A Near-Earth Object (NEO) is an asteroid or comet whose orbit brings it close to the Earth; this includes an object that will come close to the Earth at some point in its future orbital evolution. NEOs generally result from objects that have experienced gravitational perturbations from nearby planets, moving them into orbits that allow them to come near to the Earth.

The idea that a celestial body such as an asteroid or comet hitting the Earth caused the extinction of the dinosaurs has entered popular culture during the last two decades, but recently the question 'Could it happen to us?' has also become a matter of increasing public interest. Within the last few years an increased awareness of the relatively recent realisation that we could be in danger from a visitor from the skies has been punctuated by the release of various books, both fact-based and fictional, and disaster movies such as Deep Impact and Armageddon.

Every day, millions of pieces of debris hit the Earth, but most of the objects which enter the atmosphere are very tiny and burn up before reaching the ground. For objects (such as asteroids) which are composed mainly of stony material, only those larger than 100m in diameter will survive long enough to reach the ground, and will have severely diminished in size by the time they do. Iron bodies of 50m and above may survive to reach the ground. These larger bodies can penetrate the atmosphere and may hit the Earth. The majority of these larger bodies hit water¹ and go unnoticed. Even if a piece of debris of this size was to hit land, it would not cause any real damage unless it actually landed on something important. However, it is known that there are much larger bodies in our solar system, with diameters measuring in kilometres rather than metres.

Types of NEO

There are three types of near-Earth orbit: Amor, which have orbital radii between 1.017 and 1.3 AU²; Apollo, with orbital radii between 1 AU and 1.017 AU; and Athena, with orbital radii of less than 1 AU. In these orbits there are an estimated 100 million ten-metre objects, around 100,000 one-hundred-metre objects, a possible 3,000 five-hundred-metre objects and an approximate 1,000 which are one kilometre or larger. The number of smaller objects generally increases exponentially as size decreases.

This Entry at least partially answers questions such as how we know about impacts, what happens when something hits Earth, what are we doing to assess the risks and defend ourselves, and perhaps most importantly, what the chances are of a major event occurring in the near future.

Impact: Earth

In 1994, over the course of a week in July, fragments of comet P/Shoemaker-Levy 9 collided spectacularly with Jupiter. Comet fragments, each up to two kilometres in size, collided with the atmosphere of Jupiter, sending out plumes many thousands of kilometres high. This was the first event of its kind - a major collision between two bodies of the solar system - to be directly observed, and was instrumental in making scientists and the general public alike think of the possibilities of something similar happening to Earth.

Evidence of impacts

Observing the other inner planets and our own moon reveals many craters which have been caused over many millions of years by asteroids and other space debris colliding with them. These observations have been used to develop an idea of the cratering rates of the inner planets: it is thought that about 4.7 objects create craters above 20 kilometres in diameter per km² per billion years. This may not seem very frequent next to a human lifetime, but it does illustrate that large objects do collide with the inner planets, including Earth.

Inspecting our own planet produces evidence of collisions in Earth's past, although many of these craters have been eroded and disguised over time by weather and tectonic activity, but there are several notable examples of large craters still observable today. The Manicougan crater in Quebec, Canada, is about 30 million years old. The fact that it has remained clearly visible for so long helps to illustrate the changes a major impact can bring about, and also that a large body has hit the Earth in its past.

An important event which illustrates the reality of this threat occurred in Tunguska, Siberia, in 1908. What actually happened is unclear, but one theory, arguably the most widely held, is that a 60-kilometre-wide comet entered the atmosphere and detonated about eight kilometres above Tunguska with a force equivalent to 12.5 megatons of TNT - 1000 times the force of the Hiroshima atomic bomb - flattening 2000 km² of Siberian forest containing around 60 million trees.

Other evidence supporting past impacts (and therefore the possibility of future impacts) can be assumed from many early societies' preoccupation with the sky - apart from using the stars as a calendar for agricultural and religious purposes, many ancient societies and cultures have left various indications of impact activity. For instance, some medieval sources depict fiery serpents descending from the skies, and Babylonian astrology was largely fireball-based: it is easy to assume that these serpents and fireballs symbolise meteors and comets.

