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)
• January 22, 2020: Preliminary results indicate that NASA's OSIRIS-REx spacecraft successfully executed a 620 m flyover of site Nightingale yesterday as part of the mission's Reconnaissance B phase activities. Nightingale, OSIRIS-REx's primary sample collection site, is located within a crater high in asteroid Bennu's northern hemisphere. 29)
Figure 10: During the OSIRIS-REx Reconnaissance B flyover of primary sample collection site Nightingale, the spacecraft left its safe-home orbit to pass over the sample site at an altitude of 620 m. The pass, which took 11 hours, gave the spacecraft's onboard instruments the opportunity to take the closest-ever science observations of the sample site (image credit: NASA/Goddard/University of Arizona)
- The primary goal of the Nightingale flyover was to collect the high-resolution imagery required to complete the spacecraft's Natural Feature Tracking image catalog, which will document the sample collection site's surface features – such as boulders and craters. During the sampling event, which is scheduled for late August, the spacecraft will use this catalog to navigate with respect to Bennu's surface features, allowing it to autonomously predict where on the sample site it will make contact . Several of the spacecraft's other instruments also took observations of the Nightingale site during the flyover event, including the OSIRIS-REx Thermal Emissions Spectrometer (OTES), the OSIRIS-REx Visual and InfraRed Spectrometer (OVIRS), the OSIRIS-REx Laser Altimeter (OLA), and the MapCam color imager.
- A similar flyover of the backup sample collection site, Osprey, is scheduled for Feb. 11. Even lower flybys will be performed later this spring – Mar. 3 for Nightingale and May 26 for Osprey – as part of the mission's Reconnaissance C phase activities. The spacecraft will perform these two flyovers at an altitude of 250 m, which will be the closest it has ever flown over asteroid Bennu's surface.
• December 12, 2019: After a year scoping out asteroid Bennu's boulder-scattered surface, the team leading NASA's first asteroid sample return mission has officially selected a sample collection site. 30)
- The Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-Rex) mission team concluded a site designated "Nightingale" – located in a crater high in Bennu's northern hemisphere – is the best spot for the OSIRIS-REx spacecraft to snag its sample.
- The OSIRIS-REx team spent the past several months evaluating close-range data from four candidate sites in order to identify the best option for the sample collection. The candidate sites – dubbed Sandpiper, Osprey, Kingfisher, and Nightingale – were chosen for investigation because, of all the potential sampling regions on Bennu, these areas pose the fewest hazards to the spacecraft's safety while still providing the opportunity for great samples to be gathered.
- "After thoroughly evaluating all four candidate sites, we made our final decision based on which site has the greatest amount of fine-grained material and how easily the spacecraft can access that material while keeping the spacecraft safe," said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona in Tucson. "Of the four candidates, site Nightingale best meets these criteria and, ultimately, best ensures mission success."
- Site Nightingale is located in a northern crater 140 meters wide. Nightingale's regolith – or rocky surface material – is dark, and images show that the crater is relatively smooth. Because it is located so far north, temperatures in the region are lower than elsewhere on the asteroid and the surface material is well-preserved. The crater also is thought to be relatively young, and the regolith is freshly exposed. This means the site would likely allow for a pristine sample of the asteroid, giving the team insight into Bennu's history.
- Although Nightingale ranks the highest of any location on Bennu, the site still poses challenges for sample collection. The original mission plan envisioned a sample site with a diameter of 50 meters. While the crater that hosts Nightingale is larger than that, the area safe enough for the spacecraft to touch is much smaller – approximately 16 meters in diameter, resulting in a site that is only about one-tenth the size of what was originally envisioned. This means the spacecraft has to very accurately target Bennu's surface. Nightingale also has a building-size boulder situated on the crater's eastern rim, which could pose a hazard to the spacecraft while backing away after contacting the site.
- The mission also selected site Osprey as a backup sample collection site. The spacecraft has the capability to perform multiple sampling attempts, but any significant disturbance to Nightingale's surface would make it difficult to collect a sample from that area on a later attempt, making a backup site necessary. The spacecraft is designed to autonomously "wave-off" from the site if its predicted position is too close to a hazardous area. During this maneuver, the exhaust plumes from the spacecraft's thrusters could potentially disturb the surface of the site, due to the asteroid's microgravity environment. In any situation where a follow-on attempt at Nightingale is not possible, the team will try to collect a sample from site Osprey instead.
