Transcript Asteroids, Meteoroids and Comets

Evil Stars (?)
When comets and
asteroids get too
close for
comfort…
Dr Paul Callanan, Department of Physics, UCC.
"I looked, and behold, there was a great earthquake
and the Sun became black as sackcloth, and the full
Moon became like blood, and the stars fell to the
Earth"
Revelations 6:12-13
The skies have been surveyed for signs of the Second
Coming, or pending catastrophies, down through the
ages.
For example, the word “disaster”, loosely
translated means “evil star”
(or the evil influence of a star or planet).
For example, the Irish Annals - which provide an annual
summary of events from a monastic perspective from the
time of the arrival of christianity in Ireland to the 16th
century - contain many astronomical observations of
eclipses (e.g. "the colour of blood upon the Moon",
aurorae ("dragons in the sky") , comets, etc.
The Irish monks observed in 1066, for example, that
"A hairy star, strange, enormous, was seen in the air ...
Such was its size and brightness that men said it
was a Moon."
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Only 12 years earlier, the Annals record a "Round tower of fire
seen in the air... over Ros Ela on Sunday the feast of S. George
for 5 hours of the day.” We now know this to be the Crab
Supernova Explosion.
Ros Ela is nowadays Rostalla,
Co. Westmeath, 2 miles from
Durrow, the 6th century
monastic foundation of
St. Columcille.
Yet despite such powerful apparitions, the second coming never
came: probably for this reason these astronomical observations
ceased soon afterwards (from ~1133 onwards).
Irish monks were not the only astronomers at the time…
How can we explain these phenomena ?
We will focus on three types of object:
Asteroids, Meteoroids and Comets .
Asteroids (never visible with the naked eye)
rocky objects bigger than ~100 m.
Meteoroids/Meteorites, etc
smaller rocky objects: when they enter the Earths
atmosphere they are called meteors, and if they hit the
ground they are called meteorites, where they can become
collectors items (if they are found !).
Comets
Mostly icy objects with rocky cores.
Before we proceed, a question:
How much extra-terrestrial matter do you think hits
Ireland every year ?
Several tens of thousands of tonnes of meteorites and
interplanetary dust fall on the Earth per year. This
means that, for Ireland alone, of the order of 1.5
tonnes of extraterrestrial material falls every year.
Most of this material consists of microscopic particles.
Asteroids
Most asteroids inhabit a region between the orbits of
Mars and Jupiter: all asteroids orbit the Sun.
They are fragments of debris from the formation of the
inner (rocky) planets of the solar system.
They can sometimes be seen as fast moving objects
against the background of star.
Some have been studied by passing satellites.
Asteroid Ida and friend
Boulders of surface of asteroid Eros
The population of known asteroids in the solar system has
increased dramatically over recent years, thanks to
systematic monitoring campaigns.
Of the ~hundred thousand known, some cross the orbit of
the Earth, and a very small fraction of these could,
potentially, collide with the Earth (more later)…
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A meteoroid is a much smaller piece of rock, visible
as it enters the Earth’s atmosphere (we will deal
with meteorites later).
Meteors, when they occur in showers, often appear to
originate from the same point in the sky.
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Leonids and
Ayres
Rock
You see the same effect when
driving through e.g. snow…
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Comets
These are usually discovered as faint, fuzzy patches
in the sky. Comets consist of a
rocky core
surrounded
by ice: they are
heated as they
approach the
Sun, which
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a “tail” of material
extending many
millions of km.
Most comets originate from the so-called Oort Cloud, a
vast reservoir of material, left over from the formation of
the Solar System, extending half way from the Sun to the
next nearest star.
The cloud has been described as the "Siberia of the Solar
System, a vast, cold frontier populated with exiles of the
Sun's inner empire and only barely under sway of central
authority". At this distance, the Sun appears about as
bright as does Venus to us on the Earth.
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What causes comets to leave the Oort cloud and fall
towards the Sun ?
Occasionally, a passing star perturbs the cloud, causing
some comets to fall towards the Sun. Typically, about a
dozen stars pass close enough to do this every million years.
Very close stellar encounters may increase the number of
comets by as much as ~300...
By comparison, in 1.4 million years time the small, red dwarf
star Gliese 710 will pass relatively near, increasing the
number of comets passages in the inner Solar system by
~25 %.
As comets orbit the Sun they often leave a trail of rocky
debris behind. Sometimes the Earth passes through this
material, which causes meteor showers to be observed in
the upper atmosphere.
When asteroids and
comets get too close for
comfort:
Collisions big and small…
• f
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A meteorite…
It turns out that Antarctica is one of the best places to
find meteorites…
Barringer Crater, Arizona
Tunguska: an explosion over the Siberian forest in
in 1908: caused by an asteroid ~40 m across.
Tunguska area of destruction
superimposed on map of London
The Moon bears testimony to a history of
bombardment…
Phobos…
…as does
the
surface of
Mars.
From such impacts,
a small number of
martian rocks fall
as meteorites to
the Earth.
Life on Mars ?
Controversial evidence from
ALH84001…
The jury is out.
Shoemaker-Levy 9: impact
on Jupiter…
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Torn apart by Jupiters gravitational pull as it neared…
… a rising plume of gas
above the Jovian limb…
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More dramatic when
observed in other parts of
the spectrum.
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Craters on Callisto
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Impacts and their effect on evolution:
Dinosaur Extinction.
It is generally believed
that asteroid/comet
impacts could have
played a large role
in the extinction of,
for example, the dinosaurs.
The Chicuxlub “crater”
indicates that a massive impact
occurred ~65 million years ago,
contemporaneous with the extinction of the dinosaurs.
The asteroid or comet slammed into Earth with the force of a
hundred million hydrogen bombs. A huge tidal wave would have
moved more than 1,000 kilometers inland. Much of North
American life may have been wiped out almost immediately.
Not long after, the hot debris raining around the rest of the
world ignited fires that killed many other living organisms.
The longer-term effects were even more severe. Dust and
smoke remained in the atmosphere for weeks or months,
blocking sunlight and causing temperatures to fall as if Earth
were experiencing a global and extremely harsh winter. The
reduced sunlight would have stopped photosynthesis for up to
a year, killing large numbers of species throughout the food
chain.
The impact may have released large amounts of carbon
dioxide into the atmosphere. The added carbon dioxide
would have strengthened the greenhouse effect, and the
months of global winter may have been followed by decades
or longer of global summer.The impact probably also caused
chemical reactions in the atmosphere that produced large
quantities of harmful compounds, such as nitrous oxides.
These compounds dissolved in the oceans, where they
probably were responsible for killing vast numbers of marine
organisms. Acid rain may have been another by-product,
killing vegetation and acidifying lakes around the world.
Maybe the monks were right after all…
Because of the potential effects of such
collisions, attempts have been made to quantify
the probably of significant collisions between the
Earth and a large asteroid or comet:
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Hence, catastrophic collisions appear to be very rare indeed.
Comet/asteroid collisions with the Earth may not
always be bad news.
We know from observations of distant gas clouds and
nearer comets that organic
molecules exist in interstellar
and interplanetary space.
Several astronomers have speculated that some of the
essential building blocks of life were brought to Earth
by comets and asteroids - ie that the origin of life
might have been given a kick start by such
interplanetary interference. It has also been
suggested that most of the water in the oceans was
brought to the Earth via comets.
If you find this difficult to imagine, remember that
most of the material you are made of started off in
the core of a distant star that exploded eons ago,
spreading its gas into space.
So if comets and
asteroids have helped
us out biologically in
the distant past,
maybe even “evil
stars” have a good
side.