Chapter 14: Solar System Debris Asteroids, Comets, and Meteorites Solar System Debris
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Solar System Debris Debris: (1) scattered remains of something broken or destroyed; (2) accumulation of fragments of rock. Solar system: 1 large object several medium-sized objects lots of debris asteroids, meteoroids, comets, dust What is this debris? The classification of is a matter of minor controversy. Traditionally, the solar system has been divided into planets (the big bodies orbiting the Sun), their satellites (variously sized objects orbiting planets), asteroids (small dense objects orbiting the Sun) and comets (small icy objects with highly eccentric orbits). Unfortunately, the solar system has been found to be more complicated than this would suggest: there are several small moons that are probably captured asteroids; comets sometimes fizzle out and become indistinguishable from asteroids; the Kuiper Belt objects and others like Chiron don't fit this scheme well; Pluto/Charon system sometimes considered "double planet“ and like Kupier Belt objects. How much debris is out there and where is it? Most asteroids have been found orbiting the Sun between Mars and Jupiter, but they also share orbits with planets and cross into the inner solar system. Mass of all known asteroids < 1/10 mass of Moon. Most comets have been found to orbit the Sun in two regions beyond the orbit of Neptune. Kupier Belt, 30-100 AU, never inside orbits of jovian planets Oort cloud, up to 50,000 AU from Sun Mass of all comets probably comparable to mass of terrestrial planets. Why Debris is Important? Early history of life on Earth pieced together from fossil record. To study early history of solar system, need cosmic fossils - materials that have remained relatively unchanged since the solar system was very young. planets: melted, battered by impacts, tectonically active, and altered Look to smaller objects, asteroids, and comets for clues. Asteroids Asteroids are believed to be left over from the beginning of the solar system 4.6 billion years ago. Asteroids Rocky and metallic objects too small to be considered planets. Range in size from Ceres (diameter of ~1000 km), down to objects a few centimeters or less across. Name asteroids, meaning "star-like", derives from the fact that they are more star-like in appearance than comets. Asteroids: Discovery Too small be seen without a telescope. Ceres, largest of asteroids, orbiting Sun at 2.8 AU was discovered first by Giovanni Piazzi in 1801. He was searching for the missing planet predicted to be between Mars and Jupiter by Titius-Bode law. In next 6 years, three more objects found in region. Currently, more that 10,000 asteroids have well-determined orbits. Each given a number for order of discovery & a name. Most orbits lie between those of Mars and Jupiter Asteroids: Formation Believed to represent material left over from formation of solar system. Although sometimes suggested that asteroids are remains of a planet that was destroyed in a massive collision, it is more likely that they represent material that never coalesced into a planet. Highest concentration of asteroids in asteroid belt, the region lying between orbits of Mars and Jupiter. Likely that origin of the asteroid belt is linked to gravitational perturbation by Jupiter, which kept these planetisimals from coalescing into larger bodies. Asteroids are classified into types according to their spectra (and hence their chemical composition) Asteroids: Classes Compared S-type Appear bright (reflectivity 15-20%) Predominately silicate materials C-type (e.g., Ceres and Pallas) Appear very dark (reflectivity 3-4%) Carbon–rich silicate materials composition thought to be similar to the Sun, depleted in hydrogen, helium, and other volatiles. M-type (e.g., Psyche) Relatively bright (reflectivity 10-18%) Metals like iron and nickel Rare Mathilde: C-type asteroid Vesta: An Unusual Asteroid HST resolves features as small as 50 miles across, allowing astronomers to map Vesta's geologically diverse terrain. The surface is a complex record of Vesta's four billion-year history. Features include ancient lava flows, and a gigantic impact basin that is so deep, it exposes the asteroid's subsurface, or mantle. Asteroids: Abundance & Location Not all classes of asteroids are equally abundant and different classes of asteroids found at different distances from Sun. 17% S-type dominate inner belt region 75% C-type dominate outer belt region 8% other types