april 18, 2006astronomy 20101 chapter 12: comets and asteroids: debris of the solar system

April 18, 2006April 18, 2006 Astronomy 2010Astronomy 2010 11

Chapter Chapter 12: 12:

Comets Comets andand

AsteroidsAsteroids: Debris : Debris of the of the Solar Solar SystemSystem


Discovery of AsteroidsDiscovery of Asteroids Most asteroid orbits lie in the asteroid belt Most asteroid orbits lie in the asteroid belt – between Mars and Jupiter.– between Mars and Jupiter.

Too small to be visible without a telescope.Too small to be visible without a telescope. First discovered when astronomers were First discovered when astronomers were hunting for a planet between Mars and Jupiter hunting for a planet between Mars and Jupiter

11stst discovered in the 1801 discovered in the 1801– Name: Ceres Name: Ceres – Distance from the Sun : 2.8 AUDistance from the Sun : 2.8 AU– Discoverer : Giovanni PiazziDiscoverer : Giovanni Piazzi

Followed in subsequent years by the discovery Followed in subsequent years by the discovery of other small planets in similar orbitsof other small planets in similar orbits

By 1890, more than 300 objects had been By 1890, more than 300 objects had been discovered.discovered.

More than 10000 asteroids now have well More than 10000 asteroids now have well determined orbits.determined orbits.


Asteroid NomenclatureAsteroid Nomenclature Asteroids are given a number and a Asteroids are given a number and a namename

Names originally chosen from Names originally chosen from Greek/Roman goddesses; other female Greek/Roman goddesses; other female names; all names go!names; all names go!

Asteroids 2410, and 4859 named Asteroids 2410, and 4859 named after Morrison and Fraknoiafter Morrison and Fraknoi


Asteroid CensusAsteroid Census Total number of asteroids in the solar system Total number of asteroids in the solar system very large.very large.

Must be estimated on the basis of systematic Must be estimated on the basis of systematic sampling of the sky.sampling of the sky.

Studies indicate there are Studies indicate there are 101066 asteroids with asteroids with diameters greater than 1 km!diameters greater than 1 km!

Largest: Ceres - Diameter: ~1000 kmLargest: Ceres - Diameter: ~1000 km Pallas, and Vesta – Diameter: ~ 500 kmPallas, and Vesta – Diameter: ~ 500 km 15 more larger than 250 km.15 more larger than 250 km. 100 times more objects of 10 km size than 100 100 times more objects of 10 km size than 100 km.km.

Total mass of asteroids is less than the mass Total mass of asteroids is less than the mass of the Moonof the Moon


Asteroid OrbitsAsteroid Orbits

Revolve around the sun in Revolve around the sun in west-to-east.west-to-east.

Most lie in or near the Most lie in or near the ecliptic.ecliptic.

Asteroid beltAsteroid belt defined as defined as region that contains all region that contains all asteroids with semi-major asteroids with semi-major axes 2.2 to 3.3 au.axes 2.2 to 3.3 au.

Periods: 3.3 to 6 years.Periods: 3.3 to 6 years. 75% of known asteroids in 75% of known asteroids in

the main belt.the main belt. Not closely spaced – Not closely spaced –

typically >million km typically >million km between them.between them.

Japanese astronomer K. Japanese astronomer K. Hirayama found in 1917 that Hirayama found in 1917 that asteroids fall into asteroids fall into families.families.


Asteroid FamiliesAsteroid Families Groups with similar characteristicsGroups with similar characteristics Each family may result from Each family may result from explosion of larger body (most explosion of larger body (most likely by a collision)likely by a collision)

In a family, asteroids have similar In a family, asteroids have similar velocitiesvelocities

Several dozen families are found.Several dozen families are found. Physical similarities between Physical similarities between largest asteroids of given largest asteroids of given families.families.


Asteroid Physical Asteroid Physical AppearanceAppearance

Majority: very dark Majority: very dark – Do not reflect much light.Do not reflect much light.– Reflectivity ~ 3-4%.Reflectivity ~ 3-4%.

Some: Some: – Sizable group Sizable group – Typical reflectivity ~ 15-20% (similar to Typical reflectivity ~ 15-20% (similar to Moon)Moon)

Few: Few: – Reflectivity ~ 60%Reflectivity ~ 60%

Understanding of the reasons for the Understanding of the reasons for the above difference provided by spectral above difference provided by spectral analysis.analysis.


