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Lecture #24: Comets
The Main Point
• Comets!
•What are they?
Comets are small icy objects from the
outer solar system that evaporate
spectacularly as they get heated when
their orbits carry them closer to the
Sun
•Where do they come from?
•Relationship to asteroids and planets...
• Reading:
– Chapter 12.2
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Comet Lingo
• "Comet" comes from the Greek word
"kometes", meaning long-haired
• The solid inner part of a comet is
called the nucleus. Most comet
nuclei are only a few km in diameter,
and are composed of rock and ice.
• The "fuzzy" bright region around the
nucleus is called the "coma" ("hair"),
and is composed of gas and dust.
• Solar heating leads to the formation
of an extended region of gas and
dust, called the tail.
A part of the Bayeux Tapestry, commemorating the Norman
conquest of 1066 and also archiving the appearance of a
"portentous star" in the sky. The star is now known to have been
Halley's comet
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• An enormous hydrogen cloud
surrounds the coma of most comets
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Comet Tails
The life
of a Comet
Gas
Dust
• Dust tails are small pieces (micron to mm sized) of dust,
rock, and ice that are shedding off the nucleus and
following Keplerian orbits
• Ion tails (or plasma tails) are gases evaporated from the
nucleus, ionized, and pushed in the anti-Sun direction by
the solar wind
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A Model Comet...
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Other Cometary Properties
• A comet is essentially a "dirty snowball"
• Comet nuclei are irregularly-shaped, like asteroids, but smaller
– Model proposed by astronomer Fred Whipple in 1950
• Cometary activity is caused by sublimation of ices
• The activity appears to be confined to isolated jets on the nucleus
• Comet nuclei are very dark, typically reflecting < 4% of the
incident sunlight (as dark as a charcoal briquet)
• Comet nuclei have very low density (0.1 to 0.25 g/cm3), and thus
appear to be mostly made of loosely packed (porous or "fluffy") ice
• Comets have more than just two types of tails; one is primarily dust
and ice, but several others are composed of charged particles
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Cometary Composition
Halley's Comet Up Close!
Atoms, Molecules, and Ions Identified in Comets
Atoms: H,C,O,S,Na,Fe,K,Ca,V,Cr,Mn,Co,Ni,Cu,Si,Mg,Al,Ti
Molecules:OH,S2,OCS,H2S,H2O,HDO,H2CO,C2,C3,CH,CN,CO,CO2,
CS,CH4,C2H2,C2H6,NH,NH2,HCN,CH3CN,N2,NH3
Ions: H2O+,OH+,H3O+,CO+,CO2+,CH+,CN+,N2+,C+,Ca+
• Images and other data from the VEGA and Giotto
spacecraft (1986 flybys)
Sketch map
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Giotto image
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•
The best measured is comet Halley, with a composition of nearly 80%
water ice. But Halley is highly evolved, and may not be typical
•
Farther from the Sun, at temperatures below the evaporation
temperature of water ice, evidence for CO, CO2, and other ices is seen
•
The dark surface material may be complex organic molecules formed
from the residual cometary material after the ices are melted away
(from direct spacecraft measurements, as well as spectroscopy from
telescopes)
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Comet Orbits
•
Short-Period Comets
There are two populations of comet orbits:
•
– short period comets orbit the Sun in < 200 years,
often have low inclinations, and only travel out as far
as the outer planets
•
• Examples:
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– long period comets take thousands of years or more
to orbit the Sun, and come from all directions in the
sky
•
• Examples:
– Hyakutake (32,000 years!)
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Recent discoveries of a disk of small bodies beyond Neptune, the Kuiper Belt,
appears to provide a source for the short period comets
They are brought closer to the Sun by
frequent encounters with Jupiter and
other giant planets
– Fragments, remnants become the
small short period comets
– Halley (76 years)
– Encke (3.3 years)
• These two populations must have
different sources
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Only the largest and brightest
Kuiper Belt Objects (KBOs) have
been discovered so far
There may be 100,000 or more KBOs
larger than a few hundred km in size
There may be hundreds of millions of
KBOs that are comet-sized (few km)
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Long-Period Comets
Comet McNaught
• Orbits are traced back in time...
