Abstract
Asteroid and comet impact remains an extreme case of the ‘super hazard’ because they are both the least likely, but also the most dreadful, of all known natural catastrophes. However, the increasing ability of telescopes to search space for such Near-Earth Objects (NEOs), coupled with the recognition of an increasing number of fossil crater sites, has changed attitudes. A major watershed in understanding was crossed in 1980 with the suggestion that an extraterrestrial impact was responsible for the massive extinction of life known to have occurred about 65 million years of age at the K/T geological boundary (Alvarez et al., 1980).
This theory was subsequently linked to the discovery of the Chicxulub Basin, Gulf of Mexico, now buried beneath later sediments, but recognised as a large impact crater formed at the same time as the mass extinction. According to McGuire et al. (2002), at least 165 impact sites have now been identified, and more are likely to be discovered.
The risk of impact from comets is assessed at 10-30 per cent of that for asteroids. These hazards can be accommodated, at least partially, into conventional disaster management strategies. Forecasting and warning are certainly possible. For example, a lead-time of 250-500 days between detection and impact has been given for long-period comets (Marsden and Steel, 1994), while the period for asteroids might extend to decades or centuries.
Introduction
Asteroid and comet impact remains an extreme case of the ‘super hazard’ because they are both the least likely, but also the most dreadful, of all known natural catastrophes. However, the increasing ability of telescopes to search space for such Near-Earth Objects (NEOs), coupled with the recognition of an increasing number of fossil crater sites, and has changed attitudes. A major watershed in understanding was crossed in 1980 with the suggestion that an extraterrestrial impact was responsible for the massive extinction of life known to have occurred about 65 million years of age at the K/T geological boundary (Alvarez et al., 1980).
This theory was subsequently linked to the discovery of the Chicxulub Basin, Gulf of Mexico, now buried beneath later sediments, but recognised as a large impact crater formed at the same time as the mass extinction. According to McGuire et al. (2002), at least 165 impact sites have now been identified, and more are likely to be discovered. Only 13 per cent of these occur in a marine environment.
Although they have yet to greatly impact human society, natural phenomena emanating from beyond the Earth must also be seen to represent a security threat. ‘An asteroid of size 1 km or more hitting our world at the minimum possible velocity (11 km/s – the escape velocity of the Earth) would release at least as much energy as 100,000 one-megaton hydrogen bombs’ (Kitchin 2001:54). Asteroids are minor planets within our solar system which vary in size from a diameter of a thousand to less than one kilometre. Most lie between the orbits of Mars and Jupiter, but some, the ‘Earth Crossing Asteroids’ (ECAs), can cross this planet’s orbit with the sun.
The ECAs, together with comets and meteoroids (debris from asteroids or comets) which pass close to the Earth are collectively referred to as ‘Near-Earth Objects’ (NEOs). The possibility of one of these celestial objects striking the Earth, and the likely effects, has been the subject of increasing speculation in recent years, and some measures have been taken to improve the capacity to predict if such a collision could occur and to initiate thinking on how it could be avoided.
There are no validated records of human deaths due to NEO collisions, but there is evidence that such collisions have occurred. Meteoroids regularly enter the Earth’s atmosphere (what are referred to as meteors), where most burn up and disappear, but some survive long enough to strike the surface (meteorites) or explode close to the surface (bolides). Evidence that comets can collide with planets was provided in 1994 when Shoemaker-Levy 9 was observed crashing into Jupiter.
The ‘Cretaceous/Tertiary Impact’, caused by either a comet or an asteroid, 65 million years ago created the 250 km wide Chicxulub crater in the Gulf of Mexico and is widely held responsible for the extinction of the dinosaurs and various other life forms. A bolide was believed to be responsible for the 1908 phenomenon around the River Tunguska in Siberia when over 1000 square kilometres of uninhabited forest were flattened (Chyba et al. 1993).
Critical review/discussion
There are thousands more “near-Earth objects” (NEOs) on Dr Marsden’s PHA list. Most of them seldom give cause for immediate concern. But it does mean that an object on the PHA list can come within a few million kilometres of Earth and “that the object is large enough to have a global effect.” Austen Atkinson even suggests there is “an international conspiracy of silence” to conceal the danger of impacts.
