Far-infrared array reciever (FAR) for SOFIA

In this paper, we present the design for a 16-channel heterodyne array receiver for use on SOFIA. The array will be capable of using either hot-electron bolometers or membrane mounted Schottky diodes in efficient, low-cost waveguide mounts. Focal plane arrays will be constructed to target astrophysically important lines between approximately 1.9 and 3 THz. Due to the prevailing physical conditions in the interstellar medium, this frequency range is one of the richest in the FIR portion of the spectrum. An array receiver designed for this wavelength range will make excellent use of the telescope and the available atmospheric transmission, and will provide a new perspective on stellar, chemical, and galaxy evolution in the present as well as past epochs. The proposed system uses the most sensitive detectors available in an efficient optical system

Photometry of Particles Ejected From Active Asteroid (101955) Bennu

AbstractNear‐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

Episodes of particle ejection from the surface of the active asteroid (101955) Bennu

Active asteroids are those that show evidence of ongoing mass loss. We report repeated instances of particle ejection from the surface of (101955) Bennu, demonstrating that it is an active asteroid. The ejection events were imaged by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) spacecraft. For the three largest events, we estimated the ejected particle velocities and sizes, event times, source regions, and energies. We also determined the trajectories and photometric properties of several gravitationally bound particles that orbited temporarily in the Bennu environment. We consider multiple hypotheses for the mechanisms that lead to particle ejection for the largest events, including rotational disruption, electrostatic lofting, ice sublimation, phyllosilicate dehydration, meteroid impacts, thermal stress fracturing, and secondary impacts

OSIRIS‐REx Visible and Near‐Infrared Observations of the Moon

The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission observed the Moon during the spacecraft's Earth gravity assist in 2017. From the spacecraft view, the lunar phase was 42 degrees, and the in-view hemisphere was dominated by anorthositic highlands terrain. Lunar spectra obtained by the OSIRIS-REx Visible and InfraRed Spectrometer show evidence of several candidate absorption features. We observe the 2.8-mu m hydration band, confirming the spectral results from other missions, but detected in full-disk spectra. We also tentatively identify weak spectral features near 0.9 and 1.3 mu m, consistent with lunar regolith containing a mixture of plagioclase and orthopyroxene minerals, as expected for highlands terrain.NASA [NNM10AA11C]; Italian Space Agency [2017-37-H.0]Public domain articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Disk-resolved photometric modeling and properties of asteroid (101955) Bennu

International audienceOSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) is a NASA mission to return a sample of asteroid (101955) Bennu. Photometric modeling of Bennu's surface is a key element of both sample site characterization and our broader scientific understanding of the asteroid. Bennu's heterogeneous surface presents substantial variation in reflectance and produces a scattered dataset that poses a challenge to photometric modeling. We show that the resolution of the shape model with which we calculate photometric angles strongly affects the accuracy of the analysis, as well as the efficacy of subsequent photometric corrections. We use global imaging data to fit empirical photometric models of the surface. These models represent the average behavior of Bennu's surface and can be used beyond this work to photometrically correct panchromatic and color basemaps of Bennu and perform albedo analyses of individual features on Bennu's surface. Bennu's global photometry reveals a moderate opposition effect and detectable phase reddening, both of which suggest a macroscopically rough surface, which is confirmed by centimeter-scale images of the asteroid

The Formation of Terraces on Asteroid (101955) Bennu

International audienceThe surface of the rubble-pile asteroid (101955) Bennu has been characterized in detail by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) mission. By examining global and local digital terrain models, we observed that Bennu possesses terraces, that is, a series of roughly latitude-parallel, step-like slope breaks. These partially circumscribe the poles and extend east-west over several longitudinal quadrants at mid- to high (≥30°) latitudes. The terraces are subtle in amplitude, with heights ranging from 1 to 5 m. They often exhibit back-wasting that results in V-shaped scarps that open downslope in some locations. When boulders >5-10 m are absent at or near a terrace, the steeper portion (the drop) of the terrace lacks rocks, whereas the flatter portion (the bench) of the terrace has accumulations of rocks at its crest. When boulders >5-10 m are present, their steep downslope faces often make up the drop from the terrace crest, and they retain debris upslope, thereby enhancing the terrace structure. A geotechnical stability analysis indicates that Bennu's surface is likely unstable and that surface cohesion is <0.6 Pa. Bennu's terraces strongly resemble scarps generated in laboratory and numerical simulations of a cohesionless granular bed as the slope of the bed increases quasi-statically. We conclude that terraces are probably actively forming on Bennu as its surface slowly fails owing to creep induced by spin acceleration

Photometry of Particles Ejected From Active Asteroid (101955) Bennu

©2020. The Authors. 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\u27s 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

Photometry of Particles Ejected From Active Asteroid (101955) Bennu

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 similar to 10(4) g per orbit. The largest event took place on 6 January 2019 and consisted of similar to 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 similar to 70 degrees and 120 degrees, 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 similar to 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.Agenzia Spaziale ItalianaOpen access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Photometry of Particles Ejected From Active Asteroid (101955) Bennu

©2020. The Authors. 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\u27s 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