Minimize OSIRIS-REx

OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security‚ÄíRegolith Explorer)

Spacecraft    Launch    Mission Status    Sensor Complement    TAG Phase    Flight Dynamics    References 

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.

OsirisREx_Auto4D

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.

Key OSIRIS-REx science objectives include: 4) 5) 6) 7)

• 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.

OsirisREx_Auto4C

Figure 2: Schedule of the OSIRIS-REx project (image credit: NASA)




Spacecraft:

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.

OsirisREx_Auto4B

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.

OsirisREx_Auto4A

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.

OsirisREx_Auto49

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.

OsirisREx_Auto48

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)

OsirisREx_Auto47

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.

Phase name

Description

Start time

Launch

Launch on an EELV from Cape Canaveral on an Earth-escape trajectory

Sept. 2016

Outbound cruise

Perform deep space maneuver; Earth flyby & gravity assist; instrument calibration & checkout

Oct. 2016

Approach

Perform braking maneuvers; survey the Bennu orbital environment for natural satellites; collect the first resolved images

Aug. 2018

Preliminary survey

Estimate the mass of Bennu; refine shape and spin state models

Nov. 2018

Orbital A

Demonstrate orbital flight; transition to landmark-based optical navigation

Dec. 2018

Detailed survey

Spectrally map the entire Bennu surface; collect images and lidar data for global shape and spin state models; search for dust plumes

Jan. 2019

Orbital B

Collect lidar and radiometric data for high resolution topographic map and gravity model; observe candidate sampling sites and downselect for reconnaissance

Mar. 2019

Reconnaissance

Conduct sorties for closer look at up to 4 candidate sampling sites and select 1

May 2019

TAG rehearsal

Systematically and deliberately practice steps of sample collection sequence

Aug. 2019

Sample collection

Collect >60g (Level 2 requirement) of pristine bulk regolith and 26 cm2 of surface material, and stow it in the SRC (Sample Return Capsule)

Sept. 2019

Quiescent operations

Remain in Bennu's heliocentric orbit; monitor spacecraft health

Oct. 2019

Return Cruise

Transport the sample back to the vicinity of the Earth

Mar. 2021

Earth Return & Recovery

Get the sample safely to the ground and to the curation facility in late September 2023

Sept. 2023

Table 1: OSIRIS-REx mission phases

OsirisREx_Auto46

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)




Mission status

• June 3, 2021: NASA's OSIRIS-REx spacecraft is 328,000 miles, or 528,000 kilometers, away from the asteroid Bennu, having fired its engines on May 10 to initiate a return trip to Earth. The spacecraft is on track to deliver an asteroid sample to Earth on September 24, 2023. 29)

- Mission engineers had planned to do a small thruster firing last week to ensure the spacecraft stays on the correct path back to Earth. But, the May 10 departure maneuver was calculated and executed so precisely, the mission team decided not to do a clean-up maneuver last week.

Figure 10: OSIRIS-REx Heads Home with Sample of Asteroid Bennu. After nearly five years in space, NASA’s OSIRIS-REx spacecraft is on its way back to Earth with an abundance of rocks and dust from the near-Earth asteroid Bennu (video credit: NASA's Goddard Space Flight Center)

May 10, 2021: After nearly five years in space, NASA’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) spacecraft is on its way back to Earth with an abundance of rocks and dust from the near-Earth asteroid Bennu. 30)

- On Monday, May 10, at 4:23 p.m. EDT the spacecraft fired its main engines full throttle for seven minutes – its most significant maneuver since it arrived at Bennu in 2018. This burn thrust the spacecraft away from the asteroid at 600 miles per hour (nearly 1,000 km/hr), setting it on a 2.5-year cruise towards Earth.

- After releasing the sample capsule, OSIRIS-REx will have completed its primary mission. It will fire its engines to fly by Earth safely, putting it on a trajectory to circle the sun inside of Venus’ orbit.

- After orbiting the Sun twice, the OSIRIS-REx spacecraft is due to reach Earth Sept. 24, 2023. Upon return, the capsule containing pieces of Bennu will separate from the rest of the spacecraft and enter Earth’s atmosphere. The capsule will parachute to the Utah Test and Training Range in Utah's West Desert, where scientists will be waiting to retrieve it.

- “OSIRIS-REx’s many accomplishments demonstrated the daring and innovate way in which exploration unfolds in real time,” said Thomas Zurbuchen, associate administrator for science at NASA Headquarters. “The team rose to the challenge, and now we have a primordial piece of our solar system headed back to Earth where many generations of researchers can unlock its secrets.”

- To realize the mission’s multi-year plan, a dozen navigation engineers made calculations and wrote computer code to instruct the spacecraft when and how to push itself away from Bennu. After departing from Bennu, getting the sample to Earth safely is the team’s next critical goal. This includes planning future maneuvers to keep the spacecraft on course throughout its journey.

- “Our whole mindset has been, ‘Where are we in space relative to Bennu?’” said Mike Moreau, OSIRIS-REx deputy project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Now our mindset has shifted to ‘Where is the spacecraft in relation to Earth?’”

- The navigation cameras that helped orient the spacecraft in relation to Bennu were turned off April 9, after snapping their last images of the asteroid. With Bennu in the rearview mirror, engineers are using NASA’s Deep Space Network of global spacecraft communications facilities to steer the OSIRIS-REx by sending it radio signals. By measuring the frequency of the waves returned from the spacecraft transponder, engineers can tell how fast OSIRIS-REx is moving. Engineers measure how long it takes for radio signals to get from the spacecraft back to Earth in order to determine its location.

Exceeding Mission Expectations

- The May 10 departure date was precisely timed based on the alignment of Bennu with Earth. The goal of the return maneuver is to get the spacecraft within about 6,000 miles (approximately 10,000 km) of Earth in September 2023. Although OSIRIS-REx still has plenty of fuel remaining, the team is trying to preserve as much as possible for a potential extended mission to another asteroid after returning the sample capsule to Earth. The team will investigate the feasibility of such a mission this summer.

- The spacecraft’s course will be determined mainly by the Sun’s gravity, but engineers will need to occasionally make small course adjustments via engine burns.

- “We need to do regular corrections to bring the trajectory increasingly closer to Earth’s atmosphere for the sample release, and to account for small errors that might have accumulated since the last burn,” said Peter Antreasian, OSIRIS-REx navigation lead at KinetX Aerospace, which is based in Simi Valley, California.

- The team will perform course adjustments a few weeks prior to Earth re-entry in order to precisely target the location and angle for the sample capsule’s release into Earth’s atmosphere. Coming in too low could cause the capsule to bounce out of the atmosphere like a pebble skipping off a lake; too high and the capsule could burn up due to friction and heat from the atmosphere. If OSIRIS-REx fails to release the capsule, the team has a backup plan to divert it away from Earth and try again in 2025.

- “There’s a lot of emotion within the team about departure,” Moreau said. “I think everyone has a great sense of accomplishment, because we faced all these daunting tasks and were able to accomplish all the objectives thrown at us. But there’s also some nostalgia and disappointment that this part of the mission is coming to an end.”

- OSIRIS-REx exceeded many expectations. Most recently, in the midst of a global pandemic, the team flawlessly executed the most mission’s critical operation, collecting more than 2 ounces (60 grams) of soil from Bennu’s surface.

- Leading up to sample collection, a number of surprises kept the team on its toes. For example, a week after the spacecraft entered its first orbit around Bennu, on Dec. 31, 2018, the team realized that the asteroid was releasing small pieces of rock into space.

- “We had to scramble to verify that the small particles being ejected from the surface did not present a hazard to the spacecraft,” Moreau said.

- Upon arrival at the asteroid, team members also were astonished to find that Bennu is littered with boulders.

- “We really had this idea that we were arriving on an asteroid with open real estate,” said Heather Enos, OSIRIS-REx deputy principal investigator, based at the University of Arizona, Tucson. “The reality was a big shocker.”

- To overcome the extreme and unexpected ruggedness of Bennu’s surface, engineers had to quickly develop a more accurate navigation technique to target smaller-than-expected sites for sample collection.

- The OSIRIS-REx mission was instrumental in both confirming and refuting several scientific findings. Among those confirmed was a technique that used observations from Earth to predict that the minerals on the asteroid would be carbon-rich and show signs of ancient water. One finding that proved unsuccessful was that Bennu would have a smooth surface, which scientists predicted by measuring how much heat radiated off its surface.

- Scientists will use the information gleaned from Bennu to refine theoretical models and improve future predictions.

- “This mission emphasizes why we have to do science and exploration in multiple ways – both from Earth and from up-close in space – because assumptions and models are just that,” Enos said.

- 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. The university leads the science team and the mission's science observation planning and data processing. Lockheed Martin Space in Littleton, Colorado, 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, managed by NASA's Marshall Space Flight Center in Huntsville, Alabama, for the agency's Science Mission Directorate Washington.

• April 15, 2021: Like boot prints on the Moon, NASA's OSIRIS-REx spacecraft left its mark on asteroid Bennu. Now, new images — taken during the spacecraft's final fly-over on April 7 — reveal the aftermath of its historic encounter with the asteroid. 31)

- The spacecraft flew within 2.3 miles (3.7 km) of the asteroid — the closest it has been since the TAG (Touch-and-Go) sample collection event on Oct. 20, 2020. During TAG, the spacecraft's sampling head sunk 1.6 feet (48.8 cm) into the asteroid's surface and simultaneously fired a pressurized charge of nitrogen gas, churning up surface material and driving some into the collection chamber. The spacecraft's thrusters also launched rocks and dust during the maneuver to reverse course and safely back away from the asteroid.

Figure 11: Like boot prints on the Moon, NASA's OSIRIS-REx spacecraft left its mark on asteroid Bennu. Now, new images — taken during the spacecraft's final fly-over on April 7, 2021 — reveal the aftermath of the historic Touch-and-Go (TAG) sample acquisition event from Oct. 20, 2020 (video credit: NASA's Goddard Space Flight Center)

- Comparing the two images reveals obvious signs of surface disturbance. At the sample collection point, there appears to be a depression, with several large boulders evident at the bottom, suggesting that they were exposed by sampling. There is a noticeable increase in the amount of highly reflective material near the TAG point against the generally dark background of the surface, and many rocks were moved around.

- Where thrusters fired against the surface, substantial mass movement is apparent. Multiple sub-meter boulders were mobilized by the plumes into a campfire ring–like shape — similar to rings of boulders seen around small craters pocking the surface.

- Jason Dworkin, the mission's project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, noticed that one boulder measuring 4 feet (1.25 meters) across on the edge of the sampling site seemed to appear only in the post-TAG image. “The rock probably weighs around a ton, with a mass somewhere between a cow and a car.”

Figure 12: Bennu’s surface was disturbed in three different ways: by the force of the spacecraft touching down; by the sampling mechanism, which collected material by blowing gas into its collection filter; and by four of the spacecraft's back-away thrusters, which moved the spacecraft away from the sample site (marked with a red "X" in the second of these two images) and agitated dust and boulders on the surface. The image above shows the TAG site and highlights (red circle) a large boulder thrown about 40 feet (about 12 meters), image credits: NASA/Goddard/University of Arizona

- Dante Lauretta, of the University of Arizona and the mission's principal investigator, later pointed out that this boulder is likely one of those present in the pre-TAG image, but much nearer the sampling location, and estimates it was thrown a distance of 40 feet (about 12 meters) by the sample collection event.

