22 February 2005

Asteroids & Comets

AST 2010: Chapter 12 1

Debris of the Solar System

Asteroids

are rocky or metallic objects orbiting the Sun that are smaller than a major planet, but that show no evidence of an atmosphere and contain little volatile (easily evaporated) material

Comets

are icy bodies that revolve around the Sun and are smaller than a major planet, but that contain frozen water and other volatile materials 22 February 2005 AST 2010: Chapter 12 2

Discovery of Asteroids

Most asteroid orbits lie in the asteroid belt, between Mars and Jupiter Too small to be visible without a telescope First discovered when astronomers were hunting for a planet between Mars and Jupiter 1st discovered in the 1801 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 By 1890, more than 300 objects had been discovered More than 20,000 asteroids now have well determined orbits 22 February 2005 AST 2010: Chapter 12 3

Asteroid Nomenclature

Given a number and a name Names originally chosen from Greek/Roman goddesses; other female names; all names go!

Asteroids 2410, and 4859 named after Morrison and Fraknoi 22 February 2005

Mathilde

AST 2010: Chapter 12

Gaspra Ida

4

Asteroid Census

Total number of asteroids in the solar system very large Must be estimated on the basis of systematic sampling of the sky Studies indicate there are 106 asteroids with diameters greater than 1 km!

Largest: Ceres, with diameter of ~1000 km Pallas and Vesta have diameter of ~500 km 15 more larger than 250 km across 100 times more objects 10-km across than 100-km across Total mass of asteroids is less than the mass of the Moon 22 February 2005 AST 2010: Chapter 12 5

Asteroid Orbits

Revolve around the sun in west-to-east direction Most lie in or near the ecliptic Asteroid belt defined as region that contains all asteroids with semi-major axes of 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 22 February 2005 AST 2010: Chapter 12 6

Asteroid Families

The

families

are groups with similar orbital characteristics

Each family may have resulted from a breakup of a larger body, or from the collision of two asteroids Members of each family have similar speeds Physical similarities between largest asteroids of given families

Several dozen families are found

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Asteroid Physical Appearance

Majority: very dark Do not reflect much light Reflectivity ~3-4% Some: Sizable group Typical reflectivity ~ 15-20% (similar to Moon) Few: Reflectivity ~60% Understanding of the reasons for the above difference provided by spectral analysis 22 February 2005 AST 2010: Chapter 12 8

Asteroid Classification - 1

Dark asteroids

Believed to be primitive bodies Chemically unchanged since beginning of solar system Composed of silicates with dark organic carbon compounds Ceres, Pallas, and most objects in outer third of the belt

Most primitive asteroids part of class C

C stands for carbonaceous (carbon-rich) 22 February 2005 AST 2010: Chapter 12 9

Asteroid Classification - 2

Class S S stands for “stony” composition No dark carbons Higher reflectivity Most asteroids of this type believed to be also primitive Class M M stands for “metal” Identification difficult Done by radar for the largest asteroids such as Psyche 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 22 February 2005 AST 2010: Chapter 12 10

Asteroid Classification - 3

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Trojan Asteroids

Located far beyond main belt ~5.2 AU, nearly same distance as Jupiter Collectively called Trojans (from Homer’s Illiad) Have stable orbits because of Jupiter Two points in Jupiter’s orbit where asteroids can stay indefinitely 2 points make equilateral triangle with Jupiter and the Sun Since first discovery in 1906, several hundreds have been found Dark, primitive appear faint, but some are nonetheless sizeable 22 February 2005 AST 2010: Chapter 12 12

Asteroids in Outer Solar System

Many asteroids with orbits beyond Jupiter Examples: Chiron, just inside the orbit of Saturn, to almost the distance of Uranus Pholus, found in 1992, at 33 AU, red surface, of unknown composition Named after centaurs (half horse, half human) 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 Chiron is however much bigger than comets 22 February 2005 AST 2010: Chapter 12 13

Earth-Approaching Asteroids

1989 – a 200-m object passed within 800,000 km of the Earth 1994 – a 10-m object passed 105,000 km away Some of these objects have collided with the Earth in the past, and some are likely to do so again in the future Referred to as Near-Earth Objects (NEOs)

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NEOs

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 None of the known NEOs will end up crashing into the Earth in the foreseeable future… Larger impacts likely to generate environmental catastrophes A good argument for further investigation of NEOs 22 February 2005 AST 2010: Chapter 12 15

NEO observation

5-km NEO

Toutatis

approached the Earth at 3 million km in 1992 less than 3 times the distance to the Moon Radar images show it is a double object (two irregular lumps) 3- and 2-km objects squashed together 22 February 2005 AST 2010: Chapter 12 16

Comets

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 22 February 2005 AST 2010: Chapter 12 17

Comet Orbits

Scientific study of comets dates back to Newton who first recognized their orbits are elongated ellipses Edmund Halley (a contemporary of Newton) in 1705 calculated/published 24 cometary orbits Noted that the orbits of bright comets seen in 1531, 1607, and 1682 were quite similar — and could belong to 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 22 February 2005 AST 2010: Chapter 12 18

Comet Halley

It has been observed/recorded on every passage at intervals from 74 to 79 years since 239 B.C.

