Many moons

Natural History, Dec, 2003 by Stephan Reebs

Astronomers have long known that Mercury and Venus have no moons, Earth has but one, and Mars two. The count for the other planets in the solar system, however, is far from fixed. New moons are constantly being discovered, and the pace has picked up dramatically in recent years.

In the first nine months of 2003 alone, the tally of discoveries totaled twenty-one new satellites for Jupiter, one for Saturn, three for Uranus, and one for Neptune (also announced in 2003, but discovered earlier, were two more for Uranus and one more for Neptune). That brings the known totals to sixty-one for Jupiter, thirty-one for Saturn, twenty-seven for Uranus, and thirteen for Neptune.

Why have so many moons eluded detection until now? Simply put, they're small and far away. Some measure little more than half a mile across, and the newest discovery, a Neptunian moon, is almost 3 billion miles from Earth. But state-of-the-art cameras, together with sophisticated computer programs that can quickly calculate the orbits of moving specks of light, are lifting the veil on the stealthy satellites.

Besides being small, the new moons have irregular shapes and highly elliptical orbits. Astronomers think they started life as wandering asteroids--chunks of rock--and eventually got caught in the planets' gravitational fields. (Up-to-date tabulations at www.ifa.hawaii.edu/sheppard/satellites/, maintained by David C. Jewitt and Scott S. Sheppard, both astronomers at the University of Hawai'i, in Honolulu)