What is the difference between a comet and an asteroid?

The primary differences between comets and asteroids are rooted in their composition, formation locations, and behavioral characteristics.

Composition

• Comets are composed of ice, dust, and rock. They are often referred to as "dirty snowballs" due to their mixture of volatile ices (such as water ice) and silicate and organic dust particles245.
• Asteroids are made of rocky materials and metals. They can be classified into various types, including those rich in carbon compounds (C-type asteroids), silicate rocks, and those composed primarily of metal135.

Formation and Location

• Comets formed in the outer reaches of the Solar System, such as the Kuiper belt and beyond, where the cold temperatures allowed ices to remain solid145.
• Asteroids formed closer to the Sun, primarily in the asteroid belt between Mars and Jupiter, where it was too warm for ices to remain solid145.

Orbital Characteristics

• Comets have highly eccentric orbits, which take them from the outer Solar System to close proximity with the Sun. This close approach causes the comet's ices to melt and vaporize, forming a coma and tail245.
• Asteroids typically have more elliptical but less eccentric orbits, mostly confined to the asteroid belt between Mars and Jupiter. They do not produce a coma or tail145.

Visibility and Behavior

• Comets are visible from Earth when they approach the Sun, due to the formation of a dust tail and an ion tail. The dust tail is white or yellow and trails behind the comet, while the ion tail is bluish and points away from the Sun24.
• Asteroids do not produce such tails and are generally less visible from Earth without a telescope. However, pieces of both comets and asteroids can become visible as meteors if they enter Earth's atmosphere14.

Size and Structure

• Comets are generally smaller, with nuclei ranging from 1 to 10 kilometers in diameter4.
• Asteroids can range from a few meters to hundreds of kilometers in diameter. Larger asteroids may have enough gravity to form spherical shapes, while smaller ones can be irregular or even "rubble piles"34.

These distinctions highlight the unique characteristics of each type of celestial body and their roles in understanding the early history of the Solar System.