The period variations are caused by the jovian planets In 1910 the Earth was brushed by the comet’s tail, causing much needless public concern Its last appearance in our skies was in 1986 met by several spacecraft It is predicted to return in 2061 Its nucleus approximately 16x8x8 km 3 22 February 2005 AST 2010: Chapter 12 19

Comet Census

Records exist for ~1000 comets Comets are discovered at an average rate of 5 to 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) Comet Hale-Bopp (1997) 22 February 2005 AST 2010: Chapter 12 20

Comet Components (1)

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 22 February 2005 AST 2010: Chapter 12 21

Comet Components (2)

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Nucleus and Coma of Comet

The

nucleus

is composed of ancient ice, dust, and gaseous core material The nucleus has low gravity cannot keep dust and gas from escaping The

coma

of the comet, as seen from the Earth is the bright head The coma is a temporary atmosphere of gas and dust around the nucleus The coma is 100,000's of kilometers across Halley's nucleus Halley's coma 22 February 2005 AST 2010: Chapter 12 23

Ion Tail of Comet

Sun spews out charged particles, called the solar wind The solar wind travels along solar magnetic field lines extending radially outward from the Sun Ultraviolet (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 22 February 2005 AST 2010: Chapter 12 24

Dust Tail and Hydrogen Cloud of Comet

The

dust tail

the coma forms when solar photons collide with the dust in The 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 than the ions by the solar wind and do not reach the same high speeds as ions The

hydrogen cloud

from the nucleus is dissociated by UV sunlight into oxygen and hydrogen forms when water vapor ejected in the jets The hydrogen clouds can be tens of millions of kilometers across They are the largest things in the solar system! 22 February 2005 AST 2010: Chapter 12 25

Origin and Evolution of Comets

Comets originate from very great distances The aphelia of new comets are typically around 50,000 AU This clustering of aphelia was first noted by Dutch astronomer Jan Oort in 1950 He then proposed an idea for the origin of those comets, which is still accepted by most astronomers today

Oort’s model

or 1 LY of comet origin: A star’s sphere of influence extends a little beyond 50,000 AU, Objects in orbit about the Sun at this distance can be easily perturbed by passing stars The new comets are some of these objects whose orbits have been disturbed, bringing them much closer to the Sun The reservoir of ancient icy objects from which such comets are presumably derived is called the

Oort comet cloud

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22 February 2005 AST 2010: Chapter 12 27

Oort Comet Cloud

Astronomers estimate that there may be about a trillion (10 cloud the Sun 12 ) comets in the Oort cloud In addition, 10 times this number of comets could be orbiting the Sun between the planets and the Oort Such cometary objects remain undiscovered probably because they are too faint to be seen directly and because their stable orbit do not bring them closer to The total number of comets within the sphere of influence of our Sun could therefore be on the order of ten trillion (10 13 )! Their total mass would be similar to that of 1000 Earths Cometary material could thus be the most important constituent of the solar system after the Sun itself 22 February 2005 AST 2010: Chapter 12 28

The Kuiper Belt

Another possible source of comets lies just beyond the orbit of Neptune The existence of this region was first suggested by Gerard Kuiper in 1951 The first object from this region, now called the

Kuiper belt

, was discovered in 1992 The object is ~200 km across Since then, several hundred more

Kuiper-belt objects

(KBOs) have been found It appears that these KBO are heavily influenced by the gravity of Neptune Many of the known KBOs have orbits like that of Pluto Some astronomers have therefore suggested that Pluto can be considered the largest member of the Kuiper belt For this reason, KBOs are sometimes called

plutinos

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Fate of Comets

Most comets probably spend nearly all their existence in the Oort cloud or Kuiper belt at a temperature near absolute zero But once a comet enters the inner solar system, its life likely changes dramatically!

If it survives the initial passage near the Sun, it will return towards the cold aphelion

and may follow a fairly stable orbit for a “while”

It may impact the Sun It may be completely vaporized as it flies by the Sun It may interact with one or more planets with three possible fates:

It is destroyed after impacting a planet It gets speeded up and ejected, leaving the solar system forever It is perturbed into an orbit of shorter period

Each time a comet approaches the Sun, it loses part of its material A few comets end their life catastrophically by breaking apart

Shoemaker-Levy 9 broke into ~20 pieces when it passed close to Jupiter in July 1992 The fragments were then captured into a very elongated 2-year orbit around Jupiter, before crashing into it in July 1994

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Comet Shoemaker-Levy 9

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