Questions 10-19

　　No two comets ever look identical, but they have basic features in common, one of the

　　most obvious of which is a coma. A coma looks like a misty, patch of light with one or more

　　tails often streaming from it in the direction away from the Sun.

　　Line At the heart of a comet's coma lies a nucleus of solid material, typically no more than

　　(5) 10 kilometers across. The visible coma is a huge cloud of gas and dust that has escaped

　　from the nucleus, which it then surrounds like an extended atmosphere. The coma can extend as far as a million kilometers outward from the nucleus. Around the coma there is often an even larger invisible envelope of hydrogen gas.

　　The most graphic proof that the grand spectacle of a comet develops from a relatively

　　(10) small and inconspicuous chunk of ice and dust was the close-up image obtained in 1986 by the European Giotto probe of the nucleus of Halley's Comet. It turned out to be a bit like a

　　very dark asteroid, measuring 16 by 8 kilometers. Ices have evaporated from its outer layers

　　to leave a crust of nearly black dust all over the surface. Bright jets of gas from evaporating

　　ice burst out on the side facing the Sun, where the surface gets heated up, carrying dust

　　(15) with them. This is how the coma and the tails are created.

　　Comets grow tails only when they get warm enough for ice and dust to boil off. As a

　　comet's orbit brings it closer to the Sun, first the coma grows, then two distinct tails usually

　　form. One, the less common kind, contains electrically charged (i.e., ionized) atoms of gas,

　　which are blown off directly in the direction away from the Sun by the magnetic field of

　　(20) the solar wind. The other tail is made of neutral dust particles, which get gently pushed back by the pressure of the sunlight itself. Unlike the ion tail, which is straight, the dust tail

　　becomes curved as the particles follow their own orbits around the Sun.

　　I0. The passage focuses on comets primarily in terms of their

　　(A) orbital patterns

　　(B) coma and tails

　　(C) brightness

　　(D) size

　　11. The word "identical" in line I is closest in meaning to

　　(A) equally fast

　　(B) exactly alike

　　(C) near each other

　　(D) invisible