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the world is not coming to an end in 2012
The world is not coming to an end in 2012.
  • The story started with claims that a planet called Nibiru is headed toward Earth. This catastrophe was initially predicted for May 2003, but when nothing happened the doomsday date was moved forward to December 2012. Then this was linked to the end of one of the cycles in the ancient Mayan calendar at the winter solstice in 2012 -- hence the predicted doomsday date of December 21, 2012.
  • Nibiru and other stories about wayward planets are an Internet hoax. There is no factual basis for these claims. If Nibiru or Planet X were real and headed for an encounter with the Earth in 2012, astronomers would have been tracking it for at least the past decade, and it would be visible by now to the naked eye.
  • The Mayan calendar isn’t ending on December 21, 2012 any more than our calendar ends on December 31. The first day of a new cycle will begin on December 22, 2012.

Compared with planets, the comets and asteroids are unevolved objects. They are much as they were when they formed 4.6 billion year ago.
  • By studying comets and asteroids we can learn about the conditions in the solar nebula from which the planets formed.
    • Comets are the icy remains of the solar nebula.
    • Asteroids are the rocky remains of the solar nebula.
    • Meteors are the fragments of comets and asteroids that fall into Earth’s atmosphere.
Meteoroid = fragment of a comet or asteroid in space
  • Meteor = meteoroid colliding with Earth and producing a visible light trace in the sky
  • Meteorite = meteor that survives the plunge through the atmosphere to strike the ground


Comets leave a trail of debris behind them as they orbit the sun.

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

A meteor shower occurs when Earth passes through the orbital path of a comet. The comet may still exist or have been destroyed.

meteor showers


Meteor Showers

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

All of the meteors in a given shower have the same origin.

Shower Date R.A. Dec. Associated


Perseids Aug. 10-14 3h4m 58o 1982 III

Leonids Nov. 14-19 10h12m 22o 1866 I Temp

Geminids Dec. 10-13 7h28m 32o

radiants of meteor showers


Radiants of Meteor Showers

Tracing the tracks of meteors in a shower backwards, they appear to come from a common origin, the radiant.

Common direction of motion through space.

Most meteors we see, whether or not there is a shower, come from comets. Therefore, they are small specks of matter that burn up in the atmosphere.



Sizes from microscopic dust to a few centimeters.

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).

analysis of meteorites


Analysis of Meteorites

3 broad categories:

  • Iron meteorites
  • Stony meteorites
  • Stony-iron meteorites
Iron Meteorites
    • Dense and heavy
    • Dark rusted surfaces
    • When sliced, polished, and etched with nitric acid, they reveal Widmanstatten patterns caused by crystals of nickel-iron alloys that have grown large. This indicates that the meteorite cooled slowly.
  • Stony-iron meteorites are a mixture of iron and stone. They appear to have formed when a mixture of molten iron and rock cooled and solidified.
Stony Meteorites
    • Chondrites
      • Contain chondrules (rounded bits of glassy rock ranging from microscopic to pea size.)
        • They formed from droplets of molten rock that cooled and hardened rapidly when the solar system was young.
        • Their presence indicates that the meteorites have not melted since they formed.
      • Some chondrites only have a few volatiles indicating they were heated slightly, which caused them to lose their volatiles, but not heated enough to destroy the chondrules.
      • Carbonaceous chondrites contain both chondrules and volatile compounds including carbon. They have not been heated since the formation of the solar system.
    • Achondrites contain no chondrules and lack volatiles. They appear to have been heated. They are similar to Earth’s lavas.
(Volatiles, are elements and compounds with low boiling points. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen and methane.)

While meteorites can tell us about the origin of the solar system, they also contain grains of interstellar matter that predate our solar system.

the origins of meteorites


The Origins of Meteorites
  • Probably formed in the solar nebula, ~ 4.6 billion years ago.
  • Almost certainly not from comets (in contrast to meteors in meteor showers!).
  • Probably fragments of stony-iron planetesimals


Planetesimals cool and differentiate;

Forming iron-nickel cores and rocky mantles.

Collisions broke up the bodies to form different kinds of meteorites:

Iron meteorites – iron cores

Stony Meteorites – stony mantel.

Meteorites can not have been broken up from planetesimals very long ago

Remains of planetesimals should still exist.





Small, irregular objects, mostly in the apparent gap between the orbits of Mars and Jupiter.

Last remains of planetesimals that built the planets 4.6 billion years ago!

evidence for collisions


Evidence for Collisions

Hirayama families: Groups of asteroids sharing the same orbits and spectroscopic characteristics – apparently result of common origin through collisions.

