Comets
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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.
Increasingly, 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 years.
Because 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.
However, 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.
Before 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:
"what 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. The 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 comets." 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. |
Serious 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 UK.
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
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An 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. News 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
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The 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. Team 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
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Halley's 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 name. It's 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, 1994
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Between 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. This 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
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In 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. Its 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
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Over 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. Text 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
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This 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. It 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
Impact Craters on Earth | Skywatching | Spaceguard Projects | Night-Sky Software for PCs
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
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