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an image of a meteor flashing through the sky

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revolving globe showing current sea levels since the last 

ice age, before which many ancient societies like Atlantis 

flourished all over planet Earth on what are now sunken lands.

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The standard picture of the physical make-up of comets is one where the core, or nucleus, is composed of minute dust particles and assorted substances held together by water-ice and other ices. This model of cometary composition has become known as the “dirty snowball” theory, and came about in the 1940s. At that time, spectroscopic analysis of ‘the heads’ of comets, (which are extended balls of gas that grow around the nucleus of comets as they approach the sun) showed the presence of certain molecules derived from water (H2O), methane (CH4), ammonia (NH3) and carbon dioxide (CO2).

The term

“dirty snowball” was coined

by an American astronomer, Fred L. Whipple, in 1949, though the idea that

comets were made of ice goes back to Laplace in the 19th century, and

neither is it the only model of cometary composition that exists. The

Giotto spacecraft, which was one of several spacecraft which flew close

to Halley’s Comet in March 1986, sent back images which suggested that

many larger rocks were also present, and that the nucleus of a comet is

better viewed as an “icy rock pile”

than a “dirty snowball”. But

even this model has been challenged, and there is emerging a large school

of thought which favours the idea that the centre of the nucleus of comets

may in fact be a ‘warm liquid core’ in which organic molecules and even

complex compunds, including live viruses, thrive.


as technological advances allow more and more cometary and solar system

dust particles to be gathered and analysed, astronomers and astro-physicists

are warming to the ideas of “Panspermia”,

as organic molecules are retrieved in meteor showers by high altitude

balloons as well as in near-earth space. The theory of panspermia holds

that ‘life’ is transported and spread throughout the universe by comets

which crash into rocky bodies, such as our moon is now, and thereby deliver

the water, gasses and organic matter necessary for ‘life’ to get started.

As comets

approach the Sun the ices begin to vaporise in the increasing heat, and

along with small bits of dust particles begin to form the spectacular

tails that can be seen from the Earth. These ‘tails’

are in reality ‘trails’ or

‘streams’, and periodically the

orbit of the Earth intersects the orbital paths of the various cometary

debris trails.

When this

occurs the bits in these ‘trails’

and ‘streams’ enter the Earth’s

atmosphere and burn up creating the shooting-star phenomena known as ‘meteor

showers’. The same phenomena occurs when the Earth’s orbital

path encounters

inter-planetary dust clouds, which can also contain much larger

bodies composed of metal and rock.

For example,

on February 28 1998, the comet Temple-Tuttle was at its closest point

to the Sun, and even though it has now moved away again it left behind

a trail, or cloud, of debris which has been added to that left by previous

passages of this comet through our

solar system. The Earth passes through this trail of cometary

debris each November 15th to 21st, and as the resulting ‘shooting

stars’ appear to be coming from the direction of the constellation

Leo they are known as the ‘Leonid meteor shower’.

It has been one of the best observed meteor showers for over 1000



the Earth passed very close to the orbital path of comet Tempel-Tuttle

in 1998 and 1999 the meteor showers were expected to be more spectacular

than in most years, and be more like a ‘meteor

storm. In fact the Earth passes this close only every

33 years, and rather than the usual annual 10 to 15 meteors per hour seen

most Novembers, in 1999 they were expected to increase to anywhere from

200 to 15,000 meteors

per hour, and the same number was expected in 2000. As it turned out the

‘peaks’ were not all that spectacular,

but predictions by astronomers at Armagh Observatory in the north of Ireland

were that the Earth’s passages through this debris stream in 2001

and 2002

could produce ‘shooting stars’

numbering around 10,000 to 100,000 at the peaks in those two years.



since that last major Earth orbit encounter with the greatest concentration

of Leonid cometary debris in 1966, many more satellites have been launched

into space, and there was growing concern that some the dust particles

could literally ‘sand blast’

them into oblivion. The speed of these particles is approx. 155,000 miles

per hour which is a hundred times faster than a bullet, and could cause

major damage to satellites which are not protected by Earth’s atmosphere.

On April 27 and

28 1998, in Manhatten Beach, California, the “Leonid

Meteoroid Storm and Satellite Threat Conference” was held.

