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10 Hygiea

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10 Hygiea Astronomical symbol for 10 Hyhiea
10 Hygiea 2MASS.jpg
Hygiea by 2MASS
Discovered byA. de Gasparis
Discovery timeApril 12, 1849
Other namesnone
GroupMain belt (Hygiea family)
Reference date July 14, 2004 (JD 2453200.5)
Longest distance from the Sun525.311 Gm (3.511 AU)
Shortest distance from the Sun413.378 Gm (2.763 AU)
Longest distance from the center of its orbital path
("semi-major axis")
469.345 Gm (3.137 AU)
How egg-shaped its orbit is
How long it takes to complete an orbit2029.776 d (5.56 a)
Average speed16.76 km/s
Mean anomaly273.788°
Angle above the reference plane
Longitude of where it comes up through the reference plane283.646°
Angle between its shortest distance from what it orbits around and where it comes up through the reference plane
("argument of periapsis")
Size and Other Qualities
Measures500×385×350 km [1][2]
Mass8.6 ± 0.7 ×1019 kg [3][4][5]
Average density2.4 g/cm³
Gravity at its surface0.091 m/s²
Slowest speed able to escape into space
("escape velocity")
0.21 km/s
How long it takes to turn around one time1.1510 d [6]
How much light it reflects0.072 (geometric)[1]
Avg. surface temp.~164 K
max: 247K (−26° C) [7]
Light-band group
("spectral type")
C-type asteroid
Seeming brightness
("apparent magnitude")
9.1 to 11.97
True brightness
("absolute magnitude")
Seeming size
("angular diameter")
0.318" to 0.133"

10 Hygiea is the fourth biggest object in the asteroid belt. It is somewhat non-spherical in shape, with diameters of 350 – 500 km. Its mass is estimated to be 3% of the total mass of the asteroid belt.

It is the biggest of the class of dark C-type asteroids with a carbonaceous surface that are dominant in the farther part main belt, which lies beyond the Kirkwood gap at 2.82 AU, and is also the biggest body in this region. Its dark surface and bigger than average distance from the Sun makes it very dim for such a big asteroid when seen 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, which is as much as four orders fainter than Vesta, and will need at least a 4-inch telescope to resolve. At a perihelic opposition, however, Hygiea can reach +9.1 and may just be resolvable with 10x50 binoculars, unlike the fifth and sixth biggest asteroids 704 Interamnia and 511 Davida which are always beyond binocular visibility.


Hygiea was found 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 asteroid was named by the director of the Naples observatory, Ernesto Capocci. He called 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 wrote 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.


Size comparison: the first 10 asteroids profiled against Earth's Moon. Hygiea is farthest right

Hygiea's surface is made 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%).

Generally Hygiea's properties are the least 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 bigger 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 spins unusually slow, taking 27 hours and 37 minutes to complete one rotation, whereas 6 to 12 hours are more typical for big asteroids.

At least 5 stellar occultations by Hygiea were tracked by Earth-based observers, but all with few seeing 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 bigger than about 16 km in diameter [10]


  1. 1.0 1.1 Supplemental IRAS Minor Planet Survey[dead link]
  2. M. Kaasalainen et al. Models of Twenty Asteroids from Photometric Data, Icarus, Vol. 159, p. 369 (2002).
  3. S. R. Chesley et al. The Mass of Asteroid 10 Hygiea, abstract for American Astronomical Society, DDA meeting #36, #05.05 (2005) (on ADS).
  4. G. Michalak Determination of asteroid masses, Astronomy & Astrophysics, Vol. 374, 703 (2001).
  5. Yu. Chernetenko, O. Kochetova, and V. Shor Masses and densities of minor planets Update 1 Sept. 2005. (webpage) Archived 25 May 2011 at WebCite
  6. PDS lightcurve data Archive copy at the Internet Archive
  7. 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).
  8. The Brightest Asteroids
  9. John Russell Hind (1852). The solar system: a descriptive treatise upon the sun, moon, and planets, including an account of all the recent discoveries. G. P. Putnam. p. 126.
  10. A. Storrs et al., Imaging Observations of Asteroids with Hubble Space Telescope, Icarus, Vol. 137, p. 260 (1999).

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