|Discovery site||Lincoln Lab's ETS|
|Discovery date||11 September 1999|
|MPC designation||(101955) Bennu|
|Apollo · NEO · PHA|
|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)|
|1.1264 AU (168.51 Gm)|
|1.20 yr (436.65 d)|
Average orbital speed
|28,000 metres per second (63,000 mph)|
|0° 49m 28.056s / day|
|Earth MOID||0.0032228 AU (482,120 km)|
|Jupiter MOID||3.87795 AU (580.133 Gm)|
|Jupiter Tisserand parameter||5.525|
|Proper orbital elements|
Proper mean motion
|301.1345 deg / yr|
Proper orbital period
|±10 m 275|
|Mass||6.0×1010 kg to 7.76×1010 kg|
|1.26 ± 0.070 g/cm3|
Equatorial surface gravity
|4.288 h (0.1787 d)|
|176 ± 2°|
101955 Bennu (provisional designation 1999 RQ36) is a carbonaceous asteroid in the Apollo group that was discovered by the LINEAR Project on September 11, 1999. It is a potential Earth impactor that is listed on the Sentry Risk Table with the third-highest rating on the Palermo Technical Impact Hazard Scale, due to a 1-in-2,700 chance of impacting Earth in the late 22nd century. It is the planned target of the OSIRIS-REx mission which is intended to return samples to Earth in 2023 for further study.
101955 Bennu has a mean diameter of approximately 492 m (1,614 ft; 0.306 mi) and has been observed extensively with the Arecibo Observatory Planetary Radar and the Goldstone Deep Space Network.
Asteroid Bennu has a roughly spheroidal shape, which resembles a spinning top. The direction of rotation about its axis is retrograde with respect to its orbit. Bennu has a fairly smooth shape with one prominent boulder on its surface, in the southern hemisphere. 10–20 m
There is a well-defined ridge along the equator of asteroid Bennu. The presence of this ridge suggests that fine-grained regolith particles have accumulated in this area, possibly due to its low gravity and fast rotation.
Observations of this minor planet by the Spitzer Space Telescope in 2007 gave an effective diameter of ±10 m, which is in line with other studies. It has a low visible geometric albedo of 484±0.005. The 0.046thermal inertia was measured and found to vary by ±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 longitudinally. No emission from a potential dust coma has been detected around asteroid 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 ±1.5 m/year. Analysis of the gravitational and thermal effects has given a bulk density of ρ = 284±70 kg/m3, which is only slightly denser than water. Therefore, the predicted macroporosity is 1260±10%, suggesting that the interior has a rubble pile structure. The estimated mass is 40±0.9)×1010 kg. (7.8
||This section may be too technical for most readers to understand. (September 2017) (Learn how and when to remove this template message)|
Photometric observations of 101955 Bennu in 2005 yielded a synodic rotation period of ±0.0065 h. It has a 4.2905B-type classification, which is a sub-category of carbonaceous asteroids. 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.
Origin and evolution
The carbonaceous material that compose asteroid Bennu originally came from dying stars such as red giants and supernovae. According to the accretion theory, this material came together 4.5 billion years ago during the birth of the Solar System.
Asteroid 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 into more complex minerals. Bennu probably began in the inner main 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.
Possible Earth impact
On average, an asteroid with a diameter of 500 m (1,600 ft; 0.31 mi) 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 not found to 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)[update] 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 1200 megatons in TNT equivalent.
Long term fate
In the span of time of concern to present-day people, the fate of 101955 Bennu that looms most significant is collision with the Earth. In the much longer term (many millions of years), it is more likely that 101955 Bennu will be destroyed by some other cause. Lauretta et al. report the results of a computer simulation:
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 1000 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.
Selection for OSIRIS-REx mission
The OSIRIS-REx mission of NASA's New Frontiers Program was launched towards 101955 Bennu on September 8, 2016. It is expected to reach the asteroid in August 2018 and return samples to Earth in 2023.
Bennu was selected from over 000 known asteroids by the OSIRIS-REx selection committee. The primary restraint for selection was close proximity to Earth, due to the low 500Δv required to reach it from Earth orbit. The criteria stipulated an asteroid in an orbit with low eccentricity, low inclination, and an orbital radius of AU. 0.8–1.6 
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, organic compounds and amino acids 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 due to its potentially hazardous orbit.
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.  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 resembles the Egyptian deity, which is typically depicted as a heron.
Animation of OSIRIS-REx collecting a sample from asteroid Bennu
- Nolan, M. C.; Magri, C.; Howell, E. S.; Benner, L. A. M.; Giorgini, J. D.; Hergenrother, C. W.; Hudson, R. S.; Lauretta, D. S.; Margot, J. L.; Ostro, S. J.; Scheeres, D. J. (2013). "Shape model and surface properties of the OSIRIS-REx target Asteroid (101955) Bennu from radar and lightcurve observations". Icarus. 226 (1): 629–640. Bibcode:2013Icar..226..629N. ISSN 0019-1035. doi:10.1016/j.icarus.2013.05.028.
- "Goldstone Delay-Doppler Images of 1999 RQ36". Asteroid Radar Research. Jet Propulsion Laboratory.
- "JPL Small-Body Database Browser: 101955 Bennu (1999 RQ36)" (2013-01-20 last obs.). Jet Propulsion Laboratory. Retrieved 20 August 2016.
- "(101955) Bennu". NEODyS. University of Pisa. Retrieved 1 December 2015.
- "101955 1999 RQ36: Earth Impact Risk Summary". NASA. Jet Propulsion Laboratory. August 5, 2010. Retrieved 24 November 2013.
