News Services University of Arizona Contact: Erich Karkoschka, Lunar and Planetary Lab University of Arizona Tucson, AZ 85721-0092 Phone: (520) 621-3994 Fax: (520) 621-4933 E-mail: erich@pirl.lpl.arizona.edu May 18, 1999 UNIVERSITY OF ARIZONA RESEARCHER DISCOVERS 18TH MOON ORBITING URANUS An Arizona scientist has discovered an 18th moon orbiting the planet Uranus, the International Astronomical Union announced today. Until now, Saturn has been the only planet in our solar system known to have as many as 18 satellites. Erich Karkoschka, a researcher at the Lunar and Planetary Lab of The University of Arizona in Tucson, made the discovery. The newly found moon is the first satellite of Uranus discovered in 1999 but will still be designated as Satellite 1986 U 10 (short S/1986 U 10). "This discovery is very unusual," Karkoschka said. "Typically, satellites are found within days after the discovery image has been taken. In this case, the discovery image is more than 13 years old." The interplanetary spacecraft Voyager 2 took seven images of the new satellite when it flew by Uranus in late January, 1986. These images have been publicly available in digital format. However, nobody recognized the satellite until Karkoschka investigated these images recently. He has studied the Uranian satellites based on images taken with the NASA/European Space Agency Hubble Space Telescope (HST) and found the new satellite when he compared his HST results with images taken by Voyager 2. The image shows Uranus with its ring system and the 10 innermost satellites. All but S/1986 U 10 were known at the time the image was taken, based on Voyager images taken in January 1986. Arrows at the edge of the image point to the 10 satellites. The other dots of light are background stars of the constellation Sagittarius. (If you know enough astronomy to want to look this up on a star chart, the bright star next to Juliet is Kaus Borealis, Lambda Sagittarii.) Belinda and S/1986 U 10, imaged near the upper right hand corner, circle Uranus in almost identical orbits. They pass each other once a month. This is the first example of two satellites in nearby orbits passing each other so slowly. Astronomers began discovering moons around other planets in our solar system in the 17th century. Throughout the approximately 60 satellite discoveries made over the four centuries since, either Jupiter or Saturn has had the most known satellites. Saturn is known to have 18 moons. Jupiter has 16 known satellites, not counting the Galileo spacecraft, an artificial satellite around that planet. Many scientists had thought that these two largest planets in our solar system would have more moons than smaller planets. Smaller Uranus now dispells that belief. The other known satellites orbit Neptune (8 satellites), Mars (2 satellites), and Earth, Pluto, and asteroids Ida and Eugenia (1 each). Soon after England's William Herschel discovered Uranus in 1781, he found its two largest satellites, which are about half the size of our moon. In 1851, English astronomer William Lassell detected two more Uranian satellites. In 1951, Gerard Kuiper of the University of Chicago -- later founder of the UA Lunar and Planetary Lab -- discovered Uranus' fifth satellite. The Voyager team found 10 more Uranian satellites in 1985-86. Two years ago, Philip Nicholson of Cornell University discovered the 16th and 17th satellites. These two satellites are some 100 times farther away from Uranus than are the satellites discovered by the Voyager team. Nicholson and Karkoschka have been the only scientists to find new moons around a major planet in the 1990s. However, during this decade, the first two satellites of minor planets (Ida and Eugenia) were imaged. The new satellite is about 25 miles (40 km) in diameter, similar in size to comet Hale-Bopp, and it may also have similar composition as the comet, Karkoschka said. "Hale-Bopp was a spectacular sight when it crossed the inner part of the solar system two years ago," he said. "On the other hand, the new satellite will never get spectacular since it will remain in the dark, frigid parts of the solar system. It will remain a tiny speck of light." Uranus may well have more than 18 satellites, Karkoschka noted: Jupiter and Saturn have satellites of about half the size of the new Uranian satellite. No such small satellite has yet been discovered around Uranus since the dim sunlight at Uranus makes the detection of such small satellites very hard. Based on the detection in seven images, Karkoschka concluded that the satellite orbits Uranus once every 15 hours and 18 minutes. This is similar to the rotation period of Uranus. The satellite hovers 32,000 miles (51,000 km) above the clouds of Uranus, or the same distance as the diameter of Uranus. The motions of satellites of Uranus can be viewed at the Space Telescope Science Institute from the link below. When Voyager 2 took the discovery image on January 23, 1986, it was 650,000 miles (1 million km) from Uranus. This is 2,500 times closer than the Earth ever gets to Uranus. Nineteen hours later, Voyager 2 flew by Uranus. The 10 satellites shown in the new discovery picture have been in continuous sunlight during the last 25 years. Starting next year, Cordelia, the satellite inside the ring system, will enter the shadow of Uranus during every orbit. The other satellites will follow. By 2002, all 10 innermost moons will enter the shadow of Uranus every orbit. These eclipses occur during two intervals within the 84-year long Uranus-year. Similarly, lunar eclipses on Earth occur twice within each Earth-year, currently in January and July. The new discovery image is a mosaic of 10 exposures, Karkoschka said. The exposures of Uranus had shorter exposure times than the exposures of the surrounding area containing the rings and satellites. Since Uranus is a million times brighter than its satellites, Karkoschka retained the darker planet image so the satellites would be visible. "To an astronaut on board the Voyager spacecraft, the satellites would have appeared as faint stars while Uranus in the center would have been blazingly bright, as bright as the full moon on Earth," Karkoschka said. "In visible light, Uranus seems to be a bland, quiet place. Only two faint little cloud features can be found upon close examination of the image. The true activity in the atmosphere of Uranus is only revealed in infrared light." While the Voyager camera was not sensitive to infrared light, the Hubble Space Telescope has imaged atmospheric activity. The HST image can be viewed from the link below. The colors in the image are close to realistic, he added. The newly discovered moon won't be stuck with the name "S/1986U10" forever, Karkoschka noted. The International Astronomical Union names satellites and asteroids a year or more after discovery. In the past, the IAU has often adopted a name suggested by the discoverer if that name fits in the context of the system's previously named satellites. However, anyone can suggest what the new moon might be named, Karkoschka added. Brush up your Shakespeare. Uranus' other nine innermost moons are Belinda, Bianca, Juliet, Rosalind, Ophelia, Cressida, Portia, Cordelia and Desdemona. LINKS: http://science.opi.arizona.edu/pics/disc1.jpg http://science.opi.arizona.edu/pics/disc2.jpg http://oposite.stsci.edu/pubinfo/pr/1999/11/animations.html http://oposite.stsci.edu/pubinfo/pr/1998/35/index.html