In May of 2011, NASA selected the
O
rigins,
S
pectral
I
nterpretation,
R
esource
I
dentification, and
S
ecurity–
R
egolith
Ex
plorer (OSIRIS-REx) asteroid sample return mission as the third mission ...in the New Frontiers program. The other two New Frontiers missions are
New Horizons
, which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on January 1, 2019, and
Juno
, an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The spacecraft departed for near-Earth asteroid (101955) Bennu aboard an United Launch Alliance Atlas V 411 evolved expendable launch vehicle at 7:05 p.m. EDT on September 8, 2016, on a seven-year journey to return samples from Bennu. The spacecraft is on an outbound-cruise trajectory that will result in a rendezvous with Bennu in November 2018. The science instruments on the spacecraft will survey Bennu to measure its physical, geological, and chemical properties, and the team will use these data to select a site on the surface to collect at least 60 g of asteroid regolith. The team will also analyze the remote-sensing data to perform a detailed study of the sample site for context, assess Bennu’s resource potential, refine estimates of its impact probability with Earth, and provide ground-truth data for the extensive astronomical data set collected on this asteroid. The spacecraft will leave Bennu in 2021 and return the sample to the Utah Test and Training Range (UTTR) on September 24, 2023.
During its approach to asteroid (101955) Bennu, NASA’s Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft surveyed Bennu’s immediate ...environment, photometric properties, and rotation state. Discovery of a dusty environment, a natural satellite, or unexpected asteroid characteristics would have had consequences for the mission’s safety and observation strategy. Here we show that spacecraft observations during this period were highly sensitive to satellites (sub-meter scale) but reveal none, although later navigational images indicate that further investigation is needed. We constrain average dust production in September 2018 from Bennu’s surface to an upper limit of 150 g/s averaged over 34 min. Bennu’s disk-integrated photometric phase function validates measurements from the pre-encounter astronomical campaign. We demonstrate that Bennu’s rotation rate is accelerating continuously at 3.63 ± 0.52 × 10^(–6) degrees/sq. day, likely due to the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, with evolutionary implications.
The REgolith X-ray Imaging Spectrometer (REXIS) instrument on board NASA’s
OSIRIS-REx
mission to the asteroid Bennu is a Class-D student collaboration experiment designed to detect fluoresced X-rays ...from the asteroid’s surface to measure elemental abundances. In July and November 2019 REXIS collected ∼615 hours of integrated exposure time of Bennu’s sun-illuminated surface from terminator orbits. As reported in Hoak et al. (Results from the REgolith X-ray Imaging Spectrometer (REXIS) at Bennu,
2021
) the REXIS data do not contain a clear signal of X-ray fluorescence from the asteroid, in part due to the low incident solar X-ray flux during periods of observation. To support the evaluation of the upper limits on the detectable X-ray signal that may provide insights for the properties of Bennu’s regolith, we present an overview of the REXIS instrument, its operation, and details of its in-flight calibration on astrophysical X-ray sources. This calibration includes the serendipitous detection of the transient X-ray binary MAXI J0637-430 during Bennu observations, demonstrating the operational success of REXIS at the asteroid. We convey some lessons learned for future X-ray spectroscopy imaging investigations of asteroid surfaces.
We analyzed high-angular rate streaks first recorded by OSIRIS-REx’s MapCam during a 2017 search for Earth Trojan asteroids. We interpret them as water-ice particles that translated across the ...imager’s field of view, originating from the spacecraft itself. Their translation velocities approximated 0.1–1 m/s based on reasonable conclusions about their range. Pursuing several lines of investigation to seek a coherent hypothesis, we conclude that the episodic releases of the water ice particles are associated with spacecraft attitudes that resulted in solar illumination of previously shadowed regions. This correlation suggests that the OSIRIS-REx spacecraft itself possesses micro-climatic zones consisting of hot regions and cold traps that may temporarily potentially pass volatiles back and forth before losing most of them.
