20 July 2005 AST 2010: Chapter 12 1

AsterAsteroids oids &&

ComeCometsts

20 July 2005 AST 2010: Chapter 12 2

Debris of the Solar SystemDebris 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

20 July 2005 AST 2010: Chapter 12 3

Discovery of Discovery of AsteroidsAsteroidsMost asteroid orbits lie in the

asteroid belt, between Mars and Jupiter

too small to be visible without a telescope

They were first discovered when astronomers were hunting for a missing planet between Mars and Jupiter The 1st asteroid, named Ceres and initially thought to be the “missing planet”, was discovered by Giovanni Piazzi in 1801

It orbits at 2.8 AU from the SunThe discovery of other “minor planets” in similar orbits followed in subsequent yearsNow, more than 20,000 asteroids are known to have well-determined orbits

20 July 2005 AST 2010: Chapter 12 4

Asteroid Asteroid NomenclatureNomenclature

Asteroids are given both a number and a nameThe names were originally chosen from Greek/Roman goddesses, then other female names were used, and finally all names go!Asteroids 2410 and 4859 are named after Morrison and Fraknoi

Mathilde IdaGaspra

20 July 2005 AST 2010: Chapter 12 5

Asteroid CensusAsteroid CensusThe total number of asteroids in the solar system is very large

It must be estimated on the basis of systematic sampling of the sky

Studies indicate that there are 106 asteroids with diameters greater than 1 km!

The largest is Ceres, with a diameter of ~1000 kmPallas and Vesta have diameters of ~500 km15 more are larger than 250 km acrossThere are 100 times more objects 10-km across than 100-km across

The total mass of asteroids is less than the mass of the Moon

20 July 2005 AST 2010: Chapter 12 6

Asteroid OrbitsAsteroid OrbitsAll asteroids revolve around the Sun in west-to-east direction, like the planetsMost of their orbits lie near the plane in which the Earth and the other planets circleThe asteroid belt is defined as the region that contains all asteroids with semi-major axes in the range from 2.2 to 3.3 AU

Their orbital periods range from 3.3 to 6 years

75% of known asteroids are in the main belt

But they are not closely spaced

20 July 2005 AST 2010: Chapter 12 7

Asteroid FamiliesAsteroid FamiliesJapanese astronomer Kiyotsuga Hirayama found in 1917 that some asteroids fall into 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 speedsThere are physical similarities among the larger members of a given familySeveral dozen families are found

20 July 2005 AST 2010: Chapter 12 8

Asteroid Physical AppearanceAsteroid Physical AppearanceThe majority of asteroids are very dark

They do not reflect much lightTheir reflectivities are only 3 to 4 percent

There is, however, a sizable group that is not very dark

Its typical reflectivities are 15 to 20 percent (similar that of the Moon)

A few asteroids even have reflectivities as high as 60%To understand the reasons for these differences, astronomers performed spectral analysis of the light reflected by asteroids

20 July 2005 AST 2010: Chapter 12 9

Asteroid Classification (1)Asteroid Classification (1)The dark asteroids

are believed to be primitive bodieschemically unchanged since the beginning of the solar system

are composed of silicates mixed with dark, organic carbon compoundsinclude Ceres, Pallas, and most objects in the outer third of the asteroid belt

Most of the primitive asteroids are classed as C asteroids

C stands for carbonaceous (carbon rich)

20 July 2005 AST 2010: Chapter 12 10

Asteroid Classification (2)Asteroid Classification (2)The second most populous group is the S asteroids

S stands for a “stony” or silicate compositionThey have no dark carbon compounds and hence higher reflectivitiesMost S asteroids seem to be also primitive

The third group is the M asteroidsM stands for “metal”Their identification is difficult

done by radar for the largest asteroids such as Psyche

They are much less numerousEach may have come from a parent body that had earlier differentiated and later shattered in a collisionThere is enough metal in a 1-km M-type asteroid to supply the world with iron for a long period of time

20 July 2005 AST 2010: Chapter 12 11

Asteroid Classification (3)Asteroid Classification (3)

20 July 2005 AST 2010: Chapter 12 12

Trojan AsteroidsTrojan AsteroidsThe Trojans are located far beyond main belt

at ~5.2 AU from the Sun, nearly the same distance as Jupiterdark, primitive objects, like some other asteroids

They have stable orbits because of JupiterIn Jupiter’s orbit, there are two points near which an asteroid can stay almost indefinitely

They make equilateral triangles with Jupiter and the Sun

Since their first discovery in 1906, several hundreds have been foundThe larger Trojans can be up to ~200 km across

20 July 2005 AST 2010: Chapter 12 13

Animation: the TrojansAnimation: the Trojans

GreenGreen circles indicate main-belt circles indicate main-belt asteroids and asteroids and blueblue dots on the dots on the outermost circle are the Trojansoutermost circle are the Trojans

20 July 2005 AST 2010: Chapter 12 14

Asteroids in Outer Solar Asteroids in Outer Solar SystemSystemThere are asteroids with orbits that carry them far

beyond JupiterThey are hard to detect and only a few have been found

Examples:Chiron is 200 km across and the largest of them, with a path carrying it from just inside the orbit of Saturn to almost the distance of UranusPholus, with an orbit that takes it 33 AU from the Sun, beyond Neptune, has the reddest surface of any object in the solar system, with unknown composition

They are named after centaurs (mythological half horse, half human) because these objects have some of the properties of both comets and asteroidsIn 1988, on its closest approach to the Sun, Chiron’s brightness doubled, much like the comets

