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. 8)
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. 9) 10)
• 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. 11)
• January 8, 2016: The student-built REXIS (Regolith X-Ray Imaging Spectrometer) instrument of MIT/SSL has been integrated onto the OSIRIS-Rex spacecraft. 12)
• 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. 13)
• 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. 14) 15)
- 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. 16) 17)
• 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. 18)
• 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. 19)
• 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. 20)
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) 21)
• 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. 22)
• 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. 23) 24) 25)
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. 26)
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. 27) 28)
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)
Figure 9: NASA's OSIRIS-REx: Mission to Bennu (video credit: NASA)
• On 12 June 2019, NASA's OSIRIS-REx spacecraft performed another significant navigation maneuver—breaking its own world record for the closest orbit of a planetary body by a spacecraft. 29)
- The maneuver began the mission's new phase, known as Orbital B, and placed the spacecraft in an orbit 680 m above the surface of asteroid Bennu. The previous record—also set by the OSIRIS-REx spacecraft—was approximately 1.3 km above the surface.
Figure 10: Altitude comparison between Orbital A and B phase orbits (image credit: University of Arizona/Heather Roper)
- Upon arrival at Bennu, the team observed particles ejecting into space from the asteroid's surface. To better understand why this is occurring, the first two weeks of Orbital B will be devoted to observing these events by taking frequent images of the asteroid's horizon. For the remaining five weeks, the spacecraft will map the entire asteroid using most of its onboard science instruments: the OSIRIS-REx Laser Altimeter (OLA) will produce a full terrain map; PolyCam will form a high-resolution, global image mosaic; and the OSIRIS-REx Thermal Emission Spectrometer (OTES) and the REgolith X-ray Imaging Spectrometer (REXIS) will produce global maps in the infrared and X-ray bands. All of these measurements are essential for selecting the best sample collection site on Bennu's surface.
- OSIRIS-REx will remain in Orbital B until the second week of August, when it will transition to the slightly higher Orbital C for additional particle observations. During Orbital C, the spacecraft will be approximately 1.3 km above the asteroid's surface.
- The OSIRIS-REx team will also use data collected from Orbital B phase to assess the safety and sample-ability (the likelihood that a sample can be collected) of each potential sample collection site. The team will then choose four possible sample sites to be thoroughly evaluated this fall during the Reconnaissance phase of the mission. Data from the Reconnaissance phase will be used to evaluate the candidate sites for further down-selection, as well as provide the closeup imaging required to map the features and landmarks necessary for the spacecraft's autonomous navigation to the asteroid's surface.
- Several safety requirements must be considered before sample collection. For instance, any candidate site must be clear enough of large rocks or boulders so that the spacecraft can navigate to the surface without encountering dangerous terrain. Additionally, to keep OSIRIS-REx upright during sample collection, the chosen site can't be tilted too much compared to the sampling arm. Bennu's unexpectedly rocky surface has made it more challenging than originally predicted to identify sites that meet both of these safety requirements. In response, the team is evaluating both spacecraft and navigation performance capabilities, which will likely enable greater precision guidance to target more confined sites.
• April 29, 2019: The study of a tiny grain of stardust - older than our solar system - is shining new light on how planetary systems are formed. The microbe-sized extraterrestrial particle, which originated from a nova explosion more than 4.5 billion years ago, was discovered inside a meteorite collected in Antarctica by the National Aeronautics and Space Administration (NASA). 30)
Figure 11: Researchers found a grain of stardust (inset image) that survived the formation of our solar system. The carbon-rich graphite grain (red) revealed an embedded speck (blue) of oxygen-rich material (image credit: UT, illustration by Heather Roper/University of Arizona)
- Alongside planetary scientists at the University of Arizona (UA), the grain was studied last year at the atomic level by Associate Professor Jane Howe of the Faculty of Applied Science & Engineering while she was a senior scientist at Hitachi High Technologies.
- "This grain is presolar," says Howe. "It originated before the formation of the sun. It's just amazing to analyze such an anomaly."
- Using advanced ion and electron microscopes, Howe and the researchers observed the arrangement of carbon atoms and its variants, known as carbon isotope anomalies, and discovered the presolar graphite grain contained oxygen-rich silicates – something they did not expect to see.
- The researchers' observation gives new insights into the conditions of a dying star. It also contradicts the scientific hypothesis that the two types of stardust material, oxygen- and carbon-rich – which are presolar building blocks in the formation of a solar system – could not form in the same nova outburst, under the same conditions.
- The international collaboration, which includes Howe, planetary scientists, astronomers and material scientists at the University of Arizona, Washington University in St. Louis, Polytechnic University of Catalonia in Spain, and Hitachi High Technologies in the U.S. and Japan, published their findings today in Nature Astronomy. 31)
Figure 12: Dr. Pierre Haenecour (left) of the Lunar and Planetary Laboratory at the University of Arizona and U of T Associate Professor Jane Howe, analyze images of stardust particles with Hitachi's SU9000 low-voltage STEM/SEM electron microscope (photo courtesy of Maria Schuchardt, University of Arizona)
- "Sometimes research is about satisfying your curiosity. One of the greatest curiosities is how the universe was formed and how life started," says Howe. "And this weirdo particle showed us something we didn't know before."
- Howe, who joined U of T Engineering in January, is currently using her electron microscopy expertise to study materials to advance renewable energy, and also plans to expand her work to include meteoritic materials science research.
- "I thought this research project was really exciting, and I'm a curious person by nature. At the time, it was just part of my job assignment, but now it's starting to become part of my research portfolio," says Howe.
- She hopes to further her collaboration with researchers at the University of Arizona. In addition, she recently began a collaboration with Kim Tait, an associate professor in the department of Earth sciences who is also the senior curator of mineralogy at the Royal Ontario Museum, to study its collection of meteorites.
- And, in September 2023 when the University of Arizona-led NASA OSIRIS-Rex mission returns to Earth after taking samples of carbon-rich asteroid, Bennu, Howe will be among the team of Canadian researchers to analyze its samples.
- "This kind of research, it's part of a much larger debate of how life started on Earth. We all care about who we are and where we came from," says Howe."I'm so excited to be part of advancing our knowledge in this."
• April 4, 2019: From Feb. 12 through 17, OLA (OSIRIS-REx Laser Altimeter) made more than 11 million measurements of the distance between OSIRIS-REx and Bennu's surface as the spacecraft flew less than 2 km above the surface – the closest orbit ever achieved by spacecraft. OLA obtained these measurements by firing laser pulses at Bennu and measuring the amount of time it takes for the light to bounce off the asteroid's surface and return to the instrument. That time measurement is then translated into altitude data. Using this data, the OLA team created the 3-D model of Bennu's surface. The colors represent the distance from the center of Bennu: dark blue areas lie approximately 60 meters lower than peaks indicated in red. Some parts of the asteroid have not yet been measured, which creates gaps in the image. OLA will take nearly a billion more measurements throughout 2019 to complete the first-ever high-resolution 3D lidar map of a near-Earth asteroid. 32)
Figure 13: This three-dimensional view of asteroid Bennu was created by the OSIRIS-REx Laser Altimeter (OLA), contributed by the Canadian Space Agency, on NASA's OSIRIS-REx spacecraft (image credit: NASA/University of Arizona/CSA/York/MDA)
• March 19, 2019: NASA's OSIRIS-REx spacecraft made the first-ever close-up observations of particle plumes erupting from an asteroid's surface. Bennu also revealed itself to be more rugged than expected, challenging the mission team to alter its flight and sample collection plans, due to the rough terrain. 33)
- Asteroid Bennu is the target of NASA's OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) mission, which began orbiting the asteroid on 31 December 2018. Bennu, which is only slightly wider than the height of the Empire State Building, may contain unaltered material from the very beginning of our solar system.
- "The discovery of plumes is one of the biggest surprises of my scientific career," said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. "And the rugged terrain went against all of our predictions. Bennu is already surprising us, and our exciting journey there is just getting started."