Near‐Earth asteroid (101955) Bennu is an active asteroid experiencing mass loss in the form of ejection events emitting up to hundreds of millimeter‐ to centimeter‐scale particles. The close ...proximity of the Origins, Spectral Interpretations, Resource Identification, and Security–Regolith Explorer spacecraft enabled monitoring of particles for a 10‐month period encompassing Bennu's perihelion and aphelion. We found 18 multiparticle ejection events, with masses ranging from near zero to hundreds of grams (or thousands with uncertainties) and translational kinetic energies ranging from near zero to tens of millijoules (or hundreds with uncertainties). We estimate that Bennu ejects ~104 g per orbit. The largest event took place on 6 January 2019 and consisted of ~200 particles. The observed mass and translational kinetic energy of the event were between 459 and 528 g and 62 and 77 mJ, respectively. Hundreds of particles not associated with the multiparticle ejections were also observed. Photometry of the best‐observed particles, measured at phase angles between ~70° and 120°, was used to derive a linear phase coefficient of 0.013 ± 0.005 magnitudes per degree of phase angle. Ground‐based data back to 1999 show no evidence of past activity for Bennu; however, the currently observed activity is orders of magnitude lower than observed at other active asteroids and too low be observed remotely. There appears to be a gentle decrease in activity with distance from the Sun, suggestive of ejection processes such as meteoroid impacts and thermal fracturing, although observational bias may be a factor.
Plain Language Summary
We measured the brightness of pebble‐sized particles in the vicinity of near‐Earth asteroid Bennu to better understand their physical characteristics and the events that launched them from Bennu's surface. Our measurements spanned 10 months, encompassing Bennu's closest and farthest distances from the Sun, so that we could assess how the level of ejection activity changes with solar distance. We observed 18 multiparticle ejection events containing anywhere from a few to 200+ particles. Individual particles ranged from millimeters to centimeters in diameter. The energy of the events and a possible decrease in activity with larger distances from the Sun suggest that meteoroid impacts, fracturing of surface boulders due to solar heating, or both may be responsible for ejecting the particles. We estimate that Bennu releases ~10,000 g of material over one orbit or 1.2 years. Although mass loss has been remotely observed for other asteroids, the comparatively low level of particle ejection activity at Bennu was only observable thanks to the close proximity of the Origins, Spectral Interpretations, Resource Identification, and Security–Regolith Explorer spacecraft.
Key Points
Asteroid (101955) Bennu is active from perihelion through aphelion with a possible decrease in activity further from the Sun
Bennu's activity is less than that detected by telescope for other active asteroids and is only observable up close
The particles' shallow phase functions resemble those of similarly sized individual rocks rather than those of ensemble asteroid surfaces
The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft arrived at the near-Earth asteroid (101955) Bennu in December 2018 and executed a ...science observation campaign to comprehensively characterize the asteroid. Proximity operations at Bennu included orbital phases and flyby phases with various viewing geometries and altitudes. The complexity of the mission plan, integrated instrument operations, and the challenges of spacecraft navigation in the microgravity environment required an intricate planning and implementation process that included participation and coordination among all mission elements. The Science Planning Team (SPT) and the Implementation Team (IpT) at the University of Arizona planned and implemented all science and most optical navigation observations. Prior to the formal planning process, science requirements were mapped to mission phases and observation geometry constraints. During development of the mission phases, the navigation team produced a spacecraft trajectory, and the SPT developed the pointing and attitude profile to meet the specified constraints. In the strategic planning process, which began three months prior to execution, the SPT conducted sensitivity analysis of the observation designs against a set of perturbed trajectories delivered by the navigation team to ensure that they were robust to navigational uncertainties. Planning of the specific observations to occur within each phase was divided into units of weeks, and the plans for each week were developed and implemented on a rolling eight-week tactical planning and implementation cycle, ending with execution and data downlink. This cycle included a standardized schedule of activities and gateways to ensure that every observation plan underwent a full suite of analysis, verification, and approval in the allocated timeframe. Checklists guided the SPT and IpT through the build and verification process to confirm plan safety and fidelity. The SPT led the first four weeks of the tactical process, with participation from the IpT and other stakeholders. During the first two weeks, the SPT gathered information from stakeholders, conducted preliminary planning to confirm the science observations were feasible and obeyed spacecraft constraints, and determined how to integrate instrument commanding with the spacecraft pointing profile. The SPT started the final observation design and planning six weeks prior to execution. Once complete, plan walkthroughs were conducted with stakeholders, which culminated in a go/no-go decision to proceed with implementation at the four-week point. In the last four weeks of the tactical planning and implementation process, the IpT led the final processing of science plans with participation from stakeholders. The IpT compiled the plans, performed comprehensive safety checks against established spacecraft and instrument flight rules, and generated flight products and artifacts. After IpT delivered the flight products, the spacecraft team integrated them with the spacecraft sequencing, performed ground testing, and produced an integrated report. IpT reviewed the report, verifying instrument health and safety and confirming nominal plan execution in the ground simulation. The final flight products were uplinked to the spacecraft a few days prior to the execution week. During execution, the IpT and other stakeholders monitored instrument performance and viewed science and navigation data. Resulting science data products were used for operational decisions and science investigations.
