Transcript What are comets? - The Astronomy Workshop

What are comets?
In 1704 Sir Edmond Halley hypothesized
that the comets of 1456, 1532, 1607, 1682
were the same object.
 He calculated a 75.7 year orbit and
predicted that it would return in 1758.
 Halley died; but comet returned.
 perihelion=0.59 AU; aphelion=35.3 AU
(closest to Sun)
(farthest from Sun)
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Halley’s comet was at perihelion in 1986
It will return in 2061.
Comets are dirty iceballs!
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Nucleus (very small, about 10 km across)
rock & ices (mostly H2O and CO2 [dry ice],
some methane CH4 & ammonia NH3)
Coma (103 to 105 km across) is gaseous. As
comet approaches Sun ices sublime,
change from solid to gas, dust grains
loosen and move away
Tails (105 km to 1 AU long)—solar wind
(steady stream of solar particles) pushes
gas away; dust continues to orbit Sun
1997—Comet Hale-Bopp
Can you spot the coma, gas tail, and curved dust tail?
Asteroids show up as
streaks on photos.
Since they orbit
the Sun, they are
observed to move
with respect to the
stars.
Most asteroids lie in the asteroid
belt.
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1801 Ceres was discovered first. Approx.
100,000 lie between Mars and Jupiter.
Asteroids are typically cratered and
irregularly shaped.
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Composition
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Carbonaceous—containing carbon
Rocky—mostly silicates
Metallic—iron and nickel rich
NEAR probe orbited Eros (2000) and crash
landed on it (2001)
Eros is a near-Earth asteroid
Orbit brings it close to Earth!
View of 3 asteroids from spacecraft
50 km
Impacts on Earth
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Meteors burn up in the atmosphere;
meteorites don’t; impacts can and do
happen
Meteor Crater, Arizona
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Diameter is 1.2 km; 50,000 years old
Projectile was 50 meter diameter, metallic
asteroid
Crater diameter is 1.2 km, 200 m deep
How does a small body produce a very
large impact crater?
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A huge amount of energy is released as
the body explodes on impact.
The energy released depends on a body’s
mass (m) and velocity (v).
Energy of motion, kinetic energy (KE) is
given by the formula KE = ½ m v2
The projectile that exploded on impact and produced Meteor
Crater was equivalent to the energy of 1000 Hiroshima bombs.
20 megatons!
In 1908, an 80 meter stony asteroid exploded over
Tunguska Siberia. The air blast knocked down trees
over 30 km. Windows were broken 600 km away.
People heard the noise 1000 km away
Italian clay—65 million years old
Iridium, shocked quartz, & soot in
layer support impact hypothesis
(Italian coin is the size of a quarter)
Did an impact cause mass
extinctions 65 million yrs. ago?
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Disappearance of 70% species from the
fossil record, including dinosaurs
Evidence for 10 km asteroid impact in
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Worldwide 65 million yr old clay layer with:
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Iridium (element #77) rare in crust, but in clay
Shocked quartz—indicates heat & pressure
Soot—global wildfires
What happens when a large (10 km)
asteroid or comet strikes?
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Object explodes (energy = billion megatons)
on impact and vaporizes a portion of crust
Molten rock is thrown into atmosphere; it falls
back down as a fiery rain igniting forests
Lighter dust stays suspended=>dark & cold
Photosynthesis & food chain disrupted
Soot, shocked quartz, iridium dust settles
creating 65 million year old clay layer
exposed at over 100 places on Earth
Chicxulub Crater is identified off
Yucatan Peninsula
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300 km diameter ringed basin
About 65 million year old
Could this happen again?
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Yes! It is estimated that more than 10,000
objects approach Earth
Tunguska/Meteor Crater events happen about
every 1000 years
A “Big One” predicted once every 100 million
years
Crater is buried beneath Yucatan Penisula and
Gulf of Mexico. This map was made by
studying gravity measurements made in the
region. Compressed layers show crater.