101955 Bennu

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101955 Bennu
Mosaic image of 101955 Bennu consisting of 12 PolyCam images collected on 2 December 2018 by OSIRIS-REx from a range of 24 km (15 mi).
Discovered by LINEAR
Discovery site Lincoln Lab's ETS
Discovery date11 September 1999
MPC designation (101955) Bennu
Pronunciation /bɛˈn/
Named after
1999 RQ36
Apollo  · NEO  · PHA
Orbital characteristics [1]
Epoch 31 July 2016 (JD 2457600.5)
Uncertainty parameter 0
Observation arc 13.36 yr (4880 days)
Aphelion 1.3559  au (202.84  Gm)
Perihelion 0.89689 au (134.173 Gm)
1.1264 au (168.51 Gm)
Eccentricity 0.20375
1.20 yr (436.65 d)
Average orbital speed
28,000 metres per second (63,000  mph)
 49m 28.056s / day
Inclination 6.0349°
Earth  MOID 0.0032228 au (482,120 km)
Venus  MOID 0.194 au (29,000,000 km) [2]
Mars  MOID 0.168 au (25,100,000 km) [2]
Jupiter  MOID 3.877 au (580.0 Gm)
TJupiter 5.525
Proper orbital elements [3]
Proper eccentricity
Proper inclination
Proper mean motion
301.1345  deg  / yr
1.19548 yr
(436.649 d)
Physical characteristics
Mean radius
262.5±37.5 m [4]
Equatorial radius
262.5±37.5 m [4]
Mass 6.0×1010 kg [5] to 7.8±0.9×1010 kg [6]
Mean density
1.26±0.070 g/cm3 [6]
Equatorial surface gravity
10 micro-g [7]
4.288  h (0.1787  d)
176 ± 2° [8]
0.046±0.005 [1]
0.059±0.003 [9]
Surface temp. minmeanmax
Kelvin [10] 236259279
B [1]
F [9]

    101955 Bennu (provisional designation 1999 RQ36) [11] 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. [12] It has a cumulative 1-in-2,700 chance of impacting Earth between 2175 and 2199. [5] [13] It is named after the Bennu, the ancient Egyptian mythological bird associated with the Sun, creation, and rebirth.

    C-type asteroid asteroid spectral type

    C-type (carbonaceous) asteroids are the most common variety, forming around 75% of known asteroids. They are distinguished by a very low albedo because their composition includes a large amount of carbon, in addition to rocks and minerals. They occur most frequently at the outer edge of the asteroid belt, 3.5 astronomical units (AU) from the Sun, where 80% of the asteroids are of this type, whereas only 40% of asteroids at 2 AU from the Sun are C-type. The proportion of C-types may actually be greater than this, because C-types are much darker than most other asteroid types except for D-types and others that are mostly at the extreme outer edge of the asteroid belt.

    Apollo asteroid Earth-crossing asteroid that has an orbital semi-major axis greater than that of the Earth (> 1 AU) but perihelion distance less than the Earths aphelion distance (q < 1.017 AU)

    The Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, discovered by German astronomer Karl Reinmuth in the 1930s. They are Earth-crossing asteroids that have an orbital semi-major axis greater than that of the Earth but perihelion distances less than the Earth's aphelion distance.

    Lincoln Near-Earth Asteroid Research research project, collaboration between the U.S. Air Force, NASA, and the Massachusetts Institute of Technologys Lincoln Laboratory

    The Lincoln Near-Earth Asteroid Research (LINEAR) project is a collaboration of the United States Air Force, NASA, and the MIT's Lincoln Laboratory for the systematic detection and tracking of near-Earth objects. LINEAR was responsible for the majority of asteroid discoveries from 1998 until it was overtaken by the Catalina Sky Survey in 2005. As of 15 September 2011, LINEAR had detected 231,082 new small Solar System bodies, of which at least 2,423 were near-Earth asteroids and 279 were comets. The instruments used by the LINEAR program are located at Lincoln Laboratory's Experimental Test Site (ETS) on the White Sands Missile Range (WSMR) near Socorro, New Mexico.


    101955 Bennu has a mean diameter of approximately 492 m (1,614 ft; 0.306 mi) and has been observed extensively with the Arecibo Observatory planetary radar and the Goldstone Deep Space Network. [4] [14] [15]

    Arecibo Observatory Radio telescope in Arecibo, Puerto Rico (U.S.)

