101955 Bennu (provisional designation ) 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 and has the second highest cumulative rating on the Palermo scale. It has a cumulative chance of around 1 in 1,750 of impacting Earth 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, creation, and rebirth.
has a mean diameter of and has been observed extensively by the Arecibo Observatory planetary radar and the Goldstone Deep Space Network.
Bennu was the target of the OSIRIS-REx 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 and the Goldstone Deep Space Network 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 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 are named after birds and bird-like creatures in mythology.
Physical characteristics
Bennu has a roughly spheroidal shape, resembling a spinning top. Bennu's axis of rotation is tilted 178 degrees to its orbit; the direction of rotation about its axis is retrograde with respect to its orbit. 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 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.
An analysis published in 2026 of "spacecraft observations revealed that Bennu is blanketed by boulders of differing physical properties" and implying "cracks in rocks resulting from geological processes within the parent body or, more recently, micrometeoroid impacts and thermal fatigue".
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 1 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 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 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 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 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 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, 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, "This finding is in agreement with pre-encounter measurements and consistent with CI and CM chondrites." dominated by phyllosilicates. "Report Card" at 25:15 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 × 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 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, "Bennu is an Active Asteroid!" sporadically emitting plumes of particles and rocks as large as (not dust, defined as tens of micrometers). 3. Multifaceted Scientific Dust Observations Scientists hypothesize the releases may be caused by thermal fracturing, volatile release through dehydration of phyllosilicates, pockets of subsurface water, and/or 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. Sec. 4.3 Dormancy and Rejuvenation B-type asteroids displaying a blue color in particular, may be dormant comets, similar to Ryugu but at an earlier stage.
Surface features
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 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. + Final four OSIRIS-REx candidate sample sites Name Location Description Nightingale 56°N 43°E Abundant fine-grained material with a large variation in color. Primary sample collection site. Kingfisher 11°N 56°E A relatively new crater with the highest water signature of all four sites. Osprey 11°N 80°E Located on a low albedo patch with a large variety of rocks. Backup sample collection site. Sandpiper 47°S 322°E Located between two young craters, located in rough terrain. Minerals vary in brightness with hints of hydrated minerals.
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