85 Io

85 Io
Discovered by C. H. F. Peters
Discovery date September 19, 1865
Alternative names A899 LA; A899 UA
Minor planet
Main belt
Orbital characteristics
Epoch March 6, 2006 (JD 2453800.5)
Aphelion 473.341 Gm (3.164 AU)
Perihelion 320.334 Gm (2.141 AU)
Semi-major axis 396.837 Gm (2.652 AU)
Eccentricity 0.193
Orbital period 1578.081 d (4.32 a)
Average orbital speed 18.12 km/s
Mean anomaly 206.947°
Inclination 11.967°
Longitude of ascending node 203.440°
Argument of perihelion 122.293°
Physical characteristics
Dimensions 180×160×160 km[1][4]
Mass ~3.4×1018 (estimate)
Mean density ~1.4 g/cm³ (estimate)[5]
Equatorial surface gravity ~0.028 m/s² (estimate)
Escape velocity ~0.07 km/s (estimate)
Rotation period 0.2864 d (6.875 h) [2]
Albedo 0.067 [3]
Temperature ~172 K
max: 272K (-2° C)
Spectral type C
Absolute magnitude 7.61

85 Io (pronounced /ˈaɪoʊ/ eye'-oh) is a large, dark Main belt asteroid of the C spectral class. It is probably a primitive body composed of carbonates. Like 70 Panopaea it orbits within the Eunomia asteroid family but it is not related to the shattered parent body.

Io is a retrograde rotator, with its pole pointing towards one of ecliptic coordinates (β, λ) = (-45°, 105°) or (-15°, 295°) with a 10° uncertainty[1]. This gives an axial tilt of about 125° or 115°, respectively. Its shape is quite regular.

It was discovered by C. H. F. Peters on September 19, 1865 and named after Io, a lover of Zeus in Greek mythology.

An Ionian diameter of 178 kilometres was measured from an occultation of a star on December 10, 1995 [4].

Io is also the name of the volcanic satellite of Jupiter. With a two-digit number and a two-letter name, 85 Io has the shortest designation of all minor planets.


* shape model deduced from lightcurve


1. J. Torppa et al Shapes and rotational properties of thirty asteroids from photometric data, Icarus, Vol. 164, p. 346 (2003).

2. PDS lightcurve data

3. Supplemental IRAS Minor Planet Survey

4. A. Erikson Photometric observations and modelling of the asteroid 85 Io in conjunction with data from an occultation event during the 1995-96 apparition, Planetary and Space Science, Vol. 47, p. 327 (1999).

5. G. A. Krasinsky et al Hidden Mass in the Asteroid Belt, Icarus, Vol. 158, p. 98 (2002).

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