, that rubbed against a burred screw head that had likely been there since Columbia’s manufacture. The incident resulted in significant changes to ground processes during shuttle inspections and repairs. With regard to the pin ejected during engine ignition that damaged the hydrogen cooling tubes, investigators found that those pins never passed any acceptance testing. Since STS-93 marked the last flight of that generation of main engines, newer engines incorporated a different configuration, requiring no design or other changes.

Enjoy the crew narrate a video about the STS-93 mission. Read Hawley’s recollections of the STS-93 mission in his oral history with the JSC History Office.

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      As extraordinary as these events were, their impacts on the stratosphere were spectacularly eclipsed by the impact of the January 2022 eruption of the Hunga Tonga-Hunga Ha’apai  (Hunga) volcano in the Pacific Ocean. The Hunga eruption lofted about 150 Tg of water vapor into the stratosphere – with initial injections reaching into the mesosphere. The eruption almost instantaneously increased total stratospheric water vapor by about 10%. MLS was the only sensor able to track the plume in the first weeks following the eruption. The Hunga humidity enhancement resulted in an envelope-redefining, low-temperature anomaly in the stratosphere, in turn inducing changes in stratospheric circulation. Repartitioning of southern midlatitude Cl also occurred, though to a lesser degree than following the ANY fires and in a manner broadly consistent with known chemical mechanisms. The Hunga water vapor enhancement has not substantially declined in the 2.5 years since the eruption, and studies indicate that it will likely endure for several more years.
      Impacts of the Hunga humidity on polar O3 loss have also been investigated. The timing and location of the eruption were such that the plume reached high southern latitudes only after the 2022 Antarctic winter vortex had developed. Since the strong winds at the vortex edge present a transport barrier, polar stratospheric cloud (PSC) formation and O3 hole evolution were largely unaffected. When the vortex broke down at the end of the 2022 Antarctic winter, moist air flooded the southern polar region, increasing humidity in the region. Cold, moist conditions led to unusually early and vertically extensive PSC formation and Cl activation, but chemical processing ran to completion by mid-July, as typically occurs in southern winter. The cumulative chemical O3 losses ended up being unremarkable throughout the lower stratosphere. The Hunga plume was also largely excluded from the 2022–2023 Arctic vortex. The 2023–2024 Arctic O3 loss season was characterized by conditions that were dynamically disturbed and not persistently cold, and springtime O3 was near or above average. The extraordinary stratospheric hydration from Hunga has so far had minimal impact on chemical processing and O3 loss in the polar vortices in either hemisphere – see Figure 2.
      Figure 2. The evolution of MLS water vapor anomalies (deviations from the baseline 2005–2021 climatology) from January 2019 through December 2023 as a function of equivalent latitude at 700 K potential temperature in the middle stratosphere at ~27 km altitude (17 mi). Black contours mark the approximate edge of the polar vortex. The green triangle marks the time of the main Hunga eruption at latitude 20.54°S on January 15, 2022. Figure credit: Updated and adapted from a 2023 paper in Geophysical Research Letters With the end of Aura and MLS, the future for stratospheric limb sounding observations is unclear. While stratospheric O3 and aerosol will continue to be measured on a daily, near-global basis by the Ozone Mapping and Profiler Suite (OMPS) Limb Profiler (OMPS-LP) instruments on the Suomi National Polar-orbiting Partnership (Suomi NPP) and Joint Polar Satellite System (JPSS-2, -3, and -4) satellites, there are no confirmed plans for daily, near-global observations of either long-lived trace gases or halogenated species – both of which are needed to diagnose observed changes in O3. The only other sensor making such measurements, the Canadian Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE–FTS), is itself older than MLS and, as a solar occultation instrument, measures only 30 profiles-per-day, taking around a month to cover all latitudes. Similarly, no other sensor is set to provide daily, near-global measurements of stratospheric water vapor until the launch of the Canadian High-altitude Aerosols, Water vapour and Clouds (HAWC) mission in the early 2030s. Some potential new mission concepts are under consideration by both NASA and ESA, but they are subject to competition. Even if both instruments are ultimately selected, gaps in the records of many species measured by MLS are inevitable. The MLS PI is leading an effort to develop new technologies that would allow an instrument that could restart MLS measurements to be built in a far smaller mass/power footprint (e.g., 60 kg, 90 W vs. 500 kg, 500 W for Aura MLS), and technologies exist for yet-smaller MLS-like instruments that could assume the legacy of the highly impactful MLS record at low cost in future decades.
