243 Ida (left) and Dacytl (right), as photographed by Galileo.
243 (1) Dactyl, is a tiny asteroid (diameter 1.4 km) that orbits asteroid 243 Ida with a period of 1.54 days at an average distance of 108 km, with an inclination of 9° to Ida's equator. The orbit is not very accurately known because the Galileo probe coincidentally passed very nearly in its plane whilst taking the images. It was imaged by the Galileo probe on August 28, 1993; Galileo mission member Ann Harch, while examining the delayed image downloads, discovered it on February 17, 1994. Its provisional designation was S/1993 (243) 1.
The origins of Dactyl are unclear. The two main hypotheses are that it and Ida formed at the same time, and that it was knocked loose by a later impact. Both hypotheses have problems and cannot explain the situation satisfactorily.
Ida is a heavily cratered, irregularly shaped asteroid in the main asteroid belt between Mars and Jupiter -- the 243rd asteroid to be discovered since the first one was found at the beginning of the 19th century.
Dactyl is made more or less from the same kind of material as Ida. As an S-type asteroid, Ida is composed mostly of silicate rocks.
This view of the asteroid 243 Ida was acquired by the Galileo spacecraft at ranges of 3,057 to 3,821 kilometers (1,900 to 2,375 miles) on August 28, 1993, about 3.5 minutes before the spacecraft made its close approach to the asteroid.
Asteroid, one of the many small or minor rocky planetoids that are members of the solar system and that move in elliptical orbits primarily between the orbits of Mars and Jupiter.
Asteroids that intersect the orbit of Mars are called Amors; asteroids that intersect the orbit of Earth are known as Apollos; and asteroids that have orbits smaller than Earth’s orbit are called Atens.
Three-quarters of the asteroids visible from Earth, as well as the dwarf planet 1 Ceres, belong to the C type, which appear to be related to a class of stony meteorites known as carbonaceous chondrites.
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