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.

A German amateur astronomer and post office headmaster, Hencke was looking for 4 Vesta when he stumbled on Astraea.

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)[16] 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.

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.

An stellar occultation on 6 June 2008 allowed Astraea's diameter to be estimated; it was found to be 115 ± 6 km.