found in middle belt region Known Asteroids Asteroids: Orbits about the Sun Asteroids orbit the Sun in many regions at different distances. They are often grouped by characteristics of their orbits. Asteroid Belt: between Mars and Jupiter Trojan asteroids: co-orbital with Jupiter Apollo and Aten asteroids: Earth-crossing asteroids Amor asteroids: Mars-crossing asteroids Other asteroids The Asteroid Belt All orbit Sun in west-to-east direction (same as planets). Most orbits lie near plane of ecliptic. The asteroid belt defined as region containing asteroids with semi-major axes in the range of 2.2-3.3 AU. Asteroids in belt take 3.3-6 years to orbit Sun. Contains 75% of known asteroids. Spacing of asteroids in belt ~ several million km. Many classified into families - groups with similar orbital and physical characteristics . Asteroids: Size and Location > 100,000 asteroids lie in the asteroid belt. Asteroids differ from planets in both their orbits and their size. generally move on quite eccentric trajectories, few are >300 km in diameter, and most are far smaller (as small as 1/10 km across). Taken together, mass of known asteroids amounts to < 1/10 mass of Moon. flew by main-belt asteroids. Galileo Gaspra S-type 7 hour rotation period 16 x 11 x 10 km, irregular shape sparse crater count implies 200 million years old Ida larger S-type more heavily cratered, ~1 billion years old satellite, Dactyl, 1.5 km diameter period = 24 hours; orbital distance = 100 km Ida’s density ~ 2.5 g/cm3 Phobos images of Phobos Mars Global Surveyor Asteroids: Orbital Characteristics The Asteroid Belt Most asteroids orbit in a zone between the orbits of Mars and Jupiter called the Asteroid Belt Ranges 2 - 3.5 A.U. from the Sun. About 5,000 orbits have been calculated, but 100,000 asteroids may exist. Orbits are elliptical but nearly circular and near the plane of the ecliptic. Their orbits are very similar to the planets' orbits. The Kirkwood gaps are found in the orbits of belt asteroids and are formed by Jupiter's strong gravitational influence. Orbits of other Asteroids Asteroids: Orbital Characteristics Trojan Asteroids Found in the same orbit with Jupiter, but are 60o ahead and behind the planet. They are stable positions in Jupiter's orbit where the gravity of the Sun and Jupiter cancel. Such positions called Lagrange points. Apollo Asteroids Orbits cross Earth's orbit. About 50 known Apollo asteroids, but may be as many as 1,000. All potential "Earth-colliders". Eros is an example. It is about 30 kilometers across. Asteroids: Orbital Characteristics Other asteroid groups A few stray asteroids have been found that lie completely outside of the asteroid belt. Chiron is the most famous example. Its orbit carries it between Saturn and Uranus. Chiron may actually be a dormant comet that has lost most of its volatiles. When it is closest to the Sun, a very diffuse atmosphere forms around it. If Chiron is a comet, it is the largest one known with a diameter of about 180 kilometers. Chiron Chiron is unusual has a detectable coma, indicating that it is a cometary body, over 50,000 times the characteristic volume of a comet, a size more commensurate with a large asteroid its curious orbit is unstable on time scales of a million years, indicating that it hasn't been in its present orbit long. Mt. Wilson Observatory - California 02 April 1995 Charles Morris, TIE telescope Centaurs Chiron was the first of four bodies discovered so far with similar orbits and properties. These bodies have been designated Centaurs, after the race of half-man/half-horse beings from Greek mythology, in recognition of their dual comet/asteroid nature. It is believed that the Centaurs may be objects which have escaped from the Kuiper belt. Kirkwood Gaps Distribution of asteroid semi-major axes shows some prominent gaps caused by resonances with Jupiter's orbital motion. These are known as the Kirkwood Gaps . Asteroid in resonance with Jupiter receives a strong gravitational tug from planet each time they are close together. If asteroid's period is in a well-defined ratio with that of Jupiter’s, the effects reinforce each other. Trojan Asteroids Although most asteroids orbit in the main belt, 2-3 A.U. from the Sun, an additional class of asteroids orbit at the distance of Jupiter and are called the Trojan asteroids. Trojan asteroids are locked into a 1:1 orbital resonance with Jupiter. Several hundred such asteroids are now known; it is estimated that there may be a thousand or more altogether. Trojan Asteriods: Lagrange