- Shortly after the discovery of the particle plumes on 6 January, the mission science team increased the frequency of observations, and subsequently detected additional particle plumes during the following two months. Although many of the particles were ejected clear of Bennu, the team tracked some particles that orbited Bennu as satellites before returning to the asteroid's surface.
- The OSIRIS-REx team initially spotted the particle plumes in images while the spacecraft was orbiting Bennu at a distance of about 1.6 km. Following a safety assessment, the mission team concluded the particles did not pose a risk to the spacecraft. The team continues to analyze the particle plumes and their possible causes.
- "The first three months of OSIRIS-REx's up-close investigation of Bennu have reminded us what discovery is all about — surprises, quick thinking, and flexibility," said Lori Glaze, acting director of the Planetary Science Division at NASA Headquarters in Washington. "We study asteroids like Bennu to learn about the origin of the solar system. OSIRIS-REx's sample will help us answer some of the biggest questions about where we come from."
- OSIRIS-REx launched in 2016 to explore Bennu, which is the smallest body ever orbited by spacecraft. Studying Bennu will allow researchers to learn more about the origins of our solar system, the sources of water and organic molecules on Earth, the resources in near-Earth space, as well as improve our understanding of asteroids that could impact Earth.
- The OSIRIS-REx team also didn't anticipate the number and size of boulders on Bennu's surface. From Earth-based observations, the team expected a generally smooth surface with a few large boulders. Instead, it discovered Bennu's entire surface is rough and dense with boulders.
- The higher-than-expected density of boulders means that the mission's plans for sample collection, also known as Touch-and-Go (TAG), need to be adjusted. The original mission design was based on a sample site that is hazard-free, with a radius of 25 m. However, because of the unexpectedly rugged terrain, the team hasn't been able to identify a site of that size on Bennu. Instead, it has begun to identify candidate sites that are much smaller in radius.
- The smaller sample site footprint and the greater number of boulders will demand more accurate performance from the spacecraft during its descent to the surface than originally planned. The mission team is developing an updated approach, called Bullseye TAG, to accurately target smaller sample sites.
- "Throughout OSIRIS-REx's operations near Bennu, our spacecraft and operations team have demonstrated that we can achieve system performance that beats design requirements," said Rich Burns, the project manager of OSIRIS-REx at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Bennu has issued us a challenge to deal with its rugged terrain, and we are confident that OSIRIS-REx is up to the task."
- The original, low-boulder estimate was derived both from Earth-based observations of Bennu's thermal inertia — or its ability to conduct and store heat — and from radar measurements of its surface roughness. Now that OSIRIS-REx has revealed Bennu's surface up close, those expectations of a smoother surface have been proven wrong. This suggests the computer models used to interpret previous data do not adequately predict the nature of small, rocky, asteroid surfaces. The team is revising these models with the data from Bennu.
- The OSIRIS-REx science team has made many other discoveries about Bennu in the three months since the spacecraft arrived at the asteroid, some of which were presented Tuesday at the 50th Lunar and Planetary Conference in Houston and in a special collection of papers issued by the journal Nature.
Figure 14: This view of asteroid Bennu ejecting particles from its surface on January 19 was created by combining two images taken on board NASA's OSIRIS-REx spacecraft. Other image processing techniques were also applied, such as cropping and adjusting the brightness and contrast of each image (image credit: NASA/Goddard/University of Arizona/Lockheed Martin)
- The team has directly observed a change in the spin rate of Bennu as a result of what is known as the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect. The uneven heating and cooling of Bennu as it rotates in sunlight is causing the asteroid to increase its rotation speed. As a result, Bennu's rotation period is decreasing by about one second every 100 years. Separately, two of the spacecraft's instruments, the MapCam color imager and the OSIRIS-REx Thermal Emission Spectrometer (OTES), have made detections of magnetite on Bennu's surface, which bolsters earlier findings indicating the interaction of rock with liquid water on Bennu's parent body.
- Goddard provides overall mission management, systems engineering, and the safety and mission assurance for OSIRIS-REx. Dante Lauretta of the University of Arizona, Tucson, is the principal investigator, and the University of Arizona also leads the science team and the mission's science observation planning and data processing. Lockheed Martin Space in Denver built the spacecraft and is providing flight operations. Goddard and KinetX Aerospace are responsible for navigating the OSIRIS-REx spacecraft. OSIRIS-REx is the third mission in NASA's New Frontiers Program, which is managed by NASA's Marshall Space Flight Center in Huntsville, Alabama, for the agency's Science Mission Directorate in Washington.
• March 14, 2019: This trio of images acquired by NASA's OSIRIS-REx spacecraft shows a wide shot and two close-ups of a region in asteroid Bennu's northern hemisphere. The wide-angle image (left), obtained by the spacecraft's MapCam camera, shows a 180 m wide area with many rocks, including some large boulders, and a "pond" of regolith that is mostly devoid of large rocks. The two closer images, obtained by the high-resolution PolyCam camera, show details of areas in the MapCam image, specifically a 15 m boulder (top) and the regolith pond (bottom). The PolyCam frames are 31 m across and the boulder depicted is approximately the same size as a humpback whale. 34)
Figure 15: The images were taken on 25 February while the spacecraft was in orbit around Bennu, approximately 1.8 km from the asteroid's surface (image credit: NASA/Goddard/University of Arizona)
- The observation plan for this day provided for one MapCam and two PolyCam images every 10 minutes, allowing for this combination of context and detail of Bennu's surface.
• February 20, 2019: During the mission's orbital phase, OSIRIS-REx circles the asteroid near Bennu's terminator line. While this positioning helps maintain the spacecraft in a stable orbit, the half-light/half-dark view of the asteroid creates challenging conditions for science imaging. 35)
Figure 16: This image of OCAMS (MapCam) shows a region near asteroid Bennu's north pole on the terminator line between the asteroid's day and night sides. The OSIRIS-REx spacecraft's MapCam camera obtained the image on Feb. 20 while in orbit around the asteroid from a distance of 1.8 km. At this distance, each pixel covers approximately 12 cm of Bennu's surface. The largest boulder, located slightly left of the center, measures around 16 meters across, which, for scale, is the length of the trailer on a semi-truck (image credit: NASA/Goddard/University of Arizona)
• December 31, 2018: At 2:43 p.m. EST on December 31, while many on Earth prepared to welcome the New Year, NASA's OSIRIS-REx spacecraft, 70 million miles (110 million kilometers) away, carried out a single, eight-second burn of its thrusters – and broke a space exploration record. The spacecraft entered into orbit around the asteroid Bennu, and made Bennu the smallest object ever to be orbited by a spacecraft. 36)
Figure 17: Artist's concept of OSIRIS-REx entering orbit at asteroid Bennu (image credit: Heather Roper/University of Arizona)
- "The team continued our long string of successes by executing the orbit-insertion maneuver perfectly," said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. "With the navigation campaign coming to an end, we are looking forward to the scientific mapping and sample site selection phase of the mission."
- The gravity of Bennu is so small, forces like solar radiation and thermal pressure from Bennu's surface become much more relevant and can push the spacecraft around in its orbit much more than if it were orbiting around Earth or Mars, where gravity is by far the most dominant force.
- Inching around the asteroid at a snail's pace, OSIRIS-REx's first orbit marks a leap for humankind. Never before has a spacecraft from Earth circled so close to such a small space object – one with barely enough gravity to keep a vehicle in a stable orbit.
- Now, the spacecraft will circle Bennu about 1.75 km from its center, closer than any other spacecraft has come to its celestial object of study. (Previously the closest orbit of a planetary body was in May 2016, when the Rosetta spacecraft orbited about 7 km from the center of the comet 67P/Churyumov-Gerasimenko. The comfortable distance is necessary to keep the spacecraft locked to Bennu, which has a gravity force only 5-millionths as strong as Earth's. The spacecraft is scheduled to orbit Bennu through mid-February at a leisurely 62 hours per orbit.
- Now that the OSIRIS-REx spacecraft is closer to Bennu, physical details about the asteroid will leap into sharper focus, and the spacecraft's tour of this rubble pile of primordial debris will become increasingly detailed and focused.
