IXPE (Imaging X-ray Polarimetry Explorer)
NASA has selected a science mission that will allow astronomers to explore, for the first time, the hidden details of some of the most extreme and exotic astronomical objects, such as stellar and supermassive black holes, neutron stars and pulsars. Objects such as black holes can heat surrounding gases to more than a million degrees. The high-energy X-ray radiation from this gas can be polarized – vibrating in a particular direction. The Imaging X-ray Polarimetry Explorer (IXPE) mission will fly three space telescopes with cameras capable of measuring the polarization of these cosmic X-rays, allowing scientists to answer fundamental questions about these turbulent and extreme environments where gravitational, electric and magnetic fields are at their limits. 1) 2) 3)
The mission, slated for launch in 2020/21, will cost $188 million. This figure includes the cost of the launch vehicle and post-launch operations and data analysis. Principal Investigator Martin Weisskopf of NASA/MSFC (Marshall Space Flight Center) in Huntsville, Alabama, will lead the mission. Ball Aerospace in Broomfield, Colorado, will provide the spacecraft and mission integration. ASI ( Italian Space Agency) will contribute the polarization sensitive X-ray detectors, which were developed in Italy through INFN (National Institute for Nuclear Physics) and the INAF (National Institute of Astrophysics). 4)
The goal of the IXPE mission is to expand understanding of high-energy astrophysical processes and sources, in support of NASA’s first science objective in Astrophysics: “Discover how the universe works.” Polarization uniquely probes physical anisotropies—ordered magnetic fields, aspheric matter distributions, or general relativistic coupling to black-hole spin—that are not otherwise measurable. It does this by expanding our understanding of high energy astrophysical processes, specifically the polarimetry of cosmic sources with special emphasis on objects such as neutron stars and black holes. By obtaining X-ray polarimetry and polarimetric imaging of cosmic sources, IXPE addresses two specific science objectives: 5) 6) 7)
• Determine the radiation processes and detailed properties of specific cosmic X-ray sources or categories of sources.
• Explore general relativistic and quantum effects in extreme environments.
NASA’s Astrophysics Roadmap, “Enduring Quests, Daring Visions”, also recommends such measurements. — IXPE uses X-ray polarimetry to expand dramatically X-ray observation space, which historically has been limited to imaging, spectroscopy, and timing. This advance will provide new input to our understanding as to how X-ray emission is produced in astrophysical objects, especially systems under extreme physical conditions — such as neutron stars and black holes. Polarization uniquely probes physical anisotropies — ordered magnetic fields, aspheric matter distributions, or general relativistic coupling to black-hole spin — that are not otherwise easily measurable. Hence, IXPE complements all other investigations in high-energy astrophysics by adding the important and relatively unexplored dimensions of polarization to the parameter space for exploring cosmic X-ray sources and processes, and for using extreme astrophysical environments as laboratories for fundamental physics.
The primary science objectives of IXPE are (Ref. 3):
• Enhance our understanding of the physical processes that produce X-rays from and near compact objects such as neutron stars and black holes.
• Explore the physics of the effects of gravity, energy, electric and magnetic fields at their extreme limits.
IXPE addresses key questions in High Energy Astrophysics:
• What is the spin of a black hole?
• What are the geometry and magnetic-field strength in magnetars?
• Was our Galactic Center an Active Galactic Nucleus in the recent past?
• What is the magnetic field structure in synchrotron X-ray sources?
• What are the geometries and origins of X-rays from pulsars?
Figure 1: Artist's rendition of the IXPE spacecraft [image credit: HEASARC (High Energy Astrophysics Science Archive Center)] 8)
Hundreds of galactic and extragalactic sources are amenable to meaningful X-ray polarimetry with IXPE. 9) IXPE is 100X more efficient than the polarimeter that first measured the Crab’s polarization.
Legend to Figure 2: The top axis identifies the all-sky number of extragalactic sources above the limiting flux on the bottom axis. Text near the top of each dashed line also gives the number LMXB (Low Mass X-ray Binary) and HMXB (High Mass X-ray Binary) at that limiting flux. The green line denotes the Crab Nebula, with the green dot marking the time required for the OSO-8 (Orbiting Solar Observatory-8) polarimeter to achieve a 3% detectable polarization at 99%-confidence for the Crab without pulsar contamination.
