OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security‒Regolith Explorer)
OSIRIS-REx is an 'Asteroid Sample Return Mission' NASA's New Frontiers Program. The objective is to rendezvous and thoroughly characterize near-Earth asteroid Bennu (previously known as 1019551999 RQ36). The rendezvous with Bennu is planned for October 2018 . After several months of proximity operations to characterize the asteroid, OSIRIS-REx flies a TAG (Touch-And-Go) trajectory to the asteroid's surface to collect at least 60 gram of pristine regolith sample for Earth return. — This asteroid is both the most accessible carbonaceous asteroid and the most potentially hazardous asteroid known. Knowledge of its nature is fundamental to understanding planet formation and the origin of life. Only by understanding the organic chemistry and geochemistry of an asteroid sample can this knowledge be acquired.
OSIRIS-REx brings together all of the pieces essential for a successful asteroid sample return mission, — The University of Arizona's (Tucson, AZ) leadership in planetary science and experience operating the Mars Phoenix Lander; Lockheed Martin's (Denver, CO) unique experience in sample-return mission development and operations; NASA/GSFC's (Greenbelt, MD) expertise in project management, systems engineering, safety and mission assurance, and visible-near infrared spectroscopy; KinetX's (Tempe, AZ) experience with spacecraft navigation; and Arizona State University's (Tempe, AZ) knowledge of thermal emission spectrometers. The Canadian Space Agency (CSA) is providing a laser altimeter, building on the strong relationship established during the Phoenix Mars mission. In addition, MIT and Harvard College Observatory are providing an imaging X-ray spectrometer as a Student Collaboration Experiment. The science team includes members from the United States, Canada, France, Germany, Great Britain, and Italy. 1) 2) 3)
Bennu is a time capsule from 4.5 billion years ago. A pristine, carbonaceous asteroid containing the original material from the solar nebula, from which our Solar System formed. This is the first U.S. mission to return samples from an asteroid to Earth, addressing multiple NASA Solar System Exploration objectives to understand not just the origin of the Solar System, but the origin of water and organic material on Earth.
Bennu is a near-Earth object with a mean diameter in of ~492 m and a mass of ~7.8 x 1010 kg. It completes an orbit of the Sun every 436.604 days (1.2 years). This orbit takes it close to the Earth every six years. Although the orbit is reasonably well known, scientists continue to refine it.
Figure 1: Simulated image of asteroid Bennu (image credit: NASA)
The OSIRIS-REx Mission seeks answers to questions that are central to the human experience: Where did we come from? What is our destiny? OSIRIS-REx is going to Bennu, a carbon-rich asteroid that records the earliest history of our Solar System, and bringing a piece of it back to Earth. Bennu may contain the molecular precursors to the origin of life and the Earth's oceans. Bennu is also one of the most potentially hazardous asteroids. It has a relatively high probability of impacting the Earth late in the 22nd century. OSIRIS-REx will determine Bennu's physical and chemical properties. This will be critical for future scientists to know when developing an impact mitigation mission.
• Return and analyze a sample of pristine carbonaceous asteroid regolith in an amount sufficient to study the nature, history, and distribution of its constituent minerals and organic material.
• Map the global properties, chemistry, and mineralogy of a primitive carbonaceous asteroid to characterize its geologic and dynamic history and provide context for the returned samples.
• Document the texture, morphology, geochemistry, and spectral properties of the regolith at the sampling site in situ at scales down to the submillimeter.
• Measure the orbit deviation caused by non-gravitational forces; determine the Yarkovsky effect on a potentially hazardous asteroid and constrain the asteroid properties that contribute to this effect.
• Characterize the integrated global properties of a primitive carbonaceous asteroid to allow for direct comparison with ground-based telescopic data of the entire asteroid population.
OSIRIS-REx will launch from Earth and travel for about two years to the asteroid Bennu. Upon arrival, OSIRIS-REx will map the total surface, creating a detailed shape model of the asteroid. OSIRIS-REx will also measure the magnitude of the Yarkovsky effect, a factor in the orbits of asteroids that may pose a threat to Earth. The craft will then approach — not land upon — Bennu, and extend a robotic arm to obtain a sample of pristine surface material (at least 60 gram).
Returning to Earth in a Sample Return Capsule, a proven model originally used during the NASA Stardust mission, the material will then be studied by scientists at the NASA/JSC ( Johnson Space Center) and from around the world for clues about the composition of the very early Solar System, the source of what may have made life possible on Earth. The data collected at the asteroid will aid our understanding of asteroids that pose an impact hazard to Earth, and the OSIRIS-REx spacecraft will be a pathfinder for future spacecraft that perform reconnaissance on any newly-discovered threatening objects.
OSIRIS-REx is scheduled for launch in 2016. As planned, the spacecraft will reach its asteroid target in 2018 and return a sample to Earth in 2023.
NASA/GSFC will provide overall mission management, systems engineering and safety and mission assurance for OSIRIS-REx. The PI (Principal Investigator) of the mission is Dante Lauretta of the University of Arizona. Lockheed Martin Space Systems in Denver will build the spacecraft. OSIRIS-REx is the third mission in NASA's New Frontiers Program. NASA/MSFC (Marshall Space Flight Center) in Huntsville, AL, manages New Frontiers for the agency's Science Mission Directorate in Washington.
Figure 2: Schedule of the OSIRIS-REx project (image credit: NASA)
The spacecraft is a derivative of the MRO (Mars Reconnaissance Orbiter) and MAVEN (Mars Atmosphere and Volatile EvolutioN) missions, leveraging the key heritage design components of these two missions. Healthy resource margins across the vehicle, fully redundant spacecraft subsystems with extensive cross strapping, and high heritage hardware enable flexibility throughout the spacecraft development and during flight operations.
The OSIRIS-REx flight system is made up of the spacecraft bus (which includes the structure, and all of the various subsystem components to control and operate the vehicle), the TAGSAM (Touch-And-Go Sample Acquisition Mechanism), the SRC (Sample Return Capsule), and the five science instruments.
EPS (Electrical Power Subsystem): The EPS includes two rigid solar arrays, gimballed about the spacecraft Y and Z axes. In addition, two batteries are utilized for off-sun maneuvering, including the critical TAG mission phase.
PPS (Propulsion Subsystem): The high heritage propulsion subsystem is a single fault tolerant monopropellant system of Aerojet Rocketdyne, a subsidiary of Aerojet Rocketdyne Holdings, Inc. The propulsion subsystem includes main engines, trajectory correction maneuver thrusters, attitude control system thrusters, and low thrust reaction engine assemblies. The propulsion devices on the spacecraft include four MR-107S 222 N thrusters, six MR-106L 22 N thrusters, 16 MR-111G 4.4 N thrusters and two MR-401 0.44 N thrusters. — Aerojet Rocketdyne propulsion is involved in every phase of the mission, including the Earth-departure phase to fine tune the Earth escape velocity; the cruise phase to adjust trajectory and ensure a perfectly accurate trajectory for the Earth swing-by and arrival at Bennu.
GN&C (Guidance, Navigation and Control): The GN&C subsystem includes four RWAs (Reaction Wheel Assemblies) for performing spacecraft slewing and low jitter pointing during science operations. These reaction wheels also store system momentum between desaturation events. The GN&C subsystem is responsible for commanding all of the thrusters on the spacecraft including executing trajectory correction maneuvers and RWA desaturations. The GN&C subsystem utilizes an IMU (Inertial Measurement Unit) and flight-proven star trackers to determine and propagate on-board attitude knowledge. Sun sensors additionally support spacecraft autonomous safing operations. Two GN&C sensors provide measurements used for relative navigation: a GN&C lidar is used for ranging to the surface to support TAG operations, a TAGCAMS (TAG Camera System) supports ground based navigation throughout proximity operations and autonomous on-board optical based navigation during the TAG phase.
Figure 3: Artist's rendition of NASA's OSIRIS-REx spacecraft preparing to take a sample from asteroid Bennu (image credit: NASA)
RF communications: This subsystem utilizes X-band communications, using a MAVEN build-to-print high gain antenna and MRO heritage traveling wave tube amplifier for science high data rate downlink. A medium gain antenna is utilized during the TAG mission phase. Also two low gain antennas are available for TAG but also used for nominal (and safe-mode) engineering data downlink and uplink commanding.
Figure 4: OSIRIS-REx flight system – optimized for an Asteroid Sample Return Mission (image credit: OSIRIS-REx collaboration)
SRC (Sample Return Capsule):
To safely return the collected sample to Earth, OSIRIS-Rex capitalizes on the success of NASA's Stardust mission. The proven Stardust SRC technology and capsule, mission operations, and mission design are all reused on OSIRIS-Rex for Bennu sample return.
