5 Astraea

Discovery and name

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.

Hencke's symbol for Astraea is an inverted anchor, encoded in Unicode 17.0 as U+1F778 🝸 ([[File:Astraea symbol (fixed width).svg|12px|class=skin-invert]]),{{cite web | access-date = September 17, 2025 | url-status = live This symbol is no longer used. The astrological symbol is a percent sign, encoded specifically at U+2BD9 ⯙: it is simply shift-5 on the keyboard, because Astraea was the fifth asteroid discovered. 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. 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 reclassification of the four original asteroids (which were identified as planets at the time), as it became apparent that these were only the largest of a new type of celestial body with thousands of members.

Characteristics

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. This gives an axial tilt of about 33°. 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.{{cite journal |display-authors=6 |access-date=26 January 2012 |archive-url=https://web.archive.org/web/20160303214312/http://astro.troja.mff.cuni.cz/projects/asteroids3D/download/durech_et_al_2011_occ_paper.pdf |archive-date=3 March 2016 |url-status=dead

Right: The orbit of 5 Astraea in white compared with those of Earth, Mars and Jupiter.

Notes

References

References

1. "Astraea". [[Oxford University Press]].
2. "AstDyS-2 Astraea Synthetic Proper Orbital Elements". Department of Mathematics, University of Pisa, Italy.
3. M. J. López-Gonzáles. (2005). "Lightcurves and poles of seven asteroids". Planetary and Space Science.
4. [https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27Astraea%27&START_TIME=%272049-01-29%27&STOP_TIME=%272049-01-30%27&STEP_SIZE=%272%20day%27&QUANTITIES=%279,19,20,23,29%27 Perihelic opposition]
5. "Dawn Community". [[NASA]].
6. (18 September 2023). "Unicode request for historical asteroid symbols". Unicode.
7. Faulks, David. (2016-05-28). "L2/16-080: Additional Symbols for Astrology".
8. (1849). "The Planet Hygea". spaceweather.com.