TUCSON, Ariz. - Astronomers at the University of Arizona and their Massachusetts colleagues say they have found a 17th moon orbiting Jupiter.
If confirmed, the 3-mile-diameter moon would be the smallest known satellite of a major planet and the first Jovian moon discovered in 21 years.
''It's exciting. When you realize that you were the first person to lay eyes on something that had not been seen before, that's kind of a good feeling,'' University of Arizona astronomer Jeff Larsen, who made the first observations of the moon in October, said in Saturday's Arizona Daily Star.
Larsen works with the university's Spacewatch project, which uses a 79-year-old Kitt Peak telescope to survey the solar system for comets and asteroids.
The group usually avoids the region around Jupiter because the solar system's largest planet is bright, and reflected light can swamp the telescope's sensitive electronic detectors. But last October, Spacewatch observer Jim Scotti started a search for undiscovered Jovian moons.
When Jupiter was less than 370 million miles from Earth - about as close to our planet as it gets during its 12-year journey around the sun - the observers found what appeared to be a tiny moon.
The group contacted the Smithsonian Astrophysical Observatory in Cambridge, Mass., and scientists there analyzed the data and reached the same conclusion.
The search was spurred by the May 1999 discovery of an 18th moon circling Uranus. That find was made by another University of Arizona researcher, Erich Karkoschka, who reviewed 13-year-old pictures from the Voyager 2 spacecraft.
''That's what motivated Jim to say, hey, let's go back and look again,'' Larsen said from the Spacewatch telescope on Kitt Peak, 55 miles southwest of Tucson.
For now, the object is designated S/1999 J1. If follow-up observations confirm it is a moon, S/1999 J1 will get a name.
At 3 miles in diameter, it would be the smallest moon. Among Jupiter's 16 confirmed satellites, half are smaller than 32 miles in diameter. Mars has the smallest confirmed moon, Deimos, which is about 7.5 miles in diameter. Earth's moon is 2,160 miles in diameter by comparison.
Jupiter and its moons are currently too close to the sun to allow follow-up observations immediately, but they should be accessible within the next couple months, said University of Arizona planetary scientist Robert S. McMillan, principal investigator of Spacewatch.
S/1999 J1 belongs to a sub-group of outer satellites that travel around Jupiter in irregular orbits, he said. Those outer moons - believed to be captured asteroids or comets - take about two years to complete an orbit, and they are an average distance of 15 million miles from Jupiter.
The last outer moon of Jupiter was discovered by Charles Kowal in 1974 and named Leda. In 1979, the Voyager mission uncovered three new inner satellites of Jupiter, Adrastea, Metis and Thebe.