A student question came my way asking me which object holds the title of “largest known asteroid.” Out in the main asteroid belt, beyond Mars and inside Jupiter’s orbit, over a million asteroids represent the leftovers of solar system formation–small fragments of rock and dust and a little ice that never coalesced into a planet. Altogether, the mass of the asteroid belt is about five percent of the mass of Earth’s moon. About sixty percent of the asteroid belt mass is contained in the four largest asteroids: Ceres, Vesta, Pallas, and Hygiea.
The largest asteroid (in fact it’s known as a minor planet) is 1 Ceres, about 950 km in diameter, roughly the size of Texas. No other asteroid is larger than 600 km in diameter. NASA’s Dawn spacecraft rendezvoused with asteroid 4 Vesta in 2011-2012, then reached Ceres orbit in 2015, where it still orbits today, its mission concluded.
Asteroid 4 Vesta, seen from Dawn on July 24, 2011. (NASA)
Dawn’s mapping and exploration mission finally concluded in 2018, with the spacecraft running out of fuel. Here’s an image of Ceres captured by Dawn:
Dwarf planet Ceres, the largest asteroid in our solar system, is shown in this false-color rendering of Dawn imagery, highlighting differences in surface materials. (NASA)
Ceres shows extensive evidence of mineral alteration by liquid water, and even today it may harbor subsurface pockets of liquid water. Small icy areas are exposed at the surface, along with bright white salt deposits. Ceres’ density is so low that it may still contain large amounts of ice and water.
While the Dawn mission ended five years ago, NASA continues its asteroid exploration program with the Lucy mission on its way to Jupiter’s mysterious, dark-red Trojan asteroid groups. In late 2023 NASA hopes to launch the Psyche mission to the large asteroid 16 Psyche, whose surface may be mostly metallic iron and nickel.
Our study of these large main belt asteroids helps us to understand the composition and origin of their many fragments, nudged by collisions and Jupiter’s dominant gravity into Earth-approaching orbits, where they may someday threaten a collision with Earth. Asteroid exploration both helps us understand the story of the solar system’s birth, and the hazards and resources stemming from near-Earth asteroids, which have long influenced the path of life on our world.
Dan Durda’s painting showing a human expedition to a near-Earth asteroid. (Dan Durda, SWRI)