IXPE mission collaborations:
In June 2017, a new partnership between NASA and ASI (Italy's Space Agency) was formed. Robert Lightfoot, NASA's acting administrator, signed an agreement on June 20 with Roberto Battiston, president of ASI, defining the terms of cooperation for the IXPE (Imaging X-ray Polarimetry Explorer) mission during a ceremony at the Paris Air Show in Le Bourget, France. 10)
The IXPE mission will fly three telescope systems capable of measuring the polarization of X-rays emitted by cosmic sources. ASI will contribute IXPE's sophisticated "eyes" — three polarization-sensitive X-ray detectors which were developed in Italy — and the use of its equatorial ground station located at Malindi, Kenya.
NASA will supply the X-ray telescopes and use of its facilities to perform end-to-end X-ray calibration and science operations.
Ball Aerospace in Broomfield, Colorado, will provide the spacecraft and mission integration. Ball Aerospace will also operate the flight system with support from LASP (Laboratory for Atmospheric and Space Physics) at the University of Colorado at Boulder.
Other partners include Stanford University, McGill University and MIT (Massachusetts Institute of Technology).
IXPE is next in the line of NASA SMEX (Small Explorer) program missions. NASA/GSFC (Goddard Space Flight Center) in Greenbelt, Maryland, manages the Explorers Program. NASA’s Marshall Space Flight Center in Huntsville, Alabama, leads the mission for the agency’s Science Mission Directorate in Washington.
Figure 3: The international team executing the IXPE mission takes advantage of team member expertise (image credit: IXPE Team, Ref. 7)
Project Roles and Responsibilities
MSFC provides the X-ray optics [11), 12)] (mirror module assemblies (MMA)) and Science Operations Center (SOC) along with mission management and systems engineering. IAPS/INAF and INFN provide the instrument consisting of unique polarization-sensitive detectors [13) 14) 15)] within the detector units (DU), the detectors service unit (DSU) and interconnecting harness. Ball is responsible for the spacecraft, payload, mechanical elements, along with payload, spacecraft, and system I&T, followed by launch and operations.
The Mission Operations Center (MOC) is located at CU/LASP. CU/LASP operates IXPE under contract to Ball using existing facilities, similar to the way the Ball-built Kepler [ 16) 17)] and K2 missions were operated for NASA. The IXPE Observatory communicates with the ASI-contributed Malindi ground station via S-band link. The near Earth network (NEN) ground station in Singapore is the back-up ground station. The science team generates and archives IXPE data products in NASA's HEASARC (High Energy Astrophysics Science Archive Research Center).
Figure 4: International relationships, clear institutional roles with well-defined interfaces (image credit: NASA, Ball)
IXPE’s payload is a set of three identical, imaging, X-ray polarimetry systems mounted on a common optical bench and co-aligned with the pointing axis of the spacecraft.18) 19) Each system, operating independently, comprises a 4-m-focal length Mirror Module Assembly (grazing incidence x-ray optics) that focuses X-rays onto a polarization-sensitive imaging detector. The focal length is achieved using a deployable boom. Each DU (Detector Unit) contains its own electronics, which communicate with the payload computer that in turn interfaces with the spacecraft. Each DU has a multi-function filter wheel assembly for in-flight calibration checks and source flux attenuation (Ref. 6).
Designing an instrument of appropriate sensitivity to accomplish the science objectives summarized above involved a trade of MMA (Mirror Module Assembly) design, detector design, and the number of telescope systems, all versus focal length, and considered boundary conditions of mass and power that are within spacecraft and launch vehicle constraints. These trades were completed and the result is the three telescope system described here which meets science objectives and requirements with margin while placing reasonable and achievable demands on the spacecraft, launch vehicle, and the deployable optical bench. Specifically, three identical systems provide redundancy, a range of detector clocking angles to mitigate against any detector biases, shorter focal length for given mirror graze angles (i.e., given energy response) and thinner/lighter mirrors compared to a single telescope system.