A few weeks later William H. Ailor, Ph.D. was one of several experts who

gave testimony before the US House of Representatives. He is the Director

of the Center

for Orbital and Re-entry Debris Studies.


the Committee on Science, Subcommittee on Space and Aeronautics Hearing

on “Asteroids: Perils and Opportunities”,

on May 21 1998, the discussion explored the dangers to telecommunications,

the Global Positioning System which many ships rely on for directions

in cloudy weather, and also on what lessons could be learned for the future.

The main focus was on

“The Upcoming Leonid Meteoroid Storm and its Effect on Satellites.


Since these

conferences and subcommittee hearings, major advances have been made in

the ability of astronomers to predict the exact timing of the ‘peaks’

in meteor activity of the Leonid meteor showers by astro-physicist, David

Asher, of Armagh Observatory in the north of Ireland. Working

with Robert

McNaught of the Australian National University, they predicted

that the 1999 Leonids would ‘peak’

at 02:08 am on the morning of November 18. The actual peak was in fact

just a few minutes earlier, and they successfully predicted the times

of the several ‘flurries of shooting-stars’

for November 16 through 18 2000.

For November

18/19 2001,

the Asher/McNaught model predicted around 10,000 meteors per hour (ZHR)

as our planet encountered the clouds of cometary debris laid down when

the comet Tempel-Tuttle passed through the solar system in the years 1767

and 1866. In 2002

our planet passed directly through the middle of these same two dust clouds

in the early hours of November 19th – just two of the many dust clouds

that make up the ‘Leonid Complex’.

Comets sometimes

disintegrate during their passage through our solar system, and astronomers

retrocalculated that sometime in 1992 the comet

Shoemaker-Levy 9 did just that due to the gravitational forces

of the gas giant planet Jupiter. Splitting into more than 20 large fragments

which impacted on the surface of Jupiter from July 16 to 22nd 1994, they

were watched with awe by astronomers on Earth, and for the first time

the fragility of our planet began to really sink in as scientists and

the public at large beagn to ask the inevitable question:


could have happened if those massive chunks of cometary debris had impacted

the Earth instead?”.

Copyright BBC 2002

Besides the comets,

which enter our solar system in a variety of ways, there are many

other Near Earth Objects (NEOs)

such as

asteroids in our solar system with the potential for

becoming dislodged from their orbits and heading onto collision

courses with the Earth.


near miss of asteroid 2002 MN on June 14 was the closest recorded

by astronomers since asteroid 1994 XM1 came slightly closer eight

years ago.

In recognition

of this threat there has been increasing concern over the past twenty

years, and even before the impacts of cometary fragments on Jupiter in

July 1994, NASA held a ‘Near-Earth-Object

Detection Workshop’ which discussed the problems in January

1992. Topics for ranged from “Hazard of Cosmic

Impacts” and “The Near-Earth-Object

Population”, to ideas and proposals for a ‘Search

Strategy’ and follow-up observations. The workshop concluded

with calls for International co-operation, and laid plans for what has

since become known as

“The Spaceguard Survey”.

Since the

1992 Nasa workshop on NEOs there have been many other organisations setting

up research projects, and

planetary defence worshops. A number of new organisations have

come into being to complement the work already being done by long established

universities, observatories – including

infra-red satellite detection. There is also

2025, a study designed to comply with a directive issued by

the chief of staff of the US Air Force.

So great

is the concern about ‘impact probability’

estimates that an International Spaceguard Workshop was held at the beginning

of June 1999 in Torino, Italy. In attendance were many of those who are

directly concerned on a daily basis with the ‘impact

hazard’, ranging from astro-physicists to those involved

with commercial and military satellites. What resulted from the workshop

was the creation of the Torino

Scale, which has been described on the

Nasa Impact Website in the following words:

Copyright BBC 2002

“The Torino Scale is a ‘Richter Scale’ for categorizing the

Earth impact hazard associated with newly discovered asteroids and


In January 2000

the Lord Sainsbury, Minister for Science, announced the setting

up of a Task Force to look into the hazard from Near Earth Objects

(NEOs). They reported

back to the UK Govt. on September 18 2000. The full report can be

accessed here.


and sensible reactions and comments about the report from the international

astronomical community have been archived on the Cambridge Conference

Network (CCNet),

and more info is available at Spaceguard



the links below will take you to just a small sample of some recent

projects and related expeditions, to sites giving more detailed information

about individual comets

click on the images to the left to go directly to those sites

Tycho Brahe Planetarium 1998 Greenland expedition

The Tycho 

Brahe Planetarium 1998 Greenland Expedition image/link


expedition set off in 1998 to Greenland to search for meteorites

from a recent impact there which occurred late in 1997. The expedition

left Copenhagen University for Greenland on July 22nd. 1998.


and images from the expedition were posted via satellite every day

on the Tycho Brahe Expedition website, though the English page was

a day older than the Danish page. You can access all the web pages

of the expedition by clicking on the Greenland Expedition image.


