page_title: 101955 Bennu
section_title: Physical characteristics
hierarchical_section_title: 101955 Bennu / Physical characteristics
caption_attribution_description: English: OSIRIS-REx Arrives at Bennu After traveling through space for more than two years and over two billion miles, NASA’s Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) spacecraft arrived at its destination, asteroid Bennu, on Monday, Dec. 3, 2018. The spacecraft will spend almost a year surveying the asteroid with five scientific instruments with the goal of selecting a location that is safe and scientifically interesting to collect the sample. OSIRIS-REx will return the sample to Earth in September 2023. This series of images taken by the OSIRIS-REx spacecraft shows Bennu in one full rotation from a distance of around 50 miles (80 km). The spacecraft’s PolyCam camera obtained the thirty-six 2.2-millisecond frames over a period of four hours and 18 minutes.
context_page_description: 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.
  101955 Bennu has a mean diameter of 490 m 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 600 metres from Bennu's surface.
context_section_description: 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 10–20 m 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 10 m on the surface, the largest of which is 58 m 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 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 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 284±1.5 meters/year. Analysis of the gravitational and thermal effects has given a bulk density of ρ = 1190±13 kg/m³, which is only slightly denser than water. Therefore, the predicted macroporosity is 40±10%, suggesting the interior has a rubble pile structure. The estimated mass is (7.329±0.009)×10¹⁰ kg.
caption: Image sequence showing the rotation of Bennu, imaged by OSIRIS-REx at a distance of around 80 km (50 mi).