    The Arecibo Observatory is a radio telescope in the municipality of Arecibo, Puerto Rico. This observatory is operated by University of Central Florida, Yang Enterprises and UMET, under cooperative agreement with the US National Science Foundation (NSF). The observatory is the sole facility of the National Astronomy and Ionosphere Center (NAIC), which is the formal name of the observatory. From its construction in the 1960s until 2011, the observatory was managed by Cornell University.

    Goldstone Deep Space Communications Complex observatory

    The Goldstone Deep Space Communications Complex (GDSCC), commonly called the Goldstone Observatory, is a satellite ground station located in the Mojave Desert near Barstow in the U.S. state of California. Operated by NASA's Jet Propulsion Laboratory (JPL), its main purpose is to track and communicate with space missions. It is named after Goldstone, California, a nearby gold-mining ghost town.

    NASA Deep Space Network network of radio communication facilities run by NASA

    The NASA Deep Space Network (DSN) is a worldwide network of U.S. spacecraft communication facilities, located in the United States (California), Spain (Madrid), and Australia (Canberra), that supports NASA's interplanetary spacecraft missions. It also performs radio and radar astronomy observations for the exploration of the Solar System and the universe, and supports selected Earth-orbiting missions. DSN is part of the NASA Jet Propulsion Laboratory (JPL). Similar networks are run by Russia, China, India, Japan and the European Space Agency.

    Bennu is the target of the OSIRIS-REx mission which is intended to return samples to Earth in 2023 for further study. [16] [17] [18] On 3 December 2018, the OSIRIS-REx spacecraft arrived at Bennu after a two-year journey. [19] 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. [20]

    OSIRIS-REx Third mission of the New Frontiers program; orbital reconnaissance and sample return from 101955 Bennu

    The OSIRIS-REx is a NASA asteroid study and sample-return mission. The mission's main goal is to obtain a sample of at least 60 grams (2.1 oz) from 101955 Bennu, a carbonaceous near-Earth asteroid, and return the sample to Earth for a detailed analysis. The material returned is expected to enable scientists to learn more about the formation and evolution of the Solar System, its initial stages of planet formation, and the source of organic compounds that led to the formation of life on Earth. If successful, OSIRIS-REx will be the first U.S. spacecraft to return samples from an asteroid. The Lidar instrument used aboard the OSIRIS-REx was built by Lockheed Martin, in conjunction with the Canadian Space Agency.

    Sample-return mission space mission to retrieve tangible samples from an extraterrestrial location and return with them to Earth for analysis

    A sample-return mission is a spacecraft mission with the goal of collecting and returning samples from an extraterrestrial location to Earth for analysis. Sample-return missions may bring back merely atoms and molecules or a deposit of complex compounds such as loose material ("soil") and rocks. These samples may be obtained in a number of ways, such as soil and rock excavation or a collector array used for capturing particles of solar wind or cometary debris.

    Discovery and observation

    Series of Goldstone radar images showing Bennu's rotation. Bennu rotation.gif
    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). [2] The asteroid was designated 1999 RQ36 and classified a near-Earth asteroid. Bennu approached close to Earth and it 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. [4] [14]


    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] [11] 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]

    University of Arizona Public university in Tucson, Arizona, United States

    The University of Arizona is a public research university in Tucson, Arizona. Founded in 1885, the UA was the first university in the Arizona Territory. As of 2017, the university enrolls 44,831 students in 19 separate colleges/schools, including the University of Arizona College of Medicine in Tucson and Phoenix and the James E. Rogers College of Law, and is affiliated with two academic medical centers. The University of Arizona is governed by the Arizona Board of Regents. The University of Arizona is one of the elected members of the Association of American Universities and is the only representative from the state of Arizona to this group.

    The Planetary Society nonprofit organization

    The Planetary Society is an American internationally active, non-governmental, nonprofit foundation. It is involved in research, public outreach, and political advocacy for engineering projects related to astronomy, planetary science, and space exploration. It was founded in 1980 by Carl Sagan, Bruce Murray, and Louis Friedman, and has about 60,000 members from more than 100 countries around the world.

    North Carolina State of the United States of America

    North Carolina is a state in the southeastern region of the United States. It borders South Carolina and Georgia to the south, Tennessee to the west, Virginia to the north, and the Atlantic Ocean to the east. North Carolina is the 28th-most extensive and the 9th-most populous of the U.S. states. The state is divided into 100 counties. The capital is Raleigh, which along with Durham and Chapel Hill is home to the largest research park in the United States. The most populous municipality is Charlotte, which is the second-largest banking center in the United States after New York City.

