243 Ida

243 Ida
	Galileo image of 243 Ida. The tiny dot to the right is its moon, Dactyl.
Discovery
Discovered by	Johann Palisa
Discovery date	September 29, 1884
Designations
Alternative designations	A910 CD; 1988 DB1
Minor planet category	Main belt (Koronis)
Orbital characteristics
	Epoch October 22, 2004 (JD 2453300.5)
Aphelion	447.843 Gm (2.994 AU)
Perihelion	408.207 Gm (2.729 AU)
Semi-major axis	428.025 Gm (2.861 AU)
Eccentricity	0.046
Orbital period	1767.724 d (4.84 a)
Average orbital speed	17.60 km/s
Mean anomaly	245.469°
Inclination	1.138°
Longitude of ascending node	324.218°
Argument of perihelion	108.754°
Known satellites	Dactyl
Physical characteristics
Dimensions	53.6×24.0×15.2 km
Mean radius	15.7 km
Mass	4.2×1016 kg 1
Mean density	2.6 g/cm³ 1
Equatorial surface gravity	0.0109 m/s²
Equatorial escape velocity	0.0185 km/s
Rotation period	0.1931 d (4 h 37 min) 2
Geometric albedo	0.2382 3
Spectral type	S
Absolute magnitude (H)	9.94

243 Ida is a Main belt asteroid with pictures taken by the Galileo spacecraft on August 28, 1993. It was the first binary asteroid to be found and is a member of the Koronis family of asteroids.

Discovery and name

Ida was found by Johann Palisa on September 29, 1884 in Vienna. It is named after Ida, a Cretan nymph in Greek mythology who lived on a mountain that has her name (see Mount Ida, Crete).

Moon

Ida has a small moon, Dactyl, which was found on February 17, 1994 by Galileo mission member Ann Harch, while examining the delayed image downloads. The moon was given the designation S/1993 (243) 1 and later named (243) Ida I Dactyl. The pictures where it was first seen were taken on 28 August 1993, hence the designation. The moon was reported on 12 March 1994.

Dactyl is only 1.4 km in diameter. Some researchers believe that Dactyl formed from debris blown off from Ida because of cratering, while others suggest that Ida and Dactyl formed as a pair a billion or more years ago when Ida's parent body was disrupted. Both of these theories present difficulties that are unresolved at this time. Since Dactyl was found, other sightings have suggested that asteroid moons are common.

Although Ida's dimensions, measured along the principal axes (based on its rotation) are 53.6×24.0×15.2 km, the best-fit ellipsoid measures 60.0×25.2×18.6 km. Since its mass is well known thanks to its little moon, one can calculate that its surface gravity can be between 0.0031 and 0.0324 m/s². The rotation axis is within one degree of the shorter dimension axis, which means the centrifugal effect can reach a value as high as 0.0042 m/s² — at the tips of its longest axes, Ida is actually under tension.

243 Ida Media

Animation of Galileo's trajectory from 19 October 1989 to 30 September 2003 Galileo · Jupiter · Earth · Venus · 951 Gaspra · 243 Ida
Trajectory of Galileo from launch to Jupiter orbital insertion
Images from the flyby, starting 5.4 hours before closest approach and showing Ida's rotation
Polished section of an ordinary chondrite meteorite
Size comparison of Ida, several other asteroids, the dwarf planet Ceres, and Mars
Successive images of a rotating Ida
Mosaic of images recorded by Galileo 3.5 minutes before its closest approach
Overview and zoom of a 150 m ejecta block on 243 Ida, coordinates 24.8°S × 2.8°. See Ejecta Blocks on 243 Ida and on Other Asteroids.
Asymmetric 1.5 km (0.93 mi) wide crater Fingal at 13.2°S, 39.9°E
Orbit and positions of Ida and five planets as of 9 March 2009