      Ozone Monitoring Instrument
      The Ozone Monitoring Instrument (OMI) continues the Total Ozone Mapping Spectrometer (TOMS) record for total O3 and other atmospheric parameters related to O3 chemistry and climate. It employs hyperspectral imaging in a push-broom mode to observe solar backscatter radiation in the visible and ultraviolet.
      OMI is a Dutch–Finnish contribution to the Aura mission, and its remarkable stability and revolutionary two-dimensional (2D) detector (spatial in one dimension and spectral in the other) has produced a two-decade record of science- and trend-quality datasets of atmospheric column observations. OMI continues the long-term record of total column O3 measurements begun in 1979, and its observations of nitrogen dioxide (NO2), sulfur dioxide (SO2), formaldehyde (CH2O), and absorbing aerosols provided exceptional spatial resolution for study of anthropogenic and natural trends and variations of these pollutants around the world. Its radiometric and spectral stability has made it a valuable contributor for solar spectral irradiance measurements to complement dedicated solar instruments on other satellites. The many achievements made possible with OMI are documented in a review article.
      OMI’s multidecade data records have revolutionized the ability to monitor air quality changes around the world, even at the sub-urban level. In particular, OMI NO2 data have been transformative. Recently, these data were used to track changes in air pollution associated with efforts to control the spread of SARS-CoV-2. OMI’s long, stable data record allowed for changes in pollution levels in 2020 – at the height of global lockdowns – to be put into historical perspective, especially within the envelope of typical year-to-year variations associated with meteorological variability. Many research studies assessed the impact of the pandemic lockdowns on air pollution, supporting novel uses of OMI data for socioeconomic-related research. For example, OMI NO2 data were shown to serve as an environmental indicator to evaluate the effectiveness of lockdown measures and as a significant predictor for the deceleration of COVID-19 spread. OMI NO2 data were also used as a proxy for the economic impact of the pandemic as NO2 is emitted during fossil fuel combustion, which is another proxy for economic activity since most global economies are driven by fossil fuels – see Animation.
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      Animation. OMI data show changes in average levels of NO2 from March 20 to May 20 for each year from 2015 to 2023 over the northeast U.S. Levels in 2020 were ~30%  lower relative to previous years because of efforts to slow the spread of COVID-19. OMI data indicate similar reductions in NO2 in cities across the globe in early 2020 and a gradual recovery in pollutant emissions in late 2020 into 2023. Additional images for other world cities and regions are available through the NASA Science Visualization Studio website and the Air Quality Observations from Space website. Animation credit: NASA Science Visualization Studio OMI’s datasets are being continued by successor 2D detector array instruments, such as the previously mentioned Copernicus Sentinel-5P TROPOMI mission, the Republic of Korea’s Geostationary Environment Monitoring Spectrometer (GEMS), and NASA’s Tropospheric Emissions: Monitoring of Pollution (TEMPO). All of these missions have enhanced spatial resolution relative to OMI, but have benefited from the innovative retrieval algorithms pioneered by OMI’s retrieval teams.
      Tropospheric Emission Spectrometer
      The Tropospheric Emission Spectrometer (TES) provided vertically-resolved distributions of a number of tropospheric constituents, e.g., O3, methane (CH4), and various volatile organic compounds. The instrument was decommissioned in 2018 due to signs of aging associated with a failing Interferometer Control System motor encoder bearing. Nevertheless, TES measurements led to a number of key results regarding changes in atmospheric composition that were published over the past 10 years.
      Measurements from TES, OMI, and MLS showed that transport of O3 and its precursors from East Asia offset about 43% of the decline expected in O3 over the western U.S., based on emission reductions observed there over the period 2005–2010. TES megacity measurements revealed that the frequency of high-O3 days is particularly pronounced in South Asian megacities, which typically lack ground-based pollution monitoring networks. TES water vapor and semi-heavy water measurements indicated that water transpired from Amazonian vegetation becomes a significant moisture source for the atmosphere, during the transition from dry to wet season. The increasing water vapor provides the fuel needed to start the next rainy season. Measurements of CH4 from TES and carbon monoxide (CO) from Measurements of Pollution in the Troposphere (MOPITT) on Terra showed that CH4 emissions from fires declined at twice the rate expected from changes in burned area from 2004–2014. This finding helped to balance the CH4 budget for this period, because it offset some of the large increases in fossil fuel and wetland emissions. Through direct measurement of the O3 greenhouse gas effect, TES instantaneous radiative kernels revealed the impact of hydrological controls on the O3 radiative forcing and were used to show substantial radiative bias in Intergovernmental Panel on Climate Change (IPCC) chemistry–climate models. The TES team pioneered the retrieval of a number of species, such as peroxyacetyl nitrate, carbonyl sulfide, and ethylene.
