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The Cosmos, 4th ed. by Pasachoff & Filippenko Chapter 8 Concept Review » Pluto, which emerged in 1999 from inside Neptune’s orbit, is so small and so far away that we know little about it (Sec. 8.1). The discovery of its moon, Charon, allowed us to calculate that Pluto contains only 1/500 the mass of the Earth (Sec. 8.1a). Mutual occultations of Pluto and Charon revealed sizes and surface structures of each. Pluto has a very thin nitrogen atmosphere, at least when it is closest to the Sun in its orbit (Sec. 8.1b). » Because Eris was found, at first thought to be bigger than Pluto, Pluto lost its designation as a genuine planet (Sec. 8.1c). It is now a dwarf planet, along with Eris, Haumea, and Makemake. These dwarf planets are the largest of the Kuiper‐belt objects, which are also called Trans‐Neptunian Objects (Sec. 8.2). In 2011, it was found that Eris and Pluto are virtual twins, and it is not clear which is larger, without considering Pluto’s atmosphere. More than 1000 Kuiper‐belt objects are now known, and they appear to be left over from the formation of the Solar System. » In a comet, a long tail that points away from the Sun may extend from a bright head (Sec. 8.3). The head consists of the nucleus, which is like a dirty snowball, and the gases of the coma (Sec. 8.3a). The dust tail contains dust particles released from the dirty ices of the nucleus. The gas tail, also known as an ion tail, is blown out behind the comet by the solar wind. Comets we see come from a huge spherical Oort comet cloud far beyond Pluto’s orbit or from a more flattened belt of icy objects called the scattered disk, somewhat beyond Neptune’s orbit (Sec. 8.3b). » Spacecraft imaged the nucleus of Halley’s Comet up close in 1986 and showed it to be about 16 km long and 8 km across (Sec. 8.3c). The impact of the tidally disrupted Comet Shoemaker‐Levy 9 with Jupiter in 1994 produced temporary Earth‐sized scars in Jupiter’s atmosphere (Sec. 8.3d). Comet Hale‐Bopp in 1997 was not only spectacular for the general public but also allowed many scientific studies (Sec. 8.3e). The Deep Impact mission crashed into a comet in 2005 (Sec. 8.3f ), providing information on its composition. » Meteoroids are chunks of rock in space up to about 50 m across, but most are small, like grains of sand (Sec. 8.4). When one hits the Earth’s atmosphere, we see the streak of light from its vaporization as a meteor, often incorrectly called a shooting star or a falling star. A fragment that survives and reaches Earth’s surface is a meteorite. Bits of space dust are micrometeorites (Sec. 8.4a). Most meteorites that are found are made of an iron–nickel alloy, but when meteorites are seen to fall, they are most often stony. The meteorites bring us primordial material to study and occasionally pieces of the Moon or Mars. » Many meteors are seen in showers, which occur when the Earth crosses the path of a defunct or disintegrating comet (Sec. 8.4b); these meteors appear to come from a single radiant in the sky. Sporadic meteors are not associated with showers and appear at the rate of a few each hour. © Cambridge University Press, 2013 Page 1 The Cosmos, 4th ed. by Pasachoff & Filippenko » Asteroids are minor planets (Sec. 8.5). Most asteroids are in the asteroid belt between the orbits of Mars and Jupiter. They were prevented from forming a planet by Jupiter’s gravitational tugs. Asteroids range up to about 1,000 km across (Sec. 8.5a), but the vast majority are much smaller, down to about 50 m. Most of them are stony, while some contain carbon and others consist largely of iron and nickel. The Galileo, Hyabusa, Rosetta, Dawn, and Near Earth Asteroid Rendezvous (renamed NEAR Shoemaker) missions have studied asteroids up close, and NEAR Shoemaker landed on the asteroid Eros (Sec. 8.5b). It found a solid object of primitive composition, in contrast with a rubble pile like the asteroid Mathilde. » It is plausible that an impact of a large asteroid or comet threw so much dust into the Earth’s atmosphere that it led to the extinction of most living species, including dinosaurs, 65 million years ago (Sec. 8.5b and A Closer Look 8.4). We worry about a Near‐ Earth Object (an asteroid or a comet) hitting the Earth, which could lead to the demise of civilization, or of the more likely collision with the Earth of a smaller object, which could cause substantial damage, killing thousands or millions of people. © Cambridge University Press, 2013 Page 2