101955 Bennu

101955 Bennu
Asteroid Bennu, imaged by the OSIRIS-REx probe (3 December 2018)
Discovered byLINEAR
Discovery siteLincoln Lab's ETS
Discovery date11 September 1999
MPC designation(101955) Bennu
Named after
1999 RQ36
Apollo · NEO · PHA
Orbital characteristics[2]
Epoch 31 July 2016 (JD 2457600.5)
Uncertainty parameter 0
Observation arc13.36 yr (4880 days)
Aphelion1.3559 au (202.84 Gm)
Perihelion0.89689 au (134.173 Gm)
1.1264 au (168.51 Gm)
1.20 yr (436.65 d)
28,000 metres per second (63,000 mph)
0° 49m 28.056s / day
Earth MOID0.0032228 au (482,120 km)
Venus MOID0.194 au (29,000,000 km)[1]
Mars MOID0.168 au (25,100,000 km)[1]
Jupiter MOID3.877 au (580.0 Gm)
Proper orbital elements[3]
301.1345 deg / yr
1.19548 yr
(436.649 d)
Physical characteristics
Mean radius
262.5±37.5 m[4]
Equatorial radius
262.5±37.5 m[4]
Mass6.0×1010 kg[5] to 7.8±0.9×1010 kg[6]
Mean density
1.26±0.070 g/cm3[6]
Equatorial surface gravity
10 micro-g[7]
4.288 h (0.1787 d)
176 ± 2°[8]
Surface temp.minmeanmax

101955 Bennu (provisional designation 1999 RQ36)[11] 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.[12] It has a cumulative 1-in-2,700 chance of impacting Earth between 2175 and 2199.[5][13] It is named in honor to Bennu, an ancient Egyptian deity linked with the Sun, creation, and rebirth.

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.[4][14][15]

Bennu is the target of the OSIRIS-REx mission which is intended to return samples to Earth in 2023 for further study.[16][17][18] On 3 December 2018, the OSIRIS-REx spacecraft arrived at Bennu after a two-year journey.[19] 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.[20]

Physical description

This mosaic image of asteroid Bennu is composed of 12 PolyCam images collected on Dec. 2 by the OSIRIS-REx spacecraft from a range of 15 miles (24 km).

Asteroid Bennu has a roughly spheroidal shape, resembling 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 10–20 m boulder on its surface, in the southern hemisphere.[13]

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 because of its low gravity and fast rotation.[13]

Observations of this minor planet 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 ±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 asteroid Bennu, which puts a limit of 106 g of dust within a radius of 4750 km.[21]

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 284±1.5 meters/year. Analysis of the gravitational and thermal effects has given a bulk density of ρ = 1260±70 kg/m3, which is only slightly denser than water. Therefore, the predicted macroporosity is 40±10%, suggesting that the interior has a rubble pile structure. The estimated mass is (7.8±0.9)×1010 kg.[6]

Photometry and spectroscopy

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. Polarimetric observations show that Bennu belongs to the rare F subclass of carbonaceous asteroids, which is usually associated with cometary features.[9] 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.[22]

Preliminary spectroscopic surveys of the asteroid's surface by OSIRIS-REx spacecraft, detected the presence of the hydroxyl group likely bonded in the clay material of the asteroid. While researchers suspect that Bennu was too small to host water, these hydroxyl may have come from water presence in its parent body before Bennu split off.[23]

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