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

[14][15][16] The spacecraft, launched in September 2016, arrived at the asteroid two years later and mapped its surface in detail, seeking potential sample collection sites.

[1][28] Third-grade student Michael Puzio from North Carolina proposed the name in reference to the Egyptian mythological bird Bennu.

[5] The boulders contain veins of high albedo carbonate minerals believed to have formed prior to the formation of the asteroid due to hot water channels on the much larger parent body.

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).

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.

[34] 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.

[36] 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.

[37] Before OSIRIS-REx, spectroscopy indicated a correspondence with the CI and/or CM carbonaceous chondrite meteorites,[38][39][40] including carbonaceous-chondrite mineral magnetite.

[41][42][43] Magnetite, a spectrally prominent[44][45] water product[46][47][48] but destroyed by heat,[48] is an important proxy of astronomers[49][50][51] including OSIRIS-REx staff.

[52] According to Dante Lauretta,[53] 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".

[54][55] Predicted beforehand,[56] Dante Lauretta (University of Arizona) reiterates that Bennu is water-rich- already detectable while OSIRIS-REx was still technically in approach.

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.

[73][74][75] The German SAL (Sample Analysis Laboratory) is preparing to receive cosmochemical water from Ryugu, Bennu, and other airless bodies.

[76] Bennu is an active asteroid,[77][78][79][80] sporadically emitting plumes of particles[81][82] and rocks as large as 10 cm (3.9 in),[83][84] (not dust, defined as tens of micrometers).

Bennu's basic mineralogy and chemical nature would have been established during the first 10 million years of the Solar System's formation, where the carbonaceous material underwent some geologic heating and chemical transformation inside a much larger planetoid or a proto-planet capable of producing the requisite pressure, heat and hydration (if need be)—into more complex minerals.

[108] 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.

[118] 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[27] made possible an improved estimate of the Yarkovsky acceleration and a revised assessment of the impact probability.

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.

[109] As an active asteroid with a small minimum orbit intersection distance from Earth, Bennu may be the parent body of a weak meteor shower.

[17] 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.

Scientists commented that they "found black dust and debris on the avionics deck of the OSIRIS-REx science canister" on the initial opening.

[131][132] On 11 January 2024, NASA reported finally fully opening, after three months of trying, the recovered container with samples from the Bennu asteroid.

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.

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.

Mineralogical analysis shows that the sample is rich in hydrated minerals, particularly Mg-rich phyllosilicates, confirming predictions from remote sensing data.

[141][142] In January 2025, it was reported that a wide range of carbon- and nitrogen-rich organic compounds have been identfied 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.