5 Astraea

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5 Astraea

Discovery A
Discoverer	Karl Ludwig Hencke
Discovery date	December 8, 1845
Alternate designations B	1969 SE
Category	Main belt
Orbital elements C
Epoch November 26, 2005 (JD 2453700.5)
Eccentricity (e)	0.193
Semi-major axis (a)	384.945 Gm (2.573 AU)
Perihelion (q)	310.688 Gm (2.077 AU)
Aphelion (Q)	459.202 Gm (3.070 AU)
Orbital period (P)	1507.676 d (4.13 a)
Mean orbital speed	18.39 km/s
Inclination (i)	5.369°
Longitude of the ascending node (Ω)	141.690°
Argument of perihelion (ω)	357.530°
Mean anomaly (M)	194.442°
Physical characteristics D
Dimensions	167×123×82 km [1][3]
Mass	~2.4×1018 kg
Density	~2.7 g/cm³ [2]
Surface gravity	~0.023 m/s²
Escape velocity	~0.062 km/s
Rotation period	0.700 03 d (16.801 h) [3]
Spectral class	S-type asteroid
Absolute magnitude	6.85
Albedo (geometric)	0.227 [1]
Mean surface temperature	~167 K max: 263 K (-10 °C)
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5 Astraea (as-tree'-ə (key); 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.

Size comparison: the first 10 asteroids profiled against Earth's Moon. Astrea is the fifth from the left.

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.^{[citation needed]}

Photometry indicates prograde rotation, that the north pole points in the direction of right ascension 9 h 52 min, declination 73° with a 5° uncertainty ^[3]. This gives an axial tilt of about 33°.

Astrea 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. After the discovery of Astraea, thousands of other asteroids would follow. Indeed, the discovery of Astrea 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.

There has been only one observed stellar occultation by Astraea (February 2, 1991).

[edit] Aspects

[edit] External links

• AN 23 (1846) 393 (in German)
• MNRAS 7 (1846) 27

[edit] References

1. Supplemental IRAS Minor Planet Survey
2. G. A. Krasinsky et al Hidden Mass in the Asteroid Belt, Icarus, Vol. 158, p. 98 (2002).
3. M. J. López-Gonzáles & E. Rodríguez Lightcurves and poles of seven asteroids, Planetary and Space Science, Vol. 53, p. 1147 (2005).