Chapter 14 Solar System Debris This typical asteroid, Itokawa, was photographed in 2005 by the uncrewed Japanese spacecra< Hayabusa. Orbi?ng between Mars and Jupiter, the asteroid resides some 300 million kilometers from Earth and resembles a potato about 500 meters long and 300 meters wide. The goal of this space mission is to decipher the terrain, composi?on, and trajectory of asteroids and other nearby cosmic debris, especially given the poten?al threat they pose to life on Earth. (JAXA)
Units of Chapter 14 14.1 Asteroids What Killed the Dinosaurs? 14.2 Comets 14.3 Beyond Neptune 14.4 Meteoroids
14.1 Asteroids Asteroids are quite small, and most have eccentric orbits in the asteroid belt between Mars and Jupiter. The inset shows Ceres, the largest known asteroid. Figure 14-1. Inner Solar System (a) The main asteroid belt, along with the orbits of Earth, Mars, and Jupiter (drawn obliquely, that is neither face- on nor edge- on). Note the Trojan asteroids at two locafons in Jupiter s orbit. Some Apollo (Earth- crossing) and Amor (Mars- crossing) orbits are shown. (We will learn more about these classes of asteroids later in the chapter.) (b) An ultraviolet image of the largest known asteroid, the dwarf planet Ceres, as seen by the Hubble telescope. LiNle surface detail is evident, although image processing reveals what seems to be a large impact crater some 250 km across near the center of the frame. (NASA/SWRI)
14.1 Asteroids Asteroids are rocky; over 500,000 have been identified so far Three largest Ceres Pallas Vesta Diameter 940 km 580 km 540 km
14.1 Asteroids Vesta shows evidence of volcanism; the reason is not understood Asteroids are classified in types: C-type: carbonaceous, dark S-type: silicate (rocky) M-type: metallic; iron and nickel
14.1 Asteroids Two small S-type asteroids, Gaspra and Ida, were visited by the Galileo probe. Gaspra (left) is in false color; it is really gray. Note that Ida (right) has a small moon, Dactyl. Figure 14-2. Gaspra and Ida (a) The S- type asteroid Gaspra, as seen from a distance of 1600 km by the space probe Galileo on its way to Jupiter. (b) The S- type asteroid Ida, photographed by Galileo from a distance of 3400 km. (Ida s moon, Dactyl, is visible at right.) The resolufon in these photographs is about 100 m. True- color images show the surfaces of both bodies to be a fairly uniform shade of gray. Sensors on board the spacecraw indicated that the amount of infrared radiafon absorbed by these surfaces varies from place to place, probably because of variafons in the thickness of the dust layer blankefng them. (NASA)
14.1 Asteroids The NEAR spacecraft visited the C-type asteroid Mathilde, on its way to its main target, Eros. Mathilde, like many other asteroids, has a very low density and is probably not solid. Figure 14-3. Asteroid Mathilde The C- type asteroid Mathilde, imaged by the NEAR spacecraw en route to the near- Earth asteroid Eros. Mathilde measures some 60 50 km and rotates every 17.5 days. The largest craters visible in this image are about 20 km across much larger than those seen on Gaspra or Ida. The reason may be this asteroid s low density (approximately 1400 kg/m3) and rather sow composifon. (JHU/NASA)
14.1 Asteroids Eros does seem to be solid Figure 14-4. Asteroid Eros A mosaic of detailed images of the asteroid Eros, as seen by the NEAR spacecraw (which actually landed on this asteroid). Craters of all sizes, ranging from 50 m (the resolufon of the image) to 5 km, pit the surface. The inset shows a close- up image of a young secfon of the surface, where loose material from recent impacts has apparently filled in and erased all trace of older craters. (JHU/NASA)
14.1 Asteroids Some asteroids have orbits so eccentric that they cross Earth s orbit. They are called Apollo asteroids and raise the concern of a possible collision. 6500 such asteroids have been discovered so far, of which about 1000 have been designated as potentially hazardous, due to their size.
14.1 Asteroids Some asteroids, called Trojan asteroids, orbit at the L 4 and L 5 Lagrangian points of Jupiter s orbit Figure 14-6. Lagrangian Points The Lagrangian points of the Jupiter Sun system, where a third body could orbit in synchrony with Jupiter on a circular trajectory. Only the L4 and L5 points are stable. They are the locafons of the Trojan asteroids (see Figure 14.1).
