Which object in the solar system is known for being primarily composed of ice and dust?

The object known for being primarily composed of ice and dust is a comet. Comets are often described as "dirty snowballs" because they contain a significant mixture of frozen gases, water ice, and rocky or metallic materials. When a comet approaches the sun, the heat causes the ice to vaporize, creating a glowing coma (a cloud of gas and dust) around the nucleus and often forming a spectacular tail that points away from the sun due to solar wind.

In contrast, asteroids are predominantly composed of rock and metal, with little to no ice, making them quite different from comets. Planets typically have a solid surface or gaseous envelope and do not fit the icy composition characteristic of comets. Satellites, or moons, may have various compositions but are not defined primarily by the presence of ice and dust. Thus, the defining characteristics of a comet—a mixture of ice, dust, and other materials—make it the correct answer in this context.