- In order to compare the before and after images, the team had to meticulously plan this final flyover. "Bennu is rough and rocky, so if you look at it from a different angle or capture it at a time when the sun is not directly overhead, that dramatically changes what the surface looks like," says Dathon Golish, a member of the OSIRIS-REx image processing working group, headquartered at the University of Arizona. "These images were deliberately taken close to noon, with the Sun shining straight down, when there's not as many shadows."

- "These observations were not in the original mission plan, so we were excited to go back and document what we did," Golish said. "The team really pulled together for this one last hurrah."

- The spacecraft will remain in Bennu's vicinity until departure on May 10, when the mission will begin its two-year return cruise back to Earth. As it approaches Earth, the spacecraft will jettison the Sample Return Capsule (SRC) that contains the sample from Bennu. The SRC will then travel through Earth’s atmosphere and land under parachutes at the Utah Test and Training Range on Sept. 24, 2023.

- Once recovered, the capsule will be transported to the curation facility at NASA’s Johnson Space Center in Houston, where the sample will be removed for distribution to laboratories worldwide, enabling scientists to study the formation of our solar system and Earth as a habitable planet. NASA will set 75% of the sample aside for future generations to study with technologies not invented yet.

- The OSIRIS-REx mission is the first NASA mission to visit a near-Earth asteroid, survey the surface, and collect a sample to deliver to Earth.

• April 7, 2021: NASA’s OSIRIS-REx completed its last flyover of Bennu around 6 a.m. EDT (4 a.m. MDT) April 7 and is now slowly drifting away from the asteroid; however, the mission team will have to wait a few more days to find out how the spacecraft changed the surface of Bennu when it grabbed a sample of the asteroid. 32)

- The OSIRIS-REx team added this flyby to document surface changes resulting from the Touch and Go (TAG) sample collection maneuver Oct. 20, 2020. “By surveying the distribution of the excavated material around the TAG site, we will learn more about the nature of the surface and subsurface materials along with the mechanical properties of the asteroid,” said Dr. Dante Lauretta, principal investigator for OSIRIS-REx at the University of Arizona.

- During the flyby, OSIRIS-REx imaged Bennu for 5.9 hours, covering more than a full rotation of the asteroid. It flew within 2.1 miles’ (3.5 km) distance to the surface of Bennu – the closest it’s been since the TAG sample collection event.

- It will take until at least April 13 for OSIRIS-REx to downlink all of the data and new pictures of Bennu’s surface recorded during the flyby. It shares the Deep Space Network antennas with other missions like Mars Perseverance, and typically gets 4–6 hours of downlink time per day. “We collected about 4,000 MB of data during the flyby,” said Mike Moreau, deputy project manager of OSIRIS-REx at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Bennu is approximately 185 million miles from Earth right now, which means we can only achieve a downlink data-rate of 412 kbit/s, so it will take several days to download all of the flyby data.”

- Once the mission team receives the images and other instrument data, they will study how OSIRIS-REx jumbled up Bennu’s surface. During touchdown, the spacecraft’s sampling head sunk 1.6 feet (48.8 cm) into the asteroid’s surface and simultaneously fired a pressurized charge of nitrogen gas. The spacecraft’s thrusters kicked up a large amount of surface material during the back-away burn – launching rocks and dust in the process.

- OSIRIS-REx, with its pristine and precious asteroid cargo, will remain in the vicinity of Bennu until May 10 when it will fire its thrusters and begin its two-year cruise home. The mission will deliver the asteroid sample to Earth Sept. 24, 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 (Origins Spectral Interpretation Resource Identification Security - Regolith Explorer). 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.

OsirisREx_Auto45

Figure 13: This image shows a top-down view of asteroid Bennu, with a portion of the asteroid’s equatorial ridge and northern hemisphere illuminated. It was taken by the PolyCam camera on NASA’s OSIRIS-REx spacecraft on March 4, 2021, from a distance of about 186 miles (300 km). The spacecraft’s cameras are pointed directly at Bennu’s north pole. Two large equatorial craters are visible on the asteroid’s edge (center and center left). The image was obtained during the mission’s Post-TAG Operations phase, as the spacecraft slowly approached Bennu in preparation for a final observational flyby on April 7 (image credit: NASA/Goddard/University of Arizona)

• February 8, 2021: On April 7, NASA’s OSIRIS-REx mission will give asteroid Bennu one last glance before saying farewell. Before departing for Earth on May 10, the OSIRIS-REx spacecraft will perform a final flyby of Bennu – capturing its last images of sample collection site Nightingale to look for transformations on Bennu’s surface after the Oct. 20, 2020, sample collection event. 33)

- The OSIRIS-REx mission team recently completed a detailed safety analysis of a trajectory to observe sample site Nightingale from a distance of approximately 2.4 miles (3.8 km). The spacecraft’s flight path is designed to keep OSIRIS-REx a safe distance from Bennu, while ensuring the science instruments can collect precise observations. The single flyby will mimic one of the observation sequences conducted during the mission’s Detailed Survey phase in 2019. OSIRIS-REx will image Bennu for a full 4.3-hour rotation to obtain high-resolution images of the asteroid’s northern and southern hemispheres and its equatorial region. The team will then compare these new images with the previous high-resolution imagery of Bennu obtained during 2019.

- This final flyby of Bennu was not part of the original mission schedule, but the observation run will provide the team an opportunity to learn how the spacecraft’s contact with Bennu’s surface altered the sample site. Bennu’s surface was considerably disturbed after the Touch-and-Go (TAG) sample collection event, with the collector head sinking 1.6 feet (48.8 cm) into the asteroid’s surface while firing a pressurized charge of nitrogen gas. The spacecraft’s thrusters also mobilized a substantial amount of surface material during the back-away burn.

- During this new mission phase, called the Post-TAG Observation (PTO) phase, the spacecraft will perform five separate navigation maneuvers in order to return to the asteroid and position itself for the flyby. OSIRIS-REx executed the first maneuver on Jan. 14, which acted as a braking burn and put the spacecraft on a trajectory to rendezvous with the asteroid one last time. Since October’s sample collection event, the spacecraft has been slowly drifting away from the asteroid, and ended up approximately 1,635 miles (2,200 km) from Bennu. After the braking burn, the spacecraft is now slowly approaching the asteroid and will perform a second approach maneuver on Mar. 6, when it is approximately 155 miles (250 km) from Bennu. OSIRIS-REx will then execute three subsequent maneuvers, which are required to place the spacecraft on a precise trajectory for the final flyby on Apr. 7.

- OSIRIS-REx is scheduled to depart Bennu on May 10 and begin its two-year journey back to Earth. The spacecraft will deliver the samples of Bennu to the Utah Test and Training Range on September 24, 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.

OsirisREx_Auto44

Figure 14: This artist's concept shows the planned flight path of NASA’s OSIRIS-REx spacecraft during its final flyby of asteroid Bennu, scheduled for April 7 (image credits: University of Arizona)

• January 26, 2021: On May 10, NASA's OSIRIS-REx spacecraft will say farewell to asteroid Bennu and begin its journey back to Earth. During its Oct. 20, 2020, sample collection event, the spacecraft collected a substantial amount of material from Bennu’s surface, likely exceeding the mission’s requirement of 2 ounces (60 grams). The spacecraft is scheduled to deliver the sample to Earth on Sept. 24, 2023. 34)

OsirisREx_Auto43

Figure 15: This illustration shows the OSIRIS-REx spacecraft departing asteroid Bennu to begin its two-year journey back to Earth (image credits: NASA/Goddard/University of Arizona)

- “Leaving Bennu’s vicinity in May puts us in the ‘sweet spot,’ when the departure maneuver will consume the least amount of the spacecraft’s onboard fuel,” said Michael Moreau, OSIRIS-REx deputy project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Nevertheless, with over 593 miles per hour (265 meters per second) of velocity change, this will be the largest propulsive maneuver conducted by OSIRIS-REx since the approach to Bennu in October 2018.”

- The May departure also provides the OSIRIS-REx team with the opportunity to plan a final spacecraft flyby of Bennu. This activity was not part of the original mission schedule, but the team is studying the feasibility of a final observation run of the asteroid to potentially learn how the spacecraft’s contact with Bennu’s surface altered the sample site.

- If feasible, the flyby will take place in early April and will observe the sample site, named Nightingale, from a distance of approximately 2 miles (3.2 km). Bennu’s surface was considerably disturbed after the Touch-and-Go (TAG) sample collection event, with the collector head sinking 1.6 feet (48.8 cm) into the asteroid’s surface. The spacecraft’s thrusters also disturbed a substantial amount of surface material during the back-away burn.

- The mission is planning a single flyby, mimicking one of the observation sequences conducted during the mission’s Detailed Survey phase in 2019. OSIRIS-REx would image Bennu for a full rotation to obtain high-resolution images of the asteroid’s northern and southern hemispheres and equatorial region. The team would then compare these new images with the previous high-resolution imagery of Bennu obtained during 2019.

- "OSIRIS-REx has already provided incredible science,” said Lori Glaze, NASA's director of planetary science at the agency's headquarters in Washington. "We're really excited the mission is planning one more observation flyby of asteroid Bennu to provide new information about how the asteroid responded to TAG and to render a proper farewell.”

- These post-TAG observations would also give the team a chance to assess the current functionality of science instruments onboard the spacecraft – specifically the OSIRIS-REx Camera Suite (OCAMS), OSIRIS-REx Thermal Emission Spectrometer (OTES), OSIRIS-REx Visible and Infrared Spectrometer (OVIRS), and OSIRIS-REx Laser Altimeter (OLA). It is possible dust coated the instruments during the sample collection event and the mission wants to evaluate the status of each. Understanding the health of the instruments is also part of the team’s assessment of possible extended mission opportunities after the sample is delivered to Earth.

- The spacecraft will remain in asteroid Bennu’s vicinity until May 10, when the mission will enter its Earth Return Cruise phase. As it approaches Earth, OSIRIS-REx will jettison the Sample Return Capsule (SRC). The SRC will then travel through the Earth’s atmosphere and land under parachutes at the Utah Test and Training Range.

- Once recovered, NASA will transport the capsule to the curation facility at the agency’s Johnson Space Center in Houston and distribute the sample to laboratories worldwide, enabling scientists to study the formation of our solar system and Earth as a habitable planet.

- Goddard provides overall mission management, systems engineering, and the safety and mission assurance for OSIRIS-REx. Dante Lauretta of the University of Arizona in 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 Littleton, Colorado, 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 NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages for the agency’s Science Mission Directorate in Washington.

• October 30, 2020: Impact craters left by space debris in the boulders on asteroid Bennu's rugged surface allowed researchers to reconstruct the history of the near-Earth object in unprecedented detail. 35)

OsirisREx_Auto42

Figure 16: This image shows four views of asteroid Bennu along with a corresponding global mosaic. The images were taken on Dec. 2, 2018, by the OSIRIS-REx spacecraft’s PolyCam camera, which is part of the OCAMS instrument suite designed by UArizona scientists and engineers (image credit: NASA/Goddard/University of Arizona)

- By studying impact marks on the surface of asteroid Bennu – the target of NASA's OSIRIS-REx mission – a team of researchers led by the University of Arizona has uncovered the asteroid's past and revealed that despite forming hundreds of millions of years ago, Bennu wandered into Earth's neighborhood only very recently.

- The study, published in the journal Nature, provides a new benchmark for understanding the evolution of asteroids, offers insights into a poorly understood population of space debris hazardous to spacecraft, and enhances scientists' understanding of the solar system. 36)

- The researchers used images and laser-based measurements taken during a two-year surveying phase in which the OSIRIS-REx spacecraft, about the size of a 15-passenger van, orbited Bennu and broke the record for the smallest spacecraft to orbit a small body.