Discovery 14-1: What Killed the Dinosaurs? Asteroid impact? Possibly Time scale is about right Evidence exists for impact crater of proper age, and iridium layer indicates asteroid Did asteroid cause extinction out of the blue, accelerate ongoing extinction, or?
14.2 Comets Comets that come close enough to the Sun to be detectable from Earth have very eccentric orbits Figure 14-7. Distant Orbit Comets move in highly eccentric paths that carry them far beyond the known planets.
14.2 Comets Comets have a very small nucleus, a coma of gas and dust that is the most visible part and can be very large, a hydrogen envelope, a dust tail, and an ion tail Figure 14-8. Comet Structure (a) Diagram of a typical comet, showing the nucleus, coma, hydrogen envelope, and tail. The tail is not a sudden streak in Fme across the sky, as in the case of meteors or fireworks. Instead, it travels along with the rest of the comet (as long as the comet is sufficiently close to the Sun for the tail to exist). Note how the invisible hydrogen envelope is usually larger than the visible extent of the comet; it is owen even much larger than drawn here. (b) Halley s Comet in 1986, about 1 month before it rounded the Sun at perihelion. (NOAO)
14.2 Comets The comet s tail always points away from the Sun, due to the solar wind. The ion tail is straighter than the dust tail. Figure 14-9. Comet Tails (a) A comet with a primarily ion tail. Called comet Giacobini Zinner and seen here in 1959, its coma measured 70,000 km across; its tail was well over 500,000 km long. (b) Photograph of a comet having both an ion tail (dark blue) and a dust tail (white blue), both marked in the inset, showing the gentle curvature and inherent fuzziness of the dust. This is comet Hale Bopp in 1997. At the comet s closest approach to the Sun, its tail stretched nearly 40 across the sky. (U.S. Naval Observatory; Aaron Horowitz/Corbis)
14.2 Comets The comet s tail develops as it approaches the Sun and disappears as it moves away from the Sun. The ion tail always points away from the Sun; the dust tail curves a bit as the comet gets ahead of it in its orbit. Figure 14-10. Comet Trajectory As it approaches the Sun, a comet develops an ion tail, which is always directed away from the Sun. Closer in, a curved dust tail, also directed generally away from the Sun, may appear. NoFce that the ion tail always points directly away from the Sun on both the inbound and the outgoing porfons of the orbit. The dust tail has a marked curvature and tends to lag behind the ion tail. (Compare this figure with a photo of a real comet, for example Figure 14.9.)
14.2 Comets Halley s Comet is one of the most famous; it has a period of 76 years and has been observed since antiquity. Its most recent visit, in 1986, was not spectacular. Left: The comet in 1910, as seen with the naked eye Right: The comet in 1986, as seen through a telescope Figure 14-11. Halley s Comet (a) Halley s comet as it appeared in 1910. Top, on May 10, with a 30 tail, bonom, on May 12, with a 40 tail. (b) Halley on its return and photographed with higher resolufon on March 14, 1986. (Caltech; Mt. Stromlo and Siding Springs Observatories)
14.2 Comets Halley s Comet has a shorter period than most comets, but its orbit is not in the plane of the solar system, probably due to an encounter with a larger object Figure 14-12. Halley s Orbit Halley s comet has a smaller orbital path and a shorter period than most comets, but its orbital orientafon is not typical of a short- period comet. SomeFme in the past, this comet must have encountered a jovian planet (probably Jupiter itself), which threw it into a Fghter orbit that extends not to the Oort cloud, but merely a linle beyond Neptune. Edmund Halley applied Newton s law of gravity to predict this comet s return.
