22 February 2005AST 2010: Chapter 121 Asteroids & Comets

22 February 2005 AST 2010: Chapter 12 1

Asteroids Asteroids & & CometsComets


Debris of the Solar SystemAsteroids 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) materialComets are icy bodies that revolve around the Sun and are smaller than a major planet, but that contain frozen water and other volatile materials


Discovery of AsteroidsMost asteroid orbits lie in the asteroid belt, between Mars and JupiterToo small to be visible without a telescopeFirst 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 AUDiscoverer: Giovanni Piazzi

Followed in subsequent years by the discovery of other small planets in similar orbitsBy 1890, more than 300 objects had been discoveredMore than 20,000 asteroids now have well determined orbits


Asteroid NomenclatureGiven a number and a nameNames originally chosen from Greek/Roman goddesses; other female names; all names go!Asteroids 2410, and 4859 named after Morrison and Fraknoi

Mathilde IdaGaspra


Asteroid CensusTotal number of asteroids in the solar system very largeMust be estimated on the basis of systematic sampling of the skyStudies indicate there are 106 asteroids with diameters greater than 1 km!Largest: Ceres, with diameter of ~1000 kmPallas and Vesta have diameter of ~500 km15 more larger than 250 km across100 times more objects 10-km across than 100-km acrossTotal mass of asteroids is less than the mass of the Moon


Asteroid OrbitsRevolve around the sun in west-to-east directionMost lie in or near the eclipticAsteroid belt defined as region that contains all asteroids with semi-major axes of 2.2 to 3.3 AUPeriods: 3.3 to 6 years75% of known asteroids in the main beltNot closely spaced

typically >million km between them

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


Asteroid FamiliesThe 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 asteroidsMembers of each family have similar speedsPhysical similarities between largest asteroids of given families

Several dozen families are found


Asteroid Physical AppearanceMajority: very dark

Do not reflect much lightReflectivity ~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


Asteroid Classification - 1Dark asteroids

Believed to be primitive bodiesChemically unchanged since beginning of solar system

Composed of silicates with dark organic carbon compoundsCeres, Pallas, and most objects in outer third of the belt

Most primitive asteroids part of class C C stands for carbonaceous (carbon-rich)


Asteroid Classification - 2Class S

S stands for “stony” compositionNo dark carbonsHigher reflectivityMost asteroids of this type believed to be also primitive

Class MM stands for “metal”Identification difficult

Done by radar for the largest asteroids such as Psyche

Much less numerousSuspected to originate from collision of a parent body that had previously differentiatedEnough metal in 1-km M-type asteroid to supply the world with iron for a long period of time


Asteroid Classification - 3


Trojan AsteroidsLocated far beyond main belt

~5.2 AU, nearly same distance as JupiterCollectively 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 foundDark, primitive

appear faint, but some are nonetheless sizeable


Asteroids in Outer Solar SystemMany asteroids with orbits beyond JupiterExamples:

Chiron, just inside the orbit of Saturn, to almost the distance of UranusPholus, 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 asteroids1988, 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


Earth-Approaching Asteroids1989 – a 200-m object passed within 800,000 km of the Earth1994 – a 10-m object passed 105,000 km awaySome of these objects have collided with the Earth in the past, and some are likely to do so again in the futureReferred to as Near-Earth Objects (NEOs)


NEOs640 NEOs larger than 1 km located by the end of 2002Actual population more likely to be > millionsUnstable orbitsFate:

Collide with our planet – and be destroyedBe ejected from the solar system

Probability of impact: once every 100 million yearsNone 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


NEO observation5-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


CometsObserved since antiquityTypical 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 SunAs they get “very close” they may develop a faint, nebulous tail extending far from the main body of the cometAppearance seemingly unpredictableTypically remain visible for periods from a few days to a few months


Comet OrbitsScientific study of comets dates back to Newton who first recognized their orbits are elongated ellipsesEdmund 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 yearsPredicted a return in 1758When the comet did appear in 1758, it was given the name Comet Halley


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 concernIts last appearance in our skies was in 1986

met by several spacecraft

It is predicted to return in 2061Its nucleus approximately 16x8x8 km3


Comet CensusRecords exist for ~1000 cometsComets are discovered at an average rate of 5 to 10 per yearMost visible only on photos made with large telescopesEvery few years, a comet appears that is bright enough to be seen with the naked eyeRecent flybys:

Comet Hyakutake, long tail, visible for about a month, March (1996)Comet Hale-Bopp (1997)


Comet Components (1)Nucleus: relatively solid and stable, mostly ice and gas, with a small amount of dust and other solidsComa: dense cloud of water, carbon dioxide, and other neutral gases sublimed off of the nucleusHydrogen cloud: huge (millions of km in diameter), but very sparse, envelope of neutral hydrogenDust 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


Comet Components (2)

dust tail

ion tail


Nucleus and Coma of Comet

The nucleus is composed of ancient ice, dust, and gaseous core materialThe nucleus has low gravity

cannot keep dust and gas from escaping

The coma is the bright head of the comet, as seen from the EarthThe coma is a temporary atmosphere of gas and dust around the nucleusThe coma is 100,000's of kilometers across

Halley's coma

Halley's nucleus


Ion Tail of CometSun 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 comaThese 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 sockThe 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


Dust Tail and Hydrogen Cloud of Comet

The dust tail forms when solar photons collide with the dust in the coma

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 sunlightBoth 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 forms when water vapor ejected in the jets from the nucleus is dissociated by UV sunlight into oxygen and hydrogenThe hydrogen clouds can be tens of millions of kilometers across

They are the largest things in the solar system!


Origin and Evolution of CometsComets originate from very great distancesThe aphelia of new comets are typically around 50,000 AU

This clustering of aphelia was first noted by Dutch astronomer Jan Oort in 1950He then proposed an idea for the origin of those comets, which is still accepted by most astronomers today

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



Oort Comet CloudAstronomers estimate that there may be about a trillion (1012) comets in the Oort cloudIn addition, 10 times this number of comets could be orbiting the Sun between the planets and the Oort cloudSuch 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 SunThe total number of comets within the sphere of influence of our Sun could therefore be on the order of ten trillion (1013)!

Their total mass would be similar to that of 1000 EarthsCometary material could thus be the most important constituent of the solar system after the Sun itself


The Kuiper BeltAnother 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 foundIt appears that these KBO are heavily influenced by the gravity of NeptuneMany 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 beltFor this reason, KBOs are sometimes called plutinos


Fate of CometsMost comets probably spend nearly all their existence in the Oort cloud or Kuiper belt

at a temperature near absolute zeroBut 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 SunIt may be completely vaporized as it flies by the SunIt may interact with one or more planets with three possible fates:

It is destroyed after impacting a planetIt gets speeded up and ejected, leaving the solar system foreverIt 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 1992The fragments were then captured into a very elongated 2-year orbit around Jupiter, before crashing into it in July 1994


Comet Shoemaker-Levy 9





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22 February 2005AST 2010: Chapter 121 Asteroids & Comets