Asteroid Classification - Asteroid Classification - 11

Primitive bodiesPrimitive bodies– Dark asteroidsDark asteroids– Chemically unchanged since beginning of Chemically unchanged since beginning of Solar SystemSolar System

– Composed of silicates with dark organic Composed of silicates with dark organic carbon compoundscarbon compounds

– Ceres, Pallas, and most object in outer Ceres, Pallas, and most object in outer third of the belt.third of the belt.

– Most primitive asteroids part of Class “C” Most primitive asteroids part of Class “C” Where C stands for carbonaceous – carbon-richWhere C stands for carbonaceous – carbon-rich



Class “S”Class “S” S stands for “Stony” composition.S stands for “Stony” composition. No dark carbons.No dark carbons. Higher reflectivity.Higher reflectivity. Most asteroids of this type Most asteroids of this type believed to be also primitive.believed to be also primitive.



Class “M”Class “M” M stands for “metal”M stands for “metal” Identification difficultIdentification difficult

– Done by radar for the largest asteroids such as Done by radar for the largest asteroids such as Psyche.Psyche.

Much less numerousMuch less numerous Suspected to originate from collision of a Suspected to originate from collision of a parent body that had previously parent body that had previously differentiated.differentiated.

Enough metal in 1-km M-type asteroid to Enough metal in 1-km M-type asteroid to supply the world with iron for a long period supply the world with iron for a long period of time.of time.

Mines in Sudbury, ON, Canada originate from Mines in Sudbury, ON, Canada originate from collision with class-M asteroid.collision with class-M asteroid.


Trojan AsteroidsTrojan Asteroids Located far beyond main beltLocated far beyond main belt ~ 5.2 AU, nearly same distance as Jupiter~ 5.2 AU, nearly same distance as Jupiter Unstable orbits because of Jupiter.Unstable orbits because of Jupiter. Two points on the orbit where asteroids Two points on the orbit where asteroids can stay indefinitely.can stay indefinitely.– 2 points make equilateral triangle with Jupiter 2 points make equilateral triangle with Jupiter and the Sunand the Sun

– Collectively called trojans (Homer – Illiad)Collectively called trojans (Homer – Illiad) Discovered 1906-Discovered 1906- Several hundreds found.Several hundreds found. Dark, primitive, appear faint, but are Dark, primitive, appear faint, but are nonetheless sizeable. nonetheless sizeable.


Outer Solar System Outer Solar System Asteroids Asteroids Many asteroids with orbits beyond JupiterMany asteroids with orbits beyond Jupiter

Example:Example:– Chiron, just inside the orbit of Saturn, to Chiron, just inside the orbit of Saturn, to almost the distance of Uranusalmost the distance of Uranus

– Pholus (1992) 33 AU, red surface, of unknown Pholus (1992) 33 AU, red surface, of unknown composition.composition.

– Named after Centaurs (half horse, half human)Named after Centaurs (half horse, half human) so named because these objects have some attributes so named because these objects have some attributes of comets, and asteroids.of comets, and asteroids.

1988, on closest approach to the Sun, 1988, on closest approach to the Sun, Chiron’s brightness doubled, much like the Chiron’s brightness doubled, much like the comets, which contain abundant volatile comets, which contain abundant volatile materials such as water ice, or carbon materials such as water ice, or carbon monoxide ice.monoxide ice.

Chiron is however much bigger than comets.Chiron is however much bigger than comets.


Earth-Approaching Earth-Approaching AsteroidsAsteroids

1989 – a 200-m object passed within 1989 – a 200-m object passed within 800000 km of the Earth.800000 km of the Earth.

1994 – a 10-m object passed 105000 1994 – a 10-m object passed 105000 km away.km away.

Some of these objects have collided Some of these objects have collided with the Earth in the past, some with the Earth in the past, some are likely to do so again in the are likely to do so again in the future.future.

Referred to as Near-Earth Objects Referred to as Near-Earth Objects (NEOs)(NEOs)


Near Earth Objects (NEOs)Near Earth Objects (NEOs) 640 NEOs larger than 1km located by the end of 640 NEOs larger than 1km located by the end of 20022002

Actual population more likely to be > millions.Actual population more likely to be > millions. Unstable orbitsUnstable orbits Fate: Fate:

– Collide with our planet – and be destroyedCollide with our planet – and be destroyed– Be ejected from the Solar SystemBe ejected from the Solar System

Probability of impact – once every 100 million Probability of impact – once every 100 million years.years.

