Asteroid Bennu

Updated: Apr 9


Series of Goldstone radar images showing Bennu's rotation
Series of Goldstone radar images showing Bennu's rotation

Bennu is a near-earth asteroid that scientists believe may contain important information about life on earth. The reason why Nasa thinks it is vital for understanding life on earth is because of Bennu’s age. Bennu is old and it could be made of a material containing molecules that were present when life first formed on Earth. Bennu is known as 101955 Bennu or provisional designation name is 1999 RQ36. Bennu is a carbonaceous asteroid in the Apollo group discovered by the Near-Earth asteroid survey by the Lincoln Near-Earth Asteroid Research (LINEAR) Project on 11 September 1999. It is a potentially hazardous object and it is listed on the Sentry Risk Table with the second-highest cumulative rating on the Palermo Technical Impact Hazard Scale. It has a cumulative 1 in 2,700 chance of impacting Earth between 2175 and 2199. By comparison, the smallest planet Mercury is more than 3,000 miles across.


It is named after the Bennu, the ancient Egyptian mythological bird associated with the Sun, creation, and rebirth. Bennu is tall as the Empire State building but is 20-40% empty inside.


According to Nasa, Bennu is a primitive asteroid which had formed in the first 10 million years of our solar system’s history over 4.5 billion years ago. Its birthplace is most likely in the asteroid belt between Jupiter and Mars but Bennu absorbs sunlight and re-emits that energy as heat, which is also known as the Yarkovsky effect, it has come closer, and now classified as a near-earth asteroid. The best evidence suggests that within 10 million years of our solar system’s formation, Bennu’s present-day chemistry and mineralogy were already established. Because its materials are so old, Bennu represents a type of building block of our solar system’s rocky planets. Bennu likely was broken off from a much larger carbon-rich asteroid about 700 million to 2 billion years ago, which is relatively recent in geological time. Scientists think that a cataclysmic collision caused a carbon-rich asteroid 60 to 130 miles (100-200 kilometers) in diameter, roughly the size of Connecticut, to break apart, scattering pieces including Bennu.


According to Nasa, Bennu is a primordial artefact preserved in the vacuum of space, orbiting among planets and moons and asteroids and comets. Nasa also has pointed out that Bennu could be rich in resources like platinum and gold and the mission will allow scientists to study whether asteroid mining during deep-space exploration and travel is feasible.


101955 Bennu has been observed extensively with the Arecibo Observatory planetary radar and the Goldstone Deep Space Network.


  • Discovered Date: 11 September 1999

  • Discovered by: Lincoln Laboratory Near-Earth Asteroid Research (LINEAR) project

  • B-type classification (a sub-category of carbonaceous asteroids)

  • 1.2 light-years away from Earth (1.524 AU)

  • No Moons

  • Namesake: Ancient Egyptian bird God

  • MPC designation: (101955) Bennu

  • Named after: Bennu

  • Alternative designations: 1999 RQ36

  • Minor planet category: Apollo | NEO | PHA | risk listed

  • Diameter: 490 m (1,610 ft; 0.30 mi)

  • Average Orbit Distance: 158,000,000km

  • Average orbital distance from the Sun: 105 million miles (168 million kilometers), which is only slightly farther than Earth’s average orbital distance of 93 million miles.​

  • It makes one full rotation on its axis every 4.3 hours

  • Its orbital path is tilted about 5 degrees relative to Earth’s

  • Mean orbit Velocity: 100,000km/h

  • Orbit eccentricity: .20375

  • Equatorial Inclination: 175 degrees

  • Equatorial Radiation: .25km

  • Equatorial Circumference: 1.6km

  • Volume: .0623km^3

  • Density: 1.26g/cm^3

  • Mass: 78,000,000,000kg

  • Surface Area: .79km^2

  • Surface Gravity: .00005m/s^2

  • Escape Velocity: .54 to .7km/h

  • Atmospheric Constituents: none

Orbital characteristics

  • 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)

  • Semi-major axis: 1.1264 au (168.51 Gm)

  • Eccentricity: 0.20375

  • Orbital period: 1.1955 yr (436.65 d)

  • Average orbital speed: 28.0 km/s (63,000 mph)

  • Mean anomaly: 101.7039°

  • Mean motion: 0° 49m 28.056s / day

  • Inclination: 6.0349°

  • Longitude of ascending node: 2.0609°

  • Argument of perihelion: 66.2231°

  • Earth MOID: 0.0032228 au (482,120 km)

  • Venus MOID: 0.194 au (29,000,000 km)[3]

  • Mars MOID: 0.168 au (25,100,000 km)[3]

  • Jupiter MOID: 3.877 au (580.0 Gm)

  • TJupiter: 5.525

Proper orbital elements

  • Proper eccentricity: 0.21145

  • Proper inclination: 5.0415°

  • Proper mean motion: 301.1345 deg / yr

  • Proper orbital period: 1.19548 yr (436.649 d)

Physical characteristics


Dimensions: 565 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

Surface area: 0.782±0.004 km2

Volume: 0.0615±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[5]

