According to mission scientists at the Jet Propulsion Laboratory, this image, together with data from Galileo's near-infrared mapping spectrometer, is the first conclusive evidence that natural satellites of asteroids exist.
The photo, of asteroid 243 Ida and its newly discovered natural satellite, was taken by Galileo as the spacecraft flew past Ida last August 28. It was not transmitted to Earth until recently because the spacecraft is sending back data at a very slow rate via its low-gain antenna because the high-gain antenna never completely deployed.
The discovery gives scientists an intriguing new clue in deciphering the origins and evolution of these ancient, rocky bodies, most of which orbit the sun in the main asteroid belt between 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 the details of that orbit-- are still unclear.
"It was previously thought that natural satellites of asteroids could form, but they probably weren't common," said Dr. Torrence Johnson, Galileo project scientist. "Having found one fairly quickly, we can say that they're probably more common than previously thought."
From the photo and spectrometer data, team scientists estimate that the natural satellite is about 1.5 kilometers (1 mile) across in this view, and appears to be at a distance of about 100 kilometers (60 miles), plus or minus 50 kilometers (30 miles), from Ida's center. The position will be more accurately determined as new data are analyzed. Ida itself is about 56 x 24 x 21 kilometers (35 x 15 x 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. Combined with measurements of Ida's size and volume, that can tell scientists the asteroid's density, offering more clues to what it is made of.
The data from Galileo's near-infrared mapping spectrometer, which scans space objects at a variety of wavelengths to reveal their chemical composition, suggest that Ida's moon is made more or less from the same kind of material as Ida. An S-type asteroid, Ida is composed mostly of silicate rocks. Scientists are certain, in any event, that the moon's surface is not composed mostly of carbonaceous material, as are the many asteroids that are termed C-type asteroids. More information on the object's composition will become available as color pictures and more detailed data from the spectrometer are transmitted to Earth over the next few months.
Galileo scientists believe the moon may have been created at the same time as Ida, when an older, larger asteroid was shattered in a collision with another asteroid, giving birth to dozens of smaller asteroids.
Ida is a member of the Koronis family of asteroids, which scientists believe was created when a larger body perhaps 200-300 kilometers in diameter was smashed relatively recently or at least considerably after the solar system formed some 4.5 billion years ago. (The family was named for Koronis, one of the asteroids that belongs to it.)
Alternatively, it is possible that Ida was hit by a smaller object even more recently, leaving a crater on the asteroid and throwing off the material that became the small moon.
"Ida's age is baffling, because the craters visible on its surface suggest that it is old, but being a part of the Koronis family suggests it is younger," said Johnson. "In any event, we don't believe that Ida and its moon could go back to the formation of the solar system," he added. "It's generally thought that a small object like that moon could not survive this long; sooner or later it would itself be broken up in a high-speed collision with an even smaller object."
Galileo scientists also believe it is virtually impossible that the moon is a captured object, something created completely separately from Ida that happened to wander near the asteroid and was caught by its gravitational field. According to the laws of celestial mechanics, such an event would deflect the smaller object, but it would not be captured into orbit unless a third force of some kind slowed it down. "Once we have determined the object's orbit, we can estimate timescales and make better guesses as to where it came from," said Johnson.
Launched in October 1989, Galileo made its closest approach to Ida at a distance of 2400 kilometers (1500 miles) last August while flying through the asteroid belt en route to its final destination, the giant planet Jupiter, where it will go into orbit in December 1995.
Because Galileo is sending data back to Earth through its low-gain radio antenna, it must transmit at slow rates. One portrait of Ida, a mosaic of five separate frames, was received shortly after the flyby, but later pictures had to wait because telecommunications conditions became unfavorable as Galileo's distance from Earth increased. In the meantime they were stored on Galileo's onboard tape recorder, awaiting playback this spring. The newly found moon was outside the boundaries of the picture of Ida received last September. Ground controllers instructed Galileo to send back more portions of photos and other data beginning in February as the spacecraft's distance from Earth decreased and radio communications with the spacecraft improved.
In preparation for complete playback, they commanded the spacecraft to transmit strips of each image--called "jail bars" by the project's engineers and scientists--so that they could locate Ida accurately in images stored on Galileo's recorder. Later, portions of an image containing Ida could be selected for playback in their entirety.
On February 17, a day after the first of these "jail bars" was sent back from Galileo, evidence of the natural satellite was noticed in one set of image strips by Ann Harch, a Galileo imaging team associate at JPL. It took several days to verify that what appeared to be a moon was not, in fact, an artificial effect of some kind. On February 23, scientists examining similar preliminary data from a chemical map obtained by the near-infrared mapping spectrometer discovered an unusual object in their data. By February 28, scientists from both the camera and spectrometer teams concluded that they had a moon.
Amateur astronomers for many years have observed the light of stars blinking off and on as objects such as asteroids pass in front of them in events called stellar occultations. Some have reported "blinkouts" that suggest that some asteroids have moons, but such reports have never been confirmed by definite second sightings. Galileo's discovery is thus the first unambiguous evidence of an asteroid moon.
Other images that may show the asteroid moon are still stored on Galileo's tape recorder and will be played back later this spring. Among them is an image that is expected to be at least three times sharper than the first image received.
The newly found moon has been provisionally designated "1993 (243) 1," the first natural satellite discovered in 1993 at Ida, which was the 243rd asteroid discovered over the past two centuries. The moon will be formally named later by the International Astronomical Union.