- "Bennu has challenged OSIRIS-REx with extraordinarily rugged terrain," said Rich Burns, OSIRIS-REx project manager at NASA's Goddard Space Flight Center. "The team has adapted by employing a more accurate, though more complex, optical navigation technique to be able to get into these small areas. We'll also arm OSIRIS-REx with the capability to recognize if it is on course to touch a hazard within or adjacent to the site and wave-off before that happens."
- With the selection of final primary and backup sites, the mission team will undertake further reconnaissance flights over Nightingale and Osprey, beginning in January and continuing through the spring. Once these flyovers are complete, the spacecraft will begin rehearsals for its first "touch-and-go" sample collection attempt, which is scheduled for August. The spacecraft will depart Bennu in 2021 and is scheduled to return to Earth in September 2023.
- NASA's Goddard Space Flight Center in Greenbelt, Maryland, 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 provides 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 11: This image shows sample site Nightingale, OSIRIS-REx's primary sample collection site on asteroid Bennu. The image is overlaid with a graphic of the OSIRIS-REx spacecraft to illustrate the scale of the site (image credit: NASA/Goddard/University of Arizona)
• December 5, 2019: Shortly after NASA's OSIRIS-REx spacecraft arrived at asteroid Bennu, an unexpected discovery by the mission's science team revealed that the asteroid could be active, or consistently discharging particles into space. The ongoing examination of Bennu – and its sample that will eventually be returned to Earth – could potentially shed light on why this intriguing phenomenon is occurring. 31)
- The OSIRIS-REx team first observed a particle ejection event in images captured by the spacecraft's navigation cameras taken on Jan. 6, just a week after the spacecraft entered its first orbit around Bennu. At first glance, the particles appeared to be stars behind the asteroid, but on closer examination, the team realized that the asteroid was ejecting material from its surface. After concluding that these particles did not compromise the spacecraft's safety, the mission began dedicated observations in order to fully document the activity.
- "Among Bennu's many surprises, the particle ejections sparked our curiosity, and we've spent the last several months investigating this mystery," said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. "This is a great opportunity to expand our knowledge of how asteroids behave."
- After studying the results of the observations, the mission team released their findings in a Science paper published Dec. 6. The team observed the three largest particle ejection events on Jan. 6 and 19, and Feb. 11, and concluded that the events originated from different locations on Bennu's surface. The first event originated in the southern hemisphere, and the second and third events occurred near the equator. All three events took place in the late afternoon on Bennu. 32)
- The team found that, after ejection from the asteroid's surface, the particles either briefly orbited Bennu and fell back to its surface or escaped from Bennu into space. The observed particles traveled up to 10 feet (3 meters) per second, and measured from smaller than an inch up to 4 inches (10 cm) in size. Approximately 200 particles were observed during the largest event, which took place on Jan. 6.
- The team investigated a wide variety of possible mechanisms that may have caused the ejection events, and narrowed the list to three candidates: meteoroid impacts, thermal stress fracturing, and released of water vapor.
- Meteoroid impacts are common in the deep space neighborhood of Bennu, and it is possible that these small fragments of space rock could be hitting Bennu where OSIRIS-REx is not observing it, shaking loose particles with the momentum of their impact.
Figure 12: This animation illustrates the modeled trajectories of particles that were ejected from Bennu's surface on January 19. After ejecting from the asteroid's surface, the particles either briefly orbited Bennu and fell back to its surface or escaped away from Bennu and into space (video credit: NASA/Goddard/University of Arizona/Lauretta & Hergenrother et al., Science 10.1126)
- The team also determined that thermal fracturing is another reasonable explanation. Bennu's surface temperatures vary drastically over its 4.3-hour rotation period. Although it is extremely cold during the night hours, the asteroid's surface warms significantly in the mid-afternoon, which is when the three major events occurred. As a result of this temperature change, rocks may begin to crack and break down, and eventually particles could be ejected from the surface. This cycle is known as thermal stress fracturing.