Figure 5 shows the IXPE observatory with key payload elements. The payload uses a deployable X-ray shield to prevent off-axis X-rays from striking the detectors. The deployable boom is covered with a thermal sock (not shown) to maintain a more constant thermal environment. A metrology system consisting of a deployed section-mounted camera which images a metrology target (diode string) on the spacecraft top deck is used to monitor motions between the two ends of the Observatory during science observations.
MSFC provides the X-ray optics MMAs (Mirror Module Assemblies) and SOC (Science Operations Center) along with mission management and systems engineering. IAPS/INAF and INFN provide the instrument consisting of unique polarization-sensitive detectors within the DUs (Detector Units), the detector service units (DSU) and interconnecting harness. Ball is responsible for the Spacecraft, Payload mechanical elements and flight metrology along with Payload, Spacecraft and system I&T followed by launch and operations.
The MOC (Mission Operations Center) is located at CU/LASP (University of Colorado/Laboratory for Atmospheric and Space Physics). CU/LASP operates IXPE under contract to Ball using existing facilities similar to the way the Ball-built Kepler and K2 missions have been operated for NASA. The IXPE Observatory communicates with the ASI-contributed Malindi ground station via S-band link. The science team generates and archives IXPE data products in HEASARC.
MSFC provides grazing-incidence MMAs to focus X-ray photons onto the polarization-sensitive detectors. The high-heritage IXPE design achieves 230 cm2 effective area at 2.3 keV and 249 cm2 at 4.5 keV with 24 concentrically nested X-ray-mirror shells in each 300-mm-diameter optics module. The X-ray optics deflect X-ray photons onto the detector through two grazing incidence reflections in the parabolic and hyperbolic sections. The chosen packing of the mirror shells reduces stray X-radiation impinging on the detector from sources outside the field of view (FOV) – via single reflections off the hyperbolic mirror surfaces – by more than 2.5 orders of magnitude. This ensures that observations of faint extended sources are not compromised by a nearby bright source just outside the field of view. These mirrors enable imaging, key for IXPE science, and also provide a large amount of background reduction by concentrating the source flux into a small detector area.
Instrument: The ASI-provided instrument consists of three DUs and the DSU along with the interconnecting cabling. At the very heart of each DU is a polarization-sensitive imaging detector that allows broad-band X-ray polarimetry with low net background and minimal, if any, systematic effects. These GPDs (Gas Pixel Detectors) were invented and developed by the Italian members of the team and refined over the past 15 years to a high level of maturity. The GPDs utilize the anisotropy of the emission direction of photoelectrons produced by polarized photons to gauge with high sensitivity the polarization state of X-rays interacting in the GPD gaseous medium. X-rays in the energy range of 2–8 keV are the focus of IXPE investigations. The GPDs are supported by electronics within the DU to operate and collect the data from the GPDs. A FCW (Filer Calibration Wheel) is included in each DU and includes polarized and unpolarized X-ray sources to check calibration on orbit. A collimator sits on top of the DUs which, in combination with the X-ray shield around the MMAs, blocks off-axis radiation (not passing through the X-ray optics) from entering the detectors. The DSU provides the needed secondary power lines to the DUs, controls each DU, manages their FCW and high voltage operations, provides the thermal control of the GPD, collects the housekeeping, processes and formats the scientific data, and interfaces to the spacecraft avionics.
X-Ray Telescopes: The IXPE Observatory is based in three X-ray telescopes. Each telescope consists of an MMA and a DU. The MMAs and DUs are paired during calibration testing at MSFC. The defined MMA—DU pairs are then integrated and aligned at Ball as matched sets during payload integration and test. Each MMA—DU has an individual FOV of 11 arcmin. The Observatory FOV, the overlapping FOVs of the 3 telescopes, is 9 arcmin.
Metrology: An onboard metrology system is used to monitor initial on-orbit alignment positions, and residual motions of the boom for post-facto science data processing. This consists of a space-qualified Ball-built visible camera (VisCam) and several light-emitting diodes (LEDs) which form the metrology target. The camera tracks the motion of the spacecraft top deck via the relative positions of the diode array (where the DUs are mounted) with respect to the X-ray mirror platform (where the camera is mounted) in three degrees of freedom—two translational motions and rotation. This information is used in science data post-processing to help refine the specific pointing locations on the sky for the collected X-ray photons.
