(2) Pallas

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( 2) Pallas is with a mean diameter of 546 km is the largest asteroid and the second largest celestial bodies in the main asteroid belt, probably a dwarf planet.

Discovery

Pallas was discovered on March 28, 1802 by Heinrich Wilhelm Olbers and named after the Greek goddess Pallas Athena. The discovery was made by accident when Olbers discovered a year ago and now classified as a dwarf planet Ceres wanted to visit. After the discovery of Ceres Olbers had, like the other astronomers of his time, not expected yet to find another planet between Mars and Jupiter, since it was believed, found the predicted by the Titius - Bode series planet in this region of the solar system have to. Olbers therefore expressed the hypothesis that it constitutes fragments of a larger, broken planet Ceres and Pallas.

The discovery inspired the activities of the so-called Himmelspolizey of European Observatories; it was founded in 1800 to systematically search for suspected minor planets. With Olbers, and Harding two members of this organization in 1804 and 1807 succeeded the discovery of the asteroid ( 3) Juno and (4) Vesta.

Just as Ceres, Pallas was seen immediately after its discovery as a full-fledged planet, so were nine celestial bodies in 1802 as a planet. After the discovery of Juno and Vesta, the number of "planets" rose to 1807 to eleven. Here it remained until 1845. Then the fifth asteroid was discovered with ( 5) Astraea, and also first performed this as a planet. After the discovery of the "real " planet Neptune in 1846, even 13 heavenly bodies were regarded as planets. However, since 1847 the accumulated discoveries of asteroids, this henceforth of the " major planets " were distinguished by their number again dropped to eight.

William Herschel, who discovered Uranus in 1781, tried the diameter of Ceres and Pallas to determine what caused him some difficulties: The two objects appeared in his telescope virtually star-shaped (ie without measurable extent as the classical planets ). Herschel already suggested, therefore before 1802, Ceres and Pallas as " asteroids " (Greek for " star-like " ) to refer to in order to differentiate from the large planet. His view was shared but at this time only a few astronomers. It was not until around 1850, the number of celestial bodies found between Mars and Jupiter rising rapidly, they were under the names of " asteroids ", " asteroids ", " Tiny Planets " or " minor planets " summarized.

Discovered in 1803, chemical element palladium was named after this asteroid.

Orbit

Pallas moves at an average distance of 2.77 AU in 4.62 years of the sun. The orbital eccentricity is relatively large with 0.23: The distance from the sun, therefore, varies between 2.14 AU at perihelion and 3.41 AU at aphelion. The orbit is 34.9 ° also highly inclined to the ecliptic, making it often happens that Pallas far away from the ecliptic is staying in the sky.

While the opposition they reached an apparent magnitude of up to 6.5 mag. It is thus by ( 4) Vesta asteroid the second brightest in the night sky, but not visible to the naked eye.

Properties

The rotation period of Pallas is 7.8 hours. Its surface has an average albedo of 0.16. Through observations with the Hubble Space Telescope, the size of Pallas could be determined: The shape of Pallas becomes one triaxial ellipsoid with axis lengths of 582 ± 10 km, 556 ± 3 km and 500 ± 9 km. The mass was 1.18 × 10-10 solar masses determined (2.34 × 1020 kg ). From observations of the Infrared Astronomical Satellite and from occultations an average diameter was derived from about 523 km before. Pallas is now the single largest Ceres celestial bodies of the classical main belt, but is still surpassed in mass by Vesta.

Visibility

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