- Water release may also explain the asteroid's activity. When Bennu's water-locked clays are heated, the water could begin to release and create pressure. It is possible that as pressure builds in cracks and pores in boulders where absorbed water is released, the surface could become agitated, causing particles to erupt.
- But nature does not always allow for simple explanations. "It could be that more than one of these possible mechanisms are at play," said Steve Chesley, an author on the paper and Senior Research Scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "For example, thermal fracturing could be chopping the surface material into small pieces, making it far easier for meteoroid impacts to launch pebbles into space."
- If thermal fracturing, meteoroid impacts, or both, are in fact the causes of these ejection events, then this phenomenon is likely happening on all small asteroids, as they all experience these mechanisms. However, if water release is the cause of these ejection events, then this phenomenon would be specific to asteroids that contain water-bearing minerals, like Bennu.
- Bennu's activity presents larger opportunities once a sample is collected and returned to Earth for study. Many of the ejected particles are small enough to be collected by the spacecraft's sampling mechanism, meaning that the returned sample may possibly contain some material that was ejected and returned to Bennu's surface. Determining that a particular particle had been ejected and returned to Bennu might be a scientific feat similar to finding a needle in a haystack. The material returned to Earth from Bennu, however, will almost certainly increase our understanding of asteroids and the ways they are both different and similar, even as the particle ejection phenomenon continues to be a mystery whose clues we'll also return home with in the form of data and further material for study.
- Sample collection is scheduled for summer 2020, and the sample will be delivered to Earth in September 2023.
• December 4, 2019: NASA's OSIRIS-REx mission is just days away from selecting the site where the spacecraft will snag a sample from asteroid Bennu. After a lengthy and challenging process, the team is finally ready to down-select from the four candidate sites to a primary and backup site. 33)
- OSIRIS-REx is NASA's first asteroid sample return mission, so this decision of a sample collection site is key for asteroid operations and mission success.
- After selecting the four candidate sample sites – Sandpiper, Osprey, Kingfisher, and Nightingale – in July, the mission completed its Reconnaissance A phase. During Recon A, the OSIRIS-REx spacecraft performed a month-long series of four flyovers – one over each potential sample collection site. This mission phase provided the team with high-resolution imagery in order to thoroughly examine the sampleability (fine-grained material), topography, albedo, and color of each site. The data collected from these high-altitude flyovers is central for determining which site is best-suited for sample collection.
Figure 13: These images show the four candidate sample collection sites on asteroid Bennu: Nightingale, Kingfisher, Osprey and Sandpiper. One of these four sites will ultimately be the location on which NASA's OSIRIS-REx spacecraft will touch down to collect a sample (image credit: NASA/Goddard/University of Arizona)
- While the mission is one step closer to collecting a sample, Recon A observations have revealed that even the best candidate sites on Bennu pose significant challenges to sample collection, and the choice before the site selection board is not an easy one.
- "Sample site selection really is a comprehensive activity. It requires that we look at many different types of data in many different ways to ensure the selected site is the best choice in terms of spacecraft safety, presence of sampleable material, and science value," said Heather Enos, OSIRIS-REx deputy principal investigator at the University of Arizona, Tucson, and chair of the sample site selection board. "Our team is incredibly innovative and integrated, which is what makes the selection process work."
- The most recent images show that while there is fine-grained material (smaller than 2.5 cm in diameter), much of it may not be easily accessible. The mission was originally designed for a beach-like surface, with "ponds" of sandy material, not for Bennu's rugged terrain. In reality the potential sample sites are not large, clear areas, but rather small spaces surrounded by large boulders, so navigating the spacecraft in and out of the sites will require a bit more fine-tuning than originally planned.
- Starting in Bennu's southern hemisphere, site Sandpiper was the first flyover of the Recon A mission phase. Sandpiper is one of the "safer" sites because it is located in a relatively flat area, making it easier for the spacecraft to navigate in and out. The most recent images show that fine-grained material is present, but the sandy regolith is trapped between larger rocks, which makes it difficult for the sampling mechanism to operate.
- Site Osprey was the second site observed during Recon A. This site was originally chosen based on its strong spectral signature of carbon-rich material and because of a dark patch in the center of the crater, which was thought to possibly be fine-grained material. However, the latest high-resolution imagery of Osprey suggests that the site is scattered with material that may be too large to ingest for the sampling mechanism.