None of the known NEOs will end up crashing None of the known NEOs will end up crashing into the Earth in the foreseeable future… into the Earth in the foreseeable future…

Larger impacts likely to generate environmental Larger impacts likely to generate environmental catastrophescatastrophes

A good argument towards further investigation A good argument towards further investigation of NEOs.of NEOs.


NEO observationNEO observation 5-km NEO 5-km NEO ToutatisToutatis, ,

– approached the Earth at 3 approached the Earth at 3 million km in 1992 million km in 1992

– less than 3 times the less than 3 times the distance to the Moondistance to the Moon

Radar images show it is a Radar images show it is a double object (two double object (two irregular lumps) 3 and 2 irregular lumps) 3 and 2 km objects squashed km objects squashed together.together.


CometCometss


Appearance of CometsAppearance of Comets Observed since antiquityObserved since antiquity Typical comets appear as rather faint, diffuse Typical comets appear as rather faint, diffuse spot of light – smaller than the Moon, and many spot of light – smaller than the Moon, and many times less brilliant.times less brilliant.

Small chunk of icy material that develop an Small chunk of icy material that develop an atmosphere as they get closer to the Sun.atmosphere as they get closer to the Sun.

As they get “very close” they may develop a As they get “very close” they may develop a faint, nebulous tail extending far from the faint, nebulous tail extending far from the main body of the comet.main body of the comet.

Appearance seemingly unpredictableAppearance seemingly unpredictable Typically remain visible for periods from a few Typically remain visible for periods from a few days to a few months.days to a few months.


Comet OrbitsComet Orbits Scientific study of comets dates back to Scientific study of comets dates back to Newton who first recognized their orbits Newton who first recognized their orbits are elongated ellipses.are elongated ellipses.

Edmund Halley (a contemporary of Newton) Edmund Halley (a contemporary of Newton) calculated/published 24 cometary orbits calculated/published 24 cometary orbits (1705).(1705).

Noted that the orbits of bright comets Noted that the orbits of bright comets seen in 1531, 1607, 1682 were quite seen in 1531, 1607, 1682 were quite similar – and could be the same comet – similar – and could be the same comet – returning to the perihelion every 76 returning to the perihelion every 76 years. Predicted a return in 1758.years. Predicted a return in 1758.

When the comet did appear in 1758, it was When the comet did appear in 1758, it was given the name Comet Halley.given the name Comet Halley.


Comet HalleyComet Halley

Observed/Recorded on every passage at Observed/Recorded on every passage at intervals from 74 to 79 years since 239 intervals from 74 to 79 years since 239 B.C.B.C.

Period variations caused by Jovian planetsPeriod variations caused by Jovian planets 1910, Earth was brushed by the comet tail. 1910, Earth was brushed by the comet tail. – causing much public concern…– causing much public concern…

Last appearance in our skies – 1986.Last appearance in our skies – 1986.– Met by several spacecraftsMet by several spacecrafts

Return in 2061.Return in 2061. Nucleus approximately 16x8x8 kilometers. Nucleus approximately 16x8x8 kilometers.


Comet CensusComet Census Records exist for ~1000 cometsRecords exist for ~1000 comets Comets are discovered at an average rate Comets are discovered at an average rate of 5- 10 per year.of 5- 10 per year.

Most visible only on photos made with Most visible only on photos made with large telescopes.large telescopes.

Every few years, a comet appears that is Every few years, a comet appears that is bright enough to be seen with the naked bright enough to be seen with the naked eye.eye.

Recent flybys: Recent flybys: – Comet Hyakutake, long tail, visible for Comet Hyakutake, long tail, visible for about a month, March (1996)about a month, March (1996)

– Hale-Bopp (1997)Hale-Bopp (1997)


Comet StructureComet Structure nucleus: nucleus:

relatively solid and stable, mostly ice and gas with a relatively solid and stable, mostly ice and gas with a small amount of dust and other solidssmall amount of dust and other solids

comacoma:: dense cloud of water, carbon dioxide and other neutral dense cloud of water, carbon dioxide and other neutral gases sublimed off of the nucleusgases sublimed off of the nucleus

hydrogen cloudhydrogen cloud::huge (millions of km in diameter) but very sparse huge (millions of km in diameter) but very sparse envelope of neutral hydrogenenvelope of neutral hydrogen

dust taildust tail:: up to 10 million km long composed of smoke-sized dust up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases; this particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the unaided eyeis the most prominent part of a comet to the unaided eye

ion tailion tail::as much as several hundred million km long composed of as much as several hundred million km long composed of plasma and laced with rays and streamers caused by plasma and laced with rays and streamers caused by interactions with the solar windinteractions with the solar wind


