First asteroid with a moon

The first asteroid found to have a moon is the main-belt asteroid 243 Ida. In 1993, NASA's Galileo probe made a brief diversion on its way to Jupiter so researchers could study the asteroid, making its closest approach on 29 August. Ida's tiny moon Dactyl was discovered in the pictures taken during this pass on 17 February 1994 by Ann Harch of the Galileo Imaging Team.

243 Ida was discovered in 1884 by Johann Palisa of the Vienna Observatory in Austria. It is an irregularly shaped S-type (stony) asteroid that measures 59.8 x 25.4 x 18.6 km (37.1 x 15.7 x 11.5 mi). Ida is part of what's called the Koronis Family, a group of 12 asteroids that are thought to have their origins in a collision that broke apart a larger body. The family is named for the largest surviving fragment of the original parent body, 158 Koronis.

The Galileo spacecraft was launched on 18 October 1989. Its mission was to enter orbit around Jupiter and carry out a comprehensive survey of the planet and its moons. Ida and another asteroid called 951 Gaspra were added to the mission as targets of opportunity because their orbits would put them close to Galileo's path as it moved through the asteroid belt. The probe took 96 images as it flew past the asteroid at 44,640 km/h (27,738 mph) on 29 August, and came within 2,391 km (1,485 mi) of Ida during its closest approach.

Galileo's high-gain antenna had failed to deploy properly, which meant that researchers had to make do with the restricted bandwidth of its low-gain antenna (10 bits per second) for the duration of the mission. Because of this low downlink speed it was not until February 1994 that the pictures taken during the pass started to come through. Dactyl was spotted in what was called a "jailbar" image (which only showed 3 out of every 20 scan lines to speed up transmission) by assistant science coordinator Ann Harch of Cornell University.

Dactyl measures 1,600 x 1,400 x 1,200 m (5,249 ft x 4,593 x 3,937 ft) and was 85 km from Ida at the time of Galileo's pass. It has the same density as Ida, which has led some astronomers to suggest that it may be a piece of the asteroid that was broken off by an impact. As the time from the beginning to the end of Galileo's encounter was just seven hours, we don't know much about the moon's orbit.