Asteroids and comets have several things in common, including a relatively small size as compared to planets and a lack of a viable atmosphere. Asteroids range in size from a few feet to several miles in diameter. Comets are generally smaller, with a rocky core. Both comets and asteroids don't have enough gravity to keep atmospheres.
Scientists believe comets and asteroids are leftover fragments from when the solar system formed 4.5 billion years ago. Asteroids are closer to the sun, usually between the orbits of Mars and Jupiter in the asteroid belt. Comets mainly exist at the outer edge of the solar system.
Another commonality between asteroids and comets is the way the objects hurtle toward the inner solar system. Collisions with nearby objects and gravitational pulls from distant bodies push or pull asteroids and comets closer to the sun. Some of these objects get close enough to Earth to be seen with the naked eye. Both comets and asteroids pose a threat to life on Earth if they collide with the planet.
Asteroids are chunks of rock made of heavy minerals. Comets are mostly dust and ice crystals. When comets get close to the sun, particles are jettisoned behind these objects and produce a luminous tail. Asteroids don't glow when they get closer to the sun.