Comet StructureComet Structure

dust tail

ion tail


Nucleus and ComaNucleus and Coma Nucleus: ancient ice, Nucleus: ancient ice, dust and gaseous core dust and gaseous core materialmaterial

nucleus has low nucleus has low gravity – cannot keep gravity – cannot keep dust and gas from dust and gas from escapingescaping

Coma: the bright head Coma: the bright head of the comet – seen of the comet – seen from the Earth. from the Earth.

The coma is a The coma is a temporary atmosphere temporary atmosphere of gas and dust of gas and dust around the nucleus. around the nucleus.

The coma is 100,000's The coma is 100,000's of kilometers acrossof kilometers across

halley's coma

halley's nucleus


Ion TailIon Tail Sun spews out charged particles, called the Sun spews out charged particles, called the solar solar windwind. The solar wind travels along solar magnetic . The solar wind travels along solar magnetic field lines extending radially outward from the Sun. field lines extending radially outward from the Sun.

UV sunlight ionizes gases in the coma. These UV sunlight ionizes gases in the coma. These ionsions (charged particles) are pushed by solar wind (charged particles) are pushed by solar wind particles along magnetic field lines to form the ion particles along magnetic field lines to form the ion tail millions of kilometers long. tail millions of kilometers long.

The blue ion tail acts like a "solar" wind sock. The The blue ion tail acts like a "solar" wind sock. The ion tail ion tail always points directly away from the Sunalways points directly away from the Sun, , because the ions move at very high speed. because the ions move at very high speed.

When the comet is moving away from the Sun, its ion When the comet is moving away from the Sun, its ion tail will be almost in front of it! tail will be almost in front of it!

The blue color is mostly from the light emitted by The blue color is mostly from the light emitted by carbon monoxide ions but other types of ions also carbon monoxide ions but other types of ions also contribute to the light. Since the gas is so contribute to the light. Since the gas is so diffuse, the observed spectrum is an emission-line diffuse, the observed spectrum is an emission-line spectrum. spectrum.


Dust Tail and Hydrogen Dust Tail and Hydrogen CloudCloud The dust tail forms when solar The dust tail forms when solar photonsphotons collide with the collide with the

dust in the coma. Ejected dust particles form a long, dust in the coma. Ejected dust particles form a long, curved tail that lies slightly farther our from the Sun curved tail that lies slightly farther our from the Sun than the nucleus' orbit. than the nucleus' orbit.

The dust tail has a yellow-white color from reflected The dust tail has a yellow-white color from reflected sunlight. Both of the tails will stretch for millions sunlight. Both of the tails will stretch for millions of kilometers. of kilometers.

The dust tail curves gently away from comet’s head, The dust tail curves gently away from comet’s head, because dust particles are more massive than individual because dust particles are more massive than individual ions. They are accelerated more gently by the solar ions. They are accelerated more gently by the solar wind and do not reach the same high speeds as ions. wind and do not reach the same high speeds as ions.

The hydrogen cloud forms when water vapor in the jets The hydrogen cloud forms when water vapor in the jets from the nucleus is dissociated by solar UV into oxygen from the nucleus is dissociated by solar UV into oxygen and hydrogen. and hydrogen.

The hydrogen cloud can be tens of millions of The hydrogen cloud can be tens of millions of kilometers across – the largest things in the solar kilometers across – the largest things in the solar system! system!

All of this is coming from a dirty snowball the size of All of this is coming from a dirty snowball the size of a city! a city!


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Stardust MissionStardust Mission


Origin and Evolution of Origin and Evolution of CometsComets

Originate from very great distancesOriginate from very great distances ApheliaAphelia of new comets ~ 50000 AU of new comets ~ 50000 AU Clustering of aphelia first noted by Dutch Clustering of aphelia first noted by Dutch astronomer Jan Oort (1950).astronomer Jan Oort (1950).

Oort’s Comet Origin ModelOort’s Comet Origin Model– Star’s sphere of influence extends a little beyond Star’s sphere of influence extends a little beyond 50000 AU or 1 LY50000 AU or 1 LY

– Objects in orbit about the Sun at this distance can Objects in orbit about the Sun at this distance can be easily perturbed by passing Stars. be easily perturbed by passing Stars.

