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IMAGES OF ASTEROIDS IDA & DACTYL

This set of color images of asteroid 243 Ida was taken by the imaging system on the Galileo spacecraft as it approached and raced past the asteroid on August 28, 1993.
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Asteroid 243 Ida
August 28, 1993

These images were taken through the 4100-angstrom (violet), 7560-angstrom (infrared) and 9680- angstrom (infrared) filters and have been processed to show Ida as it would appear to the eye in approximately natural color. The stark shadows portray Ida's irregular shape, which changes its silhouetted outline when seen from different angles. More subtle shadings reveal surface topography (such as craters) and differences in the physical state and composition of the soil ('regolith'). Analysis of the images show that Ida is 58 kilometers long and 23 kilometers wide (36 x 14 miles). Ida is the first asteroid discovered to have a natural satellite, Dactyl (not shown here). Both Ida and Dactyl are heavily cratered by impacts with smaller asteroids and comets, including some of the same populations of small objects that bombard Earth. These data, combined with reflectance spectra from Galileo's near-infrared mapping spectrometer, may allow scientists to determine whether Ida is a relatively unaltered primitive object made of material condensed from the primordial Solar Nebula at the origin of the Solar System or whether it has been altered by strong heating--evidence interpreted so far suggests that Ida is a piece of a larger object that has been severely heated. Whereas heating and melting of large planets is well understood, the cause of heating of small asteroids is more enigmatic--it may have involved exotic processes that occurred only for a short time after the birth of the Sun and its planets.

This view of the asteroid 243 Ida is a mosaic of five image frames acquired by the Galileo spacecraft's solid-state imaging system at ranges of 3,057 to 3,821 kilometers (1,900 to 2,375 miles) on August 28, 1993, about 3-1/2 minutes before the spacecraft made its closest approach to the asteroid.
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Asteroid 243 Ida
August 28, 1993

Galileo flew about 2,400 kilometers (1,500 miles) from Ida at a relative velocity of 12.4 km/sec (28,000 mph). Asteroid and spacecraft were 441 million kilometers (274 million miles) from the Sun. Ida is the second asteroid ever encountered by a spacecraft. It appears to be about 52 kilometers (32 miles) in length, more than twice as large as Gaspra, the first asteroid observed by Galileo in October 1991. Ida is an irregularly shaped asteroid placed by scientists in the S class (believed to be like stony or stony iron meteorites). It is a member of the Koronis family, presumed fragments left from the breakup of a precursor asteroid in a catastrophic collision. This view shows numerous craters, including many degraded craters larger than any seen on Gaspra. The extensive cratering seems to dispel theories about Ida's surface being geologically youthful. This view also seems to rule out the idea that Ida is a double body. The south pole is believed to be in the dark side near the middle of the asteroid. The camera's clear filter was used to produce this extremely sharp picture. Spatial resolution is 31 to 38 meters (roughly 100 feet) per pixel. A 30-frame mosaic was taken to assure capturing Ida; its position was somewhat uncertain before the Galileo encounter. Galileo shuttered and recorded a total of 150 images in order to capture Ida 21 different times during a five hour period (about one rotation of the asteroid). Color filters were used at many of these times to allow reconstruction of color images. Playback to Earth of the remaining images is planned for April through June 1994. The Galileo project, whose primary mission is the exploration of the Jupiter system in 1995-97, is managed for NASA's Office of Space Science by the Jet Propulsion Laboratory.

