5 Astraea

5 Astraea (pronounced /əˈstriːə/,[6] or as Greek: Αστραία; 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. Astraea 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 1,200 marks for the discovery.[7]

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.[2] This gives an axial tilt of about 33°.
The orbit of 5 Astraea compared with the orbits of Earth, Mars and Jupiter

Astraea 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.[8] In terms of maximum brightness, it is indeed only the seventeenth brightest main belt asteroid, being fainter than 192 Nausikaa and even, at rare near-perihelion oppositions, the highly eccentric carbonaceous 324 Bamberga[1].

After the discovery of Astraea, thousands of other asteroids would follow. Indeed, 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)[8] 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).

References

* Yeomans, Donald K.. "Horizons system". NASA JPL. http://ssd.jpl.nasa.gov/?horizons. Retrieved 2007-03-20. — Horizons can be used to obtain a current ephemeris

1. ^ a b Supplemental IRAS Minor Planet Survey
2. ^ a b c M. J. López-Gonzáles & E. Rodríguez Lightcurves and poles of seven asteroids, Planetary and Space Science, Vol. 53, p. 1147 (2005).
3. ^ Michalak, G. (2001). "Determination of asteroid masses". Astronomy & Astrophysics 374: 703–711. doi:10.1051/0004-6361:20010731. http://www.aanda.org/index.php?option=article&access=standard&Itemid=129&url=/articles/aa/abs/2001/29/aa10228/aa10228.html. Retrieved 2008-11-10.
4. ^ (Mass estimate of Astra 0.015 / Mass of Ceres 4.75) * Mass of Ceres 9.43E+20 = 2.977E+18
5. ^ Michalak2001 (Table 6) assumed masses of perturbing asteroids used in calculations of perturbations of the test asteroids.
6. ^ In US dictionary transcription, us dict: ə·strē′·ə.
7. ^ "Dawn Community". NASA. http://dawn.jpl.nasa.gov/DawnCommunity/flashbacks/fb_09.asp. Retrieved 2009-04-17.
8. ^ a b "The Planet Hygea". spaceweather.com. 1849. http://spaceweather.com/swpod2006/13sep06/Pollock1.jpg. Retrieved 2008-04-18.

External links

* 2 Telescope images of 5 Astraea
* AN 23 (1846) 393 (in German)
* MNRAS 7 (1846) 27