CHAPTER 10
Dwarf Planets and Solar System Debris
CHAPTER OUTLINE
The Discovery of Pluto
1. An analysis of the orbital data of Uranus indicated that 98% of its orbital variation could be accounted for by the presence of Neptune; the remaining unexplained 2% variation led to the search for Planet X.
2. In 1905 Lowell initiated a search for Planet X. Clyde Tombaugh finally discovered Planet X—Pluto—in early 1930.
3. Tombaugh used a blink comparator to compare two photos of the sky taken a few days apart. A moving object such as a planet will appear to jump from one spot to another as the observer quickly changes views from the first photo to the second.
4. Pluto was discovered 6° from where Lowell had predicted it.
5. Pluto, however, is too small to cause the irregularities that had been seen in Uranus’s orbit. Later it was shown that these irregularities were not caused by another planet but were variations due to the limited accuracy of the data available.
6. Pluto’s discovery was an accident, though without Lowell’s predictions, the search would not have been so vigorously pursued.
10-1 Pluto
1. The New Horizons spacecraft will pass by Pluto in 2015 and give us the first close-up images of the planet and its moons, and then continue its exploration of objects in the Kuiper belt at the outer solar system until 2020.
Pluto as Seen from Earth
1. In 1989 Pluto was as close to the Earth as it has been for 248 years. (From 1979 to 1999 Pluto was inside Neptune’s orbit.)
2. Pluto’s average distance from the Sun is 40 AU, but its eccentric orbit causes it to vary in distance from 30 AU to about 50 AU.
3. Pluto’s orbit is tilted 17° to the ecliptic (no other planet is tilted more than 7°).
4. Stellar occultations indicate that Pluto has an atmosphere (nitrogen, methane, and carbon monoxide).At aphelion it is probably too cold for the methane to remain gaseous. The atmosphere is undergoing global cooling, while the planet’s surface is getting slightly warmer.
5. On Pluto’s surface the temperatures range from 235C to 210C.
Pluto and its Moons
1. In 1978, James Christy discovered that Pluto has a moon, named Charon.
2. Charon’s orbit is tilted at 61° to Pluto’s orbit around the Sun.
3. Charon orbits Pluto every 6.4 days, the same as Pluto’s and Charon’s rotational period. Thus, these two objects always keep the same face toward each other.
4. Pluto’s atmosphere limits an accurate determination of its size, which probably ranges from 2,300 km to 2,340 km.Charon’s diameter is about 1,200 km.
5. Pluto’s mass is about 8 times Charon’s, but only 1/5 of our Moon’s. Pluto’s density is about 2 g/cm3, Charon’s is about 1.6 g/cm3, indicating both contain rock and ice.
6. Two additional moons were discovered in 2005 by the Hubble Space Telescope, and a fourth moon in 2011.
7. Because Pluto is small and has an eccentric orbit, some theorized that it is a former moon of Neptune that was somehow ejected. The discovery of its moons made it seem less likely that Pluto was once Neptune’s moon.
10-2 Solar System Debris
1. Pluto and Charon are now classified as two of the largest examples of a group of objects orbiting in a swathe of the outer solar system (the Kuiper belt).
2. Apart from the Sun (a large object) and the planets and moons (medium-sized objects), most of the other objects in the solar system can be classified as debris—an accumulation of rock fragments.
3. Solar system debris comes in a number of forms, including asteroids, meteoroids, comets, and dust. It is material that did not become part of the Sun or the planets when the solar system was formed
10-3 Asteroids
1. About 200 years after the first asteroid, Ceres, was discovered, the orbits of thousands of asteroids have been accurately determined.
2. Astronomers estimate that hundreds of thousands of asteroids have been captured on photographic surveys of the sky. Asteroids are also known as minor planets.
3. Ceres, at 930 km (580 mi) in diameter and now classified as a dwarf planet, is the largest asteroid and makes up about a third of the mass of all asteroids.
4. Pallas and Vesta have diameters greater than 500 km. About 30 more asteroids have diameters between 200 and 300 km. About 100 are between 100 and 200 km. All the rest are under 100 km in diameter.
5. Observations and theoretical models suggest Ceres and Vesta have layered structures consisting of core, mantle, and crust. Ceres’ mantle may consist of ice. NASA’s Dawn mission is currently exploring these objects.
