Asteroid
An asteroid is a rocky or metallic body, smaller than a planet, that orbits the Sun; most are found in the asteroid belt between Mars and Jupiter.
Definition
An asteroid is a chunk of rock or metal that orbits the Sun, much smaller than a planet and usually lumpy rather than round, with most found in a wide ring between Mars and Jupiter. Asteroids range from a few meters to several hundred kilometers across; most orbit in the main belt at about $2.2$ to $3.2$ AU, but near-Earth asteroids cross or approach Earth's orbit, and unlike comets they contain little ice and grow no tails. They are remnant planetesimals that never gathered into a planet, largely because Jupiter's gravity kept collision speeds in the belt too high for growth. They are classified spectrally as C-type (carbonaceous, dark, most common), S-type (stony), and M-type (metallic), and resonances with Jupiter carve the Kirkwood gaps and push asteroids into planet-crossing orbits, the source of most meteorites and of the impact hazard to Earth.
Example
Vesta is one of the biggest asteroids at about $525$ kilometers across, while some are only car-sized. Ceres, the largest object in the belt at about $940\ \text{km}$, is round enough to count as a dwarf planet, and although more than a million asteroids are cataloged, their combined mass is less than the Moon's. NASA's DART spacecraft struck the small asteroid Dimorphos in September 2022 and shortened its orbit around Didymos by about $33$ minutes, the first demonstration that a kinetic impactor can change an asteroid's path.
Key Insight
Asteroids are leftovers from when the planets formed, so studying them is like studying the solar system's original building blocks. The belt is not the crowded obstacle course of movies: asteroids are separated by about a million kilometers on average, and spacecraft pass through without dodging. Some asteroids, like Vesta, melted and separated into core and mantle early on while others never did, and comparing them reveals how much heat from short-lived radioactive isotopes was available in the first few million years.