Rotation period: 4.296057±0.000002 h

Axial tilt: 177.6±0.11°

North pole right ascension: +85.65±0.12°

North pole declination: −60.17±0.09°

Geometric albedo: 0.044±0.002

Surface temp min mean max

Kelvin 236 259 279

Fahrenheit -34.6 6.8 42.8

Celsius -37 -14 6

Spectral type: B, F

Absolute magnitude (H): 20.9


Bennu is an active asteroid sporadically emitting plumes of particles and rocks as large as 10 cm (3.9 in) (not dust, defined as tens of micrometers). Scientists hypothesize the releases may be caused by thermal fracturing, volatile release through dehydration of phyllosilicates, pockets of subsurface water, and meteoroid 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. If the IAU declares Bennu to be a dual-status object, its comet designation would be P/1999 RQ36 (LINEAR).


Image sequence showing the rotation of Bennu, imaged by OSIRIS-REx at a distance of around 80 km (50 mi).
Image sequence showing the rotation of Bennu, imaged by OSIRIS-REx at a distance of around 80 km (50 mi).


How Bennu got its Name?


The name Bennu was selected from more than eight thousand student entries from different 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.


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. The origins of its minerals and atoms are to be found in dying stars such as red giants and supernovae. 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 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. 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.


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 comet 3200 Phaethon, originally discovered as, and still numbered as an asteroid.


OSIRIS-REx mission ( Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer)


Bennu is the target of the OSIRIS-REx mission and the mission will return its samples to Earth in 2023 for further study of Bennu. The OSIRIS-REx mission of NASA's New Frontiers program was launched towards 101955 Bennu on September 8, 2016. On 3 December 2018, the OSIRIS-REx spacecraft arrived at Bennu after a two-year journey. 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.


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.


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


On 20 October 2020, OSIRIS-REx descended to the asteroid and pogo-sticked off, while successfully collecting a sample. OSIRIS-REx is expected to return samples to Earth in 2023 via a capsule drop by parachute from the spacecraft to the Earth's surface in Utah, expected on September 24.


The mission is essentially a seven-year-long voyage and will conclude when at least 60 grams of samples are delivered back to the Earth.


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 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.


Final four OSIRIS-REx candidate sample sites

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.

The final four candidate OSIRIS-REx sample sites
The final four candidate OSIRIS-REx sample sites

Bennu Characteristics

  • Roughly spheroidal shape, resembling a spinning top

  • The Axis of rotation is tilted 178 degrees to its orbit

  • The direction of rotation about its axis is retrograde with respect to its orbit.

  • 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. 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. 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.

  • 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 have 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 made 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.

  • 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 or even hollow.[30] The estimated mass is (7.329±0.009)×1010 kg.

Asteroid Bennu ejecting particles

Particle trajectories from four 2019 ejection events


Photometry and spectroscopy


Photometric observations of Bennu in 2005 yielded a synodic rotation period of 4.2905±0.0065 h. 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.


Before OSIRIS-REx, spectroscopy indicated a correspondence with the CI or CM carbonaceous chondrite meteorites, including carbonaceous-chondrite mineral magnetite. Magnetite, a spectrally prominent water product but destroyed by heat, is an important proxy of astronomers including OSIRIS-REx staff.


Water


Predicted beforehand, Dante Lauretta (University of Arizona) then stated 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. Phyllosilicates, among others, hold water. Bennu's water spectra were detectable on an 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 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.


Surface


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 candidates 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).


Close-up examinations by OSIRIS-REx revealed that Bennu’s surface is much rougher than scientists had estimated based on radar observations from Earth. Bennu is completely covered in rocks and large boulders, some as big as 72 feet (22 meters) across. Since the arrival of OSIRIS-REx at the Bennu, the OSIRIS-REx team has observed more than 300 occasions when rocky particles were ejected from the asteroid’s surface. Some particles escape into space, others briefly orbit the asteroid and most fall back onto its surface after being launched. Expanding water vapor, impacts by small space rocks known as meteoroids, and rocks cracking from thermal stress are all possible explanations.


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)
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
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.
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.

IAU named features


Map of Bennu showing the locations of the IAU-named surface features
Map of Bennu showing the locations of the IAU-named surface features

10 Things About Bennu

  • The asteroid was discovered by the Lincoln Near-Earth Asteroid Research (LINEAR) survey on September 11, 1999.

  • Bennu’s original name was 1999 RQ36. In 2013, A nine-year-old, a third-grade student named Michael Puzio won a contest to name the asteroid.

  • Bennu has drifted into near-Earth space because of gravitational interactions with giant planets and the gentle push of heating from the Sun.

  • Bennu's density is low and it's only about 30 percent more than water. This suggests the asteroid is probably a loose collection of rocks, like a pile of rubble.

  • Bennu has a close approach to Earth every six years.

  • Scientists estimate Bennu has a 1‐in‐2,700 chance of impacting the Earth during one of its close approaches to the Earth in the late 22nd century. The asteroid stays in the potentially hazardous asteroid

  • Bennu may burn up in the Sun. Over millions of years, of all of the planets, Bennu is most likely to hit Venus

  • The boulder that juts from Bennu's south pole is about 164 feet (50 meters) high and 180 feet (55 meters) wide.

