243 Ida

Ida

Galileo image of 243 Ida. Pola Regio is located on the right "tip" of the asteroid. The dot to the right is its moon Dactyl.
Discovery[1]
Discovered by	Johann Palisa
Discovery site	Vienna Observatory
Discovery date	September 29, 1884
Designations
MPC designation	(243) Ida
Pronunciation	/ˈaɪdə/[2]
Named after	Ida (nurse of Zeus)
Minor planet category	Main belt (Koronis family)[3]
Adjectives	Idean (Idæan) /aɪˈdiːən/[4]
Orbital characteristics[5]
Epoch 31 July 2016 (JD 2457600.5)
Aphelion	2.979 AU (4.457×1011 m)
Perihelion	2.743 AU (4.103×1011 m)
Semi-major axis	2.861 AU (4.280×1011 m)
Eccentricity	0.0411
Orbital period (sidereal)	1,767.644 days (4.83955 a)
Average orbital speed	0.2036°/d
Mean anomaly	38.707°
Inclination	1.132°
Longitude of ascending node	324.016°
Argument of perihelion	110.961°
Known satellites	Dactyl
Physical characteristics
Dimensions	59.8 × 25.4 × 18.6 km[6]
Mean radius	15.7 km[7]
Mass	4.2 ± 0.6 ×1016 kg[7]
Mean density	2.6 ± 0.5 g/cm3[8]
Equatorial surface gravity	0.3–1.1 cm/s2[9]
Synodic rotation period	4.63 hours (0.193 d)[10]
North pole right ascension	168.76°[11]
North pole declination	−2.88°[11]
Geometric albedo	0.2383[5]
Temperature	200 K (−73 °C)[3]
Spectral type	S[12]
Absolute magnitude (H)	9.94[5]

Ida, minor planet designation 243 Ida, is an asteroid in the Koronis family of the asteroid belt. It was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at Vienna Observatory and named after a nymph from Greek mythology. Later telescopic observations categorized Ida as an S-type asteroid, the most numerous type in the inner asteroid belt. On 28 August 1993, Ida was visited by the uncrewed Galileo spacecraft while en route to Jupiter. It was the second asteroid visited by a spacecraft and the first found to have a natural satellite.

Ida's orbit lies between the planets Mars and Jupiter, like all main-belt asteroids. Its orbital period is 4.84 years, and its rotation period is 4.63 hours. Ida has an average diameter of 31.4 km (19.5 mi). It is irregularly shaped and elongated, apparently composed of two large objects connected together. Its surface is one of the most heavily cratered in the Solar System, featuring a wide variety of crater sizes and ages.

Ida's moon Dactyl was discovered by mission member Ann Harch in images returned from Galileo. It was named after the Dactyls, creatures which inhabited Mount Ida in Greek mythology. Dactyl is only 1.4 kilometres (0.87 mi) in diameter, about 1/20 the size of Ida. Its orbit around Ida could not be determined with much accuracy, but the constraints of possible orbits allowed a rough determination of Ida's density and revealed that it is depleted of metallic minerals. Dactyl and Ida share many characteristics, suggesting a common origin.

The images returned from Galileo and the subsequent measurement of Ida's mass provided new insights into the geology of S-type asteroids. Before the Galileo flyby, many different theories had been proposed to explain their mineral composition. Determining their composition permits a correlation between meteorites falling to the Earth and their origin in the asteroid belt. Data returned from the flyby pointed to S-type asteroids as the source for the ordinary chondrite meteorites, the most common type found on the Earth's surface.