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Lecture 24
Comets
Asteroids and Trojans
Asteroids are planetesimals left over from the
formation of the solar system that orbit mostly
between the orbits of Mars and Jupiter
4. Planet f
Mercury,Venus, Earth and Mars
+ Asteroids
Jupiter, Saturn, Uranus and Neptune
+
Pluto
Does the solar system end at Neptune?
(a dwarf planet)
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Comet Halley
Comet Hyakutake
Comet Hale-Bopp
Comet West
Two reservoirs of comets exist beyond the orbit
of Neptune and are the sources of the comets
that we see in the inner solar system
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Comet Kohoutek
Comet Giacobini-Zinner
Comets are discovered in the inner solar system
but do not originate there
Comets that come close enough to the Sun to be
detectable from Earth have very eccentric orbits
Observation of orbits tells us that comets
originate from the distant outer solar system
beyond the orbit of Neptune
(In the above diagram, the comet should not
have a tail - it is too far from the Sun)
The outermost reservoir of comets is the
Oort Cloud
Extends to 100, 000 AU
The Oort comet cloud
Extends to 100,000 AU
More than 200 billion comets hibernate in the remote Oort comet cloud, shown here in cross
section. It is located in the outer fringes of the solar system, at distances of about 100,000 AU
from the Sun. By comparison, the distance to the nearest star, Proxima Centauri, is 0.27 million
AU, while Neptune orbits the Sun at a mere 30 AU. The planetary realm therefore appears as an
insignificant dot when compared to the comet cloud, and has to be magnified by a factor of
1,000 in order to be seen. This comet reservoir is named after the Dutch astronomer Jan H.
Oort (1900-1992) who, in 1950, first postulated its existence.
The Oort cloud has never been seen - it’s existence has
been deduced from an analysis of the orbits of new
comets
Nearest star is 270,000 AU
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outside the orbit of Neptune
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100 million to 10 billion comets
More than 200 billion comets hibernate in the remote Oort comet cloud, shown here in cross
section. It is located in the outer fringes of the solar system, at distances of about 100,000 AU
from the Sun. By comparison, the distance to the nearest star, Proxima Centauri, is 0.27 million
AU, while Neptune orbits the Sun at a mere 30 AU. The planetary realm therefore appears as an
insignificant dot when compared to the comet cloud, and has to be magnified by a factor of
1,000 in order to be seen. This comet reservoir is named after the Dutch astronomer Jan H.
Oort (1900-1992) who, in 1950, first postulated its existence.
A repository of frozen, comet-sized worlds resides in the outer precincts of the planetary
system, just beyond the orbit of Neptune and near the orbital plane of the planets. Known as
the Kuiper belt, it is thought to contain 100 million to 10 billion, or 108 to 1010, comets. Many
short-period comets are tossed into the inner solar system from the Kuiper belt.
We have now seen about 1000 Kuiper Belt objects
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What you see when looking at a comet depends on how you look at it.
The nucleus of a comet is usually invisible, unless a spacecraft is sent
in to take a glimpse. A comet first becomes visible when it develops
a coma of gas and dust. When the comet passes closer to the Sun, long
ion and dust tails become visible, streaming out of the coma in the
direction opposite to the Sun.
The comet’s tail always points
away from the Sun, due to the
solar wind. The ion tail is straighter
than the dust tail.
Exit
The comet’s tail develops
as it approaches the Sun
and disappears as it moves
away from the Sun. The
ion tail always points away
from the Sun; the dust tail
curves a bit as the comet
gets ahead of it in its orbit.
Approaching Sun
In 1984 IRAS - the infrared satellite - detected
cometary TRAILS as well as TAILS
Comet Trail and Tail
This is an artist's concept of
a comet dust trail and dust tail.
The trail can only be seen in
the light of radiated heat. The
dust trail is made of particles
that are the size of sand grains
and pebbles. They are large
enough that they are not
affected much by the Sun's
light and solar wind.
The dust tail, on the other hand,
is made of grains the size of
cigarette-smoke particles. These
grains are blown out of the dust
coma near the comet nucleus by
the Sun's light.
When a comet nears the Sun, its ices can sublimate into gas and carry off
dust, creating a coma and long tails.
