Impacts with Space Objects

Moon shows many impact scars though most are prior to 3.8 billion years ago

Earth destroys most evidence over time. Why?

ASTEROIDSOriginate in the asteroid belt between Mars and Jupiter

Mostly pieces of rock and metals that never coalesced to form a planet

• 200 asteroids with diameters >100km

• ~1000 asteroids >30km diameter

• ~1,000,000 >1km diameter

COMETSShort-period comets

They are part of the Kuiper belt, a flattened disk of comets made up of icy debris left over from the formation of the outer planets.

Make a complete orbit in <200 years.

Haley’s Comet every ~75 years

Comet Orbits

• http://www.st-andrews.ac.uk/~bds2/ltsn/ljm/JAVA/COMETORB/COMET.HTM

Long-period cometsThey are part of the Oort cloud, a vast diffuse envelope

of encircling comets. Period may thousands of years of more.

Comet Hale Bopp – visible for 18 months in 1996 - 1997

• All comets are mainly made of ice and rocky debris.

IMPACTSEnergy release depends on speed and size.

Asteroids may impact at >30,000mph.Long-period comets >150,000mph.M-16 rifle ~2250 mph.

Given equal size, the mass of an asteroid is greater than that of a comet.

Atmosphere may heat the asteroid to 3,000oC.

Impact instantly released enormous heat and energy, melting and vaporizing not only rock but also much of the asteroid itself.

Crater may end up filled with crushed and shattered rock.

Comet Shoemaker Levy 9 impact with Jupiter July 16-22, 1994

Fragment Q impacts on Jupiter. There were 21 discernable fragments up to 2km diameter.

Meteor Crater Arizona>1km wide (4,000 ft.) impact crater created ~50,000

years ago.

METEOR CRATER, ARIZONA

Impact from nickel-iron metallic meteorite (asteroid).

Diameter ~40m (120ft.).

Speed at impact ~27,000mph.

Crater floor holds 365m (870ft.) of shattered rock.

Tektites – glassy spherules formed by in-air-cooling of impact melted rock.

Happens with large asteroid impacts.

Wolfe Creek Australia – age ~300,000 years

Manicouagan Impact Site ~215.5 million years B.P.

~5km (3mi) diameter asteroid~70km (40mi) diameter crater

Extinction of Dinosaurs ~65 Million Years B.P.

KT boundary clay layer and iridium

KT boundary layer Raton Basin

Figure 17.25

Tunguska, June 30, 1908

TUNGUSKA

Probably a stony meteorite or comet possibly smaller than 40m in diameter.

Exploded 5 – 10km (3 – 6mi.) above the earth’s surface.

Flattened trees over 2,150km2 (830mi2)

Figure 17.26

• Tunguska originally thought to be ~60m in diameter. If so, similar impacts every ~2,000 years.

• But, with new estimate of 40m or less, once every ~300 years.

• Total potential hazard from impacts has increased.

NEAR-EARTH OBJECTS (NEOs) PROJECT

Monitors and identifies near earth-objects for potential impact of objects >1km (.62mi.) diameter.

Average time interval between impacts of this size >100,000 years.

However, objects 40m(or less) – 100m in diameter can cause devastating local effects (like Tunguska).

Smaller potentially damaging events occur perhaps every couple of centuries.

Would we have warning before an impact?

Maybe, maybe not.

November 2011 Asteroid

http://neo.jpl.nasa.gov/news/news171.html

Near Earth Objects Tracking

http://neo.jpl.nasa.gov/risk/#legend