5 Astraea

5 Astraea
Images of 5 Astraea (bottom) compared with 3D models based on lightcurve data (top)
Discovery
Discovered byK. L. Hencke
Discovery siteDriesen Obs.
Discovery date8 December 1845
Designations
Pronunciation/æˈstriːə/
Named after
Astraea (Greek goddess)
1969 SE
main-belt · (middle)
Astraea
AdjectivesAstraean
Symbol🝸 (historical astronomical)
% (modern astrological)
Orbital characteristics
Epoch 23 March 2018 (JD 2458200.5)
Uncertainty parameter 0
Observation arc171.93 yr (62,799 d)
Aphelion3.0659 AU
Perihelion2.0810 AU
2.5735 AU
Eccentricity0.1914
4.13 yr (1,508 d)
186.83°
0° 14m 19.32s / day
Inclination5.3677°
141.58°
2028-Jun-27
358.75°
Proper orbital elements
2.5761849 AU
0.1980486
4.5118628°
87.046396 deg / yr
4.13573 yr
(1510.574 d)
Precession of perihelion long.
52.210903 arcsec / yr
Precession of asc. node
−57.357951 arcsec / yr
Physical characteristics
Dimensions169 km × 125 km × 83 km
114 ± 4 km
48 900 km2
Volume920 000 km3
Mass(2.29±0.13)×1018 kg
Mean density
2.95±0.36 g/cm3
16.80060 h (0.700025 d)
Equatorial rotation velocity
6.49 m/s
125° ± 3°
39° ± 3°
0.227
S
9.0 to 13.0
6.85
0.15" to 0.041"

5 Astraea (/æˈstriːə/) is an asteroid in the asteroid belt. This object is orbiting the Sun at a distance of 385 million kilometres (2.5735 AU) with a period of 4.13 yr and an orbital eccentricity of 0.19. The orbital plane is inclined at an angle of 5.37° to the plane of the ecliptic. 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. It is an S-type asteroid in the Tholen classification system.

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 🝸 (), though given Astraea's role with justice and precision, it is perhaps a stylized set of scales, or a typographic substitute for one. This symbol is no longer used. The astrological symbol is a percent sign, encoded specifically at U+2BD9 ⯙, or a standard percent sign ("%") can also be used. 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 ecliptic coordinates (125°, 39°), with a 3° uncertainty. 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).

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

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.

See also