The fledgling International Monitoring System (IMS), as having an Earth-monitoring as well as a missile-detection role, spotted a speeding meteoroid that crashed into the atmosphere over the Pacific on 23 April 2001, where it produced a blast nearly as powerful as the Hiroshima bomb. On 27 October 2001, there was an enormous flash, and explosions were heard in many areas of England’s east coast. An astronomer at the London Planetary Forum, Jacquelin Mitton, said these were meteoroid fragments following close on the tail of a comet. An asteroid up to 16,000 feet wide gave the Earth a close shave on 7 January 2002, passing less than twice the distance from the moon to our planet.
It was spotted by a NEAT telescope on Mount Palomar in California. Benny Peiser, an asteroid expert at Liverpool John Moores University, said that “such an object could wipe out a medium-sized country if it impacted.” In February 2002, NASA’s Spaceguard survey said that in the year 2001, over 100 rocks in near space more than one kilometre across were discovered, bringing the definite known total to 587 of the estimated 1,743 out there.
There are a number of smaller meteoroids, up to 5,000 a year, that are often as big as a potato—a few can be tens of feet across—which plummet to Earth and can cause severe damage to whatever they strike, including aircraft and satellites. Some scientists believe that there are potentially millions of meteors out there in near space. Indeed, not a year goes by without thousands of these potato-sized objects actually landing intact on Earth’s surface. Press reports frequently give examples of vehicles and buildings being hit by rocks from outer space.
Meteors are themselves thought to be the product of the larger-sized asteroids that have been busted apart. Those meteors landing on Earth’s surface (strictly speaking, meteorites) are usually slowed to about 500 mph but can still cause a giant plume of vaporised stone to shoot up from the impact site, blasting a hole through the atmosphere to eject hot and luminous debris. As it cools, the rocky material drifts back as tiny pellets of stone, which are heated by the air to glow hot pink.
According to Lockheed Martin’s Sandia weapons and space vehicle research laboratory in California, a half-mile-wide comet, if it were solid and struck Earth, would release energy akin to that generated by 300,000 million tons of TNT. Sandia is now effectively able to calculate and correlate the amount of kinetic energy produced by a space object of any given mass and size and the amount of damage it can do. They use teraflop computers that can perform 54 million cell calculations using parallel processing.
Brian Marsden pointed out that even with current technology, it is unlikely that the warning time for a “long-period” comet (one that doesn’t return to Earth regularly every few hundred years as the “short-period” ones do) would be any more than a few weeks. Austen Atkinson, in his book Impact Earth, says that we might indeed have only a few hours’ warning of such comets, or perhaps none at all.
The connection between comets and meteoroids becomes clearer when we consider the much-discussed “meteor showers” and how often Earth passes through these. These showers have names and leitmotifs and appear seasonally or at certain times of the year. They are themselves triggered by contact with the debris or gases in the tail of a large comet.
Comets are thought to have formed from the gravitational effects of the sun as the solar system passed through interstellar dust clouds. In June 1995, the Hubble telescope had confirmed that the icy fringes of the solar system were the most likely places where comets resided, known as the “Kuiper Belt” (a more distant source of comets is the Oort cloud). Objects in the Kuiper Belt are thought to be chaotically orbiting, short-period comets zooming around the sun every 200 years.
Many would be much larger than commonly supposed, although the vast majority of the many tens of thousands per year that reach Earth would have nuclei no bigger than an average boulder. There might be nearly 5,000 of them up to six miles across that have Earth-crossing orbits on a short-period 200-year cycle, which means that every 50 years, one of them could be on a very close fly-by indeed.
A comet with an “Earth-crossing” orbit is one that could bring any object into Earth’s gravitational field with possibly disastrous consequences since it either misses us or it doesn’t. Alternatively, it could simply be one that zips in and out of the solar system as it blindly follows its own trajectory. They are occasionally nudged towards Earth, and about 70 of them make return visits to our skies at the end of a long, hyperbolic tether, at intervals of approximately ten years. Some 40 others zoom in from further out and can take up to 1,000 years to return, much diminished in size after their tenuous nucleus is worn away by the sun’s rays.
