|Discovered by||K. L. Hencke|
|Discovery site||Driesen Obs.|
|Discovery date||8 December 1845|
|Astraea (Greek goddess)|
|main-belt · (middle)|
|Epoch 23 March 2018 (JD 2458200.5)|
|Uncertainty parameter 0|
|Observation arc||171.93 yr (62,799 d)|
|4.13 yr (1,508 d)|
|0° 14m 19.32s / day|
|Proper orbital elements|
Proper semi-major axis
Proper mean motion
|87.046396 deg / yr|
Proper orbital period
Precession of perihelion
|52.210903 arcsec / yr|
Precession of the ascending node
|−57.357951 arcsec / yr|
|Dimensions||167 km × 123 km × 82 km|
|48 300 km2|
|Volume||882 000 km3|
|0.700 03 d (16.801 h)|
Equatorial rotation velocity
|8.74 to 12.89|
|0.15" to 0.041"|
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.
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 looks like an inverted anchor, though given Astraea's role with justice and precision, it is perhaps a stylized set of scales. This symbol is no longer used. The astrological symbol is U+2BF0 ⯰. The modern astronomical symbol is a simple encircled 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.
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).
Der Planet hat mit Einwilligung des Entdeckers den Namen Astraea erhalten, und sein Zeichen wird nach dem Wunsche des Hr. Hencke ein umgekehrter Anker sein.