Figure 5: Illustration of the deployed OSIRIS-REx spacecraft components (image credit: NASA)
Project development status:
• May 22, 2016: The OSIRIS-REx satellite was flown to NASA's Kennedy Space Center from prime contractor Lockheed Martin's facility near Denver, Colorado via Buckley Air Force Base. It arrived safely inside its shipping container on May 20 aboard an Air Force C-17 at the Shuttle Landing Facility. 8) 9)
• March 8, 2016: NASA's OSIRIS-REx spacecraft is in thermal vacuum testing, designed to simulate the harsh environment of space and see how the spacecraft and its instruments operate under ‘flight-like' conditions. 10)
• January 8, 2016: The student-built REXIS (Regolith X-Ray Imaging Spectrometer) instrument of MIT/SSL has been integrated onto the OSIRIS-Rex spacecraft. 11)
• Dec. 17, 2015: The Canadian-built OLA (OSIRIS-REx Laser Altimeter) of CSA was delivered to Lockheed Martin Space Systems facilities near Denver, Colorado. OLA was built by MDA (MacDonald, Dettwiler and Associates Ltd.) and its partner, Optech. In the coming months, OLA will be integrated onto the spacecraft and undergo spacecraft-level testing in preparation for launch in September 2016. 12)
• October 21, 2015: Lockheed Martin has completed the assembly of NASA's OSIRIS-REx spacecraft. The spacecraft is now undergoing environmental testing at the company's Space Systems facilities near Denver, CO. 13) 14)
- Over the next five months, the spacecraft will be subjected to a range of rigorous tests that simulate the vacuum, vibration and extreme temperatures it will experience throughout the life of its mission. Specifically, OSIRIS-REx will undergo tests to simulate the harsh environment of space, including thermal vacuum, launch acoustics, separation and deployment shock, vibration, and electromagnetic interference and compatibility.
- OSIRIS-REx is scheduled to ship from Lockheed Martin's facility to NASA's Kennedy Space Center next May, where it will undergo final preparations for launch.
Figure 6: The high gain antenna and solar arrays were installed on the OSIRIS-REx spacecraft prior to it moving to environmental testing (image credit: Lockheed Martin Corporation)
• August 29, 2015: The assembly of the OSIRIS-REx spacecraft continues, with many elements integrated onto the spacecraft ahead of schedule. Last month both OTES and OVIRS were delivered to Lockheed Martin and installed on the science deck. OTES had the honor of being the first science instrument to be placed on the spacecraft. Both OTES and OVIRS came in ahead of schedule, despite some adversity in their development. 15) 16)
• July 8, 2015: The OVIRS (OSIRIS-REx Visible and Infrared Spectrometer) instrument arrived at Lockheed Martin Space Systems in Denver for installation onto the OSIRIS-REx spacecraft. 17)
• June 22, 2015: With the launch only 15 months away, the team of the OSIRIS-REx asteroid sample return mission, led by the University of Arizona, is preparing to deliver its instruments for integration with the spacecraft over the next several months. 18)
• March 31, 2015: The spacecraft structure has been integrated with the propellant tank and propulsion system and is ready to begin system integration at Lockheed Martin. The OSIRIS-REx project officially received authorization to transition into the next phase of the mission, Phase D, after completing a series of independent reviews verifying that the program's technical, schedule and cost elements are all on course. The key decision meeting was held at NASA Headquarters in Washington on March 30 and chaired by NASA's Science Mission Directorate. The next major milestone is the Mission Operations Review, scheduled for completion in June. 19)
Figure 7: In a clean room facility of Lockheed Martin near Denver, technicians began assembling the OSIRIS-REx spacecraft (image credit: Lockheed Martin Corporation, Universe Today) 20)
• Feb. 27, 2015: OSIRIS-REx mission completes system integration review. The team met at the Lockheed Martin facility in Littleton, Colorado during the week of February 23, 2015 to review the plan for integrating all of the systems on the spacecraft, such as the scientific instrumentation, electrical and communication systems, and navigation systems. Successful completion of this System Integration Review means that the project can proceed with assembling and testing the spacecraft in preparations for launch in September 2016. Assembly and testing operations for the spacecraft are on track to begin next month at the Lockheed Martin facilities in Littleton. 21)
• In early April 2014, the OSIRIS-REx program completed the comprehensive CDR (Critical Design Review) of the mission and has been given approval to begin building the spacecraft, flight instruments and ground system. The review was performed by an independent review board, comprised of experts from NASA and several external organizations, that validated the detailed design of the spacecraft, instruments and ground system. 22) 23) 24)
Launch: The OSIRIS-REx spacecraft was launched on September 8, 2016 (23:05 UTC) on an Atlas V 411 vehicle of ULA (United Launch Alliance) from the Space Launch Complex 41, Cape Canaveral, FL. 25)
The OSIRIS-REx launch window opens on September 3, 2016. The launch period will last for 39 days, with a 30 minute window available each day. OSIRIS-REx will leave Cape Canaveral, Florida on an Atlas V rocket in the 411 configuration. Throughout the 39 days the characteristic energy (C3) is fixed at 29.3km2/s2, for a launch vehicle capability of 1955 kg. 26) 27)
Following an Earth flyby and gravity assist in Sept 2017, OSIRIS-REx cruises for 11 months and starts the optical search for Bennu in Aug 2018, marking the beginning of the Approach phase. Rendezvous occurs in Oct 2018, followed by a month of slow approach to allow the flight system to search for moons around Bennu and to refine its shape and spin state models.
Table 1: OSIRIS-REx mission phases
Figure 8: Earth range, Sun range, and SPE angle from launch to Earth return (image credit: NASA, Lockheed)
• December 31, 2017: The purpose of NASA's OSIRIS-REx spacecraft is to map and return samples from asteroid Bennu, a carbon-rich hunk of rock that might contain organic materials or molecular precursors to life. It is also an asteroid that could someday make a close pass or even a collision with Earth, though not for several centuries. 28)
- At the philosophical level, OSIRIS-REx is a mission to figure out where we came from, as asteroids are remnants from the formation of our solar system. But while the spacecraft might tell us some things about where we have been and where we are headed, it also can remind us of where we are right now.
- On October 2, 2017, the MapCam instrument on OSIRIS-REx captured the data for a composite image of the Earth and Moon (Figure 9). The spacecraft was approximately 5 million km from Earth at the time, about 13 times the distance between the Earth and Moon. (Click here to see the geometry of the shot.) Three images (different color wavelengths) were combined and color-corrected to make the composite, and the Moon was "stretched" (brightened) to make it more easily visible.
Figure 9: Composite image of the Earth and Moon system, captured on 2 Oct. 2017 by the MapCam instrument on OSIRIS-REx (image credit: OSIRIS-REx team and the University of Arizona, story by Mike Carlowicz)
• December 13, 2017: OSIRIS-REx is continuing outbound cruise operations, en route to arrival in August of 2018 at asteroid Bennu. The spacecraft is currently 47.6 million km from Earth and is executing a program designed to study and reduce the presence of water on the spacecraft. 29)
During routine in-flight testing of the spacecraft's thermal properties earlier this year, the mission's navigation team noticed an unexpected minor acceleration of the spacecraft when the SRC (Sample Return Capsule) was exposed to sunlight. The mission team determined that this small thrust was caused by the outgassing of water that had been adsorbed by the SRC's heat shield and backshell before launch.
- Retention of water in blanketing and other materials - and the subsequent outgassing of this water - occurs with all spacecraft. For OSIRIS-REx, it was determined that when the SRC was exposed to the Sun at a distance of less than 1 Astronomical Unit (1 AU = approximately 150 million km), this trapped water escaped and imparted a small thrust.
- While this small thrust would not be a problem for other missions, the gravity at the target asteroid Bennu is low enough that even this small amount of thrust could make orbital operations more difficult for OSIRIS-REx.
- To better understand the outgassing effects on the spacecraft's trajectory - and to bake out much of the remaining water before the spacecraft arrives at Bennu - the OSIRIS-REx mission team designed an outgassing program for execution starting earlier this fall.
- The choice of timing took into account both the spacecraft's proximity to the Sun (less than 1 AU) and the fact that there were no science activities planned during this period. The outgassing program is being run concurrently with outbound cruise operations and does not affect the timing of the spacecraft's arrival at Bennu.
- Starting in mid-October, the spacecraft has been placed into various attitudes to expose different parts of the SRC to direct sunlight and initiate outgassing. Priority is given to the portions of the SRC that will face the Sun during asteroid proximity operations. The mission team has been able to detect and measure the rate of outgassing at each attitude and has determined that water is being removed as expected.
- The goal is to reduce the outgassing to the point where the spacecraft can fly the planned baseline trajectories around Bennu without modifications, and preliminary indications show that the program is progressing toward this goal. The program is scheduled to run through early January 2018.
• September 28, 2017: NASA's OSIRIS-REx asteroid mission captured a lovely ‘Blue Marble' image of our Home Planet during the Sept. 22 successful gravity assist swing-by sending the probe hurtling towards asteroid Bennu for a rendezvous next August on a round trip journey to snatch pristine soil samples. 30)
Figure 10: A color composite image of Earth taken on Sept. 22, 2017 by the MapCam camera on NASA's OSIRIS-REx spacecraft just hours after the spacecraft completed its EGA (Earth Gravity Assist) maneuver at a range of approximately 170,000 km (image credit: NASA/Goddard/University of Arizona)
Legend to Figure 10: The image is centered on the Pacific Ocean and shows several familiar landmasses, including Australia in the lower left, and Baja California and the southwestern United States in the upper right.
- The spacecraft conducted a post flyby science campaign by collecting images and science observations of Earth and the Moon that began four hours after closest approach in order to test and calibrate its onboard suite of five science instruments and help prepare them for OSIRIS-REx's arrival at Bennu in late 2018.
