The image above shows the somewhat oblate shape of the B-type asteroid designated 2 Pallas, which is the second asteroid to be discovered, and the third most massive, containing about 7% of all the mass in the entire asteroid belt. Based on its diameter of 512 kilometres, Pallas was originally thought to be a dwarf planet, but it is no longer seen as such because its shape differs considerably from the typically spherical shape of such objects. It should be noted that the Pallasite class of meteorites are not connected to the asteroid in any way; these meteorites were named after Peter Simon Pallas, a German naturalist.
• Aphelion: 3.412605509 AU
• Perihelion: 2.13060671 AU
• Eccentricity: 0.23127363
• Orbital period: 4.614296176 years (1,685.371678 days)
• Equatorial rotation velocity: 65 m/sec
• Average orbital speed: 17.65 km/sec
• Mean proper motion: 78.041654 degrees / year
• Dimensions: 550 ± 8 × 516 ± 6 × 476 ± 6 km
• Volume: 7.1 ± 0.3 ×107 cubic kilometres
• Mass: 2.11 ± 0.26 × 1020 kilograms
• Surface area: 8.3 ± 0.2 ×105 square kilometres
• Mean density: 3.0 ± 0.5 gram / cubic centimetre
• Escape velocity: ≈ 0.33 km/sec
• Apparent magnitude: Variable from 6.49 to 10.65
• Satellites: None
Although Pallas is significantly further away from Earth, and has a much lower albedo (reflective index) than the asteroid Vesta, its mean magnitude at opposition can be high as +8.0, which places it well within the brightness range that can be observed with 10×50 binoculars. However, when it is at even smaller elongations, its brightness can drop to as low as +10.6, which means that at least medium-sized telescopes are required to observe it. Nonetheless, Pallas’ brightness can also increase to about +6.4 or even brighter during some rare perihelic oppositions; one such instance occurred in February of 2014, when it shone at magnitude +6.96.
Although it is by no means certain, Pallas is thought by some investigators to have undergone at least some thermal alteration and subsequent partial differentiation. If this turns out to have been the case, Pallas is likely the remains of a protoplanet that failed to fully accrete into a full-sized planet. Current models that describe the processes of planet formation hold that objects the size of Pallas and Vesta would have occurred in large numbers, but also that any objects that were not incorporated into full-fledged planets would have been destroyed by repeated collisions with similarly sized protoplanets.
Settling the question of whether or not Pallas is indeed a protoplanetary remnant is unlikely to be reached any time soon, since there are currently no plans by any space agency to explore the asteroid from up close.
Since the orbit of Pallas is unusually inclined with respect to the ecliptic and nearly as eccentric as that of Pluto, it is effectively out of reach of spacecraft. Therefore, the asteroid can only be studied though spectroscopic analysis, and the image above was obtained only because the Dawn mission team was granted observing time on the Hubble telescope to obtain comparative data for the asteroids Ceres and Vesta.
Nonetheless, spectroscopic analysis has shown that the surface of Pallas consists primarily of silicate that contains negligible amounts of water and iron. Silicates of this type typically include pyroxene and olivine, which are commonly found in carbonaceous chondrite (CR) meteorites, although the amount of hydrous minerals in these meteorites is typically lower than in CM-type* chondrite meteorites.
* CM-type meteorites are named after the Mighei meteorite that fell in the area of Ukraine in 1889. This type of meteorite typically contains about 10% water, and many complex organic compounds. In fact, one example of a CM-type meteorite, the Murchison meteorite that fell in Murchison, Australia in 1969 was found to contain more than 230 different amino acids. Since only 20 amino acids occur naturally on Earth, most investigators believe that the organic material in CM-type meteorites represent primitive interstellar material from outside of the solar system.
Not much is known about what the surface of Pallas might look like, since light curves of the asteroid taken in visible and infrared frequencies are almost flat, except for a slight rise towards the blue part of the spectrum. While Hubble images taken in 2007 have a resolution of about 70 km (43 miles) per pixel, the low, 0.12 albedo of the asteroid places the variations that occur between pixels that might suggest surface features at the lower end of detectability. However, light curves taken in ultraviolet light show distinct color variations in the surface at about 75° west longitude, although the nature of the variations remains unclear.