5 Astraea is an asteroid in the asteroid belt. This object is orbiting the Sun at a distance of 2.5735 AU with a period of 1508 days and an orbital eccentricity of 0.19. The orbital plane is inclined at an angle of 5.37° to the plane of the ecliptic. It is spinning with a period of 16.8 h. The surface of Astraea is highly reflective 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 Astraea, a Greek 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 1200 marks for the discovery. Hencke's symbol for Astraea is an inverted anchor, in the pipeline for Unicode 17.0 as U+1F778 🝸, 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 a percent sign, encoded specifically at U+2BD9 ⯙. The modern astronomical symbol is a simple encircled 5 (⑤). For 38 years after the discovery of the fourth known asteroid, Vesta, in 1807, no further asteroids were discovered. After the discovery of Astraea, 8 more were discovered in the following 5 years, and 24 were found in the 5 years after that. 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 new type of celestial body with thousands of members.

Characteristics

Photometry indicates prograde rotation, that the north pole points in the direction of right ascension 115° or 310° and declination 55°, with a 5° uncertainty. This gives an axial tilt of about 33°. With an apparent magnitude of 8.7 (on a favorable opposition on 15 February 2016), it is only the seventeenth-brightest main-belt asteroid, and fainter than, for example, 192 Nausikaa or even 324 Bamberga (at rare near-perihelion oppositions). An stellar occultation on 6 June 2008 allowed Astraea's diameter to be estimated; it was found to be 115 ± 6 km.