• Most lead to a vast but
unobserved spherical cloud of
small objects:
– This is the Oort Cloud
– 20,000 to 100,000 AU
• Stars passing "close" to the Sun
can jostle these objects and
cause them to fall inwards
• There may be 1011 to 1012
comets in the outer Oort cloud,
and perhaps ten times that
many comets in the cloud as a
whole!
• Comet mass > 1000 Earths??
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Comet McNaught swung by
the Sun in mid-January 2007.
Fierce solar heat turned it into
the brightest comet in 40
years. Now it is receding back
into the outer solar system.
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Meteors: Pieces of Comets?
Comet 17P/Holmes
• The short period comets appear to be the sources of several
meteor showers, as the Earth crosses through their debris
•The Hubble image taken
Nov. 4, 2007 shows the heart
of the comet.
•The composite color image,
taken Nov. 1 by an amateur
astronomer, shows the
complex structure of the
entire coma.
•Comet Holmes brightened
by nearly a millionfold in a
24-hour period beginning
Oct. 23, 2007.
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Why are Comets Important?
Spectacular Comet Deaths!
• Comets can be thought of as big celestial chemistry sets!
• Comets hitting the Sun
– Heat, light, water, organic chemicals...
– Aren't these the building blocks of life?
– SOHO satellite data
– "Sun grazers"
– Do these events
influence solar
activity?
• Comets must occasionally impact the Sun & planets...
– They deliver volatiles and organic molecules (Bringers of Life!)
– They can also cause massive ecological destructions (Bringers of Death!)
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Spectacular Comet Deaths!
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Current & Future Comet Missions
• Comets smashing into
Jupiter
•
Completed: Stardust (NASA)
Stardust re-entry
– Mission to study and return samples from comet Wild-2
– Launched in 1999, Comet encounter Jan. 2, 2004, Earth
sample return Jan. 15, 2006
– Shoemaker-Levy 9, July
1994
•
Completed: Deep Impact (NASA)
– Mission to study and expose the subsurface of comet
Tempel 1
– Launch Jan. 2005, encounter comet in July 2005
•
Earth for scale
Comet impact site
Underway: Rosetta (ESA)
– Mission to orbit and send landers to an active comet
– Launched March 2, 2004; encounter Comet
67P/Churyumov-Gerasimenko in the year 2014
•
Proposed: CONTOUR 2 (Cornell)
– Mission to study diversity of comets
– Reflight of Cornell’s CONTOUR mission lost in 2002
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Deep Impact: A Smashing Success
Summary
• Comets are small, irregular, icy and rocky objects that
evaporate spectacularly as they approach the Sun
• Comets fall into two main classes:
This movie taken by Deep Impact's flyby
spacecraft shows the flash that occurred when
comet Tempel 1 ran over the spacecraft's probe.
It was taken by the flyby craft's high-resolution
camera over a period of about 40 seconds.
– Short period comets (P < 200 yr.); source = Kuiper Belt
– Long period comets (P > 1000 yr.); source = Oort cloud
Arrows point to a large,
smooth region. The
impact site is indicated
by the large arrow at
the bottom of this
image.
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• The main parts of a comet are the nucleus, coma, tail, and
extended hydrogen cloud
• Comets are essentially "dirty snowballs" composed of water
and other ices, silicate minerals, and organic compounds
• Comet impacts may have brought substantial quantities of
water, other volatiles, and organics to the planets...
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Next Lecture...
Impacts!
• Will asteroids and comets hit Earth?
• What is the magnitude of the risk?
• What would be the consequences?
• What can be done about it?
Reading: Chapter 12.4
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