Between 20,000 to 50,000 years ago, a huge iron-nickel asteroid about 80 feet in diameter hurtling at about 40,000 miles per hour, struck the rocky plain of Northern Arizona with an explosive force greater than 20 million tons of TNT.
In less than a few seconds, left a crater 700 feet deep and over 4000 feet across. Large blocks of limestone, some the size of small houses were heaved onto the rim. Flat-lying beds of rock in the crater walls were overturned in fractions of a second and uplifted permanently as much as 150 feet.
What was the impact of Meteor Crater? From the point of impact, a blast wave swept across the landscape producing instantaneous overpressures at every point on the ground as the shock front moved out. The amount of damage due to air blasts has been studied with surface and atmospheric nuclear explosions. Damage was found to be a function of the yield and height of the explosion. Dave Kring used a range of 20 to 40 megaton explosive energy and an altitude of zero in his analysis of the magnitude and radial extent of blast wave conditions for the Meteor Crater event (see table below.)
At the point of impact, the plants and animals, rock, and most of the meteorite were vaporized. Underlying bedrock was ejected and overturned, burying the land and anything else not already blown away by the air blast, out to a distance of between 1 and 2 km. The animals within 3 to 4 km of the impact site would have been subjected to winds exceeding 2000 km/hour and killed. A 50% casualty rate would occur between 9 and 14 km of the impact site due simply to bodies being picked up by the air blast and accelerated to a few to tens of kilometers per hour before being slammed back down again.
Overpressures (the pressure above normal atmospheric pressure)
would cause death to anything living within a radius of 2.7 to 3.2 km of the impact site
and cause lung damage within a radius of 6.5 to 9.3 km for a 20 megaton explosion. In the
case of a 40 megaton explosion, these distances would increase by an additional 1 to 2 km.
Animals as far away as 16 to 24 km would have been injured severely. Vegetation would have
been almost completely destroyed over an area of 800 to 1500 km2 around the
Meteor Crater impact site. Fortunately, as Kring points out, the impact effects would have
been severe only within that 800 to 1500 km2 area. No global extinction would
Magnitudes of pressures and wind velocities as a function of distance for the Meteor Crater impact event
|20 megatons||40 megatons|
|Source: Kring, D. A., 1997.|
and comets are remnants of the formation of the Solar System about 4.5 billion years ago
and can range in size from pebbles or lumps of ice, to rocky or icy worlds nearly 1,000
kilometres across. An asteroid or comet may become a Near Earth Object when its trajectory
intersects the orbit of the Earth or is within 0.3 Astronomical Units (astronomers call
the Earth to Sun distance one Astronomical Unit or 1AU). An object is said to
be potentially hazardous when its orbit comes even closer to the Earth, to within 0.05 AU
(7.5 million kilometres, or about 20 times the Earth to Moon distance) and when it is at
least 150 metres in diameter. So far, 258 potentially hazardous objects have been
What are Near Earth Objects?
Asteroids are made up of carbonaceous (carbon containing) materials, rocks (silicates) or metal. They may comprise of piles of boulders held together only by their own very weak gravitational forces, or be solid lumps of stone or slabs of iron; They are not spherical and may spin or tumble as they go. There are many subcategories of asteroids: each behaves differently when entering the Earths atmosphere, and in its reaction to countermeasures.
Comets are essentially dirty snow-balls and include many particles of dust. It is only when a comet comes near the Sun that these gases evaporate, freeing the dust that forms the tail sometimes seen by the naked eye.
Maybe more than you expect. (But not so high that you should spend your retirement savings early!) The current best estimate is that impacts large enough to cause a global catastrophe occur on average every few hundred thousand years. These impacts are the ones large enough to cause significant global climatic disruption, widespread crop failures and probable societal collapse. This translates into a probability of a little less than one chance in a few thousand of such an impact occurring during a typical human lifespan. Smaller objects, of course, are more numerous, and impact more frequently. Impacts capable of causing severe local or regional disasters occur once every few hundred years, depending on how you define "local" and "regional." Fortunately most of the Earth is uninhabited so the likelihood of a impact near a populated area is far less.
What would happen if there was an impact?
If an object were to penetrate the Earth's atmosphere, the type of impact at the surface would depend on a number of factors: their size, their composition, their velocity and their angle of approach. The main effects of impacts are blast waves: Tsunamis (or ocean waves), injection of material into the atmosphere, and electromagnetic changes near the surface. Depending on its size, a particular Near Earth Object can have one or more of these effects.
