Pallas

second largest asteroid in the asteroid belt and the second such object to be discovered, by the German astronomer and physician Wilhelm Olbers on March 28, 1802, following the discovery of Ceres the year before. It is named after Pallas Athena, the Greek goddess of wisdom.

Pallas's orbital inclination of 34.8° is rather large, but its moderate orbital eccentricity (0.23), mean distance from the Sun of 2.77 astronomical units (about 414 million km [257 million miles]), and orbital period of 4.61 years are typical for asteroids located between the orbits of Mars and Jupiter. The discoveries of Ceres and Pallas, together with that of two more asteroids (Juno and Vesta) over the next five years, gave rise to the surprisingly long-lived but no longer generally accepted idea that the asteroids are remnants of the “missing” planet between Mars and Jupiter predicted by Bode's law—i.e., that they were pieces of an actual planet that broke up.

Pallas has an ellipsoidal shape with radial dimensions of 285 × 262 × 250 km, equivalent to a sphere with a diameter of 530 km—i.e., about 15 percent of the diameter of the Moon. Its diameter and that of Vesta are so nearly the same that the two bodies can exchange titles of “second largest” and “third largest” when new measurements are published. Pallas's albedo (reflectivity) is 0.15. Its mass is about 2.2 × 10²⁰ kg, and its density is about 2.9 grams per cubic cm (nearly 90 percent that of the Moon). Pallas turns once on its axis every 7.8 hours. Compositionally, Pallas resembles the carbonaceous chondrite meteorites. Its surface is known to contain hydrated minerals.

Edward F. Tedesco