AST 111 Asteroids and Comets

Asteroids and Meteorites Planets have changed since formation – Developed layers – Geological Activity Many small bodies haven’t really changed

Asteroids Leftover planetesimals from Solar System formation Did not see any for two centuries after invention of the telescope 400,000 cataloged

Asteroids Ceres is largest (600 miles) – Nearly spherical – (Remember planet rules!) – Dwarf planet 1 million+ asteroids with 1 km+ diameter Mass of all asteroids totals up to smaller than the Moon

Asteroids Composition determined by spectra Mostly metal and rock (condensed before the frost line) – Many are carbon-rich; formed near frost line Mass and density measured by fly-bys and asteroids’ small moons

Asteroids Main asteroid belt between Mars and Jupiter – Asteroids thousands to millions of km apart! Trojan Asteroids follow / lag Jupiter by 60 o – “Lagrange points”

Gaps and Groupings Orbital resonances cause Jupiter to line up with asteroids periodically Can force increasing oscillation in asteroid orbits – Can kick them out, or – Can keep them in place

Why is there an asteroid belt? Young Jupiter disrupted many of the planetesimals that were forming a planet Happened through orbital resonances The mass of the asteroid belt is less than that of any terrestrial planet

Meteorites Asteroids constantly collide – Small pieces get broken off Ones that hit Earth and survive to the ground are meteorites. – Usually break into fragments

Primitive Meteorites Primitive meteorites: formed with Solar System – 4.6 billion years old (age of Solar System) Stony primitive meteorites – Rocky minerals with metallic flakes Carbon-rich primitive meteorites – Carbon compounds and even water

Primitive Meteorites Life of primitive meteorite: – Created in solar nebula – Orbited the Sun more or less undisturbed – Fell to Earth Stony ones from inner asteroid belt (warmer) Carbon ones from outer asteroid belt (colder)

Processed Meteorites Once part of a larger object Younger than primitive meteorites They can be: Metal-rich (iron and nickel) – Much like terrestrial planet cores Rocky – Made of rock similar to terrestrial mantles and crusts

Processed Meteorites Larger asteroids could undergo differentiation – Interiors melted – Metals sank, rocks rose Rocky meteorites from surfaces of differentiated asteroids Metal meteorites from cores of shattered asteroids – Like part of a “dissected planet”

Comets Planetesimals formed beyond the frost line

Comets “Comet” refers to any leftover icy planetesimal Majority of comets live in: – Kuiper Belt – Oort Cloud Typically 20 km in diameter If it’s in our sky: – It got deflected in here by a planet

History of Comets Thought to be in Earth’s atmosphere – Tycho Brahe: at least beyond the Moon – Newton: they orbit the Sun – Edmond Halley: calculated orbit, showed same comet comes back

Halley’s Comet First spacecraft sent to Halley’s Comet in 1980’s Size and mass suggest either: – A porous structure, or – A “rubble pile”

Comet Discoveries Many made by amateurs SOHO (solar observatory) has found 1250! – They either swing around the Sun or die trying

What are they made of? ICE! – Condensed H-compounds Along with some “rocky dust” – “Dirty Snowballs” Spectra show CO – Only condenses in coldest, farthest parts

What’s inside? Deep Impact mission Fired impactor, collided at 37,000 km/hr 10+ meters of dust Porous interior – Density similar to fresh snow

Why the tail? If an icy object crosses the frost line, it melts! Two tails: – Plasma tail caused by solar wind (strong push) – Dust tail caused by sunlight pressure (weak push)

A Comet’s Orbit p. 378

How long do they live? If they hang around inner solar system: – Loses ice (0.1% per pass) – Can only make a few hundred passes

Meteor Showers These happen when Earth passes through a comet’s path Sand and pebble – sized pieces of the comet burn up in the atmosphere – 70 km / s!

Where exactly do comets come from? Two major sources of comets: – Kuiper Belt – Oort Cloud Comets that visit inner solar system have very elliptical orbits – Because they were thrown by Jovian planets Orbits up to a quarter of the distance to the nearest star!

Kuiper Belt vs. Oort Cloud The “regulars” in inner solar system are from Kuiper Belt – AU – 100,000+ comets – Same plane as planets Oort Cloud: – Out to 50,000 AU – One trillion + comets!! – Random orbits

Origins of Comets Planetesimals that hung around Jovian worlds collided or got launched – These are what make up the Oort Cloud Plenetesimals beyond Jovian planets were “safe” – Orbital resonances sometimes launch one at the Sun

Jovian Planets: Asteroid Launchers Every asteroid or comet that has hit Earth since heavy bombardment was sent by the Jovian planets – But remember… they sent hundreds of thousands of comets to the Oort Cloud, decreasing the number hanging around the planets

Jovian Planets: Asteroid Launchers Can civilizations only arise in Solar Systems with an “asteroid launcher” such as Jupiter to clear out comets?