- Site Kingfisher was selected because it is located in a small crater – meaning that it may be a relatively young feature compared to Bennu's larger craters (such as the one in which Sandpiper is located). Younger craters generally hold fresher, minimally-altered material. High-resolution imagery captured during the Recon A flyover revealed that while the original crater may be too rocky, a neighboring crater appears to contain fine-grained material.
- Recon A concluded with a flyover of site Nightingale. Images show that the crater holds a good amount of unobstructed fine-grained material. However, while the sampleability of the site ranks high, Nightingale is located far to the north where the lighting conditions create additional challenges for spacecraft navigation. There is also a building-size boulder situated on the crater's eastern rim, which could be a hazard to the spacecraft when backing away after contacting the site.
Figure 14: This flat projection mosaic of asteroid Bennu shows the relative locations of the four candidate sample collection sites on the asteroid: Nightingale, Kingfisher, Osprey and Sandpiper. NASA's OSIRIS-REx spacecraft is scheduled to touch down on one of these four sites to collect a sample in summer 2020 (image credit: NASA/Goddard/University of Arizona)
- Bennu has also made it a challenge for the mission to identify a site that won't trigger the spacecraft's safety mechanisms. During Recon A, the team began cataloguing Bennu's surface features to create maps for the Natural Feature Tracking (NFT) autonomous navigation system. During the sample collection event, the spacecraft will use NFT to navigate to the asteroid's surface by comparing the onboard image catalog to the navigation images it will take during descent. In response to Bennu's extremely rocky surface, the NFT system has been augmented with a new safety feature, which instructs it to wave-off the sampling attempt and back away if it determines the point of contact is near a potentially hazardous surface feature. With Bennu's building-sized boulders and small target sites, the team realizes that there is a possibility that the spacecraft will wave-off the first time it descends to collect a sample.
- "Bennu's challenges are an inherent part of this mission, and the OSIRIS-REx team has responded by developing robust measures to overcome them," said Mike Moreau, OSIRIS-REx deputy project manager at Goddard. "If the spacecraft executes a wave-off while attempting to collect a sample, that simply means that both the team and the spacecraft have done their jobs to ensure the spacecraft can fly another day. The success of the mission is our first priority."
Figure 15: The team is mere months away from a sample collection attempt at the asteroid surface (video credit: NASA's Goddard Space Flight Center)
- Whichever site wins the race, the OSIRIS-REx mission team is ready for whatever new challenges Bennu may bring. Next spring, the team will undertake further reconnaissance flights over the primary and backup sample sites, and will then start spacecraft rehearsals for touchdown. Sample collection is scheduled for summer 2020, and the sample will return to Earth in September 2023.
- NASA's Goddard Space Flight Center in Greenbelt, Maryland 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.
• August 29, 2019: A made-in-Canada laser aboard NASA's OSIRIS-REx spacecraft has produced high-resolution topographic maps of the four locations on asteroid Bennu that mission scientists have identified as candidates for sample collection. 34)
- OLA (OSIRIS-REx Laser Altimeter) is equipped with two lasers that scanned the asteroid's surface to produce detailed images of the boulders, craters and other geological features at each of the four sites. These maps will be crucial in helping mission scientists select the safest and most scientifically interesting of the approximately 10-meter-wide candidates – known as Nightingale, Kingfisher, Osprey, and Sandpiper.
- OLA's high-resolution results follow the activation of the instrument's LELT (Low-Energy Laser Transmitter) at the beginning of July 2019. The LELT is designed to fire 10,000 light pulses per second at the asteroid, and operates at a range of less than 1 km above Bennu's surface.
Figure 16: These detailed views of four potential sample sites on asteroid Bennu (complete with boulders, craters and other geological features) are based on a series of measurements taken by the OSIRIS-REx Laser Altimeter (OLA), the Canadian laser instrument aboard NASA's OSIRIS-REx spacecraft (video creation: Michael Daly, Centre for Research in Earth and Space Science, York University; credit: NASA/University of Arizona/Canadian Space Agency/York University/MDA)
- In previous mission phases, OLA's HELT (High-Energy Laser Transmitter) – firing 100 pulses per second from greater distances – collected data that enabled the creation of the first 3D lidar map of the asteroid in April.
