What are comets?

• A. Balls of ice, rock, and dust in space with a tail
• B. A bright streak of light in the night sky caused by a meteor entering Earth's atmosphere
• C. A large planet with rings
• D. A rocky body that orbits the Sun

Answer: (A)

Comets are icy bodies made of ice, rock, and dust that travel through space. When they swing close to the Sun, the heat causes the ices to sublimate into gas, creating a surrounding cloud called the coma and ejecting dust. Solar radiation and the solar wind then push this material outward, forming a glowing tail that points away from the Sun as the comet moves. These features—an icy composition and a visible tail formed by solar heating—make comets distinct from rocky asteroids, which are simply rocky bodies that orbit the Sun, and from meteors, which are streaks of light caused by meteoroids burning up in Earth’s atmosphere. Comets originate from distant realms like the Kuiper Belt and the Oort Cloud and follow highly elongated orbits that bring them into the inner solar system and back out again.