Points There are exactly five places in the solar system where a small body can orbit the Sun in synchrony with Jupiter. These places are known as the Lagrange points of Jupiter's orbit. All five Lagrange points revolve around the Sun at the same rate as Jupiter. Near Earth Asteroids (NEAs) Three groups: : semi-major axes <1.0 AU; aphelion distance >0.983 AU Atens :semi-major axes >1.0 AU; perihelion distance <1.017 AU Apollos : perihelion distances between 1.017 and 1.3 AU and only cross Mars’ orbit Amors NEAs are asteroids that closely approach the Earth. Most are small, diameter < 1 km Earth-Crossing Asteroids Certain groups of asteroids have elliptical orbits that cross the orbit of Earth and other inner planets. About 300 asteroids are known to cross Earth's orbit. However, about 1500 unknown NEO’s are estimated to exist. The good news is that none of the known asteroids will strike the Earth. The bad news is that we have discovered only a fraction of the total number of Earth-crossing asteroids, so there are many for which we do not know the orbit. Animation of Aten asteriod Near Earth Asteroid Rendezvous NEAR spacecraft launched in 1996. Goals fly by Mathilde, C-type in main belt gravity measurements yield density of 1.6 g/cm3 match orbit with Eros, a C-type asteroid and largest of Earth-approaching asteroids map surface determine surface composition and density Mathilde and Rhode Island Close to Earth:Binary Asteroids Binary asteroids -- two rocky objects orbiting about one another appear to be common in Earth-crossing orbits. If one is ever found headed our way, it could be double trouble. http://www.jpl.nasa.gov/releases/2002/release_2002_83.html Another near miss. http://www.jpl.nasa.gov/releases/2002/release_2002_79.html ‘Falling Stars’ Meteoroids, Meteors, & Meteorites Meteoroids are simply smaller versions of asteroids. may be chunks that have been broken off asteroids by impacts. Meteors are streaks of light across the sky caused by a meteoroid entering the Earth's upper atmosphere and burning up in the process. Sometimes called "shooting" or "falling stars". Typically, 5 or 6 meteors are visible per hour across the sky (sporadic meteors). Sometimes a portion of a large meteoroid may survive its passage through the atmosphere and reach the Earth's surface. This rock is called a meteorite. Meteorites provided astronomers with the first good estimate of the age of the Solar System. Radiometric dating of meteorites gives them an age of about 4.5 billion years. Meteorites Meteorites are bits of the solar system that have fallen to the Earth. most come from asteroids, including few are believed to have come specifically from Vesta; a few probably come from comets a small number of meteorites have been shown to be of lunar (23 finds) or Martian origin (22). Types of Meteorites Iron Primarily iron and nickel; similar to type M asteroids Stony Iron Mixtures of iron and stony material like type S asteroids Chondrite by far the largest number of meteorites fall into this class; similar in composition to the mantles and crusts of the terrestrial planets Carbonaceous Chondrite Achondrite similar to terrestrial basalts; the meteorites believed to have originated on the Moon and Mars are achondrites Martian Meteor Meteor Showers At certain times of the year, many more meteors are observed to be radiating from a particular point in space. These so-called meteor showers are now known to be associated with comet orbits. When the Earth crosses the "dusty" trail of a comet, many more meteors per hour can be observed. There are several major meteor showers each year. Comet Debris Meteor Shower Radiant Leonids, 1998 Literature and Astronomy Meteor showers can be very impressive. Samuel Taylor Coleridge's famous lines from The Rime of the Ancient Mariner may have been inspired by the Leonid meteor shower that he witnessed in 1797. The upper air burst into life! And a hundred fire-flags sheen, To and fro they were hurried about! And to and fro, and in and out, The wan stars danced between. And the coming wind did roar more loud, And the sails did sigh like sedge; And the rain poured down from one black cloud; The Moon was at its edge . Meteorite Hunting Impact Sites Antarctica Meteor Craters The End? Predicted Annual Number of Meteorite Falls
Educated guesses about consequences of impacts of various sizes: from 'The Impact Hazard', by Morrison, Chapman and Slovic, published in Hazards due to Comets and Asteroids
