10 Hygiea
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Discovery
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| Discovered by | A. de Gasparis |
| Discovery date | April 12, 1849 |
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Designations
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| Alternative names | none |
| Minor planet category |
Main belt (Hygiea family) |
| Epoch July 14, 2004 (JD 2453200.5) | |
| Aphelion | 525.311 Gm (3.511 AU) |
| Perihelion | 413.378 Gm (2.763 AU) |
| Semi-major axis | 469.345 Gm (3.137 AU) |
| Eccentricity | 0.119 |
| Orbital period | 2029.776 d (5.56 a) |
| Average orbital speed | 16.76 km/s |
| Mean anomaly | 273.788° |
| Inclination | 3.842° |
| Longitude of ascending node | 283.646° |
| Argument of perihelion | 313.557° |
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Physical characteristics
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| Dimensions | 500×385×350 km [1] |
| Mass | 8.6 ± 0.7 ×1019 kg [2][3][4] |
| Mean density | 2.4 g/cm³ |
| Equatorial surface gravity | 0.091 m/s² |
| Escape velocity | 0.21 km/s |
| Rotation period | 27.623 h [5] |
| Albedo | 0.0717 (geometric)[5] |
| Temperature | ~164 K max: 247 K (−26° C) [6] |
| Spectral type | C-type asteroid |
| Apparent magnitude | 9.1[7] to 11.97 |
| Absolute magnitude | 5.43 |
| Angular diameter | 0.318" to 0.133" |
10 Hygiea (pronounced /haɪˈdʒiːə/, Greek: ‘Υγιεία) is the fourth largest object in the asteroid belt, somewhat oblong with diameters of 350–500 km and a mass estimated to be 3% of the total mass of the asteroid belt.
It is the largest of the class of dark C-type asteroids with a carbonaceous surface that are dominant in the outer main belt, which lies beyond the Kirkwood gap at 2.82 AU, and is also the largest body in this region. Its dark surface and larger than average distance from the Sun makes it very dim for such a large asteroid when observed from Earth. In fact it is the third dimmest of the first twenty-three asteroids discovered, with only 13 Egeria and the quite small 17 Thetis having lower mean opposition magnitudes.[8]
At most oppositions, Hygiea has a magnitude of around +10.2,[8] which is as much as four orders fainter than Vesta, and will require at least a 4-inch telescope to resolve. At a perihelic opposition, however, Hygiea can reach +9.1[7] and may just be resolvable with 10x50 binoculars, unlike the fifth and sixth largest asteroids 704 Interamnia and 511 Davida which are always beyond binocular visibility.
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[edit] Discovery
Hygiea was discovered by Annibale de Gasparis on April 12, 1849 in Naples, Italy. It was the first of his nine asteroid discoveries. It is named after Hygieia, the Greek goddess of health, daughter of Asclepius (Aesculapius for the Romans).
The director of the Naples observatory, Ernesto Capocci, named the asteroid. He chose to call it Igea Borbonica ("Bourbon Hygieia") in honor of the ruling family of the Kingdom of the Two Sicilies where Naples was located. However, by 1852, John Russell Hind would write that "it is universally termed Hygeia, the unnecessary appendage 'Borbonica' being dropped."[9]
The name was often spelled Hygeia in the nineteenth century, for example in the Monthly Notices of the Royal Astronomical Society.
[edit] Characteristics
Hygiea's surface is composed of primitive carbonaceous material similar to the chondrite meteorites. It is the main member of the Hygiea family and contains almost all the mass in this family (well over 90%). The primitive present surface composition would indicate that Hygiea had not been melted during the early period of Solar system formation, in contrast to other large planetesimals like 4 Vesta. Hygiea appears to have a noticeably oblong shape, much more so than the other objects in the "big four" (the dwarf planet Ceres and the asteroids 2 Pallas and 4 Vesta). Aside from being the smallest of the four, another important factor to this end is Hygiea's relatively low density, which is comparable to the icy satellites of Jupiter or Saturn more than to the terrestrial planets or the stony asteroids.
Generally Hygiea's properties are the most poorly known out of the "big four" objects in the main belt. Its orbit is much closer to the plane of the ecliptic than those of Ceres, Pallas or Interamnia, but is less circular than Ceres or Vesta with an eccentricity of around 12%. Its perihelion is at a quite similar longitude to those of Vesta and Ceres, though its ascending and descending nodes are opposite the corresponding ones for those objects. Although its perihelion is extremely close to the mean distance of Ceres and Pallas, a collision between Hygiea and its larger companions is impossible because at that distance they are always on opposite sides of the ecliptic. At aphelion Hygiea reaches out to the extreme edge of the asteroid belt at the perihelia of the Hilda family which is in 3:2 resonance with Jupiter.
It is an unusually slow rotator, taking 27 hours and 37 minutes for a revolution, whereas 6 to 12 hours are more typical for large asteroids. Its direction of rotation is unknown at present, due to a twofold ambiguity in lightcurve data that is exacerbated by its long rotation period, which makes single-night telescope observations span at best only a fraction of a full rotation. Lightcurve analysis indicates that Hygiea's pole points towards either ecliptic coordinates (β, λ) = (30°, 115°) or (30°, 300°) with a 10° uncertainty [1]. This gives an axial tilt of about 60° in both cases.
At least 5 stellar occultations by Hygiea were tracked by Earth-based observers, but all with few observing independent measurements so that much was not learned of its shape. The Hubble Space Telescope was able to resolve the asteroid, and to rule out the presence of any orbiting companions greater than about 16 km in diameter [10].
[edit] References
- ^ a b M. Kaasalainen et al Models of Twenty Asteroids from Photometric Data, Icarus, Vol. 159, p. 369 (2002).
- ^ S. R. Chesley et al The Mass of Asteroid 10 Hygiea, abstract for American Astronomical Society, DDA meeting #36, #05.05 (2005) (on ADS).
- ^ G. Michalak Determination of asteroid masses, Astronomy & Astrophysics, Vol. 374, 703 (2001).
- ^ Yu. Chernetenko, O. Kochetova, and V. Shor Masses and densities of minor planets Update 1 Sept. 2005. (webpage)
- ^ a b NASA JPL Small-Body Database Browser on 10 Hygiea
- ^ L.F. Lim et al Thermal infrared (8–13 µm) spectra of 29 asteroids: the Cornell Mid-Infrared Asteroid Spectroscopy (MIDAS) Survey, Icarus Vol. 173, p. 385 (2005).
- ^ a b Bright Minor Planets 2000. Minor Planet Center. Retrieved on 2008-05-21.
- ^ a b Moh'd Odeh. The Brightest Asteroids. The Jordanian Astronomical Society. Retrieved on 2008-05-21.
- ^ John Russell Hind, The Solar System, p. 126 (1852). [1]
- ^ A. Storrs et al, Imaging Observations of Asteroids with Hubble Space Telescope, Icarus, Vol. 137, p. 260 (1999).
[edit] External links
- shape model deduced from lightcurve
- A simulation of the orbit of Hygiea
- Yeomans, Donald K.. Horizons system. NASA JPL. Retrieved on 2007-03-20. — Horizons can be used to obtain a current ephemeris.
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