The OSIRIS-REx mission to asteroid (101955) Bennu comprised multiple phases, each with objectives that posed unique complexities for planning and implementation of science and optical navigation ...observations. In response to these challenges, the Science Planning Team (SPT) and the Implementation Team (IpT) at the University of Arizona amassed a toolkit composed of both in-house and commercial tools and scripts to accomplish reliable production of safe, high-fidelity flight products on a rapid, tactical planning timeline. J-Asteroid was the primary planning and implementation tool used by SPT and IpT. Built upon the geospatial information system JMARS (Java Mission-planning and Analysis for Remote Sensing), it was developed by Arizona State University's Mars Space Flight Facility. Configurable to the mission design, J-Asteroid modeled ephemerides, asteroid shape and spin state, spacecraft trajectory, navigation uncertainty, slew and attitude profile, instrument systems, and instrument command sequencing for new observation plans. A front-end interface enabled design of spacecraft scan patterns, ability to incorporate automated commanding or an ingested library of blocks, and two- and three-dimensional visualization of instrument footprints over a global Bennu shape model. J-Asteroid checked plans against a comprehensive database of flight rules and constraints and against perturbed trajectories. J-Asteroid's Integrated Commanding Tool converted finalized plans into delivery packages. JPlanner2, a Java-based tool, automated the processing of J-Asteroid delivery packages through scripts to generate, compile, and test flight products. JPlanner2 calculated data volume, generated activity plots, performed flight rule and soft constraint checks, and produced build reports and artifacts for delivery to the Spacecraft Operations Team at Lockheed Martin. Activity Inspector was an in-house tool that produced an integrated spacecraft attitude profile and confirmed compliance with spacecraft and instrument pointing constraints, which allowed continuous and rapid verification of J-Asteroid plans. Activity Inspector generated standardized pointing telemetry products for stakeholder communication. The tool performed analyses of Monte Carlo sets of perturbed trajectories, allowing for assessment of plan stability and produced visualizations by compiling the transmitted plan against the reconstructed trajectory. SPOCFlight was an in-house, server-based data system with a web interface. This event-driven system managed, parsed, calibrated, and packaged ground, spacecraft, and science data, and enabled users to monitor data, generate reports, and execute Activity Inspector. Several planning and implementation tools and processes leveraged data products available on SPOCFlight. A library of instrument command blocks was developed to perform repeated functions, such as instrument calibrations and optical navigation imaging. Complex blocks worked with J-Asteroid to produce observation-specific sequencing. Validated blocks were stored in a spacecraft library and initiated from onboard instrument sequences. Python scripts for analysis parsed text and JSON reports output by J-Asteroid to produce tables, graphs, and additional computations, further enabling evaluation of observation plans. The scripts provided increased user efficiency. Git repositories were used to configuration-manage SPT scripts and IpT-generated flight products. JIRA, a commercial off-the-shelf work management software, was used to organize unique issue types for both SPT and IpT. The JIRA workflow included gated reviews as observations moved through planning, implementation, execution, and downlink. The OSIRIS-REx science operations toolkit of in-house and commercial tools enhanced efficiency, reduced risk, and increased flight product robustness.
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