- Presented at the opening day of the American Astronomical Society's Division for Planetary Sciences meeting on Oct. 26, the paper details the first observations and measurements of impact craters on individual boulders on an airless planetary surface since the Apollo missions to the moon 50 years ago, according to the authors.

- The publication comes just a few days after a major milestone for NASA's University of Arizona-led OSIRIS-REx mission. On Oct. 20, the spacecraft successfully descended to asteroid Bennu to grab a sample from its boulder-scattered surface – a first for NASA. The sample has now been successfully stowed and will be returned to Earth for study in 2023, where it could give scientists insight into the earliest stages of the formation of our solar system.

Impact Craters on Rocks Tell a Story

- Although Earth is being pelted with more than 100 tons of space debris each day, it is virtually impossible to find a rockface pitted by impacts from small objects at high velocities. Courtesy of our atmosphere, we get to enjoy any object smaller than a few meters as a shooting star rather than having to fear being struck by what essentially amounts to a bullet from outer space.

- Planetary bodies lacking such a protective layer, however, bear the full brunt of a perpetual cosmic barrage, and they have the scars to show for it. High-resolution images taken by the OSIRIS-REx spacecraft during its two-year survey campaign allowed researchers to study even tiny craters, with diameters ranging from a centimeter to a meter, on Bennu's boulders.

- The team found boulders of 1 meter or larger to be scarred, on average, by anywhere from one to 60 pits – impacted by space debris ranging in size from a few millimeters to tens of centimeters.

- "I was surprised to see these features on the surface of Bennu," said the paper's lead author, Ronald Ballouz, a postdoctoral researcher in the UArizona Lunar and Planetary Laboratory and a scientist with the OSIRIS-REx regolith development working group. "The rocks tell their history through the craters they accumulated over time. We haven't observed anything like this since astronauts walked on the moon."

- For Ballouz, who grew up during the 1990s in post-civil war Beirut, Lebanon, the image of a rock surface pitted with small impact craters evoked childhood memories of building walls riddled with bullet holes in his war-torn home country.

- "Where I grew up, the buildings have bullet holes all over, and I never thought about it," he said. "It was just a fact of life. So, when I looked at the images from the asteroid, I was very curious, and I immediately thought these must be impact features."

- The observations made by Ballouz and his team bridge a gap between previous studies of space debris larger than a few centimeters, based on impacts on the moon, and studies of objects smaller than a few millimeters, based on observations of meteors entering Earth's atmosphere and impacts on spacecraft.

- "The objects that formed the craters on Bennu's boulders fall within this gap that we don't really know much about," Ballouz said, adding that rocks in that size range are an important field of study, mainly because they represent hazards for spacecraft in orbit around Earth. "An impact from one of these millimeter to centimeter-size objects at speeds of 45,000 miles per hour can be dangerous."

- Ballouz and his team developed a technique to quantify the strength of solid objects using remote observations of craters on the surfaces of boulders – a mathematical formula that allows researchers to calculate the maximum impact energy that a boulder of a given size and strength could endure before being smashed. In other words, the crater distribution found on Bennu today keeps a historical record of the frequency, size and velocity of impact events the asteroid has experienced throughout its history.

- "The idea is actually pretty simple," Ballouz said, using a building exposed to artillery fire as an analogy to boulders on an asteroid. "We ask, 'What is the largest crater you can make on that wall before the wall disintegrates?' Based on observations of multiple walls of the same size, but with different sized craters, you can get some idea of the strength of that wall."

- The same holds true for a boulder on an asteroid or other airless body, said Ballouz, who added that the approach could be used on any other asteroid or airless body that astronauts or spacecraft may visit in the future.

- "If a boulder gets hit by something larger than an object that would leave a certain size cater, it would just disappear," he explained. In other words, the size distribution of boulders that have persisted on Bennu serve as silent witnesses to its geologic past.

OsirisREx_Auto41

Figure 17: This composite image of a boulder on Bennu’s surface shows the cascading rim of one of the asteroid’s ancient craters that originated while Bennu resided in the asteroid belt. The image combines photos from OSIRIS-REx and reconstructed shape models built from the OSIRIS-REx laser altimeter instrument. The overlaid colors highlight the topography of the boulder (warmer colors are higher elevation), image credit: University of Arizona/Johns Hopkins APL/York University

A Newcomer to Earth's Neighborhood

- Applying the technique to boulders ranging in size from pebbles to parking garages, the researchers were able to make inferences about the sizes and type of impactors to which the boulders were exposed, and for how long.

- The authors conclude that the largest craters on Bennu's boulders were created while Bennu resided in the asteroid belt, where impact speeds are lower than in the near-Earth environment, but are more frequent and often near the limit of what the boulders could withstand. Smaller craters, on the other hand, were acquired more recently, during Bennu's time in near-Earth space, where impact speeds are higher but potentially disruptive impactors are much less common.

- Based on these calculations, the authors determine that Bennu is a relative newcomer to Earth's neighborhood. Although it is thought to have formed in the main asteroid belt more than 100 million years ago, it is estimated that it was kicked out of the asteroid belt and migrated to its current territory only 1.75 million years ago. Extending the results to other near-Earth objects, or NEOs, the researchers also suggest that these objects likely come from parent bodies that fall in the category of asteroids, which are mostly rocky with little or no ice, rather than comets, which have more ice than rock.

- While theoretical models suggest that the asteroid belt is the reservoir for NEOs, no observational evidence of their provenance was available other than meteorites that fell to Earth and were collected, Ballouz said. With these data, researchers can validate their models of where NEOs come from, according to Ballouz, and get an idea of how strong and solid these objects are – crucial information for any potential missions targeting asteroids in the future for research, resource extraction or protecting Earth from impact.

• October 29, 2020: NASA's OSIRIS-REx mission has successfully stowed the spacecraft’s Sample Return Capsule (SRC) and its abundant sample of asteroid Bennu. On Wednesday, Oct. 28, the mission team sent commands to the spacecraft, instructing it to close the capsule – marking the end of one of the most challenging phases of the mission. 37)

OsirisREx_Auto40

Figure 18: The left image shows the OSIRIS-REx collector head hovering over the Sample Return Capsule (SRC) after the TAGSAM (Touch-And-Go Sample Acquisition Mechanism) arm moved it into the proper position for capture. The right image shows the collector head secured onto the capture ring in the SRC. Both images were captured by the StowCam camera (image credits: NASA/Goddard/University of Arizona/Lockheed Martin)

- “This achievement by OSIRIS-REx on behalf of NASA and the world has lifted our vision to the higher things we can achieve together, as teams and nations,” said NASA Administrator Jim Bridenstine. “Together a team comprising industry, academia and international partners, and a talented and diverse team of NASA employees with all types of expertise, has put us on course to vastly increase our collection on Earth of samples from space. Samples like this are going to transform what we know about our universe and ourselves, which is at the base of all NASA’s endeavors.”

- The mission team spent two days working around the clock to carry out the stowage procedure, with preparations for the stowage event beginning Oct. 24. The process to stow the sample is unique compared to other spacecraft operations and required the team’s continuous oversight and input over the two-day period. For the spacecraft to proceed with each step in the stowage sequence, the team had to assess images and telemetry from the previous step to confirm the operation was successful and the spacecraft was ready to continue. Given that OSIRIS-REx is currently more than 205 million miles (330 million km) from Earth, this required the team to also work with a greater than 18.5-minute time delay for signals traveling in each direction.

- Throughout the process, the OSIRIS-REx team continually assessed the TAGSAM's wrist alignment to ensure the collector head was being placed properly into the SRC. Additionally, the team inspected images to observe any material escaping from the collector head to confirm that no particles would hinder the stowage process. StowCam images of the stowage sequence show that a few particles escaped during the stowage procedure, but the team is confident that a plentiful amount of material remains inside of the head.

- “Given the complexity of the process to place the sample collector head onto the capture ring, we expected that it would take a few attempts to get it in the perfect position,” said Rich Burns, OSIRIS-REx project manager at NASA's Goddard Space Flight Center in Greenbelt, Maryland. “Fortunately, the head was captured on the first try, which allowed us to expeditiously execute the stow procedure.”

- By the evening of Oct. 27, the spacecraft’s TAGSAM arm had placed the collector head into the SRC. The following morning, the OSIRIS-REx team verified that the collector head was thoroughly fastened into the capsule by performing a “backout check.” This sequence commanded the TAGSAM arm to attempt to back out of the capsule – which tugged on the collector head and ensured the latches are well secured.

- “I want to thank the OSIRIS-REx team from the University of Arizona, NASA Goddard, Lockheed Martin, and their partners, and also especially the SCaN and Deep Space Network people at NASA and JPL, who worked tirelessly to get us the bandwidth we needed to achieve this milestone, early and while still hundreds of millions of miles away,” said Thomas Zurbuchen, NASA’s associate administrator for science at the agency’s headquarters in Washington. “What we have done is a real first for NASA, and we will benefit for decades by what we have been able to achieve at Bennu.”

- On the afternoon of Oct. 28, following the backout check, the mission team sent commands to disconnect the two mechanical parts on the TAGSAM arm that connect the sampler head to the arm. The spacecraft first cut the tube that carried the nitrogen gas that stirred up the sample through the TAGSAM head during sample collection, and then separated the collector head from the TAGSAM arm itself.

- That evening, the spacecraft completed the final step of the sample stowage process –closing the SRC. To secure the capsule, the spacecraft closed the lid and then fastened two internal latches. As of late Oct. 28, the sample of Bennu is safely stored and ready for its journey to Earth.

- “I’m very thankful that our team worked so hard to get this sample stowed as quickly as they did,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. “Now we can look forward to receiving the sample here on Earth and opening up that capsule.”

- The stowage process, originally scheduled to begin in early November, was expedited after sample collection when the mission team received images that showed the spacecraft’s collector head overflowing with material. The images indicated that the spacecraft collected well over 2 ounces (60 grams) of Bennu’s surface material, and that some of these particles appeared to be slowly escaping from the head. A mylar flap designed to keep the sample inside the head appeared to be wedged open by some larger rocks. Now that the head is secure inside the SRC, pieces of the sample will no longer be lost.

- The OSIRIS-REx team will now focus on preparing the spacecraft for the next phase of the mission – Earth Return Cruise. The departure window opens in March 2021 for OSIRIS-REx to begin its voyage home, and the spacecraft is targeting delivery of the SRC to Earth on Sep. 24, 2023.

• October 27, 2020: NASA’s OSIRIS-REx mission is ready to perform an early stow on Tuesday, Oct. 27, of the large sample it collected last week from the surface of the asteroid Bennu to protect and return as much of the sample as possible. 38)

- On Oct. 22, the OSIRIS-REx mission team received images that showed the spacecraft’s collector head overflowing with material collected from Bennu’s surface – well over the two-ounce (60 gram) mission requirement – and that some of these particles appeared to be slowly escaping from the collection head, called the Touch-And-Go Sample Acquisition Mechanism (TAGSAM).

OsirisREx_Auto3F

Figure 19: This illustration shows NASA’s OSIRIS-REx spacecraft stowing the sample it collected from asteroid Bennu on Oct. 20, 2020. The spacecraft will use its TAGSAM arm to place the TAGSAM collector head into the Sample Return Capsule (SRC), image credits: NASA/University of Arizona, Tucson

- A mylar flap on the TAGSAM allows material to easily enter the collector head, and should seal shut once the particles pass through. However, larger rocks that didn’t fully pass through the flap into the TAGSAM appear to have wedged this flap open, allowing bits of the sample to leak out.

