Characteristics
Both Vesta and Pallas have assumed the title of second-largest asteroid from time to time. However, while Pallas is slightly larger than 4 Vesta in volume, it is significantly less massive. The mass of Pallas is only 22% of Ceres, and about 0.3% that of the Moon.
Pallas is farther from the Earth and has a much lower albedo than Vesta, and consequently appears dimmer. Indeed, the much smaller 7 Iris marginally exceeds Pallas in mean opposition magnitude. Pallas's mean opposition magnitude is +8.0, which is well within the range of 10×50 binoculars, but unlike Ceres and Vesta, it will require more powerful optical aid to view at small elongations, when its magnitude can drop as low as +10.6. During rare perihelic oppositions, Pallas can reach a magnitude of +6.4, right on the edge of naked-eye visibility. During late February 2014, Pallas will shine at magnitude 6.96.
Pallas has unusual dynamic parameters for such a large body. Its orbit is highly inclined and somewhat eccentric, despite being at the same distance from the Sun as the central part of the asteroid belt. Furthermore, its axial tilt is very high, either 78±13° or 65±12° (based on ambiguous lightcurve data, the pole points towards either ecliptic coordinates (β, λ) = (−12°, 35°) or (43°, 193°) with a 10° uncertainty; data from the Hubble Space Telescope obtained in 2007 as well as the observations by the Keck telescope in 2003–2005 favour the first solution.) This means that, every Palladian summer and winter, large parts of the surface are in constant sunlight or constant darkness for a time of the order of an Earth year.
Based on spectroscopic observations, the primary component of the Palladian surface material is a silicate that is low in iron and water. Minerals of this type include olivine and pyroxene, which are found in CM chondrules. The surface composition of Pallas is very similar to the Renazzo carbonaceous chondrite (CR) meteorites, which are even lower in hydrous minerals than the CM type. The Renazzo meteorite was discovered in Italy in 1824 and is one of the most primitive meteorites known.
Very little is known of Palladian surface features. Hubble images from 2007 show pixel-to-pixel variation (pixel resolution is ≈70 km), but Pallas's 12% albedo placed such features at the lower end of detectability. There is little variability between lightcurves obtained through visible-light and infrared filters, but significant deviations in the ultraviolet, suggesting large surface or compositional features near 285° (75° west longitude). Rotation appears to be prograde.
Pallas is believed to have undergone at least some degree of thermal alteration and partial differentiation, which suggests that it was a protoplanet. During the planetary formation stage of the Solar System, objects grew in size through an accretion process to approximately this size. Many of these objects were incorporated into larger bodies, which became the planets, while others were destroyed in collisions with other protoplanets. Pallas and Vesta are likely survivors from this early stage of planetary formation.
Pallas was among the "candidate planets" in an early draft of the IAU's 2006 definition of planet, but does not qualify in the final definition because it has not "cleared the neighborhood" around its orbit. In the future, it is possible that Pallas may be classified as a dwarf planet, if it is found to have a surface shaped by hydrostatic equilibrium.
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