Jan. 7, 1610, was a red-letter day for astronomy. That evening, as Jupiter rose above the domes of Padua’s majestic San Antonio Basilica, Galileo Galilei took his usual window seat on the top floor of his house. Peering through a small, handcrafted telescope, Galileo discovered three small objects hovering near the planet. A week later, he reported a fourth body “wandering around Jupiter, as do Venus and Mercury around the sun.” Galileo was the first to recognize moons orbiting Jupiter.
Those four moons—the only ones big enough for Galileo to see—have 1,000 times the girth of the latest Jovian find. Astronomers last week reported Jupiter’s 17th known moon, an object some 5 kilometers wide. If confirmed, it would rank as the tiniest satellite identified for any planet, as well as the first Jovian moon discovered in 25 years.
The astronomers made their find with a 79-year-old telescope, just 36 inches in diameter, atop Kitt Peak in Arizona. Known as Spacewatch, the telescope scans the solar system for asteroids and comets. Spacewatch researcher James V. Scotti of the University of Arizona in Tucson arranged for the telescope to examine a region of sky near Jupiter for several nights last October. Scotti knew Jupiter was near its closest approach to Earth, making the time ideal for finding any undiscovered moons.
Yet he and his colleagues didn’t notice that one of the newly observed objects, which appeared to be in the main asteroid belt between the orbits of Mars and Jupiter, was moving too slowly to be an asteroid. Scotti followed the standard procedure of forwarding the data to the International Astronomical Union’s (IAU) Minor Planet Center at the Smithsonian Astrophysical Observatory in Cambridge, Mass. Researchers there designated the object an asteroid, 1999 UX18.
On July 18, an astronomer at the center, Tim Spahr, used the 1999 Spacewatch measurements to test a new computer program designed to identify slow-moving objects. It revealed 1999 UX18 in additional Spacewatch images over a 16-day span last fall.
When Spahr tried to construct an orbit around the sun for 1999 UX18, he couldn’t get a satisfactory fit. He wondered if the body might instead be circling Jupiter.
Calculations by the center’s director, Brian G. Marsden, and associate director, Gareth V. Williams, revealed that a Jovian orbit fit all the Spacewatch observations. Williams’ analysis showed that the object isn’t one of Jupiter’s 16 previously known moons. He calculates that the moon belongs to a group of moons that lies some 24 million km from Jupiter, takes about 2 years to orbit the planet, and moves in a direction opposite to the other Jovian satellites.
Researchers at Spacewatch and the center announced their findings in a July 20 circular of the IAU. Even with this moon, Jupiter’s retinue would still lag behind that of Saturn, with 18 known moons, and Uranus, which has 20.
With Jupiter now rising in the morning sky, large telescopes can now observe the 17th moon, in the morning sky. The Galileo spacecraft, which has toured Jupiter since 1995, may attempt to view the object and obtain spectra next February, when it comes within 12 million km of the satellite.