101955 Bennu
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101955 Bennu ( provisional designation ) is a carbonaceous asteroid in the
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group discovered by the
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Project on 11 September 1999. It is a potentially hazardous object that is listed on the Sentry Risk Table and has the second highest cumulative rating on the Palermo scale. It has a cumulative 1-in-1,750 chance of impacting
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
between 2178 and 2290 with the greatest risk being on 24 September 2182. It is named after Bennu, the ancient Egyptian mythological bird associated with the
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
, creation, and rebirth. has a mean diameter of and has been observed extensively by the
Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...
planetary radar and the Goldstone
Deep Space Network The NASA Deep Space Network (DSN) is a worldwide Telecommunications network, network of spacecraft communication ground segment facilities, located in the United States (California), Spain (Madrid), and Australia (Canberra), that supports NASA' ...
. Bennu was the target of the
OSIRIS-REx OSIRIS-REx was a NASA asteroid-study and sample-return mission that visited and collected samples from 101955 Bennu, a C-type asteroid, carbonaceous near-Earth object, near-Earth asteroid. The material, returned in September 2023, is expected ...
mission that returned samples of the asteroid to Earth. The spacecraft, launched in September 2016, arrived at the asteroid two years later and mapped its surface in detail, seeking potential sample collection sites. Analysis of the orbits allowed calculation of Bennu's mass and its distribution. In October 2020, OSIRIS-REx briefly touched down and collected a sample of the asteroid's surface. A capsule containing the sample was returned and landed on Earth in September 2023, with distribution and analysis of the sample ongoing. On 15 May 2024, an overview of preliminary analytical studies on the returned samples was reported.


Discovery and observation

Bennu was discovered on 11 September 1999 during a Near-Earth asteroid survey by the Lincoln Near-Earth Asteroid Research (LINEAR). The asteroid was given the provisional designation and classified as a near-Earth asteroid. Bennu was observed extensively by the
Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...
and the Goldstone
Deep Space Network The NASA Deep Space Network (DSN) is a worldwide Telecommunications network, network of spacecraft communication ground segment facilities, located in the United States (California), Spain (Madrid), and Australia (Canberra), that supports NASA' ...
using radar imaging as Bennu closely approached Earth on 23 September 1999.


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 University of Arizona (Arizona, U of A, UArizona, or UA) is a Public university, public Land-grant university, land-grant research university in Tucson, Arizona, United States. Founded in 1885 by the 13th Arizona Territorial Legislature, it ...
,
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, and the LINEAR Project in 2012. Third-grade student Michael Puzio from
North Carolina North Carolina ( ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, South Carolina to the south, Georgia (U.S. stat ...
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.


Physical characteristics

Bennu has a roughly spheroidal shape, resembling a
spinning top A spinning top, or simply a top, is a toy with a squat body and a sharp point at the bottom, designed to be rotation, spun on its vertical Axis of rotation, axis, balancing on the tip due to the gyroscopic effect. Once set in motion, a top will ...
. Bennu's
axis of rotation Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis intersect ...
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 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 on the surface, the largest of which is across. The boulders contain veins of high
albedo Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...
carbonate mineral Carbonate minerals are those minerals containing the carbonate ion, . Carbonate divisions Anhydrous carbonates *Calcite group: trigonal **Calcite CaCO3 **Gaspéite (Ni,Mg,Fe2+)CO3 **Magnesite MgCO3 **Otavite CdCO3 **Rhodochrosite MnCO3 **Sider ...
s believed to have formed prior to the formation of the asteroid due to hot water channels on the much larger parent body. The veins range from 3 to 15 centimeters wide, and can be over one meter in length, much bigger than carbonate veins seen in
meteorite A meteorite is a rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...
s. 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 (about once every 4.3 hours). Observation 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. 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 , which is in line with other studies. It has a low visible geometric albedo of . The
thermal inertia Thermal inertia is a term commonly used to describe the observed delays in a body's temperature response during heat transfers. The phenomenon exists because of a body's ability to both store and transport heat relative to its environment. Sinc ...
was measured and found to vary by approximately 19% during each rotational period. It was based on this observation that 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.
Astrometric Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way. History ...
observations between 1999 and 2013 have demonstrated that 101955 Bennu is influenced by the Yarkovsky effect, causing the
semimajor axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the long ...
of its orbit to drift on average by  meters/year. Analysis of the gravitational and thermal effects has given a bulk density of ρ =  kg/m3, which is only slightly denser than water. Therefore, the predicted macroporosity is %, suggesting the interior has a
rubble pile In astronomy, a rubble pile is a celestial body that consists of numerous pieces of debris that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the various chunks that make the ...
structure or even hollows. The estimated mass is . The sample of Bennu revealed that some of the makings of living things are present on Bennu.


