Dactyl ( ), formal designation (243) Ida I, is a small asteroid moon , in size) that orbits

243 Ida 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 Ida (nurse of Zeus), a nymph from Greek mythology. Later telesc ...

, a main-belt asteroid. It was imaged by the ''

Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...

'' spacecraft on August 28, 1993; Dactyl was discovered while examining the delayed image downloads from Galileo on February 17, 1994. It was provisionally designated S/1993 (243) 1. The satellite was named after the

mythical Myth is a genre of folklore consisting primarily of narratives that play a fundamental role in a society. For scholars, this is very different from the vernacular usage of the term "myth" that refers to a belief that is not true. Instead, the ...

creatures called dactyls, who, according to

Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of ancient Greek folklore, today absorbed alongside Roman mythology into the broader designation of classical mythology. These stories conc ...

, lived on Mount Ida.

Orbit

Dactyl's orbit around Ida is not precisely known. ''Galileo'' was in the plane of Dactyl's orbit when most of the images were taken, which made determining its exact orbit difficult. Dactyl orbits in the prograde direction and is inclined about 8° to Ida's equator. Based on computer simulations, Dactyl's pericenter must be more than about from Ida for it to remain in a stable orbit. The range of orbits generated by the simulations was narrowed down by the necessity of having the orbits pass through points at which ''Galileo'' observed Dactyl to be at 16:52:05 UT on 28 August 1993, about from Ida at longitude 85°. On 26 April 1994, the

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

observed Ida for eight hours and was unable to spot Dactyl. It would have been able to observe it if it were more than about from Ida. If in a circular orbit at the distance at which it was seen, Dactyl's orbital period would be about 20 hours. Its orbital speed is roughly , "about the speed of a fast run or a slowly thrown baseball".

Origin

The origins of Dactyl are unclear, but two main hypotheses exist. The first is that Dactyl and Ida formed at the same time, and the second is that Dactyl was knocked loose by a later impact. Dactyl was the first asteroid moon discovered. Its discovery settled the long debate over the existence of asteroid moons.

References

Works cited

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External links

243 Ida and Dactyl
from ''The Nine Planets''

from ''Views of the Solar System''

announcing Dactyl's discovery

announcing Dactyl's naming
Ida and Dactyl
in false coloring, to reveal surface differences {{Solar System moons (compact) Asteroid satellites Minor planets visited by spacecraft

Orbit

Origin

See also

References

Works cited

External links