101955 Bennu
101955 Bennu 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. It has a cumulative 1-in-2,700 chance of impacting Earth between 2175 and 2199. It is named after the Bennu, the ancient Egyptian mythological bird associated with the Sun, creation, and rebirth.
has a mean diameter of and has been observed extensively with the Arecibo Observatory planetary radar and the Goldstone Deep Space Network.
Bennu is the target of the OSIRIS-REx mission which is intended to return samples to Earth in 2023 for further study. On 3 December 2018, the OSIRIS-REx spacecraft arrived at Bennu after a two-year journey. 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.
On 18 June 2019, NASA announced that the OSIRIS-REx spacecraft managed to get even closer and capture an image at a distance of from Bennu's surface.
Discovery and observation
Bennu was discovered on 11 September 1999 during a Near-Earth asteroid survey by the Lincoln Near-Earth Asteroid Research. The asteroid was given the provisional designation 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.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 will be 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.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. Observations 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, 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 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. 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.
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 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. The estimated mass is.
Photometry and spectroscopy
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.
Preliminary spectroscopic surveys of the asteroid's surface by OSIRIS-REx then confirmed magnetite and the meteorite-asteroid linkage, dominated by phyllosilicates. Phyllosilicates, among others, hold water.
Bennu's water spectra were detectable on approach, reviewed by outside scientists, then confirmed from orbit. This is defined as water, by geologists astrogeologists, geochemists, meteoriticists, and astronomers, as such specimens generate water under, e. g., the space environment or via mining.
Activity
Bennu is an active asteroid, sporadically emitting plumes of particles and rocks as large as,.Scientists hypothesize the releases may be caused by thermal fracturing, volatile release through dehydration of phyllosilicates, 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. B-type asteroids displaying a blue color in particular, may be dormant comets. If the IAU declares Bennu to be a dual-status object, its comet designation would be P/.
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 March 6, 2020 the IAU announced the first official names for 12 Bennu surface features, including regiones, craters, dorsa, fossae and saxa.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 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.
IAU named features
| Name | Named after | Location |
| Aellopus Saxum | Aello, one of the half-bird half-woman Harpy sisters from Greek mythology | 25.44°N 335.67°E |
| Aetos Saxum | Aetos, childhood playmate of the god Zeus who was turned into an eagle from Greek mythology | 3.46°N 150.36°E |
| Amihan Saxum | Amihan, bird deity from Philippine mythology | 17.96°S 256.51°E |
| Benben Saxum | Benben, Ancient Egyptian primordial mound that arose from the primordial waters Nu | 45.86°S 127.59°E |
| Boobrie Saxum | Boobrie, shapeshifting entity from Scottish mythology that often takes the form of a giant water bird | 48.08°N 214.28°E |
| Camulatz Saxum | Camulatz, one of four birds in the K'iche' creation myth in Maya mythology | 10.26°S 259.65°E |
| Celaeno Saxum | Celaeno, one of the half-bird half-woman Harpy sisters from Greek mythology | 18.42°N 335.23°E |
| Ciinkwia Saxum | Ciinkwia, thunder beings from Algonquian mythology that look like giant eagles | 4.97°S 249.47°E |
| Dodo Saxum | Dodo, a dodo bird character from Alice's Adventures in Wonderland | 32.68°S 64.42°E |
| Gamayun Saxum | Gamajun, prophetic bird from Slavic mythology | 9.86°N 105.45°E |
| Gargoyle Saxum | Gargoyle, dragon-like monster with wings | 4.59°N 92.48°E |
| Gullinkambi Saxum | Gullinkambi, rooster from Norse mythology that lives in Valhalla | 18.53°N 17.96°E |
| Huginn Saxum | Huginn, one of two ravens that accompany the god Odin in Norse mythology | 29.77°S 43.25°E |
| Kongamato Saxum | Kongamato, giant flying creature from Kaonde mythology | 5.03°N 66.31°E |
| Muninn Saxum | Muninn, one of two ravens that accompany the god Odin in Norse mythology | 29.34°S 48.68°E |
| Ocypete Saxum | Ocypete, one of the half-bird half-woman Harpy sisters from Greek mythology | 25.09°N 328.25°E |
| Odette Saxum | Odette, princess that turns into the White Swan in Swan Lake | 44.86°S 291.08°E |
| Odile Saxum | Odile, the Black Swan from Swan Lake | 42.74°S 294.08°E |
| Pouakai Saxum | Poukai, monstrous bird from Māori mythology | 40.45°S 166.75°E |
| Roc Saxum | Roc, giant bird of prey from Arabic mythology | 23.46°S 25.36°E |
| Simurgh Saxum | Simurgh, benevolent bird that possesses all knowledge from Iranian mythology | 25.32°S 4.05°E |
| Strix Saxum | Strix, bird of ill omen from Roman mythology | 13.4°N 88.26°E |
| Thorondor Saxum | Thorondor, the King of the Eagles in Tolkien's Middle-earth | 47.94°S 45.1°E |
| Tlanuwa Regio | Tlanuwa, giant birds from Cherokee mythology | 37.86°S 261.7°E |
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. 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.
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. This includes rock comet 3200 Phaethon, originally discovered as, and still numbered as an asteroid.
Orbit
Possible Earth impact
On average, an asteroid with a diameter of can be expected to impact Earth about every 130,000 years or so. 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 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 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.
2060 close approach
Bennu will pass from Earth on 23 September 2060,, while Moon's average orbital distance is today and 384,404 km in 50 years time.It will be too dim to be seen with common binoculars. The close approach of 2060 causes in the close approach of 2135. On 25 September 2135, the nominal approach distance is from Earth, but Bennu could pass as close as. 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 2175 approach is in February 2175 at a distance of roughly. 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 2175–2199.
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, 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, followed by the Earth and Mercury. The odds of Bennu striking Mars are only 0.8% and there is a 0.2% chance that Bennu will eventually collide with Jupiter.
| Asteroid | Date | Nominal approach distance | Min. distance | Max. distance | Absolute magnitude | Size |
| 1914-12-31 | 0.606 | 0.604 | 0.608 | 19.4 | 279–900 | |
| 1918-09-17 | 0.911 | 0.909 | 0.913 | 17.9 | 556–1795 | |
| 1925-08-30 | 0.903 | 0.901 | 0.905 | 18.5 | 443–477 | |
| 2001-11-08 | 0.454 | 0.318 | 3.436 | 20.7 | 153–494 | |
| 2028-06-26 | 0.647 | 0.647 | 0.647 | 18.2 | 921–943 | |
| 99942 Apophis | 2029-04-13 | 0.0981 | 0.0963 | 0.1000 | 19.7 | 310–340 |
| 2065-05-28 | 0.865 | 0.856 | 0.874 | 20.7 | 153–494 | |
| 101955 Bennu | 2135-09-25 | 0.780 | 0.308 | 1.406 | 20.19 | 472–512 |
| 2140-12-01 | 0.634 | 0.631 | 0.637 | 19.3 | 427–593 |
Meteor shower
As an active asteroid with a small Earth-MOID, 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.OSIRIS-REx
The OSIRIS-REx mission of NASA's New Frontiers program was launched towards on September 8, 2016. On December 3, 2018, the spacecraft arrived at the asteroid Bennu after a two-year journey. 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. OSIRIS-REx is expected to return samples to Earth in 2023.Selection
Bennu was selected from over known asteroids by the OSIRIS-REx selection committee. The primary constraint for selection was close proximity to Earth, since proximity implies low impulse 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 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.