- "Our orbit design is highly dependent on Bennu's physical properties, such as its mass and gravity field, which we didn't know before we arrived," said OSIRIS-REx's flight dynamics system manager Mike Moreau, who is based at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Up until now, we had to account for a wide variety of possible scenarios in our computer simulations to make sure we could safely navigate the spacecraft so close to Bennu. As the team learned more about the asteroid, we incorporated new information to hone in on the final orbit design," he said.
- The simulations have played a critical role. The OSIRIS-REx mission, after all, was designed based on complex computer programs that predicted — quite accurately, as it turns out — the properties of Bennu and how the spacecraft's trajectory would behave. This diligent preparation allowed the team to navigate the vehicle safely to Bennu in December and put some questions to rest (there are, indeed, signs of ancient water preserved in Bennu's rocks) and to fly over its poles and equator in a preliminary survey that led to some surprises (Bennu has many large boulders).
- Having completed the preliminary survey of Bennu with a flyby of its south pole on 16 December, the spacecraft moved to a safe 50 km away from the asteroid to give the navigation team a chance to regroup and prepare for orbit insertion. Next, Lockheed Martin engineers programmed the spacecraft to begin moving back to a position about 15 km over Bennu's north pole to prepare for three burns of its thrusters over the course of 10 days that would place the spacecraft into orbit.
- Even though OSIRIS-REx is in the most stable orbit possible, Bennu's gravitational pull is so tenuous that keeping the spacecraft safe will require occasional adjustments, said Dan Wibben, OSIRIS-REx maneuver and trajectory design lead at KinetX Aerospace in Simi Valley, California.
- The OSIRIS-REx navigation team will use "trim" maneuvers to slightly thrust the spacecraft in one direction or another to correct its orbit and counter these small forces. If the spacecraft drifts away from Bennu, or some other problem forces it into safe mode, it has been programmed to fly away from the asteroid to stay safe from impact.
- "It's simple logic: always burn toward the Sun if something goes wrong," said Coralie Adam, OSIRIS-REx lead optical navigation engineer at KinetX. Engineers can navigate the spacecraft back into orbit if it drifts away, Adam said, though that's unlikely to happen.
Figure 18: This series of images were taken over the course of about four hours and 19 minutes on 4 December 2018, as OSIRIS-REx made its first pass over Bennu's north pole (image credit: NASA/Goddard, University of Arizona)
- The navigation and spacecraft operations teams are focused on the first orbital phase. Their primary goal is to transition away from star-based navigation, which allowed the team to locate the spacecraft based on pictures of the star formations around it taken by the cameras onboard. Navigators use methods like this since there is no GPS in deep space and we can't see the spacecraft from Earth-based telescopes. From this point forward, though, the OSIRIS-REx team will rely on landmarks on Bennu's surface to track OSIRIS-REx, a more precise technique that will ultimately guide them to a sample-collection site clear of boulders and large rocks, said Adam.
- "After conducting a global imaging and mapping campaign during our recent preliminary survey phase, the science team has created 3-D models of Bennu's terrain that we're going to begin using for navigation around the asteroid," she said.
- Another critical objective of this orbital phase, Adam said, is to get a better handle on Bennu's mass and gravity, features that will influence the planning of the rest of the mission, notably the short touchdown on the surface for sample collection in 2020. In the case of Bennu, scientists can only measure these features by getting OSIRIS-REx very close to the surface to see how its trajectory bends from Bennu's gravitational pull.
- "The Orbital A phase will help improve our detailed models for Bennu's gravity field, thermal properties, orientation, and spin rate," said Wibben. "This, in turn, will allow us to refine our trajectory designs for the even more challenging flight activities we will perform in 2019."
- The 31 December maneuver to place the spacecraft into orbit about Bennu is the first of many exciting navigation activities planned for the mission. The OSIRIS-REx team will resume science operations in late February. At that point, the spacecraft will perform a series of close flybys of Bennu for several months to take high-resolution images of every square inch of the asteroid to help select a sampling site. During the summer of 2020, the spacecraft will briefly touch the surface of Bennu to retrieve a sample. The OSIRIS-REx mission is scheduled to deliver the sample to Earth in September 2023.
• December 10, 2018: Recently analyzed data from NASA's OSIRIS-REx mission has revealed water locked inside the clays that make up its scientific target, the asteroid Bennu. 37)
- During the mission's approach phase, between mid-August and early December, the spacecraft traveled 2.2 million km on its journey from Earth to arrive at a location 19 km from Bennu on 3 December. During this time, the science team on Earth aimed three of the spacecraft's instruments towards Bennu and began making the mission's first scientific observations of the asteroid.
- Data obtained from the spacecraft's two spectrometers, the OVIRS (OSIRIS-REx Visible and Infrared Spectrometer) and the OTES (OSIRIS-REx Thermal Emission Spectrometer), reveal the presence of molecules that contain oxygen and hydrogen atoms bonded together, known as "hydroxyls." The team suspects that these hydroxyl groups exist globally across the asteroid in water-bearing clay minerals, meaning that at some point, Bennu's rocky material interacted with water. While Bennu itself is too small to have ever hosted liquid water, the finding does indicate that liquid water was present at some time on Bennu's parent body, a much larger asteroid.
- "The presence of hydrated minerals across the asteroid confirms that Bennu, a remnant from early in the formation of the solar system, is an excellent specimen for the OSIRIS-REx mission to study the composition of primitive volatiles and organics," said Amy Simon, OVIRS deputy instrument scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "When samples of this material are returned by the mission to Earth in 2023, scientists will receive a treasure trove of new information about the history and evolution of our solar system."
- Additionally, data obtained from OCAMS (OSIRIS-REx Camera Suite) corroborate ground-based telescopic observations of Bennu and confirm the original model developed in 2013 by OSIRIS-REx Science Team Chief Michael Nolan and collaborators. That model closely predicted the asteroid's actual shape, with Bennu's diameter, rotation rate, inclination, and overall shape presented almost exactly as projected.
- One outlier from the predicted shape model is the size of the large boulder near Bennu's south pole. The ground-based shape model calculated this boulder to be at least 10 meters in height. Preliminary calculations from OCAMS observations show that the boulder is closer to 50 meters in height, with a width of approximately 55 meters.
- Bennu's surface material is a mix of very rocky, boulder-filled regions and a few relatively smooth regions that lack boulders. However, the quantity of boulders on the surface is higher than expected. The team will make further observations at closer ranges to more accurately assess where a sample can be taken on Bennu to later be returned to Earth.
- "Our initial data show that the team picked the right asteroid as the target of the OSIRIS-REx mission. We have not discovered any insurmountable issues at Bennu so far," said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. "The spacecraft is healthy and the science instruments are working better than required. It is time now for our adventure to begin."
- The mission currently is performing a preliminary survey of the asteroid, flying the spacecraft in passes over Bennu's north pole, equator, and south pole at ranges as close as 7 km to better determine the asteroid's mass. The mission's scientists and engineers must know the mass of the asteroid in order to design the spacecraft's insertion into orbit because mass affects the asteroid's gravitational pull on the spacecraft. Knowing Bennu's mass will also help the science team understand the asteroid's structure and composition.
- This survey also provides the first opportunity for OLA (OSIRIS-REx Laser Altimeter), an instrument contributed by the Canadian Space Agency, to make observations, now that the spacecraft is in proximity to Bennu.
- The spacecraft's first orbital insertion is scheduled for 31 December, and OSIRIS-REx will remain in orbit until mid-February 2019, when it exits to initiate another series of flybys for the next survey phase. During the first orbital phase, the spacecraft will orbit the asteroid at a range of 1.4-2.0 km from the center of Bennu — setting new records for the smallest body ever orbited by a spacecraft and the closest orbit of a planetary body by any spacecraft.