Photometry and spectroscopy

Photometric observations of Bennu in 2005 yielded a synodic
rotation period In astronomy, the rotation period or spin period of a celestial object (e.g., star, planet, moon, asteroid) has two definitions. The first one corresponds to the '' sidereal rotation period'' (or ''sidereal day''), i.e., the time that the objec ...
of . 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. Before OSIRIS-REx, spectroscopy indicated a correspondence with the CI and/or CM carbonaceous chondrite meteorites, including carbonaceous-chondrite mineral
magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...
. Magnetite, a spectrally prominent water product but destroyed by heat, is an important proxy of astronomers including OSIRIS-REx staff.


Water

According to Dante Lauretta, OSIRIS-REx Principal Investigator, "Bennu appears to be a very water-rich target, and water is the most interesting and perhaps the most lucrative commodity that you would mine from an asteroid". Predicted beforehand, Dante Lauretta (University of Arizona) reiterates that Bennu is water-rich- already detectable while OSIRIS-REx was still technically in approach. Preliminary spectroscopic surveys of the asteroid's surface by OSIRIS-REx confirmed magnetite and the meteorite-asteroid linkage, dominated by
phyllosilicates Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, the crystalline forms of silica (silicon dio ...
. Phyllosilicates, among others, hold water. Bennu's water spectra were detectable on approach, reviewed by outside scientists, then confirmed from orbit. 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. A separate estimate, including other forms of water storage, is 6.2 wt%.
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
and university sample facilities are preparing to secure, study, and curate the sample, predicted to be rich in water and organic compounds. The German SAL (Sample Analysis Laboratory) is preparing to receive cosmochemical water from Ryugu, Bennu, and other airless bodies.


Activity

Bennu is an active asteroid, sporadically emitting plumes of particles and rocks as large as ,No One Knows Why Rocks Are Exploding From Asteroid Bennu.
Daniel Oberhaus, ''Wired''. 5 December 2019.
. (not
dust Dust is made of particle size, fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian processes, aeolian process), Types of volcan ...
, defined as tens of micrometers). Scientists hypothesize the releases may be caused by thermal fracturing, volatile release through dehydration of
phyllosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, the crystalline forms of silica (silicon dio ...
s, pockets of subsurface water, and/or
meteoroid A meteoroid ( ) is a small rocky or metallic body in outer space. Meteoroids are distinguished as objects significantly smaller than ''asteroids'', ranging in size from grains to objects up to wide. Objects smaller than meteoroids are classifie ...
impacts. Before the arrival of OSIRIS-REx, Bennu had displayed polarization consistent with Comet Hale-Bopp and 3200 Phaethon, a rock comet. Bennu, Phaethon, and inactive Manx comets are examples of active asteroids. B-type asteroids displaying a blue color in particular, may be dormant comets, similar to Ryugu but at an earlier stage. If the
IAU The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...
declares Bennu to be a dual-status object, its comet designation would be P/ (LINEAR).


Surface features

All geological features on Bennu are named after various species of
birds Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
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 6 March 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). Analysis showed that the particles making up Bennu's exterior are loosely packed and lightly bound to each other; "The spacecraft would have sunk into Bennu had it not fired its thrusters to back away immediately after it grabbed dust and rock from the asteroid's surface." Analysis also revealed that the Sun's heat fractures rocks on Bennu in just 10,000 to 100,000 years instead of millions of years as was thought before.