Figure 11: NASA's OSIRIS-REx spacecraft OTES spectrometer captured these infrared spectral curves during Earth Gravity Assist on Sept. 22 2017, hours after the spacecraft's closest approach (image credit: NASA/Goddard/University of Arizona/Arizona State University)
Figure 12: NASA's OSIRIS-REx spacecraft OVIRS spectrometer captured this visible and infrared spectral curve, which shows the amount of sunlight reflected from the Earth, after the spacecraft's Earth Gravity Assist on Sept. 22, 2017 (image credit: NASA/Goddard/University of Arizona)
• September 22, 2017: NASA's asteroid sample return spacecraft successfully used Earth's gravity on Friday to slingshot itself on a path toward the asteroid Bennu, for a rendezvous next August. 31)
- At 12:52 p.m. EDT on September 22, the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security – Regolith Explorer) spacecraft came within 17,237 km of Antarctica, just south of Cape Horn, Chile, before following a route north over the Pacific Ocean.
- OSIRIS-REx launched from Cape Canaveral Air Force Station in Florida on September 8, 2016, on an Atlas V 411 rocket. Although the rocket provided the spacecraft with the all the momentum required to propel it forward to Bennu, OSIRIS-REx needed an extra boost from the Earth's gravity to change its orbital plane. Bennu's orbit around the Sun is tilted six degrees from Earth's orbit, and this maneuver changed the spacecraft's direction to put it on the path toward Bennu.
- As a result of the flyby, the velocity change to the spacecraft was 3.778 km/s.
- "The encounter with Earth is fundamental to our rendezvous with Bennu," said Rich Burns, OSIRIS-REx project manager at NASA/GSFC in Greenbelt, Maryland. "The total velocity change from Earth's gravity far exceeds the total fuel load of the OSIRIS-REx propulsion system, so we are really leveraging our Earth flyby to make a massive change to the OSIRIS-REx trajectory, specifically changing the tilt of the orbit to match Bennu."
- The mission team also is using OSIRIS-REx's Earth flyby as an opportunity to test and calibrate the spacecraft's instrument suite. Approximately four hours after the point of closest approach, and on three subsequent days over the next two weeks, the spacecraft's instruments will be turned on to scan Earth and the Moon. These data will be used to calibrate the spacecraft's science instruments in preparation for OSIRIS-REx's arrival at Bennu in late 2018.
- "The opportunity to collect science data over the next two weeks provides the OSIRIS-REx mission team with an excellent opportunity to practice for operations at Bennu," said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. "During the Earth flyby, the science and operations teams are co-located, performing daily activities together as they will during the asteroid encounter."
• August 25, 2017: NASA's OSIRIS-REx spacecraft fired its thrusters to position itself on the correct course for its upcoming Earth flyby. The spacecraft, which is on a two-year outbound journey to asteroid Bennu, successfully performed a precision course adjustment on Aug. 23 to prepare for the gravity slingshot on Sept. 22. 32)
- This trajectory correction maneuver was the first to use the spacecraft's ACS (Attitude Control System) thrusters in a turn-burn-turn sequence. In this type of sequence, OSIRIS-REx's momentum wheels turn the spacecraft to point the ACS thrusters toward the desired direction for the burn, and the thrusters fire. After the burn, the momentum wheels turn the spacecraft back to its previous orientation. The total thrust is monitored by an on-board accelerometer that will stop the maneuver once the desired thrust is achieved.
- High-precision changes in velocity, or speed and direction, will be critical when the OSIRIS-REx spacecraft operates near Bennu. Because Bennu is so small, it has only a weak gravity field. Therefore, it will only require tiny changes in velocity to do many of the maneuvers that are planned to explore and map the asteroid.
- The Aug. 23 maneuver began at 1 p.m. EDT and lasted for approximately one minute and 17 seconds. Preliminary tracking data indicate that the maneuver was successful, changing the velocity of the spacecraft by 47.9 cm/s and using approximately 0.46 kg of fuel.
- OSIRIS-REx will fly by Earth on Sept. 22 to use our planet's gravity to propel the spacecraft onto Bennu's orbital plane. As of Aug. 25, the spacecraft is about 16.6 million km from Earth.
• March 24, 2017: During an almost two-week search, NASA's OSIRIS-REx mission team activated the spacecraft's MapCam imager and scanned part of the surrounding space for elusive Earth-Trojan asteroids — objects that scientists believe may exist in one of the stable regions that co-orbits the sun with Earth. Although no Earth-Trojans were discovered, the spacecraft's camera operated flawlessly and demonstrated that it could image objects two magnitudes dimmer than originally expected. 33)
- The spacecraft, currently on its outbound journey to asteroid Bennu, flew through the center of Earth's fourth Lagrangian area — a stable region 60º in front of Earth in its orbit where scientists believe asteroids may be trapped, such as asteroid 2010 TK7, discovered by NASA's WISE (Wide-field Infrared Survey Explorer) satellite in 2010. Though no new asteroids were discovered in the region that was scanned, the spacecraft's cameras MapCam and PolyCam successfully acquired and imaged Jupiter and several of its moons, as well as Main Belt asteroids.
- "The Earth-Trojan Asteroid Search was a significant success for the OSIRIS-REx mission," said OSIRIS-REx Principal Investigator Dante Lauretta of the University of Arizona, Tucson. "In this first practical exercise of the mission's science operations, the mission team learned so much about this spacecraft's capabilities and flight operations that we are now ahead of the game for when we get to Bennu."
- The Earth Trojan survey was designed primarily as an exercise for the mission team to rehearse the hazard search the spacecraft will perform as it approaches its target asteroid Bennu. This search will allow the mission team to avoid any natural satellites that may exist around the asteroid as the spacecraft prepares to collect a sample to return to Earth in 2023 for scientific study.
- The spacecraft's MapCam imager, in particular, performed much better than expected during the exercise. Based on the camera's design specifications, the team anticipated detecting four Main Belt asteroids. In practice, however, the camera was able to detect moving asteroids two magnitudes fainter than expected and imaged a total of 17 Main Belt asteroids. This indicates that the mission will be able to detect possible hazards around Bennu earlier and from a much greater distance than originally planned, further reducing mission risk.
• February 9, 2017: OSIRIS-REx begins its search for an enigmatic class of near-Earth objects known as Earth-Trojan asteroids. OSIRIS-REx, currently on a two-year outbound journey to the asteroid Bennu, will spend almost two weeks searching for evidence of these small bodies. 34)
- Trojan asteroids are trapped in stable gravity wells, called Lagrange points, which precede or follow a planet. OSIRIS-REx is currently traveling through Earth's fourth Lagrange point (L4), which is located 60 degrees ahead in Earth's orbit around the sun, about 150 million km from our planet. The mission team will use this opportunity to take multiple images of the area with the spacecraft's MapCam camera in the hope of identifying Earth-Trojan asteroids in the region.
Figure 13: Lagrange points in the Sun–Earth system
- Although scientists have discovered thousands of Trojan asteroids accompanying other planets, only one Earth-Trojan has been identified to date, asteroid 2010 TK7. Scientists predict that there should be more Trojans sharing Earth's orbit, but they are difficult to detect from Earth as they appear near the sun on the Earth's horizon.
- "Because the Earth's fourth Lagrange point is relatively stable, it is possible that remnants of the material that built Earth are trapped within it," said Dante Lauretta (the PI of OSIRIS-REx of the University of Arizona). "So this search gives us a unique opportunity to explore the primordial building blocks of Earth."
- The search commences Feb. 9 and continues through Feb. 20. On each observation day, the spacecraft's MapCam camera will take 135 survey images that will be processed and examined by the mission's imaging scientists at the University of Arizona, Tucson. The study plan also includes opportunities for MapCam to image Jupiter, several galaxies, and the main belt asteroids 55 Pandora, 47 Aglaja and 12 Victoria.
- Whether or not the team discovers any new asteroids, the search is a beneficial exercise. The operations involved in searching for Earth-Trojan asteroids closely resemble those required to search for natural satellites and other potential hazards around Bennu when the spacecraft approaches its target in 2018. Being able to practice these mission-critical operations in advance will help the OSIRIS-REx team reduce mission risk once the spacecraft arrives at Bennu.
• December 28, 2016: NASA's OSIRIS-REx spacecraft executed its first Deep Space Maneuver today, putting it on course for an Earth flyby in September 2017. The team will continue to examine telemetry and tracking data as it becomes available at the current low data rate and will have more information in January. 35)
• October 7, 2016: The OSIRIS-REx spacecraft fired its TCM (Trajectory Correction Maneuver) thrusters for the first time in order to slightly adjust its trajectory on the outbound journey from Earth to the asteroid Bennu. The TCM-1 lasted for about 12 seconds, changing the spacecraft velocity for about 0.5m/s. The spacecraft is currently 14.5 million km from Earth. 36)
- TCM-1 was originally included in the spacecraft's flight plan to fine-tune its trajectory if needed after the mission's Sept. 8 launch. The ULA Atlas V's launch performance was so accurate, however, that the spacecraft's orbit had no practical need for correction. Instead, the OSIRIS-REx mission team used the Oct. 7 maneuver to test the TCM thrusters and as practice to prepare for a much larger propulsive maneuver scheduled in December.