The U.S. National Research Council warned in a report, which included experts from the U.S., Japan, Canada, Russia, and Germany, that if nothing was done about space debris, inner space could become so clogged with high-speed orbiting flotsam that it would become a “death zone,” presenting a potentially lethal hazard to spacecraft of all types, manned and unmanned. In the following year, 1996, the ESA declared that mitigation policies must include the removal of rocket launchers after mission completions.
In 1991 the International Astronomical Union formed a Working Group on Near-Earth Objects (NEOs). A year later, a subcommittee was set up to investigate “asteroid hazards” in conjunction with the International Institute of Problems of Asteroid Hazards (IIPAH). It was the continued fear of an impact, or the threat of serious disruption to spacecraft flights, that goaded the U.S. Defense Department to issue a document and present it to the USAF in 1996.
This document, in turn, resulted in a directive to the chiefs of staff regarding the technologies they would have to use to maintain their domination of the skies. The document, in effect, deplored the “current lack of adequate means of detection, command and control…in terms of the courses of action in the event of a likely impact by an ECO (Earth-crossing-object).” A NASA-DoD working group was established a year later, and a U.S. Government Orbital Debris Mitigation Standard Practices was set up. An Orbital Debris Workshop for Industry was set up a year after that.
NASA, the Marshall Space Flight Center in Huntsville, Alabama, the Jet Propulsion Laboratory (JPL) at Pasadena, and the Langley Research Center in Virginia pointed out the strategic and scientific importance of comsats and ELINT satellites, which must be protected from plasma and particulate sky static. Mitigation studies were done at the USAF Space Warfare Center at Shriever AFB near Colorado Springs and also at Kirkland AFB in Albuquerque, New Mexico. Important debris studies were conducted by private concerns like ITT Industries, Lockheed Martin, Teledyne Brown Engineering, Kaman Sciences Corporation, Boeing North American, Batelle, and others.
A paper by Dr Nigel Holloway of the Atomic Weapons Establishment in Britain reviewed the risk of NEO impacts in the context of the “tolerability of risk,” a concept introduced by the U.K. safety regulators of the Health and Safety Executive, following the Sizewell B public inquiry.
Many scientific institutions and observatories take part in NEO research. The detector GORID was successfully launched on the board of the Russian Express-2 comsat in September 1996 with a Proton rocket from Baikonur, mainly to check on asteroid debris. It is expected to last at least seven years in orbit, according to a joint team of experts from Holland, Germany, and Russia headed by Dr G. Drolshagen of the Dutch space debris research organisation, ESTEC (the European Space Research and Technology Centre).
They said that during the first 118 days of operation, several hundred events were recorded, “many showing signs of real impacts.” Now spacecraft protection has gone hand in hand with new designs based on new guidelines and “basic technologies” derived from a database that includes results of hypervelocity impacts and state-of-the-art space vehicle protection systems. Hypervelocity impact research bears on many areas of modern physics, geoscience, and astronomy itself. New types of accelerators have been made to emulate projectile impacts and include electrostatic and EM-propulsion and explosion propulsion techniques. Studies have been done, as we have seen, with “light gas guns” that can fire projectiles at a velocity of up to 3 kilometres per second.
The British government took the threat to Britain seriously and indeed the world of the consequences of a large meteoroid impact—and the need for them to be deflected by military-type systems.
In September 2000, it published its Task Force report and recommended, prior to considering the military option, the building of a new 3-meter-class survey telescope to search for Near-Earth Objects (NEOs). It drew attention to other surveillance regimes actually in operation that could be utilised for more focussed NEO observations, especially the ESA’s Gaia mission and NASA’s Space Infrared Telescope Facility (SIRTF). It also recommended the global coordination of sky searches with inexpensive microsatellites and the coordination with a major British input of other NEO research and observation studies and forums to formulate Spaceguard strategies.
One of its recommendations does indeed suggest that the United Kingdom and other governments “set in hand studies to look into the practical possibilities of mitigating the results of impacts and deflecting incoming objects.”
