SUMMARY OF KEY IDEAS

• Pieces of solar system debris larger than about 10 m (11 yd) and composed primarily of rock and metal are called asteroids.

• Tens of thousands of belt asteroids with diameters larger than a kilometer are known to orbit the Sun between the orbits of Mars and Jupiter. The gravitational attraction of Jupiter depletes certain orbits within the asteroid belt. The resulting Kirkwood gaps occur at simple fractions of Jupiter’s orbital period.

• A planet’s gravity and the Sun’s gravity can combine to capture Trojan asteroids in two locations, called stable Lagrange points, along a planet’s orbit. Jupiter, Neptune, Uranus, Mars, and Earth are known to have Trojan asteroids.

  265

• The Apollo asteroids move in highly elliptical orbits that cross the orbit of Earth. Many of these asteroids will eventually strike the inner planets.

• A belt asteroid (Ceres) and four KBOs (Pluto, Eris, Haumea, and Makemake) are classified as dwarf planets.

• Pluto, a KBO and dwarf planet, is an icy world that may well resemble the moon Triton.

• Comet nuclei are fragments of ice and rock often orbiting at a great inclination to the plane of the ecliptic.

• Many comet nuclei orbit the Sun in the Kuiper belt, a doughnut-shaped region beyond Neptune. Trillions of cometary nuclei are also believed to exist in the spherical Oort cloud located far beyond the Kuiper belt. Some get nudged into orbits taking them close to the Sun, which is when we see their tails.

• In the Kuiper belt and Oort cloud, comets have fairly circular orbits. When close to the Sun, they generally move in highly elliptical orbits.

• As an icy comet nucleus approaches the Sun, it develops a luminous coma surrounded by a vast hydrogen envelope. A gas (or ion) tail and a dust tail extend from the comet, pushed away from the Sun by the solar wind and radiation pressure.

• Boulder-sized and smaller pieces of rock and metal in space are called meteoroids. When a meteoroid enters Earth’s atmosphere, it produces a fiery trail, and it is then called a meteor. If parts of the object survive the fall, the fragments that reach Earth’s surface are called meteorites.

• Meteorites are grouped into three major classes according to their composition: iron, stony-iron, and stony meteorites. Rare stony meteorites, called carbonaceous chondrites, may be relatively unmodified material from the primordial solar nebula. These meteorites often contain organic hydrocarbon compounds, including amino acids.

• Fragments of rock from comets that have vaporized all their ices produce meteor showers.

• An analysis of the Allende meteorite suggests that a nearby supernova explosion may have been involved in the formation of the solar system some 4.6 billion years ago.

• An asteroid that struck Earth 65 million years ago probably contributed to the extinction of the dinosaurs and many other species. Another impact may have caused the Great Dying of life 250 million years ago. Such devastating impacts occur on average every 100 million years.