Figure 14: Artist's conception of the OSIRIS-REx spacecraft in cruise configuration (image credit: University of Arizona, Heather Roper)
• On September 22, 2016, two weeks after launch, the OSIRIS-REx spacecraft switched on the TAGCAMS (Touch and Go Camera System) to demonstrate proper operation in space. This image of the spacecraft was captured by the StowCam portion of the system when it was 6.17 million km away from Earth and traveling at a speed of 30 km/s around the Sun.
Figure 15: StowCam first light: Visible in the lower left hand side of the image is the radiator and sun shade for another instrument (SamCam) onboard the spacecraft. Featured prominently in the center of the image is the SRC (Sample Return Capsule), showing that our asteroid sample's ride back to Earth in 2023 is in perfect condition. In the upper left and upper right portions of the image are views of deep space. No stars are visible due to the bright illumination provided by the sun (image credit: NASA)
• As of Sept. 15, 2016, OSIRIS-REx was approximately 3.2 million km from Earth. All of the spacecraft's subsystems are operating as expected. 37)
• OSIRIS-REx separated from its United Launch Alliance Atlas V rocket at 59 minutes after liftoff. The solar arrays deployed and are now powering the spacecraft (Ref. 25).
Sensor complement: (OCAMS, OLA, OVIRS, OTES, REXIS, TAGSAM)
OSIRIS-REx delivers its science using five instruments and radio science along with the TAGSAM (Touch-And-Go Sample Acquisition Mechanism). All of the instruments and data analysis techniques have direct heritage from flown planetary missions. 38)
OCAMS (OSIRIS-REx Camera Suite)
OCAMS is composed of three cameras. PolyCam provides long-range Bennu acquisition and high-resolution imaging of Bennu's surface. MapCam supports optical navigation during proximity-operations, global mapping, and sample-site reconnaissance. SamCam performs sample-site characterization and sample-acquisition documentation. The OCAMS camera suite is being developed at LPL (Lunar and Planetary Lab) of UA (University of Arizona). 39) 40)
These cameras will "see" asteroid Bennu as the spacecraft first approaches it. OCAMS will then provide global image mapping and sample site imaging and characterization. Finally OCAMS will record the entire sampling event during the TAG (Touch-And-Go) maneuver. Specifically:
• PolyCam, a 20 cm telescope, is the first to "see" the asteroid from 2 million km away. Once the spacecraft is closer, it will image Bennu at high resolution. FOV = 0.8º. The asteroid is first acquired through the PolyCam, an 8 arcsec Richey-Chretien telescope capable of detecting up to 12th mag objects limited by spacecraft jitter. As features on the asteroid become resolvable, this telescope is used for preliminary mapping at a surface resolution of <25 cm.
• MapCam searches for satellites and outgassing plumes. It maps the asteroid in 4 different colors, informs our model of asteroid shape, and provides high resolution imaging of the sample-site. FOV = 4º.
• SamCam will continuously document the sample acquisition event and TAG maneuver. SanCam gives the context for the recovered sample with a FOV of 21º.
All cameras use identical detector arrays but are characterized by focal lengths separated by a factor of 5.
Figure 16: The 3 cameras are seen on the instrument deck with the Sample Return Capsule in the background. In the center from left to right: SamCam, MapCam, and PolyCam. Notice the electronics control module underneath the deck (image credit: LPL of UA)
Post-launch calibration of the cameras is performed during the 2-year cruise that includes an Earth flyby. Five sources are used for calibration: stellar clusters (geometric distortion); solar-type stars (radiometric calibration); blocking filter (dark current evolution in the radiation environment); illumination lamps (pixel-to-pixel fixed pattern noise); and the Earth-Moon system (operational preparation).
Within 500,000 km of asteroid Bennu, PolyCam aides the navigation team by locating the asteroid against background stars. The approach affords an opportunity to verify the phase curve, the rotation rate, and other properties that have been measured using ground-based telescopes. A search for potentially hazardous secondaries will assure a safe approach. In addition, Poly-Cam collects images for a preliminary shape model.
Survey: After approaching the asteroid and accomplishing flybys of the polar regions, a series of observing positions allows the mapping of Bennu from various phase angles and latitudes throughout its 4.5 hour rotation. Both high resolution and color-ratio maps are generated over at least 80% of the surface. The data sets are combined to make a solid model of the asteroid shape forming the basis for detailed mapping. These maps are used to delineate craters, large boulders, and linear features. The maps will also be examined to determine 12 potential sampling sites of diameter 25 m.
Orbital Phase: The navigation team guides the spacecraft to a polar orbit above the terminator where the gravitational attraction is balanced by radiation pressure from the Sun. The 1 km orbit puts the cameras several hundred meters above the surface of the 275 m radius object. From this vantage point the 12 sites are more closely examined (>5 cm objects resolved by PolyCam) and the top 4 sites are selected for further investigation.
Reconnaissance: Fly-overs from the safe home orbit allow sub-cm imaging for the 4 finalists. The high resolution is accomplished by refocusing the PolyCam, effectively converting a telescope to a microscope. These reconnaissance fly-overs permit final assurance that the surface materials are neither hazardous to sample collection (>21 cm) nor devoid of small regolith particles that can be collected by the sampling arm (<2 cm).
Sampling: Using all information, a final site is selected and a series of rehearsals takes place to practice each step of the sampling process. At each rehearsal the MapCam monitors the surface motions with images at a range of 30 m from the surface,a distance that allows the rotational velocity to be matched.
From this Matchpoint the final sampling event is initiated. Slowly descending toward the surface with its arm extended, the OSIRIS-REx spacecraft prepares to collect a sample of asteroid Bennu. The SamCam records the event at about a frame/sec, its wide field encompassing the sampling head near the center of the frame.
These images document the context of the undisturbed surface, then the post-collection morphology, that help the team decide if a sufficiently large sample has been taken. It is important to be certain that the sample is in the collection chamber before returning to Earth. The SamCam images the sample head when the spacecraft has reached a safe distance away from the asteroid to provide visual confirmation of the sample within the sample head. With these final images the mission for the cameras is completed. An overview of OCAMS resolution vs range for the mission phases is shown in Figure 17.
Figure 17: The 3 cameras have overlapping capabilities and can accommodate the loss of a camera. Notice the dots for SamCam and MapCam at their closest approaches, these are modifications of the focal length using a diopter lens in the filter wheel (image credit: LPL of UA)
OLA (OSIRIS-REx Laser Altimeter)
OLA is a scanning LIDAR (Light Detection and Ranging) instrument to provide high-resolution topographical information. OLA's high-energy laser transmitter is used for ranging from 1–7.5 km that supports Radio Science and provides scaling information for images and spectral spots. OLA's low-energy transmitter is used for rapid ranging and LIDAR imaging at 500 m to 1 km, providing a global topographic map of Bennu as well as local maps of candidate sample sites. The OLA instrument is a contribution of CSA (Canadian Space Agency) and is manufactured, assembled and tested by MDA (MacDonald Dettwiler & Associates) and Optech Inc. 41) 42) 43) 44) 45)
OLA will deliver high density 3D point cloud data, enabling reconstruction of an asteroid shape model at the highest density yet recorded on any small body, and providing much needed slope information (Figure 18) at the sample site leading up to acquisition. These data will be important for determining the geological context of the samples obtained by the OSIRIS-REx mission, as well as help minimizing the risk of encountering hazards during sampling. In addition, OLA will be important for the accurate determination of the gravity field of Bennu by providing an accurate measure of the distance between the spacecraft and asteroid in support of radio science. Finally, OLA will provide ranging in support of other instruments and navigation.
Figure 18: Slope distribution on simulated asteroid Bennu (image credit: MDA)
The OLA system is based on a heritage design of the scanning lidar system built by MDA for the US AFRL (Air Force Research Laboratory) XSS-11 mission. The base system will be augmented with a second higher energy laser transmitter for increased range capability that is based on the heritage of the MDA built 2008 Phoenix Mars Lidar.
The OLA system is a scanning, time-of-fight lidar, based on a flight proven space lidar augmented with a 2nd high energy laser transmitter. This dual laser design, coupled with a 2-axis scanning mirror, complete with an integrated real-time data acquisition and processing platform provides a powerful, yet flexible, scientific instrument. OLA's unique capabilities allow it to support both OSIRIS-REx mission operational tasks (navigation ranging, self-triangulation of the spacecraft to surface features) as well as supporting science objectives (3D shape, topography, block and crater distributions, asteroid volume, etc).
OLA operations concept:
The OSIRIS-REx mission has a number of phases. Each phase has a typical range and spacecraft motion. OLA's scanning mirror provides operational flexibility to efficiently measure the asteroid's surface. The OSIRIS-REx mission phases, along with the scanner and spacecraft motions are depicted in Figure 19.
A survey of Bennu includes four major phases:
• Preliminary Survey (~7 km) — OLA operates in a pushbroom mode by scanning perpendicular to the S/C motion.
• Orbital A (~1.5km) — No OLA activity.
• Detailed Survey (~3.5km-5km) — OLA scans perpendicular to the S/C's North-South slew.
