Science · Grades 3–6

Comets, Asteroids, and Meteoroids

Reading comprehension lesson about comets, asteroids, and meteoroids — the rocky and icy wanderers of our solar system.

Reading Passage

Comets, Asteroids, and Meteoroids

The solar system contains far more than just the sun and eight planets. Billions of smaller objects — comets, asteroids, and meteoroids — orbit the sun, leftover material from the solar system’s formation 4.6 billion years ago. These objects have played important roles in the history of Earth, delivering water and organic molecules in the early solar system and causing mass extinctions when large ones have struck our planet. Understanding these objects helps scientists piece together how the solar system formed and assess the risk they pose to Earth today.

Asteroids are rocky or metallic objects that orbit the sun, mostly in the asteroid belt between Mars and Jupiter. They range in size from tiny pebbles to Ceres, the largest asteroid, which is about 590 miles across and classified as a dwarf planet. Most asteroids are irregularly shaped, cratered worlds with no atmosphere. The asteroid belt contains millions of asteroids, but they are spread across such an enormous volume of space that spacecraft can pass through without hitting anything. Some asteroids have orbits that bring them close to Earth — these are called near-Earth asteroids, and scientists track them carefully for any potential collision threat.

Comets are icy bodies that originate in the outer solar system — either the Kuiper Belt beyond Neptune or the more distant Oort Cloud. When a comet’s orbit brings it close to the sun, solar radiation heats the ice, causing it to vaporize and release dust and gas in a process called outgassing. This creates the comet’s glowing head (coma) and its distinctive tails — a dust tail that curves along the comet’s path and an ion tail that always points directly away from the sun, pushed by the solar wind. Halley’s Comet, the most famous comet, orbits the sun about every 76 years and has been observed by humans for at least 2,000 years.

A meteoroid is a small rocky or metallic particle in space. When a meteoroid enters Earth’s atmosphere, friction with air molecules heats it to incandescence, creating a streak of light called a meteor — commonly called a shooting star. Most meteors burn up completely before reaching the ground. Meteor showers occur when Earth passes through the debris trail left by a comet. If a meteoroid survives its passage through the atmosphere and lands on Earth’s surface, it is called a meteorite. Scientists study meteorites because they are primitive samples of the early solar system, sometimes containing amino acids and other organic molecules that may have seeded the chemistry necessary for life.

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