101955 Bennu

101955 Bennu (provisional designation 1999 RQ36)[9] is a carbonaceous asteroid in the Apollo group discovered by the LINEAR Project on 11 September 1999. It is a potentially hazardous object that is listed on the Sentry Risk Table with the second-highest cumulative rating on the Palermo Technical Impact Hazard Scale.[10] It has a cumulative 1-in-2,700 chance of impacting Earth between 2175 and 2199.[11][12] It is named after the Bennu, the ancient Egyptian mythological bird associated with the Sun, creation, and rebirth.

101955 Bennu
Mosaic image of Bennu consisting of 12 PolyCam images collected on 2 December 2018 by OSIRIS-REx from a range of 24 km (15 mi).
Discovery[1]
Discovered byLINEAR
Discovery siteLincoln Lab's ETS
Discovery date11 September 1999
Designations
(101955) Bennu
Pronunciation/ˈbɛnuː/[2]
Named after
Bennu
1999 RQ36
Apollo · NEO · PHA · risk listed
Orbital characteristics[1]
Epoch 31 July 2016 (JD 2457600.5)
Uncertainty parameter 0
Observation arc13.36 yr (4880 days)
Aphelion1.3559 au (202.84 Gm)
Perihelion0.89689 au (134.173 Gm)
1.1264 au (168.51 Gm)
Eccentricity0.20375
1.1955 yr (436.65 d)
28.0 km/s (63,000 mph)
101.7039°
0° 49m 28.056s / day
Inclination6.0349°
2.0609°
66.2231°
Earth MOID0.0032228 au (482,120 km)
Venus MOID0.194 au (29,000,000 km)[3]
Mars MOID0.168 au (25,100,000 km)[3]
Jupiter MOID3.877 au (580.0 Gm)
TJupiter5.525
Proper orbital elements[4]
0.21145
5.0415°
301.1345 deg / yr
1.19548 yr
(436.649 d)
Physical characteristics[5]
Dimensions565 m × 535 m × 508 m[1]
Mean radius
245.03±0.08 m
Equatorial radius
282.37±0.06 m
Polar radius
249.25±0.06 m
0.782±0.004 km2
Volume0.0615±0.0001 km3
Mass(7.329±0.009)×1010 kg
Mean density
1.190±0.013 g/cm3
Equatorial surface gravity
6.27 micro-g[6]
4.296057±0.000002 h
177.6±0.11°
North pole right ascension
+85.65±0.12°
North pole declination
−60.17±0.09°
0.044±0.002
Surface temp. min mean max
Kelvin[7] 236 259 279
Fahrenheit -34.6 6.8 42.8
Celsius -37 -14 6
B[1][5]
F[8]
20.9

    101955 Bennu has a mean diameter of 490 m (1,610 ft; 0.30 mi) and has been observed extensively with the Arecibo Observatory planetary radar and the Goldstone Deep Space Network.[5][13][14]

    Bennu is the target of the OSIRIS-REx mission which is intended to return its samples to Earth in 2023 for further study.[15][16][17] On 3 December 2018, the OSIRIS-REx spacecraft arrived at Bennu after a two-year journey.[18] It orbited the asteroid and mapped out Bennu's surface in detail, seeking potential sample collection sites. Analysis of the orbits allowed calculation of Bennu's mass and its distribution.[19]

    On 18 June 2019, NASA announced that the OSIRIS-REx spacecraft had closed in and captured an image from a distance of 600 metres (2,000 ft) from Bennu's surface.[20]

    In October 2020, OSIRIS-REx successfully touched down on the surface of Bennu, collected a sample using an extendable arm,[21] secured the sample and prepared for a return trip to Earth.[22][23]

    Discovery and observation
    Series of Goldstone radar images showing Bennu's rotation.

