Dactyl (moon)

Physical characteristics

Dactyl is an "egg-shaped" but "remarkably spherical" object measuring 1.6 by 1.4 by 1.2 kilometres (0.99 by 0.87 by 0.75 mi). It is oriented with its longest axis pointing towards Ida. Like Ida, Dactyl's surface exhibits saturation cratering. It is marked by more than a dozen craters with a diameter greater than 80 m (260 ft), indicating that the moon has suffered many collisions during its history. At least six craters form a linear chain, suggesting that it was caused by locally produced debris, possibly ejected from Ida. Dactyl's craters may contain central peaks, unlike those found on Ida. These features, and Dactyl's spheroidal shape, imply that the moon is gravitationally controlled despite its small size. Like Ida, its average temperature is about 200 K (−73 °C; −100 °F).

Dactyl shares many characteristics with Ida. Their albedos and reflection spectra are very similar. The small differences indicate that the space weathering process is less active on Dactyl. Its small size would make the formation of significant amounts of regolith impossible. This contrasts with Ida, which is covered by a deep layer of regolith.

The two largest imaged craters on Dactyl were named Acmon and Celmis , after two of the mythological Dactyls. Acmon is the largest crater in the above image, and Celmis is near the bottom of the image, mostly obscured in shadow. The craters are 300 and 200 meters in diameter, respectively.

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 65 km (40 mi) 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 90 km (56 mi) from Ida at longitude 85°. On 26 April 1994, the Hubble Space Telescope 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 700 km (430 mi) 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 10 m/s (33 ft/s), "about the speed of a fast run or a slowly thrown baseball".

Age and origin

Dactyl may have originated at the same time as Ida, from the disruption of the Koronis parent body. However, it may have formed more recently, perhaps as ejecta from a large impact on Ida. It is extremely unlikely that it was captured by Ida. Dactyl may have suffered a major impact around 100 million years ago, which reduced its size.

Dactyl was the first asteroid moon discovered. Its discovery settled the long debate over the existence of asteroid moons.