B-type asteroids are a relatively uncommon type of carbonaceous asteroid, falling into the wider C-group; the 'B' indicates these objects are spectrally blue. In the asteroid population, B-class objects can be found in the outer asteroid belt, and also dominate the high-inclination Pallas family which includes the third-largest asteroid 2 Pallas. They are thought to be primitive, volatile-rich remnants from the early Solar System. There are 65 known B-type asteroids in the SMASS classification, and 9 in the Tholen classification as of March 2015.
Generally similar to the C-type objects, but differing in that the ultraviolet absorption below 0.5 μm is small or absent, and the spectrum is rather slightly bluish than reddish. The albedo also tends to be greater than in the generally very dark C type. Spectroscopy of B-class objects suggests major surface constituents of anhydrous silicates, hydrated clay minerals, organic polymers, magnetite, and sulfides. The closest matches to B-class asteroids have been obtained on carbonaceous chondrite meteorites that have been gently heated in the laboratory.
The majority of asteroids that have been observed to display cometary-like activity are B-types. Some of these objects display indications of aqueous alteration in the past, and they may have incorporated significant amounts of water ice.
Asteroid 101955 Bennu is a B-type asteroid which is the target of the OSIRIS-REx mission. The mission seeks to characterize the asteroid by mapping the surface, studying the Yarkovsky effect, and retrieving a sample of the asteroid to return in 2023. The spacecraft was launched in 2016 and has been at Bennu since December 2018.
Other B-type asteroids include:
2 Pallas
24 Themis
47 Aglaja
59 Elpis
85 Io
88 Thisbe
142 Polana
213 Lilaea
241 Germania
282 Clorinde
335 Roberta
372 Palma
383 Janina
431 Nephele
531 Zerlina
541 Deborah
555 Norma
560 Delila
702 Alauda
704 Interamnia
767 Bondia
895 Helio
1021 Flammario
1331 Solvejg
1474 Beira
1484 Postrema
1539 Borrelly
1655 Comas Sola
1705 Tapio
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