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Comets

A comet is an icy, small Solar System body that warms and releases gases when passing close to the Sun, creating a glowing cloud called the coma and sometimes an extended tail pointing away from the Sun. Comets are composed of loose collections of ice, dust, and rocky particles. They have highly eccentric elliptical orbits bringing them periodically near the Sun. Comets matter because they reveal clues about the early Solar System's composition and are spectacular objects visible to the naked eye. Space missions like NASA's Deep Impact and the European Space Agency's Rosetta have studied comets up close, advancing our understanding of these ancient cosmic visitors.

Sources

• en.wikipedia.org: “A comet is an icy, small Solar System body or interstellar object that warms and begins to release gases when passing close to the Sun, a process called outgassing.”
• en.wikipedia.org: “This produces an extended, gravitationally unbound atmosphere or coma surrounding the nucleus, and sometimes a tail of gas and dust gas blown out from the coma.”
• en.wikipedia.org: “Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles.”
• en.wikipedia.org: “Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years.”