    Bennu was discovered on 11 September 1999 during a Near-Earth asteroid survey by the Lincoln Near-Earth Asteroid Research (LINEAR).[3] The asteroid was given the provisional designation 1999 RQ36 and classified as a near-Earth asteroid. Bennu was observed extensively by the Arecibo Observatory and the Goldstone Deep Space Network using radar imaging as Bennu closely approached Earth on 23 September 1999.[24][13]

    Naming

    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.[1][9] 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.[1]

    Its features will be named after birds and bird-like creatures in mythology.[25]


    Physical characteristics
    Image sequence showing the rotation of Bennu, imaged by OSIRIS-REx at a distance of around 80 km (50 mi).
    The original view (left) of Bennu asteroid surface is curated with advanced noise elimination filter to get a pristine view (middle) of the raw surface and the surface speckles are presented separately on the right image.
    Bennu pristine surface is reconstructed from the original raw image as taken by RSETTA. (B) is a ranbow color map of intensity of the re-constructed pristine surface and (C) is the grayscale image of (B)
    Surface reconstruction of Bennu speckle layer(high intensity pixels) to have a visual intuition abut the distribution of speckles on the surface

    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.[5] 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,[12] 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.[5] The boulders contain veins of high albedo carbonate minerals believed to have formed prior to the formation of the asteroid due to hot water channels on the much larger parent body.[26][27] The veins range from 3 to 15 centimeters wide, and can be over one meter in length, much bigger than carbonate veins seen in meteorites.[27]

    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.[12] Observations by the OSIRIS-REx spacecraft has shown that Bennu is rotating faster over time.[28] This change in Bennu's rotation is caused by the Yarkovsky–O'Keefe–Radzievskii–Paddack effect, or the YORP effect.[28] 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.[28]

    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. It was based on this observation scientists (incorrectly) estimated a moderate regolith grain size, ranging from several millimeters up to a centimeter, 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.[29]

    Astrometric observations between 1999 and 2013 have demonstrated that 101955 Bennu is influenced by the Yarkovsky effect, causing the semimajor axis of its orbit 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.[5]

    Asteroid Bennu regolith surface
    Wide angle shot of the Northern Hemisphere of Bennu, imaged by OSIRIS-REx at an altitude of approximately 1.8 km (1.1 mi)
    Bennu's regolith-covered surface as imaged by OSIRIS-REx
    The Nightingale sample site imaged by OSIRIS-REx at touchdown. The circular TAGSAM head in the center of the frame is 1 ft (0.30 m) in diameter.

    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.[8] 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.[30]

    Before OSIRIS-REx, spectroscopy indicated a correspondence with the CI and/or CM carbonaceous chondrite meteorites,[31][32][33] including carbonaceous-chondrite mineral magnetite.[34][35][36] Magnetite, a spectrally prominent[37][38] water product[39][40][41] but destroyed by heat,[41] is an important proxy of astronomers[42][43][44] including OSIRIS-REx staff.[45]

    Water

    Predicted beforehand,[46] Dante Lauretta (University of Arizona) then stated that Bennu is water-rich- already detectable while OSIRIS-REx was still technically in approach.[47][48]

    Preliminary spectroscopic surveys of the asteroid's surface by OSIRIS-REx confirmed magnetite and the meteorite-asteroid linkage,[49][50][51] dominated by phyllosilicates.[52][53][54] Phyllosilicates, among others, hold water.[55][56][57] Bennu's water spectra were detectable on approach,[50][58] reviewed by outside scientists,[59][37] then confirmed from orbit.[34][60][61][62]

    OSIRIS-REx observations have resulted in a (self-styled) conservative estimate of about 7 x 108 kg water in one form alone, neglecting additional forms. This is a water content of ~1 wt.%, and potentially much more. In turn this suggests transient pockets of water beneath Bennu’s regolith. The surficial water may be lost from the collected samples. However, if the sample return capsule maintains low temperatures, the largest (centimeter-scale) fragments may contain measurable quantities of adsorbed water, and some fraction of Bennu's ammonium compounds.[62]

    Further information: Asteroidal water

    Activity

    Bennu is an active asteroid,[63][64][65][66] sporadically emitting plumes of particles[67][68] and rocks as large as 10 cm (3.9 in),[69][70] (not dust, defined as tens of micrometers).[71][72] Scientists hypothesize the releases may be caused by thermal fracturing, volatile release through dehydration of phyllosilicates, pockets of subsurface water,[62] and/or meteoroid impacts.[70]

    Before the arrival of OSIRIS-REx, Bennu had displayed polarization consistent with Comet Hale-Bopp and 3200 Phaethon, a rock comet.[8] Bennu, Phaethon, and inactive Manx comets[73] are examples of active asteroids.[74][75][65] B-type asteroids displaying a blue color in particular, may be dormant comets.[76][77][78][79][62] If the IAU declares Bennu to be a dual-status object, its comet designation would be P/1999 RQ36 (LINEAR).[80]