Figure 19: This mosaic image of asteroid Bennu is composed of 12 PolyCam images collected on 2 December by the OSIRIS-REx spacecraft from a range of 24 km (image credit: NASA/Goddard/University of Arizona)
• December 6, 2018: On 3 December, after traveling billions of kilometers from Earth, NASA's OSIRIS-REx spacecraft reached its target, Bennu, and kicked off a nearly two-year, up-close investigation of the asteroid. It will inspect nearly every square inch of this ancient clump of rubble left over from the formation of our solar system. Ultimately, the spacecraft will pick up a sample of pebbles and dust from Bennu's surface and deliver it to Earth in 2023. 38)
- Generations of planetary scientists will get to study pieces of the primitive materials that formed our cosmic neighborhood and to better understand the role asteroids may have played in delivering life-forming compounds to planets and moons.
Figure 20: This artist's concept shows the OSIRIS-REx (Origins Spectral Interpretation Resource Identification Security - Regolith Explorer) spacecraft contacting the asteroid Bennu with the TAGSAM (Touch-And-Go Sample Arm Mechanism). The mission aims to return a sample of Bennu's surface coating to Earth for study as well as return detailed information about the asteroid and it's trajectory (image credit: NASA's Goddard Space Flight Center)
- But it's not just history that the mission to Bennu will help uncover. Scientists studying the rock through OSIRIS-REx's instruments in space will also shape our future. As they collect the most detailed information yet about the forces that move asteroids, experts from NASA's Planetary Defense Coordination Office, who are responsible for detecting potentially hazardous asteroids, will improve their predictions of which ones could be on a crash-course with our planet.
Here is how the OSIRIS-REx mission will support this work:
- How scientists predict Bennu's whereabouts: About 500 m in size, Bennu is large enough to reach Earth's surface; many smaller space objects, in contrast, burn up in our atmosphere. If it impacted Earth, Bennu would cause widespread damage. Asteroid experts at the Center for Near-Earth Object Studies (CNEOS) at NASA's Jet Propulsion Laboratory in Pasadena, California, project that Bennu will come close enough to Earth over the next century to pose a 1 in 2,700 chance of impacting it between 2175 and 2196. Put another way, those odds mean there is a 99.963 percent chance the asteroid will miss the Earth. Even so, astronomers want to know exactly where Bennu is located at all times.
- Astronomers have estimated Bennu's future trajectory after observing it several times since it was discovered in 1999. They've turned their optical, infrared and radio telescopes toward the asteroid every time it came close enough to Earth, about every six years, to deduce features such as its shape, rotation rate and trajectory.
- "We know within a few kilometers where Bennu is right now," said Steven Chesley, senior research scientist at CNEOS and an OSIRIS-REx team member whose job it is to predict Bennu's future trajectory.
- Why Bennu's future trajectory predictions get fuzzy: Scientists have estimated Bennu's trajectory around the Sun far into the future. Their predictions are informed by ground observations and mathematical calculations that account for the gravitational nudging of Bennu by the Sun, the Moon, planets and other asteroids, plus non-gravitational factors.
- Given these parameters, astronomers can predict the next four exact dates (in September of 2054, 2060, 2080 and 2135) that Bennu will come within 5 million miles (7.5 million kilometers or .05 astronomical units) of Earth. That's close enough that Earth's gravity will slightly bend Bennu's orbital path as it passes by. As a result, the uncertainty about where the asteroid will be each time it loops back around the Sun will grow, causing predictions about Bennu's future orbit to become increasingly hazy after 2060.
- In 2060, Bennu will pass Earth at about twice the distance from here to the Moon. But it could pass at any point in a 30-kilometer window of space. A very small difference in position within that window will get magnified enormously in future orbits and make it increasingly hard to predict Bennu's trajectory.
- As a result, when this asteroid comes back near Earth in 2080, according to Chesley's calculations, the best window we can get on its whereabouts is nearly 9,000 miles (14,000 kilometers) wide. By 2135, when Bennu's shifted orbit is expected to bring it closer than the Moon, its flyby window grows wider, to nearly 100,000 miles (160,000 kilometers). This will be Bennu's closest approach to Earth over the five centuries for which we have reliable calculations.
- "Right now, Bennu has the best orbit of any asteroid in our database," Chesley said. "And yet, after that encounter in 2135, we really can't say exactly where it is headed."
- There's another phenomenon nudging Bennu's orbit and muddying future impact projections. It's called the Yarkovsky effect. Having nothing to do with gravity, the Yarkovsky effect sways Bennu's orbit because of heat from the Sun.
- "There are a lot of factors that might affect the predictability of Bennu's trajectory in the future, but most of them are relatively small," says William Bottke, an asteroid expert at the Southwest Research Institute in Boulder, Colorado, and a participating scientist on the OSIRIS-REx mission. "The one that's most sizeable is Yarkvovsky."
- This heat nudge was named after the Polish civil engineer who first described it in 1901: Ivan Osipovich Yarkovsky. He suggested that sunlight warms one side of a small, dark asteroid and some hours later radiates that heat away as the asteroid rotates its hot side into cold darkness. This thrusts the rock pile a bit, either toward the Sun or away from it, depending on the direction of its rotation.
- In Bennu's case, astronomers have calculated that the Yarkovsky effect has shifted its orbit about 0.18 miles (284 meters) per year toward the Sun since 1999. In fact, it helped deliver Bennu to our part of the solar system, in the first place, from the asteroid belt between Mars and Jupiter over billions of years. Now, Yarkovsky is complicating our efforts to make predictions about Bennu's path relative to Earth.
- Getting face-to-face with the asteroid will help: The OSIRIS-REx spacecraft will use its suite of instruments to transmit radio tracking signals and capture optical images of Bennu that will help NASA scientists determine its precise position in the solar system and its exact orbital path. Combined with existing, ground-based observations, the space measurements will help clarify how Bennu's orbit is changing over time.
- Additionally, astronomers will get to test their understanding of the Yarkovksy effect on a real-life asteroid for the first time. They will instruct the spacecraft to follow Bennu in its orbit about the Sun for about two years to see whether it's moving along an expected path based on gravity and Yarkovsky theories. Any differences between the predictions and reality could be used to refine models of the Yarkovsky effect.
- But even more significant to understanding Yarkovsky better will be the thermal measurements of Bennu. During its mission, OSIRIS-REx will track how much solar heat radiates off the asteroid, and where on the surface it's coming from-data that will help confirm and refine calculations of the Yarkovsky effect on asteroids.
- The spacecraft also will address some open questions about the Yarkovsky theory. One of them, said Chesley, is how do boulders and craters on the surface of an asteroid change the way photons scatter off of it as it cools, carrying away momentum from the hotter side and thereby nudging the asteroid in the opposite direction? OSIRIS-REx will help scientists understand by mapping the rockiness of Bennu's surface.
- "We know surface roughness is going to affect the Yarkovsky effect; we have models" said Chesley. "But the models are speculative. No one has been able to test them."
- After the OSIRIS-REx mission, Chesley said, NASA's trajectory projections for Bennu will be about 60 times better than they are now.
• December 3, 2018: NASA's OSIRIS-REx spacecraft completed its 1.2 billion-mile (2 billion-kilometer) journey to arrive at the asteroid Bennu Monday. The spacecraft executed a maneuver that transitioned it from flying toward Bennu to operating around the asteroid. 39)
- Now, at about 19 km from Bennu's Sun-facing surface, OSIRIS-REx will begin a preliminary survey of the asteroid. The spacecraft will commence flyovers of Bennu's north pole, equatorial region, and south pole, getting as close as nearly 7 km above Bennu during each flyover.
- The primary science goals of this survey are to refine estimates of Bennu's mass and spin rate, and to generate a more precise model of its shape. The data will help determine potential sites for later sample collection.
- OSIRIS-REx's mission will help scientists investigate how planets formed and how life began, as well as improve our understanding of asteroids that could impact Earth. Asteroids are remnants of the building blocks that formed the planets and enabled life. Those like Bennu contain natural resources, such as water, organics and metals. Future space exploration and economic development may rely on asteroids for these materials.
- "As explorers, we at NASA have never shied away from the most extreme challenges in the solar system in our quest for knowledge," said Lori Glaze, acting director for NASA's Planetary Science Division. "Now we're at it again, working with our partners in the U.S. and Canada to accomplish the Herculean task of bringing back to Earth a piece of the early solar system."