  • Bennu doesn't have any moon.

  • NASA’s OSIRIS-REx mission studied Bennu in unprecedented detail. It collected a sample in 2020 and will return it to Earth in 2023.

Potential for Life and Atmosphere


Bennu does not have the conditions necessary for life and the temperatures range from a toasty 240 degrees Fahrenheit (116 Celsius) to a frigid100 degrees. Because there is no atmospheric pressure, liquid water cannot exist on or under its surface. Bennu doesn’t have enough gravity to have an atmosphere. Bennu is so dark, it tends to absorb the sun's radiation.


Orbit


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


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 the physical properties of Bennu that were poorly known at the time but were 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) 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 0.015 megaton).


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.
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: Blue

Bennu: Pink


Bennu will pass 0.005 au (750,000 km, 460,000 mi) from Earth on 23 September 2060, 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.


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 trajectory has the asteroid more than 1 AU from Earth on that date. 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 and 2199.


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, they tracked 1,000 virtual Bennus for an interval of 300 Myr with the gravitational perturbations of the planets Mercury, Neptune included. Their 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.


Structure


Bennu looks like a spinning top, a shape whose origin scientists don’t fully understand. Some other asteroids have similar equatorial ridges.


In terms of its composition, because of Bennu’s resemblance to carbon-rich meteorites found on Earth, scientists think the asteroid is made of some of the solar system’s oldest materials. These materials were forged in large dying stars, including supernova explosions, long before our solar system formed. The asteroid’s materials would have been altered by heat when its parent body broke apart in the giant collision. Meteorites that seem similar to Bennu in color or spectral properties often contain organic material, which does not necessarily come from a biological source.


Bennu is thought to be a rubble pile asteroid, meaning smaller fragments from the original large asteroid came together to form Bennu. The small pieces of Bennu stay together because of gravity and another force scientists call cohesion, which makes soils and sands stick together. But based on existing measurements, Bennu is about 20 to 40 percent empty space inside. If Bennu were placed (gently) on the surface of Earth, our planet’s gravity would cause the whole structure to fall apart.


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. The meteors are expected to be near the naked eye limit and only produce a Zenith hourly rate of less than 1.


Exploration


In 1999, scientists collected data about Bennu through radar imaging and ranging from radio telescopes, and through spectroscopy, a technique to explore the composition of an object by examining the kind of light it reflects.


From 2005 to 2007, when it approached Earth again, various space-based and ground-based telescopes observed Bennu. NASA’s Spitzer Space Telescope measured the temperature and brightness of the asteroid in 2008.


NASA’s Hubble and Spitzer space telescopes, the European Space Agency’s Herschel Space Observatory, and a variety of ground-based observatories focused on Bennu again from 2011 to 2012. These observations helped scientists refine its rotation period and shape. Radio observations from 2011, as well as 2005 and 1999, have enabled astronomers to calculate speeds and distances for Bennu at various times.


Observations from 1999-2000 and 2005-6 were especially fruitful because Bennu was so close


Timeline


  • Sept. 11, 1999: The LINEAR survey discovers the asteroid 1999 RQ36.

  • 2005-2006: The asteroid makes a close approach to Earth and scientists observe it with space-based and ground-based telescopes.

  • 2011-2012: The asteroid makes another close approach. Radio observations and space observations help scientists define its rotation period and model its shape.

  • 2013: A third-grader named Michael Puzio names the asteroid Bennu through a contest.

  • 2014: Scientists calculate the mass of Bennu by measuring its thermal properties and how much it deviates from its predicted orbit, the first time that the mass of an asteroid has been found in this way.

  • 2016: NASA’s OSIRIS-REx spacecraft launches, en route to study Bennu up close and return a sample.

  • 2018: OSIRIS-REx approached Bennu and began to map its surface.

  • 2020: OSIRIS-REx collected a sample of material from Bennu’s surface.

  • 2023: OSIRIS-REx will return to Earth with the sample from Bennu.

Heterogeneous mass distribution of the rubble-pile asteroid (101955) Bennu


The gravity field of a small body provides insight into its internal mass distribution. We used two approaches to measure the gravity field of the rubble-pile asteroid (101955) Bennu:

(i) tracking and modeling the spacecraft in orbit about the asteroid and

(ii) tracking and modeling pebble-sized particles naturally ejected from Bennu’s surface into sustained orbits.


These approaches yield statistically consistent results up to degree and order 3, with the particle-based field being statistically significant up to degree and order 9. Comparisons with a constant-density shape model show that Bennu has a heterogeneous mass distribution. These deviations can be modeled with lower densities at Bennu’s equatorial bulge and center. The lower-density equator is consistent with recent migration and redistribution of material. The lower-density center is consistent with a past period of rapid rotation, either from a previous Yarkovsky-O’Keefe-Radzievskii-Paddack cycle or arising during Bennu’s accretion following the disruption of its parent body.


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