Radar images of asteroids reveal irregular shapes, sometimes peanut-like shapes:

Evidence for low-velocity collisions between asteroids on very similar orbits.

colors of asteroids


Colors of Asteroids

“Colors” to be interpreted as albedo (reflectivity) at different wavelengths.

M-type: Brighter, less reddish asteroids, probably made out of metal-rich materials; probably iron cores of fragmented asteroids

S-type: Brighter, redder asteroids, probably made out of rocky materials; very common in the inner asteroid belt

C-type: Dark asteroids, probably made out of carbon-rich materials (carbonaceous chondrites); common in the outer asteroid belt



Distribution: C-type asteroids in the outer asteroid belt; S-type asteroids in inner asteroid belt  may reflect temperatures during the formation process.

However, more complex features found:

Vesta shows evidence for impact crater and lava flows.

Images of the Asteroid Vesta show a complex surface, including a large impact crater.

Heat for existence of lava flows probably from radioactive decay of 26Al.

Meteorite probably fragmented from Vesta

dawn mission to vesta
Dawn Mission to Vesta

Not all asteroids are in the asteroid belt.
  • A few thousand asteroids larger than 1 km cross Earth’s orbit.
    • Near Earth Objects (NEOs)
    • Searches are underway to find these NEOs.
the origin of the asteroids
The Origin of the Asteroids
  • Ray blasts from Death Stars are unlikely to cause planets to explode as in Star Wars.
  • Besides, the total mass of all the asteroids is only ~ 1/20 that of the moon.
  • The asteroids probably are not the result of a planet exploding.
  • Asteroids are probably the remains of a planet that did not form at 2.8 Au from the sun due to Jupiter’s gravity.
  • Therefore, asteroids are probably fragments of left over planetesimals.
    • The ones in the outer belt formed where the solar nebula was cooler so carbon could condense. That’s why type C asteroids are in the outer belt and type S are in the inner belt.



Comet C/2001 Q4


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

Five spacecraft flew past the nucleus of Comet Halley when it visited the inner solar system in 1985 and 1986.
  • Since then, spacecraft have visited the nuclei of Comet Borrelly, Comet Wild 2, and Comet Temple 1.
the geology of comet nuclei


The Geology of Comet Nuclei

Comet nuclei contain ices of water, carbon dioxide, methane, ammonia, etc.:

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.

deep impact spacecraft 2005
Deep Impact Spacecraft 2005
  • Released a probe that Comet Temple 1 slammed into.
  • The nucleus is rich in dust finer than talcum powder. It is not solid rock, but has the density of fresh fallen snow.
stardust spacecraft
Stardust spacecraft

Collected particles from Comet Wild 2 that were parachuted back to Earth.

When a comet is far from the sun, it’s just the nucleus. When it gets close enough to the sun, it begins to sublime and a coma and tail form.

The coma of a comet is the cloud of gas and dust that surrounds the nucleus. It can be over a million km in diameter, which is bigger than the sun.


Two Types of Tails


gas 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

Comet tails point generally away from the sun, but their precise direction depends on the flow of the solar wind and the orbital motion of the nucleus.

Comet Mrkos in

1957 shows how

The gas tail can

change from

night to night

due to changes

in the magnetic

field in the

solar wind.

fragmenting comets


Fragmenting Comets

Comet Linear apparently completely vaporized during its sun passage in 2000.

Only small rocky fragments remained.

fragmentation of comet nuclei


Fragmentation of Comet Nuclei

Comet 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.

comets holmes eruption
Comets Holmes Eruption

Spectacular outbursting Comet Holmes exploded in size and brightness on October 24, 2007. It expanded in size until it was bigger than the Sun. This amazing eruption of the comet is produced by dust ejected from a tiny solid nucleus.


Composite of 19 snapshots of Comet Holmes, showing its changing brightness and position spanning the period from October 2007 until March 2008. During its outburst in late October 2007, the comet brightened by a factor of about 500,000 as a large pocket of volatile material exploded through its crust and spread into space.

the origin of comets
The Origin of Comets
  • Long period comets
    • Most comets are long period comets
    • Have long elliptical orbits with periods greater than 200 years
    • Orbits are randomly inclined
    • Some orbit the sun clockwise and some orbit the sun counter clockwise.
  • Short period comets
    • About 100 known
    • Periods less than 200 years
    • Inclinations within 30 degrees of the plane of the solar system.
    • Most revolve counterclockwise.
Comets cannot last more than 100 to 1000 orbits around the sun before all their ice is gone and there is nothing left but dust and rock.
  • The comets we see today cannot have been orbiting close to the sun for 4.6 billion years.
  • Where do new comets come from?
the origin of comets1


The Origin of Comets

Many comets are believed to originate in the Oort cloud:

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

Gravitational influence of occasional passing stars may perturb some orbits and draw them towards the inner solar system.

