Asteroids are small, rocky bodies that orbit the Sun, remnants from the early formation of the solar system that were prevented from coalescing into a full-sized planet. The majority of these minor planets reside in the main asteroid belt, a region between the orbits of Mars and Jupiter. This belt contains millions of objects, ranging from dust particles to bodies hundreds of kilometers in diameter. The first of these bodies was discovered at the beginning of the 19th century, with Italian astronomer Giuseppe Piazzi spotting the largest among them in 1801.
The Definition and Context of the “Largest” Asteroids
The four largest objects in the main belt—Ceres, Vesta, Pallas, and Hygiea—collectively account for approximately 60% of the entire belt’s mass. Their classification is complex, as Ceres is officially recognized as a dwarf planet due to its size and spherical shape. Nevertheless, Ceres remains the undisputed largest object of the asteroid belt and is included in discussions of the largest asteroids. Vesta, Pallas, and Hygiea follow in size, designated (4) Vesta, (2) Pallas, and (10) Hygiea, reflecting the order in which they were formally numbered.
Ceres: The Largest Object in the Asteroid Belt
Ceres is the dominant body in the main belt, with a mean diameter of roughly 940 kilometers. Its gravity was sufficient to pull the body into a nearly perfect sphere, fulfilling a key requirement for its dwarf planet classification. The NASA Dawn mission revealed that Ceres is internally differentiated, separating into a rocky core and a mantle rich in water ice and hydrated minerals. This low density suggests that about 25% of its mass is composed of water.
The surface of Ceres displays evidence of past and potentially ongoing geological activity involving water. Bright spots inside the Occator Crater are deposits of highly reflective salts, primarily sodium carbonate, which may have been brought to the surface by briny liquid welling up from below. Features like the mountain Ahuna Mons are believed to be cryovolcanoes, formed by the slow eruption of ice and salt water instead of molten rock. The presence of ammoniated phyllosilicates also suggests Ceres may have formed in the colder, outer solar system before migrating inward.
Vesta, Pallas, and Hygiea: The Next Three Giants
Vesta, the second most massive body, is a remnant protoplanet. Unlike Ceres, Vesta is not spherical but is a highly differentiated body with a basaltic, or volcanic, surface and an iron core. Fragments of its surface have reached Earth as howardite-eucrite-diogenite (HED) meteorites, offering a direct sample of a protoplanet. The body is dominated by the colossal Rheasilvia impact basin at its south pole, a crater so large it defines Vesta’s shape.
Pallas is the third most massive object, characterized by its highly inclined orbit, tilted at about 34.8 degrees relative to the main plane of the asteroid belt. This severe inclination makes Pallas difficult to reach for spacecraft missions, requiring significant changes in velocity and fuel. Pallas is classified as a B-type asteroid, indicating a primitive, carbonaceous composition similar to carbonaceous chondrite meteorites, and its surface is heavily saturated with impact craters.
Hygiea is the fourth largest body, and recent high-resolution observations have shown it to be nearly spherical, making it a strong candidate for dwarf planet status. It is the largest of the dark C-type asteroids, possessing a low density that suggests a composition rich in carbonaceous material and volatiles. Hygiea is the parent body of one of the largest asteroid families, believed to be the result of a massive collision that shattered and re-accreted the material into its current, rounded form.