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Source: Adapted from http://www.spaceguarduk.com/asteroids.htm (last accessed 9 November 2005).

and dust during their active phases (which occur when their orbits bring them close to the Sun). They have two known main reservoirs - the Kuiper Belt and associated scattered disk (beyond Neptune's orbit) and the 'much more distant spherical halo of comets' called the Oort Cloud (Chapman 2004). Their orbits are highly elliptical, with a perihelion distance of less than 1 AU, and an average aphelion distance of about 10,000 AU. They are shortlived, surviving about a hundred perihelion passages. Comets that take more than 200 years to orbit the Sun are 'long-period' comets (this includes comets that are not periodic at all, coming for the first time from the Oort Cloud and being perturbed right out of the Solar System). Comets that take less than 20 years are 'short-period' or Jupiter-family comets; those that take 20-200 years are 'intermediate-period' or Halley-family comets. By 1994, astronomers had discovered 26 short-period active Earth-crossing comets, of which 13 belong to the Jupiter family and 13 to the Halley family, two extinct short-period comets, and 411 and long-period comets (Marsden and Steel 1994).

Space debris as a hazard

Comets, asteroids, and meteoroids escape slowly from their reservoirs. They do so chiefly owing to chaotic dynamics near planetary resonances, which are distances from the Sun at which a small body has an orbital period that is a simple fraction of the orbital period of a planet (Chapman 2004). Collisions and other minor orbital perturbations abet their escape. Dislodged bodies that arrive in the inner Solar System - the terrestrial planet zone -become near-Earth objects (NEOs), which include comets and near-Earth asteroids (NEAs). NEOs pose a potential threat to the Earth, which has led to considerable recent research into the risk involved. Comets contribute about 1 per cent of the total risk, near-Earth asteroids and their associated meteoroids 99 per cent.

The magnitude and frequency of impact events is calculable from the size distribution of craters on other planets and satellites. The frequency of collision with comets, asteroids, and meteoroids is inversely proportional to the size of the colliding body (Figure 3.2). It ranges from the continuous rain of meteoritic dust that enters the atmosphere, through the common strikes by small meteorites, to the occasional strike, once every million years, by asteroids with a diameter of about 1 km, to the exceedingly rare strike, just once every 50 million years or thereabouts, by a mountain-sized asteroid or comet. The rate of collision is not necessarily constant: there are theoretical reasons, supported by some empirical evidence, for supposing that bombardment tends to occur as episodic showers lasting a few million years (Hut et al. 1987). The showers themselves seem to occur roughly every 30 million years (Clube and Napier 1982; Napier 1987).

Cratering

The immediate effect of a bolide impact is the production of a crater, sometimes called an astrobleme (p. 34). It is impossible in a laboratory to replicate the processes by which large impact craters form by sudden releases of huge quantities of energy, and no such structure has formed during recorded human history (French 1998, 17). Researchers gain knowledge of large impact structures indirectly, by combining theoretical and experimental studies of shock waves and geological studies of larger terrestrial impact structures. All seem to agree that cratering is a complex process, which still has many uncertain details.

Cratering processes depend on bolide size. As French (1998, 17-18) explained, smaller bolides, a few metres or less in size, lose most or all of their original velocity and kinetic

Impact energy (Mt of TNT equivalent energy)

Impact energy (Mt of TNT equivalent energy)

Bolide diameter (km)

Figure 3.2 Size distribution for the cumulative number of NEAs larger than a particular size. Adapted from Chapman (2004).

Bolide diameter (km)

Figure 3.2 Size distribution for the cumulative number of NEAs larger than a particular size. Adapted from Chapman (2004).

