This picture of asteroid 951 Gaspra is a mosaic of two images taken by the Galileo spacecraft from a range of 5,300 kilometers (3,300 miles), some 1y the Galileo spacecraft from a range of 5,300
kilometers (3,300 miles), some 10 minutes before closest approach on October 29, 1991. The Sun is shining from the right; phase angle is 50 degrees. The resolution, about 54 meters/pixel, is the highest for the Gaspra encounter and is about three times better than that in the view released in November 1991.
Gaspra is an irregular body with dimensions about 19 x 12 x 11 kilometers (12 x 7.5 x 7 miles). The portion illuminated in tbody with dimensions about 19 x 12 x 11 kilometers (12 x 7.5 x 7 miles). The portion illuminated in this view is about 18 kilometers (11 miles) from lower left to upper right. The north pole is located at upper left; Gaspra rotates counterclockwise every 7 hours. The large concavity on the lower-right limb is about 6 kilometers (3.7 miles) across, the prominent crater on the terminator, center left, about 1.5 kilometers (1 mile).
A striking feature of Gaspra's surface is the abundance of small craters. More than 600 craters, 100-500 meters (330-1650 feet) in diameter are visible here. The number of such small craters compared to larger ones is much greater for Gaspra than for previously studied bodies of comparable size such as the satellites of Mars.
Gaspra's very irregular shape suggests that the asteroid was derived from a larger body by nearly catastrophic collisions. Consistent with such a history is the prominence of groove-like linear features, believed to be related to fractures. These linear depressions, 100-300 meters wide and tens of meters deep, are in two crossing groups with slightly different morphology, one group wider and more pitted than the other. Grooves had previously been seen only on Mars's moon Phobos, but were predicted for asteroids as well. Gaspra also shows a variety of enigmatic curved depressions and ridges in the terminator region at left.
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This picture of asteroid 951 Gaspra is a combination of the highest-resolution morphology and color ii AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
PHOTO CAPTION nformation obtained by the Galileo spacecraft during its approach to the asteroid on October 29, 1991. The Sun is shining from the right; phase angle is 50 degrees. The base image is the best black-and-white view of Gaspra (resolution 54 meters/pixel) on which are superimposed the subtle color variations constructed from violet, green, and near-infrared (1000 nanometers) inages taken in an earlier sequence at a resolution about 164 meters/pixel.
The very subtle color variations on Gaspa's surface have been artificially exaggerated here; to first order Gaspra's color is fairly homogeneous over the surface. However, subtle albedo and color variations do occur and are correlated with surface topography in a significant way. In this false-color view, the bluish areas represent regions of slightly higher albedo, which are also regions of slightly stronger spectral absorption near 1000 nanometers, probably due to the mineral olivine. These bluish areas tend to be associated with some of the crisper craters and with ridges. The slightly reddish areas, apparently concentrated in topographic lows, represent regions of somewhat lower albedo and weaker absorption near 1000 nanometers. In general, such patterns can be explained in terms of greater exposure of fresher rock in the brighter bluish areas and the accumulation of some regolith materials in the darker reddish areas.
Gaspra is an irregular body with dimensions about 19 x 12 x 11 kilometers (12 x 7.5 x 7 miles). The portion illuminated in this view is about 18 kilometers (11 miles) from lower left to upper right.
This color picture results from a joint effort by image processing groups at the U. S. Geological Survey in Flagstaff,
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PUBLIC INFORMATION OFFICE
JET PROPULSION LABORAT This montage of 11 images taken by the Galileo spacecraft as it flew by the asteroid Gaspra on October 29, 1991, shows Gaspra growing progressively larger in the field of view of Galileo's solid-ssively larger in the field of view of Galileo's
solid-state imaging camera as the spacecraft approached the asteroid. Sunlight is coming from the right. Gaspra is roughly 17 kilometers (10 miles) long, 10 kilometers (6 miles) wide.
The earliest view (upper left) was taken 5 3/4 hours before closest approach when the spacecraft was 164,000 kilometers (102,000 miles) from Gaspra, the last (lower right)at a range of 16,000 kilometers (10,000 miles), 30 minutes before closest approach.
Gaspra spins once in roughly 7 hours, so these images capture almost one full rotation of the asteroid. Gaspra spins counterclockwise; its north pole is to the upper left, and the "nose" which points upward in the first image, is seen rotating back into shadow, emerging at lower left, and rotating to upper right. Several craters are visible on the newly seen sides of Gaspra, but none approaches the scale of the asteroid's radius. Evidently, Gaspra lacks the large craters common on the surfaces of many planetary satellites, consistent with Gaspra's comparatively recent origin from the collisional breakup of a larger body.
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This montage shows asteroid 951 Gaspra (top) compared with Deimos (lower left) anddATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
PHOTO Phobos (lower right), the moons of Mars. The three bodies are shown at the same scale and nearly the same lighting conditions. Gaspra is about 17 kilometers (10 miles) long.
All three bodies have irregular shapes, due to past catastrophic conditions. However their surfaces appear remarkably different, possibly because of differences in composition but most likely because of very different impact histories.
The Phobos and Deimos images were obtained by the Viking Orbiter spacecraft in 1977; the Gaspra image is the best of a series obtained by the Galileo spacecraft on October 29, 1991. Galileo is scheduled to add the detailed view of another asteroid when it flies by Ida in August 1993.
