When comets and asteroids get too close for comfort … Evil Stars (?) Dr Paul Callanan, Department...

When comets and asteroids get too close for comfort…

Evil Stars (?)

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 itwas a Moon."

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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, etcsmaller 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 !).CometsMostly 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.

AsteroidsMost 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)…

QuickTime™ and aMicrosoft Video 1 decompressorare needed to see this picture.

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.

Leonids and Ayres Rock

You see the same effect when driving through e.g. snow…

Comets

These are usually discovered as faint, fuzzy patches in the sky. Comets consist of arocky core surrounded by ice: they are heated as they approach the Sun, which generates 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.

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



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 fromALH84001…The jury is out.



Shoemaker-Levy 9: impact on Jupiter…

Torn apart by Jupiters gravitational pull as it neared…



… a rising plume of gas above the Jovian limb…





More dramatic when observed in other parts of the spectrum.



Craters on Callisto

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 impactoccurred ~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:



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 organicmolecules 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.

When comets and asteroids get too close for comfort … Evil Stars (?) Dr Paul Callanan, Department...

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