Chapter 12 Comets and Asteroids Debris of the

Chapter 12: Comets and Asteroids: Debris of the Solar System April 18, 2006 Astronomy 2010 1

Discovery of Asteroids Most asteroid orbits lie in the asteroid belt – between Mars and Jupiter. n Too small to be visible without a telescope. n First discovered when astronomers were hunting for a planet between Mars and Jupiter n 1 st discovered in the 1801 n – – – Name: Ceres Distance from the Sun : 2. 8 AU Discoverer : Giovanni Piazzi Followed in subsequent years by the discovery of other small planets in similar orbits n By 1890, more than 300 objects had been discovered. n More than 10000 asteroids now have well determined orbits. n April 18, 2006 Astronomy 2010 2

Asteroid Nomenclature n Asteroids are given a number and a name n Names originally chosen from Greek/Roman goddesses; other female names; all names go! n Asteroids 2410, and 4859 named after Morrison and Fraknoi April 18, 2006 Astronomy 2010 3

Asteroid Census Total number of asteroids in the solar system very large. n Must be estimated on the basis of systematic sampling of the sky. n n n n Studies indicate there are 106 asteroids with diameters greater than 1 km! Largest: Ceres - Diameter: ~1000 km Pallas, and Vesta – Diameter: ~ 500 km 15 more larger than 250 km. 100 times more objects of 10 km size than 100 km. Total mass of asteroids is less than the mass of the Moon April 18, 2006 Astronomy 2010 4

Asteroid Orbits n n n n Revolve around the sun in west-toeast. Most lie in or near the ecliptic. Asteroid belt defined as region that contains all asteroids with semi -major axes 2. 2 to 3. 3 au. Periods: 3. 3 to 6 years. 75% of known asteroids in the main belt. Not closely spaced – typically >million km between them. Japanese astronomer K. Hirayama found in 1917 that asteroids fall into families. April 18, 2006 Astronomy 2010 5

Asteroid Families n Groups with similar characteristics n Each family may result from explosion of larger body (most likely by a collision) n In a family, asteroids have similar velocities n Several dozen families are found. n Physical similarities between largest asteroids of given families. April 18, 2006 Astronomy 2010 6

Asteroid Physical Appearance n Majority: very dark – Do not reflect much light. – Reflectivity ~ 3 -4%. n Some: – Sizable group – Typical reflectivity ~ 15 -20% (similar to Moon) n Few: – Reflectivity ~ 60% n Understanding of the reasons for the above difference provided by spectral analysis. April 18, 2006 Astronomy 2010 7

Asteroid Classification - 1 n Primitive bodies – – – Dark asteroids Chemically unchanged since beginning of Solar System Composed of silicates with dark organic carbon compounds Ceres, Pallas, and most object in outer third of the belt. Most primitive asteroids part of Class “C” § Where C stands for carbonaceous – carbon-rich April 18, 2006 Astronomy 2010 8

Asteroid Classification - 2 n Class “S” n S stands for “Stony” composition. n No dark carbons. n Higher reflectivity. n Most asteroids of this type believed to be also primitive. April 18, 2006 Astronomy 2010 9

Asteroid Classification - 3 Class “M” n M stands for “metal” n Identification difficult n – Done by radar for the largest asteroids such as Psyche. n n Much less numerous Suspected to originate from collision of a parent body that had previously differentiated. Enough metal in 1 -km M-type asteroid to supply the world with iron for a long period of time. Mines in Sudbury, ON, Canada originate from collision with class-M asteroid. April 18, 2006 Astronomy 2010 10

Trojan Asteroids Located far beyond main belt n ~ 5. 2 AU, nearly same distance as Jupiter n Unstable orbits because of Jupiter. n Two points on the orbit where asteroids can stay indefinitely. n – 2 points make equilateral triangle with Jupiter and the Sun – Collectively called trojans (Homer – Illiad) Discovered 1906 n Several hundreds found. n Dark, primitive, appear faint, but are nonetheless sizeable. n April 18, 2006 Astronomy 2010 11