Payload Structures and Mechanisms: The three IXPE MMAs are mounted to a MMSS (Metallic Mirror Module Support Structure) deck. The MMSS deck contains a deployable X-ray shield that, in combination with the collimator tube atop each detector, blocks virtually all X-rays from entering the detector that have not passed through the MMA. Three bipods attached to the MMSS deck support launch loads.
Additionally, the MMSS interfaces with the coilable, deployable boom30 through a TTR (Tip/Tilt/Rotation) mechanism to provide compensation for any boom deployment errors and relaxes some aspects of on-ground alignment. If on deployment, the X-ray image is not within the required position range of the detector center point, the X-ray image can be re-aligned by using the TTR mechanism, while observing a bright source. Note that all three MMAs are moved in unison. This is possible because the forward star tracker is mounted with the optics, so that this adjustment effectively re-aligns the pointing axis with the new payload axis. Co-alignment of the individual MMAs with respect to each other and the star tracker, is performed during payload integration and test.
Figure 6: IXPE Observatory stowed in a Pegasus XL fairing envelope (image credit: IXPE Team)
Figure 7 shows the IXPE payload with key elements highlighted. The payload uses a deployable X-ray shield to prevent off-axis X-rays from striking the detectors. The deployable boom is covered with a thermal sock to minimize temperature gradients and thermal distortion between the longerons. A metrology system, with a camera mounted on the underside of the MMSS (Metallic Mirror Module Support Structure) which images a metrology target (diode string) on the spacecraft top deck, is used to monitor motions between the two ends of the Observatory during science observations. A tip/tilt/rotate mechanism allows on-orbit adjustability between the deployed X-ray optics and the spacecraft top deck-mounted DUs (Detector Units), providing system tolerance to variations in deployed geometry.
Figure 8: Mirror module design (image credit: IXPE Team)
GPD (Gas Pixel Detector)
The GPD, a contribution of ASI, has the following features/characteristics:
• Detection uses photoelectric effect
• Photoelectron emission aligned with X-ray polarization vector
• Electron multiplier with pixelated detector.
Figure 9: Measurement concept of the incoming X-ray radiation (image credit: IXPE Team)
Figure 10: Illustration of the GPD elements (image credit: IXPE Team)
The IXPE Observatory is based on the BCP-100 (Ball Commercial Platform) spacecraft architecture. The modular design allows for concurrent payload and spacecraft development with a well-defined, clean interface that reduces technical and schedule risk. The BCP-100 design supports the project goal of incorporating a low-risk spacecraft by using flight-proven components, a simple structural design, and significant design and software reuse from prior missions. The design balances a low-cost and low-risk approach with significant spacecraft capability and flexibility. IXPE is leveraging the BCP-100’s flexibility for science payload accommodation. The IXPE payload is mounted on the spacecraft top deck. The IXPE Observatory is designed to launch on a Pegasus XL or larger launch vehicle.
Some background: The BCP-100 design, based on the STP (Space Test Program) Standard Interface Vehicle, supports the project goal of incorporating a low-risk spacecraft by using flight-proven components, a simple structural design, and significant design and software reuse from prior missions. The design balances a low-cost and low-risk approach with significant spacecraft capability and flexibility. The BCP-100 capabilities support a variety of potential small payloads. The standard capability spacecraft can operate over a wide range of low earth orbit altitudes (400 – 850 km) and inclinations (0º to sun-synchronous). The spacecraft design provides the required power over the full range of sun angles. A star tracker is a key element of the attitude determination and control system. It is mounted directly to and aligned with the deployed payload to minimize alignment errors between the spacecraft and payload.
STP Satellite -2 (STPSat-2) was the first use of the STP vehicle and was launched 19 November 2010 on a Minotaur IV from the Kodiak Launch Complex, Alaska. It accommodates 2 separate SERB payloads . STPSat-2 continues extended operations well beyond its 13 month design life, and achieved 6 years on-orbit in December 2016.
IXPE is the fourth build of the BCP-100 class spacecraft. IXPE is leveraging the flexibility of the BCP-100 architecture to accommodate the IXPE science payload. It is re-configured for launch on a Pegasus XL launch vehicle with the IXPE payload mounted on the spacecraft top deck. The solar array wraps around the spacecraft body and payload.