- By June, OLA's HELT had collected about 9 million additional measurements to complete coverage of the entire asteroid, compiling the first global map of asteroid Bennu's topography (see Figure 23).
- Mission scientists anticipate that high volumes of data collected by OLA's LELT – in the order of several billion measurements – will enable the creation of a new, higher-resolution global map, featuring one data point/7 cm and offering an unprecedented level of detail over Bennu's entire surface.
- High-resolution maps of the four potential sample sites, like that of the Sandpiper site (Figure 17), will allow OSIRIS-REx scientists to:
a) assess the safety and accessibility of each region
b) locate landmarks that will help the spacecraft navigate during sample collection
c) identify areas of fine-grained material compatible with OSIRIS-REx's sampling device.
- OLA's LELT will continue to work in tandem with other instruments on the spacecraft to gather crucial data about the surface of the asteroid. A primary and a backup site will be announced in December 2019, and the spacecraft is scheduled to begin rehearsing sampling maneuvers in early 2020.
Figure 17: The same area of asteroid Bennu's surface – a potential sample site known as Sandpiper – was measured by each of OLA's lasers. OLA's high-energy laser transmitter (HELT) captured its measurements from a distance of 5 km (top right). OLA's low-energy laser transmitter (LELT) captured the details of the site's boulders and craters from a distance of only 700 m (bottom right); image creation: Michael Daly, Centre for Research in Earth and Space Science, York University, credit: NASA/University of Arizona/Canadian Space Agency/York University/MDA)
• August 12, 2019: After months grappling with the rugged reality of asteroid Bennu's surface, the team leading NASA's first asteroid sample return mission has selected four potential sites for the Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) spacecraft to "tag" its cosmic dance partner. 35)
- Since its arrival in December 2018, the OSIRIS-REx spacecraft has mapped the entire asteroid in order to identify the safest and most accessible spots for the spacecraft to collect a sample. These four sites (Figure 18) now will be studied in further detail in order to select the final two sites – a primary and backup – in December.
- The team originally had planned to choose the final two sites by this point in the mission. Initial analysis of Earth-based observations suggested the asteroid's surface likely contains large "ponds" of fine-grain material. The spacecraft's earliest images, however, revealed Bennu has an especially rocky terrain. Since then, the asteroid's boulder-filled topography has created a challenge for the team to identify safe areas containing sampleable material, which must be fine enough – less than 1 inch (2.5 cm) diameter – for the spacecraft's sampling mechanism to ingest it.
- "We knew that Bennu would surprise us, so we came prepared for whatever we might find," said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. "As with any mission of exploration, dealing with the unknown requires flexibility, resources and ingenuity. The OSIRIS-REx team has demonstrated these essential traits for overcoming the unexpected throughout the Bennu encounter."
- The original mission schedule intentionally included more than 300 days of extra time during asteroid operations to address such unexpected challenges. In a demonstration of its flexibility and ingenuity in response to Bennu's surprises, the mission team is adapting its site selection process. Instead of down-selecting to the final two sites this summer, the mission will spend an additional four months studying the four candidate sites in detail, with a particular focus on identifying regions of fine-grain, sampleable material from upcoming, high-resolution observations of each site. The boulder maps that citizen science counters helped create through observations earlier this year were used as one of many pieces of data considered when assessing each site's safety. The data collected will be key to selecting the final two sites best suited for sample collection.
- In order to further adapt to Bennu's ruggedness, the OSIRIS-REx team has made other adjustments to its sample site identification process. The original mission plan envisioned a sample site with a radius of 82 feet (25 m). Boulder-free sites of that size don't exist on Bennu, so the team has instead identified sites ranging from 16 to 33 feet (5 to 10 m) in radius. In order for the spacecraft to accurately target a smaller site, the team reassessed the spacecraft's operational capabilities to maximize its performance. The mission also has tightened its navigation requirements to guide the spacecraft to the asteroid's surface, and developed a new sampling technique called "Bullseye TAG (Touch and Go)," which uses images of the asteroid surface to navigate the spacecraft all the way to the actual surface with high accuracy. The mission's performance so far has demonstrated the new standards are within its capabilities.
