1Vagabonds of the Solar SystemChapter 17Guiding Questions1. How and why were the asteroids first discovered?2. Why didn’t the asteroids coalesce to form a single planet?3. What do asteroids look like?4. How might an asteroid have caused the extinction of the dinosaurs?5. What are the differences among meteoroids, meteors, and meteorites?6. What do meteorites tell us about the way in which the solar system formed?7. Why do comets have tails?8. Where do comets come from?9. What is the connection between comets and meteor showers?A search for a planet between Mars and Jupiter led to the discovery of asteroids• Astronomers first discovered the asteroids while searching for a “missing planet”• Thousands of asteroids with diameters ranging from a few kilometers up to 1000 kilometers orbit within the asteroid belt between the orbits of Mars and Jupiter2The asteroids are the relics of planetesimalsthat failed to accrete into a full-sized planet, thanks to the effects of Jupiter and other Mars-sized objects.3Jupiter’s gravity helped shape the asteroid belt• Even today, gravitational perturbations by Jupiter deplete certain orbits within the asteroid belt• The resulting gaps, called Kirkwood gaps, occur at simple fractions of Jupiter’s orbital periodJupiter’s gravity also captures asteroids in two locations, called Lagrangian points, along Jupiter’s orbit4Asteroids occasionally collide with one another• Asteroids undergo collisions with each other, causing them to break up into smaller fragments5Asteroids are found outside the asteroid belt—and have struck the Earth• Some asteroids, called near-Earth objects, move in elliptical orbits that cross the orbits of Mars and Earth• If such an asteroid strikes the Earth, it forms an impact crater whose diameter depends on both the mass and the speed of the asteroidAn asteroid may have struck the Earth 65 million years ago, possibly causing the extinction of the dinosaurs and many other species6Small rocks in space are called meteoroids • If a meteoroid enters the Earth’s atmosphere, it produces a fiery trail called a meteor• If part of the object survives the fall, the fragment that reaches the Earth’s surface is called a meteoriteMeteorites are classified as stones, stony irons,or irons, depending on their composition7Irons and stony irons are fragments of the core of an asteroid that was large enough and hot enough to have undergone chemical differentiation, just like a terrestrial planetSome meteorites retain traces of the early solar system• Some stony meteorites come from the crust of such differentiated meteorites, while others are fragments of small asteroids that never underwent differentiation• Rare stony meteorites called carbonaceous chondrites may be relatively unmodified material from the solar nebula• These meteorites often contain organic material and may have played a role in the origin of life on Earth• Analysis of isotopes in certain meteorites suggests that a nearby supernova may have triggered the formation of the solar system 4.56 billion years ago8A comet is a dusty chunk of ice that partiallyvaporizes as it passes near the Sun• A comet is a chunk of ice with imbedded rock fragments that generally moves in a highly elliptical orbit about the SunAs a comet approaches the Sun, its icy nucleus develops a luminous coma, surrounded by a vast hydrogen envelope9An ion tail and a dust tail extend from the comet, pushed away from the Sun by the solar wind and radiation pressure10Comets originate either from a belt beyond Plutoor from a vast cloud in near interstellar space• The Oort cloud contains billions of comet nuclei in a spherical distribution that extends out to 50,000 AU from the Sun• Intermediate period and long-period comets are thought to originate in the Oort cloud• As yet no objects in the Oort cloud have been detected directly11• The Kuiperbelt lies in the plane of the ecliptic at distances between 30 and 500 AU from the Sun• It is thought to contain many tens of thousands of comet nuclei• Many Jupiter-family comets probably come from the Kuiper belt, and hundreds of larger objects have been observed in the Kuiper beltComets eventually break apart, and their fragments give rise to meteor showers12• Fragments of “burned out” comets produce meteoritic swarms• A meteor shower is seen when the Earth passes through a meteoritic swarmKey Words• amino acids• asteroid• asteroid belt• carbonaceous chondrite• coma (of a comet)• comet• differentiated asteroid• dust tail• fusion crust• Hirayama family• hydrogen envelope• intermediate-period comet• iron meteorite (iron)• ion tail• Jupiter-family comet• Kirkwood gaps• Kuiper belt• long-period comet• meteor• meteor shower• meteorite• meteoritic swarm• meteoroid• minor planet• near-Earth object (NEO)• nucleus (of a comet)• Oort cloud• radiant (of a meteor shower)• radiation pressure• stable Lagrange points• stony iron meteorite• stony meteorite (stone)• supernova• tail (of a comet)• Trojan asteroid• undifferentiated asteroid• Widmanstätten
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