The Observatory is designed to support IXPE measurement requirements. Key design drivers include pointing stability in the presence of various disturbances, particularly gravity gradient, and minimization of SAA passes which makes the zero degree inclination orbit the best available choice. A nominal IXPE target list is known in advance with targets distributed over the sky. The observatory has observational access to an annulus normal to the Sun line at any given time with a width ±30° from Sun-normal. This orientation allows the payload to collect all necessary science data during the mission while keeping the solar arrays oriented toward the sun and maintaining sufficient power margins. Typically, each science target is visible over an approximate 60 day window and can be observed continuously for a minimum time of 56.7 minutes each orbit. Changes in the IXPE orbit over mission lifetime are sufficiently small, eliminating the need for a propulsion system and its resulting operational complexity.
A view of the deployed IXPE Observatory is shown in Figure 12, while Figure 13 shows the Observatory stowed in a Pegasus XL launch vehicle fairing. When deployed, IXPE is 5.2 m from the bottom of the spacecraft structure to the top of the payload and is 1.1 m in diameter. The solar panels span 2.7 m when deployed. The Observatory launch mass is approximately 325 kg.
The payload is mounted on the +Z face of the spacecraft structure (top deck). This simplifies alignment and integration, and minimizes mass by providing the shortest possible load paths. The star tracker OH (Optical Heads) are mounted on opposite ends of the Observatory anti-boresighted from one another to prevent simultaneous Earth obscuration. One OH is mounted on top of the telescope support structure, co-located and boresighted with the X-ray optics. The second OH is mounted on the bottom of the spacecraft top deck looking out through the PAF ring. Two hemispherical S-band low-gain antennas are mounted on opposite sides of the spacecraft and coupled together to provide omnidirectional communications coverage.
Figure 11: A set of three MMA (Mirror Module Assemblies) of IXPE focus X-rays onto three corresponding focal plane detector units (image credit: IXPE Team)
IXPE has substantial timeline and technical margins:
• Design Reference Mission (DRM) targets studied in detail during Year 1
• Year 2 is available for follow-up observations, targets of opportunity, survey of additional sources.
Table 1: Overview of the IXPE mission parameters and margins
The IXPE spacecraft subsystems consist of command and data handling (C&DH), flight software (FSW), telecommunications, mechanical & structural, mechanisms, thermal control, attitude determination and control (ADCS), electrical power and harnessing. The IXPE C&DH subsystem consists of the integrated avionics unit and provides all C&DH functionality including FSW hosting, uplink/downlink data handling, data storage, Payload interfaces, and all electrical interfaces. The C&DH system uses a RAD750 single board computer. IXPE’s telecom subsystem is built around a simple, direct-to-ground S-band architecture using omnidirectional antennas, also capable of providing a downlink through TDRSS for critical events monitoring. The power system maintains positive power balance for all mission modes and orientations and is based on a simple, robust direct energy transfer architecture. The battery clamps the operating voltage. The ADCS provides a 3-axis stabilized platform controlled by reaction wheels and torque rods. The primary attitude sensor is a pair of star tracker optical heads augmented by coarse sun sensors and a magnetometer. GPS is used for timing as well as spacecraft orbital ephemeris.
The thermal control system employs well characterized passive and active-heater thermal control to maintain all Observatory components within allowable temperatures. The spacecraft hexagonal structure is built up from machined aluminum plates and closed out with a honeycomb aluminum top deck. Spacecraft and Payload components are mounted on the internal surfaces of the spacecraft side walls and both sides of the top deck.
Power Interface: During normal mission operations, the spacecraft generates 300 W orbit average power (OAP); the payload uses ~100 W between the different payload elements including thermal control. The payload is provided with switched power feeds. Each power feed provides unregulated 28 ±6 VDC from the spacecraft. In addition, the spacecraft provides over-current protection on each power line provided to the payload.
Thermal Interface: The spacecraft monitors and controls the temperature of selected payload element interfaces using temperature sensors and heaters mounted to the spacecraft top deck and distributed among the payload elements. The spacecraft top deck is maintained at a temperature of 20ºC ±5ºC supporting the DUs. The MMAs are maintained to a fairly tight tolerance of 20ºC ±5ºC. FSW-controlled heaters maintain the MMAs, DUs, MMSS deck and spacecraft panels at stable temperatures throughout the orbit and seasonal changes to minimize distortions along the telescopes lines of sight. The temperature measurements are provided to the ground as part of spacecraft state of health (SOH) data.
