The solar system also includes debris such as

comets, asteroids, and meteoroids.

VOCABULARYSolar-System Debris

The Planets and the Solar System

Haley’s Comet

Asteroid 243 Ida

comet

asteroid

meteor

meteorite

meteor shower

Comets

Throughout history, comets have been considered as portents of doom, even until very recently:

Appearances of comet Kohoutek (1973), Halley (1986), and Hale-Bopp (1997) caused great concern

among superstitious.

Comet Hyakutake in 1996

Appearance of 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.

Comet structure

• Theory of comet structure first proposed by Fred Whipple, Harvard, 1950.

• Nucleus is – Solid object a few kilometers across– Composed of mainly water ice, with traces of other ices,

mixed with silicate grains and dust.– Model known as the “dirty snowball” model.

• Water vapor + other volatiles escape from the nucleus when heated by sunlight.

• No large fragments of solid matter from a comet ever survived passage through Earth’s atmosphere – full composition of the nuclei - not known.

Comet Orbits• 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. He predicted a return of the comet in 1758.

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

•Scientific study of comets dates back to Newton who first recognized their orbit as elongated ellipse.

Comet Halley

• 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.• Nucleus approximately 16x8x8 kilometers.

Comet Components• 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.

Two Types of Tails

Ion tail: Ionized gas pushed away from the

comet by the solar wind. Pointing straight away

from the sun.

Dust tail: Dust set free from vaporizing ice in

the comet; carried away from the comet by the

sun’s radiation pressure. Lagging behind the

comet along its trajectory

Gas and Dust Tails of Comet Mrkos in 1957

Comet Hale-Bopp in 1997

Discovery of C/Hale-Bopp

• Discovered in 1995 by Alan Hale (professional astronomer in New Mexico, and Thomas Bopp (amateur astronomer in Arizona)

• Both were observing at their home locations on the evening of July 22nd-23rd, 1995 with their amateur telescopes

• Hale-Bopp located at 7.15 AU, just outside the Jupiter orbit!

Alan Hale

“C” for Long Period

• ~ 4200 yrs ago since last appearance…~2380 yrs for next appearance

• Closest approach:– Earth: March 27, 1997 @

1.315 AU– Sun: April 1, 1997 @

0.914 AU

Some Properties of the Nucleus of Hale-Bopp

• Known from measurements:– R 30 km (2nd largest comet!)– Spin Period 11.5– Obliquity 86 degrees

• Unknown, but guess:– Density 700 kg m-3

– Specific heat 1400 J kg-1 K-1

– Bond Albedo 0.04– Emissivity 0.9

Top: Blue filter image. Bottom: false color version. Image credit: http://www2.jpl.nasa.gov/comet/ampo145.html

The Geology of Comet NucleiComet nuclei contain ices of water, carbon dioxide, methane, ammonia,

Materials that should have condensed from the outer solar nebula.

Those compounds

sublime (transition from solid directly to gas phase) as

comets approach the

sun.

Densities of comet nuclei: ~ 0.1 – 0.25 g/cm3

Not solid ice balls, but fluffy material with significant

amounts of empty space.

Fragmentation of Comet NucleiComet nuclei are very fragile and are easily fragmented.

Comet Shoemaker-Levy was disrupted by tidal forces of Jupiter

Two chains of impact craters on Earth’s moon and on Jupiter’s moon Callisto may have been caused by fragments of a comet.

Fragmenting CometsComet Linear

apparently vaporized during its sun passage

in 2000.

Only small rocky fragments remained.

The Origin of CometsComets are believed to originate in the Oort cloud:

Spherical cloud of several trillion icy bodies, ~ 10,000 – 100,000 AU from the sun.

10,000 – 100,000 AU

Oort Cloud

Gravitational influence of occasional passing stars may perturb some orbits and draw them towards the inner solar system. Interactions with planets may perturb orbits further, capturing comets in short-period orbits.

Oort Cloud• 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.

The Kuiper BeltSecond source of small, icy bodies in the outer solar system:

Kuiper Belt, at ~ 30 – 100 AU

from the sun.

Pluto and Charon may be

captured Kuiper-Belt

objects.

Kuiper Belt• First object discovered in 1992.

– Diameter ~ 200 km.– Period ~ 300 years.

• 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.

Fate of Comets• Comets spent nearly all their existence in the Oort cloud or Kuiper belt

– At a temperature near absolute zero.• As comet enter the 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.

• 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.– Fragments of Shoemaker-Levy captured into a very elongated 2 year

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

Conclusion• New examples are detected using spectra

analysis at an average rate of 5- 10 per year.• Comparison of abundances to interstellar

sources showed similarities between comets• There is a strong link between comets and

interstellar ices.• Comets give clues about the origins of life,

despite their historical role as omens of death and destruction.

Meteorites

Distinguish between:

• Meteoroid = small body in space

• Meteor = meteoroid colliding with Earth and producing a visible light trace in the sky

(shooting star)• Meteorite = meteor that survives the

plunge through the atmosphere to strike the ground

Meteorites

About 2 meteorites large enough to produce visible

impacts strike the Earth every day.

Statistically, one meteorite is expected

to strike a building somewhere on Earth

every 16 months.