– Some perturbed object take on orbits that bring them Some perturbed object take on orbits that bring them much closer to the Sun.much closer to the Sun.

– Reservoir of ancient icy objects from which comets Reservoir of ancient icy objects from which comets are derived is called are derived is called Oort Comet CloudOort Comet Cloud..


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Oort CloudOort Cloud Estimated 10Estimated 101212 comets in the Oort cloud. comets in the Oort cloud. 10 times this number of comets could be 10 times this number of comets could be orbiting the Sun between the planets and the orbiting the Sun between the planets and the Oort cloud.Oort cloud.

Such objects undiscovered because to small, to Such objects undiscovered because to small, to reflect sufficient light to be detectable at reflect sufficient light to be detectable at large distances, and because their stable orbit large distances, and because their stable orbit do not bring them closer to the Sun.do not bring them closer to the Sun.

Total number of comets in the sphere of Total number of comets in the sphere of influence of our Sun could be of the order of influence of our Sun could be of the order of 10101313! !

Represents a mass the order of 1000 Earths.Represents a mass the order of 1000 Earths.


Kuiper BeltKuiper Belt Second source of comets just beyond the Second source of comets just beyond the orbit of Pluto.orbit of Pluto.

First object discovered in 1992.First object discovered in 1992.– Diameter ~ 200 km.Diameter ~ 200 km.– Period ~ 300 years.Period ~ 300 years.

60 objects found since then.60 objects found since then. Share orbital resonance with Neptune – Share orbital resonance with Neptune – two orbits completed for three by two orbits completed for three by Neptune.Neptune.

Nicknamed Plutinos for this reason.Nicknamed Plutinos for this reason. Speculated that Pluto is the largest Speculated that Pluto is the largest example of this group. example of this group.

They may share a common origin.They may share a common origin.


Fate of CometsFate of Comets Comets spend nearly all their existence in the Oort cloud or Comets spend nearly all their existence in the Oort cloud or

Kuiper beltKuiper belt– At a temperature near absolute zero.At a temperature near absolute zero.

As comet enters the inner Solar System, their “life” changes As comet enters the inner Solar System, their “life” changes altogether!altogether!– If they survive the initial passage near the Sun, they return If they survive the initial passage near the Sun, they return

towards the cold aphelia – and may follow a quasi-stable orbit for towards the cold aphelia – and may follow a quasi-stable orbit for a “while”.a “while”.

– May impact the SunMay impact the Sun– May be completely vaporized as they fly by the SunMay be completely vaporized as they fly by the Sun– May interact with a planetMay interact with a planet

Final impactFinal impact Speed up and ejectionSpeed up and ejection Perturbed into an orbit of shorter period.Perturbed into an orbit of shorter period.

Each flyby the Sun reduces the size and mass of the nucleus of Each flyby the Sun reduces the size and mass of the nucleus of the comets. the comets.

Some comets end their life catastrophically by breaking apart.Some comets end their life catastrophically by breaking apart.– Shoemaker-Levy 9 broke into ~20 pieces when it passed close to Shoemaker-Levy 9 broke into ~20 pieces when it passed close to

Jupiter in July 1992.Jupiter in July 1992.– Fragments of Shoemaker-Levy captured into a very elongated 2 year Fragments of Shoemaker-Levy captured into a very elongated 2 year

around Jupiter – In 1994 the comet fragments crashed into Jupiter.around Jupiter – In 1994 the comet fragments crashed into Jupiter.


Comet Shoemaker-Levy 9Comet Shoemaker-Levy 9


Impact!Impact! Recorded Recorded by HST-by HST-WFP in WFP in different different wavelengtwavelengthshs


Shoemaker-Levy 9 Impact Shoemaker-Levy 9 Impact SiteSite


Sequence of Impact SitesSequence of Impact Sites The rotation The rotation of Jupiter of Jupiter left a trail left a trail of impacts.of impacts.

We see the We see the debris left debris left in the upper in the upper atmosphere.atmosphere.

Debris Debris fields would fields would cover Earth!cover Earth!


Time Evolution of Debris Time Evolution of Debris FieldsFields

april 18, 2006astronomy 20101 chapter 12: comets and asteroids: debris of the solar system

Documents

discovery of asteroids

known asteroids

fraknoi asteroids

asteroid orbits

asteroid belt

moon total mass of asteroids

similar orbits

determined orbits