5 Astraea
Astraea (minor planet designation: 5 Astraea) is a large asteroid from the asteroid belt. Its surface is highly reflective (bright) and its composition is probably a mixture of nickel–iron with silicates of magnesium and iron. It is an S-type asteroid in the Tholen classification system. Discovery and name. Astraea was the fifth asteroid discovered, on 8 December 1845, by Karl Ludwig Hencke and named for "Astræa", a goddess of justice named after the stars. It was his first of two asteroid discoveries. The second was 6 Hebe. A German amateur astronomer and post office headmaster, Hencke  was looking for 4 Vesta when he stumbled on Astraea. The King of Prussia awarded him an annual pension of 1,200 marks for the discovery. Hencke's symbol for Astraea is an inverted anchor, though given Astraea's role with justice and precision, it is perhaps a stylized set of scales, or a typographic substitute for one. This symbol is no longer used. The astrological symbol is U+2BD9 ⯙ (), a stylized percent sign. The modern astronomical symbol is a simple encircled 5 (⑤). After the discovery of Astraea, thousands of other asteroids would follow. Indeed, the discovery of Astraea proved  to be the starting point for the eventual demotion of the four original asteroids (which were regarded as planets at the time) to their current status, as it became apparent that these four were only the largest of a whole new type of celestial body. Characteristics. Photometry indicates prograde rotation, that the north pole points in the direction of right ascension 9 h 52 min, declination 73° with a 5° uncertainty. This gives an axial tilt of about 33°. Astraea is physically unremarkable but notable mainly because for 38 years (after the discovery of Vesta in 1807 ) it had been thought that there were only four asteroids. With an apparent magnitude of 8.7 (on a favorable opposition on 15 February 2016), it is indeed only the "seventeenth-brightest" main-belt asteroid, and fainter than, for example, 192 Nausikaa or even 324 Bamberga (at rare near-perihelion oppositions). An occultation on 6 June 2008 produced an effective diameter (silhouette) of . Astraea has been studied by radar. Arecibo observed Astraea in March 2012.