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↑^ Jump up to: a b c d e Nolan, M. C.; Magri, C.; Howell, E. S.; Benner, L. A. M.; Giorgini, J. D.; Hergenrother, C. W.; Hudson, R. S.; Lauretta, D. S.; Margot, J. L.; Ostro, S. J.; Scheeres, D. J. (2013). "Shape model and surface properties of the OSIRIS-REx target Asteroid (101955) Bennu from radar and lightcurve observations". Icarus 226 (1): 629–640. Bibcode:2013Icar..226..629N. doi:10.1016/j.icarus.2013.05.028. ISSN 0019-1035.
↑^ Hudson, R. S. ; Ostro, S. J. ; Benner, L. A. M. "Recent Delay-Doppler Radar Asteroid Modeling Results: 1999 RQ36 and Craters on Toutatis". Bulletin of the American Astronomical Society (American Astronomical Society) 32: 1001. Bibcode:2000DPS....32.0710H.
Bennu is the target of the OSIRIS-REx mission which is intended to return samples to Earth in 2023 for further study. On 3 December 2018, the OSIRIS-REx spacecraft arrived at Bennu after a two-year journey. Before attempting to obtain a sample from the asteroid, it will map out Bennu's surface in detail and orbit the asteroid to calculate its mass.
On 18 June 2019, NASA announced that the OSIRIS-REx spacecraft managed to get even closer and capture an image at a distance of 600 metres (2,000 ft) from Bennu's surface.
The name Bennu was selected from more than eight thousand student entries from dozens of countries around the world who entered a "Name That Asteroid!" contest run by the University of Arizona, The Planetary Society, and the LINEAR Project in 2012. Third-grade student Michael Puzio from North Carolina proposed the name in reference to the Egyptian mythological bird Bennu. To Puzio, the OSIRIS-REx spacecraft with its extended TAGSAM arm resembled the Egyptian deity, which is typically depicted as a heron.
Its features will be named after birds and bird-like creatures in mythology.
Image sequence showing the rotation of Bennu, imaged by OSIRIS-REx at a distance of around 80 km (50 mi).
Wide angle shot of the Northern Hemisphere of Bennu, imaged by OSIRIS-REx at an altitude of approximately 1.1 mi (1.8 km).
Bennu has a roughly spheroidal shape, resembling a spinning top. Bennu's axis of rotation is tilted 178 degrees to its orbit; the direction of rotation about its axis is retrograde with respect to its orbit. While the initial ground based radar observations indicated that Bennu had a fairly smooth shape with one prominent 10–20 m boulder on its surface, high resolution data obtained by OSIRIS-REx revealed that the surface is much rougher with more than 200 boulders larger than 10 m on the surface, the largest of which is 58 m across.
There is a well-defined ridge along the equator of Bennu. The presence of this ridge suggests that fine-grained regolith particles have accumulated in this area, possibly because of its low gravity and fast rotation. Observations by the OSIRIS-REx spacecraft has shown that Bennu is rotating faster over time. This change in Bennu's rotation is caused by the Yarkovsky–O'Keefe–Radzievskii–Paddack effect, or the YORP effect. Due to the uneven emission of thermal radiation from its surface as Bennu rotates in sunlight, the rotation period of Bennu decreases by about one second every 100 years.
Observations of this minor planet by the Spitzer Space Telescope in 2007 gave an effective diameter of 484±10 m, which is in line with other studies. It has a low visible geometric albedo of 0.046±0.005. The thermal inertia was measured and found to vary by approximately 19% during each rotational period. The data suggest that the regolith grain size is moderate, ranging from several millimeters up to a centimeter, and evenly distributed. No emission from a potential dust coma has been detected around Bennu, which puts a limit of 106 g of dust within a radius of 4750 km.
Astrometric observations between 1999 and 2013 have demonstrated that 101955 Bennu is influenced by the Yarkovsky effect, causing the semimajor axis to drift on average by 284±1.5 meters/year. Analysis of the gravitational and thermal effects has given a bulk density of ρ = 1190±13 kg/m3, which is only slightly denser than water. Therefore, the predicted macroporosity is 40±10%, suggesting the interior has a rubble pile structure. The estimated mass is (7.329±0.009)×1010 kg.