6. Asteroids are classified in a number of ways including by position (e.g., asteroid belt, near-Earth, and Trojans) and by type (bright, metallic and reddish S-type, bright, metallic and non-reddish M-type, and dark C-type).
7. Asteroids are irregular in shape, and thus reflect different amounts of light toward Earth as they rotate.
8. Gaspra, a rocky S-type asteroid 19 12 11 km, was imaged by the Galileo spacecraft in 1990.
9. Some asteroids like Gaspra have a reddish tint and are fairly bright. Others are dark like a lump of coal.
10. In 2010, The Hayabusa mission returned samples from the surface of S-type asteroid Itokawa back to Earth. Study of those samples support the idea that the S-type asteroids are composed of the same material as the ordinary chondrite meteorites.
The Orbits of Asteroids
1. The asteroids revolve around the Sun in a counterclockwise direction like the planets.
2. Most asteroids orbit in or near the plane of the ecliptic.
3. Most asteroids orbit the Sun at distances from 2.2 to 3.3 AU (between Mars and Jupiter) in what is called the asteroid belt.
4. Asteroids are not evenly distributed across the asteroid belt. At certain distances—2.5 and 3.28 AU—gaps appear and are related, respectively, to 1/3 and 1/2 of Jupiter’s orbital period. These Kirkwood gaps are due to synchronous tugs from Jupiter.
5. Gaps in the asteroid belt also appear corresponding to 2/5 and 3/7 of Jupiter’s orbital period.
6. About 7000 asteroids have near-Earth orbits. Apollo asteroids are some 2000 asteroids that have eccentric orbits that cross the Earth’s orbit. About 70 of them are potentially hazardous to Earth because they are larger than 1 km in diameter and get closer than 0.05 AU to Earth.
The Origin of Asteroids
1. Astronomers originally thought that asteroids were the remains of the explosion of a planet. However, there is no known mechanism for making a planet explode.
2. If all the asteroids were combined into one object, they would only form a body about 1,500 km in diameter, much smaller than our Moon.
3. It is most likely that asteroids are primordial material that never formed into a planet because of Jupiter’s gravitational influence.
Advancing the Model: The Mission to Eros
- The NEAR mission was the first to orbit an asteroid, Eros, and then land on its surface.
- Eros has a composition similar to the most primitive rocks in the solar system, the chondritic meteorites. It is a relic from the dawn of our solar system, a planetesimal that was not captured by a growing protoplanet.
Tools of Astronomy: You Can Name an Asteroid
- Only about 3000 of the perhaps 100,000 asteroids that on photographs have been named.
- To name an asteroid, you must know its orbit accurately, which requires searching for it in many photographs over time. After you have determined the orbit, you must wait until the asteroid has completed at least one more cycle around the Sun to check your predicted orbit.
- The object’s official name will include not only the name you give, but a number indicating the order of discovery. For example, 1 Ceres and 2 Pallas.
10-4 Comets
1. Comets are among the most spectacular sights available to the naked eye.
Cometary Orbits: Isaac Newton and Edmund Halley
1. Newton proposed that comets orbit the Sun according to his laws of motion and universal gravity. He concluded that since comets were visible for only short periods of time, their orbits were very eccentric, i.e., elongated.
2. Halley, a friend of Newton, used Newton’s methods, his own observations, and prior comet descriptions to calculate orbits for a number of comets. He correctly surmised that the comets of 1531, 1607, and 1682 were in fact the same comet and predicted its next return in 1758. This comet was named in his honor his honor (Comet Halley) and is probably the most famous periodic comet.
3. Comet Halley has a period of approximately 76 years. Because the comet is slightly affected by the gravity of the planets during its return from deep space, its period varies slightly one appearance to the next. Halley was the first to recognize this effect of other bodies on comets’ orbits.
4. The planes of revolution of comets are not limited to the ecliptic but are randomly oriented. Comets sweep past the Sun from all directions.
