Leftover building blocks
Asteroids & Comets
Between and beyond the planets, the solar system is full of smaller worlds — rocky asteroids, icy comets, and the dust they leave behind. They are the raw ingredients left over from the solar system's birth.
Asteroids
Asteroids are rocky, airless leftovers from the formation of the solar system. Most orbit the Sun in the asteroid belt between Mars and Jupiter, but despite what movies show, they are spread so far apart that spacecraft fly through the belt without danger. The largest object in the belt, Ceres, is big enough to be a dwarf planet. NASA missions like OSIRIS-REx have even collected samples from asteroids and returned them to Earth.
Comets
Comets are balls of ice, dust, and rock, often described as "dirty snowballs." They spend most of their lives in the cold outer solar system — in the Kuiper Belt or the far more distant Oort Cloud. When a comet's orbit brings it near the Sun, its ices turn to gas, forming a glowing coma and two tails (one of dust, one of charged gas) that always point away from the Sun. Halley's Comet, the most famous, returns about every 76 years.
Meteors, meteoroids, and meteorites
A meteoroid is a small piece of rock or metal in space. If it enters Earth's atmosphere and burns up as a streak of light, it's a meteor (a "shooting star"). If a piece survives and lands on the ground, it becomes a meteorite. Meteor showers happen when Earth passes through the dusty trail left behind by a comet.
Why they matter
Asteroids and comets are time capsules: they preserve the original material from which the planets formed 4.6 billion years ago. Comets may have delivered some of Earth's water. Scientists also track near-Earth asteroids for planetary defense — NASA's DART mission deliberately crashed into the asteroid moonlet Dimorphos in 2022 and successfully changed its orbit, the first test of deflecting a space rock.