243 Ida
First asteroid found to have a natural satellite.
243 Ida, a member of the Koronis family within the main asteroid belt, was first spotted on 29 September 1884 by Austrian astronomer Johann Palisa at the Vienna Observatory. Its name comes from a nymph in Greek mythology. Later ground-based observations classified it as an S-type asteroid, the most common type in the inner belt. On 28 August 1993, the uncrewed Galileo spacecraft, en route to Jupiter, flew past Ida—making it only the second asteroid ever visited by a spacecraft and the first confirmed to have a natural satellite.
Like all main-belt asteroids, Ida orbits between Mars and Jupiter. Its orbital period is 4.84 years, and it rotates once every 4.63 hours. With an average diameter of 31.4 kilometers (19.5 miles), Ida is irregularly shaped and elongated, appearing to consist of two large objects fused together. Its surface is among the most heavily cratered in the Solar System, displaying craters of widely varying sizes and ages.
Ida’s moon, Dactyl, was discovered by mission member Ann Harch in images sent back by Galileo. Named after the Dactyls—figures from Greek mythology who lived on Mount Ida—Dactyl is just 1.4 kilometers (0.87 miles) across, roughly one-twentieth the size of Ida. Its orbit around Ida could not be precisely determined, but the possible orbital constraints allowed a rough estimate of Ida’s density, revealing that the asteroid is depleted in metallic minerals. Dactyl and Ida share many characteristics, suggesting they formed together.
The images from Galileo and the subsequent measurement of Ida’s mass offered new insights into the geology of S-type asteroids. Before the flyby, many theories existed about their mineral composition. Determining that composition helps link meteorites found on Earth to their origins in the asteroid belt. Data from the flyby pointed to S-type asteroids as the source of ordinary chondrite meteorites, the most common type found on Earth’s surface.
**Discovery and observations**
Ida was discovered on 29 September 1884 by Johann Palisa at the Vienna Observatory—his 45th asteroid discovery. It was named by Moriz von Kuffner, a Viennese brewer and amateur astronomer. In Greek mythology, Ida was a nymph of Crete who raised the god Zeus. In 1918, Kiyotsugu Hirayama recognized Ida as a member of the Koronis family, a group he proposed were remnants of a destroyed precursor body.
- Discovery date
- 29 September 1884
- Discoverer
- Johann Palisa
- Discovery location
- Vienna Observatory
- Orbital period
- 4.84 years
- Rotation period
- 4.63 hours
- Average diameter
- 31.4 km
- Mass
- between 3.65 and 4.99 × 10^16 kg
Lore & Background
Ida was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at the Vienna Observatory, his 45th asteroid discovery. It was named by Moriz von Kuffner after a nymph from Greek mythology who raised the god Zeus. In 1918, Kiyotsugu Hirayama recognized Ida as a member of the Koronis family, proposed to be remnants of a destroyed precursor body. Ida's reflection spectrum was measured on 16 September 1980 as part of the eight-color asteroid survey, matching S-type classification.
Ida was visited by the uncrewed Galileo spacecraft on 28 August 1993 while en route to Jupiter. The flyby occurred at a speed of 12,400 m/s relative to the asteroid, with closest approach of 2,390 km. About 95% of Ida's surface came into view. Transmission of images was delayed due to a failure in the spacecraft's high-gain antenna; the first five images were received in September 1993, and the remaining in February 1994.
Ida's moon Dactyl was discovered by mission member Ann Harch in images returned from Galileo. Dactyl is only 1.4 km in diameter, about 1/20 the size of Ida. Its orbit could not be determined with much accuracy, but constraints allowed a rough determination of Ida's density, revealing it is depleted of metallic minerals. Dactyl and Ida share many characteristics, suggesting a common origin.
Reader's Guide
The Galileo flyby of Ida provided new insights into the geology of S-type asteroids, the most numerous type in the inner asteroid belt. Before the flyby, many theories had been proposed to explain their mineral composition. Data from the flyby pointed to S-type asteroids as the source for ordinary chondrite meteorites, the most common type found on Earth's surface. Estimates of Ida's density, constrained to less than 3.2 g/cm³ by the long-term stability of Dactyl's orbit, all but ruled out a stony-iron composition. The Galileo images also led to the discovery of space weathering on Ida, a process causing older regions to become more red over time. The reflection spectra of freshly exposed parts of the surface resembled that of ordinary chondrite meteorites, while older regions matched S-type spectra. These discoveries—space weathering effects and low density—led to a new understanding of the relationship between S-type asteroids and ordinary chondrite meteorites, suggesting that some S-types, particularly the Koronis family, could be the source of these meteorites. Ida's surface is one of the most heavily cratered in the Solar System, and its irregular shape, apparently composed of two large objects connected together, contributes to its uneven gravitational field.
Did You Know?
- Ida was the second asteroid visited by a spacecraft, after Gaspra.
- Ida's moon Dactyl was the first confirmed satellite of an asteroid.
More in Notable Asteroids 1-24
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