- Because the first sample collection event was so successful, NASA’s Science Mission Directorate has given the mission team the go-ahead to expedite sample stowage, originally scheduled for Nov. 2, in the spacecraft’s Sample Return Capsule (SRC) to minimize further sample loss.

- "The abundance of material we collected from Bennu made it possible to expedite our decision to stow,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. “The team is now working around the clock to accelerate the stowage timeline, so that we can protect as much of this material as possible for return to Earth."

- Unlike other spacecraft operations where OSIRIS-REx autonomously runs through an entire sequence, stowing the sample is done in stages and requires the team’s oversight and input. The team will send the preliminary commands to the spacecraft to start the stow sequence and, once OSIRIS-REx completes each step in sequence, the spacecraft sends telemetry and images back to the team on Earth and waits for the team’s confirmation to proceed with the next step.

- Signals currently take just over 18.5 minutes to travel between Earth and the spacecraft one-way, so each step of the sequence factors in about 37 minutes of communications transit time. Throughout the process, the mission team will continually assess the TAGSAM’s wrist alignment to ensure the collector head is properly placed in the SRC. A new imaging sequence also has been added to the process to observe the material escaping from the collector head and verify that no particles hinder the stowage process. The mission anticipates the entire stowage process will take multiple days, at the end of which the sample will be safely sealed in the SRC for the spacecraft’s journey back to Earth.

- “I’m proud of the OSIRIS-REx team’s amazing work and success to this point,” said NASA’s Associate Administrator for Science Thomas Zurbuchen. “This mission is well positioned to return a historic and substantial sample of an asteroid to Earth, and they’ve been doing all the right things, on an expedited timetable, to protect that precious cargo.”

• October 23, 2020: Two days after touching down on asteroid Bennu, NASA’s OSIRIS-REx mission team received on Thursday, Oct. 22, images that confirm the spacecraft has collected more than enough material to meet one of its main mission requirements – acquiring at least 2 ounces (60 grams) of the asteroid’s surface material. 39)

Figure 20: Captured by the spacecraft’s SamCam camera on Oct. 22, 2020, this series of three images shows that the sampler head on NASA’s OSIRIS-REx spacecraft is full of rocks and dust collected from the surface of the asteroid Bennu. They show also that some of these particles are slowly escaping the sampler head. Analysis by the OSIRIS-REx team suggests that bits of material are passing through small gaps where the head’s mylar flap is slightly wedged open. The mylar flap (the black bulge on the left inside the ring) is designed to keep the collected material locked inside, and these unsealed areas appear to be caused by larger rocks that didn’t fully pass through the flap. Based on available imagery, the team suspects there is plentiful sample inside the head, and is on a path to stow the sample as quickly as possible (image credit: NASA)

- The spacecraft captured images of the sample collector head as it moved through several different positions. In reviewing these images, the OSIRIS-REx team noticed both that the head appeared to be full of asteroid particles, and that some of these particles appeared to be escaping slowly from the sample collector, called the Touch-And-Go Sample Acquisition Mechanism (TAGSAM) head. They suspect bits of material are passing through small gaps where a mylar flap – the collector’s “lid” – is slightly wedged open by larger rocks.

- “Bennu continues to surprise us with great science and also throwing a few curveballs,” said Thomas Zurbuchen, NASA’s associate administrator for science at the agency’s headquarters in Washington. “And although we may have to move more quickly to stow the sample, it’s not a bad problem to have. We are so excited to see what appears to be an abundant sample that will inspire science for decades beyond this historic moment.”

- The team believes it has collected a sufficient sample and is on a path to stow the sample as quickly as possible. They came to this conclusion after comparing images of the empty collector head with Oct. 22 images of the TAGSAM head after the sample collection event.

- The images also show that any movement to the spacecraft and the TAGSAM instrument may lead to further sample loss. To preserve the remaining material, the mission team decided to forego the Sample Mass Measurement activity originally scheduled for Saturday, Oct. 24, and canceled a braking burn scheduled for Friday to minimize any acceleration to the spacecraft.

- From here, the OSIRIS-Rex team will focus on stowing the sample in the Sample Return Capsule (SRC), where any loose material will be kept safe during the spacecraft’s journey back to Earth.

- “We are working to keep up with our own success here, and my job is to safely return as large a sample of Bennu as possible,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona in Tucson, who leads the science team and the mission’s science observation planning and data processing. “The loss of mass is of concern to me, so I’m strongly encouraging the team to stow this precious sample as quickly as possible.”

- The TAGSAM head performed the sampling event in optimal conditions. Newly available analyses show that the collector head was flush with Bennu’s surface when it made contact and when the nitrogen gas bottle was fired to stir surface material. It also penetrated several centimeters into the asteroid’s surface material. All data so far suggest that the collector head is holding much more than 2 ounces of regolith.

- OSIRIS-REx remains in good health, and the mission team is finalizing a timeline for sample storage. An update will be provided once a decision is made on the sample storage timing and procedures.

• NASA’s OSIRIS-REx spacecraft unfurled its robotic arm Tuesday (20 October 2020), and in a first for the agency, briefly touched an asteroid to collect dust and pebbles from the surface for delivery to Earth in 2023. 40)

OsirisREx_Auto3E

Figure 21: NASA’s OSIRIS-REx mission readies itself to touch the surface of asteroid Bennu (image credits: NASA/Goddard/University of Arizona)

- This well-preserved, ancient asteroid, known as Bennu, is currently more than 200 million miles (321 million km) from Earth. Bennu offers scientists a window into the early solar system as it was first taking shape billions of years ago and flinging ingredients that could have helped seed life on Earth. If Tuesday’s sample collection event, known as “Touch-And-Go” (TAG), provided enough of a sample, mission teams will command the spacecraft to begin stowing the precious primordial cargo to begin its journey back to Earth in March 2021. Otherwise, they will prepare for another attempt in January.

- “This amazing first for NASA demonstrates how an incredible team from across the country came together and persevered through incredible challenges to expand the boundaries of knowledge,” said NASA Administrator Jim Bridenstine. “Our industry, academic, and international partners have made it possible to hold a piece of the most ancient solar system in our hands.”

- At 1:50 p.m. EDT, OSIRIS-REx fired its thrusters to nudge itself out of orbit around Bennu. It extended the shoulder, then elbow, then wrist of its 11-foot (3.35-meter) sampling arm, known as the Touch-And-Go Sample Acquisition Mechanism (TAGSAM), and transited across Bennu while descending about a half-mile (805 meters) toward the surface. After a four-hour descent, at an altitude of approximately 410 feet (125 m), the spacecraft executed the “Checkpoint” burn, the first of two maneuvers to allow it to precisely target the sample collection site, known as “Nightingale.”

- Ten minutes later, the spacecraft fired its thrusters for the second “Matchpoint” burn to slow its descent and match the asteroid’s rotation at the time of contact. It then continued a treacherous, 11-minute coast past a boulder the size of a two-story building, nicknamed “Mount Doom,” to touch down in a clear spot in a crater on Bennu’s northern hemisphere. The size of a small parking lot, the site Nightingale site is one of the few relatively clear spots on this unexpectedly boulder-covered space rock.

- “This was an incredible feat – and today we’ve advanced both science and engineering and our prospects for future missions to study these mysterious ancient storytellers of the solar system,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate at the agency’s headquarters in Washington. “A piece of primordial rock that has witnessed our solar system’s entire history may now be ready to come home for generations of scientific discovery, and we can’t wait to see what comes next.”

- “After over a decade of planning, the team is overjoyed at the success of today’s sampling attempt,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona in Tucson. “Even though we have some work ahead of us to determine the outcome of the event – the successful contact, the TAGSAM gas firing, and back-away from Bennu are major accomplishments for the team. I look forward to analyzing the data to determine the mass of sample collected.”

- All spacecraft telemetry data indicates the TAG event executed as expected. However, it will take about a week for the OSIRIS-REx team to confirm how much sample the spacecraft collected.

- Real-time data indicates the TAGSAM successfully contacted the surface and fired a burst of nitrogen gas. The gas should have stirred up dust and pebbles on Bennu’s surface, some of which should have been captured in the TAGSAM sample collection head. OSIRIS-REx engineers also confirmed that shortly after the spacecraft made contact with the surface, it fired its thrusters and safely backed away from Bennu.

- “Today’s TAG maneuver was historic,” said Lori Glaze, Planetary Science Division director at NASA Headquarters in Washington. “The fact that we safely and successfully touched the surface of Bennu, in addition to all the other milestones this mission has already achieved, is a testament to the living spirit of exploration that continues to uncover the secrets of the solar system."

Figure 22: Captured on Aug. 11, 2020 during the second rehearsal of the OSIRIS-REx mission’s sample collection event, this series of images shows the SamCam imager’s field of view as the NASA spacecraft approaches asteroid Bennu’s surface. The rehearsal brought the spacecraft through the first three maneuvers of the sampling sequence to a point approximately 131 feet (40 meters) above the surface, after which the spacecraft performed a back-away burn (image credits: NASA/Goddard/University of Arizona)

- “It’s hard to put into words how exciting it was to receive confirmation that the spacecraft successfully touched the surface and fired one of the gas bottles,” said Michael Moreau, OSIRIS-REx deputy project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “The team can’t wait to receive the imagery from the TAG event late tonight and see how the surface of Bennu responded to the TAG event.”

- The spacecraft carried out TAG autonomously, with pre-programmed instructions from engineers on Earth. Now, the OSIRIS-REx team will begin to assess whether the spacecraft grabbed any material, and, if so, how much; the goal is at least 60 grams, which is roughly equivalent to a full-size candy bar.

- OSIRIS-REx engineers and scientists will use several techniques to identify and measure the sample remotely. First, they’ll compare images of the Nightingale site before and after TAG to see how much surface material moved around in response to the burst of gas.

- “Our first indication of whether we were successful in collecting a sample will come on October 21 when we downlink the back-away movie from the spacecraft,” Moreau said. “If TAG made a significant disturbance of the surface, we likely collected a lot of material.”

- Next, the team will try to determine the amount of sample collected. One method involves taking pictures of the TAGSAM head with a camera known as SamCam, which is devoted to documenting the sample-collection process and determining whether dust and rocks made it into the collector head. One indirect indication will be the amount of dust found around the sample collector head. OSIRIS-REx engineers also will attempt to snap photos that could, given the right lighting conditions, show the inside of the head so engineers can look for evidence of sample inside of it.

OsirisREx_Auto3D

Figure 23: These images show the OSIRIS-REx Touch-and-Go Sample Acquisition Mechanism (TAGSAM) sampling head extended from the spacecraft at the end of the TAGSAM arm. The spacecraft’s SamCam camera captured the images on Nov. 14, 2018 as part of a visual checkout of the TAGSAM system, which was developed by Lockheed Martin Space to acquire a sample of asteroid material in a low-gravity environment. The imaging was a rehearsal for a series of observations that will be taken at Bennu directly after sample collection (image credits: NASA/Goddard/University of Arizona)

- A couple of days after the SamCam images are analyzed, the spacecraft will attempt yet another method to measure the mass of the sample collected by determining the change in the spacecraft’s “moment of inertia,” a phrase that describes how mass is distributed and how it affects the rotation of the body around a central axis. This maneuver entails extending the TAGSAM arm out to the side of the spacecraft and slowly spinning the spacecraft about an axis perpendicular to the arm. This technique is analogous to a person spinning with one arm extended while holding a string with a ball attached to the end. The person can sense the mass of the ball by the tension in the string. Having performed this maneuver before TAG, and now after, engineers can measure the change in the mass of the collection head as a result of the sample inside.