• Orbital B (~750m from the surface) — OLA scans in a raster mode to create 3D "images" of the surface.
• Reconnaissance (~225m-500m) — OLA uses its scanner to spread its measurement points over the sampling ellipse while the spacecraft translates and slews.
Figure 19: OLA adapts its scan patterns to the operational mode of the spacecraft (image credit: MDA)
OLA key requirements:
The OLA instrument key requirements include:
• Operational range: 7.5 km to <500 m @ 3% Lambertian albedo
• FOR (Field of Regard): ± 7º
• Range accuracy: < 30 cm
• Range resolution: < 1 cm
• Relative pointing: ~ µ50 rad.
The OLA key requirements per OSIRIS-REx mission phase are captured in Table 2. OLA exceeds its key requirements.
Table 2: OLA key requirements per mission phase
The detailed Bennu surface scans, performed during Orbital Phase B and the Reconnaissance phases, will be processed on the ground to produce a detailed asteroid model complete with high resolution surface features. This model, when combined with a measurement of the asteroid mass and rotation rate, allows the surface geometry to be converted to topography and slopes (Figure 20).
Figure 20: Surface slope map produced in SBMT (image credit: MDA)
OLA sensor system implementation: The OLA design comprises two complementary lasers, coupled with a 2-axis FSM (Fine Scanning Mirror), a common receiver and an integrated data acquisition and processing platform (Figures 21 & 23).
Figure 21: OLA sensor implementation (image credit: MDA)
OLA's 2-axis scanning mirror directs transmitted laser pulses towards the target and the laser returns back to the field of view of the receiver. Although the design supports a variety of scan patterns, the OLA mission only requires use of raster and linear scans. Scan data (per laser pulse) includes time-tagged target range, intensity, azimuth, and elevation as well as a measure of the outgoing laser intensity. The OSIRIS-REx ground segment uses the OLA telemetry to generate the required mission data products. If required, OLA generated scan data (e.g. range and range rate data) may be used to complement other OSIRIS-REx sensor data and support other mission needs.
A unique aspect of the OLA instrument is its ability to point its laser very quickly allowing many measurements to be made without moving the spacecraft. OLA's measurement speed of up to 10,000 measurements/s, combined with a very agile scanning device, it allows "range pictures"to be taken.
Figure 22: By rapidly moving the scanning device, a 2D picture can be created. The red dots are the measurement locations and the grey lines represent the path of the scanner (image credit: UA)
OLA High Energy Stage: OLA's low pulse rate, high-energy laser transmitter ranges and maps from ~7.5 km down to ~1km. OLA's high energy laser provides high accuracy range data of Bennu for the purposes of navigation. It also provides long-range scans of the asteroid surface required to establish the shape (and model the gravity) of Bennu.
OLA Low Energy Stage: OLA's high pulse rate, low-energy transmitter ranges and maps from ~1 km down to ~500 m (and possibly 200 m if required). The low energy laser stage is used at shorter ranges to generate high resolution maps. These maps can be used to detect candidate sample sites and select the preferred TAG (Touch-And Go) site. The detailed TAG site surface map can also be used to provide context for samples collected at the TAG sample site.
OLA feature-benefit analysis: OLA's capabilities go much beyond altimetry. In addition to its large ranging capability (from 7.5 km to 500 m), OLA design provides the following capabilities in support of both science and navigation mission goals:
• Flexible user-selectable scanning over a +/-7o FOR (sub-window scan, scan window size, scan speed, scan patterns, altimetry mode) supporting various mission needs (altimetry, sparse/dense mapping, multiple-window scans).
• Robust scan data (Range, Azimuth, Elevation, Intensity, Laser Return Intensity) supporting target tracking, generation of local/global topographic maps, Range maps, Intensity maps and Surface Hazard maps. Surface features, identifiable in the dense scans, can be used to navigate the spacecraft to the TAG location. High resolution intensity maps can be combined with other camera sensor data to allow a more robust surface feature identification.
• Dual laser design, comprising high and low energy lasers with range overlap, provide ranging fault tolerance (OLA degraded ranging function following laser failure).
• The 2-axis scanning mirror provides flexible user defined scanning which can be tailored to support the known (and new) mission phase needs. The mirror may still provide degraded functionality (e.g.; altimetry) following loss-of-motion failures.
• Last, but not least, the OLA ranging capability may be used to augment other scientific and GN&C sensors by providing range data throughout the mission phases, including the final TAG mission phase. OLA provides the spacecraft with a 10 Hz real-time range data via the OLA telemetry data stream.
OLA science benefits: OLA measures the distance from the instrument to the surface of Bennu with high resolution and rate in any lighting condition. This data will be used to generate high-resolution shape models of Bennu which can be combined with a measurement of mass, pole orientation and spin rate to generate topography and geophysical models that will be used to understand the origin, evolution and present state of the asteroid.
These models will also be used for spacecraft navigation and approach planning. The high resolution topography of candidate sampling sites will be used to choose a sampling site that has a high probability of successfully sampling the asteroid while maintaining the safety of the spacecraft.
OVIRS (OSIRIS-REx Visible and Infrared Spectrometer)
OVIRS is a linear-variable point spectrometer (4 mrad FOV) with a spectral range of 0.4 – 4.3 µm, providing full-disk Bennu spectral data, global spectral maps (20 m resolution), and local spectral information of the sample site (0.08 – 2 m resolution). OVIRS spectra will be used to identify volatile- and organic- rich regions of Bennu's surface and guide sample-site selection. 47)
The OVIRS instrument uses an OAP (Off-Axis Parabolic) mirror to image the surface of the asteroid onto a field stop. The field stop selects a 4 mrad angular region of the image. The light from this 4 mrad area passes to a second OAP that recollimates it and illuminates the FPA (Focal Plane Assembly). Because the beam speed is low (~ f/50) this assembly, consisting of the array with the filter mounted in close proximity to it, is effectively at a pupil. Each detector element of the array "sees" the same spatial region of the asteroid but, as described below, different columns of the array "see" it at different wavelengths. The complete spectrum of the 4 mrad spot is obtained in a single measurement. This is somewhat different than the case for some wedged filter spectral imagers, such as LEISA, where the spectrum of a given point is built up over several frames, e.g.
In order to obtain the high SNR required for OVIRS on a very dark asteroid surface (albedo ~3%), at least 30 pixels will be averaged in each wavelength column. This conservative number, used in sensitivity modeling, is based on worst-case estimates of both spectral "smile" and scattering at segment boundaries. The actual number of pixels summed will be determined in instrument testing. The data will first be filtered using a pre-measured bad pixel map. To prevent cosmic ray events from contaminating the spectra while still reducing the data volume, pixels will be averaged in subsets before transmission to the ground. Contaminated subsets will be removed in ground processing before summing the remaining subsets at each wavelength to obtain the final spectrum.
The detector array is thermally coupled to a two-stage passive radiator to obtain focal plane temperatures of ~105 K. This reduces the dark current sufficiently that dark current noise is never the dominant noise source with more than a factor of two margin. The camera enclosure shields its contents from radiation and contaminants and mounts to the OSIRIS science deck. A cold baffle in the optical path limits the thermal background signal from the instrument enclosure. In addition, small radiators will reduce the temperature of the optics enclosure itself to less than 160 K, further reducing thermal background noise. The thermal design is such that, except for very low asteroid surface temperatures and very low solar reflectance, the measurement noise is dominated by source photon noise. For very low asteroid signal, the primary noise term is the low read noise. This is the optimum design from a noise standpoint.
Figure 24: Illustration of the OVIRS instrument (image credit: NASA/GSFC)
OVIRS will be calibrated prior to launch and the calibration will be checked throughout the mission. Spectral calibration will be accomplished using gratings to provide effective monochromatic scanned radiometric sources with R>2,000. Radiometric and relative response calibrations will be performed using NIST traceable calibrated blackbodies and flood sources. The quality of the point spread function will be assessed using collimated point and extended sources. The boresight pointing shall be measured with respect to an optical alignment cube on OVIRS.
In-flight radiometric calibration will rely on three methods: a calibrated onboard array of miniature black body sources (T ~700 K) placed at the OVIRS fieldstop and tungsten filament sources located after the secondary mirror, in-flight observations of the Earth and the Moon and absolute solar reflectance calibrations. The terrestrial and lunar calibrations will occur on the OSIRIS-REx flyby of Earth. The onboard solar reflectance calibrations will be carried out occasionally by using the spacecraft control system to point the solar calibration port at the sun. The combination of these methods will provide redundant radiometric calibration. It is expected that OVIRS will provide spectral data with at least 5% radiometric accuracy and no worse than 2% pixel-to-pixel precision. Because wedged filters are very stable, the spectral calibration is not expected to change in flight, however, the Earth and Lunar observations will also provide spectral calibrations. Spectral calibration is expected to be accurate to 0.25 of the halfwidth or better. The dark current and background flux will be measured using dark sky observations.