The Nature of Comets
1. Comets have three parts: the coma, the nucleus, and the tail. The coma and nucleus comprise the comet’s head.
2. The coma may be as large as a million km in diameter. Some tails have been as long as an astronomical unit.
3. Fred Whipple proposed in 1950 what is still the basic model of a comet: the nucleus is essentially a dirty snowball made up of water ice, frozen carbon dioxide, and small solid grains. We now include in the model a crusty layer on the surface of the nucleus.
4. The coma is made up of diffuse gas and dust. The nucleus is the solid chunk of a comet. The tail is the gas and/or dust swept away from the comet’s head.
5. As a comet approaches the Sun, it becomes warmer and the ices inside melt and vaporize.
6. The nucleus of Comet Hale-Bopp is about 25 miles across and spins once every 12 hours.As it spins, material is ejected from it in geysers and spirals away from it.
7. ESA’s Giottospacecraft revealed that Halley’s coma is billions of times less dense than the atmosphere of the Earth at sea level. It also confirmed that the comet is composed of material as primitive as the original solar nebula.
8. The presence or absence of noble gases in a comet provides a tool for measuring the thermal history of comets. Another tool is the structure of the dust that comets carry.
Comet Tails
1. Comets usually have two tails. The straight tail consists of charged atoms or molecules (ions) that are being swept away from the comet by the solar wind. The curved tail is caused by dust grains in the coma being pushed away by solar radiation pressure.
2. A comet’s tail always points away from the Sun (and thus does not always follow the comet’s head). After passing the Sun, a comet’s tail actually leads the head.
3. Comet tails are typically 107 to 108 km long and may be 1 AU long.
4. Comets “die” through gradual evaporation of their entire nuclei, or through evaporation of all their volatile materials, leaving chunks of rock, or by falling into the Sun.
Missions to Comets
1. Deep Impactreleased a probe that crashed on the nucleus of Comet Temple 1 in 2005; data showed the presence of a mix of water, organic compounds, and silicates, similar to what is observed in the long-period comets from the Oort cloud.
2. Deep Impact went on to encounter Comet Hartley 2 in 2010, and found the ratio of heavy water to water in its coma was similar to Earth’s oceans, supporting the idea that comets supplied some of the Earth’s water.
3. Stardust plunged through the coma of Comet Wild 2 in 2004, collected samples and returned them to Earth; data showed the presence of olivine and high-temperature minerals such as aluminum, calcium, and titanium.
4. Rosetta was launched in 2004 and will meet its target in 2014; its lander will drill into the crust of the comet’s nucleus and collect and analyze material from the nucleus.
Historical Note: Astronomer Maria Mitchell
- Maria Mitchell discovered a comet in 1847, resulting in her becoming the first woman elected to the American Academy of Arts and Sciences and receiving a gold medal from the king of Denmark.
- Mitchell was an early believer in women’s rights and saw astronomy and science as an avenue for liberation and a way for women to break from domestic tradition.
10-5 The Oort Cloud and the Kuiper Belt
1. In 1950 Jan Oort revivedthe idea that a comet cloud exists in a spherical shell between 20,000 and 100,000 AU from the Sun. Billions of comet nuclei are thought to exist in this
Oort cloud.
2. Long-period comets are believed to originate in the Oort cloud. Interactions between comets in the cloud or between a comet and a star passing-by could deflect some comets into the inner solar system.
3. In 1951 Gerard Kuiper proposed that a second, smaller band of comets must exist inside the Oort cloud. The Kuiper belt is a disk-shaped region beyond Neptune’s orbit, 30–1000 AU from the Sun, and presumed to be the source of short-period comets.
4. The first object in the Kuiper belt was observed in 1992. Eris, currently the largest dwarf planet, Pluto, Sedna and other trans-Neptunian objects are Kuiper-belt objects.
The Origin of Short-Period Comets
1. Long-period comets sometimes become short-period comets through the gravitational influence of Jupiter and the Sun.
2. Most comet orbits are either elliptical or parabolic (but not hyperbolic). Understanding how comets are distributed and what they are made of will put important constraints on models describing the formation and early evolution of our solar system.
3. The average distance between comets in the Oort cloud is greater than 16 AU! The Oort cloud and the Kuiper belt are far from crowded.
4. It seems that objects in the Oort cloud formed around the distance of Uranus and Neptune from the Sun, and then were deflected outward when they passed to close to those planets
Historical Note: Jan H. Oort
- Although he is best known for the cloud of comets that bears his name, his accomplishments are numerous.