- “We will use the combination of data from TAG and the post-TAG images and mass measurement to assess our confidence that we have collected at least 60 grams of sample,” said Rich Burns, OSIRIS-REx project manager at Goddard. “If our confidence is high, we'll make the decision to stow the sample on October 30.”

- To store the sample, engineers will command the robotic arm to place the sample collector head into the Sample Return Capsule (SRC), located in the body of the spacecraft. The sample arm will then retract to the side of the spacecraft for the final time, the SRC will close, and the spacecraft will prepare for its departure from Bennu in March 2021 — this is the next time Bennu will be properly aligned with Earth for the most fuel-efficient return flight.

Figure 24: This (silent) animation shows the OSIRIS-REx spacecraft deploying its Touch-and-Go Sample Acquisition Mechanism (TAGSAM) to collect a sample of regolith (loose rocks and dirt) from the surface of the asteroid Bennu. The sampler head, with the regolith safely inside, is then sealed up in the spacecraft's Sample Return Capsule, which will be returned to Earth in late 2023. Scientists will study the sample for clues about the early solar system and the origins of life (video credits: NASA/Goddard)

- If, however, it turns out that the spacecraft did not collect enough sample at Nightingale, it will attempt another TAG maneuver on Jan. 12, 2021. If that occurs, it will touch down at the backup site called “Osprey,” which is another relatively boulder-free area inside a crater near Bennu’s equator.

- OSIRIS-REx launched from Cape Canaveral Air Force Station in Florida Sept. 8, 2016. It arrived at Bennu Dec. 3, 2018, and began orbiting the asteroid for the first time on Dec. 31, 2018. The spacecraft is scheduled to return to Earth Sept. 24, 2023, when it will parachute the SRC into Utah's west desert where scientists will be waiting to collect it.

- 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.

• September 21, 2020: In an interplanetary faux pas, it appears some pieces of asteroid Vesta ended up on asteroid Bennu, according to observations from NASA’s OSIRIS-REx spacecraft. The new result sheds light on the intricate orbital dance of asteroids and on the violent origin of Bennu, which is a “rubble pile” asteroid that coalesced from the fragments of a massive collision. 41)

Figure 25: It appears some pieces of asteroid Vesta ended up on asteroid Bennu, according to observations from NASA’s OSIRIS-REx spacecraft. The new result sheds light on the intricate orbital dance of asteroids and on the violent origin of Bennu (video credit: NASA's Goddard Space Flight Center)

- “We found six boulders ranging in size from 5 to 14 feet (about 1.5 to 4.3 meters) scattered across Bennu’s southern hemisphere and near the equator,” said Daniella DellaGiustina of the Lunar & Planetary Laboratory, University of Arizona, Tucson. “These boulders are much brighter than the rest of Bennu and match material from Vesta.”

OsirisREx_Auto3C

Figure 26: During spring 2019, NASA’s OSIRIS-REx spacecraft captured these images, which show fragments of asteroid Vesta present on asteroid Bennu’s surface. The bright boulders (circled in the images) are pyroxene-rich material from Vesta. Some bright material appear to be individual rocks (left) while others appear to be clasts within larger boulders (right), image credits: NASA/Goddard/University of Arizona

- “Our leading hypothesis is that Bennu inherited this material from its parent asteroid after a vestoid (a fragment from Vesta) struck the parent,” said Hannah Kaplan of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Then, when the parent asteroid was catastrophically disrupted, a portion of its debris accumulated under its own gravity into Bennu, including some of the pyroxene from Vesta.”

- DellaGiustina and Kaplan are primary authors of a paper on this research appearing in Nature Astronomy September 21.

- The unusual boulders on Bennu first caught the team’s eye in images from the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) Camera Suite (OCAMS). They appeared extremely bright, with some almost ten times brighter than their surroundings. They analyzed the light from the boulders using the OSIRIS-REx Visible and Infrared Spectrometer (OVIRS) instrument to get clues to their composition. A spectrometer separates light into its component colors. Since elements and compounds have distinct, signature patterns of bright and dark across a range of colors, they can be identified using a spectrometer. The signature from the boulders was characteristic of the mineral pyroxene, similar to what is seen on Vesta and the vestoids, smaller asteroids that are fragments blasted from Vesta when it sustained significant asteroid impacts.

- Of course it’s possible that the boulders actually formed on Bennu’s parent asteroid, but the team thinks this is unlikely based on how pyroxene typically forms. The mineral typically forms when rocky material melts at high-temperature. However, most of Bennu is composed of rocks containing water-bearing minerals, so it (and its parent) couldn’t have experienced very high temperatures. Next, the team considered localized heating, perhaps from an impact. An impact needed to melt enough material to create large pyroxene boulders would be so significant that it would have destroyed Bennu's parent-body. So, the team ruled out these scenarios, and instead considered other pyroxene-rich asteroids that might have implanted this material to Bennu or its parent.

- Observations reveal it’s not unusual for an asteroid to have material from another asteroid splashed across its surface. Examples include dark material on crater walls seen by the Dawn spacecraft at Vesta, a black boulder seen by the Hayabusa spacecraft on Itokawa, and very recently, material from S-type asteroids observed by Hayabusa-2 at Ryugu. This indicates many asteroids are participating in a complex orbital dance that sometimes results in cosmic mashups.

- As asteroids move through the solar system, their orbits can be altered in many ways, including the pull of gravity from planets and other objects, meteoroid impacts, and even the slight pressure from sunlight. The new result helps pin down the complex journey Bennu and other asteroids have traced through the solar system.

- Based on its orbit, several studies indicate Bennu was delivered from the inner region of the Main Asteroid Belt via a well-known gravitational pathway that can take objects from the inner Main Belt to near-Earth orbits. There are two inner Main Belt asteroid families (Polana and Eulalia) that look like Bennu: dark and rich in carbon, making them likely candidates for Bennu’s parent. Likewise, the formation of the vestoids is tied to the formation of the Veneneia and Rheasilvia impact basins on Vesta, at roughly about two billion years ago and approximately one billion years ago, respectively.

- “Future studies of asteroid families, as well as the origin of Bennu, must reconcile the presence of Vesta-like material as well as the apparent lack of other asteroid types. We look forward to the returned sample, which hopefully contains pieces of these intriguing rock types,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona in Tucson. “This constraint is even more compelling given the finding of S-type material on asteroid Ryugu. This difference shows the value in studying multiple asteroids across the solar system.”

- The spacecraft is going to make its first attempt to sample Bennu in October and return it to Earth in 2023 for detailed analysis. The mission team closely examined four potential sample sites on Bennu to determine their safety and science value before making a final selection in December 2019. DellaGiustina and Kaplan’s team thinks they might find smaller pieces of Vesta in images from these close-up studies.

- The research was funded by the NASA New Frontiers Program. The primary authors acknowledge significant collaboration with the French space agency CNES and Japan Society for the Promotion of Science Core-to-core Program on this paper. 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. The late Michael Drake of the University of Arizona pioneered the study of vestoid meteorites and was the first principal investigator for OSIRIS-REx. 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. NASA is exploring our Solar System and beyond, uncovering worlds, stars, and cosmic mysteries near and far with our powerful fleet of space and ground-based missions.

• September 9, 2020: The asteroid, which is being studied by NASA's OSIRIS-REx, shows some surprising activity on its surface, and scientists are beginning to understand what might be causing it. 42)

- When NASA's OSIRIS-REx spacecraft arrived at asteroid (101955) Bennu, mission scientists knew that their spacecraft was orbiting something special. Not only was the boulder-strewn asteroid shaped like a rough diamond, its surface was crackling with activity, shedding small pieces of rock into space. Now, after more than a year and a half up close with Bennu, they're starting to better understand these dynamic particle-ejection events.

- A collection of studies in a special edition of the Journal of Geophysical Research: Planets homes in on the asteroid and these enigmatic particles. The studies provide a detailed look at how these particles act when in space, possible clues as to how they're ejected, and even how their trajectories can be used to approximate Bennu's weak gravitational field.

- Typically, we consider comets, not asteroids, to be the active ones. Comets are composed of ice, rock, and dust. As those ices are heated by the Sun, the vapor fizzes from the surface, dust and chunks of the comet nucleus are lost to space, and a long dusty tail forms. Asteroids, on the other hand, are composed mainly of rock and dust (and perhaps a smaller quantity of ice), but it turns out some of these space rocks can be surprisingly lively, too.

- "We thought that Bennu's boulder-covered surface was the wild card discovery at the asteroid, but these particle events definitely surprised us," said Dante Lauretta, the OSIRIS-REx principal investigator and a professor at the University of Arizona. "We've spent the last year investigating Bennu's active surface, and it's provided us with a remarkable opportunity to expand our knowledge of how active asteroids behave."

- Cameras on OSIRIS-REx (short for Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) spotted rock particles being repeatedly launched into space during a January 2019 survey of the asteroid, which is about a third of a mile (565 meters) wide at its equator.

- One of the studies, led by senior research scientist Steve Chesley at NASA's Jet Propulsion Laboratory in Southern California, found that most of these pebble-size pieces of rock, typically measuring around a quarter-inch (7 millimeters), were pulled back to Bennu under the asteroid's weak gravity after a short hop, sometimes even ricocheting back into space after colliding with the surface. Others took longer to return to the surface, remaining in orbit for a few days and up to 16 revolutions. And some were ejected with enough oomph to completely escape from the Bennu environs.

Figure 27: Using data collected by NASA's OSIRIS-REx mission, this animation shows the trajectories of rock particles after being ejected from asteroid (101955) Bennu's surface (video credit: M. Brozovic/NASA/JPL-Caltech/University of Arizona)

- By tracking the journeys of hundreds of ejected particles, Chesley and his collaborators were also able to better understand what might be causing the particles to launch from the surface of Bennu. The particle sizes match what is expected for thermal fracturing (as the asteroid's surface is repeatedly heated and cooled while it rotates), but the locations of the ejection events also match the modeled impact locations of meteoroids (small rocks hitting the surface of Bennu as it orbits the Sun). It may even be a combination of these phenomena, added Chesley. But to come to a definitive answer, more observations are needed.

- While their very existence poses numerous scientific questions, the particles also served as high-fidelity probes of Bennu's gravity field. Many particles were orbiting Bennu far closer than would be safe for the OSIRIS-REx spacecraft, and so their trajectories were highly sensitive to the irregular gravity of Bennu. This allowed researchers to estimate the Bennu's gravity even more precisely than was possible with OSIRIS-REx's instruments.

- "The particles were an unexpected gift for gravity science at Bennu since they allowed us to see tiny variations in the asteroid's gravity field that we would not have known about otherwise," said Chesley.

- On average, only one or two particles are ejected per day, and because they are in a very low-gravity environment, most are moving slowly. As such, they pose little threat to OSIRIS-REx, which will attempt to briefly touch down on the asteroid on Oct. 20 to scoop up surface material, which may even include particles that were ejected before dropping back to the surface.

- If all goes as planned, the spacecraft will return to Earth in September 2023 with a cache of Bennu's material for scientists to study further.

• August 6, 2020: NASA’s first asteroid sampling spacecraft is making final preparations to grab a sample from asteroid Bennu’s surface. Next week, the OSIRIS-REx mission will conduct a second rehearsal of its touchdown sequence, practicing the sample collection activities one last time before touching down on Bennu this fall. 43)

- On Aug. 11, the mission will perform its “Matchpoint” rehearsal – the second practice run of the Touch-and-Go (TAG) sample collection event. The rehearsal will be similar to the Apr. 14 “Checkpoint” rehearsal, which practiced the first two maneuvers of the descent, but this time the spacecraft will add a third maneuver, called the Matchpoint burn, and fly even closer to sample site Nightingale – reaching an altitude of approximately 40 m – before backing away from the asteroid.