This latter aspect was an appeal for spending as much as $100 million on a defence system against space objects, similar to a missile defence system, with an early warning network coupled with some means to stop an incoming threat. The astronomer Duncan Steel says that such a leading role for Britain (presumably referring only to the observational aspect) would cost U.K. taxpayers some £10 million annually but would be well worth the trade-off in view of the massive economic damage that would occur to the United Kingdom should the meteorite strike. He mentions other monitoring assets already in operation that could be linked up with the British scheme, such as the Visible and Infrared Survey Telescope for Astronomy (VISTA)
Louis Friedman of the Planetary Society pointed out that further vigilance is required because our present observations are based on only 10 per cent of possible NEOs. “Whether there is a threat or not, or what the nature of it might be, depends on details…we just don’t have.” 31 He added that the task force report adopted the right cautionary tone without resorting to “hysterical doomsday language,” although he is surprised that Britain should be pushing for its own space-based observatories since Britain’s own space science exploration programs are rather meagre compared with America’s or Russia’s, and Russia seems to have been left out of the picture in regard to the international Spaceguard effort.
The Spaceguard Foundation, formed as a result of decisions spurred by the Council of Europe’s Resolution 1080 of 1996 and of other decisions taken by the IAU, has appointed Britain’s new Spaceguard Centre in Wales, set up in 2001, as the International Spaceguard Information Bureau. Led by Jonathan Tate, it has become the leading public outreach organization, consisting of astronomers, analysts, and risk experts, as well as government officials dealing with NEOs in the world’s skies. On an interesting aside, Andrea Carusi, president of the Spaceguard
Part of the post-Cold War restructuring of NATO saw, in 1998, the establishment of a unit at its Brussels headquarters to utilize military resources to protect citizens from natural rather than military threats.
Conclusions
As with the global politics of health, the horizontal approach to securing the lives of those most prone to natural disasters has steadily gained credibility in epistemic communities and in the global polity but struggles to win the hearts and minds of governments, and the general public of countries moved to help those people. Driven by greater media exposure,
Orbital debris may represent the single, the most potentially useful window of opportunity for cooperative space arms control and regulation for the United States and the global spacefaring community through 2015. 55 The National Aeronautics and Space Administration (NASA) defines “orbital debris” as “any man-made object in orbit about the Earth which no longer serves a useful purpose”.
Human space activity has generated a lot of debris: there are over 9,000 objects larger than 10cm and an estimated 100,000-plus objects between one and 10cm in size. The largest single source of this debris has been intentional and unintentional satellite explosions in orbit. Orbital debris generally moves at very high speeds relative to operational satellites and thereby poses a risk to these systems due to its enormous kinetic energy. Only three collisions between operational systems and orbital debris are known to have occurred thus far, but concerns about this hazard are growing due to the increasing number of operational space systems and the five per cent growth rate in LEO orbital.
The push is on to expand the military’s spaceflight capabilities along all dimensions. This can be seen in the resurrection of the military space plane in its new incarnation as the Space Operations Vehicle (SOV) set for 2012. SOVs provide on-demand space access with rapid turn-around plus, in principle, great flexibility in operations.
These spacecraft are more sophisticated versions of the Transatmospheric Vehicle (TAV) concept dating back to the 1960s and the cancelled Dyna-Soar as well as the late National Aerospace Plane (NASP) of the early 1990s. Both programs represented extensions of a military flight presence, at least to low earth orbit (LEO). The Space Maneuvering Vehicles being developed extend flight operations deeper into outer space for asset recovery, providing even greater operational flexibility.
Works Cited
Alvarez, L., Alvarez, W., Asaro, F. & Michel, H. V. 1980 Extraterrestrial cause for the Cretaceous.
Bottke, W.F., Morbidelli, A. & Jedicke, R. Recent progress in interpreting the nature of the near-Earth asteroid population. Mitigation of hazardous comets and asteroids (eds. Belton, M.J.S. Morgan, T.H. Samarasinha, N. & Yeomans, D.K.), pp. 1–21, Cambridge, UK: Cambridge University Press 2004.
Chyba, C., Thomas, P. and Zahnle, K. (1993) ‘The 1908 Tunguska Explosion: Atmospheric Disruption of a Stony Asteroid’, Nature 361: 40-44.
Kitchin, C. (2001) ‘Early Discoveries’, part of feature, ‘Focus: Asteroids’, Astronomy Now 15(1): 54-55.
UK Task Force Report on Potentially Hazardous Near-Earth Objects. London: Stationery Office. (2000).
Verschuur, Gerrit. Impact: The Threat of Comets and Asteroids, Oxford University Press, 1996.