Data Interfaces: The spacecraft avionics provides the main data, command and power interfaces with the payload. All payload command, data collection, and data storage is through a payload interface card which resides within the avionics. The payload interface provides the Payload with a set of data ports for commands, and collection of high rate data, realtime data, analogs and discretes.
Both the payload high rate and realtime data are time-stamped based on a 1-PPS signal from the GPS receiver and provide accurate time knowledge of the detected X-ray photons and corresponding ancillary data. The payload interface ingests payload high rate mission data, encapsulates this data in a CCSDS (Consultative Committee for Space Data Systems) compliant CADU (Channel Access Data Unit) format and stores the formatted CADU for subsequent transmission to the ground. All high rate data is transferred via a synchronous EIA compliant RS-422 link. The total high data rate available is 2 Mbit/s. The Payload interface provides total mass memory storage of 6 GB of EDAC (Error Detection and Correction)-validated memory space.
The payload interface provides for collection of payload realtime data via an EIA-422 UART payload data port. Payload realtime data is collected and interleaved into the realtime spacecraft downlink and is also stored in the avionics for retransmit.
Figure 14: IXPE spacecraft showing major elements. The +Z star tracker and one coarse sun sensor are mounted on the deployed payload (image credit: Ball Aerospace)
Table 2: IXPE Observatory capabilities
Mission development status
• January 29, 2021: Ball Aerospace recently completed the spacecraft and payload assembly integration of NASA's Imaging X-Ray Polarimetry Explorer (IXPE) space-based astrophysics observatory at its Boulder, Colo. facility. Ball will now begin environmental testing of the integrated observatory, which includes all instruments and the spacecraft bus. 20)
- Scheduled to launch later this year, once on orbit, IXPE will measure the polarization of cosmic X-rays to improve our understanding of the fundamental physics of extreme and exotic objects in the universe, such as black holes.
- "It is truly a pleasure to work with an integrated team that includes government, industry, academia and international partners on a mission that will gather exciting and important science, supported by Ball's commitment to delivering science at any scale," said Dr. Makenzie Lystrup, vice president and general manager, Civil Space, Ball Aerospace. "Moving IXPE into environmental testing is an important step gearing up towards launch this year as it ensures the observatory will be able to withstand the effects of the launch into space."
- IXPE is a SMEX ( Small Explorer) mission which is part of NASA's Astrophysics Explorer Program. The IXPE mission is led by NASA's Marshall Space Flight Center, with support from Ball Aerospace, the Italian Space Agency (ASI), Laboratory for Atmospheric and Space Physics at University of Colorado Boulder and other partners. Dr. Martin C. Weisskopf, NASA Marshall Space Flight Center, is the principal investigator for the mission.
- Ball is responsible for providing the IXPE spacecraft, mechanical and structural elements of the payload, observatory assembly, and integration and test. The spacecraft for IXPE is based on Ball's smallest Ball Configurable Platform (BCP) model. Ball built a similar BCP for NASA's recently completed Green Propellant Infusion Mission (GPIM), which was safely deorbited on October 13, 2020, burning up during re-entry into the atmosphere, leaving no trace of space debris. Ball also built two additional BCP small satellites that are currently performing on orbit: STPSat-2, which launched in November 2010, and STPSat-3, which launched in November 2013. The two STP satellites were built for the U.S. Air Force Space Test Program's Standard Interface Vehicle (STP-SIV) project.