Some two thirds of the Earths surface is covered by the sea so that the chances of an ocean impact are greater there than on land.
Impacts from Near Earth Objects with diameters over 10 kilometres would have global consequences, possibly resulting in the extinction of most living organisms. Such events are fortunately very rare. Impacts of objects with diameters from a few kilometres in diameter to 10 kilometres are also rare.
Impacts with objects with diameters around 1 kilometre are the most dangerous because they are much more frequent and give many more casualties per impact than smaller ones. Impacts of smaller objects, with diameters of a few hundred metres, would have dramatic local consequences, but are unlikely to affect the Earth as a whole. For objects below about 50 metres in size the Earths atmosphere usually provides adequate protection.
If an impact becomes likely is there anything that can be done about it?
This depends critically on the amount of warning and to a lesser extent on the
composition and structure of the asteroid, but in principle deflection of impacting
objects will be feasible.
What is being done to monitor the situation?The United States is doing far more about Near Earth Objects than the rest of the world put together. The Minor Planet Centre is at the hub of observations world-wide. In addition, military surveillance facilitates in space and on the ground look continuously for objects and explosions in the upper atmosphere, including those from Near Earth Objects. The United States recognises that observations of Near Earth Objects bring good science as well as dealing with a practical problem.
There is no co-ordinated approach to Near Earth Objects in Europe. The European Union and its Commission have no formal policy on Near Earth Objects at present. The European Space Agency has a direct and developing interest. There is limited ground-based work on Near Earth Objects in Japan, China, Canada and Australia. The only current activity in the Southern Hemisphere is in Australia, where there are plans for a 0.6 metre telescope for operation early in 2001.Through the work of groups in about a dozen universities and other institutions, Britain has contributed to the international effort on the study of Near Earth Objects and the consequences of their impacts on Earth.
Chronology of Events
|1694||Edmond Halley suggests that cometary impacts may have caused global catastrophes, formed the Caspian Sea as an impact crater, and might be linked to the biblical flood legend. This idea revived from time to time (for example by William Whiston)|
|1790s||Pierre Simon de Laplace suggests comet impacts cause terrestrial catastrophes|
|1794||Chladni proposes that meteorites are of extra-terrestrial origin|
|1801||First asteroid discovered (Ceres); discovery of more Main Belt asteroids soon follows|
|1822||Lord Byron suggests that mankind could save itself from comet collisions by diverting them.|
|1890s||Alexander Bickerton suggests that impacts have sculpted the face of the Earth. Barringer suggests the Meteor Crater in Arizona is of impact origin|
|1898||Eros, first Earth-approacher discovered (Amor type asteroid)|
|1930s||Odessa crater in Texas shown to be an impact crater: the first proven case on Earth|
|1932||First two Earth-crossing asteroids discovered, Apollo and Adonis|
|1937||Asteroid Hermes (1937 UB), size about 800m, observed for only a few days as it misses Earth by just 670,000 kilometres (60 per cent further than Moon). Insufficient data obtained to secure orbit, so Hermes is lost; it may come back close to Earth at any time.|
|1941||Fletcher Watson estimates rate of impacts on Earth|
|1947||Minor Planet Center established by the International Astronomical Union (IAU) in Cincinnati, Ohio; moves to Cambridge Massachusetts, 1978|
|1948-51||Edgeworth (1948) and Kuiper (1951) predicted belt of comets, just beyond Neptune, much nearer Sun than Oort cloud. Now known as Edgeworth-Kuiper Belt|
|1949||Ralph Baldwin explains lunar craters as being of impact origin.|
|1949||Icarus, a close Earth-approacher discovered (Apollo type asteroid)|
|1950||Oort cloud hypothesis; billions of comets in spherical shell around solar system, 50,000 AU from Sun|
|1951||Ernst Ípik (Armagh), after earlier work, estimates cratering rates on Earth|
|1954||First Aten-type Inner-Earth asteroid discovered (1954XA) but subsequently lost|
|c. 1960||Eugene Shoemaker proves impact origin of Barringer (Meteor) Crater (Arizona).|
|1970||Eleanor Helin (JPL) and Eugene and Carolyn Shoemaker start systematic photographic survey of NEOs|
|1973||Arthur C Clarke coins term Spaceguard in his novel Rendezvous with Rama|
|1979||Movie Meteor is released|
