A three-dimensional model of Astraea based on its light curve.
|Discovered by||Karl Ludwig Hencke|
|Discovery date||December 8, 1845|
|Minor planet category||Main belt|
|Epoch November 26, 2005 (JD 2453700.5)|
|Aphelion||3.070 AU (459.202 Gm)|
|Perihelion||2.077 AU (310.688 Gm)|
|2.573 AU (384.945 Gm)|
|4.13 a (1507.676 d)|
Average orbital speed
|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|
119 km (mean)
|0.700 03 d (16.801 h)|
max: 263 K (-10 °C)
|8.74 to 12.89|
|0.15" to 0.041"|
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 object in the Tholen classification system.
Astraea was the fifth asteroid discovered, on December 8, 1845, by K. L. 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. An amateur astronomer and post office employee, 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.
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. In terms of maximum brightness, it is indeed only the seventeenth brightest main-belt asteroid, being fainter than 192 Nausikaa and even, at rare near-perihelion oppositions, the highly eccentric carbonaceous 324 Bamberga. It will be at magnitude +8.7 on a favorable opposition on February 15, 2016.
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.
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