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Figure 15: Ball Aerospace completed integration of NASA's IXPE observatory, which now moves into testing (Ball Aerospace)
• August 2020: The payload includes three polarization-sensitive, X-ray detector arrays paired with three X-ray mirror module assemblies (MMA). A deployable boom provides the correct separation (focal length) between the detector units and MMAs. Currently, the boom has been delivered, all four detectors units (DU) are complete, the detectors service unit (DSU) is complete, instrument system testing has been completed (DSU with 3 DUs), three of four MMAs is built and all spacecraft components except the solar array have been delivered along with the spacecraft and payload structures. Payload and spacecraft integration and test (I&T) started in March 2020. 21)
• July 10, 2019: The M-CDR (Mission -Critical Design Review) of NASA's IXPE (Imaging X-Ray Polarimetry Explorer) spaceborne astrophysics observatory was recently completed at Ball Aerospace's Boulder, Colorado, facility. The IXPE mission is led by NASA's Marshall Space Flight Center, with support from Ball Aerospace, the Italian Space Agency (ASI), LASP (Laboratory for Atmospheric and Space Physics) at University of Colorado Boulder and other partners. 22)
• August 8, 2018: A PDR (Preliminary Design Review) of NASA's IXPE (Imaging X-Ray Polarimetry Explorer) spaceborne astrophysics observatory was completed in late June at Ball Aerospace's Boulder, CO facility led by NASA's Marshall Space Flight Center, with support from Ball Aerospace, the Italian Space Agency (ASI) and other industry partners. 23)
- IXPE is a Small Explorer, or SMEX mission, which is part of NASA's Astrophysics Explorer Program. Dr. Martin C. Weisskopf, NASA Marshall Space Flight Center, is the principal investigator for the mission. Once launched in 2021, IXPE will measure the polarization of cosmic X-rays to improve our understanding of the fundamental physics of extreme and exotic objects in the universe.
- "The IXPE mission is an excellent example of a highly-integrated government and industry working together for a common goal," said Jim Oschmann, vice president and general manager of Civil Space, Ball Aerospace. "IXPE will explore, and for the first time discover, hidden details of some of the most unique objects in our universe, such as neutron stars and stellar and supermassive black holes."
• June 20, 2017: NASA signed a cooperation agreement with ASI (Italian Space Agency) to explore some of the most turbulent and extreme environments in our universe — from the hottest, messiest star factories to violent jets screaming away from monster black holes. 24)
• The phase B activities of the IXPE mission started in the spring of 2017.
• NASA’s Astrophysics Explorers Program selected the Imaging X-ray Polarimetry Explorer (IXPE) in January 2017 (Ref. 1).
• The IXPE Project completed its Phase A activities in July 2016 with the submission of the CSR (Concept Study Report) to the NASA Explorers Program Office. NASA considered three SMEX mission concepts for flight and selected the IXPE Project as the winner in January 2017. The Project entered Phase B on February 1, 2017 and completed SRR (Systems Requirements Review) in September 2017.
Launch: The IXPE Observatory is planned for launch in the summer of 2021 on a Falcon 9 rocket of SpaceX from Launch Complex 39A of the Kennedy Space Center in Florida. 25)
Orbit: Circular equatorial orbit, altitude ~600 km ±15 km, inclination =0.2º.
The Payload uses a single science operational mode capturing the x-ray data from the targets. The mission design follows a simple observing paradigm: pointed viewing of known x-ray sources (with known locations in the sky) over multiple orbits (not necessarily consecutive orbits) until the observation is complete. This means that the attitude determination and control subsystem design enables the IXPE Observatory to remain pointed at the same science target for up to many days at a time.
The IXPE Ground System consists of the Mission Operations Center (MOC) at CU/LASP, the IXPE Science Operations Center (SOC) at MSFC, the Malindi ground station (with Singapore (KSAT) as a backup), and the Internet and other connections between the various elements. In addition, IXPE uses the Space Network (TDRS) for launch and early operations support for critical event monitoring and orbit determination using differential one-way Doppler (DOWD) tracking. Malindi is also used during early operations support, and during boom deployment. The Flight Dynamics Facility (FDF) will provide improved inter-range vectors (IIRV) to the MOC and the ground stations until TLE data has converged with the FDF provided solutions (Ref. 7).
The MOC is responsible for operating the spacecraft (in collaboration with Ball) and the science payload (in collaboration with MSFC, I2T and Ball). The IXPE mission operations team performs mission planning, commanding, health and status monitoring, and sustaining engineering for both the Spacecraft and the payload from the MOC. MOC systems engineering integrates off-the-shelf, heritage software for front-end processing, health and status monitoring, and command uplinks with mission planning software. All software will run on commercial off the shelf (COTS) hardware and operating systems. The MOC will be located within LASP’s existing multi-mission satellite operations facility and will use hardware, software, procedures, and personnel that are already in place and used to operate AIM, QuikSCAT, K2 and SORCE. The data flow from the Observatory to the science data team is shown in Figure 16.