Candidate sample sites

After a thorough analysis of Bennu’s surface by the OSIRIS-REx mission team, using data from both MapCam and OVIRS, four candidate sites were selected for sample collection: Nightingale, Kingfisher, Osprey, and Sandpiper. Among these, Nightingale was ultimately chosen, as it exhibited a stronger spectral reddening compared to the rest of the surface (indicating fresher or less exposed terrain). Additionally, it successfully passed the safety assessment tests for the spacecraft’s descent. On 12 December 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


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 Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
, the origins of its minerals and atoms are to be found in dying stars such as red giants and
supernova A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...
e. 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 Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical mineralogy, optical) properties of minerals and mineralized artifact (archaeology), artifacts. Specific s ...
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 The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...
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. Impactors on boulders of Bennu indicate that Bennu has been in near Earth orbit (separated from the main
asteroid belt The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...
) for 1–2.5 million years. Subsequently, the orbit drifted as a result of the Yarkovsky effect and mean motion resonances with the giant planets, such as
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...
and
Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...
. 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 object A near-Earth object (NEO) is any small Solar System body orbiting the Sun whose closest approach to the Sun (Apsis, perihelion) is less than 1.3 times the Earth–Sun distance (astronomical unit, AU). This definition applies to the object's orb ...
s is . This includes rock comet 3200 Phaethon, discovered and still numbered as an asteroid.


Orbit

Bennu orbits the Sun with a period of . Earth gets as close as about 480,000 km (0.0032  au) from its orbit around 23 to 25 September. On 22 September 1999 Bennu passed 0.0147 au from Earth, and six years later on 20 September 2005 it passed 0.033 au from Earth. The next close approaches of less than 0.04 au will be 30 September 2054 and then 23 September 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 25 September 2135 around 0.0014 au (see table). In the 75 years between the 2060 and 2135 approaches, Bennu completes 64 orbits, meaning its period will have changed to . The Earth approach of 2135 will increase the orbital period to about . Before the 2135 Earth approach, Bennu will be at its maximum distance from Earth on 27 November 2045 at a distance of .


Possible Earth impact

On average, an asteroid with a diameter of can be expected to impact Earth about every 130,000 years. 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 '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. In 2014, the best estimate of the impact probability was a cumulative probability of 0.037% in the interval 2175 to 2196. This corresponds to a cumulative rating 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 TNT equivalent is a convention for expressing energy, typically used to describe the energy released in an explosion. A ton of TNT equivalent is a unit of energy defined by convention to be (). It is the approximate energy released in the de ...
(for comparison, TNT equivalent of
Tsar Bomba The Tsar Bomba (code name: ''Ivan'' or ''Vanya''), also known by the alphanumerical designation "AN602", was a Thermonuclear weapon, thermonuclear aerial bomb, and by far the most powerful nuclear weapon ever created and tested. The Soviet phy ...
, the most powerful nuclear weapon ever tested, was approximately 54 megatons, and that of the
Tunguska event The Tunguska event was a large explosion of between 3 and 50 TNT equivalent, megatons that occurred near the Podkamennaya Tunguska River in Yeniseysk Governorate (now Krasnoyarsk Krai), Russia, on the morning of 30 June 1908. The explosion over ...
, the most energetic
impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effe ...
in recorded history, has been estimated at 3–5 megatons, though another estimate is 20–30 megatons). The 2021 orbit solution extended the virtual impactors from the year 2200 to the year 2300 and slightly increased the cumulative Palermo impact hazard scale rating to −1.42. The solution included the estimated gravitational effect of 343 other asteroids, representing about 90% of the total mass of the main asteroid belt.


2060/2135 close approaches

Bennu will pass from Earth on 23 September 2060, while for comparison the
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
's average orbital distance ('' lunar distance)'' is and will only change to 384,404 km in 50 years time. Bennu 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 Earth approach distance is ±20 thousand km. 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 ~20 km wide with some keyholes being only 5 meters wide.