Planetary Society 1998 Belize expedition

an image of a comet or asteroid impacting the Earth from an original 

painting by Don Davis, NASA


Planetary Society’s third expedition to Belize went on during February

1998. Past expeditions, like this one, were a geological adventure

designed to collect data and samples of ‘ejecta’

– debris blasted from the Chicxulub crater 65 million years ago

when an asteroid or comet crashed just off the coast of Yucatan,

Mexico, finishing off the dinosaurs.


leaders were from Nasa’s Jet Propulsion Lab and Geo Eco Research.

Only in Belize has unique evidence from the impact crater been found.

Field reports and images were posted daily on the expedition website.


Halleys Comet, 1986

an image of Halleys comet taken by SEDS


comet is probably the best known of all the comets that pass periodically

through our solar sytem. It was in 1705 that Edmond Halley first

predicted it would return after his death in 1758, and so took his



orbit is inclined at 18 degrees to the ecliptic, and the major planets

in our solar system exert such a gravitational pull on it that its

orbital period varies slightly between 76.3 years (as

it was in 1066) and 76.0 years (as

it was in 1986).


Comet P/Shoemaker-Levy 9 collides with Jupiter,


an image of the cometary fragments of comet P/Shoemaker-Levy 9 impacting 

Jupiter by SEDS


the dates of July 16th. and July 22nd. 1994 the comet, Shoemaker/Levy

9, broke into 21 fragments as it headed on a collision course with

the planet Jupiter. These fragments varied in size, but many of

them were up to 2Km in diameter.


was the first time that ‘a collision

between two bodies in our solar system’ has been

observed by modern astronomers, and there are many more images like

the one to the left to be seen by clicking on the image/link to

the left.


Comet Hale-Bopp, Spring Equinox 1997

an image of comet Hale-Bopp taken on the Spring 

Equinox 1997 by JPL


1997 the comet Hale-Bopp passed through our solar system. It was

visible from many locations around the world, and the image to the

left was taken by the Nasa Jet Propulsion Lab on the Spring Equinox,

March 21st 1997.


orbital period is around 4,000 years and does not threaten to collide

with any other body in our solar system. You can access many more

images of comet Hale-Bopp at the JPL site .


1st International Hale-Bopp Conference

an image of comet Hale-Bopp taken by the Nasa Jet Propulsion Lab


150 specialists from all the major comet research organisations

met for the First International Hale-Bopp Conference, held at the

Conference Centre, Puerto de la Cruz, Tenerife, Canary Islands,

Spain on February 2nd to 5th 1998.


of the proceedings with all links can be found at the conference

website by clicking on the image to the left.


Comet Hyakutake Spring Equinox 1996

an image of comet Hyakutake taken by the Nasa Jet Propulsion Lab


comet was discovered on January 30th. 1996 by Yuji Hyakutake in

Japan with a pair of 25 x 150 binoculars. It came closest to the

Earth on March 25th 1996, and was closest to the Sun on May 1st

that year.


has an orbital period of around 40,000 years. You can access many

more images of this comet at the Nasa Jet Propulsion Lab Comet Hyakutake

website by clicking on the image to the left.


NEO Public Awareness Symposium – October 13th 2003


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The Morien Institute

see March/April 2002 images of Ikeya-Zhang the first 2002 comet


October 3, 2002

“Large Meteorite falls on the Irkutsk Region”Pravda, Russia

October 8, 2002

“Cash plea for space impact study”BBC News Online, UK

November 15, 2002

“Impact ‘showered debris over Britain'”BBC News Online, UK

June 6, 2003

“Meteorite crash site found in Siberia”Interfax, Russia

June 7, 2003

“Siberia meteorite flattens 40 sq miles”The Times, England

please take a look at our Comet & Asteroid Impact Book Pages for a selection of books about

the past and present threat to our planet from comets and asteroids, dust-loading

of our stratosphere by cometary debris, and the proposals for an

international SpaceGuard effort and Planetary Defence

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