At the far point of its orbit, a comet—a ball of frozen gases and microscopic rock particles—moseys along at less than 100 miles (161 kilometers) per hour at the outer reaches of the solar system. However, as it follows its orbit and sweeps toward the sun, the comet accelerates. By the time it passes the orbits of the inner planets, as some comets do, a comet is hurtling along at more than 100,000 miles (161,000 kilometers) per hour. Particles and gases, pushed behind the comet by solar winds and light pressure, can stretch for millions of miles in one of the solar system’s most spectacular shows.
Although they are primarily made of water and their tails are brilliantly illuminated by sunlight, the body of a comet, its nucleus, is actually among the darkest objects in the solar system, darker than coal. A primordial building block that condensed out of the original solar nebula—the nucleus is a dirty snowball of gas, dust, and ices 0.5 - 6 miles (1 - 10 kilometers) across.
People never suspected that comets return periodically, but British astronomer Sir Edmund Halley became the first to suspect and predict a comet’s return. In 1705 he predicted that one particular comet would become visible again in 1758, some 76 years after its previous appearance. Halley did not live to see his prediction come true, but Halley’s comet is now among the most famous celestial objects.
Halley used Newton’s newly formulated laws of motion to calculate the orbit of his comet. But the orbit changes because the comet passed the planets in different positions than they occupied the last time it came by. In addition, the nongravitational effects caused by gases boiling off during the comet’s pass near the sun in 1986, slightly altered the orbital period. Halley is now expected to revisit the inner solar system a year early, in 2061—just 75 years after its last visit.
Astronomer Gerard Kuiper hypothesized more than 40 years ago that a comet belt circled the solar system. In 1992, ground-based telescopes began detecting large icy objects ranging from 60-200 miles (97-322 kilometers) in diameter. Now called the Kuiper Belt, this area is a primordial comet reservoir just beyond Neptune—the spawning ground for comets like Shoemaker-Levy 9.
Asteroid
Asteroids, sometimes called minor planets, are made of rock. They can range in size from 620 miles (1,000 kilometers) in diameter to house-sized objects, at which point astronomers begin to call them meteoroids. Colliding together, large asteroids break off new fragments, which are the meteoroids. Although there are very few large asteroids, like those that inspired the doomsday movie “Armageddon,” the total estimated mass of every asteroid combined into a single object would create a body less than half the diameter of Earth’s moon.
Some asteroids follow elliptical orbits taking them inside the orbit of Earth and out again. Most, however, are concentrated in an area called the asteroid belt, which stretches between the orbits of Mars and Jupiter. Some planetary moons—such as Deimos and Phobos of Mars—are thought to be asteroids caught by a planet’s gravitational pull.
Based on studies of asteroid pieces that have fallen to Earth, more than 90 percent are made primarily of rock. The rest are made mostly of iron and nickel or a combination of rock and metal.
The asteroid Gaspra was studied by NASA’s Galileo spacecraft enroute to Jupiter. The craft determined that the potato-shaped asteroid, which is made of metal and rock, makes one full rotation every seven hours.
Meteoroid
Comets and asteroids rarely collide with Earth, but meteorites, asteroid fragments that have landed on Earth, and micrometeorites, tiny particles from asteroids and comets that are collected in the upper atmosphere or even the seabed, are valuable sources of information about the evolution and composition of the solar system.
Falling through the upper atmosphere at speeds up to 45 miles (70 kilometers) per second, meteoroids are subjected to friction against air molecules. This causes most of them to disintegrate at around 60 miles (100 kilometers) above Earth. From the ground at night, they appear as brilliant meteors or “shooting stars.”
Repeating at the same date each year, dozens to hundreds or more meteors are seen each hour in a clear dark sky, an event called a meteor shower. Among the best annual showers are the Persieds, which occur each August 11 and 12. Another periodic shower is the Leonid, which occurs each November when Earth passes through debris left behind by the comet Tempel-Tuttle.
Meteoroids that survive their fiery entry and land on the ground are called meteorites. Most meteoroids slow down quickly due to atmospheric drag, but the largest—heavier than a few tons—are slowed very little. Earth’s most recent catastrophic encounter with a space visitor—a small asteroid or comet—occurred in 1908 over the remote region of Tunguska, Siberia. The body exploded above the surface, flattening trees for an area 80 miles (129 kilometers) across.
Some 65 million years ago, a comet or asteroid blasted out a crater about a 110 miles (180 kilometers) wide on today’s Yucatán Peninsula and threw up a global cloud of dust and gas. The sun was all but blocked for years, dropping average temperatures to near freezing and killing off more than half of the plant and animal species on Earth, including dinosaurs.
Meteorites are interplanetary rubble, fragments of collisions between asteroids. Most meteorites are the remaining rubble of creation, fragments that never became a planet. Others come from very specific places. More than a dozen meteorites appear to have come from Mars. One was found in 1984 in Antarctica. Like the others, it was hurled into space when that planet was hit by a comet or asteroid. A flurry of excitement swept the globe when experts thought they found evidence of fossil microbial life in the rock, but those findings are now largely dismissed by the scientific community. Still, the incident showed that much can be learned from the rocks that fall from the sky.