This image is the most detailed picture of the recently discovered natural satellite of asteroid 243 Ida taken by the Galileo Solid-State Imaging camera during its encounter with the asteroid on August 28, 1993.
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Asteroid Dactyl
August 28, 1993

Shuttered through the camera's broadband clear filter as part of a 30-frame mosaic designed to image the asteroid itself, this frame fortuitously captured the previously unknown moon at a range of about 3,900 kilometers (2,400 miles), just over 4 minutes before the spacecraft's closest approach to Ida. Each picture element spans about 39 meters (125 feet) on the surface of the moon. More than a dozen craters larger than 80 meters (250 feet) in diameter are clearly evident, indicating that the moon has suffered numerous collisions from smaller Solar System debris during its history. The larger crater on the terminator is about 300 meters (1,000 feet) across. The satellite is approximately egg-shaped, measuring about 1.2 x 1.4 x 1.6 kilometers (0.75 x 0.87 x 1 mile). At the time this image was shuttered, Ida was about 90 kilometers (56 miles) away from the moon, outside this frame to the left and slightly below center. This image was relayed to Earth from Galileo on June 8, 1994. The Galileo project, whose primary mission is the exploration of the Jupiter system in 1995-97, is managed for NASA's Office of Space Science by the Jet Propulsion Laboratory.

This color picture is made from images taken by the imaging system on the Galileo spacecraft about 14 minutes before its closest approach to asteroid 243 Ida on August 28, 1993.
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Asteroid 243 Ida & Dactyl
August 28, 1993

The range from the spacecraft was about 10,500 kilometers (6,500 miles). The images used are from the sequence in which Ida's moon was originally discovered; the moon is visible to the right of the asteroid. This picture is made from images through the 4100-angstrom (violet), 7560 A (infrared) and 9680 A (infrared) filters. The color is 'enhanced' in the sense that the CCD camera is sensitive to near infrared wavelengths of light beyond human vision; a 'natural' color picture of this asteroid would appear mostly gray. Shadings in the image indicate changes in illumination angle on the many steep slopes of this irregular body as well as subtle color variations due to differences in the physical state and composition of the soil (regolith). There are brighter areas, appearing bluish in the picture, around craters on the upper left end of Ida, around the small bright crater near the center of the asteroid, and near the upper right- hand edge (the limb). This is a combination of more reflected blue light and greater absorption of near infrared light, suggesting a difference in the abundance or composition of iron- bearing minerals in these areas. Ida's moon also has a deeper near-infrared absorption and a different color in the violet than any area on this side of Ida. The moon is not identical in spectral properties to any area of Ida in view here, though its overall similarity in reflectance and general spectral type suggests that it is made of the same rock types basically. These data, combined with study of further imaging data and more detailed spectra from the Near Infrared Mapping Spectrometer, may allow scientists to determine whether the larger parent body of which Ida, its moon, and some other asteroids are fragments was a heated, differentiated object or made of relatively unaltered primitive chondritic material. The Galileo project, whose primary mission is the exploration of the Jupiter system in 1995-97, is managed for NASA's Office of Space Science by the Jet Propulsion Laboratory.

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Asteroid 243 Ida
August 28, 1993

This montage of 14 images (the time order is right to left, bottom to top) shows Ida as it appeared in the field of view of Galileo's camera on August 28, 1993. Asteroid Ida rotates once every 4 hours, 39 minutes and clockwise when viewed from above the north pole; these images cover about one Ida 'day.' This sequence has been used to create a 3-D model that shows Ida to be almost croissant shaped. The earliest view (lower right) was taken from a range of 240,000 kilometers (150,000 miles), 5.4 hours before closest approach. The asteroid Ida draws its name from mythology, in which the Greek god Zeus was raised by the nymph Ida.

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Mathilde, Gaspra, Ida
Comparison

These are views of the three asteroids that have been imaged at close range by spacecraft. The image of Mathilde (left) was taken by the NEAR spacecraft on June 27, 1997. Images of the asteroids Gaspra (middle) and Ida (right) were taken by the Galileo spacecraft in 1991 and 1993, respectively. All three objects are presented at the same scale. The visible part of Mathilde is 59 km wide x 47 km high (37 x 29 miles). Mathilde has more large craters than the other two asteroids. The relative brightness has been made similar for easy viewing; Mathilde is actually much darker than either Ida or Gaspra.

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