14.2 Comets Typical cometary mass: 10 12 to 10 16 kg Each trip close to the Sun removes some material; Halley s Comet, for example, is expected to last about another 40,000 years Sometimes a comet s nucleus can disintegrate violently
14.2 Comets The Stardust mission flew through the tail of comet Wild-2, gathering dust particles in detectors made of aerogel and returning them to Earth for analysis Figure 14-14. Stardust at Wild- 2 (a) The Stardust spacecraw captured this image of comet Wild- 2 in 2004, just before the craw passed through the comet s coma. (b) Onboard is a detector made of a foamlike gel (called aerogel) that is 99.8% air, yet is strong enough to stop and store cometary dust parfcles as they hit the spacecraw. (c) Upon return of the craw to Earth in 2006, analysis began of the minute tracks in the aerogel, the ends of which contain captured comet dust fragments. (NASA)
14.2 Comets The Deep Impact mission slammed a projectile into comet Tempel 1 and studied the material expelled in order to analyze the composition of the comet Figure 14-15. Deep Impact This image captures the moment of impact of a projecfle sent into Comet Tempel 1 by the Deep Impact mothership. (JPL)
14.2 Comets Most comets that enter the inner solar system reside in the Kuiper belt outside the orbit of Neptune. Occasionally a comet from the far larger Oort cloud wanders into the inner solar system as well. Figure 14-16. Comet Reservoirs (a) Diagram of the Oort cloud, showing a few cometary orbits. Most Oort cloud comets never come close to the Sun. Of all the orbits shown, only the most elongated ellipse represents a comet that will actually enter the solar system (which, on the scale of this drawing, is much smaller than the overlaid box at the center of the figure) and possibly become visible from Earth. (See also Figure 14.7.) The magnified inset at right displays more clearly the Kuiper belt, source of short- period comets, whose orbits tend to hug the plane of the eclipfc.
14.3 Beyond Neptune Pluto was discovered in 1930. It was thought to be needed to explain irregularities in the orbits of Uranus and Neptune, but it turned out that there were no such irregularities.
14.3 Beyond Neptune Pluto s orbit is eccentric and inclined to the plane of the ecliptic; it also crosses the orbit of Neptune Figure 14-17. Neptune and Pluto The orbits of Neptune and Pluto cross, although Pluto s orbital inclinafon and a 3:2 resonance prevent the planets from actually coming close to each other. Between 1979 and 1999, Pluto was actually inside Neptune s orbit.
14.3 Beyond Neptune Pluto s large moon, Charon, was discovered in 1978. It is orbitally locked to Pluto, and about a sixth as large. The additional small moons are named Nix and Hydra. Figure 14-18. Pluto and Charon (a) The photograph from which Pluto s moon, Charon, was discovered. The moon is the small blotch of light at the top right porfon of the image. The larger blob of reflected sunlight is Pluto itself. (b) The Pluto Charon system, shown to the same scale and bener resolved than in part (a), as seen by the Hubble Space Telescope. The angular separafon of the planet and its moon is about 0.9. The two addifonal small moons shown here are called Nix and Hydra. (U.S. Naval Observatory; NASA)
14.3 Beyond Neptune Observations of eclipses of Pluto and Charon allowed measurement of orbital details Figure 14-19. Pluto Charon Eclipses The orbital orientafon of Charon produced a series of eclipses between 1985 and 1991. ObservaFons of eclipses of Charon by Pluto and of Pluto by Charon have provided detailed informafon about the sizes and orbits of both bodies.
14.3 Beyond Neptune The Kuiper belt is outside the orbit of Pluto and has many icy chunks Current theory is that Pluto is the nearest, and largest, of these objects Figure 14-20. Pluto (a) A surface map of Pluto not a photograph, but rather a modeled view created by carefully combining 24 Hubble Space Telescope images with a mathemafcal descripfon of the surface. (NASA)
14.3 Beyond Neptune No objects have been observed in the Oort cloud it is simply too far away. However, some Kuiper belt objects (KBOs) have been observed over 1000 so far. Here are Pholus and Eris. Figure 14-23. Kuiper Belt Object (a) Some of the best available images of a Kuiper belt object. Known as Pholus, the object itself is the fuzzy blob (marked with an arrow) that changes posifon between one frame and the next. It may be almost 1000 km across and lies more than 40 AU from Earth. (b) The trans- Neptunian object Eris and its small moon Dysnomia (named awer the Greek goddess of discord and her daughter, goddess of chaos and lawlessness) were recently imaged in the infrared at the Keck Observatory in Hawaii. (LPL/Keck)
14.3 Beyond Neptune Comparison of several trans-neptunian objects with Earth and its moon Figure 14-22. Trans- Neptunian Objects Some large trans- Neptunian objects, including Pluto and the largest known, called Eris, with part of Earth and the Moon added for scale. Most diameters are approximate, as they have been esfmated from the object s observed brightness. (NASA; Caltech)
14.3 Beyond Neptune What happened to Pluto? In 2006, the International Astronomical Union (IAU) adopted an official definition of planet. (There had not been an official definition before.) A planet must 1. Orbit the Sun 2. Be massive enough that its gravity keeps it spherical 3. Clear its orbit of other debris Pluto does 1 and 2 but not 3.