    Physical characteristics

    Image sequence showing the rotation of Bennu, imaged by OSIRIS-REx at a distance of around 80 km (50 mi). Asteroid-Bennu-OSIRIS-RExArrival-GifAnimation-20181203.gif
    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). 2019-02-25 regolith image compilation.png
    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 176 degrees to its orbit; The direction of rotation about its axis is retrograde with respect to its orbit. [8] Bennu has a fairly smooth shape with one prominent 10–20 m boulder on its surface, in the southern hemisphere. [13]

    Top toy designed to spin rapidly on the ground, the motion of which causes it to remain precisely balanced on its tip because of its rotational inertia

    A spinning top is a toy designed to spin rapidly on the ground, the motion of which causes it to remain precisely balanced on its tip due to its rotational inertia. Such toys have existed since antiquity. Traditionally tops were constructed of wood, sometimes with an iron tip, and would be set in motion by aid of a string or rope coiled around its axis which, when pulled quickly, caused a rapid unwinding that would set the top in motion. Today they are often built of plastic, and modern materials and manufacturing processes allow tops to be constructed with such precise balance that they can be set in motion by a simple twist of the fingers and twirl of the wrist without need for string or rope.

    Retrograde and prograde motion astronomy

    Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is the central object. It may also describe other motions such as precession or nutation of the object's rotational axis. Prograde or direct motion is motion in the same direction as the primary rotates. Rotation is determined by an inertial frame of reference, such as distant fixed stars. However, retrograde and prograde can also refer to an object other than the primary if so described.

    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. [13] Observations by the OSIRIS-Rex spacecraft has shown that Bennu is rotating faster over time. [21] This change in Bennu's rotation is caused by the Yarkovsky-O'Keefe-Radzievskii-Paddack effect, or the YORP effect. [21] 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. [21]

    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. [22]

    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 = 1260±70 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 (7.8±0.9)×1010 kg. [6]

    Photometry and spectroscopy

    Image of Bennu ejecting particles from its surface, taken by OSIRIS-REx on 19 March 2019. Bennu-Particle-Ejection-Event-20190119.jpg
    Image of Bennu ejecting particles from its surface, taken by OSIRIS-REx on 19 March 2019.

    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. [9] 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. [24]

    Preliminary spectroscopic surveys of the asteroid's surface by OSIRIS-REx spacecraft, detected the presence of hydrated minerals in the form of clay. While researchers suspect that Bennu was too small to host water, the hydroxyl groups may have come from water presence in its parent body before Bennu split off. [25] [26]

    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. [27] 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 of course the hydration (if need be)—into more complex minerals. [13] 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. [28]

    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. [29]

    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 asteroids is 8%±5%. [9]


    Diagram of the orbits of Bennu and the inner planets around the Sun. Bennu Orbit.png
    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. [30] 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. [31] 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, [4] 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. [6] 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). [5]

    2060 close approach

    Animation of 101955 Bennu's orbit around Earth 2128-2138. 2135 close approach is shown.
Earth *   101955 Bennu Animation of 101955 Bennu orbit around Earth 2128-2138.gif
    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. [1] 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. [32] [5] 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. [6]

    On 25 September 2175, there is a 1 in 24,000 chance of an Earth impact, [5] 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 ). [1] The most threatening virtual impactor is on 24 September 2196 when there is a 1 in 11,000 chance of an Earth impact. [5] There is a cumulative 1 in 2,700 chance of an Earth impact between 2175–2199. [5]

    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. [29]

    Meteor shower

    The small Earth-MOID may result in a weak meteor shower around September 25 radiating from the southern constellation of Sculptor. [33] The meteors are expected to be near the naked eye limit and only produce a Zenith hourly rate of less than 1. [33]


    OSIRIS-REx's first images of Bennu. OSIRIS-REx First Image of Asteroid Bennu.gif
    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 trajectory.gif
    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 26 December 2018 to 20 March 2021
OSIRIS-REx *   101955 Bennu Animation of OSIRIS-Rex trajectory around 101955 Bennu.gif
    Animation of OSIRIS-REx 's trajectory around 101955 Bennu from 26 December 2018 to 20 March 2021
       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. [19] 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. [34] OSIRIS-REx is expected to return samples to Earth in 2023. [35]


    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. [36] The criteria stipulated an asteroid in an orbit with low eccentricity, low inclination, and an orbital radius of 0.8–1.6  au . [37] 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. [37]

    See also

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