Formation of dust TRAIL
Dust particles are “massive” compared to individual atoms. They move slowly
(recall: F = ma) away from nucleus and Sun. They lag behind the comet
because they drift into orbits that are bigger than the comet’s orbit—therefore,
they must slow down because of conservation of angular momentum
Formation of dust TRAIL
Dust particles are “massive” compared to individual atoms. They move slowly
(recall: F = ma) away from nucleus and Sun. They lag behind the comet
because they are drifting into orbits that are bigger than the comet’s orbit—
therefore, they must slow down because of conservation of angular momentum
Formation of dust TRAIL
Dust particles are “massive” compared to individual atoms. They move slowly
(recall: F = ma) away from nucleus and Sun. They lag behind the comet
because they are drifting into orbits that are bigger than the comet’s orbit—
therefore, they must slow down because of conservation of angular momentum
Note: the gas atoms are very low mass, and are pushed out so fast
that their paths are straight lines! (not curved like the dust paths)
Typical cometary mass: 1012 to 1016 kg
Each trip close to the Sun removes some material; Halley’s
Comet, for example, is expected to last about another 40,000
years
Sometimes a comet’s nucleus can disintegrate violently
Comets seen in the inner solar system must be
replenished from unseen reservoirs in the
outer solar system
Lifetime of Halley about 40,000 years
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14.2
Comets
Kuiper Belt is the dominant source
Most comets that enter the inner solar system reside in the
Kuiper belt outside the orbit of Neptune. Occasionally a
comet from the far larger Oort cloud wanders into the inner
solar system as well.
Halley’s Comet - the most famous comet
14.2 Comets
Halley’s Comet is one of the most famous; it has
a period of 76 years and has been observed
since antiquity. Its most recent visit, in 1986, was
not spectacular.
Left: The comet in 1910, as seen with the naked
eye
Right: The comet in 1986, as seen through a
telescope
Halley's Comet or Comet Halley is the best-known of the short-period comets,
and is visible from Earth every 75 to 76 years.
Halley is the only short-period comet that is clearly visible to the naked eye
from Earth, and thus the only naked-eye comet that might appear twice in a
human lifetime.
Other naked-eye comets may be brighter and more spectacular,
but will appear only once in thousands of years.
14.2 Comets
Halley’s Comet has a shorter period than most comets, but its
orbit is not in the plane of the solar system, probably due to an
encounter with a larger object
Next appearance is 2061 - 50 years from now
164 BCE
The Adoration of the Magi (circa
Observation of Halley's Comet,
recorded in cuneiform on a clay
tablet between 22–28 September
164 BCE, Babylon, Iraq.
In 1066, the comet was seen in England and
thought to be an omen: later that year
Harold II of England died at the Battle of Hastings;
it was a bad omen for Harold, but a good omen
for the man who defeated him, William the Conqueror.
The comet is represented on the Bayeux Tapestry as
a fiery star, and the surviving accounts describe it as
appearing to be four times the size of Venus and
shining with a light equal to a quarter of that of the
Moon. Halley came within 0.10!AU of the Earth at
that time.
1066
Birth
of Christ?
The Adoration of the Magi (circa 1305) by Giotto, purportedly depicting Halley.
The Adoration of the Magi (circa 1305) by Giotto, purportedly depicting H
Observation of Halley's Comet,
recorded in cuneiform on a clay
tablet between 22–28 September
164 BCE, Babylon, Iraq.
A photograph of Halley's Comet taken during its 1910 approach
Observation of Halley's Comet,
recorded in cuneiform on a clay
tablet between 22–28 September
164 BCE, Babylon, Iraq.
A photograph of Halley's Comet taken dur
Halley's comet in 1986
Nucleus of Comet Halley
Giotto Image 1986
The Earth’s orbit intersects a stream of meteoric material left along the orbit of Comet Halley,
producing two meteor showers, the Eta Aquarids in May and the Orionids in October. Other
comets intersect the Earth’s orbit just once during their trip around the Sun. Annual meteor
showers are created when the Earth enters the intersection point, such as the August Perseids
produced by debris from Comet Swift Tuttle.
A composite image of the nucleus of Comet Halley (right) obtained using images taken in
March 1986 with the camera on board the Giotto spacecraft, from a distance of 6.5 million
meters before comet dust destroyed the camera. It is compared with a schematic drawing (left)
that highlights the major features recognizable in the photograph. The nucleus is about 16
kilometers long and 8 kilometers wide. Dust and gas geyser out of narrow jets from the sunlit
side of the nucleus, but about 90 percent of the surface is inactive. The gas is mainly water
vapor sublimed from ice in the nucleus, while a significant fraction of the dust may be dark
carbon-rich matter. A dark surface crust, which insulates most of the underlying ice, is blacker
than coal, reflecting about 4 percent of the incident sunlight. “Mountains” rise about 500 meters
above the surrounding terrain, while a broad “crater” is depressed about 100 meters.
Comet Hartley 2 seen by NASA EPOXI mission
Date: 4 Nov 2010 This close-up view of comet Hartley 2 was taken by NASA's EPOXI mission
during its flyby of the comet. It was captured by the spacecraft's Medium-Resolution instrument.
Nucleus of Comet Borrelly
NASA Deep Space 1 on the way to
Comet Tempel 1
What you see when looking at a comet depends on how you look at it. The
nucleus of a comet is usually invisible, unless a spacecraft is sent in to take a
glimpse. A comet first becomes visible when it develops a comapassing
of gas and dust.