2182

The most threatening virtual impactor is on Tuesday, 24 September 2182 when there is a 1 in 2,700 chance of an Earth impact, but the asteroid could be as far as the Sun is from Earth. To impact Earth on 24 September 2182 Bennu must pass through a keyhole roughly 5 km wide on 25 September 2135.Table 3. Impact dates, keyhole centers and widths in the 2135 B-plane
(Farnocchia2021) The table reports the zeta coordinate on the B-plane, which is not the same thing as the miss distance during the 2135 encounter.
The next two biggest risks occur in 2187 (1:14,000) and 2192 (1:26,000). There is a cumulative 1 in 1,800 chance of an Earth impact between 2178 and 2290.


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.


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 A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of cosmic debris called meteoroids entering Earth's atmosphere at ext ...
. Bennu particles would radiate around 25 September from the southern
constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...
of
Sculptor Sculpture is the branch of the visual arts that operates in three dimensions. Sculpture is the three-dimensional art work which is physically presented in the dimensions of height, width and depth. It is one of the plastic arts. Durable sc ...
. The meteors are expected to be near the naked eye visibility limit and only produce a Zenith hourly rate of less than 1.


Exploration


OSIRIS-REx

The
OSIRIS-REx OSIRIS-REx was a NASA asteroid-study and sample-return mission that visited and collected samples from 101955 Bennu, a C-type asteroid, carbonaceous near-Earth object, near-Earth asteroid. The material, returned in September 2023, is expected ...
mission of NASA's New Frontiers program was launched towards on 8 September 2016. On 3 December 2018, the spacecraft arrived at the asteroid Bennu after a two-year journey. One week later, at the
American Geophysical Union The American Geophysical Union (AGU) is a 501(c)(3) nonprofit organization of Earth, Atmospheric science, atmospheric, Oceanography, ocean, Hydrology, hydrologic, Astronomy, space, and Planetary science, planetary scientists and enthusiasts that ...
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. On 20 October 2020, OSIRIS-REx descended to the asteroid and " pogo-sticked off" it while successfully collecting a sample. On 7 April 2021, OSIRIS-REx completed its final flyover of the asteroid and began slowly drifting away from it. On 10 May 2021, the departure was completed with OSIRIS-REx still managing to contain the asteroid sample. OSIRIS-REx returned samples to Earth in 2023 via a capsule-drop by parachute, ultimately, from the spacecraft to the Earth's surface in
Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...
on 24 September 2023. Shortly after the sample container was retrieved and transferred to an airtight chamber at the
Johnson Space Center The Lyndon B. Johnson Space Center (JSC) is NASA's center for human spaceflight in Houston, Texas (originally named the Manned Spacecraft Center), where human spaceflight training, research, and flight controller, flight control are conducted. ...
in Houston, Texas, the lid on the container was opened. Scientists commented that they "found black dust and debris on the avionics deck of the OSIRIS-REx science canister" on the initial opening. Further study is planned. On 11 October 2023, the recovered capsule was opened to reveal a "first look" at the asteroid sample contents. On 13 December 2023, further studies of the returned sample were reported and revealed organic molecules as well as unknown materials which require more study to have a better idea of their composition and makeup. On 11 January 2024, NASA reported finally fully opening, after three months of trying, the recovered container with samples from the Bennu asteroid. The total weight of the recovered material weighed , over twice the mission's goal. On 15 May 2024, an overview of preliminary analytical studies on the returned samples was reported.


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. The criteria stipulated an asteroid in an orbit with low eccentricity, low inclination, and an orbital radius of . 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 compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
s, 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. File:Animation of OSIRIS-REx trajectory.gif, Trajectory in the Solar System from 9 August 2016 to 24 September 2023 File:Animation of OSIRIS-Rex trajectory around 101955 Bennu.gif, Trajectory around 101955 Bennu from 25 December 2018 File:Animation of OSIRIS-REx around Bennu - touch down on Bennu.gif, Touchdown on Bennu