Outer Solar System Asteroids Many asteroids with orbits beyond Jupiter n Example: n – Chiron, just inside the orbit of Saturn, to almost the distance of Uranus – Pholus (1992) 33 AU, red surface, of unknown composition. – Named after Centaurs (half horse, half human) § so named because these objects have some attributes of comets, and asteroids. 1988, on closest approach to the Sun, Chiron’s brightness doubled, much like the comets, which contain abundant volatile materials such as water ice, or carbon monoxide ice. n Chiron is however much bigger than comets. n April 18, 2006 Astronomy 2010 12

Earth-Approaching Asteroids n 1989 – a 200 -m object passed within 800000 km of the Earth. n 1994 – a 10 -m object passed 105000 km away. n Some of these objects have collided with the Earth in the past, some are likely to do so again in the future. n Referred to as Near-Earth Objects (NEOs) April 18, 2006 Astronomy 2010 13

Near Earth Objects (NEOs) n n 640 NEOs larger than 1 km located by the end of 2002 Actual population more likely to be > millions. Unstable orbits Fate: – Collide with our planet – and be destroyed – Be ejected from the Solar System Probability of impact – once every 100 million years. n None of the known NEOs will end up crashing into the Earth in the foreseeable future… n Larger impacts likely to generate environmental catastrophes n A good argument towards further investigation of NEOs. n April 18, 2006 Astronomy 2010 14

NEO observation n 5 -km NEO Toutatis, – approached the Earth at 3 million km in 1992 – less than 3 times the distance to the Moon n Radar images show it is a double object (two irregular lumps) 3 and 2 km objects squashed together. April 18, 2006 Astronomy 2010 15

Comets April 18, 2006 Astronomy 2010 16

Appearance of Comets n n n Observed since antiquity Typical comets appear as rather faint, diffuse spot of light – smaller than the Moon, and many times less brilliant. Small chunk of icy material that develop an atmosphere as they get closer to the Sun. As they get “very close” they may develop a faint, nebulous tail extending far from the main body of the comet. Appearance seemingly unpredictable Typically remain visible for periods from a few days to a few months. April 18, 2006 Astronomy 2010 17

Comet Orbits n n Scientific study of comets dates back to Newton who first recognized their orbits are elongated ellipses. Edmund Halley (a contemporary of Newton) calculated/published 24 cometary orbits (1705). Noted that the orbits of bright comets seen in 1531, 1607, 1682 were quite similar – and could be the same comet – returning to the perihelion every 76 years. Predicted a return in 1758. When the comet did appear in 1758, it was given the name Comet Halley. April 18, 2006 Astronomy 2010 18

Comet Halley n n Observed/Recorded on every passage at intervals from 74 to 79 years since 239 B. C. Period variations caused by Jovian planets 1910, Earth was brushed by the comet tail. – causing much public concern… Last appearance in our skies – 1986. – Met by several spacecrafts Return in 2061. n Nucleus approximately 16 x 8 x 8 kilometers. n April 18, 2006 Astronomy 2010 19

Comet Census n n n Records exist for ~1000 comets Comets are discovered at an average rate of 5 - 10 per year. Most visible only on photos made with large telescopes. Every few years, a comet appears that is bright enough to be seen with the naked eye. Recent flybys: – Comet Hyakutake, long tail, visible for about a month, March (1996) – Hale-Bopp (1997) April 18, 2006 Astronomy 2010 20

Comet Structure n n 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 off of 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; this is the most prominent part of a comet to the unaided eye ion tail: as much as several hundred million km long composed of plasma and laced with rays and streamers caused by interactions with the solar wind April 18, 2006 Astronomy 2010 21

Comet Structure ion tail dust tail April 18, 2006 Astronomy 2010 22

Nucleus and Coma n n n Nucleus: ancient ice, dust and gaseous core material nucleus has low gravity – cannot keep dust and gas from escaping Coma: the bright head of the comet – seen from the Earth. The coma is a temporary atmosphere of gas and dust around the nucleus. The coma is 100, 000's of kilometers across April 18, 2006 Astronomy 2010 halley's nucleus halley's coma 23