Figure 25: Photo of the OVIRS instrument (image credit: BATC, Ref. 17)
OTES (OSIRIS-REx Thermal Emission Spectrometer)
OTES is a FTI (Fourier Transform Interferometer), point spectrometer (8 mrad FOV) that collects hyperspectral thermal infrared data over the spectral range from 4 – 50 µm with a spectral resolution of 10 cm-1. OTES provides full-disk Bennu spectral data, global spectral maps, and local sample site spectral information. 48)
OTES is being developed and built at the School of Earth and Space Exploration at ASU (Arizona State University). During several phases of the mission, OTES measures the energy emitted by Bennu over wavelengths of approximately 5 – 50 µm, the thermal infrared. At these wavelengths, virtually all minerals have unique spectral signatures that are like fingerprints, which will help the science team to understand what minerals are present on the surface of Bennu and search for minerals of particular interest, such as those that contain water. Additionally, the emitted heat energy (temperature) at these wavelengths can tell the science team about physical properties of the surface, such as the mean particle size.
OTES is an uncooled, Fourier transform infrared point spectrometer. The design of OTES is heritage from the Mars Global Surveyor TES (Thermal Emission Spectrometer) and the Mars Exploration Rovers Mini-TES instruments. The heart of the instrument is a Michelson interferometer that collects one interferogram every two seconds. OTES's spectral resolution is 10 cm-1 and its field of view is 8 mrad, achieved with a 15.2 cm f/3.91 Ritchey-Chretien telescope. A key component of OTES is its beamsplitter, which is the part of the interferometer that splits the incoming light beam into two pathways before they are recombined and measured at the detector. Unlike the TES and Mini-TES beamsplitters, which were made of CsI (Cesium Iodide) and KBr (Potassium Bromide), the OTES beamsplitter is made of CVD (Chemical Vapor Deposited) diamond. A diamond beamsplitter is physically stronger than the CsI and KBr and it is not hygroscopic, which means that it does not absorb water from the atmosphere (which will cause CsI and KBr beamsplitters to become cloudy, making them less effective).
Figure 26: The OTES beamsplitter assembly (image ASU)
OTES looks at just one spot on the asteroid's surface at a time, and it does not need to focus in the same way the human eye or a camera does. OTES's telescope collects all of the infrared energy emitted by whatever is in its field of view. The spatial resolution varies depending on the distance of the spacecraft from the target (in this case, Bennu). It's a bit like looking through the tube from a roll of paper towels – the farther away you are from what you're looking at, the more things you see; when you get closer to whatever you're looking at, you see a smaller portion of it. When the spacecraft is at a moderate distance from Bennu, such as 5 km (during the survey part of the mission), OTES sees a spot on the surface that is about 40 m in diameter. In the reconnaissance phase of the mission, OTES has a spatial resolution that is closer to 4 m. OTES has no ability to point itself – it looks straight out from the spacecraft – so to see other places on the surface, OTES relies on the spacecraft to move the OTES field of view across the surface of Bennu.
Figure 27: Drawing showing the relative positions of the beamsplitter and moving mirror assemblies (i.e., the interferometer), image credit: ASU
As OTES measures the mineral signatures and temperatures of many spots, the information from each spot is placed on a map to understand the whole of Bennu. In this way, one can look at where on the surface different minerals are found, how particle sizes change across the face of the asteroid, and obtain critically important context information for the samples that OSIRIS-REx will return to Earth.
Figure 28: Exploded" view of the OTES instrument (image credit: ASU)
Legend to Figure 28: From left to right are the sunshade, the telescope, the aft optics plate (the moving mirror assembly is at top, and the beamsplitter is the greenish circle), the electronics board (green card), and the instrument enclosure (with triangular flexure mounts for attaching OTES to the spacecraft).
REXIS (Regolith X-ray Imaging Spectrometer)
REXIS, a student collaboration experiment, is a joint venture of MIT/SSL (Massachusetts Institute of Technology/Space Systems Laboratory) and the Harvard-Smithsonian CfA (Center for Astrophysics). REXIS significantly enhances OSIRIS-REx by obtaining a global X-ray map of elemental abundance on Bennu.
REXIS is a small (2.7kg), compact X-ray imaging camera with a ~30º field of view that will measure and image the X-ray lines (fluoresced by incident solar X-rays) which reveal the surface composition (O, Mg, Si, S, Fe, etc.) of the Near-Earth asteroid Bennu. 49) 50) 51)
REXIS is comprised of two subassemblies: the Spectrometer and the SXM ( Solar X-ray Monitor). The Spectrometer observes the asteroid while the SXM observes the Sun. Because the Sun's X-ray output affects Bennu's X-ray output, we need to keep track of what the Sun is doing, including solar flares, in order to calibrate the Bennu data correctly. The Spectrometer collects X-ray photons from Bennu using four CCDs (Charge Coupled Devices) but before the photons are detected by the CCDs, they pass through a coded aperture mask. The mask is a random pattern of open and closed holes in a thin stainless steel sheet. By analyzing how the shadow of the mask pattern is shifted on the CCDs, we can determine areas of high X-ray activity on the asteroid. This is how REXIS takes "images" of Bennu without any mirrors or lenses like the other instruments on OSIRIS-REx. 52)
REXIS is a coded aperture soft X-ray (0.3 - 7.5 keV) telescope that images X-ray fluorescence line emission produced by the interaction of solar X-rays with the regolith of the asteroid. REXIS will use a 2 x 2 array of CCDs (CCID-41 with Suzaku-XIS heritage) for X-ray detection, each with their 1 k x 1k 24 µm pixels binned by a factor of 32 into 0.768 x 0.768 mm "effective" pixels. Imaging is achieved by correlating the detected X-ray image with a 64 x 64 element random mask made of gold. REXIS will store each X-ray event in order to maximize the data storage usage and to minimize the risk. The pixels will be addressed in 64 x 64 bins and the 0.3 - 7.5 keV range will be covered by 5 broad bands and 11 narrow line bands. A 24 second resolution time tag will be interleaved with the event data to account for asteroid rotation. Images will be reconstructed on the ground after downlink of the event list (an individual image has a FOV of 401 m x 401 m before co-adding). Images are formed simultaneously in 16 energy bands centered on the dominant lines of abundant surface elements from O-K (0.5 keV) to Fe-Ka (7 keV) as well the representative continuum.
Figure 29: Schematic view of the REXIS instrument (image credit: MIT, Harvard-Smithsonian CfA)
The REXIS science objective is to obtain an X-ray (0.3-7.5 keV) global map of the elemental abundance of the asteroid Bennu, thereby providing a complementary understanding of the globally representative context of the returned sample.
Figure 30: Shown at left is a simulated X-ray fluorescence spectrum from regolith of a C1 carbonaceous chondrite for the quiescent Sun for the REXIS instrument with the asteroid at a heliocentric distance of 1 AU. Increased solar activity can increase fluxes by 2 to 4 orders of magnitude over those shown. — At right is shown the minimal detectable (5σ) excess of a high concentration surface unit vs. unit radius (m) for the total flux (black) and a lines constituting 3% (blue) and 30% (red) of the total flux (c.f. 1-40% of the total for the 5 brightest observable fluorescence lines). The inset shows a simulated image from 700 m of a region containing three units with factors of 5, 6, and 10 higher concentration of O than the surrounding region reconstructed with the preliminary random mask design (image credit: MIT, Harvard-Smithsonian CfA)
TAGSAM (Touch-And-Go Sample Acquisition Mechanism)
The TAGSAM is the key flight system component, used for making contact and acquiring sample from the surface of Bennu during the TAG mission phase. TAGSAM is designed to collect greater than 150 g to provide margin to the 60 g mission requirement. The TAGSAM functions by fluidizing regolith with high pressure gaseous nitrogen flow to transport it to a sample container. The TAGSAM is made up of a single planar, articulating arm with redundant motor windings at the shoulder, elbow, and wrist and provides large structural, torque, and alignment margins, ultimately ensuring successful sample acquisition and stowage of the TAGSAM head into the SRC (Figure 31).
TAGSAM is an elegantly simple device that satisfies all sample-acquisition requirements. TAGSAM consists of two major components, a sampler head and an articulated positioning arm. The head acquires the bulk sample by releasing a jet of high-purity N2 gas that "fluidizes" the regolith into the collection chamber. The articulated arm, which is similar to, but longer than, the Stardust aerogel deployment arm, positions the head for collection, brings it back for visual documentation, and places it in the Stardust-heritage SRC (Sample Return Capsule).
Radio Science will determine the mass of Bennu and estimate the mass distribution to 2nd degree and order, with limits on the 4th degree and order distribution. Knowing the mass estimate and shape model, the team will compute the bulk density and apparent porosity of Bennu. These data are obtained by combining radiometric tracking data with optical observations, supplemented by OLA altimetry data. Together, this information constrains the internal structure. Most importantly, the gravity field knowledge provides information on regolith mobility and identifies areas of significant regolith pooling.
TAG (Touch-And-Go) phase overview:
The TAG Phase has a set of driving requirements (Table 3) to collect a sample and ensure spacecraft safety. The primary TAG activities include a set of three maneuvers to reach the surface: Orbit Departure, Checkpoint, and Matchpoint. This sequence targets the desired TAG site with the desired velocity at the correct time. Accurate position and velocity are crucial to ensure spacecraft safety and mission success (Ref. 3).
Table 3: Key TAG requirements and capabilities
The primary TAG activities include a set of three maneuvers to reach the surface: Orbit Departure, Checkpoint, and Matchpoint. This sequence targets the desired TAG site with the desired velocity at the correct time. Accurate position and velocity are crucial to ensure spacecraft safety and mission success.
