Oddball asteroid.

While scanning the sky for near-Earth asteroids last month, an astronomer made a rare find. On May 10, Brian Skiff of the Lowell Observatory in Flagstaff, Ariz., spied a space rock that takes only 6 months to go around the sun, the shortest orbital period of any known asteroid. With observatory colleagues, Skiff describes the find in a May 14 circular of the Minor Planet Center in Cambridge, Mass. The team estimates that the rock, designated 2004 JG, has a diameter between 500 meters and 1 kilometer.

Residing between the orbits of Venus and Mercury on an elongated path, 2004 JG6 is only the second known asteroid with an orbit entirely within Earth's, notes Ted Bowell, who collaborated with Skiff. Most asteroids lie between the orbits of Jupiter and Mars, and calculations by William Bottke of the Southwest Research Institute in Boulder, Colo., suggest that only 2 percent of near-Earth asteroids come as close to Earth as 2004 JG does.

The asteroid was found during the Lowell Observatory Near-Earth-Object Search, which uses a small robotic telescope to survey large areas of the sky. Over the next few weeks, 2004 JG6 will move through the constellations Cancer and Canis Minor low in the western sky at dusk.--R.C.