Comets are among the most interesting and unpredictable bodies in the solar system. They are made of frozen gases (water vapor, ammonia, methane, carbon dioxide, and carbon monoxide) that hold together small pieces of rocky and metallic materials. Many comets travel in very elongated orbits that carry them far beyond Pluto. These long-period comets take hundreds of thousands of years to complete a single orbit around the Sun. However, a few short-period comets (those having an orbital period of less than 200 years), such as Halley’s Comet, make a regular encounters with the inner solar system.
When a comet first becomes visible from Earth, it appears very small, but as it approaches the Sun, solar energy begins to vaporize the frozen gases, producing a glowing head called the coma. The size of the coma varies greatly from one comet to another. Extremely rare ones exceed the size of the Sun, but most approximate the size of Jupiter. Within the coma, a small glowing nucleus with a diameter of only a few kilometers can sometimes be detected. As comets approach the Sun, some develop a tail that extends for millions of kilometers. Despite the enormous size of their tails and comas, comets are relatively small members of the solar system.
The observation that the tail of a comet points away from the Sun in a slightly curved manner led early astronomers to propose that the Sun has a repulsive force that pushes the particles of the coma away, thereby forming the tail. Today, two solar forces are known to contribute to this formation. One, radiation pressure, pushes dust particles away from the coma. The second, known as solar wind, is responsible for moving the ionized gases, particularly carbon monoxide. Sometimes a single tail composed of both dust and ionized gases is produced, but often two tails—one of dust, the other, a blue streak of ionized gases—are observed.
As a comet moves away from the Sun, the gases forming the coma recondense, the tail disappears, and the comet returns to distant space. Material that was blown from the coma to form the tail is lost from the comet forever. Consequently, it is believed that most comets cannot survive more than a few hundred close orbits of the Sun. Once all the gases are expelled, the remaining material—a swarm of tiny metallic and stony particles—continues the orbit without a coma or a tail.
Comets apparently originate in two regions of the outer solar system. Most short-period comets are thought to orbit beyond Neptune in a region called the Kuiper belt, in honor of the astronomer Gerald Kuiper. During the past decade over a hundred of these icy bodies have been discovered. Most Kuiper belt comets move in nearly circular orbits that lie roughly in the same plane as the planets. A chance collision between two comets, or the gravitational influence of one of the Jovian planets—Jupiter, Saturn, Uranus, and Neptune—may occasionally alter the orbit of a comet in these regions enough to send it to the inner solar system and into our view.
Unlike short-period comets, long-period comets have elliptical orbits that are not confined to the plane of the solar system. These comets appear to be distributed in all directions from the Sun,
forming a spherical shell around the solar system, called the Oort cloud, after the Dutch astronomer Jan Oort. Millions of comets are believed to orbit the Sun at distances greater than 10,000 times the Earth-Sun distance. The gravitational effect of a distant passing star is thought to send an occasional Oort cloud comet into a highly eccentric orbit that carries it toward the Sun. However, only a tiny portion of the Oort cloud comets have orbits that bring them into the inner solar system.
The most famous short-period comet is Halley’s Comet, named after English astronomer Edmond Halley. Its orbital period averages 76 years, and every one of its 30 appearances since 240 B.C. has been recorded by Chinese astronomers. When seen in 1910, Halley’s Comet had developed a tail nearly 1.6 million kilometers (1 million miles) long and was visible during daylight hours. Its most recent approach occurred in 1986.
1. All of the following questions can be answered based on the facts presented in paragraph 1 EXCEPT:
A. Do any comets travel to regions of space beyond our solar system?
B. Do most short-period comets enter the inner solar system?
C.What are comets composed of?
D. Why are comets regarded as among the most unpredictable bodies in the solar system? Paragraph 1 is marked with an arrow [→]
2. The word “exceed” in the passage is closet in meaning to
B. grow to
D. go beyond
3. The word “detected” in the passage is closet in meaning to
4. Paragraph 2 supports the idea that comets appear very small when they first become visible from Earth because
A. the glow from the nucleus hides the rest of the comet from view
B.they are still too far away from the Sun to have developed a coma or a tail
C. their various gases have mostly become vaporized
D.they are relatively small when compared to the visible planets Paragraph 2 is marked with an arrow [→]
5. Paragraph 2 supports which of the following statements about comet size?
A. The size of a comet is affected by the addition of gases absorbed as the comet passes the Sun.
B. The size of a comet’s tail is less variable than the size of its coma.
C.The coma of most comets is smaller than the Sun.
D. The size of a comet cannot be accurately determined until it nears Earth. Paragraph 2 is marked with an arrow [→]
6. The word “propose” in the passage is closet in meaning to
A. offer the theory
7. According to paragraph 3, what is true about comets’ tails?
A. Their shape led early astronomers to draw false conclusions about the Sun.
B. They consist mostly of ionized gases that had been blown out of the Sun
C. Radiation pressure and solar wind both play a role in their formation.
D. Their dust content decreases as a comet nears the Sun.
8. Select TWO answer choices that according to paragraph 4 describe how a comet changes as it travels away from the Sun. To receive credit, you must select TWO answers.
A. It loses its metallic and stony particles.
B.It loses all the material that was in its tail.
C.Its orbital period becomes shorter.
D.The gases that were in its coma recondense. Paragraph 3 is marked with an arrow [→]
9. The word “roughly” in the passage is closet in meaning to
10. According to paragraphs 5 and 6, compared to the orbits of short-period comets, the orbits of long-period comets are
A. more circular
B.more likely to result in a comet’s entering the inner solar system
C. more likely to be affected by a passing star
D. more likely to lie in the same plane as the planets Paragraph 5 and 6 are marked with an arrow [→]
11. According to paragraph 6, which of the following is true about the Oort cloud?
A.It is almost 10,000 times the size of the Sun.
B. It is formed by the orbits of long-period comets orbiting far from the Sun.
C. It is produced by the gravitational effects of distant stars.
D. It can disrupt the orbit of a comet enough to send it on a path toward the Sun. Paragraph 6 is marked with an arrow [→]
12. According to paragraph 7, all of the following are true of Halley’s Comet EXCEPT:
A. On at least one appearance, it had a tail of well over a million kilometers in length.
B. Each of its last 30 appearances has been recorded by astronomers in China.
C. It is the only comet that can be observed during daylight hours.
D. It averages 76 years between appearances in the inner solar system. Paragraph 7 is marked with an arrow [→]
13. Look at the four squares [■] that indicate where the following sentence could be added to the passage. It was not until 1705, however, that Halley determined that each of these appearances was actually a reappearance of the same object.
Where would the sentence best fit? Click on a square [■] to add the sentence to the passage.
14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Drag your choices to the spaces where they belong. To review the passage, click on View Text.
hort –period comets complete their orbits in less than 200 years while long-period comets may take hundreds of thousands of years to complete an orbit. ● ● ●
1,The size of a comet’s nucleus is determined by two solar forces, radiation pressure and solar wind.
2,Comets accelerate as they move away from the Sun in part because they lose material from the coma each time they pass near the Sun.
3,Comets are at times sent into the inner solar system as a result of a collision or a gravitational disturbance.
4,Although a comet’ coma and tail may be
quite large, these features do not develop until a comet is close enough to the Sun for solar energy to vaporize its frozen gases.
5,While short-period comets have circular orbits that lie close to the plane of the planets, long-period comets have more elliptical orbits that form a spherical shell around the solar system.
6,Although studies of Halley’s Comet and other short-period comets provide some clues, astronomers do not yet fully understand how and when comets formed.