An overall summary of all the TAG activities is shown in Figure 32. The clock starts ticking for the TAG phase timeline once the final RWA momentum desaturation is performed roughly 9 days before contact. This final desaturation prevents orbital perturbations as the Flight Dynamics team performs orbit determination and refines the maneuvers for TAG. Following the final desaturation, the SMM (Sample Mass Measurement) procedure is executed to determine the baseline inertia of the sampler arm prior to collecting sample. By measuring the spacecraft inertia before and after collection, the collected mass can be measured. After a few days of orbit determination, a very small (< 1 mm/s) phasing burn is performed to tweak the orbit and align OSIRIS-REx at the right place at the right time for the orbit departure maneuver.
The DSN (Deep Space Network) ground stations of NASA provide coverage for a high gain telecom pass to upload the final TAG sequence command blocks. This command upload includes all the details to allow the spacecraft to run autonomously to perform the maneuvers, allow onboard navigation and maneuver guidance updates, fault protection and safety corridor monitoring, contact detection and sample collection, and finally the back-away maneuver and recovery reconfiguration. Without this final command upload, the spacecraft would simply remain in the 1 km Safe Home orbit to allow the team to restart the TAG clock when ready.
Figure 32: TAG Phase Overview. The final four hours prior to contact are performed autonomously onboard, beginning with the orbit departure maneuver (image credit: OSIRIS-REx collaboration)
After the orbit departure maneuver, the spacecraft slews to an attitude to allow telecom coverage for ground to assess the burn performance. While the spacecraft has all necessary tools onboard to ensure safety, it is desirable for the ground to also monitor progress and safety. However, with a roundtrip light time of ~30 min, it is necessary for the spacecraft to be very robust at autonomously ensuring safety and not relying on ground intervention. While in this attitude the TAGSAM arm is moved into the sampling configuration using onboard potentiometer checks to verify final positioning.
The spacecraft then slews into an attitude to allow asteroid imaging with the NavCam (part of the TAGCAMS suite). These images support optical navigation both on the ground for reconstruction purposes as well as onboard the vehicle if necessary. The primary onboard navigation sensors for TAG are the redundant GN&C lidars. The lidar alone provides the necessary data to update onboard navigation, perform maneuver guidance, and monitor for range and rate safety to the surface. However, OSIRIS-REx has included onboard NFT (Natural Feature Tracking) algorithms to process the NavCam images as a backup to the lidar baseline. This offers two independent, onboard navigation techniques to meet all TAG requirements.
The lidar collects range data in the look-ahead inertial-fixed attitude and also in the final TAG inertial-fixed attitude as shown in Figure 32. Onboard processing determines the time that a configurable lidar range threshold is first crossed, providing in-track orbital knowledge. Lidar range measurements close to Checkpoint provide radial knowledge, and these two pieces of information go into a simple polynomial based algorithm to provide an update to the Checkpoint orbital state. The updated state is fed through a guidance algorithm to adjust the Checkpoint and Matchpoint burns to remove known trajectory dispersion and reduce the TAG contact position and velocity errors. 53)
If NFT is used instead of lidar, the NavCam images that are collected are processed onboard to identify known surface features. The known features, as determined from ground tools utilizing a high accuracy asteroid shape model and the known TAG trajectory, are stored in a catalog and rendered onboard to represent their expected appearances. A correlation algorithm finds where the catalog features are in the images, and provides measurements to a Kalman Filter that estimates the orbital state of the spacecraft. The state estimate from NFT can be used with the same maneuver guidance algorithm to update the Checkpoint and Matchpoint burns.
Prior to the Checkpoint burn, the solar arrays are raised into the "Y-wing" configuration to minimize the chance of dust accumulation during contact, as well as provide more ground clearance in the case the spacecraft tips over (up to 45º) during contact. At this point, the spacecraft is in final TAG attitude and physical configuration ready for contact. If anything is determined out of bounds by the onboard fault protection, the spacecraft simply aborts the remaining TAG sequence and performs a back-away maneuver that escapes the asteroid's gravity (>0.5 m/s burn ensures escape) to allow ground to troubleshoot.
Upon Checkpoint burn completion, the onboard fault protection begins monitoring the approach state to ensure the spacecraft is within a safe corridor. Prior to Checkpoint, the spacecraft is on a passively safe trajectory and thus does not need to actively monitor range. Upon Matchpoint completion, the spacecraft attitude control system is set up to allow thruster control if the rates or position errors get above a deadband. This design helps mitigate unnecessary thruster firings prior to contact, thus reducing the likelihood of surface contamination from unreacted hydrazine. The design also provides the torque authority necessary to ensure spacecraft safety by not tipping over more than 45º.
All of the above activities are rehearsed prior to the actual TAG event. The Checkpoint Rehearsal demonstrates that the navigation and spacecraft configurations are achieved properly prior to the Checkpoint burn. The Matchpoint Rehearsal demonstrates the final two burns would have delivered OSIRIS-REx to the TAG site within the required accuracy. Each rehearsal takes three weeks to perform and evaluate before moving on to the next step. Thus, the system is very well characterized and understood prior to the actual TAG event. The slow orbital velocity of TAG provides these excellent rehearsal opportunities and ability to repeat events as necessary.
At 5 m above the surface during the TAG event, as determined by either the lidar or NFT, the spacecraft arms the TAGSAM gas bottle pyro valve to fire upon contact declaration. The TAGSAM arm spring assembly helps rebound the spacecraft from the surface while simultaneously keeping the head on the surface during the brief collection event. Depending on the asteroid surface properties, the contact event duration can be as short as 2 seconds or as long as 20 seconds. The spacecraft design has been rigorously analyzed to support the wide variation in surface properties that will not be fully understood until we make contact.
Once contact is declared, a timer begins to allow for up to 5 seconds of collection before the back-away maneuver initiates to safely depart the asteroid. Immediately after the back-away completes, the spacecraft slews to a nominal sun attitude and reconfigures the solar arrays and sampling arm to allow power and thermal recovery.
After sufficient recovery time, the data collected during TAG by the various spacecraft sensors, cameras and science payloads is downlinked. Images during the full TAG sequence will greatly aid in understanding exactly where contact was made and the surface/regolith response to the sampling event. The spacecraft also provides important details on sensed accelerations to understand the surface contact dynamics. The ground operators then kick off a sequence to perform a stop burn to halt the drift away from the asteroid in case it's necessary to go back for a second sample attempt. If all goes as planned on the first attempt, then the spacecraft simply waits far away from the asteroid until it's time to head back to Earth.
To further assess TAG success, the spacecraft takes images of the sampler head to qualitatively determine if sample was collected, and performs the SMM procedure to measure change in inertia and quantitatively measure the collected sample mass. If sufficient sample is collected, then the sampler head is permanently stowed inside the SRC to complete the TAG phase.
Flight Dynamics Overview:
Leading up to a TAG attempt, the spacecraft is in a 1km circular "Safe Home" orbit about the asteroid parallel to the Sun terminator plane. The orbit is designed to be slightly behind the Sun terminator plane so that the sunward component of the asteroid's gravitational pull counteracts the solar radiation pressure. This stable orbit allows for better prediction of the spacecraft state and removes the need for orbit maintenance.
Optical Navigation (OpNav) is performed using one of the redundant NavCams. During the mission phases prior to TAG, the entire asteroid surface is imaged and a full shape model is created. In the TAG phase, the OpNav process involves taking asteroid images from the terminator orbit and registering them to the asteroid model via ground processing. The registered images provide accurate measurement information that is used in the orbit determination process.
The spacecraft begins the TAG sequence in the Safe Home orbit, and the orbit departure latitude is chosen to be the negative of the TAG site latitude. A small maneuver (<1 mm/s) is used to adjust the phasing of the orbit to achieve the ideal time of orbit departure relative to the asteroid surface. When the spacecraft crosses the orbit departure latitude on the morning side of the asteroid, the orbit departure maneuver is performed with the goal of arriving at the 125 m altitude Checkpoint position 4 hours later. The trajectory sequence is depicted in Figure 32.
When the Checkpoint position is reached, the Checkpoint maneuver is performed to cancel out the majority of the surface-relative lateral velocity and begin descending towards the surface. The Checkpoint maneuver is performed to allow the spacecraft to maintain its inertial-fixed attitude.
After 10 minutes, the spacecraft reaches the Matchpoint at an altitude of 55 m. The Matchpoint maneuver reduces the rate of descent sufficiently to achieve a TAG vertical velocity of 10 cm/s. TAG occurs approximately 10 minutes after the Matchpoint maneuver.
The Flight Dynamics System has a requirement to deliver the spacecraft to within 25 m of a given TAG site with a CI (Confidence Interval) of 98.3%, which is approximately 2.85σ for a two dimensional Gaussian distribution. The 98.3% CI is an allocation of the overall mission-level requirement on the probability of successfully acquiring sufficient sample with a single TAG attempt. Three TAG attempts have been accounted for in the schedule, propellant budget, and TAGSAM gas bottles in case the first attempt is deemed unsuccessful.