    See also: Nice model
    Asteroid Bennu ejecting particles
    6 January 2019
    Particle trajectories from four 2019 ejection events (video; 0:43)
    19 January 2019
    OSIRIS-REx mission[70][28][81]

    Surface features

    All geological features on Bennu are named after various species of birds and bird-like figures in mythology.[82] 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.[83] 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).[84]

    Candidate sample sites
    The final four candidate OSIRIS-REx sample sites
    Final four OSIRIS-REx candidate sample sites[85]
    NameLocationDescription
    Nightingale56°N 43°EAbundant fine-grained material with a large variation in color. Primary sample collection site.[86]
    Kingfisher11°N 56°EA relatively new crater with the highest water signature of all four sites.
    Osprey11°N 80°ELocated on a low albedo patch with a large variety of rocks. Backup sample collection site.[86]
    Sandpiper47°S 322°ELocated 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.[86]

    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[87]
    Name Named after Location
    Aellopus Saxum Aello, one of the half-bird half-woman Harpy sisters from Greek mythology 25.44°N 335.67°E
    Aetos Saxum Aetos, childhood playmate of the god Zeus who was turned into an eagle from Greek mythology 3.46°N 150.36°E
    Amihan Saxum Amihan, bird deity from Philippine mythology 17.96°S 256.51°E
    Benben Saxum Benben, Ancient Egyptian primordial mound that arose from the primordial waters Nu 45.86°S 127.59°E
    Boobrie Saxum Boobrie, shapeshifting entity from Scottish mythology that often takes the form of a giant water bird 48.08°N 214.28°E
    Camulatz Saxum Camulatz, one of four birds in the K'iche' creation myth in Maya mythology 10.26°S 259.65°E
    Celaeno Saxum Celaeno, one of the half-bird half-woman Harpy sisters from Greek mythology 18.42°N 335.23°E
    Ciinkwia Saxum Ciinkwia, thunder beings from Algonquian mythology that look like giant eagles 4.97°S 249.47°E
    Dodo Saxum Dodo, a dodo bird character from Alice's Adventures in Wonderland 32.68°S 64.42°E
    Gamayun Saxum Gamajun, prophetic bird from Slavic mythology 9.86°N 105.45°E
    Gargoyle Saxum Gargoyle, dragon-like monster with wings 4.59°N 92.48°E
    Gullinkambi Saxum Gullinkambi, rooster from Norse mythology that lives in Valhalla 18.53°N 17.96°E
    Huginn Saxum Huginn, one of two ravens that accompany the god Odin in Norse mythology 29.77°S 43.25°E
    Kongamato Saxum Kongamato, giant flying creature from Kaonde mythology 5.03°N 66.31°E
    Muninn Saxum Muninn, one of two ravens that accompany the god Odin in Norse mythology 29.34°S 48.68°E
    Ocypete Saxum Ocypete, one of the half-bird half-woman Harpy sisters from Greek mythology 25.09°N 328.25°E
    Odette Saxum Odette, princess that turns into the White Swan in Swan Lake 44.86°S 291.08°E
    Odile Saxum Odile, the Black Swan from Swan Lake 42.74°S 294.08°E
    Pouakai Saxum Poukai, monstrous bird from Maori mythology 40.45°S 166.75°E
    Roc Saxum Roc, giant bird of prey from Arabic mythology 23.46°S 25.36°E
    Simurgh Saxum Simurgh, benevolent bird that possesses all knowledge from Iranian mythology 25.32°S 4.05°E
    Strix Saxum Strix, bird of ill omen from Roman mythology 13.4°N 88.26°E
    Thorondor Saxum Thorondor, the King of the Eagles in Tolkien's Middle-earth 47.94°S 45.1°E
    Tlanuwa Regio Tlanuwa, giant birds from Cherokee mythology 37.86°S 261.7°E

    Origin and evolution

    The carbonaceous material that composes Bennu originally came from the breakup of a much larger parent body—a planetoid or a proto-planet. But like nearly all other matter in the Solar System, the origins of its minerals and atoms are to be found in dying stars such as red giants and supernovae.[88] According to the accretion theory, this material came together 4.5 billion years ago during the formation of the Solar System.

    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.[12] 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.[89] Impactors on boulders of Bennu indicate that Bennu has been in near earth orbit (separated from the main asteroid belt) for 1–2.5 million years.[90]

    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.[91]

    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%.[8] This includes rock comet 3200 Phaethon, originally discovered as, and still numbered as an asteroid.[92][93]

    Orbit
    Diagram of the orbits of Bennu and the inner planets around the Sun.

