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.)