energy in the atmosphere through disintegration and ablation, striking the ground at low velocities, no more than a few hundred metres per second. In consequence, the bolides penetrate only a short distance into the target (depending on its velocity and the nature of the target material), the bolide's momentum excavating a pit by strictly mechanical means that is slightly larger than the bolide itself. The bolide survives, more or less intact, and much of it remains in the pit bottom. Such pits (penetration craters or penetration funnels) are typically less than a few tens of metres in diameter, an example being the pit dug by the largest piece of the Kirin (China) meteorite fall in 1976. In contrast, larger and more coherent bolides penetrate Earth's atmosphere with little or no deceleration and strike the ground at practically their original cosmic velocity, which is in excess of 11 km/s. Such bolides are relatively large, perhaps exceeding 50 m in diameter for a stony object and 20 m for a more coherent iron one. On smiting the target, they form hypervelocity impact craters, usually simply called impact craters (Figure 3.3). The size of the crater formed depends mainly on the kinetic energy of the bolide and the density of the target rocks. Typical crater diameters are about 20 km for a bolide with a diameter of 1 km, and 10 km for a bolide with a diameter of 0.5 km. Impact craters begin forming the very moment that the bolide hits the target at its original cosmic velocity. These impact velocities are much greater than the speed of sound in the target rocks, and intense shock waves radiate outwards through the target rocks from the point of impact at high velocities (sometimes over 10 km/s), and in doing so produce the crater. Ordinary geological processes cannot generate such shock waves, which produce shock pressures of up to several hundred GPa, far above the stress levels (around 1 GPa) at which terrestrial rocks undergo normal elastic and plastic deformation. As a result, the shock waves produce unique and enduring deformation effects in the rocks they pass through. On expanding, the shock waves interact with the original ground surface, setting in motion a large volume of the target rock, the upshot of which is the excavation of an impact crater. Gravity and rock mechanics then modify the crater, giving it characteristic features (see p. 44).

Hypervelocity impacts in the ocean would also produce a crater. Water near the impact would be highly compressed by the shock and vaporize upon decompression, spraying out of an expanding transient water cavity. The impact would breach the oceanic crust and upper mantle interface leaving a pronounced morphological, gravity, and magnetic structure - a hydrobleme. It is possible that an oceanic impact would also produce a system of giant waves - superwaves - that would flood lowland lying near the sea (Huggett 1989a, 1989b).

It is perhaps worth emphasizing the colossal energy involved in the formation of hyper-velocity impact craters. A larger bolide striking the Earth produces similar effects to a nuclear explosion, but bigger by many orders of magnitude owing to the enormous kinetic energy involved. The kinetic energy of a bolide is the product of bolide mass and the square of the speed at which the bolide travels through space. It is common practice to express the kinetic energy of a bolide in terms of megatons of TNT equivalent energy; that is, the kinetic energy equivalent to exploding so many megatons of TNT. The atomic bomb dropped on Nagasaki had a kinetic energy equivalent of 0.02 Mt. Even a small asteroid or comet, with a diameter of 1 km, will have the energy equivalent of about 50,000 Mt. A bolide with a diameter of 10 km will have the energy equivalent of about 50,000,000 Mt. The sudden release of terrestrial energy is small by comparison: the explosion of Krakatau in 1883 was equivalent to about 50 Mt; and a major earthquake is equivalent to some 100 to 500 Mt. The impact of a bolide with 100,000,000 Mt of energy is equivalent to exploding 10 atomic bombs, roughly the size of the one dropped on Nagasaki in 1945, on every square kilometre of the Earth's surface (McCrea 1981). This comparison is slightly misleading: a nuclear explosion and a bolide impact are not strictly comparable because they involve different intensities of energy. The energy intensity for a chemical explosion, such as that of TNT, is about 17 MJ/kg; for a bolide impact it is about 180 MJ/kg; and for a nuclear explosion it is 200,000 MJ/kg (Allaby and Lovelock 1983, 142). In practice, this means that the energy released by an impacting bolide is powerful enough to reduce molecules to atoms and atoms to ions, but unlike a nuclear explosion, not powerful enough to alter the atoms themselves.

Given the highly energetic nature of hypervelocity impacts, it is reasonable to speculate that they may trigger a number of geophysical processes including reversals of the Earth's magnetic field, continental drift, and volcanism (Napier and Clube 1979; Rampino 1989). For many years, geophysicists were sceptical about the possibility of a hefty impact triggering large-scale volcanism (e.g. Ivanov and Melosh 2003). A model built by Linda T. Elkins-Tanton and Bradford H. Hager (2005) showed that a giant bolide with 30-km diameter hitting a thin (75-km thick) lithosphere could produce flood-province-scale volcanism.

(a) Contact/compression stage Projectile

Shock wave

Mafic magma forms by immediate in situ decompression and later by convective flow beneath a dome in the lithosphere-asthenosphere boundary, which forms under the crater by instantaneous fluid flow of the lithosphere during impact and later by isostatic uplift. A 30-km bolide would form a crater with a diameter of around 300 km and impacts of that size, which occur about 10-50 times per billion years, would be necessary to produced flood-basalts volcanism. None of the craters visible on Earth today is that size, but it is possible that big craters lie underneath or close to flood basalts.

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