|before 1980||Nuclear winter calculations in context of full nuclear war: subsequently realised to be applicable to consequences of an NEO impact|
|1980||Alvarez et al propose that a massive asteroid impact led to the extinction of the dinosaurs. Later linked to event at Chicxulub|
|1981 July||NASA conference: Collision of Asteroids and Comets with Earth: Physical and Human consequences|
|1981||Spacewatch: Tom Gehrels and Bob McMillan, University of Arizona, began programme to survey NEOs including small asteroids, with electronic detection and data collection. Survey began late 1988|
|1990 Sept||US Congress House in NASA Multiyear Authorisation Act of 1990: imperative that detection rate of Earth-orbit-crossing asteroids must be increased substantially, and that means to destroy or alter the orbits should be defined and agreed internationally|
|1990||American Institute of Aeronautics and Astronautics makes recommendations concerning NEOs to US Congress|
|1990||Duncan Steels survey of asteroids begins using United Kingdom Schmidt Telescope in Australia|
|1991||US Congress House Committee on Science and Technology use NASA Authorisation Bill to direct NASA to study 1) a programme to increase detection rate of Earth-orbit-crossing asteroids addressing costs, schedule, technology and equipment (Spaceguard Survey Report); 2) systems and technologies to destroy or alter orbits of such asteroids if they should pose a danger to life on Earth (NEO Interception Workshop)|
|1992 Jan||Spaceguard Survey Report delivered to US Congress. Recommends a search programme and international collaboration to find greater than 1 kilometre objects; and the provision of six ground based telescopes, northern and southern hemisphere sites, southern hemisphere radar; half costs to come from international partners|
|1992 Jan||NEO Interception Workshop. Full investigation of counter-measures concluded that nuclear explosives in stand-off mode most likely to succeed (see 1991 above)|
|1992 March||Three witnesses testify before US Congress on results of above workshops|
|1993||European Science Foundation initiates new scientific network Impact Cratering and Evolution of Planet Earth|
|1994 Feb||US Congress House Committee on Science and Technology pass an amendment to NASA Authorisation Bill directing NASA to report within a year with a programme to identify and catalogue, with help from the Department of Defense and space agencies of other countries, within 10 years, orbital characteristics of all comets and asteroids greater than 1 kilometre diameter and in an orbit that crosses Earths|
|1994 July||Shoemaker-Levy 9 comet collides with Jupiter. At least 21 cometary fragments, with diameters up to 2 kilometres, cause massive explosions and spark public interest|
|1994||IAUs 22nd Assemblys Working Group on NEOs, present a report recommending that an international authority should take responsibility for NEO investigations|
|1995 June||Near Earth Objects Survey Workgroup Report is released with a programme to meet Congress requirements. Recommends NASA, USAF and international collaboration; two dedicated 2 metre discovery telescopes; use of two existing 1 metre telescopes for survey and follow-up; enhanced funding to obtain roughly half time on a 3 to 4 metre telescopes for physical observation; MPC enhancements|
|1995||UN host conference on NEOs attended by representatives of UN Office of Outer Space Affairs|
|1995||IAUs Working Group on NEOs workshop sets up the Spaceguard Foundation to promote international NEO discovery, follow-up and study.|
|1996 Jan||UN meeting in Colombo, Sri Lanka, resolves that an international network of telescopes under UN aegis is needed for NEO searching and tracking|
|1996||Parliamentary Assembly of the Council of Europe, Resolution 1080, detection of asteroids and comets that are potentially dangerous to mankind|
|1997||Spaceguard UK set up to promote British NEO activities|
|1998||Two new major NEO search programmes start in United States using Department of Defense facilities: NEAT in Hawaii and LINEAR in New Mexico see Annex C leading to step-increase in NEO discovery rate|
|1998||National Research Council of US National Academy of Sciences: highest priorities to NEOs|
|1998 May||US Congressional Hearings on NEOs and Planetary Defense. Recommends use of more GEODSS telescopes, states difficulties with using already existing observatories|
|1998||NASA NEO Program Office set up at JPL to help co-ordinate and provide a focal point for US studies of NEOs|
|1998 Jul-Aug||Armageddon and Deep Impact films released, both about NEO collision with the Earth|
