By Ron Cowen
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.