The world is not coming to an end in 2012.
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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.
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
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).
While meteorites can tell us about the origin of the solar system, they also contain grains of interstellar matter that predate our solar system.
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.
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” 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
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
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.
Collected particles from Comet Wild 2 that were parachuted back to Earth.
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.
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
1957 shows how
The gas tail can
night to night
due to changes
in the magnetic
field in the
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. http://www.ifa.hawaii.edu/faculty/jewitt/holmes.html
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.
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.
Can Wise Find the Hypothetical Tyche?
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.
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
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.