|1999||In a report to the ESA Council at Ministerial level, ESAs Long Term Policy Committee recommend that the Agency be involved in NEO activities, including the study of countermeasures|
|1999 March||Threat from NEOs is debated in House of Commons. Minister for Energy and Industry, John Battle, says that government will consult British astronomers and other experts on ways the UK can support NEO research|
|1999 June||Threat from NEOs discussed in House of Lords. Science Minister, Lord Sainsbury, says that Britain must co-operate internationally on this topic|
|1999 June||International Monitoring Programs for Asteroid and Comet Threat (IMPACT) conference in Italy organised by the IAU, the Spaceguard Foundation and others. Outcomes include the Torino Scale to describe risk and severity of possible impact for newly discovered objects.|
|1999 July||UNISPACE III, a UN Space conference, hold a workshop on NEOs and make recommendations to UN General Assembly. Subsequently, resolution adopted by conference, the Vienna Declaration on Space and Human Development, declares a strategy including actions that should be taken to improve knowledge of NEOs; improve international coordination of activities relating to NEOs; harmonise worldwide effort to identify, follow up and predict NEO orbits; and research safety measures on the use of nuclear power sources in outer space|
|2000 Jan||UK Government sets up a Task Force on Potentially Hazardous NEOs|
To be presented at The Vulcano Workshop: Beginning the SpaceGuard Survey (1995 September 18-22, Vulcano, Italy).
The Lowell Observatory Near-Earth-Object Search (LONEOS) is a system to survey asteroids and comets that has been under development for a little more than 2 years. Hardware consists of a 58-cm, f/1.91 Schmidt telescope; a CCD camera containing two Loral 2048 X 2048 chips (eventually, two 2048 X 4096 chips); a Silicon Graphics IRIS 4D/220GTX computer containing six processors; and other computers. The instantaneous field of view will be 10.1 square degrees.
To image the sky, the telescope will scan in declination at a rate up to 6~deg/min, corresponding to a data-acquisition rate of 1~Mb/s. At this fastest scan rate, the system will have the capability of making three scans/region, each lunation, over the entire accessible dark sky to a limiting magnitude that should exceed V_lim = 19.7 (50% detection rate in three consecutive scans at S/N ratio of 3.2), though that will probably not be the strategy adopted.
Our goal is to maximize the number of NEOs discovered that exceed 500~m to 1~km diameter. Model calculations indicate that, after ten years of full operation, about 60% of near-Earth asteroids and Jupiter-family comets larger than 1~km diameter could be discovered by LONEOS. Scans will be made on fixed regions of the sky, so images of fixed celestial sources (stars, galaxies, etc.) and repeating "cosmetic defects" (diffraction spikes, bleeding from saturated stars, etc.) will always occupy known pixels. By co-adding a number of scans, we will build high-S/N-ratio fixed-source maps, which will allow us to search for moving targets only in pixels thought to contain dark sky, thereby gaining a factor of 10^3 to 10^4 in speed over what we could achieve if we tried to detect and analyze all sources. Initially, detections will be made only on the basis of single-pixel data numbers exceeding a chosen threshold.
We will describe algorithms that maximize V_lim; i.e., minimize the S/N ratio and the false-positive detection rate. Because LONEOS images are close to being undersampled-untrailed images will typically occupy about 9, and no more than 16 pixels-the detection process is a delicate one. First, by examining unsaturated star images (about 10^5/scan), we determine the point-spread function (PSF, itself a function of zenith angle and off-axis distance in R.A.) to subpixel resolution. By moving the peak of the PSF into each subpixel of the pixel containing the peak signal, and then repixelizing, we develop a family of so-called PSF masks. After a sequence of three scans, putative moving-target detections are compared to the masks, and are accepted or rejected on the basis of chi-squared tests. The best-fit mask provides position and brightness estimates (the latter appropriately calibrated).
Approximately constant with time, our detection rate should amount to about 170~NEOs/month, of which about 50/month are larger than 1~km diameter. Over a five-year interval, we expect to discover an average of 60~NEOs/month, of which 20 exceed 1~km diameter. Thus, to discriminate the larger NEOs of interest and to secure orbits accurate enough for recovery at a subsequent apparition, we will need help in follow-up work.
Research supported, in part, by NASA grant NAGW-3397.