Destruction levels

Ballistics experts have determined the diameter of a crater made by a solid body hitting the Earth at a speed realistic for an asteroid to be around twenty times that of the solid body, so an object of one kilometre in diameter would produce a crater around 20 kilometres wide. An object of this size would pose a threat to the entire planet - the Chicxulub crater in Mexico was formed by the impact of a 10-15 kilometre comet nucleus around 65 million years ago. This coincides with the Cretaceous/Tertiary boundary which marked radical changes in climate: large amounts of water and carbon dioxide were released into the atmosphere causing large-scale global warming, and nitrogen was burned in the fireball as it came through the atmosphere which led to acid rain.

Tunguska

The Tunguska event, mentioned earlier, destroyed a largely unpopulated area of woodland. A similar object detonating over London would cause almost complete destruction of everything within the M25. This would include the death of many thousands of people, and cause greater economic unrest than the attack on the Twin Towers in New York on 11 September, 2001. If an object of such a size should actually hit the ground, the consequences would be many times worse.

Impact Effects

An impact by an asteroid of around two kilometres in size would kill a substantial fraction of the Earth's population, and would throw up dust and dirt which would block out the sun and cool the Earth, possibly even sending it into an ice age. A larger object could cause mass extinction: it is thought that a comet nucleus 10-15 kilometres in diameter caused the death of the dinosaurs, releasing energy of around 100 million megatons.

All in all, from a large impact we could expect to see major fires, massive levels of destruction, smoke and dust causing a partial blockage of the sun - and its energy - causing dramatic changes to local and global climates, affecting crop yield and destroying many of Earth's creature's natural habitats. Also governments and economies would be thrown into chaos. All these effects would also have smaller knock-on effects of their own.

The Torino scale

Similar to the Richter scale, used to measure the destructive power of Earthquakes and other tectonic activity, the Torino scale has been developed to rate the seriousness of predicted close encounters. The scale consists of ten 'levels' indicating the importance of an event, from an unlikely event involving an object which may burn up in the atmosphere being given a value of zero, up to a certain event involving an impact which could cause a global catastrophe being assigned the highest value, ten. These levels are divided into five 'zones' which indicate the likelihood and consequences of an event: white, green, yellow; orange and red. The latter three include certain events requiring immediate concern and attention.

Professor Richard P Binzel, in the Department of Earth, Atmospheric, and Planetary Sciences, at the Massachusetts Institute of Technology (MIT), developed the first version of the Torino scale, called 'A Near-Earth Object Hazard Index', and was published by Binzel in the subsequent conference proceedings (Annals of the New York Academy of Sciences, volume 822, 1997). A revised version was presented at an international conference in June 1999 on near-Earth objects. This conference was held in Torino, Italy. The revised version was accepted, and named the 'Torino Scale' in recognition of the international co-operation displayed towards efforts to research and understand the threats posed by Near-Earth Objects.

An object is assigned a value (a whole number from 1-10) on the Torino Scale based on the probability that it will collide with the Earth, and its kinetic energy. An object that may pass close to the Earth on more than one occasion will have a separate Torino Scale value associated with each approach, and may be summarised by its highest value. In short, the higher the kinetic energy of an object, and the higher the probability of collision, the higher the value assigned.

An object's Torino scale value can (and most likely will) change over time, with the exception of those given a value of 0: An object given a value of one or greater may be given a higher value if the threat is seen to be more certain, or reduced to zero if calculations show that the NEO will definitely miss the Earth as information on the NEO is updated and improved with time.

The following table explains the Torino scale in more detail:

Self Defence

Research and Tracking

There are teams of astronomers around the world whose sole purpose is finding and tracking NEOs. The search for Near-Earth Objects utilises 1-2 metre telescopes fitted with sophisticated CCD detectors. There are many such telescopes around the world with a large number based in the United States of America and operated by NASA, the US Air Force and a number of universities. The most productive current NEO surveys are:

• The LINEAR search programme of the MIT Lincoln Lab, in New Mexico, supported by the US Air Force.

• The NEAT (Near-Earth Asteroid Tracking) search programme in Hawaii, with support from the NASA Jet Propulsion Lab and the US Air Force.

• The Spacewatch survey at the University of Arizona, funded by NASA and a variety of private grants.

Other searches in the US, France, Japan and China also contribute to the search for NEOs and in 2000 a British task force was set up, while additional astronomers follow up the discoveries with supporting observations. In the last decade the total number of people involved in the effort was less than 100, but this has now grown, and is continuing to do so as governments become more aware of the possibility of collision.