- This second rehearsal will be the first time the spacecraft executes the Matchpoint maneuver to then fly in tandem with Bennu’s rotation. The rehearsal also gives the team a chance to become more familiar navigating the spacecraft through all of the descent maneuvers, while verifying that the spacecraft’s imaging, navigation and ranging systems operate as expected during the event.

- During the descent, the spacecraft fires its thrusters three separate times to make its way down to the asteroid’s surface. The spacecraft will travel at an average speed of around 0.3 km/hr during the approximately four-hour excursion. Matchpoint rehearsal begins with OSIRIS-REx firing its thrusters to leave its 870 m safe-home orbit. The spacecraft then extends its robotic sampling arm – the Touch-And-Go Sample Acquisition Mechanism (TAGSAM) – from its folded, parked position out to the sample collection configuration. Immediately following, the spacecraft rotates to begin collecting navigation images for the Natural Feature Tracking (NFT) guidance system. NFT allows OSIRIS-REx to autonomously navigate to Bennu’s surface by comparing an onboard image catalog with the real-time navigation images taken during descent. As the spacecraft approaches the surface, the NFT system updates the spacecraft’s predicted point of contact depending on OSIRIS-REx’s position in relation to Bennu’s landmarks.

- The spacecraft’s two solar panels then move into a “Y-wing” configuration that safely positions them up and away from the asteroid’s surface. This configuration also places the spacecraft’s center of gravity directly over the TAGSAM collector head, which is the only part of the spacecraft that will contact Bennu’s surface during the sample collection event.

- When OSIRIS-REx reaches an altitude of approximately 125 m, it performs the Checkpoint burn and descends more steeply toward Bennu’s surface for another eight minutes. At approximately 50 m above the asteroid, the spacecraft fires its thrusters a third time for the Matchpoint burn. This maneuver slows the spacecraft’s rate of descent and adjusts its trajectory to match Bennu’s rotation as the spacecraft makes final corrections to target the touchdown spot. OSIRIS-REx will continue capturing images of Bennu’s landmarks for the NFT system to update the spacecraft’s trajectory for another three minutes of descent. This brings OSIRIS-REx to its targeted destination around 40 m from Bennu – the closest it has ever been to the asteroid. With the rehearsal complete, the spacecraft executes a back-away burn, returns its solar panels to their original position and reconfigures the TAGSAM arm back to the parked position.

- During the rehearsal, the one-way light time for signals to travel between Earth and the spacecraft will be approximately 16 minutes, which prevents the live commanding of flight activities from the ground. So prior to the rehearsal’s start, the OSIRIS-REx team will uplink all of the event’s commands to the spacecraft, allowing OSIRIS-REx to perform the rehearsal sequence autonomously after the GO command is given. Also during the event, the spacecraft’s low gain antenna will be its only antenna pointing toward Earth, transmitting data at the very slow rate of 40 bit/s. So while the OSIRIS-REx team will be able to monitor the spacecraft’s vital signs, the images and science data collected during the event won’t be downlinked until the rehearsal is complete. The team will experience these same circumstances during the actual TAG event in October.

- Following Matchpoint rehearsal, the OSIRIS-REx team will verify the flight system’s performance during the descent, including that the Matchpoint burn accurately adjusted the spacecraft’s descent trajectory for its touchdown on Bennu. Once the mission team determines that OSIRIS-REx operated as expected, they will command the spacecraft to return to its safe-home orbit around Bennu.

- The mission team has spent the last several months preparing for the Matchpoint rehearsal while maximizing remote work as part of its COVID-19 response. On the day of rehearsal, a limited number of personnel will monitor the spacecraft from Lockheed Martin Space’s facility, taking appropriate safety precautions, while the rest of the team performs their roles remotely. The mission implemented a similar protocol during the Checkpoint rehearsal in April.

- On Oct. 20, the spacecraft will travel all the way to the asteroid’s surface during its first sample collection attempt. During this event, OSIRIS-REx’s sampling mechanism will touch Bennu’s surface for approximately five seconds, fire a charge of pressurized nitrogen to disturb the surface and collect a sample before the spacecraft backs away. The spacecraft is scheduled to return the sample to Earth on Sept. 24, 2023.

• July 15, 2020: The University of Arizona has played a role in imaging and mapping most major objects in the solar system. Now, it adds the asteroid Bennu to the list. The Bennu Global Mosaic, as the complete map of the asteroid is called, is the highest resolution map of any celestial body. 44)

OsirisREx_Auto3B

Figure 28: This global map of asteroid Bennu’s surface was created by stitching and correcting 2,155 PolyCam images. At 2 inches (5 cm) per pixel, this is the highest resolution at which a planetary body has been globally mapped (image credit: NASA/Goddard/University of Arizona)

- As NASA's OSIRIS-REx spacecraft prepares to briefly touch down and collect a sample from the asteroid Bennu in October, the mission's science team, led by the University of Arizona, has worked meticulously to create the highest resolution global map of any planetary body, including Earth. The endeavor is the latest in the university's long history of celestial imaging and mapping – one that began with the first lunar landings.

- The team stitched together 2,155 images – containing pixels that translate to two square inches on the surface – to create the Bennu Global Mosaic.

- "This is the finest spatial scale we've ever mapped of a planetary object," said Daniella DellaGiustina, OSIRIS-REx image processing lead scientist. "It's also unprecedented in the way we used it. Typically, when NASA chooses a landing site for an upcoming mission, they have an orbiter doing reconnaissance of the surface long before a separate mission contacts the surface. But we went to Bennu without that luxury. This paradigm of doing every step in close succession is unique and made things demanding."

- The spacecraft collected the images at distances ranging from 2.2 to 2.9 miles above the asteroid's surface between March 21 and April 11, 2019. The mosaic was completed in February.

- The detailed view of Bennu was used by the mission team during its selection of the primary and backup sample collection sites, dubbed Nightingale and Osprey, respectively.

Making a Mosaic

- There are a couple of important criteria that a useful map of Bennu's surface needed to meet. "It needed to contain minimal distortion and good lighting to get sense for texture and relief across surface," DellaGiustina said.

- Carina Bennett was up for the task. She has a background in photography, film and art, having earned a Bachelor of Arts in media arts and creative writing from UArizona and a Master of Arts in film and video production from the University of Iowa. She worked as a videographer in University Communications at UArizona nearly 10 years ago while simultaneously enrolled in computer science courses. Her computer science degree and the connections she forged while working for the university brought her to her first job on the OSIRIS-REx mission. She is now a senior engineer on the mission's image processing team.

- To create the Bennu Global Mosaic, the team first had to capture images of the surface using the PolyCam instrument.

- "PolyCam, one of the UArizona-developed cameras onboard the spacecraft, captured 7,000 images, and I narrowed those down to just over 2,100," Bennett said. "I looked for images that had the best geometry, meaning the best angle between the spacecraft and the part of the asteroid we were imaging and the best angle between the sun and that area."

- The spacecraft snapped photos from three predetermined orbital angles – in the northern hemisphere, at the equator and in the southern hemisphere – that made sure there were clear views of the entire asteroid surface and optimized the shadows of Bennu's features. While maps typically want to eliminate shadows, they were needed in this case to make the surface features pop.

- "We wanted a little shadow, but not too much and not weird angles. It was all just very meticulously planned," Bennett said.

- Then, using a 3D model of the asteroid that was created using a program that inferred the shape based on multiple photo angles, Bennett and her team overlaid the images.

- "We took a few images and manually matched them to sites scattered across the 3D shape model," she said. "If they're not lined up perfectly, they seem to wiggle when we toggled between the two. We carefully nudged the photos into place until we got a perfect match. Then, to lay the rest of the images, we used computer algorithms, which automatically matched surface features."

- This is where Bennett's photography and graphic design background came in. "One thing I can't do is use Photoshop. If we were to do that, it would compromise the scientific integrity. People get scientific information from brightness of the pixels, for example, so we don't want to smudge away the science," Bennett said. "Instead, I had to carefully choose where to divide the images. I cut through things like shadows or along crater rims instead of down the middle of a rock that was imaged from two different viewing angles. By carefully tracing the topography and matching images together like puzzle pieces, I was able to make the map a lot more seamless."

- The final global mosaic can serve as a base map to give context to future scientific data.

- "When scientists collect spectral (light) data reflected and emitted from Bennu to determine its composition, it just looks like squiggly lines and latitude and longitude coordinates," Bennett said. "So being able to then look at the corresponding location and features on the map is extremely helpful in interpreting that data."

- Individual images also aren't as useful as a high-definition map, DellaGiustina said. "This can provide data to unlock what kind of global patterns exist on Bennu and provide context to other datasets," she said.

- The global mosaic was also used for a citizen science project where anyone with an internet connection could map and measure Bennu's boulders, which will contribute to a global boulder census.

- Future mosaics, which will focus on smaller portions of the asteroid and be higher resolution, will be used for navigation of the primary and secondary sample sites.

• June 9, 2020: Close-up observations of asteroid Bennu by NASA’s OSIRIS-REx spacecraft contain the first evidence of thermal fracturing of rocks on an airless body, a Nature Communications paper by Planetary Science Institute Research Scientist Jamie Molaro says. 45)

OsirisREx_Auto3A

Figure 29: Exfoliation features on a cliff face (a) and on boulders (b-f) with varying size and location. The bright dome on the horizon of panel (a) is a boulder behind the exfoliating cliff (image credit: NASA/Goddard/University of Arizona)

- Thermal fracturing or thermal stress weathering occurs as rocks heat and cool each day, and mechanical stresses build up that can cause cracks to develop and grow. Over time the cracks grow larger and cause the rock to disaggregate or split into multiple pieces. For example, daytime highs on Bennu can reach about 400 degrees Kelvin (260 degrees Fahrenheit), and nighttime lows plummet to 200 degrees Kelvin (-100 degrees Fahrenheit).

- “This is the first time evidence for thermal fracturing has been definitively observed on an object without an atmosphere,” said Molaro, lead author of the paper “In situ evidence of thermally induced rock breakdown widespread on Bennu’s surface“ published June 9, 2020. “It is one piece of a puzzle that tells us what the surface used to be like, and what it will be like millions of years from now.” 46)

- “This thermally induced breakdown has long been known on Earth. The OSIRIS-REx Camera Suite (OCAMS) orbiting as close as 0.6 km (0.4 mi) has obtained images of the surface of Bennu at pixel scales down to about 1 centimeter per pixel, providing an opportunity to search over a wide range of scales for evidence of thermal breakdown occurring in situ,” Molaro said.

- “On Earth there are chemical weathering processes that help make thermal fracturing more efficient. The presence of air and moisture within cracks makes them easier to grow, and so on Earth this effect really cannot be decoupled from the effect of the thermal stresses themselves. We’ve observed evidence of thermal fracturing on Earth and on Mars, both environments where chemical weathering may play a role. Therefore, while it was theoretically possible for thermal fracturing on an airless body to occur alone, it was not clear whether or not the stresses would be strong enough to cause crack growth in absence of the chemical effects,” Molaro said.

