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Asteroid 4, the fourth asteroid to be discovered, by H. W. M. Olbers in 1807. Its mean diameter is 530 km, similar to that of Pallas. Vesta's mass is 3 × 1020 kg and its mean density is 3.9 g/cm3. Its orbit has a semimajor axis of 2.362 AU, period 3.63 years, perihelion 2.15 AU, aphelion 2.57 AU, and inclination of 7°.1. Vesta's colour changes slightly as it rotates every 5.34 hours, indicating that its surface is not uniform in composition. The Hubble Space Telescope has shown bright and dark features as small as 80 km across on Vesta, suggesting a geologically diverse terrain with an exposed mantle, ancient lava flows, and impact basins. The average albedo is 0.42. Vesta's spectrum is characteristic of the mineral pyroxene, common in lava flows. Vesta may be a source of the eucrite meteorites and some other small asteroids. It is sufficiently different from other asteroids to be assigned a special class, V (for Vesta). Its apparent mean magnitude at opposition is 6.5, but at particularly favourable oppositions it can reach magnitude 5.5, the only asteroid bright enough to be visible with the naked eye.

Subjects: Astronomy and Astrophysics.

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