Comets, Asteroids and SpaceGuard Projects

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 periodic 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

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 varies, but can be as much as 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 the 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 what was then

“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 in 2000.

For November 18/19 2001, the Asher/McNaught model predicted around 10,000 meteors per hour (ZHR) as our planet encounters the clouds of cometary debris laid down when the comet 55P/Tempel-Tuttle passed through the solar system in the years 1767 and 1866. In 2002 our planet will pass 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 in July 1992 the comet

Shoemaker-Levy 9 did just that as it appeared to be in termporary orbit around 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 own planet began to really sink in as scientists and the public at large began to ask the inevitable question:

“what could have happened if those massive chunks of

cometary debris had impacted the Earth instead?”

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 these 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 Stategy’ and follow-up observations. The workshop concluded with calls for International co-operation, and plans were laid 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 compliment the work already being done by long established universities, observatories – including infra-red satellite detection – and a wide variety of independent researchers. 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 had the concern about ‘impact probability’ estimates grown by this time, 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 satellite operations. 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:

On October 3, 2002, the United States House Committee on Science met to hear testimony about the ‘impact threat’ from asteroids, cometary debris and other celestial bodies. The testimony given before the House Committee by a number of leading authorities in the fields of astronomy and planetary defence are accessible directly below:

In the light of recent media coverage of the threat to life on our planet posed by Near Earth Objects (NEOs) such as asteroids and comets, Jonathan (Jay) R. Tate of The Spaceguard Centre in Powys, Wales, convened an informal meeting of those who provide information on NEOs, and those who report the news to the general public. The aim of the meeting was for participants to exchange experiences and views on this important subject in order to increase everyone’s understanding of the issues involved, and the ways in which it is, or should be, responsibly reported to the public at large. The report of the symposium is available below.

Since the NEO Public Awareness Symposium at the International SpaceGuard Information Centre, in Knighton, Powys, CYMRU/WALES, there has been a rapidly increasing interest in developing mitigation strategies for cosmic bodies found to be on collision courses with the Earth. Links to them appear below:

There are a number of SpaceGuard organisations around the world that have been set up over recent years. Most are affiliated to the International SpaceGuard Foundation based in Rome. Each of these organisations have developed links with researchers in universities and non-profit research groups in their respective areas, and between them their web-sites contain the most comprehensive links to all these other organisations

EU English Edition