I remember the excitement in 1993, when a photo from the Galileo spacecraft, going through the asteroid belt on its way to Jupiter, unexpectedly showed that the asteroid Ida had a tiny moon orbiting it. Twenty years later, we now know more than 160 moons orbiting different asteroids. (Asteroids are chunks of rock orbiting the Sun -- pieces of cosmic garbage left over from the messy period when the planets first formed.)
At least five asteroids are now known to have two moons each, making them triple systems. Perhaps the most famous of these is the asteroid Sylvia, named after Rhea Silvia, the mother of Romulus and Remus, founders of the city of Rome in ancient mythology. Sylvia is one of the larger objects in the main asteroid belt, which lies between Mars and Jupiter.
Recently, a team of professional and amateur astronomers, led by Franck Marchis of the SETI Institute (where I have the pleasure of serving on the Board of Trustees) has made the most accurate measurement so far of Sylvia and its two moons (which got named Romulus and Remus). Our painting shows you what the system may look like if you could get up close and personal with it.
This past January, European observers could see the triple asteroid pass in front of a faint star, hiding its light as each object moved in formation. From this, the astronomers could make estimates and models of the size and shape of each member of the triple system, even though it orbits some 325 million miles from the Sun.
For more details, you can see the announcement at:http://www.seti.org/seti-institute/press-release/telescopes-large-and-small-team-study-triple-asteroid
The very first moon discovered around an asteroid was soon named Dactyl, a term that can mean finger, or a small finger-like creature in Greek mythology, or a small unit of poetic verse. To keep up with all the moons of asteroids, you can check the website: http://www.johnstonsarchive.net/astro/asteroidmoons.html
Ida and Dactyl (NASA)