243 Ida#Moon

(1.6 × 1.4 × 1.2 ) km

Galileo Ann Harch

Dactyl is a small moon of the asteroid Ida, which belongs to the S-type main belt in the Koronis group, an asteroid belt between Mars and Jupiter.

He is the first asteroid moon discovered. Its average diameter is 1.4 km.

Discovery and designation

Dactyl was discovered on 17 February 1994 by Ann Harch from the team of the Galileo mission by reviewing recorded and later transferred images that were made during the flyby of the Galileo spacecraft on August 28, 1993 by Ida. These images provided the first direct evidence for the existence of an asteroid moon. The discovery was announced on 12 March 1994; the moon was given the provisional designation S/1993 (243 ) 1

On 26 September 1994, the Moon of the International Astronomical Union ( IAU) was officially named after the dactyls, demons from Greek mythology, who lived in the Ida Mountains on the Greek Mediterranean island of Crete. The dactyls protected the young god Zeus after he was hidden and raised by Ida on the mountain. Other mythological sources state that the dactyls are the children of Ida and Zeus himself.

Dactyl is the English singular form of " Daktyloi " (Greek for " finger ").

Web properties

Orbit

Dactyl orbiting Ida in a prograde orbit at 108 km average distance from the center (3.44 average Idaradien ). The orbital eccentricity is 0.2; the web is inclined 8 ° in relation to the equator of Ida.

However, the orbit of Dactyl is not precisely known. Galileo was in the orbital plane of the moon than most recordings were made, what the precise orbit determination difficult. In addition, the distance traveled by Dactyl at the time of shooting was too short, since Dactyl moved with only 22 km / h to Ida.

At the time of the flyby Dactyl was removed about 85 km from Ida. Based on computer simulations must be at least 65 km in order to ensure a stable orbit the perihelion of Dactylbahn. The range of the calculated orbits was limited by the need that they had to pass through the observed points from Galileo to 16:52:05 UT at 85 ° of longitude.

The exact circulation time could not be determined and is estimated at 37 hours.

Rotation

The self-rotation at least eight hours, but possibly synchronous with the orbital period. The longitudinal axis of the moon was aligned at the time of the flyby on Ida, suggesting rather a synchronous rotation.

Physical Properties

Dactyl is an irregularly shaped, but remarkably uniform ellipsoidal body with dimensions of 1.6 × 1.4 × 1.2 km, based on the assumed albedo of 21%. The deviation from a perfect ellipsoid is at most 130 meters. The mass of Dactyl is estimated to be 4 x 1012 kg.

As Ida is the surface of Dactyl saturated with craters, indicating a great age. It has at least a dozen crater, which are larger than 80 meters. The two largest were called Acmon and Celmis. Acmon located on the bestaufgelösten image exactly on the day - night boundary (138 ° E, 39 ° S ) and measures 280 meters, Celmis is slightly below the middle (220 ° E, 46 ° S ) in the shade and measures 160 meters. At least six crater of a linear chain indicate locally resulting debris that may have been blown off by Ida and smashing on the moon. Dactyls crater have in contrast to the most likely central mountains found on Ida. This and the round shape indicate that Dactyl is gravitationally bound despite its small size. The average surface temperature of Dactyl is estimated as that of Ida to -73 ° C.

In the spectrum of Ida and Dactyl are similar to the other members of the Koronis family. Small spectral differences suggest that the conditional through space weathering process is less active than on Ida on Dactyl. Its small size would make the formation of larger amounts of regolith impossible, in contrast, Ida is covered by a thick layer of this material. In addition, it is believed that in the composition of the body, the proportion of pyroxenes, and olivine at dactil is higher.

On the origin of Dactyl, there are two theories: either Dactyl was blasted off from the asteroid in comparatively recent past by wrapping another small body on Ida or Ida and Dactyl are the remnants of an even larger source body, the shattered completely in a collision with another asteroid was thereby formed the Koronis family. In addition, it is possible that about 100 million years there has been a greater impact on Dactyl, which has reduced its size.

A capture by Ida is unlikely in the highest degree. According to the laws of celestial mechanics would an object that Ida approaching, only distracted from this asteroid, but are never captured, if not a third body would be present, which would slow down this object.

Research

Galileo took 47 shots of Dactyl on in a period of 5.5 hours on 28 August 1993. When the first picture was taken, was to be seen on the Dactyl, the probe was 10,760 kilometers of Ida and Dactyl of 10,870 km away; 14 minutes later, the closest approach ( 2393 km Ida ) took place on the system. The best resolution at Dactyl was 39 meters per pixel.

On April 26, 1994, the Hubble Space Telescope observed for eight hours the system, it was not able to locate Dactyl due to the small size and proximity to Ida. This would only have been possible at a minimum distance of 700 km.

Since the discovery of Dactyl, discovered already in the second flyby of an asteroid, it is assumed that the asteroid moons are not an uncommon phenomenon in the asteroid belt. Already in November 1998, was discovered from Earth, a second asteroid moon names Petit -Prince, the (45 ) Eugenia orbits.

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