Impactor Diameter (meters) irons make craters like Meteor Crater; stones produce airbursts like Tunguska; land impacts destroy area size of city irons,stones hit ground; comets produce airbursts; land impacts destroy area size of large urban area (New York, Tokyo) land impacts destroy area size of small state; ocean impact produces mild tsunamis land impacts destroy area size of moderate state (Virginia); ocean impact makes big tsunamis land impact raises dust with global implication; destroys area size of large state (California, France) Kinetic Energy : 1/2 mv2 m = V 1 megaton = 4 x 1016 joules Comets Comets: small bodies made out of dust and ices ("dirty snowballs"). The term "comet" derives from the Greek aster kometes, which means "long-haired star"---a reference to the tail. Since the observations of Tycho Brahe, comets are known to be members of the Solar System well beyond Earth's atmosphere. Most are on long elliptical orbits (perhaps parabolic in some cases) that take them from the outer reaches of the Solar System to the vicinity of the Sun. When they come near the Sun they are heated and emit gases and dust that are swept by the Solar Wind into the characteristic tail that always points away from the Sun. Comets Dirty snowballs Long term comets most in Oort cloud (up to 50,000 AU from Sun) normally orbit far from the Sun, very few enter planetary region of solar system highly elongated orbits not confined to ecliptic, all orbital inclinations prograde and retrograde orbits roughly uniform distribution Short term comets (periods < 200 years) most originate in region beyond Neptune called Kupier belt approximately circular, prograde orbits 30-100 AU normally orbit outside jovian planets, occasionally kicked into inner solar system Parts of a Comet Comets have several distinct parts when near the Sun and active: nucleus relatively solid and stable, mostly ice and gas with a small amount of dust and other solids coma dense cloud of water, carbon dioxide and other neutral gases sublimed from the nucleus hydrogen cloud huge (millions of km in diameter) but very sparse envelope of neutral hydrogen dust tail up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases; most prominent part of a comet to the unaided eye ion tail as much as several hundred million km long composed of plasma; laced with rays and streamers caused by interactions with the solar wind. Comet Tails Tails of bright comets can be 150 million kilometers (1 AU) in length, making them the "largest" objects in the Solar System. Many comets have two tails: gas tail (or ion tail) composed of ions blown out of the comet away from the Sun by the solar wind, and dust tail composed of dust particles liberated from the nucleus as the ices are vaporized. Comet Orbits Comets interact gravitationally with the Sun and other objects. Their motion is also influenced to some degree by gases jetting out of them, so their orbits are not completely determined by gravity. Most comets’ orbits appear to be elliptical, or in some cases parabolic. The most common comets are called short-period comets that have only mildly elliptical orbits that carry them out to a region lying from Jupiter to beyond the orbit of Neptune. These are normally seen only with telescopes. Comets visible to the naked eye are rare and are thought to come from a great spherical cloud of cometary material surrounding the Solar System called the Oort Cloud. Comet Halley English astronomer Edmund Halley used Newton's new theory of gravitation to determine the orbits of comets from their recorded positions in the sky as a function of time. He found that the bright comets of 1531, 1607, and 1682 had the same orbits, and concluded that these were different appearances of the same comet. He used his calculations to predict the return of this comet in 1758. If one traces back in the historical records for recordings of bright comets and their positions in the sky, it can be concluded that Comet Halley has been observed periodically as far back as 240 B.C. Comet Shoemaker-Levy 9 In July of 1994, fragments of Comet Shoemaker-Levy 9 impacted the planet Jupiter. The points of impact could be observed by the Galileo spacecraft. Unique? Crater chain on surface of Earth’s Moon CONTOUR CONTOUR - Comet Nucleus Tour NASA Discovery mission. Planned launch July 1, 2002. Mission to visit two comets. Encke (3.2 year period inside orbit of Jupiter) Schwassmann-Wachmann 3 Will intersect orbits when comets are near Sun and most active. Comets, Asteroids, and Meteors
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