Photometry and spectroscopy
Photometric observations of Bennu in 2005 yielded a synodic rotation period of 4.2905±0.0065 h. It has a B-type classification, which is a sub-category of carbonaceous asteroids. Polarimetric observations show that Bennu belongs to the rare F subclass of carbonaceous asteroids, which is usually associated with cometary features. Measurements over a range of phase angles showed a phase function slope of 0.040 magnitudes per degree, which is similar to other near-Earth asteroids with low albedo.
All geological features on Bennu are named after various species of birds and bird-like figures in mythology. The first features to be named were the final four candidate OSIRIS-REx sample sites, which were given unofficial names by the team in August 2019. On March 6, 2020 the IAU announced the first official names for 12 Bennu surface features, including regiones (broad geographic regions), craters, dorsa (ridges), fossae (grooves or trenches) and saxa (rocks and boulders).
Abundant fine-grained material with a large variation in color. Primary sample collection site.
A relatively new crater with the highest water signature of all four sites.
Located on a low albedo patch with a large variety of rocks. Backup sample collection site.
Located between two young craters, located in rough terrain. Minerals vary in brightness with hints of hydrated minerals.
On December 12, 2019, after a year of mapping Bennu's surface, a target site was announced. Named Nightingale, the area is near Bennu's north pole and lies inside a small crater within a larger crater. Osprey was selected as the backup sample site.
IAU named features
Global mosaic of Bennu showing the locations of the first 12 named surface features
List of official IAU-named Bennu surface features
Bennu's basic mineralogy and chemical nature would have been established during the first 10 million years of the Solar System's formation, where the carbonaceous material underwent some geologic heating and chemical transformation inside a much larger planetoid or a proto-planet capable of producing the requisite pressure, heat and hydration (if need be)—into more complex minerals. Bennu probably began in the inner asteroid belt as a fragment from a larger body with a diameter of 100 km. Simulations suggest a 70% chance it came from the Polana family and a 30% chance it derived from the Eulalia family.
Subsequently, the orbit drifted as a result of the Yarkovsky effect and mean motion resonances with the giant planets, such as Jupiter and Saturn. Various interactions with the planets in combination with the Yarkovsky effect modified the asteroid, possibly changing its spin, shape, and surface features.
Cellino et al. have suggested a possible cometary origin for Bennu, based on similarities of its spectroscopic properties with known comets. The estimated fraction of comets in the population of near Earth objects is 8%±5%. This includes rock comet3200 Phaethon, originally discovered as, and still numbered as an asteroid.
Diagram of the orbits of Bennu and the inner planets around the Sun.
Possible Earth impact
On average, an asteroid with a diameter of 500 m (1,600 ft; 0.31 mi) can be expected to impact Earth about every 130,000 years or so. A 2010 dynamical study by Andrea Milani and collaborators predicted a series of eight potential Earth impacts by Bennu between 2169 and 2199. The cumulative probability of impact is dependent on physical properties of Bennu that were poorly known at the time, but was found to not exceed 0.071% for all eight encounters. The authors recognized that an accurate assessment of 101955 Bennu's probability of Earth impact would require a detailed shape model and additional observations (either from the ground or from spacecraft visiting the object) to determine the magnitude and direction of the Yarkovsky effect.
The publication of the shape model and of astrometry based on radar observations obtained in 1999, 2005, and 2011, made possible an improved estimate of the Yarkovsky acceleration and a revised assessment of the impact probability. The current (as of 2014)[update] best estimate of the impact probability is a cumulative probability of 0.037% in the interval 2175 to 2196. This corresponds to a cumulative score on the Palermo scale of −1.71. If an impact were to occur, the expected kinetic energy associated with the collision would be 1,200 megatons in TNT equivalent (for comparison, TNT equivalent of Little Boy was approx 15 kiloton).