Typically impact onto the atmosphere with 10 – 30 km/s (≈ 30 times faster than a rifle bullet).

Sizes from microscopic dust to a few centimeters.

The Origins of Meteorites

Planetesimals cool and differentiate;

Collisions eject material from different depths with different

compositions and temperatures.

Meteorites can not have been broken up from planetesimals very

long ago

→ Remains of planetesimals should still exist.

→ Asteroids

Meteorite Types

• Iron: primarily iron and nickel; similar to type M asteroids

• Chondrite: by far the largest number of meteorites fall into this class; similar in composition to the mantles and crusts of the terrestrial planets

• Stony Iron: mixtures of iron and stony material like type S asteroids

• Carbonaceous Chondritevery: similar in composition to the Sun less volatiles; similar to type C asteroids

Meteorite Types

•Achondrite: similar to terrestrial basalts; the meteorites believed to have originated on the Moon and Mars are achondrites

Meteorite Impacts on EarthOver 150 impact craters found on Earth.

Famous example: Barringer

Crater near Flagstaff, AZ:

Formed ~ 50,000 years ago by a meteorite of ~ 80 – 100 m diameter

Meteor ShowersMost meteors appear in showers, peaking periodically at specific dates of the year.

The Leonid Meteor Shower in 2002

Meteoroid Orbits

Meteoroids contributing to a meteor shower are debris particles, orbiting in the path of a comet.

Spread out all along the orbit of the comet.

Comet may still exist or have been destroyed.

Only few sporadic meteors are not associated with comet orbits.

Radiants of Meteor ShowersTracing the tracks of meteors in a shower backwards,

they appear to come from a common origin, the radiant.

↔ Common direction of

motion through space.

The Perseid Meteor Shower

Asteroids

Asteroids

Last remains of planetesimals that built the planets 4.6

billion years ago!

Asteroids• Asteroids are also categorized by their

position in the solar system: • Main Belt: located between Mars and

Jupiter roughly 2 - 4 AU from the Sun; further divided into subgroups: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas (which are named after the main asteroid in the group).

• Near-Earth Asteroids (NEAs): ones that closely approach the Earth

• Atens: semimajor axes less than 1.0 AU and aphelion distances greater than 0.983 AU;

• Apollos: semimajor axes greater than 1.0 AU and perihelion distances less than 1.017 AU

• Amors: perihelion distances between 1.017 and 1.3 AU;

Asteroids

Where do we find most asteroids in the solar system?

1. In a belt between the Earth and Mars.

2. In a belt between Mars and Jupiter.

3. In a belt far outside the orbits of the planets.

4. On highly elliptical orbits, coming as close to the sun as Mercury’s orbit, and reaching as far out as Pluto’s orbit or beyond.

5. In elliptical orbits around Jupiter.

The Asteroid Belt

Plu

toN

eptu

neUra

nus

Saturn

Jupi

terM

ars

(Distances and times reproduced to scale)

Most asteroids orbit the sun in a

wide zone between the

orbits of Mars and Jupiter.

The Asteroid Belt

Sizes and shapes of the largest asteroids, compared to the moon

Small, irregular objects, mostly in the apparent gap

between the orbits of Mars and

Jupiter.

Thousands of asteroids with

accurateely determined orbits

known today.

Non-Belt Asteroids

Apollo-Amor Objects:

Not all asteroids orbit within the asteroid belt.

Asteroids with elliptical orbits, reaching into

the inner solar system.

Some potentially

colliding with Mars or Earth.

Trojans: Sharing

stable orbits along the orbit of Jupiter.

• Trojans: located near Jupiter's Lagrange points (60 degrees ahead and behind Jupiter in its orbit). Several hundred such asteroids are now known; it is estimated that there may be a thousand or more altogether. Curiously, there are many more in the leading Lagrange point (L4) than in the trailing one (L5). (There may also be a few small asteroids in the Lagrange points of Venus and Earth (see Earth's Second Moon) that are also sometimes known as Trojans; 5261 Eureka is a "Mars Trojan".)

Non-Belt Asteroids

Asteroid Types

• C-type, includes more than 75% of known asteroids: extremely dark (albedo 0.03); similar to carbonaceous chondrite meteorites; approximately the same chemical composition as the Sun minus hydrogen, helium and other volatiles;

Asteroids are classified into a number of types according to their spectra (and hence their chemical composition and albedo:

Image of 253 Mathilde, a 66 by 48 by 46 km C-type asteroid.

• S-type, 17%: relatively bright (albedo .10-.22); metallic nickel-iron mixed with iron- and magnesium-silicates;

Asteroid TypesOn October 29, 1991, the Galileo spacecraft flew past 951 Gaspra, an S-type asteroid situated in the inner asteroid belt. Gaspra measures 19 by 11 kilometers, and its faceted shape suggests that it is a fragment from a larger object that was shattered by collision roughly 500 million years ago.

• M-type, most of the rest: bright (albedo .10-.18); pure nickel-iron.

• There are also a dozen or so other rare types.

Asteroid Types

Shape model rendering from radar data of the M-type asteroid 216 Kleopatra (NASA/JPL). The refelectance characteristics of M-type asteroids like Kleopatra suggest that they may be composed of iron-nickel which hints at a possible source for iron meteorites.

Beyond the Solar System