10,000 – 100,000 AU

Interactions with planets may perturb orbits further, capturing comets in short-period orbits.

Oort Cloud

is there a large planet lurking in the oort cloud
Is there a large planet lurking in the Oort Cloud?

Can Wise Find the Hypothetical Tyche?

While some short period comets, such as Comet Halley, may have been comets from the Oort cloud that were captured by Jupiter, this cannot be true of all the short term comets.
  • It isn’t possible for some of the short period comets to obtain the orbits they have if they were Oort cloud comets captured by a planet.
  • There must be another source of comets in our solar system then the Oort cloud.
In 1951 astronomer Gerard Kuiper proposed that the formation of the solar system should have left behind a disk shaped belt of small, icy planetesimals beyond the Jovian planets and in the plane of the solar system.
  • This is what we now call the Kuiper belt, which is at ~ 30 – 100 AU from the sun.
  • Hundreds of Kuiper belt objects have been found in orbits extending from Neptune (30 AU) out to about 50 AU.
  • Pluto and Charon are Kuiper-belt objects.
  • This is the second source of small, icy bodies in the outer solar system.
Similar belts have been detected around other stars e.g. Beta Pictoris.

Objects in the Kuiper belt can be perturbed into the inner solar system and be captured into smaller orbits becoming short term comets.

The two places where comets originate are the Oort cloud and the Kuiper belt.

how did the oort cloud and kuiper belt form
How did the Oort Cloud and Kuiper Belt Form?
  • The Kuiper belt probably formed when the solar system did.
  • The objects in the Oort cloud could not have formed where they are now from the solar nebula.
    • They are too far away.
    • They aren’t in the plane of the solar system.
    • These objects may have formed in the solar system amongst the orbits of the Jovian planets and then were ejected from the solar system by the gravity of the Jovian planets.

Impacts on Earth

  • Small meteorite impacts occur quite often.
  • Every few years a building is damaged by a meteorite.
  • A few years ago, a car was hit by a meteorite and then auctioned off for $10,000,000.
  • Really large impacts are rare.

In 1954 Mrs. E. Hulitt Hodges of Sylacauga, Alabama was hit by a meteorite while napping in her living room. This is the only known person to have been injured by a meteorite.



Over 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



Barringer Crater: ~ 1.2 km diameter; 200 m deep

Sediments from all over the Earth from 65 million years ago have an overabundance of iridium, an element common in meteorites but rare in the Earth’s crust.
  • The impact of a large meteorite at that time may have altered the atmosphere and climate on Earth, which caused the extinction of the dinosaurs and 75% of the other species on the planet.
The biggest extinction we know of occurred 250 million years ago – The Great Dying.
    • 95% of life in the oceans died out.
    • 80% of life on land died out.
  • Data indicates that a large impact occurred off the shore of Australia 250 million years ago.

The 1908 Tunguska event in Siberia destroyed an area the size of a large city. Here the area of destruction is superimposed on a map of Washington, D.C., and its surrounding beltway. In the central area, trees were burned; in the outer area, trees were blown down pointing away from the center of the blast for as far as 30 km.

the effects of a large impact on earth
The Effects of a Large Impact on Earth
  • If on land, the initial shock would be deadly.
  • If on sea, there would be tidal waves hundreds of meters high that would devastate coastal regions.
  • Lots of dust would be thrown into the atmosphere.
    • The hot dust falling back to Earth could start fires.
    • The dust left in the atmosphere would block sunlight, making temperatures cooler for a time.
In 1998, newspaper headlines read “Mile Wide Asteroid to Hit Earth in October 2028.”
  • Rumors of Earth’s demise were greatly exaggerated. The asteroid will miss Earth by 600,000 miles.
  • Now rumor is a 430 mile wide asteroid named Apophis will hit in 2029 or 2036.
    • Actually Apophis is not 430 miles in diameter but more like 250 METERS.
    • The future for Apophis on Friday, April 13 of 2029 includes an approach to Earth no closer than 29,470 km (18,300 miles, or 5.6 Earth radii from the center, or 4.6 Earth-radii from the surface) over the mid-Atlantic, appearing to the naked eye as a moderately bright point of light moving rapidly across the sky.
    • Updated computational techniques and newly available data indicate the probability of an Earth encounter on April 13, 2036, for Apophis has dropped from one-in-45,000 to about four-in-a million.