Ion Tail n n n Sun spews out charged particles, called the solar wind. The solar wind travels along solar magnetic field lines extending radially outward from the Sun. UV sunlight ionizes gases in the coma. These ions (charged particles) are pushed by solar wind particles along magnetic field lines to form the ion tail millions of kilometers long. The blue ion tail acts like a "solar" wind sock. The ion tail always points directly away from the Sun, because the ions move at very high speed. When the comet is moving away from the Sun, its ion tail will be almost in front of it! The blue color is mostly from the light emitted by carbon monoxide ions but other types of ions also contribute to the light. Since the gas is so diffuse, the observed spectrum is an emission-line spectrum. April 18, 2006 Astronomy 2010 24

Dust Tail and Hydrogen Cloud n n n The dust tail forms when solar photons collide with the dust in the coma. Ejected dust particles form a long, curved tail that lies slightly farther our from the Sun than the nucleus' orbit. The dust tail has a yellow-white color from reflected sunlight. Both of the tails will stretch for millions of kilometers. The dust tail curves gently away from comet’s head, because dust particles are more massive than individual ions. They are accelerated more gently by the solar wind and do not reach the same high speeds as ions. The hydrogen cloud forms when water vapor in the jets from the nucleus is dissociated by solar UV into oxygen and hydrogen. The hydrogen cloud can be tens of millions of kilometers across – the largest things in the solar system! All of this is coming from a dirty snowball the size of a city! April 18, 2006 Astronomy 2010 25

Stardust Mission April 18, 2006 Astronomy 2010 26

Origin and Evolution of Comets n n Originate from very great distances Aphelia of new comets ~ 50000 AU Clustering of aphelia first noted by Dutch astronomer Jan Oort (1950). Oort’s Comet Origin Model – Star’s sphere of influence extends a little beyond 50000 AU or 1 LY – Objects in orbit about the Sun at this distance can be easily perturbed by passing Stars. – Some perturbed object take on orbits that bring them much closer to the Sun. – Reservoir of ancient icy objects from which comets are derived is called Oort Comet Cloud. April 18, 2006 Astronomy 2010 27

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Oort Cloud n n n Estimated 1012 comets in the Oort cloud. 10 times this number of comets could be orbiting the Sun between the planets and the Oort cloud. Such objects undiscovered because to small, to reflect sufficient light to be detectable at large distances, and because their stable orbit do not bring them closer to the Sun. Total number of comets in the sphere of influence of our Sun could be of the order of 1013! Represents a mass the order of 1000 Earths. April 18, 2006 Astronomy 2010 29

Kuiper Belt Second source of comets just beyond the orbit of Pluto. n First object discovered in 1992. n – Diameter ~ 200 km. – Period ~ 300 years. n n n 60 objects found since then. Share orbital resonance with Neptune – two orbits completed for three by Neptune. Nicknamed Plutinos for this reason. Speculated that Pluto is the largest example of this group. They may share a common origin. April 18, 2006 Astronomy 2010 30

Fate of Comets n Comets spend nearly all their existence in the Oort cloud or Kuiper belt – At a temperature near absolute zero. n As comet enters the inner Solar System, their “life” changes altogether! – If they survive the initial passage near the Sun, they return towards the cold aphelia – and may follow a quasi-stable orbit for a “while”. – May impact the Sun – May be completely vaporized as they fly by the Sun – May interact with a planet § Final impact § Speed up and ejection § Perturbed into an orbit of shorter period. Each flyby the Sun reduces the size and mass of the nucleus of the comets. n Some comets end their life catastrophically by breaking apart. n – Shoemaker-Levy 9 broke into ~20 pieces when it passed close to Jupiter in July 1992. – Fragments of Shoemaker-Levy captured into a very elongated 2 year around Jupiter – In 1994 the comet fragments crashed into Jupiter. April 18, 2006 Astronomy 2010 31

Comet Shoemaker-Levy 9 April 18, 2006 Astronomy 2010 32

Impact! n April 18, 2006 Astronomy 2010 Recorded by HST-WFP in different wavelengths 33

Shoemaker-Levy 9 Impact Site April 18, 2006 Astronomy 2010 34

Sequence of Impact Sites The rotation of Jupiter left a trail of impacts. n We see the debris left in the upper atmosphere. n Debris fields would cover Earth! n April 18, 2006 Astronomy 2010 35

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Time Evolution of Debris Fields April 18, 2006 Astronomy 2010 37

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