14.4 Meteoroids On an average dark night, you can see a few meteors every hour. The flash is caused by heating; most meteors do not survive to reach the ground. Figure 14-24. Meteor Trails A bright streak called a meteor is produced when a fragment of interplanetary debris plunges into the atmosphere, heafng the air to incandescence. (a) A small meteor photographed against stars and the Northern Lights provide a stunning background for a bright meteor trail. (b) These meteors (one with a red smoke trail) streaked across the sky during the height of the Leonid meteor storm of November 2001. (P. Parviainen; J. Lodriguss)
14.4 Meteoroids Meteoroids are defined as being less than 100 m in diameter. Most of the smaller ones are the remnants of comets that have broken up. If the Earth s orbit intersects the comet s, meteor showers will occur every year on the same date, until the meteoroids have burned out. Figure 14-25. Meteor Showers A meteoroid swarm associated with a given comet intersects Earth s orbit at specific locafons, giving rise to meteor showers at certain fixed Fmes of the year. A porfon of the comet breaks up as it rounds the Sun, at the point marked 1. Fragments confnue along the comet s orbit, gradually spreading out (points 2 and 3). The rate at which the debris disperses around the orbit is much slower than depicted here. It actually takes many orbits for the material to disperse as shown, but eventually the fragments extend all around the orbit, more or less uniformly. If the orbit happens to intersect Earth s, the result is a meteor shower each Fme Earth passes through the intersecfon (point 4).
14.4 Meteoroids Here are the major meteor showers
14.4 Meteoroids Larger meteoroids are usually loners from the asteroid belt and have produced most of the visible craters in the solar system. The Earth has about 100 craters more than 0.1 km in diameter; erosion has made most of them hard to discern. One of the largest is in Canada. Figure 14-27. Manicouagan Reservoir This photograph, taken from orbit by the U.S. Skylab space stafon, shows the ancient impact basin that forms Quebec s Manicouagan Reservoir. A large meteorite landed there about 200 million years ago. The central floor of the crater rebounded awer the impact, forming an elevated central peak. The lake, 70 km in diameter, now fills the resulfng ring- shaped depression. (NASA)
14.4 Meteoroids Meteoroids that burn up in the Earth s atmosphere have densities of 500 to 1000 kg/m 3 and are probably comet like in composition. Meteoroids that reach the surface have densities around 5000 kg/m 3 and are similar to asteroids. Figure 14-29. Large Meteorites (a) The world s second largest meteorite, the Ahnighito, on display at the American Museum of Natural History in New York, serves as a jungle gym for curious children. This 34- ton rock is so heavy that the Museum floor had to be specially reinforced to support its weight. (b) The Wabar meteorite, discovered in the Arabian desert. Although small fragments of the original meteor had been collected more than a century before, the 2000- kg main body was not found unfl 1965. (Corbis- Blair; Jon Mandaville/Aramco World)
14.4 Meteoroids Most meteorites are rocky (left); some are iron (right) Figure 14-30. Meteorite Samples (a) A stony (silicate) meteorite owen has a dark fusion crust, created when its surface is melted by the tremendous heat generated during its passage through the atmosphere. The coin at the bonom is for scale. (b) Iron meteorites are much rarer than the stony variety and owen contain some nickel as well. Most show characterisfc crystalline panerns when their surfaces are cut, polished, and etched with acid. (Science Graphics)
Summary of Chapter 14 Most asteroids orbit in asteroid belt Total mass of all asteroids is less than mass of Earth s moon Asteroid types: S-type (silicate), M-type (metallic) and C-type (carbonaceous) A few asteroids are in Earth-crossing orbits Comets are icy and normally orbit far from the Sun
Summary of Chapter 14 Some comets have highly eccentric orbits and enter the inner solar system Most reside in the Oort cloud (cont.) The Kuiper belt is just beyond the orbit of Neptune; a number of Kuiper belt objects have recently been observed Comets begin to vaporize as they approach the Sun Comet nucleus is tiny, but coma and tails can be enormous, covering 30 40 of the sky
Summary of Chapter 14 (cont.) Meteors are the bright flashes of light from micrometeoroids hitting the atmosphere If a meteor lands on the Earth, it is called a meteorite Meteors that burn up in the atmosphere are mostly similar to comets; those that land are more like asteroids The gradual disintegration of a comet as it orbits the Sun leaves a meteoroid swarm; if the Earth encounters a swarm, we see a meteor shower