- The mission's navigation team will use the preliminary survey of Bennu to practice the delicate task of navigating around the asteroid. The spacecraft will enter orbit around Bennu on 31 December — thus making Bennu, which is only about 492 m across — or about the length of five football fields — the smallest object ever orbited by a spacecraft. It's a critical step in OSIRIS-REx's years-long quest to collect and eventually deliver at least 60 grams of regolith — dirt and rocks — from Bennu to Earth.
- Starting in October, OSIRIS-REx performed a series of braking maneuvers to slow the spacecraft down as it approached Bennu. These maneuvers also targeted a trajectory to set up Monday's maneuver, which initiates the first north pole flyover and marks the spacecraft's arrival at Bennu.
- "The OSIRIS-REx team is proud to cross another major milestone off our list — asteroid arrival," said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. "Initial data from the approach phase show this object to have exceptional scientific value. We can't wait to start our exploration of Bennu in earnest. We've been preparing for this moment for years, and we're ready."
- OSIRIS-REx mission marks many firsts in space exploration. It will be the first U.S. mission to carry samples from an asteroid back to Earth and the largest sample returned from space since the Apollo era. It's the first to study a primitive B-type asteroid, which is an asteroid that's rich in carbon and organic molecules that make up life on Earth. It is also the first mission to study a potentially hazardous asteroid and try to determine the factors that alter their courses to bring them close to Earth.
- "During our approach toward Bennu, we have taken observations at much higher resolution than were available from Earth," said Rich Burns, the project manager of OSIRIS-REx at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "These observations have revealed an asteroid that is both consistent with our expectations from ground-based measurements and an exceptionally interesting small world. Now we embark on gaining experience flying our spacecraft about such a small body."
- When OSIRIS-REx begins to orbit Bennu at the end of this month, it will come close to approximately 1.25 km to its surface. In February 2019, the spacecraft begins efforts to globally map Bennu to determine the best site for sample collection. After the collection site is selected, the spacecraft will briefly touch the surface of Bennu to retrieve a sample. OSIRIS-REx is scheduled to return the sample to Earth in September 2023.
- Goddard provides overall mission management, systems engineering and the safety and mission assurance for OSIRIS-REx. Dante Lauretta of the University of Arizona, Tucson, is the principal investigator, and the University of Arizona also leads the science team and the mission's science observation planning and data processing. Lockheed Martin Space in Denver built the spacecraft and is providing flight operations. Goddard and KinetX Aerospace are responsible for navigating the OSIRIS-REx spacecraft. OSIRIS-REx is the third mission in NASA's New Frontiers Program, which is managed by NASA's Marshall Space Flight Center in Huntsville, Alabama for the agency's Science Mission Directorate in Washington.
Figure 21: This image of Bennu was taken by the OSIRIS-REx spacecraft from a distance of around 80 km (image credits: NASA/Goddard/University of Arizona)
• November 22, 2018: Every so often, we get a reminder of just how small our planet is in the context of the vast cosmos. We also get a reminder of the great things we can achieve as a species that loves to explore. 40)
Figure 22: On 17 January 2018, the NavCam-1 camera of OSIRIS-REx looked homeward while making its way to the asteroid Bennu (image credit: NASA Goddard/University of Arizona/Lockheed Martin, story by Mike Carlowicz based on NASA news reports)
- In January 2018, one of our latest space explorers sent back a distant view of our tiny, beautiful home. The OSIRIS-REx (Origins Spectral Interpretation Resource Identification Security–Regolith Explorer) spacecraft looked back across a distance of nearly 64 million kilometers and saw the Earth and Moon as a few pixels of reflected sunlight. The image was acquired by the NavCam1 imager on January 17, 2018, as part of an engineering test. The spacecraft was moving away from Earth at a speed of 8.5 km/s (19,000 miles per hour).
- Earth is the largest, brightest spot in the center of the image, with the smaller, dimmer Moon to the right. Some stars are faintly visible, such as the five stars comprising the head of Cetus, which surround the Earth and Moon in this view.
- OSIRIS-REx was launched in September 2016, and it has spent the past two years hurtling across the solar system toward Bennu, a near-Earth asteroid formerly known as 1999 RQ36. The spacecraft is scheduled to arrive within 20 km of the surface on December 3, 2018, to begin a closeup survey. It will eventually grab a sample of the asteroid and carry it back to Earth by 2023. Through this expedition, scientists plan to investigate how planets formed and how life began, while also trying to learn more about asteroids that could impact Earth.
- Astronomer and author Phil Plait found poetry in the image and in the plight of OSIRIS-REx. "We send a piece of ourselves, a proxy, out into the night to see some of these points of light up close so that we may understand them. ... And in reality, in truth and in fact, this is just an extension of our standing outside on a clear night and watching the stars. ... It's easy to sit at a computer and forget that what you're seeing is real, it's a place, just as real and substantive as the world beneath your feet. They [the spacecraft] see what we see, and we see what they see, because they are us. An extension of us, but us nonetheless. When we send spacecraft into the solar system, we are sending pieces of ourselves."
• November 2018: Since OSIRIS-REx began Approach Phase and captured its first glimpse of asteroid Bennu in August, the spacecraft has been imaging the asteroid at higher and higher resolution. This view, captured with PolyCam on Nov. 2, shows Bennu rotating for one full revolution over the course of about four hours. At the time, Bennu was approximately 122 miles (197 km) from the spacecraft and appeared about 200 pixels wide in PolyCam's frame. 41)
Figure 23: Bennu full rotation at 200 pixels using OCAMS (PolyCam), image credit: NASA/Goddard/University of Arizona
• On 14 November 2018, NASA's OSIRIS-REx spacecraft stretched out its robotic sampling arm for the first time in space. The arm, more formally known as the TAGSAM (Touch-and-Go Sample Acquisition Mechanism), is key to the spacecraft achieving the primary goal of the mission: returning a sample from asteroid Bennu in 2023. 42)
- As planned, engineers at Lockheed Martin commanded the spacecraft to move the arm through its full range of motion – flexing its shoulder, elbow, and wrist "joints." This long-awaited stretch, which was confirmed by telemetry data and imagery captured by the spacecraft's SamCam camera, demonstrates that the TAGSAM head is ready to collect a sample of loose dirt and rock (called regolith) from Bennu's surface.
Figure 24: Over the past month, the OSIRIS-REx team conducted a series of tests to ensure that TAGSAM, the spacecraft's sampling mechanism, is ready to collect a sample from Bennu in 2020. This rehearsal marked the first time since launch that the TAGSAM arm has moved through its full range of motion (image credit: NASA/Goddard/University of Arizona)
- "The TAGSAM exercise is an important milestone, as the prime objective of the OSIRIS-REx mission is to return a sample of Bennu to Earth," said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. "This successful test shows that, when the time comes, TAGSAM is ready to reach out and tag the asteroid."
Figure 25: This image shows the OSIRIS-REx TAGSAM sampling head extended from the spacecraft at the end of the TAGSAM arm. The image was obtained by the SamCam camera on 14 November 2018 as part of a visual checkout of the spacecraft's sample acquisition system. This is a rehearsal image for an observation that will be taken at Bennu during the moment of sample collection to help document the asteroid material collected in the TAGSAM head. There are two witness plate assemblies on the top perimeter of the TAGSAM head, one of which is entirely visible in this image. These witness plates record the deposition of material on the TAGSAM head over the duration of the mission, giving scientists a record of material on the TAGSAM head that is not from Bennu (image credit: NASA/Goddard/University of Arizona)
- Years of innovation: Lockheed Martin engineers spent more than a decade designing, building, and testing TAGSAM, which includes a 3.35 m arm with three articulating joints, a round sampler head at the end of the arm that resembles the air filter in a car, and three bottles of high-pressure nitrogen gas.