As it stands, the major governments of the world are taking the threat ever more seriously and are increasing budgets accordingly, with their own risk assessments showing the ideal amount of funding to be much higher than currently being offered.

Public Awareness

A lot of the groups involved in tracking NEOs believe that the public and the media should be made aware of their work and the possibilities of impacts from space-borne objects, and therefore publicity is a high priority. Nearearthobjects.co.uk say that their aim is to:

... increase public awareness and understanding of impact hazards so that everyone, whatever their background knowledge, can make informed decisions on NEO issues. We aim to provide a platform by which NEO science is communicated to the public and media in an accurate and yet understandable way. Our services also provide a resource by which the media can find the information and experts they need.

... which goes some way towards showing how relatively new this awareness is, and how an impact could potentially affect anyone.

Defence Tactics

Defence tactics which have been discussed range from firing nuclear missiles at an offending object to evacuating the planet, but obviously some are more realistic than others.

Firing a rocket³ at an asteroid or comet may succeed in breaking it up, but this may cause further problems - if it is too close the broken pieces would rain down on the planet creating more widespread destruction, changing the cannonball into a shotgun blast⁴. A similar idea is a controlled detonation near to the NEO designed to shift its orbit so that it is no longer on a collision course with Earth. The problems introduced with this plan are much the same as for the previous idea - the relative velocities involved and the accuracy required make this more likely to be used only as a last resort. Evacuating the planet may be an option in years to come, but at the moment it would be very expensive to evacuate even a small number of people, and in the event that it would not be safe to return, where would we go?

One of the most cost-effective and realistic ideas is to land a solar-powered engine on the object, which would convert sunlight into electricity to power a small ion gun. The force exerted would be small, but if put in place while the asteroid was still far away it would be enough to change its orbit to miss the Earth by as much as 300,000 miles. The engine could be built using the technology we have today, and missions such as the Rosetta mission to a comet - and one to the asteroid Eros - are helping to develop the expertise required to land on a NEO.

How Likely is an Impact from a NEO?

Throughout its history the Earth has been hit, and it will continue to be hit in the future, but at the present time, there is no known asteroid or comet on a collision course with Earth. In March 1998 an astronomer told the press that an asteroid named 1997xf11 would come close to the Earth in 2028 and that a collision could not be ruled out.

Since then more accurate calculations have confirmed that there is no risk of a collision, although in astronomical terms this will be a near-miss. The chance of a collision with an object of one kilometre or larger within the next century is less than 1/1000, although such a collision is possible and could happen at any time. Cosmic impacts are the only known natural disaster that can be avoided by applying the appropriate technology, but only if we are given sufficient warning. The main concern is that astronomers have discovered only about 10% of even the larger NEOs, and we have no way of predicting an impact from an unknown object.

Chances of an Impact

An event capable of destroying civilisation may be expected every 1-100 million years, and a life-destroying one every 10-100 million years. Smaller impacts destroying cities or parts of countries occur slightly more often, and very small objects hit Earth many times every day without consequence.

Experts suggest that should we detect a NEO which is on a collision course with our planet we would need somewhere between two and twenty years warning (anything less than this, and there really wouldn't be anything we could do). It is possible we could spot one earlier, and conversely it is plausible we would not notice one until it hit the atmosphere. In fact, a large number of the more recent 'near-misses' have not been spotted until after they have passed us.

Conclusions

So, in conclusion, it is not likely that we shall experience a catastrophic impact by a large Near-Earth object in the near future - indeed, in any of our lifetimes - but this is not reason for us to sit back and ignore the possibility that such an event may occur. In more personal terms, the probability of you dying as a result of a collision is about 1 in 20,000⁵. Of course, such statistics are interesting and sometimes comforting, but they don't provide any way of predicting when the next major impact will occur - it could happen tomorrow, or a million years from now. It is just as likely to happen in our lifetime as in the next, but with our current technology and an increasing level of NEO observation providing sufficient warning, any potentially threatening impacts may be avoided.

Further Reading

Publications

Websites

• Comet Shoemaker-Levy 9 Collision with Jupiter

• Spaceguard UK

• The Torino Scale

• Near-Earth Objects

• Find your way round your galactic neighbourhood with The Sky at Night, the official site for the BBC's long-running astronomy programme.