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951 Gaspra: Asteroids
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Asteroid 951 Gaspra appears to be in an obliquity resonance with its spin increasing due to the YORP effect. Gaspra, an asteroid 5.8 km in radius, is a prograde rotator with a rotation period of 7.03 hours. A 3 million year integration indicates its orbit is stable over at least this time span. From its known shape and spin axis orientation and assuming a uniform density, Gaspra�s axial precession period turns out to be nearly commensurate with its orbital precession period, which leads to a resonance condition with consequent huge variations in its obliquity. At the same time its shape is such that the Yarkovsky-O�Keefe-Radzievskii-Paddack effect (YORP effect for short) is increasing its spin rate. The YORP cycle normally leads from spin-up to spin-down and then repeating the cycle; ... it appears possible that resonance trapping can at least temporarily interrupt the YORP cycle, causing spin-up until the resonance is exited. This behavior may partially explain why there is an excess of fast rotators among small asteroids.
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This picture of asteroid 951 Gaspra is a mosaic of two images taken by the Galileo spacecraft from a range of 5300 kilometers, some 10 minutes before closest approach on October 29, 1991. A striking feature of Gaspra's surface is the abundance of small craters. More than 600 craters, 100 to 500 meters 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).
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On October 29, 199 1, Galileo passed within 1,600 kilometers of asteroid 951 * Gaspra and, with incredible navigational accuracy, took pictures of it -- pictures that reveal an irregular planetary body riddled with craters and fractures. To date, over 600 craters have been identified on the asteroid, the longest measuring about 1.5 kilometers across. The type of cracks seen ‑- some more than 90‑meters long ‑- are similar to ones seen before only on the Martian moon Phobos. Some planetary scientists theorize that about 4 billion years ago, Gaspra probably measured about 97 kilometers across. But, after years of catastrophic collisions with other planetary objects, the asteroid now measures a mere 19 X 12 X 11 kilometers. This history may not be unusual for asteroids like Gaspra, many of which are believed to have accreted into relatively substantial minor planets during the formation of the solar system, only to be broken down in collisions over the ensuing billions of years.
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Explanation: Asteroid 951 Gaspra is a huge rock tumbling in space. Gaspra became one of the best-studied asteroids in 1991 when the spacecraft Galileo flew by. In the above photograph, subtle color variations have been exaggerated to highlight changes in reflectivity, surface structure and composition. Gaspra is about 20 kilometers long and orbits the Sun in the main asteroid belt between Mars and Jupiter.
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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 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.
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Gaspra (Asteroid 951) was discovered by Grigoriy N. Neujamin in 1916. Neujamin named Gaspra after a Black Sea retreat that was visited by contemporaries such as Tolstoy and Gorky. Gaspra was just another small asteroid that was given very little attention until it was discovered that the trajectory of the Galileo spacecraft would take it close to Gaspra. Following this discovery, observers through out the world made Gaspra a prime target of study. Gaspra was found to have an elongated shape with a rotational period of 7.04 hours.
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