- Chesley, Steven R.; Farnocchia, Davide; Nolan, Michael C.; Vokrouhlický, David; Chodas, Paul W.; Milani, Andrea; Spoto, Federica; Rozitis, Benjamin; Benner, Lance A.M.; Bottke, William F.; Busch, Michael W.; Emery, Joshua P.; Howell, Ellen S.; Lauretta, Dante S.; Margot, Jean-Luc; Taylor, Patrick A. (2014). "Orbit and bulk density of the OSIRIS-REx target Asteroid (101955) Bennu". Icarus. 235: 5–22. Bibcode:2014Icar..235....5C. ISSN 0019-1035. arXiv: . doi:10.1016/j.icarus.2014.02.020.
- "One of NASA’s cleanest spacecraft ever is ready to fly". Spaceflight Now. Spaceflight Now Inc. Retrieved 23 August 2016.
- Hergenrother, CW; Barucci, MA; Barnouin, O (16 Sep 2014). "The Design Reference Asteroid for the OSIRIS-REx Mission Target (101955) Bennu". Retrieved 20 August 2016.
- "Planetary Habitability Calculators". Planetary Habitability Laboratory. University of Puerto Rico at Arecibo. Retrieved 6 December 2015.
- Murphy, Diane (1 May 2013). "Nine-Year-Old Names Asteroid Target of NASA Mission in Competition Run By The Planetary Society". The Planetary Society. Retrieved 20 August 2016.
- "Sentry Risk Table". NASA/JPL Near-Earth Object Program Office. Archived from the original on 2016-09-11. Retrieved 2017-05-04.
- Lauretta, D. S.; Bartels, A. E.; et al. (April 2015). "The OSIRIS-REx target asteroid (101955) Bennu: Constraints on its physical, geological, and dynamical nature from astronomical observations". Meteoritics & Planetary Science. 50 (4): 834–849. Bibcode:2015M&PS...50..834L. doi:10.1111/maps.12353.
- Corum, Jonathan (8 September 2016). "NASA Launches the Osiris-Rex Spacecraft to Asteroid Bennu". New York Times. Retrieved 9 September 2016.
- Chang, Kenneth (8 September 2016). "The Osiris-Rex Spacecraft Begins Chasing an Asteroid". New York Times. Retrieved 9 September 2016.
- Brown, Dwayne; Neal-Jones, Nancy (31 March 2015). "RELEASE 15-056 – NASA’s OSIRIS-REx Mission Passes Critical Milestone". NASA. Retrieved 4 April 2015.
- "NASA to Launch New Science Mission to Asteroid in 2016". NASA. 25 May 2011. Retrieved 21 May 2013.
- Hudson, R. S.; Ostro, S. J.; Benner, L. A. M. "Recent Delay-Doppler Radar Asteroid Modeling Results: 1999 RQ36 and Craters on Toutatis". Bulletin of the American Astronomical Society. American Astronomical Society. 32: 1001. Bibcode:2000DPS....32.0710H.
- Emery, J.; et al. (July 2014), Muinonen, K., ed., "Thermal infrared observations and thermophysical characterization of the OSIRIS-REx target asteroid (101955) Bennu", Conference Proceedings Asteroids, Comets, Meteors 2014, Bibcode:2014acm..conf..148E.
- Hergenrother, Carl W.; et al. (September 2013), "Lightcurve, Color and Phase Function Photometry of the OSIRIS-REx Target Asteroid (101955) Bennu", Icarus, 226 (1): 663–670, Bibcode:2013Icar..226..663H, doi:10.1016/j.icarus.2013.05.044.
- Bottke, William F.; et al. (February 2015), "In search of the source of asteroid (101955) Bennu: Applications of the stochastic YORP model", Icarus, 247: 191–217, Bibcode:2015Icar..247..191B, doi:10.1016/j.icarus.2014.09.046.
- Lauretta, D. S.; et al. (April 2015), "The OSIRIS-REx target asteroid (101955) Bennu: Constraints on its physical, geological, and dynamical nature from astronomical observations", Meteoritics & Planetary Science, 50 (4): 834–849, Bibcode:2015M&PS...50..834L, doi:10.1111/maps.12353.
- Robert Marcus; H. Jay Melosh & Gareth Collins (2010). "Earth Impact Effects Program". Imperial College London / Purdue University. Retrieved 2013-02-07. (solution using density of 2,600 kg/m^3, sped of 17km/s, and impact angle of 45 degrees)
- Milani, Andrea; Chesley, Steven R.; Sansaturio, Maria Eugenia; Bernardi, Fabrizio; Valsecchi, Giovanni B.; Arratia, Oscar (2009). "Long term impact risk for (101955) 1999 RQ36". Icarus. 203 (2): 460–471. Bibcode:2009Icar..203..460M. arXiv: . doi:10.1016/j.icarus.2009.05.029.
- "101955 Bennu (1999 RQ36) Earth Impact Risk Summary". Near Earth Object Program. NASA. 8 September 2016. Retrieved 8 September 2016.
- Near-Earth Asteroid Delta-V for Space Rendezvous
- "Why Bennu?". OSIRIS-REx Mission. Arizona Board of Regents. Retrieved 10 September 2016.
- Earth Impact Risk Summary: 101955 1999 RQ36 (Years: 2169–2199) – JPL near-Earth object website
- Orbit parameters – NASA website
- Temperature History and Dynamical Evolution of (101955) 1999 RQ 36: A Potential Target for Sample Return from a Primitive Asteroid (2011 ApJ 728 L42)
- Physical Properties of OSIRIS-REx Target Asteroid (101955) 1999 RQ36 derived from Herschel, ESO-VISIR and Spitzer observations (arXiv:1210.5370 : 19 Oct 2012)
- The Design Reference Asteroid for the OSIRIS-REx Mission Target (101955) Bennu (arXiv:1409.4704 : 16 Sep 2014)
- 101955 Bennu at the JPL Small-Body Database