The maximum vertical velocity has been designed to 12 cm/s to maintain spacecraft safety. TAG is targeted to occur with 10 cm/s of vertical velocity and is required to have less than 2 cm/s of vertical velocity error (3σ). This velocity requirement supports meeting the TAG positional accuracy requirements as well as the safety requirements.
There are two other TAG contact safety requirements that are levied on Flight Dynamics. One is the horizontal velocity error must be under 2 cm/s (3σ) to prevent additional tip over and loading concerns. The other is the TAG angle due to absolute time of contact error must be less than 4.4º (3σ) (~3 minutes). Because the TAG attitude is inertial-fixed, contact timing errors create spacecraft attitude errors relative to the surface as the asteroid spins at 1.4º/min.
A Monte Carlo analysis is performed to determine the expected TAG dispersions. Many contributing error sources are modelled including departure state errors, maneuver execution errors, lidar instrument bias and noise, surface roughness effects on lidar measurements, spacecraft attitude errors, gravity model errors, solar radiation pressure errors, and asteroid spin state errors. All of these errors are applied as zero-mean Gaussian. The following graphs (Figure 33) show the dispersions for a representative Monte Carlo run with error ellipses provided.
Figure 33: TAG position and velocity errors fall within the allocated requirements (image credit: NASA)
TAG (Touch-And-Go) constraints:
The spacecraft physical configuration and system performance has been designed to provide maximum flexibility in selection of the latitude, longitude, and altitude of the TAG site on the surface of Bennu. However, within this design space, analysis to evaluate the limits of performance has been undertaken (Figure 34). Mission requirements regarding telecom during critical events, landing site tilt accommodation, and flight hardware thermal safety have been identified and analyzed across the surface of Bennu to provide insight into the relationship between the geographic and temporal variables. Results of this analysis are discussed briefly.
Figure 34: Analysis performed across all Bennu latitudes to determine margin on all TAG constraints (image credit: NASA)
Detailed surface maps of the surface of Bennu do not currently exist, and will be generated during the Preliminary and Detailed Mapping phases of the proximity operations mission prior to selection of the TAG site. To ensure the spacecraft would have the ability to sample the most scientifically interesting region of Bennu, no assumptions on sampling latitude can be made prior to launch. Since Bennu is a retrograde rotator, with its North pole roughly perpendicular to the heliocentric plane, the only limitation on sampling latitude is driven by lighting requirements. The sampling site must be illuminated with at least a 5º elevation to provide optical images of the TAG event and the sampled surface. Given approximate latitude range of 85º North to 85º South, critical spacecraft performance has been evaluated for all epochs from the earliest possible TAG date up to asteroid departure in March 2021.
The low albedo of Bennu results in a surface that heats up rapidly from solar radiation soon after local sunrise. As a result, the ideal sampling site from a thermal perspective would be immediately after sunrise, before the surface temperature has had time to climb to unacceptable levels. Unfortunately, the relative placements of the Earth and Sun result in the Earth being below the horizon during much of the proximity operations timeline for early morning sampling locations. This would prevent a communications link during the TAG critical event period. Hence, the TAG site location is placed on the opposite side of Bennu, closer to the sunset terminator, about 65º from the local noon vector allowing for a modest surface cool down later in the afternoon. In general, this places the Earth in the zenith direction, as viewed from Bennu's equator. During the majority of the mission, the spacecraft places the sun within the spacecraft's X-Z plane of symmetry with the sun always in the positive X direction. However, during TAG, this general philosophy is modified to place the Earth within the same X-Z plane. To evaluate the ability of the flight system to maintain a communications link through either its LGA (Low Gain Antenna) or MGA (Medium Gain Antenna), the antenna offpoint margin was evaluated across the range of latitudes and possible sampling dates. The angular offpoint margin, defined as the amount of additional rotation the spacecraft can incur before the Earth moves past the 3 dB limit of satisfying 40 bps, is shown in the following graphs (Figure 35).
Figure 35: Telecom off-point margin with 34 m DSN for LGA (top) and MGA (bottom) provide flexibility for TAG site tilt accommodation (image credit: NASA)
It becomes apparent from this survey that for latitudes from 60º North to 60 º South of the equator, 34 m DSN critical event coverage can be satisfied using the LGA with at least 10º of pointing margin until July 2020. Additionally, this offpoint margin can be considered a resource for sample site surface tilt accommodation. As long as a 3 dB margin is maintained for communications, the spacecraft can align with various TAG site surface tilts for sampling. For latitudes beyond 60º, critical event coverage requirements are best satisfied using the MGA. To provide for critical event coverage beyond July 2020, the mission has to use the 70 m DSN, which provides pole to pole coverage, using the LGA, with greater than 10º of offpoint margin, until Departure in March 2021. This data is shown in Figure 36.
Figure 36: 70 m DSN offers TAG critical event coverage at all possible TAG dates and latitudes (image credit: NASA)
The thermal subsystem uses a combination of radiators and heaters to keep the spacecraft components within their operating ranges. The radiators on the negative Z side of OSIRIS -REx are also protected from solar input by the addition of sun shields that keep the radiators shaded during the nominal sun pointed attitudes. However, as noted earlier, the TAG attitude is modified to place the Earth in the X-Z plane, thereby allowing the solar vector to obtain a positive or negative Y component in the body frame. To analyze this, a parametric analysis was used to determine the worst case date and latitude configuration that would result in the maximum solar input into the thermal radiators. This worst case condition happens in early April 2020, at Bennu latitude of 60º North. Analysis demonstrates the thermal subsystem can satisfy requirements at this worst case date and latitude combination. All other dates and latitude combinations are less stressing and satisfy requirements with larger margins.
Lockheed Martin designed and developed TAGSAM, which will collect ≥ 150 g of pristine asteroid regolith for return to Earth. Sampling occurs by releasing pressurized nitrogen gas into the asteroid surface, and collecting the mobilized material. The pristine nature of the sample is maintained by the precision-cleaned TAGSAM head and the use of high-purity N2 gas. Over 10 years of extensive testing demonstrates TAGSAM collects the required sample mass from a variety of surface types and particle size-frequency distributions.
Contact with the surface and injection of pressurized gas into the surface will transfer kinetic energy to near-surface asteroid material. Because of the low-gravity environment at the asteroid, material will be accelerated to speeds that exceed the escape velocity of the asteroid, and some material may travel towards the spacecraft. Key components of the spacecraft are specifically oriented away from the surface during TAG (e.g. active side of the solar arrays, star trackers). Other key components are housed behind spacecraft structure or insulation blanketing, and so are not exposed to the TAG event. For components that remain exposed, we have completed extensive studies to estimate the amount, speeds, and risk of damage. While there is a possibility that asteroid material will contact parts of the spacecraft other than TAGSAM or the sampling arm, there is little risk to the health of the spacecraft because of the low encounter speeds, and acceptable levels of maximum possible dust mass loading.
Sample verification & stowage:
The spacecraft utilizes the conservation of angular momentum to determine how much sample was collected, leveraging a technique demonstrated on the Cassini mission. OSIRIS-REx gets high sensitivity on inertia measurement changes with the advantage of the long lever arm between the spacecraft CG and the sample location when in the configuration shown in Figure 37. The spacecraft spins 360º by driving the reaction wheels in the opposite direction, both before and after TAG. With known reaction wheel inertias, the necessary wheel speeds to reach the desired spacecraft spin rate enables solving for spacecraft inertia and ultimately the sample mass. Detailed error budgets and Monte Carlo simulations show the sample can be measured to an accuracy well within the 90 g peak-to-peak requirement.
Figure 37: Sample mass measured by performing 360º rotation with arm in two different configurations to determine the delta inertia before and after TAG (image credit: Lockheed)
While the sample mass is the key factor in determining if the mission requirements were met, images of the sampler head during and after TAG will also help give confidence that the sample collection was successful. The sampler head design provides visibility into the collection chamber interior. Images are collected at various angles to inspect for any regolith on the surface as well as in the chamber (Figure 38). By design, the regolith should not protrude from the sampler head to interfere with stowage. If images reveal unallowable protrusions, contingency procedures can remove them and ensure successful stowage.
Figure 38: Sampler head imaging performed with SamCam payload to inspect for collected sample (image credit: OSIRIS-REx collaboration)
Finally, when the sample is ready to be stowed, the SRC (Sample Return Capsule) lid is opened to allow the sampler head to move into position above the SRC capture ring (Figure 39). The StowCam (part of TAGCAMS suite) and potentiometers verify alignments prior to sending commands to drive into the capture ring. Once captured, the sampler head cannot come out, so then the head is severed from the arm. The arm is then retracted into the launch configuration and the SRC lid is closed and latched for Earth Return.
Figure 39: Pre-stowage alignments and sampler head insertion to SRC capture ring imaged by StowCam (image credit: OSIRIS-REx collaboration)
OSIRIS-REx is an exciting mission to collect and return to Earth a pristine, bulk sample of asteroid regolith. After an extensive remote sensing campaign, a TAG sample site is chosen and rehearsals are performed leading up to the final TAG event. Successful Flight Dynamics execution is critical to set the spacecraft on the proper initial trajectory for TAG, and then the autonomous systems onboard take over to update the final two maneuvers (Checkpoint and Matchpoint) and monitor performance to ensure safety through the collection event.
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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).