Idamoon

PUBLIC INFORMATION OFFICEJET PROPULSION LABORATORYCALIFORNIA INSTITUTE OF TECHNOLOGYNATIONAL AERONAUTICS AND SPACE ADMINISTRATIONPASADENA, CALIF. 91109. TELEPHONE (818) 354-5011Contact: Franklin O'DonnellFOR IMMEDIATE RELEASE March 23, 1994 The first-ever photograph of a moon of an asteroid, sent toEarth by NASA's Galileo spacecraft, was released by the spaceagency today. The photo, of asteroid 243 Ida and its newly discoverednatural satellite, was taken by Galileo as the spacecraft flewpast Ida last August 28. It was not transmitted to Earth untilrecently because the spacecraft is sending back data at a veryslow rate. According to team scientists at NASA's Jet PropulsionLaboratory, the image together with data from Galileo's near-infrared mapping spectrometer are the first conclusive evidencethat natural satellites of asteroids exist. The discovery gives scientists an intriguing new clue indeciphering the origins and evolution of these ancient, rockybodies, most of which orbit the sun in the main asteroid beltbetween Mars and Jupiter. Even so, many pieces of information on the newly found moon-- where it came from, how it came to be orbiting Ida and thedetails of that orbit -- are still unclear. "It was previously thought that natural satellites ofasteroids could form, but they probably weren't common," said Dr.Torrence Johnson, Galileo project scientist. "Having found onefairly quickly, we can say that they're probably more common thanpreviously thought." From the photo and spectrometer data, team scientistsestimate that the natural satellite is about 1.5 kilometer (1mile) across in this view, and appears to be at a distance ofabout 100 kilometers (60 miles), plus or minus 50 kilometers (30miles), from Ida's center. The position will be more accuratelydetermined as new data are analyzed. Ida itself is about 56 by24 by 21 kilometers (35 by 15 by 13 miles) in size. As yet they do not know the parameters of the object's orbit-- critical information that can reveal Ida's mass. Combinedwith measurements of Ida's size and volume, that can tellscientists the asteroid's density, offering more clues as to whatit is made of. The data from Galileo's near-infrared mapping spectrometer -- which scans space objects at a variety of wavelengths toreveal their chemical composition -- suggest that Ida's moon ismade more or less from the same kind of material as Ida. As anS-type asteroid, Ida is composed mostly of silicate rocks. Scientists are certain, in any event, that the moon'ssurface is not composed mostly of carbonaceous material, as arethe many asteroids that are termed C-type asteroids. Further information on the object's composition will becomeavailable as color pictures and more detailed data from thespectrometer are transmitted to Earth over the next few months. Galileo scientists believe the moon may have been created atthe same time as Ida -- when an older, larger asteroid wasshattered in a collision with another asteroid, giving birth todozens of smaller asteroids. Ida is a member of the Koronis family of asteroids, whichscientists believe was created when a larger body perhaps 200 to300 kilometers (120 to 180 miles) in diameter was smashedrelatively recently -- at least considerably after the solarsystem formed some 4.5 billion years ago. (The family was namedfor Koronis, one of the asteroids that belongs to it.) Alternatively, it is possible that Ida was hit by a smallerobject even more recently, leaving a crater on the asteroid andthrowing off the material that became the small moon. "Ida's age is baffling, because the craters visible on itssurface suggest that it is old, but being a part of the Koronisfamily suggests it is younger," said Johnson. "In any event, we don't believe that Ida and its moon couldgo back to the formation of the solar system," he added. "It'sgenerally thought that a small object like that moon could notsurvive this long; sooner or later it would itself be broken upin a high-speed collision with an even smaller object." Galileo scientists also believe it is virtually impossiblethat the moon is a "captured object," something createdcompletely separately from Ida that happened to wander near theasteroid and be caught by its gravitational field. According tothe laws of celestial mechanics, such an event would deflect thesmaller object, but it would not be captured into orbit unless athird force of some kind slowed it down. "Once we have determined the object's orbit, we can estimatetime scales and make better guesses as to where it came from,"said Johnson. Launched in October 1989, Galileo made its closest approachto Ida at a distance of 2,400 kilometers (1,500 miles) lastAugust while flying through the asteroid belt en route to itsfinal destination -- the giant planet Jupiter, where it will gointo orbit in December 1995. Because Galileo is sending data back to Earth through itslow-gain radio antenna, it must transmit at slow rates. Oneportrait of Ida -- a mosaic of five separate frames -- wasreceived shortly after the flyby, but later pictures had to waitbecause telecommunications conditions became unfavorable asGalileo's distance from Earth increased. In the meantime theywere stored on Galileo's onboard tape recorder, awaiting playbackthis spring. Scientists say that the newly found moon was outside theboundaries of the picture of Ida released last September. Ground controllers instructed Galileo to send back moreportions of photos and other data beginning in February as thespacecraft's distance from Earth decreased and radiocommunications with the spacecraft improved. In preparation for complete playback, they commanded thespacecraft to transmit strips of each image -- called "jail bars"by the project's engineers and scientists -- so that they couldlocate Ida accurately in images stored on Galileo's recorder. Later, portions of an image containing Ida could be selected forplayback in their entirety. On February 17 -- a day after the first of these "jail bars"was sent back from Galileo -- evidence of the natural satellitewas noticed in one set of image strips by Ann Harch, a Galileoimaging team associate at JPL. It took several days to verifythat what appeared to be a moon was not, in fact, an artificialeffect of some kind. On February 23, scientists examining similar preliminarydata from a chemical map obtained by the near-infrared mappingspectrometer discovered an unusual object in their data. ByFebruary 28, scientists from both the camera and spectrometerteams concluded that they had a confirmation. Amateur astronomers for many years have observed the lightof stars blinking off and on as objects such as asteroids pass infront of them in events called stellar occultations. Some havereported "blinkouts" that suggest that some asteroids have moons,but such reports have never been confirmed by definite secondsightings. Galileo's discovery is thus the first unambiguousevidence of an asteroid moon. Other images that may show the asteroid moon are stillstored on Galileo's tape recorder, and will be played back laterthis spring. Among them is an image that is expected to be atleast three times sharper than the first image received. The newly found moon has been provisionally designated "1993 (243) 1" -- meaning that it is the first natural satellitediscovered in 1993 at Ida, which was the 243rd asteroiddiscovered over the past two centuries. The moon will beformally named later by the International Astronomical Union. JPL manages the Galileo Project for NASA's Office of SpaceScience. #####3-23-93 FOD# 9417.

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