2060 close approach
Animation of 101955 Bennu's orbit around Earth 2128–2138. 2135 close approach is shown. Earth· 101955 Bennu
Bennu will pass 0.005 au (750,000 km; 460,000 mi) from Earth on 23 September 2060, and will be too dim to be seen with common binoculars. The close approach of 2060 causes divergence in the close approach of 2135. On 25 September 2135, the nominal approach distance is 0.002 au (300,000 km; 190,000 mi) from Earth, but Bennu could pass as close as 0.0007 au (100,000 km; 65,000 mi). There is no chance of an Earth impact in 2135. The 2135 approach will create many lines of variations and Bennu may pass through a gravitational keyhole during the 2135 passage which could create an impact scenario at a future encounter. The keyholes are all less than 55 km wide.
On 25 September 2175, there is a 1 in 24,000 chance of an Earth impact, but the nominal 2175 approach is in February 2175 at a distance of roughly 0.1 au (15,000,000 km; 9,300,000 mi). The most threatening virtual impactor is on 24 September 2196 when there is a 1 in 11,000 chance of an Earth impact. There is a cumulative 1 in 2,700 chance of an Earth impact between 2175–2199.
Lauretta et al. reported in 2015 their results of a computer simulation, concluding that it is more likely that 101955 Bennu will be destroyed by some other cause:
The orbit of Bennu is intrinsically dynamically unstable, as are those of all NEOs. In order to glean probabilistic insights into the future evolution and likely fate of Bennu beyond a few hundred years, we tracked 1,000 virtual "Bennus" for an interval of 300 Myr with the gravitational perturbations of the planets Mercury–Neptune included. Our results ... indicate that Bennu has a 48% chance of falling into the Sun. There is a 10% probability that Bennu will be ejected out of the inner Solar System, most likely after a close encounter with Jupiter. The highest impact probability for a planet is with Venus (26%), followed by the Earth (10%) and Mercury (3%). The odds of Bennu striking Mars are only 0.8% and there is a 0.2% chance that Bennu will eventually collide with Jupiter.
H < 21 asteroids passing less than 1 LD from Earth
Animation of OSIRIS-REx's trajectory from 9 September 2016 to 3 December 2018. OSIRIS-REx; 101955 Bennu; Earth; Sun;
Animation of OSIRIS-REx's trajectory around 101955 Bennu from 25 December 2018 OSIRIS-REx· 101955 Bennu
The OSIRIS-REx mission of NASA's New Frontiers program was launched towards 101955 Bennu on September 8, 2016. On December 3, 2018, the spacecraft arrived at the asteroid Bennu after a two-year journey. One week later, at the American Geophysical Union Fall Meeting, investigators announced that OSIRIS-REx had discovered spectroscopic evidence for hydrated minerals on the surface of the asteroid, implying that liquid water was present in Bennu's parent body before it split off. OSIRIS-REx is expected to return samples to Earth in 2023.
Bennu was selected from over 500000 known asteroids by the OSIRIS-REx selection committee. The primary constraint for selection was close proximity to Earth, since proximity implies low impulse (Δv) required to reach an object from Earth orbit. The criteria stipulated an asteroid in an orbit with low eccentricity, low inclination, and an orbital radius of 0.8–1.6 au. Furthermore, the candidate asteroid for a sample-return mission must have loose regolith on its surface, which implies a diameter greater than 200 meters. Asteroids smaller than this typically spin too fast to retain dust or small particles. Finally, a desire to find an asteroid with pristine carbon material from the early Solar System, possibly including volatile molecules and organic compounds, reduced the list further.
With the above criteria applied, five asteroids remained as candidates for the OSIRIS-REx mission, and Bennu was chosen, in part for its potentially hazardous orbit.
A compilation of radar images of asteroid Bennu (left) and a corresponding 3D shape model. (right)
This picture, taken by the OSIRIS-REx spacecraft on November 2, 2018, was part of a sequence of frames collected to show asteroid 101955 Bennu rotating. Bennu is approximately 200 pixels wide in this shot.
^Emery, J.; et al. (July 2014), Muinonen, K. (ed.), "Thermal infrared observations and thermophysical characterization of the OSIRIS-REx target asteroid (101955) Bennu", Conference Proceedings Asteroids, Comets, Meteors 2014: 148, Bibcode:2014acm..conf..148E.
