Intense bombardment 4.6 to 3.6 billion years ago - no life could survive first billion years on Earth. Dinosaurs and 60% of all life wiped out by an asteroid collision one about 10 kilometers in size some 65 million years ago.

KT (Cretaceous - Tertiary Boundary)

(Dinosaurs - Mammals)

World-wide iridium layer at KT boundary requires collision of 10 kilometer asteroid- iridium is rare - iron loving sinks to center of Earth.

Chicxulub crater on Yucatan peninsula - dated 65 million years ago.

Global dust cloud - no sunlight reaches surface

There are 200 known and 2000 estimated Earth crossers capable of global destruction.

Dying before age five.	1 in 8 (South Asia)
Dying before age five.	1 in 100 (U.S.)
Risk of being involved in an alcohol-related traffic accident over lifetime.	2 in 5
Risk of death in car crash over lifetime.	1 in 100
Risk of global death, over a human lifetime, by asteroid collision (for everyone)	1 in 2000
Lifetime risk of death in airplane crash.	1 in 2000
Risk of dying in childbirth	1 in 12,500
Death risk by dog bite (about the same as lifetime risk of death by leaking gas, earthquakes, drinking detergent)	1 in 20 million

433 Eros = Mars crosser, will become Earth crosser and may hit Earth in next 1.14 million years.

Asteroid 1996 JAI came within 175,000 kilometers of Earth in June 1996. It was 200-300 meters in diameter. Mean distance of Moon from Earth 384,400 kilometers. In 1994 three small asteroids - about 1 meter across came three times closer than JAI.

Of all Earth crossing asteroids Near Earth Asteroid Tracking = NEAT

Air Force and NASA - all those headed toward Earth.

Bad idea pieces hit you. Note Chinese in 1996 objected to nuclear test ban treaty since they wanted to develop nuclear capability for blasting the doomsday asteroids.

With missile and rocket eagle. Near spacecraft - not in text.

NEAR = Near Earth Asteroid Rendezvous

Launched February 1996 will orbit 433 Eros in 1999 for about one year.

Determine gravity and mass - are asteroids solid rock or rubble heaps (Gravel pile).

Smaller, faster, cheaper $150 million

Also get size, shape, spin, surface composition, geology, magnetic field.

Orbits - came from asteroid belt.

Antarctica ice preserves thousands.

Formation age = 4.6 billion years usually.

Exposure age - cosmic rays - 5 to 10 million years.

Organic matter - right hand amino acids - nonbiological.

Most in asteroid belt 2.2 to 3.3 A.U. from Sun. Mars at 1.5 A.U., Jupiter at 5.2 A.U. Asteroids are small and far way. Star-like image.

No asteroids in Kirkwood gaps with orbital periods exact fraction of Jupiter's orbital period. (Due to repeated encounters with Jupiter's gravity)

Some asteroids move closer to the Sun than the asteroid belt.

Amor asteroids - cross Mars' orbit

Apollo asteroids - cross Earth's orbit (Earth crossers)

Asteroids never coalesced to form a single large planet because of rapid and nearby formation of Jupiter.

Asteroids are the left-over rubble of the formation of the solar system.

Asteroids have endured eons of destructive collisions - some with common orbits (families) are remains of larger worlds.

Small size - largest a few hundred kilometer across. Many more - thousands - are even tinier (hence asteroid = star-like image).

Rotation periods from variations in brightness 5 to 10 hours. None rotate faster than 2.5 hours period.

Combined mass of all asteroids less than mass of our Moon.

Distance S = silicate = red

From M = metallic

Sun C = Carbonaceous = black (Range of asteroid colors same as meteorites)

Increases

The view close up (not in text - Encounters with The Asteroids on the web)

Most asteroids are too small to be resolved with telescopes.

951 Gaspra - angular irregular shape, violent past, surface 200 million years old.

243 Ida - more craters older than Gaspra.

Asteroids do not have enough gravity to pull themselves into a spherical shape.

Gaspra and Ida = battered, misshapen worlds, replete with impact craters. Note asteroids are small, inert bodies that cannot generate their own heat so not shaped from internal processes.

2. Rubble pile - reaccumulated collision fragments Instead of dispersing, fragments regroup under weak mutual gravity.

Ida is an example, consisting of two parts that bear little resemplance to each other - one with large craters the other without large craters - probably reaccumulated rubble pile.

Regolith = layer of fine debris.

On Ida - debris of largest crater is young fresh blue material. Reddens with age.