IXPE utilizes S-band uplink and downlink for all communications. IXPE maximally (but rarely) downlinks ~600 MB/day at 2.0 Mbit/s, using up to 9 contacts per day, each with 8-minute contact duration including 60 s of acquire time. If any mission data are found to be incomplete after downlink, there is sufficient contact margin for retransmission of files during the following contact. Uploads occur at 2 kbit/s and need only take place once every three days. The 10 m antennas at Malindi (primary) and Singapore (backup) will be used as the ground segment for IXPE space-to-ground communications. Emergency support will utilize the 32 kbit/s safe-mode data rate.
Concept of Operations Summary
The IXPE mission consists of 4 distinct phases of operation, Table 3: Pre-launch, Launch and Commissioning, Science Operations, and End of Life decommissioning. 26) 27) The IXPE concept of operations is summarized in Figure 17.
The pre-launch phase includes design, development, integration, test, shipment, storage (if needed), and all launch site activities prior to umbilical disconnect and Falcon 9 launch vehicle lift-off.
Launch and commissioning operations are conducted during the first 30 days on-orbit. Launch begins with umbilical separation and continues through Observatory separation from the Falcon 9 second stage. At that point, the Observatory is free flying in its stowed configuration in the injected orbit. The IXPE Observatory will launch into a ≥555-km equatorial (0 degree) orbit no earlier than April 2021.
Commissioning occurs during the subsequent 30 days. Upon separation from the launch vehicle (LV), the Spacecraft autonomously detumbles, autonomously deploys the solar arrays, and performs solar acquisition. The Observatory communicates via the TDRS constellation post launch for critical event coverage. After first acquisition of the S-band telecommunications link with the Malindi ground station, spacecraft commissioning activities begin. These activities include activation and checkout of the remaining spacecraft subsystems. Payload commissioning includes boom deployment, 28) instrument activation, TTR activation (if needed), and instrument on-orbit calibration activities. Instrument calibration on-orbit includes pointing at a number of bright X-ray sources. 29) 30) None of these activities are considered time critical.
Science operations (Phase E) are planned to last at least 2 years. Science operations phase includes specific target observations, data downlink, periodic astrometric calibrations, and target-to-target slews. X-ray targets are known in advance and observed with a single science mode. IXPE mission targets are chosen based on the season and sun position to ensure that the array is pointed within ±25 degrees of sun normal.
Phase E science operations commence with uplink of the first weekly science observation sequence. Malindi coverage transitions to 3-9 passes per day of 8 minutes each (7 minutes downlink time). Based on the target list, many of the targets can be observed using one continuous observation period with 2 ground contacts per day, while other targets are data intensive and require splitting the observations into 2 to 4 observing sequences, filling the recorder and downlinking up to 9 times per day. Science and calibration data are stored in the C&DH and downlinked daily during the scheduled passes.
Downlinks are initiated and monitored by ground automation. Downlinks are through the Malindi station at a rate of 2.0 Mbit/s (Singapore is backup). The ground stations sync and decode the channels and send them to the MOC in real time and/or as files after each pass. Real time and stored state of health telemetry is monitored by the MOC for health and safety checking and trend analysis of the Spacecraft and Payload. If communications passes are missed, the data are stored in the C&DH memory and downlinked on subsequent passes.
The MOC transmits data to the Science Operations Center (SOC) for processing. The SOC at MSFC, with support from the ASI Space Science Data Center (SSDC), is responsible for IXPE science operations. The IXPE science team performs data processing and archiving of the data for community use through the HEASARC.
Decommissioning (Phase F) occurs at the end of the mission once science operations are complete. The Observatory will be shut down (passivated) in accordance with NASA end of mission guidelines. Atmospheric drag will cause the Observatory orbit to decay. The Observatory will deorbit approximately 4.4 years after launch from its nominal 555 km orbit, which occurs well after the primary mission and well before the 25-year requirement.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).