2 Pallas

2 Pallas is the third-largest asteroid in our Solar System by mass and volume.

It is considered a remnant protoplanet. A protoplanet is a large object that formed during the early stages of the Solar System. During this era, objects grew through a process called accretion. In accretion, small particles clump together to form larger bodies. Most protoplanets eventually became the major planets we know today. Others were destroyed by collisions or ejected by the gravity of larger planets. Pallas, along with Ceres and Vesta, is one of the few intact bodies from this early era surviving within the orbit of Neptune.

The composition of Pallas provides clues about the early Solar System. It is a B-type asteroid, meaning its surface is mostly silicate material. This material contains very little iron or water. Its minerals, such as olivine and pyroxene, are similar to carbonaceous chondrite meteorites. Specifically, its surface is very similar to Renazzo carbonaceous chondrite meteorites. These are some of the most primitive meteorites known. While Pallas has a mineral composition similar to Ceres, it is significantly less hydrated. This means it contains less water within its minerals than Ceres does.

Pallas has very unusual orbital characteristics. It has a high orbital inclination of 34.8 degrees. This means its path is tilted sharply relative to the plane of the asteroid belt. This tilt makes Pallas difficult for spacecraft to reach. It also has a high orbital eccentricity, which means its orbit is not a perfect circle. In fact, its eccentricity is nearly as large as that of Pluto. Because of this high inclination, Pallas can pass very close to stars that other solar objects avoid. On 9 October 2022, Pallas passed near the star Sirius. It passed only 8.5 arcminutes south of the star. No regular planet can get closer than 30 degrees to Sirius.

The history of Pallas involves several important astronomers. Charles Messier recorded Pallas on a star chart in 1779, but he thought it was a star. In 1801, Giuseppe Piazzi discovered Ceres, the first asteroid. A few months later, Heinrich Wilhelm Matthias Olbers discovered Pallas while searching for Ceres. At the time, astronomers believed a planet existed in the gap between Mars and Jupiter. The discovery of both Ceres and Pallas supported this idea. In the early 19th century, Pallas was actually classified as a planet. It was only after more asteroids were found after 1845 that scientists began using the term "minor planet." Later, in the 1950s, the term "asteroid" became more common.

The name Pallas is an epithet for the Greek goddess Athena. In some myths, Athena accidentally killed a girl named Pallas. Out of mourning, the goddess adopted her friend's name. The asteroid's name is also linked to the chemical element palladium. The element was named after the asteroid because it was discovered shortly after the object was found. The old astronomical symbol for Pallas is a spear or lance. This symbol is still used in astrology today.

The physical surface of Pallas is heavily impacted. It appears to be saturated with craters. Because of its high inclination and eccentricity, impacts on Pallas are very energetic. These impacts occur at twice the velocity of hits on Vesta or Ceres. This high speed allows smaller objects to create large craters. Craters larger than 40 km in diameter cover at least 9% of its surface.

There is a possible large impact basin at the south pole. This impact may have ejected a volume of material equal to the size of Pallas. This event might have caused the asteroid's high inclination and slowed its rotation.

Pallas is also part of a larger group called the Pallas family. In 1917, the astronomer Kiyotsugu Hirayama began studying asteroid motions. He discovered that certain asteroids move in distinct groupings. He identified a group of three asteroids associated with Pallas. Since 1994, scientists have identified more than 10 members of this family. These members have inclinations between 33 and 38 degrees. The existence of this family was confirmed in 2002 through spectral comparisons.