- This test deployment was a rehearsal for a date in mid-2020 when the spacecraft will unfold the TAGSAM arm again, slowly descend to Bennu's surface, and briefly touch the asteroid with the sampler head. A burst of nitrogen gas will stir up regolith on the asteroid's surface, which will be caught in the TAGSAM head. The TAG sequence will take about five seconds, after which the spacecraft will execute small maneuvers to carefully back away from Bennu. Afterward, SamCam will image the sampler head, as it did during the test deployment, to help confirm that TAGSAM collected at least 60 grams of regolith.
Figure 26: In mid-2020, the OSIRIS-REx spacecraft will use its TAGSAM device to stir up and collect a sample of loose material from asteroid Bennu's surface. That material will be returned to Earth for study in 2023 (video credit: NASA/Goddard/University of Arizona)
- The TAGSAM mechanism was designed for the key challenge unique to the OSIRIS-REx mission: collecting a sample from the smallest planetary body ever to be orbited by a spacecraft. "First-of-its-kind innovations like this one serve as the precursor for future missions to small bodies," said Sandy Freund, systems engineer manager and Lockheed Martin OSIRIS-REx MSA manager. "By proving out these technologies and techniques, we are going to be able to return the largest sample from space in half a century and pave the way for other missions."
- A month of testing: The unfolding of the TAGSAM arm was the latest and most significant step in a series of tests and check-outs of the spacecraft's sampling system, which began in October when OSIRIS-REx jettisoned the cover that protected the TAGSAM head during launch and the mission's outbound cruise phase. Shortly before the cover ejection, and again the day after, OSIRIS-REx performed two spins called Sample Mass Measurements. By comparing the spacecraft's inertial properties during these before-and-after spins, the team confirmed that the 1.21 kg cover was successfully ejected on Oct. 17.
- A week later, on Oct. 25, the Frangibolts holding the TAGSAM arm in place fired successfully, releasing the arm and allowing the team to move it into a parked position just outside its protective housing. After resting in this position for a few weeks, the arm was fully deployed into its sampling position, its joints were tested, and images were captured with SamCam. The spacecraft will execute two additional Sample Mass Measurements over the next two days. The mission team will use these spins as a baseline to compare with the results of similar spins that will be conducted after TAG in 2020 in order to confirm the mass of the sample collected.
- Although the sampling system was rigorously tested on Earth, this rehearsal marked the first time that the team has deployed TAGSAM in the micro-gravity environment of space.
- "The team is very pleased that TAGSAM has been released, deployed, and is operating as commanded through its full range of motion." said Rich Burns, OSIRIS-REx project manager at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "It has been restrained for over two years since launch, so it is gratifying to see it out of its shackles and performing well."
- OSIRIS-REx is scheduled to arrive at Bennu on Dec. 3. It will spend nearly one year surveying the asteroid with five scientific instruments so that the mission team can select a location that is safe and scientifically interesting to collect the sample.
- "Now that we have put TAGSAM through its paces in space and know it is ready to perform at Bennu, we can focus on the challenges of navigating around the asteroid and seeking out the best possible sample site," said Lauretta.
• October 29, 2018: NASA's OSIRIS-REx spacecraft executed its third Asteroid Approach Maneuver (AAM-3) today. The trajectory correction maneuver (TCM) thrusters fired in a series of two braking maneuvers designed to slow the spacecraft's speed relative to Bennu from approximately 11.7 mph (5.2 m/sec) to .24 mph (.11 m/sec). Due to constraints that science instruments not be pointed too closely to the Sun, this maneuver was designed as two separate burns of approximately 5.8 mph (2.6 m/sec) each, to accomplish a net change in velocity of around 11.5 mph (5.13 m/sec). The mission team will continue to examine telemetry and tracking data over the next week to verify the new trajectory. The maneuver targeted the spacecraft to fly through a corridor designed for the collection of high-resolution images that will be used to build a shape model of Bennu. 43)
• October 15, 2018: NASA's OSIRIS-REx spacecraft executed its second Asteroid Approach Maneuver (AAM-2) today. The spacecraft's main engine thrusters fired in a braking maneuver designed to slow the spacecraft's speed relative to Bennu from 315 mph (141 m/sec) to 11.8 mph (5.2 m/sec). Likewise, the spacecraft's approach speed dropped from nearly 7,580 miles (12,200 km) to 280 miles (450 km) per day. The mission team will continue to examine telemetry and tracking data and will have more information over the next week. This burn marked the last planned use of the spacecraft's main engines prior to OSIRIS-REx's departure from Bennu in March 2021. 44)
Figure 27: Illustration of NASA's OSIRIS-REx spacecraft during a burn of its main engine (image credit: University of Arizona)
• October 01, 2018: NASA's OSIRIS-REx spacecraft executed its first Asteroid Approach Maneuver (AAM-1) today putting it on course for its scheduled arrival at the asteroid Bennu in December. The spacecraft's main engine thrusters fired in a braking maneuver designed to slow the spacecraft's speed relative to Bennu from approximately 1,100 mph (491 m/s) to 313 mph (140 m/s). The mission team will continue to examine telemetry and tracking data as they become available and will have more information on the results of the maneuver over the next week. 45)
- During the next six weeks, the OSIRIS-REx spacecraft will continue executing the series of asteroid approach maneuvers designed to fly the spacecraft through a precise corridor during its final slow approach to Bennu. The last of these, AAM-4, scheduled for Nov. 12, will adjust the spacecraft's trajectory to arrive at a position 12 miles (20 km) from Bennu on Dec. 3. After arrival, the spacecraft will initiate asteroid proximity operations by performing a series of fly-bys over Bennu's poles and equator.
• September 26, 2018: Earlier this month, OSIRIS-REx searched the area around Bennu (circled in green) for any signs of dust plumes, which could present a hazard as the spacecraft approaches the asteroid. When OSIRIS-REx gets closer, it will also look for natural satellites (small moons) and conduct another dust plume search – but for now, the coast is clear.46)
Figure 28: This OCAMS (MapCam) image of the space surrounding asteroid Bennu was taken on Sept. 12, 2018, during the OSIRIS-REx mission's Dust Plume Search observation campaign. Bennu, circled in green, is approximately 1 million km from the spacecraft. The image was created by coadding 64 ten-second exposures (image credit: NASA/Goddard/University of Arizona)
- Successful cover opening for REXIS: On Sept. 14, the Frangibolt on the flight cover of REXIS – the student-built X-ray spectrometer that will help OSIRIS-REx map elements on Bennu's surface – fired as planned and opened the instrument's cover. With the cover open, REXIS now has a clear view of space and is ready to collect data as the spacecraft approaches asteroid Bennu.
- Another trip around the Sun: It's hard to believe that an entire year has passed since OSIRIS-REx made a flyby of home on 22 September 2017, cruising low over Antarctica and using Earth's gravity to boost itself upward onto asteroid Bennu's orbital plane. Since then, the spacecraft has traveled more than 900 million kilometers (560 million miles) and will arrive at Bennu later this year.
- Just under five year from now (on Sept. 24, 2023), the OSIRIS-REx Sample Return Capsule – carrying at least 60 grams of surface material from asteroid Bennu – will land at the Utah Test and Training Range west of Salt Lake City. That sample, most of which will be carefully preserved and stored at NASA's Johnson Space Center, will allow generations of scientists to study material from the early Solar System.
• August 24, 2018: After an almost two-year journey, NASA's asteroid sampling spacecraft, OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer), caught its first glimpse of asteroid Bennu last week and began the final approach toward its target. Kicking off the mission's asteroid operations campaign on 17 August, the spacecraft's PolyCam camera obtained the image from a distance of 2.2 million km. 47)
Figure 29: On Aug. 17, the OSIRIS-REx spacecraft obtained the first images of its target asteroid Bennu from a distance of 2.2 million km, or almost six times the distance between the Earth and Moon. This cropped set of five images was obtained by the PolyCam camera over the course of an hour for calibration purposes and in order to assist the mission's navigation team with optical navigation efforts. Bennu is visible as a moving object against the stars in the constellation Serpens (image credit: NASA/Goddard/University of Arizona)
- "Now that OSIRIS-REx is close enough to observe Bennu, the mission team will spend the next few months learning as much as possible about Bennu's size, shape, surface features, and surroundings before the spacecraft arrives at the asteroid," said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. "After spending so long planning for this moment, I can't wait to see what Bennu reveals to us."