- "Although OSIRIS-REx was designed to collect a sample from an asteroid with a beach-like area, the extraordinary in-flight performance to date demonstrates that we will be able to meet the challenge that the rugged surface of Bennu presents," said Rich Burns, OSIRIS-REx project manager at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "That extraordinary performance encompasses not only the spacecraft and instruments, but also the team who continues to meet every challenge that Bennu throws at us."
- The four candidate sample sites on Bennu are designated Nightingale, Kingfisher, Osprey, and Sandpiper – all birds native to Egypt. The naming theme complements the mission's two other naming conventions – Egyptian deities (the asteroid and spacecraft) and mythological birds (surface features on Bennu).
Figure 18: Pictured are the four candidate sample collection sites on asteroid Bennu selected by NASA's OSIRIS-REx mission. Site Nightingale (top left) is located in Bennu's northern hemisphere. Sites Kingfisher (top right) and Osprey (bottom left) are located in Bennu's equatorial region. Site Sandpiper (bottom right) is located in Bennu's southern hemisphere. In December, one of these sites will be chosen for the mission's touchdown event (image credit: NASA/University of Arizona)
Figure 19: Since arriving at near-Earth asteroid Bennu in December 2018, NASA's OSIRIS-REx mission has been studying this small world of boulders, rocks, and loose rubble - and looking for a place to touch down. The goal of OSIRIS-REx is to collect a sample of Bennu in mid-2020, and return it to Earth in late 2023 (video credit: NASA)
- The four sites are diverse in both geographic location and geological features. While the amount of sampleable material in each site has yet to be determined, all four sites have been evaluated thoroughly to ensure the spacecraft's safety as it descends to, touches and collects a sample from the asteroid's surface.
- Nightingale is the northern-most site, situated at 56 degrees north latitude on Bennu. There are multiple possible sampling regions in this site, which is set in a small crater encompassed by a larger crater 459 feet (140 m) in diameter. The site contains mostly fine-grain, dark material and has the lowest albedo, or reflection, and surface temperature of the four sites.
- Kingfisher is located in a small crater near Bennu's equator at 11 degrees north latitude. The crater has a diameter of 26 feet (8 m) and is surrounded by boulders, although the site itself is free of large rocks. Among the four sites, Kingfisher has the strongest spectral signature for hydrated minerals.
- Osprey is set in a small crater, 66 feet (20 m) in diameter, which is also located in Bennu's equatorial region at 11 degrees north latitude. There are several possible sampling regions within the site. The diversity of rock types in the surrounding area suggests that the regolith within Osprey may also be diverse. Osprey has the strongest spectral signature of carbon-rich material among the four sites.
- Sandpiper is located in Bennu's southern hemisphere, at 47 degrees south latitude. The site is in a relatively flat area on the wall of a large crater 207 ft (63 m) in diameter. Hydrated minerals are also present, which indicates that Sandpiper may contain unmodified water-rich material.
- This fall, OSIRIS-REx will begin detailed analyses of the four candidate sites during the mission's reconnaissance phase. During the first stage of this phase, the spacecraft will execute high passes over each of the four sites from a distance of 0.8 miles (1.29 km) to confirm they are safe and contain sampleable material. Closeup imaging also will map the features and landmarks required for the spacecraft's autonomous navigation to the asteroid's surface. The team will use the data from these passes to select the final primary and backup sample collection sites in December.
- The second and third stages of reconnaissance will begin in early 2020 when the spacecraft will perform passes over the final two sites at lower altitudes and take even higher resolution observations of the surface to identify features, such as groupings of rocks that will be used to navigate to the surface for sample collection. OSIRIS-REx sample collection is scheduled for the latter half of 2020, and the spacecraft will return the asteroid samples to Earth on Sept. 24, 2023.
• 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. 36)
- 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 20: 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). 37)
Figure 21: 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. 38)
Figure 22: 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. 39)
Figure 23: 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 (video credit: NASA/University of Arizona/CSA/York/MDA, Michael Daly)
• 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. 40)
- 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 24: 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. 41)
Figure 25: 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. 42)
Figure 26: 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. 43)
Figure 27: 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 28: 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. 44)
- 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 29: 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. 45)
- 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 30: 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.