Returned samples

The OSIRIS-REx mission successfully returned approximately 120 grams of material from Bennu to Earth in September 2023. The returned material is predominantly very dark, with reflectance values consistent with observations of Bennu's surface, though it contains some brighter inclusions and particles. Particle sizes in the sample span a wide range, from submicron dust to rocks measuring about 3.5 cm in length. Mineralogical analysis shows that the sample is rich in hydrated minerals, particularly Mg-rich phyllosilicates, confirming predictions from remote sensing data. Other major components include magnetite, sulfides, carbonates, and organic compounds. An unexpected discovery was the presence of phosphate minerals in some samples, including Mg, Na-rich phosphates found as veins and crusts in some particles. The elemental composition of the Bennu samples closely resembles that of CI chondrite meteorites. However, the Bennu material shows some distinct isotopic ratios. The average oxygen isotopic composition places Bennu in the same region of oxygen three-isotope space as CI and CY
chondrites A chondrite is a stony (non-metallic) meteorite that has not been modified by either melting or planetary differentiation, differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar Syste ...
, as well as samples from asteroid Ryugu. The carbon content of the samples (4.5–4.7 wt%) is higher than that found in known meteorites and Ryugu samples. The presence of presolar grains in the samples indicates that some of the material has remained largely unprocessed since the formation of the solar system. Presolar
silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A wide bandgap semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder a ...
and
graphite Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...
were identified, with abundances of and respectively, similar to unheated chondrite samples. Evidence suggests that the samples come from at least two different lithologies on Bennu's surface. Three predominant types of particles were identified: hummocky, angular, and mottled. These show distinct densities, with hummocky particles having the lowest average density () and mottled particles the highest (). Spectral analysis of the samples shows a redder slope from 0.4 to 2.5 μm compared to Bennu's global spectrum, potentially indicating differences in particle size, surface texture, or space weathering between the sampled material and the asteroid's surface. Since 3 November 2023, a part of the sample is exhibited at the Hall of Meteorites of the
National Museum of Natural History The National Museum of Natural History (NMNH) is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. It has free admission and is open 364 days a year. With 4.4 ...
(Washington, DC). Another portion of the sample was exhibited by NASA at the International Astronautical Congress in
Milan Milan ( , , ; ) is a city in northern Italy, regional capital of Lombardy, the largest city in Italy by urban area and the List of cities in Italy, second-most-populous city proper in Italy after Rome. The city proper has a population of nea ...
, Italy, from 14 to 18 October 2024. In January 2025, it was reported that a wide range of carbon- and nitrogen-rich organic compounds have been identified in samples returned from Bennu, including 14 of the 20 amino acids that make up proteins in terrestrial organisms, as well as all five nucleobases (adenine, thymine, cytosine, guanine, and uracil) that are the essential building blocks of DNA and RNA.


See also

*
List of minor planets and comets visited by spacecraft The following tables list all minor planets and comets that have been visited by robotic spacecraft. List of minor planets visited by spacecraft A total of 19 minor planets (asteroids, dwarf planets, and Kuiper belt objects) have been visi ...
* 162173 Ryugu, an asteroid being studied by JAXA concurrent with the NASA mission to 101955 Bennu


References


External links


Video (2:53)
– Asteroid Bennu Mission Overview (
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
; 11 May 2021).
Video (01:12) – Asteroid Bennu ejecting material into space
( CNN; 5 December 2019)
Video (01:32) – OSIRIS REx's approach to asteroid Bennu
(
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
; 7 January 2019)
Earth Impact Risk Summary: 101955 1999 RQ36
(Years: 2175–2199) –
Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a Federally funded research and development centers, federally funded research and development center (FFRDC) in La Cañada Flintridge, California, Crescenta Valley, United States. Founded in 1936 by Cali ...
near-Earth object site
NEODyS-2 Ephemerides for 2135
(step size: 10 days) * * *
Nominal and impacting solution for 2182
* * {{DEFAULTSORT:101955 Apollo asteroids Potentially hazardous asteroids Active asteroids Discoveries by LINEAR Bennu Potential impact events caused by near-Earth objects Minor planets visited by spacecraft Bennu Articles containing video clips 19990911 B-type asteroids (SMASS)