      The spirit of TES lives on through the NASA TRopospheric Ozone and its Precursors from Earth System Sounding (TROPESS) project, which generates data products of O3 and other atmospheric constituents by processing data from multiple satellites through a common retrieval algorithm and ground data system. TROPESS builds upon the success of TES and is considered a bridge to allow the development of a continuous record of O3 and other trace gas species as a follow-on to TES.
      Future of Aura
      In April 2023, Aura’s mission operations team performed the last series of maneuvers to maintain its position in the A-Train constellation of satellites. Since then, Aura has begun drifting. As of July 2024, Aura has descended ~5 km (3 mi) in altitude from ~700 km (435 mi) and its equator crossing time has increased by ~9 min from ~1:44 PM local time. This amount of drift is small, and the Aura MLS and OMI retrieval teams are ensuring the science- and trend-quality of the datasets.
      As Aura continues to drift, the amount of sunlight reaching its solar panels will slowly decrease and will no longer be able to generate sufficient power to operate the spacecraft and instruments by mid-2026. At this point, the amount of local time drift will still be relatively small – less than one hour – so the retrieval teams will be able to ensure quality for most data products until this time.
      In the remaining years, Aura’s aging but remarkably stable instruments will continue to add to the unprecedented two decades of science- and trend-quality data of numerous key tropospheric and stratospheric constituents. Aura data will be key for monitoring the evolution of the Hunga volcanic plume and understanding its continued impact on the chemistry and dynamics of the stratosphere. Observations from MLS and OMI will also be used to evaluate data from new and upcoming instruments (e.g., ESA’s Atmospheric Limb Tracker for Investigation of Upcoming Stratosphere (Altius); NASA’s TEMPO, Plankton, Aerosol, Cloud, ocean Ecosystem (PACE), and Total and Spectral Solar Irradiance Sensor-2 (TSIS-2) missions, or at least used to help minimize the gaps between data collections.
      Aura’s Scientific Legacy
      The Aura mission has been nothing short of transformative for atmospheric research and applied sciences. The multidecade, stable datasets have furthered process-based understanding of the chemistry and dynamics of atmospheric trace gases, especially those critical for understanding the causes of trends and variations in Earth’s protective ozone layer.  
      The two decades that Aura has flown have been marked by profound atmospheric changes and numerous serendipitous events, both natural and man-made. The data from Aura’s instruments have given scientists and applied scientists an unparalleled view – including at the sub-urban scale – of air pollution around the world, clearly showing the influence of rapid industrialization, environmental regulations designed to improve air quality, seasonal agricultural burning, catastrophic wildfires, and even a global pandemic, on the air we breathe. The Aura observational record spans the period that includes the decline of O3-destroying substances, and Aura data illustrate the beginnings of the recovery of the Antarctic O3 hole, a result of unparalleled international cooperation to reduce these substances.
      Aura’s datasets have given a generation of scientists the most comprehensive global view to date of critical gases in Earth’s atmosphere and the chemical and dynamic processes that shape their concentrations. Many, but not all, of these datasets are being/will be continued by successor instruments that have benefited from the novel technologies incorporated into the design of Aura’s instruments as well as the innovative retrieval algorithms pioneered by Aura’s retrieval teams.
      Acknowledgements
      The author wishes to acknowledge the decades of hard work of the many hundreds of people who have contributed to the success of the international Aura mission. There are too many to acknowledge here and I’m sure that many names from the early days are lost to time. I would like to offer special thanks to those scientists who, back in the 1980s, first dreamed of the mission that would become Aura.
      Bryan Duncan
      NASA’s Goddard Space Flight Center (GSFC)
      bryan.n.duncan@nasa.gov
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