5 Astraea

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.^[10]

Hencke's symbol for Astraea is an inverted anchor, in the pipeline for Unicode 17.0 as U+1F778 🝸 ( ),^[11][12] though given Astraea's role with justice and precision, it is perhaps a stylized set of scales, or a typographic substitute for one.^[13][14] This symbol is no longer used. The astrological symbol is a percent sign, encoded specifically at U+2BD9 ⯙.^[15] 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.^[16] 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)^[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.^[7] This gives an axial tilt of about 33°.^{[citation needed]} 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.^[17]

Left: A size comparison of the first 10 numbered asteroids profiled against Earth's Moon.
Right: The orbit of 5 Astraea in white compared with those of Earth, Mars and Jupiter.

• Former classification of planets

• 2 Telescope images of 5 Astraea
• MNRAS 7 (1846) 27
• Physical characteristics of (5) Astraea at the Small Bodies Data Ferret
• Discovery Circumstances: Numbered Minor Planets (1)-(5000) – Minor Planet Center
• Dictionary of Minor Planet Names, Google books
• 5 Astraea at AstDyS-2, Asteroids—Dynamic Site
  • Ephemeris · Observation prediction · Orbital info · Proper elements · Observational info
• 5 Astraea at the JPL Small-Body Database
  • Close approach · Discovery · Ephemeris · Orbit diagram · Orbital elements · Physical parameters