- “Like any weathering process, thermal fracturing can cause the evolution of boulders and planetary surfaces over time; from changing the shape and size of individual boulders, to producing pebbles or fine-grained regolith, to breaking down crater walls,” Molaro said. “How quickly this occurs relative to other weathering processes tells us how quickly the surface has changed. It is one piece of a puzzle that tells us what the planetary surface used to be like, and what it will be like millions of years from now. We don’t have good constraints yet on breakdown rates from thermal fracturing, but we can get them now that we can actually observe evidence for it for the first time in-situ.

- “We show observations of boulder morphologies and fractures on Bennu that are consistent with models of thermally induced rock breakdown, and not easily explained by other weathering mechanisms. Boulders on Bennu exhibit many possible signs of thermal fracturing, but the clearest is images showing exfoliation, where thin layers of material flake off boulder surfaces,” Molaro said. “These findings provide substantive and compelling evidence that thermal fracturing plays an important role on airless body surfaces, which has major implications for understanding the evolution of asteroid surfaces, orbits, and populations.”

- Molaro’s research was funded by a grant to PSI from NASA’s Participating Scientist program.

OsirisREx_Auto39

Figure 30: Examples of disaggregation (top) and linear fractures (bottom) in boulders of varying sizes on Bennu (image credit: NASA/Goddard/University of Arizona)

• June 3, 2020: This view of sample site Osprey on asteroid Bennu is a mosaic of images collected by NASA’s OSIRIS-REx spacecraft on May 26. A total of 347 PolyCam images were stitched together and corrected to produce the mosaic, which shows the site at 0.2 inches (5 mm) per pixel at full size. The spacecraft took these images during an 820-foot (250-meter) reconnaissance pass over the site, which is the closest Osprey has been imaged. The pass was designed to provide high-resolution imagery to identify the best areas within the site to collect a sample. 47)

OsirisREx_Auto38

Figure 31: The sample site is located in the crater at the bottom of the image, just above the dark patch at the crater’s center. The long, light-colored boulder to the left of the dark patch, named Strix Saxum, is 17 ft (5.2 m) in length. The mosaic is rotated so that Bennu’s east is at the top of the image (image credit: NASA/Goddard/University of Arizona)

- Osprey is the backup sample collection site for the OSIRIS-REx mission. OSIRIS-REx is scheduled to make its first sample collection attempt at primary site Nightingale on Oct. 20.

• May 20, 2020: NASA’s first asteroid sample return mission is officially prepared for its long-awaited touchdown on asteroid Bennu’s surface. The Origins, Spectral Interpretation, Resource Identification and Security – Regolith Explorer (OSIRIS-REx) mission has targeted Oct. 20 for its first sample collection attempt. 48)

- “The OSIRIS-REx mission has been demonstrating the very essence of exploration by persevering through unexpected challenges,” said Thomas Zurbuchen, NASA’s associate administrator for science. “That spirit has led them to the cusp of the prize we all are waiting for – securing a sample of an asteroid to bring home to Earth, and I’m very excited to follow them through the home stretch.”

- From discovering Bennu’s surprisingly rugged and active surface, to entering the closest-ever orbit around a planetary body, OSIRIS-REx has overcome several challenges since arriving at the asteroid in December 2018. Last month, the mission brought the spacecraft 213 ft (65 m) from the asteroid’s surface during its first sample collection rehearsal — successfully completing a practice run of the activities leading up to the sampling event.

OsirisREx_Auto37

Figure 32: This illustration shows NASA’s OSIRIS-REx spacecraft descending towards asteroid Bennu to collect a sample of the asteroid’s surface (image credit: NASA/Goddard/University of Arizona)

- Now that the mission is ready to collect a sample, the team is facing a different kind of challenge here on Earth. In response to COVID-19 constraints and after the intense preparation for the first rehearsal, the OSIRIS-REx mission has decided to provide its team with additional preparation time for both the final rehearsal and the sample collection event. Spacecraft activities require significant lead time for the development and testing of operations, and given the current requirements that limit in-person participation at the mission support area, the mission would benefit from giving the team additional time to complete these preparations in the new environment. As a result, both the second rehearsal and first sample collection attempt will have two extra months for planning.

- “In planning the mission, we included robust schedule margin while at Bennu to provide the flexibility to address unexpected challenges,” said Rich Burns, OSIRIS-REx project manager at NASA’s Goddard Space Flight Center. “This flexibility has allowed us to adapt to the surprises that Bennu has thrown at us. It's now time to prioritize the health and safety of both team members and the spacecraft.”

- The mission had originally planned to perform the first Touch-and-Go (TAG) sample collection event on Aug. 25 after completing a second rehearsal in June. This rehearsal, now scheduled for Aug. 11, will bring the spacecraft through the first three maneuvers of the sample collection sequence to an approximate altitude of 131 ft (40 m) over the surface of Bennu. The first sample collection attempt is now scheduled for Oct. 20, during which the spacecraft will descend to Bennu’s surface and collect material from sample site Nightingale.

- “This mission’s incredible performance so far is a testament to the extraordinary skill and dedication of the OSIRIS-REx team,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. “I am confident that even in the face of the current challenge, this team will be successful in collecting our sample from Bennu.”

- During the TAG event, OSIRIS-REx’s sampling mechanism will touch Bennu’s surface for approximately five seconds, fire a charge of pressurized nitrogen to disturb the surface, and collect a sample before the spacecraft backs away. The mission has resources onboard for three sample collection opportunities. If the spacecraft successfully collects a sufficient sample on Oct. 20, no additional sampling attempts will be made. The spacecraft is scheduled to depart Bennu in mid-2021, and will return the sample to Earth on Sept. 24, 2023.

• April 15, 2020: After the successful completion of its “Checkpoint” rehearsal, NASA’s first asteroid-sampling spacecraft is one step closer to touching down on asteroid Bennu. Yesterday, NASA’s OSIRIS-REx spacecraft performed the first practice run of its sample collection sequence, reaching an approximate altitude of 246 feet (75 meters) over site Nightingale before executing a back-away burn from the asteroid. Nightingale, OSIRIS-REx’s primary sample collection site, is located within a crater in Bennu’s northern hemisphere. 49)

- The four-hour Checkpoint rehearsal took the spacecraft through the first two of the sampling sequence’s four maneuvers: the orbit departure burn and the Checkpoint burn. Checkpoint is so named because it is the location where the spacecraft autonomously checks its position and velocity before adjusting its trajectory down toward the location of the event’s third maneuver (Figure 33).

- Four hours after departing its 0.6-mile (1-km) safe-home orbit, the spacecraft performed the Checkpoint maneuver at an approximate altitude of 410 feet (125 meters) above Bennu’s surface. From there, the spacecraft continued to descend for another nine minutes on a trajectory toward – but not reaching – the location of the sampling event’s third maneuver, the “Matchpoint” burn. Upon reaching an altitude of approximately 246 ft (75 m) – the closest the spacecraft has ever been to Bennu – OSIRIS-REx performed a back-away burn to complete the rehearsal.

- During the rehearsal, the spacecraft successfully deployed its sampling arm, the Touch-And-Go Sample Acquisition Mechanism (TAGSAM), from its folded, parked position out to the sample collection configuration. Additionally, some of the spacecraft’s instruments collected science and navigation images and made spectrometry observations of the sample site, as will occur during the sample collection event.

- This first rehearsal provided the mission team with practice navigating the spacecraft through both the orbit departure and Checkpoint maneuvers and with an opportunity to verify that the spacecraft’s imaging, navigation and ranging systems operated as expected during the first part of the descent sequence. Checkpoint rehearsal also gave the team confirmation that OSIRIS-REx’s Natural Feature Tracking (NFT) guidance system accurately estimated the spacecraft’s position and speed relative to Bennu as it descended toward the surface.

- The mission team has maximized remote work over the last month of preparations for the Checkpoint rehearsal, as part of the COVID-19 response. On the day of rehearsal, a limited number of personnel monitored the spacecraft’s telemetry from Lockheed Martin Space’s facility, NASA’s Goddard Space Flight Center and the University of Arizona, taking appropriate safety precautions, while the rest of the team performed their roles remotely.

- “This rehearsal let us verify flight system performance during the descent, particularly the autonomous update and execution of the Checkpoint burn,” said Rich Burns, OSIRIS-REx project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Executing this monumental milestone during this time of national crisis is a testament to the professionalism and focus of our team. It speaks volumes about their ‘can-do’ attitude and hopefully will serve as a bit of good news in these challenging times.”

- The spacecraft will travel all the way to the asteroid’s surface during its first sample collection attempt, scheduled for Aug. 25. During this event, OSIRIS-REx’s sampling mechanism will touch Bennu’s surface for approximately five seconds, fire a charge of pressurized nitrogen to disturb the surface and collect a sample before the spacecraft backs away. The spacecraft is scheduled to return the sample to Earth on Sept. 24, 2023.

• April 9, 2020: In August, a robotic spacecraft will make NASA’s first-ever attempt to descend to the surface of an asteroid, collect a sample, and ultimately bring it safely back to Earth. In order to achieve this challenging feat, the OSIRIS-REx mission team devised new techniques to operate in asteroid Bennu’s microgravity environment – but they still need experience flying the spacecraft in close proximity to the asteroid in order to test them. So, before touching down at sample site Nightingale this summer, OSIRIS-REx will first rehearse the activities leading up to the event. 50)

- On April 14, the mission will pursue its first practice run – officially known as “Checkpoint” rehearsal – which will also place the spacecraft the closest it’s ever been to Bennu. This rehearsal is a chance for the OSIRIS-REx team and spacecraft to test the first steps of the robotic sample collection event.

- During the full touchdown sequence, the spacecraft uses three separate thruster firings to make its way to the asteroid’s surface. After an orbit departure burn, the spacecraft executes the Checkpoint maneuver at 410 ft (125 m) above Bennu, which adjusts the spacecraft’s position and speed down toward the point of the third burn. This third maneuver, called “Matchpoint," occurs at approximately 164 ft (50 m) from the asteroid’s surface and places the spacecraft on a trajectory that matches the rotation of Bennu as it further descends toward the targeted touchdown spot.

- The Checkpoint rehearsal allows the team to practice navigating the spacecraft through both the orbit departure and Checkpoint maneuvers, and ensures that the spacecraft’s imaging, navigation and ranging systems operate as expected during the first part of the descent sequence. Checkpoint rehearsal also gives the team a chance to confirm that OSIRIS-REx’s Natural Feature Tracking (NFT) guidance system accurately updates the spacecraft’s position and velocity relative to Bennu as it descends towards the surface.

- Checkpoint rehearsal, a four-hour event, begins with the spacecraft leaving its safe-home orbit, 0.6 miles (1 km) above the asteroid. The spacecraft then extends its robotic sampling arm – the Touch-And-Go Sample Acquisition Mechanism (TAGSAM) – from its folded, parked position out to the sample collection configuration. Immediately following, the spacecraft slews, or rotates, into position to begin collecting navigation images for NFT guidance. NFT allows the spacecraft to autonomously guide itself to Bennu’s surface by comparing an onboard image catalog with the real-time navigation images taken during descent. As the spacecraft descends to the surface, the NFT system updates the spacecraft’s predicted point of contact depending on OSIRIS-REx’s position in relation to Bennu’s landmarks.

- Before reaching the 410-ft (125-m) Checkpoint altitude, the spacecraft’s solar arrays move into a “Y-wing” configuration that safely positions them away from the asteroid’s surface. This configuration also places the spacecraft’s center of gravity directly over the TAGSAM collector head, which is the only part of the spacecraft that will contact Bennu’s surface during the sample collection event.

- In the midst of these activities, the spacecraft continues capturing images of Bennu’s surface for the NFT navigation system. The spacecraft will then perform the Checkpoint burn and descend toward Bennu’s surface for another nine minutes, placing the spacecraft around 243 ft (75 m) from the asteroid – the closest it has ever been.