- As OSIRIS-REx approaches the asteroid, the spacecraft will use its science instruments to gather information about Bennu and prepare for arrival. The spacecraft's science payload comprises the OCAMS camera suite (PolyCam, MapCam, and SamCam), the OTES thermal spectrometer, the OVIRS visible and infrared spectrometer, the OLA laser altimeter, and the REXIS x-ray spectrometer.
- During the mission's approach phase, OSIRIS-REx will:
a) regularly observe the area around the asteroid to search for dust plumes and natural satellites, and study Bennu's light and spectral properties;
b) execute a series of four asteroid approach maneuvers, beginning on Oct. 1, slowing the spacecraft to match Bennu's orbit around the Sun;
c) jettison the protective cover of the spacecraft's sampling arm in mid-October and subsequently extend and image the arm for the first time in flight; and
d) use OCAMS to reveal the asteroid's overall shape in late-October and begin detecting Bennu's surface features in mid-November.
- After arrival at Bennu, the spacecraft will spend the first month performing flybys of Bennu's north pole, equator and south pole, at distances ranging between 19 and 7 km from the asteroid. These maneuvers will allow for the first direct measurement of Bennu's mass as well as close-up observations of the surface. These trajectories will also provide the mission's navigation team with experience navigating near the asteroid.
- "Bennu's low gravity provides a unique challenge for the mission," said Rich Burns, OSIRIS-REx project manager at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "At roughly 500 m in diameter, Bennu will be the smallest object that any spacecraft has ever orbited."
- The spacecraft will extensively survey the asteroid before the mission team identifies two possible sample sites. Close examination of these sites will allow the team to pick one for sample collection, scheduled for early July 2020. After sample collection, the spacecraft will head back toward Earth before ejecting the Sample Return Capsule for landing in the Utah desert in Sept. 2023.
• August 20, 2018: After traveling for two years and millions of kilometers from Earth, the OSIRIS-REx probe is only a few months away from its destination: the intriguing asteroid Bennu. When it arrives in December, OSIRIS-REx will embark on a nearly two-year investigation of this clump of rock, mapping its terrain and finding a safe and fruitful site from which to collect a sample. 48)
- The spacecraft will briefly touch Bennu's surface around July 2020 to collect at least 60 grams of dirt and rocks. It might collect as much as 2 kg, which would be the largest sample by far gathered from a space object since the Apollo Moon landings. The spacecraft will then pack the sample into a capsule and travel back to Earth, dropping the capsule into Utah's west desert in 2023, where scientists will be waiting to collect it.
- This years-long quest for knowledge thrusts Bennu into the center of one of the most ambitious space missions ever attempted. But the humble rock is but one of about 780,000 known asteroids in our solar system. So why did scientists pick Bennu for this momentous investigation? Here are 10 reasons.
1) It's close to Earth: Unlike most other asteroids that circle the Sun in the asteroid belt between Mars and Jupiter, Bennu's orbit is close in proximity to Earth's, even crossing it. The asteroid makes its closest approach to Earth every 6 years. It also circles the Sun nearly in the same plane as Earth, which made it somewhat easier to achieve the high-energy task of launching the spacecraft out of Earth's plane and into Bennu's. Still, the launch required considerable power, so OSIRIS-REx used Earth's gravity to boost itself into Bennu's orbital plane when it passed our planet in September 2017.
2) It's the right size: Asteroids spin on their axes just like Earth does. Small ones, with diameters of 200 m or less, often spin very fast, up to a few revolutions per minute. This rapid spinning makes it difficult for a spacecraft to match an asteroid's velocity in order to touch down and collect samples. Even worse, the quick spinning has flung loose rocks and soil, material known as "regolith" — the stuff OSIRIS-REx is looking to collect — off the surfaces of small asteroids. Bennu's size, in contrast, makes it approachable and rich in regolith. It has a diameter of 492 m, which is a bit larger than the height of the Empire State Building in New York City, and rotating once every 4.3 hours.
Figure 30: Comparison of Bennu's size (image credit: NASA/GSFC)
3) It's really old: Bennu is a leftover fragment from the tumultuous formation of the solar system. Some of the mineral fragments inside Bennu could be older than the solar system. These microscopic grains of dust could be the same ones that spewed from dying stars and eventually coalesced to make the Sun and its planets nearly 4.6 billion years ago. But pieces of asteroids, called meteorites, have been falling to Earth's surface since the planet formed. So why don't scientists just study those old space rocks? Because astronomers can't tell (with very few exceptions) what kind of objects these meteorites came from, which is important context. Furthermore, these stones, that survive the violent, fiery decent to our planet's surface, get contaminated when they land in the dirt, sand, or snow. Some even get hammered by the elements, like rain and snow, for hundreds or thousands of years. Such events change the chemistry of meteorites, obscuring their ancient records.
4) It's well preserved: Bennu, on the other hand, is a time capsule from the early solar system, having been preserved in the vacuum of space. Although scientists think it broke off a larger asteroid in the asteroid belt in a catastrophic collision between about 1 and 2 billion years ago, and hurtled through space until it got locked into an orbit near Earth's, they don't expect that these events significantly altered it.
5) It might contain clues to the origin of life: Analyzing a sample from Bennu will help planetary scientists better understand the role asteroids may have played in delivering life-forming compounds to Earth. We know from having studied Bennu through Earth- and space-based telescopes that it is a carbonaceous, or carbon-rich, asteroid. Carbon is the hinge upon which organic molecules hang. Bennu is likely rich in organic molecules, which are made of chains of carbon bonded with atoms of oxygen, hydrogen, and other elements in a chemical recipe that makes all known living things. Besides carbon, Bennu also might have another component important to life: water, which is trapped in the minerals that make up the asteroid.
6) It contains valuable materials: Besides teaching us about our cosmic past, exploring Bennu close-up will help humans plan for the future. Asteroids are rich in natural resources, such as iron and aluminum, and precious metals, such as platinum. For this reason, some companies, and even countries, are building technologies that will one day allow us to extract those materials. More importantly, asteroids like Bennu are key to future, deep-space travel. If humans can learn how to extract the abundant hydrogen and oxygen from the water locked up in an asteroid's minerals, they could make rocket fuel. Thus, asteroids could one day serve as fuel stations for robotic or human missions to Mars and beyond. Learning how to maneuver around an object like Bennu, and about its chemical and physical properties, will help future prospectors.
7) It will help us better understand other asteroids: Astronomers have studied Bennu from Earth since it was discovered in 1999. As a result, they think they know a lot about the asteroid's physical and chemical properties. Their knowledge is based not only on looking at the asteroid, but also studying meteorites found on Earth, and filling in gaps in observable knowledge with predictions derived from theoretical models. Thanks to the detailed information that will be gleaned from OSIRIS-REx, scientists now will be able to check whether their predictions about Bennu are correct. This work will help verify or refine telescopic observations and models that attempt to reveal the nature of other asteroids in our solar system.
8) It will help us better understand a quirky solar force ...: Astronomers have calculated that Bennu's orbit has drifted about 280 m/year toward the Sun since it was discovered. This could be because of a phenomenon called the Yarkovsky effect, a process whereby sunlight warms one side of a small, dark asteroid and then radiates as heat off the asteroid as it rotates. The heat energy thrusts an asteroid either away from the Sun, if it has a prograde spin like Earth, which means it spins in the same direction as its orbit, or toward the Sun in the case of Bennu, which spins in the opposite direction of its orbit. OSIRIS-REx will measure the Yarkovsky effect from close-up to help scientists predict the movement of Bennu and other asteroids. Already, measurements of how this force impacted Bennu over time have revealed that it likely pushed it to our corner of the solar system from the asteroid belt.