Chiron, however, is much bigger than comets

20 July 2005 AST 2010: Chapter 12 15

Earth-Approaching AsteroidsEarth-Approaching AsteroidsSome asteroids may stray far outside the main belt and travel inward along paths that come close to or cross Earth’s orbitSuch asteroids, and other objects that come close to the Earth, are collectively known as Near-Earth Objects (NEOs)

Needless to say, they are of great interest to usSome of these NEOs have collided with the Earth in the past, and some others are likely to do so in the future

In 1994, a 1-km object passed closer than the MoonBy the end of 2002, more than 640 NEOs larger than 1 km in diameter had been discovered

Astronomers have estimated that there are probably 500 or so NEOs larger than 1 km in diameter that have not yet been found

20 July 2005 AST 2010: Chapter 12 16

NEOsNEOsNEOs generally have unstable orbitsEach NEO will meet one of 2 fates:

collide with one of the terrestrial planets — and be destroyedbe ejected gravitationally from the inner solar system after a near-encounter with a planet

The probability for impact is once every 100 million years

Hence the likelihood is very remote than any one of the known NEOs will end up crashing into the Earth in the foreseeable future …

The larger of these impacts will likely generate environmental catastrophes for our planet

This is a good reason for further investigation of NEOs

20 July 2005 AST 2010: Chapter 12 17

An NEO ObservationA 5-km-long NEO called Toutatis

approached to within 3 million km of the Earth in 1992

which is less than 3 times the distance to the Moon

Radar images indicate that it is a double object

consisting of 2 irregular lumps, with diameters of 3 km and 2 km, squashed together

Animation

Radar images

20 July 2005 AST 2010: Chapter 12 18

Rendezvous with ErosRendezvous with ErosEros is an Earth-approaching S-type asteroid

potato-shaped, 34 km long, and 11 km wideheavily cratered, suggesting that the surface is old

Movies of Eros captured by the NEAR-Shoemaker spacecraft, which orbited and then landed on it in 2000

20 July 2005 AST 2010: Chapter 12 19

CometsCometsThey have been observed since antiquityTypical comets appear as rather faint, diffuse spots of light

smaller than the Moon and many times less brilliant

They are small chunks of icy material that develop atmospheres as they get closer to the SunAs a comet gets “very close” to the Sun, the comet may develop a faint, nebulous tail extending far from the main body of the cometTheir appearance is seemingly unpredictableComets typically remain visible for periods from a few days to a few months

20 July 2005 AST 2010: Chapter 12 20

Comet OrbitsComet OrbitsThe scientific study of comets dates back to Newton who first recognized that their orbits were very elongated ellipsesEdmund Halley (a contemporary of Newton) in 1705 calculated/published 24 cometary orbits

He 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 yearsHe predicted a return in 1758When the comet did appear in 1758, it was given the name Comet Halley

20 July 2005 AST 2010: Chapter 12 21

Comet HalleyComet HalleyIt has been observed and recorded on every passage near the Sun 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 is approximately 16 x 8 x 8 km3

20 July 2005 AST 2010: Chapter 12 22

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

Comet Hyakutake, with a very long tail, was visible for about a month in March 1996Comet Hale-Bopp appeared in 1997

20 July 2005 AST 2010: Chapter 12 23

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

20 July 2005 AST 2010: Chapter 12 24

Comet Components (2)Comet Components (2)

dust tail

ion tail

20 July 2005 AST 2010: Chapter 12 25

Nucleus and Coma of CometNucleus and Coma of CometThe nucleus is composed of ancient ice, dust, and gaseous core materialThe nucleus has low gravity

It 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

20 July 2005 AST 2010: Chapter 12 26

Nucleus of Comet Wild 2Nucleus of Comet Wild 2

The images were captured by NASA's Stardust spacecraft The images were captured by NASA's Stardust spacecraft

20 July 2005 AST 2010: Chapter 12 27

The Sun spews out charged particlesThis solar wind occurs along the 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 field lines to form a tail millions of km long

The blue ion tail acts like a "solar" wind-sockThe 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, the comet’s 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

Comet’s Ion TailComet’s Ion Tail

20 July 2005 AST 2010: Chapter 12 28

Origin and Evolution of Origin and Evolution of CometsCometsComets originate from very great distances

The aphelia of new comets are typically ~50,000 AUThis 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:The Sun’s sphere of influence extends only a little beyond 50,000 AU, or about 1 LYObjects in orbit about the Sun at this distance can be easily perturbed by the gravity of passing starsThe comets are some of the perturbed objects, which take on orbits that bring 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

20 July 2005 AST 2010: Chapter 12 29

20 July 2005 AST 2010: Chapter 12 30

Oort Comet CloudOort 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 orbits 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

20 July 2005 AST 2010: Chapter 12 31

Kuiper BeltKuiper 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 acrossSince then, several hundred more Kuiper-belt objects (KBOs) have been foundIt appears that these KBOs 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

20 July 2005 AST 2010: Chapter 12 32

Fate of CometsFate 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 Sun, or be completely vaporized as it flies by the SunIt may interact with one or more planets with 3 possible fates:

destroyed after impacting a planetspeeded up and ejected, leaving the solar system foreverperturbed into an orbit of shorter period

Each time it approaches the Sun, a comet loses part of its material It may end its life catastrophically by breaking apart

20 July 2005 AST 2010: Chapter 12 33

Comet Shoemaker-Levy 9 Some comets die very spectacularlyWhen comet Shoemaker-Levy 9 passed close to Jupiter in July 1992, the comet broke into about 20 pieces

perhaps due Jupiter’s tidal forces

Fragments of the comets then orbited Jupiter until July 1994 when they crashed into Jupiter, experiencing violent destruction

Animation