5 Astraea (, Αστραία, written Astræa in the early literature) is a large main belt asteroid. Its surface is highly reflective (bright) and its composition is probably a mixture of nickel-iron with magnesium- and iron-silicates. The adjectival form of the name, although unused, would be Astraean (which also designates a genus of star corals).
Astraea was the fifth asteroid discovered, on December 8, 1845 by K. L. Hencke. 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 with an annual pension of 300 US$ (1968 dollars) 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
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.