    Bennu currently orbits the Sun with a period of 1.1955 years. Earth gets as close as about 480,000 km (0.0032 au) from its orbit around the 23rd to 25 September. On September 22, 1999 Bennu passed 0.0147 au from Earth, and six years later on September 20, 2005 it passed 0.033 au from Earth. The next close approaches of less than 0.09 au will be September 30, 2054 and then September 23, 2060, which will perturb the orbit slightly. Between the close approach of 1999 and that of 2060, Earth completes 61 orbits and Bennu 51. An even closer approach will occur on September 23, 2135 between 0.0008 and 0.0036 au (see below).[1] In the 75 years between the 2060 and 2135 approaches, Bennu completes 64 orbits, meaning its period will have changed to about 1.17 years.

    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.[94] 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.[95] 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[24] made possible an improved estimate of the Yarkovsky acceleration and a revised assessment of the impact probability. The current (as of 2014) best estimate of the impact probability is a cumulative probability of 0.037% in the interval 2175 to 2196.[96] 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 0.015 megaton).[11]

    2060 close approach
    Animation of 101955 Bennu's position relative to the Earth, as both orbit the Sun, in the years 2128 to 2138. 2135 close approach is shown near the end of the animation.
       Earth ·   101955 Bennu

    Bennu will pass 0.005 au (750,000 km; 460,000 mi) from Earth on 23 September 2060,[1] while the Moon's average orbital distance (Lunar Distance, LD) is 384,402 km (238,856 mi) today and will be 384,404 km in 50 years time. It will be too dim to be seen with common binoculars.[97] 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).[1] There is no chance of an Earth impact in 2135.[98][11] 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.[96]

    On 25 September 2175, there is a 1 in 24,000 chance of an Earth impact,[11] but the nominal trajectory has the asteroid more than 1 AU from Earth on that date.[99] The most threatening virtual impactor is on 24 September 2196 when there is a 1 in 11,000 chance of an Earth impact.[11] There is a cumulative 1 in 2,700 chance of an Earth impact between 2175 and 2199.[11]

    Long term

    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.[91]

    Asteroids of absolute magnitude less than 21 passing less than 1 lunar distance from Earth
    Asteroid Date Nominal approach distance (LD) Min. distance (LD) Max. distance (LD) Absolute magnitude (H) Size (meters)
    (152680) 1998 KJ91914-12-310.6060.6040.60819.4279–900
    (458732) 2011 MD51918-09-170.9110.9090.91317.9556–1795
    (163132) 2002 CU111925-08-300.9030.9010.90518.5443–477
    2017 VW132001-11-080.4540.3183.43620.7153–494
    (153814) 2001 WN52028-06-260.6470.6470.64718.2921–943
    99942 Apophis2029-04-130.0981 (GEO is 0.109)0.09630.100019.7310–340
    2005 WY552065-05-280.8650.8560.87420.7153–494
    101955 Bennu2135-09-250.7800.3081.40620.19472–512
    (153201) 2000 WO1072140-12-010.6340.6310.63719.3427–593

    Meteor shower

    As an active asteroid with a small minimum orbit intersection distance from Earth, Bennu may be the parent body of a weak meteor shower. Bennu particles would radiate around September 25 from the southern constellation of Sculptor.[100] The meteors are expected to be near the naked eye limit and only produce a Zenith hourly rate of less than 1.[100]

    OSIRIS-REx
    Main article: OSIRIS-REx
    OSIRIS-REx's first images of Bennu.
    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.[18] 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.[101][5] On 20 October 2020, OSIRIS-REx descended to the asteroid and "pogo-sticked off"[21] it while successfully collecting a sample.[102] OSIRIS-REx is expected to return samples to Earth in 2023[103] via a capsule-drop by parachute, ultimately, from the spacecraft to the Earth's surface in Utah on September 24.[21]

    Selection

    The asteroid Bennu was selected from over half a million 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.[104] The criteria stipulated an asteroid in an orbit with low eccentricity, low inclination, and an orbital radius of 0.8–1.6 au.[105] 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.[105]

    • 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.
    • 3D shape model of Bennu made from OSIRIS-REx images.

    See also

    References
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    2. "Bennu". Dictionary.com Unabridged. Random House.
    3. "(101955) Bennu = 1999 RQ36 Orbit". Minor Planet Center. Retrieved 21 March 2018.
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