9) ... and to keep asteroids at bay: One reason scientists are eager to predict the directions asteroids are drifting is to know when they're coming too-close-for-comfort to Earth. By taking the Yarkovsky effect into account, they've estimated that Bennu could pass closer to Earth than the Moon is in 2135, and possibly even closer between 2175 and 2195. Although Bennu is unlikely to hit Earth at that time, our descendants can use the data from OSIRIS-REx to determine how best to deflect any threatening asteroids that are found, perhaps even by using the Yarkovsky effect to their advantage.
10) It's a gift that will keep on giving: Samples of Bennu will return to Earth on September 24, 2023. OSIRIS-REx scientists will study a quarter of the regolith. The rest will be made available to scientists around the globe, and also saved for those not yet born, using techniques not yet invented, to answer questions not yet asked.
• July 3, 2018: New tracking data confirms that NASA's OSIRIS-REx spacecraft successfully completed its second Deep Space Maneuver (DSM-2) on June 28. The thruster burn put the spacecraft on course for a series of asteroid approach maneuvers to be executed this fall that will culminate with the spacecraft's scheduled arrival at asteroid Bennu on Dec. 3. 49)
- The DSM-2 burn, which employed the spacecraft's Trajectory Correction Maneuver (TCM) thruster set, resulted in a 37 miles per hour (16.7 m/s) change in the vehicle's velocity and consumed 12.8 kg of fuel.
- Tracking data from the Deep Space Network provided preliminary confirmation of the burn's execution, and the subsequent downlink of telemetry from the spacecraft shows that all subsystems performed as expected.
- DSM-2 was OSIRIS-REx's last deep space maneuver of its outbound cruise to Bennu. The next engine burn, Asteroid Approach Maneuver 1 (AAM-1), is scheduled for early October. AAM-1 is a major braking maneuver designed to slow the spacecraft's speed from approximately 506.2 to 144.4 m/s relative to Bennu and is the first of four asteroid approach maneuvers scheduled for this fall.
• May 2018: Asteroid Operations for the OSIRIS-REx mission begin in August 2018 – when the spacecraft will capture its first image of Bennu from a distance of two million km – and continue until March 2021 – when the spacecraft begins its return trip to Earth. During this period, OSIRIS-REx will survey and map Bennu, navigate in close proximity to the asteroid, and ultimately touch the surface for five seconds to gather a sample of the asteroid. 50)
- Asteroid Operations are divided into nine phases, which are each specifically designed to allow the mission team to build its knowledge of the asteroid, learn how to safely navigate the spacecraft in microgravity, and identify the best sample site (Figure 31).
Figure 31: OSIRIS-REx asteroid operations timeline (image credit: University of Arizona)
- Approach: Approach Phase begins on August 17, 2018, when the spacecraft is still two million km away from Bennu, and it continues until the spacecraft arrives at the asteroid. The primary goals of Approach are to visually locate Bennu for the first time, survey the surrounding area for potential hazards, and collect enough imagery of Bennu for scientists to generate a detailed shape model of the asteroid, assign a coordinate system, and understand its spin state.
- Preliminary Survey: Preliminary Survey Phase begins with the spacecraft's arrival at Bennu on December 3, 2018, and marks the first time that the OSIRIS-REx spacecraft will operate around the asteroid. The spacecraft will make a total of five passes over the north pole, equator, and south pole at a range of 7 km. The primary science goals of Preliminary Survey are to estimate Bennu's mass, refine the asteroid's spin state model, and generate a global shape model at a resolution of 75 cm.
- Orbital A: In Orbital A Phase, the spacecraft will be placed into a gravitationally-bound orbit around Bennu for the first time. There are no science requirements for Orbital A, as this phase is designed to provide the mission team with experience navigating in close proximity to a small body. The spacecraft will circle Bennu at a distance between 1.5 and 2.0 km, and each orbit will last about 50 hours.
- Detailed Survey: Baseball Diamond: The in-depth study of Bennu begins in earnest during Detailed Survey: Baseball Diamond Phase. OSIRIS-REx will make multiple passes around Bennu to produce the wide range of viewing angles necessary to fully observe the asteroid. The spacecraft will also use its OTES spectrometer to map the chemical composition of Bennu's entire surface. Images obtained during this phase will be of high enough resolution to produce digital terrain maps and global image mosaics for proposed sample sites. Bennu's terrain will be surveyed in bulk and sections will be classified as either "safe" or "unsafe," with the results visualized on a hazard map.
- Detailed Survey: Equatorial Stations: During Detailed Survey: Equatorial Stations Phase, the spacecraft will make scientific observations needed to help the team hone in on the best location on Bennu to collect a sample of regolith (loose surface material). To obtain this data, the spacecraft will execute a series of slews between Bennu's north and south poles while taking observations from seven different stations above the equator. These data will be studied to understand the geology of Bennu. The spacecraft will also conduct searches for dust and gas plumes.
- Orbital B: At the end of Detailed Survey, the spacecraft will enter a close orbit (with a radius of 1 km) around Bennu and begin Orbital B Phase. This phase marks the closest that a spacecraft has ever orbited around a small body. The primary science activities for this phase are global mapping of Bennu, the development of shape modeling based on OLA data, and the execution of a Radio Science experiment. These data are used to evaluate potential sample sites for three key elements: safety, sampleability and science value. Orbital B concludes with the team narrowing in on a primary and a back-up sample site.
- Recon: During Recon Phase, the spacecraft will make a series of low-altitude reconnaissance observations of the two final sample site candidates. These observations, obtained from 225 m above the surface, will show objects on the ground that are as small as 2 cm. Context images of the sites will also be taken during higher passes at an elevation of 525 m. Both sites will be fully studied so that the team can immediately begin planning sample collection at the back-up site if it becomes necessary.
- Rehearsal: Because sample collection is a critical event, the mission has planned for at least two rehearsals prior to final execution. In the first rehearsal OSIRIS-REx will practice leaving its orbit, maneuvering to a pre-defined Checkpoint located 125 m above the sample site, and then returning to orbit. The second rehearsal will take the spacecraft from orbit to a Matchpoint, where it will hover over the sampling location before a return to orbit. During each rehearsal, the spacecraft will collect and analyze tracking data, LIDAR ranges, and OCAMS and TAGCAMS imagery so that the team can verify the flight system's performance before the actual sample collection maneuver.
- TAG (Touch-And-Go): When it is time, OSIRIS-REx will use the TAGSAM (Touch-and-Go-Sample-Acquisition-Mechanism) instrument to collect a sample of regolith from Bennu. TAGSAM is an articulated arm on the spacecraft with a round sampler head at the end. During the Touch-and-Go maneuver (TAG), the sampler head will be extended toward Bennu, and the momentum of the spacecraft's slow, downward trajectory will push it against the asteroid's surface for about five seconds—just long enough to obtain a sample. At contact, nitrogen gas will blow onto the surface to roil up dust and small pebbles, which will then be captured in the TAGSAM head.
• 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. 51)
- 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 32). 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 32: 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. 52)
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. 53)
Figure 33: 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 33: 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 34: 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 35: 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. 54)
- 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. 55)
- 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. 56)
- 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. 57)
- 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 36: 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. 58)
• 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. 59)
- 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 37: 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 38: 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. 60)
• 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. 26).
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. 61)
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). 62) 63)
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 39: 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 40.
Figure 40: 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. 64) 65) 66) 67) 68)
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 41) 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 41: 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 42.
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 42: 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 43).
Figure 43: 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 44 & 46).
Figure 44: 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 45: 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. 70)
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 47: 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 48: Photo of the OVIRS instrument (image credit: BATC, Ref. 18)
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. 71)
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 49: 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 50: 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 51: Exploded" view of the OTES instrument (image credit: ASU)
Legend to Figure 51: 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. 72) 73) 74)
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. 75)
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 52: 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 53: 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 54).
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 55. 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 55: 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 55. 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. 76)
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 55.
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 56) show the dispersions for a representative Monte Carlo run with error ellipses provided.
Figure 56: 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 57). 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 57: 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 58).
Figure 58: 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 59.
Figure 59: 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 60. 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 60: 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 61). 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 61: 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 62). 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 62: 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).