When the comet passes closer to the Sun, long ion and dust tails become visible,
streaming out of the coma in the direction opposite to the Comet
Sun. WhenBorrelly
looking at a
comet in ultraviolet light, the hydrogen atoms in its huge hydrogen
cloud are
in 2001
detected.
A camera on board the Deep Space 1 spacecraft peered into the icy heart of Comet
Borrelly on 22 September 2001, taking this image from a distance of 3.4 million
meters. The nucleus is shaped like a gigantic bowling pin, with a length of about 8
kilometers and a width of roughly half that size. A dark veneer of material covers
most of the nucleus, reflecting only 4 percent of the incident sunlight on average.
Rugged terrain is found on both ends of the nucleus, while bright smooth plains
are present in the middle. Jets of gas and dust shot out from all sides of the
comet’s nucleus as it rotated, producing a flow of ions that was not centered on
the nucleus.
14.2 Comets
The Deep Impact mission slammed a projectile into comet
Tempel 1 and studied the material expelled in order to analyze
the composition of the comet
This allowed analysis of fresh material below
the dark crust
Comet Tempel 1
Date: 6 Sep 2005
The impact site has the highest resolution
because images were acquired until about
4 sec from impact or a few meters from
the surface.
Arrows a and b point to large, smooth
regions. The impact site is indicated
by the third large arrow.
The scale bar is 1 km and the two
arrows above the nucleus point
to the sun and the rotational axis
of the nucleus.
Impact Site
Bull's Eye!
Date: 4 Jul 2005
This image shows the initial ejecta that resulted
when NASA's Deep Impact probe collided with
comet Tempel 1 at 10:52 p.m. Pacific time,
July 3 (1:52 a.m. Eastern time, July 4) .
Name
Dimensions
km
Density
g/cm3
Halley's Comet 15!"!8!"!8
0.6
Tempel 1
7.6 x 4.9
0.62
19P/Borrelly
8"4"4
0.3
81P/Wild
5.5"4.0"3.3
0.6
Densities are less than water ice - must be porous
The albedos (reflectivity) are low - a few percent
- comet nucleii are black!
14.2 Comets
The Stardust mission flew
through the tail of comet
Wild-2, gathering dust
particles in detectors made
of aerogel and returning
them to Earth for analysis
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4. Planet fo
End of Tour
Mercury,Venus, Earth and Mars
Jupiter, Saturn, Uranus and Neptune
KEPLER mission is searching for habitable planets
NASA Ames Research Center/W. Stenzel (OSC)/Artist’s Concept
National Aeronautics and Space Administration
Kepler Mission: A Search for Habitable Planets
www.nasa.gov
Kepler in Brief
A Nutshell Description of the Kepler Mission
Why?
The Kepler Mission is a NASA Discovery Program for detecting potentially life-supporting
planets around other stars. All of the extrasolar planets detected so far by other projects are
giant planets, mostly the size of Jupiter and bigger. Kepler is poised to find planets 30 to 600
times less massive than Jupiter.
How?
By a method known as the transit method of planet finding. When we see a planet pass in front
of its parent star it blocks a small fraction of the light from that star.When that happens, we say
that the planet is transiting the star. If we see repeated transits at regular times, we have
discovered a planet! From the brightness change we can tell the planet size. From the time
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Kepler was launched in March 2009.
Why?
The Kepler Mission is a NASA Discovery Program for detecting potentially life-supporting
planets around other stars. All of the extrasolar planets detected so far by other projects are
giant planets, mostly the size of Jupiter and bigger. Kepler is poised to find planets 30 to 600
times less massive than Jupiter.
How?
By a method known as the transit method of planet finding. When we see a planet pass in front
of its parent star it blocks a small fraction of the light from that star.When that happens, we say
that the planet is transiting the star. If we see repeated transits at regular times, we have
discovered a planet! From the brightness change we can tell the planet size. From the time
between transits, we can tell the size of the planet's orbit and estimate the planet's
temperature.These qualities determine possibilities for life on the planet.
What?
The Kepler satellite has a 0.95-meter diameter telescope that is a photometer having a field of
view a bit over 10 degrees square (and area of sky the size of about two open hands). It is
designed to continuously and simultaneously monitors brightnesses of 100,000 stars brighter
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KEPLER star field
Milky Way photo by Carter Roberts
National Aeronautics and Space Administration
The Kepler Mission Star Field
www.nasa.gov
Astronomers see more planets than stars in
galaxy
By SETH BORENSTEIN
AP Science Writer
Published: Wednesday, January 11, 2012 at 4:04 p.m.
Last Modified: Wednesday, January 11, 2012 at 4:04 p.m.
The more astronomers look for other worlds, the more they find that it's a crowded and crazy
cosmos. They think planets easily outnumber stars in our galaxy and they're even finding them
in the strangest of places.
Keep watching this space