Pallas

Pallas ( The asteroid )

Large asteroid of the main asteroid belt

Pallas ( minor planet designation: 2 Pallas ) is the second asteroid to have been discovered after, 1 Ceres. It is the third largest asteroid in the solar system by both volume and mass and is a likely remnant protoplanet. With an estimated 7% of the mass of the Asteroid belt it is the three equator the mass of four Vesta and slightly under a quarter the mass of Ceres. It is about 510 kilometres ( 320 mi ) in diameter, slightly smaller than Vesta. It was discovered on 28 March 1802. When Pallas were discovered by the German astronomer Heinrich Wilhelm Matthaus Olbers, it was counted as a planet as were other asteroids in the early 19 century. The discovery of many more asteroid after 1845 eventually led to their reclassification. Pallas surface is most likely composed of a silicate material its spectrum and estimated density resemble carbonaceous chondrite meteorites. With an orbital inclination of 34.8°, Pallas orbit is unusually highly inclined to the plane of the asteroid belt, making Pallas relatively inaccessible to spacecraft, and it's orbital eccentricity is nearly as large as that of Pluto. 

It was considered as a potential dwarf planet in 2006 but it has since been determined that its shape depart significantly from an ellipsoid disqualifying it.

On the night of 5 April 1779 Charles Messier recorded Pallas on start chat you to track the path of comet (  now known as C/1779, ( Bode) ) that he observed in the spring of 1779, bit apparently  assumed it was nothing more than a 🌟 star. In 1801 the astronomer Giuseppe Piazzi discovered an object which he initially believed to be a comet.  Shortly thereafter he announced his observations of this object, nothing that he slow, uniform motion was uncharacteristic of a comet , suggesting it was a different type of object.  This was lost from sight for several months, but was recovered later that year by the Baron Von Zach and Heinrich Wilhelm Matthaus Olbers after a preliminary orbit was computed by Carl Friedrich Gauss.

Later observations

In 1917, the Japanese astronomer Kiyotsugu Hirayama began to study asteroid motions. By plotting the mean orbital motion, inclination and eccentricity of a set of asteroid he discovered several distinct grouping. In a later paper he reported a group of three asteroid associated with Pallas, which became named the Pallas family after the largest member of the group. Since 1994 more than 10 member of this family have been identified with semi major axis between 2.50 - 2.82 AU and inclination of 33 - 38°. The validity of the family was confirmed in 2002 by a comparison of their spectra. 

Pallas has been observed occulting 🌟 stars several times including the best observed of all asteroid occultation events, by 140 observer on 29 May 1983. These measurements resulted in the first accurate calculations on 29 May 1979 the discovery of a possible tiny satellite with a diameter of about 1km was reported which was never confirmed. 

Orbit and rotation

Pallas has a high eccentricity and a highly inclined orbit 

Pallas has unusual dynamic parameters for such a large body. It's orbit is highly inclined and moderately eccentric, despite being at a same distance from the Sun as the central part of the asteroid belt. This means that every Palladian summer and winter, large parts of the surface are in constant sunlight or constant darkness for a time on the order of an Earth year, with areas near the poles experiencing continuous sunlight for as long as two years. 

Near resonances

Pallas is in a likely coincidental near -1:1 orbitals resonances with Ceres. Pallas also has a near 18:7 resonances ( 91,000 year period ) and an approximate 5:2 resonance ( 83 years ) with Jupiter.

Transits of planets from Pallas 

From Pallas, the planets Mercury, Venus, Mars and, Earth can occasionally appear to transit, or pass in front of the Sun. Earth last did so in 1968 and 1998, and will next transit in 2224 .  Mercury did in October 2009. The last and next by Venus are in 1677 and 2123 and for Mars they are in 1597 and 2759. 

Physical characteristics

Relative sizes of the four largest asteroids. Pallas is second from right.

Pallas is farther from Earth and has much lower albedo than Vesta, and hence is dimmer as seen from Earth. Pallas is a B type asteroid. Based on spectroscopic observations, the primary component of the material on Pallas surface is silicate containing little iron and water.

Pallas is thought to have undergone at least some degree of thermol alteration and partial differentiation, which suggests that it is a remnant protoplanet. Pallas and Vesta are likely survivors from this early stage of planetary formation.

Pallas was on a "watchlist" of objects possibly meeting a provisional definition of "planet" in an early draft of the IAU 2006 definition of planet.

Satellites

A small 🌒 Moon about 1km in diameter was suggested based on occultation data from 29 May 1978. In 1980, speckle interferometry suggested a much larger satellite , whose existence was later refuted a few years later with occultation data.

Exploration

Pallas has not been visited by spacecraft. A flyby after the Dawn probe visits to 4Vesta and 1Ceres was discussed but was not possible. The proposed Athena small sat  mission, if funded , would be launched in 2022 as a secondary payload of the psyche mission and would travel on separate trajectory to a flyby encounter with 2 Pallas.

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