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Asteroids vs. comets: How do they differ, and do they pose a threat to Earth?

Asteroids are _______________ chunks of rock that measure in size from a few feet to hundreds of miles in diameter. NASA has identified more than a million asteroids, and more than 150 of them have their own moons. In 2022, astronomers published evidence that the asteroid Elektra has as many as _______ chunks of rock orbiting it—making it the first known quadruple asteroid.

 For many years, astronomers considered Ceres to be the largest asteroid at about 590 miles wide. In 2006, however, the International Astronomical Union reclassified Ceres as a dwarf planet—making Vesta the largest asteroid at 329 miles wide. Yet this remains a point of contention among some astronomers, and the U.S. Geological Survey’s Gazetteer of Planetary Nomenclature defines Ceres as both an asteroid and a dwarf planet.

Comets are balls of ice and rock whose glowing tails can sometimes be seen from Earth as they streak through the night sky. Comets grow those tails whenever their orbits bring them close to the sun, causing the icy objects to heat up and _________ a trail of gas and dust. The sun illuminates the tail, giving it a majestic glow.

Compared to asteroids, comets tend to have more elliptical, or oval-shaped, orbits. They also contain more chemical compounds that vaporize when heated, such as water. And when observed through a telescope, comets appear fuzzier than asteroids.

(Fonte: National Geographic – adaptado.)