- Some of the discoveries he contributed to include that our galaxy rotates, the location and orbit of the Sun within the galaxy, and the link between the Crab nebula and an historic supernova.
10-6 Meteors and Meteor Showers
1. A meteor is the phenomenon of a streak in the sky caused partially by the burning of a rock or dust particle as it falls into our atmosphere.
2. A meteoroid is an interplanetary chunk of matter smaller than an asteroid.
3. A meteorite is an interplanetary chunk of matter after it has hit a planet or moon.
4. The first confirmation of a fall of rocks from an exploding meteor occurred in France in 1803.
Meteors
1. A fireball is an extremely bright meteor.
2. A meteoroid’s typical speed is 50 km/s (100,000 mph), so when it hits the Earth’s atmosphere, it heats up and begins to vaporize.
3. A typical meteor begins to glow at a height of 130 km (80 mi) and burns out at about 80 km (50 mi).
Meteoroids
1. Most meteors are produced by meteoroids with masses ranging from a few milligrams (grain of sand) to a few grams (marble-size rock).
2. Since a meteor can be seen only if it is within 150–200 km of the viewer, it is calculated that over the entire Earth there must be about 25 million meteors a day visible to the naked eye.
3. It is estimated that 1,000 tons of meteoritic material hit the Earth every day.
4. It is estimated that only 1 in 1 million meteoroids that hit the atmosphere survives to reach the surface.
5. Unlike most asteroids, meteoroids may orbit the Sun in any orientation.
6. It is thought that many small meteoroids are debris from asteroid collisions.
7. Many meteors come from material evaporated from a comet’s nucleus.
Meteor Showers
1. A meteor shower is the phenomenon of a large group of meteors seeming to come from a particular area of the celestial sphere. It is actually caused by the Earth passing through a swarm of small meteoroids.
2. Meteor showers are named after the constellation from which they seem to originate.
3. The radiant of a meteor shower is the point in the sky from which the meteors of a shower appear to radiate.
4. Most of the major meteor showers are associated with comets.
5. Some showers change in intensity from year to year because the particles that cause the shower clump together in one region of the comet’s orbit.
6. The best time to observe a meteor shower is in the early morning hours. This results from the Earth’s rotation and its motion through the swarm of particles left behind by the comet.
10-7 Meteorites and Craters
1. Meteorites are classified into 3 categories:
(a)Irons—iron meteorites that are made up of 80%–90% iron (with some nickel).
(b)Stones—stony meteorites that can contain flakes of iron and nickel.
(c)Stonyirons—meteorites that are half stone and half iron.
2. About 95% of all meteorites are stones but if you find one it will likely be an iron.
3. Chondrites comprise an important class of stony meteorites; they make up 91% of the about 24,000 known meteorites. They formed early in the history of the solar system and they are thought to be the building blocks of the planets. They consist mostly of chondrules, small rock spheres in a mix of other mineral or metal grains.
4. Carbonaceous chondrites are the most important subclass of chondrites. They contain high levels of water, organic compounds and minerals.
5. The Hoba meteorite in Namibia weighs 65 tons and is the largest meteorite ever found.
6. The second largest (34 tons) is on display in New York City at the American Museum of Natural History.
7. One of the most prominent impact craters on Earth is Meteor Crater near Winslow, Arizona. It is nearly a mile across, 180 m (600 ft) deep and has a rim rising 45 m (150 ft) above the surrounding desert.
8. The meteorite that formed Meteor Crater is estimated to have had a total mass of 300 million kg (300,000 tons) and to have been about 45 m across. It struck about 25,000 years ago at a speed of about 13 km/s (28,600 mph), releasing an energy equivalent to a 2.5-megaton bomb.
9. It is estimated that a meteorite larger than 1 km in diameter strikes the Earth on average once every few hundred thousand years.
10. A hit by a 1-km meteorite would produce a crater 10-km in diameter and be equivalent to a 5000-megaton bomb.
11. There is compelling evidence that an asteroid some 10 km in diameter struck the Earth (near the Yucatan peninsula) 65 million years ago and led to the subsequent extinction of the dinosaurs.