A small object in the solar system that orbits the sun. Comets are clumps of rocks, ice, dust and frozen gases. They may come from the Kuiper Belt — a strip of rocky clumps beyond Neptune. Comets may also come from the Oort cloud — the cloud of icy objects at the very edge of the solar system. Comet orbits can be very variable. Some comets are visible from Earth once every few years. Others might take thousands of years to revisit our planet.
A comet differs from an asteroid because it has a tiny bit of gassy atmosphere surrounding it; asteroids are mostly rock. A comet’s frozen ice and gases are what causes the most spectacular feature: When a comet passes close to the sun, these frozen chemicals melt, surrounding the comet and trailing after it as it streaks across the sky.
In a sentence
The European Space Agency sent a craft called Rosetta to orbit a comet named 67P/Churyumov-Gerasimenko (or just 67P). Rosetta ended its mission by landing on the comet’s surface.