^King, A; Solomon, J; Schofield, P; Russell, S (December 2015). "Characterising the CI and CI-like carbonaceous chondrites using thermogravimetric analysis and infrared spectroscopy". Earth, Planets and Space. 67: 1989. Bibcode:2015EP&S...67..198K. doi:10.1186/s40623-015-0370-4.
^Takir, D; Emery, J; Hibbits, C (2017). 3-μm Spectroscopy Of Water-Rich Meteorites And Asteroids: New Results And Implications. Hayabusa Symposium 2017.
^Bates, H; Hanna, K; King, A; Bowles, N (2018). Thermal Infrared Spectra of Heated CM and C2 Chondrites and Implications for Asteroid Sample Return Missions. Hayabusa Symposium 2018.
^All About Bennu: A Rubble Pile with a Lot of Surprises. Kimberly M. S. Cartier, EOS Planetary Sciences. 21 March 2019. "In terms of spectra and minerology, Bennu’s rocks 'look a lot like the rarest, most fragile meteorites in our collection,' Lauretta said, referring to the CM carbonaceous chondrites"
^Q&A: Water Found on Asteroid. Interview with Professor Beth Ellen Clark, OSIRIS-REx mission scientist. Ithaca College. 13 December 2018. Quote:"Third, the asteroid mining community has set a commercial price point for mining water on asteroids, and if Bennu's water is held in clays and other water-rich minerals on the surface, that would make asteroids like Bennu attractive for mining water."
^Connolly, H; Jawin, E; Ballouz, R; Walsh, K; McCoy, T; Dellagiustina, D (2019). OSIRIS-REx sample science and the geology of active asteroid Bennu. 82nd Meteoritical Society Meeting. p. 2157.
^Lim, L (2019). OSIRIS-REx update. 21st NASA Small Bodies Assessment Group. "Bennu is an Active Asteroid!"
^ abBarrucci, M; Michel, P (September 2019). Asteroid-Comet continuum: no doubt but many questions. 2019 EPSC-DPS conference. p. 202–1.
^Hergenrother, C; Adam, C; Antreasian, P; Al Asad, M; Balram-Knutson, S (September 2019). (101955) Bennu is an active asteroid. 2019 EPSC-DPS conference. p. 852–1.
^ abcLauretta, D. S.; Hergenrother, C. W.; Chesley, S. R.; Leonard, J. M.; Pelgrift, J. Y.; et al. (6 December 2019). "Episodes of particle ejection from the surface of the active asteroid (101955) Bennu". Science. 366 (6470): eaay3544. doi:10.1126/science.aay3544. PMID31806784..
^Boe, B; Jedicke, R; Wiegert, P; Meech, K; Morbidelli, A (September 2019). Distinguishing Between Solar System Formation Models with Manxes (or not). 2019 EPSC-DPS conference. p. 626–2.
^Gounelle, M (2012). The Asteroid-Comet Continuum: Evidence from Extraterrestrial Samples. 2012 European Planetary Science Congress. p. 220.
^Rickman, H (2018). Origin and Evolution of Comets: Ten Years after the Nice Model, One Year after Rosetta. Singapore: World Scientific. pp. 162–68. Sec. 4.3 Dormancy and Rejuvenation
^Robert Marcus; H. Jay Melosh & Gareth Collins (2010). "Earth Impact Effects Program". Imperial College London / Purdue University. Retrieved 7 February 2013. (solution using density of 2,600 kg/m^3, sped of 17km/s, and impact angle of 45 degrees)
^ abChesley, Steven R.; Farnocchia, Davide; Nolan, Michael C.; Vokrouhlický, David; Chodas, Paul W.; Milani, Andrea; Spoto, Federica; Rozitis, Benjamin; Benner, Lance A.M.; Bottke, William F.; Busch, Michael W.; Emery, Joshua P.; Howell, Ellen S.; Lauretta, Dante S.; Margot, Jean-Luc; Taylor, Patrick A. (2014). "Orbit and bulk density of the OSIRIS-REx target Asteroid (101955) Bennu". Icarus. 235: 5–22. arXiv:1402.5573. Bibcode:2014Icar..235....5C. doi:10.1016/j.icarus.2014.02.020. ISSN0019-1035.