- Upon reaching this targeted point, the spacecraft will execute a back-away burn, then return its solar arrays to their original position and reconfigure the TAGSAM arm back to the parked position. Once the mission team determines that the spacecraft successfully completed the entire rehearsal sequence, they will command the spacecraft to return to its safe-home orbit around Bennu.

- Following the Checkpoint rehearsal, the team will verify the flight system’s performance during the descent, and that the Checkpoint burn accurately adjusted the descent trajectory for the subsequent Matchpoint burn.

- The mission team has maximized remote work over the last month of preparations for the checkpoint rehearsal, as part of the COVID-19 response. On the day of rehearsal, a limited number of personnel will command the spacecraft from Lockheed Martin Space’s facility, taking appropriate safety precautions, while the rest of the team performs their roles remotely.

- The mission is scheduled to perform a second rehearsal on Jun. 23, taking the spacecraft through the Matchpoint burn and down to an approximate altitude of 82 ft (25 m). OSIRIS-REx’s first sample collection attempt is scheduled for Aug. 25.

- 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.

OsirisREx_Auto36

Figure 33: This artist’s concept shows the trajectory and configuration of NASA’s OSIRIS-REx spacecraft during Checkpoint rehearsal, which is the first time the mission will practice the initial steps for collecting a sample from asteroid Bennu (image credit: NASA/Goddard/University of Arizona)

• March 9, 2020: This summer, the OSIRIS-REx spacecraft will undertake NASA’s first-ever attempt to touch the surface of an asteroid, collect a sample of it, and safely back away. But since arriving at asteroid Bennu over a year ago, the mission team has been tackling an unexpected challenge: how to accomplish this feat at an asteroid whose surface is blanketed in building-sized boulders. 51)

- Using these hazardous boulders as signposts, the mission team developed a new precision navigation method to overcome the challenge.

Figure 34: In late August, the OSIRIS-REx spacecraft will navigate to asteroid Bennu’s surface for its first sample collection attempt. To do this, it will use an onboard image software known as Natural Feature Tracking (NFT) — a form of optical navigation that is completely autonomous (video credits: NASA's Goddard Space Flight Center/Scientific Visualization Studio)

- The OSIRIS-REx team had originally planned to use a LIDAR system to navigate to Bennu’s surface during the Touch-And-Go (TAG) sample collection event. LIDAR is similar to radar, but it uses laser pulses rather than radio waves to measure distance. The OSIRIS-REx Guidance, Navigation, and Control (GNC) LIDAR is designed to navigate the spacecraft to a relatively hazard-free surface. The mission had originally envisioned a touchdown site 50 meters in diameter, but the largest safe areas on Bennu are much smaller. The biggest site is just 52 ft (16 m) wide, or roughly 10% of the safe area envisioned. The team realized that they needed a more precise navigation technique that would allow the spacecraft to accurately target very small sites while dodging potential hazards.

- In the face of this challenge, the OSIRIS-REx team switched to a new navigation method called NFT (Natural Feature Tracking). NFT provides more extensive navigation capabilities than LIDAR, and is key for executing what the team is calling “Bullseye TAG,” which delivers the spacecraft to the much smaller sampling area. As an optical navigation technique, it requires the creation of a high-resolution image catalog onboard the spacecraft.

- Earlier this year, the spacecraft made reconnaissance passes over the mission’s primary and backup sample collection sites, designated Nightingale and Osprey, flying as close as 0.4 miles (625 m) over the surface. During these flyovers, the spacecraft collected images from different angles and lighting conditions to complete the NFT image catalog. The team uses this catalog to identify boulders and craters unique to the sample site region, and will upload this information to the spacecraft before the sample collection event. NFT autonomously guides the spacecraft to Bennu’s surface by comparing the onboard image catalog with the real-time navigation images taken during descent. As the spacecraft descends to the surface, NFT updates its predicted point of contact depending on the spacecraft’s position in relation to the landmarks.

- On the ground, team members created “hazard maps” for both the Nightingale and Osprey sites to document all of the surface features that could potentially harm the spacecraft, like large rocks or steep slopes. The team used the image catalog in conjunction with data from the OSIRIS-REx Laser Altimeter (OLA) to create 3D maps that closely model Bennu’s topography. As part of NFT, these maps document boulder heights and crater depths, and guide the spacecraft away from potential hazards while targeting a very small site. During descent, if the spacecraft predicts it will touch unsafe terrain, it will autonomously wave-off and back away from the surface. However, if it sees that the area is free of hazards, it will continue to descend and attempt to collect a sample.

- NFT will be used in April to navigate the spacecraft during its first sample collection rehearsal. The operations team performed preliminary testing during the Orbital B mission phase in late 2019, and the results demonstrated that NFT works in real-life conditions as designed. NFT will also be used for navigation during the second rehearsal planned for June.

OsirisREx_Auto35

Figure 35: During the sample collection event, NFT will guide NASA’s OSIRIS-REx spacecraft to asteroid Bennu’s surface. The spacecraft takes real-time images of the asteroid’s surface features as it descends, and then compares these images with an onboard image catalog. The spacecraft then uses these geographical markers to orient itself and accurately target the touchdown site (image credit: NASA/Goddard/University of Arizona)

- OSIRIS-REx’s first sample collection attempt is scheduled for late August. The spacecraft will depart Bennu in 2021 and is scheduled to deliver the sample to Earth in September 2023.

• February 28,2020: University students and researchers working on a NASA mission orbiting a near-Earth asteroid have made an unexpected detection of a phenomenon 30 thousand light years away. Last fall, the student-built Regolith X-Ray Imaging Spectrometer (REXIS) onboard NASA’s OSIRIS-REx spacecraft detected a newly flaring black hole in the constellation Columba while making observations off the limb of asteroid Bennu. 52)

OsirisREx_Auto34

Figure 36: This image shows the X-ray outburst from the black hole MAXI J0637-043, detected by the REXIS instrument on NASA's OSIRIS-REx spacecraft. The image was constructed using data collected by the X-ray spectrometer while REXIS was making observations of the space around asteroid Bennu on Nov. 11, 2019. The outburst is visible in the center of the image, and the image is overlaid with the limb of Bennu (lower right) to illustrate REXIS’s field of view (image credit: NASA/Goddard/University of Arizona/MIT/Harvard)

- REXIS, a shoebox-sized student instrument, was designed to measure the X-rays that Bennu emits in response to incoming solar radiation. X-rays are a form of electromagnetic radiation, like visible light, but with much higher energy. REXIS is a collaborative experiment led by students and researchers at MIT and Harvard, who proposed, built, and operate the instrument.

- On Nov. 11, 2019, while the REXIS instrument was performing detailed science observations of Bennu, it captured X-rays radiating from a point off the asteroid’s edge. “Our initial checks showed no previously cataloged object in that position in space,” said Branden Allen, a Harvard research scientist and student supervisor who first spotted the source in the REXIS data.

Figure 37: Last fall, the student-built Regolith X-Ray Imaging Spectrometer (REXIS) aboard NASA’s OSIRIS-REx spacecraft detected a newly flaring black hole in the constellation Columba while making observations off the limb of asteroid Bennu (video credit: NASA's Goddard Space Flight Center)

- The glowing object turned out to be a newly flaring black hole X-ray binary – discovered just a week earlier by Japan’s MAXI telescope – designated MAXI J0637-430. NASA's Neutron Star Interior Composition Explorer (NICER) telescope also identified the X-ray blast a few days later. Both MAXI and NICER operate aboard NASA's International Space Station and detected the X-ray event from low Earth orbit. REXIS, on the other hand, detected the same activity millions of miles from Earth while orbiting Bennu, the first such outburst ever detected from interplanetary space.

- “Detecting this X-ray burst is a proud moment for the REXIS team. It means our instrument is performing as expected and to the level required of NASA science instruments,” said Madeline Lambert, an MIT graduate student who designed the instrument’s command sequences that serendipitously revealed the black hole.

- X-ray blasts, like the one emitted from the newly discovered black hole, can only be observed from space since Earth’s protective atmosphere shields our planet from X-rays. These X-ray emissions occur when a black hole pulls in matter from a normal star that is in orbit around it. As the matter spirals onto a spinning disk surrounding the black hole, an enormous amount of energy (primarily in the form of X-rays) is released in the process.

Figure 38: This visualization simulates an X-ray outburst from the black hole MAXI J0637-043, detected by the REXIS instrument on NASA's OSIRIS-REx spacecraft, as it moves through REXIS’s line of sight. At first, the outburst is visibly intense, but it gradually fades as it subsides. The animation was constructed using data collected by the X-ray spectrometer while REXIS was making observations of the space around asteroid Bennu on Nov. 11, 2019 (image credits: NASA/Goddard/University of Arizona/MIT/Harvard)

- “We set out to train students how to build and operate space instruments,” said MIT professor Richard Binzel, instrument scientist for the REXIS student experiment. “It turns out, the greatest lesson is to always be open to discovering the unexpected.”

- The main purpose of the REXIS instrument is to prepare the next generation of scientists, engineers, and project managers in the development and operations of spaceflight hardware. Nearly 100 undergraduate and graduate students have worked on the REXIS team since the mission’s inception.

• On February 11, NASA’s OSIRIS-REx spacecraft safely executed a 620 m flyover of the backup sample collection site Osprey as part of the mission’s Reconnaissance B phase activities. Preliminary telemetry, however, indicates that the OSIRIS-REx Laser Altimeter (OLA) did not operate as expected during the 11-hour event. The OLA instrument was scheduled to provide ranging data to the spacecraft’s PolyCam imager, which would allow the camera to focus while imaging the area around the sample collection site. Consequently, the PolyCam images from the flyover are likely out of focus. 53)

- The other science instruments, including the MapCam imager, the OSIRIS-REx Thermal Emissions Spectrometer (OTES), and the OSIRIS-REx Visual and InfraRed Spectrometer (OVIRS), all performed nominally during the flyover. These instruments and the spacecraft continue in normal operations in orbit around asteroid Bennu.

- The mission team is currently reviewing the available data from the flyover in order to fully assess the OLA instrument. The entire data set from the flyover, including the PolyCam images, will be completely downlinked from the spacecraft next week and will provide additional insight into any impact that the loss of the OLA data may have.

- OLA has already completed all of its principal requirements for the OSIRIS-REx mission. Last year, OLA’s scans of Bennu’s surface were used to create the high-resolution 3D global maps of Bennu’s topography that were crucial for selecting the primary and backup sample collection sites last fall.

• 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. 54)

OsirisREx_Auto33

Figure 39: 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. 55)

- 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.

OsirisREx_Auto32

Figure 40: 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. 56)

- 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. 57)

- 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 41: 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. 58)

- 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.

OsirisREx_Auto31

Figure 42: 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.

OsirisREx_Auto30

Figure 43: 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 44: 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. 59)

- 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 45: 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 52).

- 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 46), 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.

OsirisREx_Auto2F

Figure 46: 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. 60)

- 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 47) 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).

OsirisREx_Auto2E

Figure 47: 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 48: 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. 61)

- 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.

OsirisREx_Auto2D

Figure 49: 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). 62)

OsirisREx_Auto2C

Figure 50: 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. 63)

OsirisREx_Auto2B

Figure 51: 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. 64)

Figure 52: 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. 65)

- 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.

OsirisREx_Auto2A

Figure 53: 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. 66)

OsirisREx_Auto29

Figure 54: 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. 67)

OsirisREx_Auto28

Figure 55: 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)