The bombardment hypothesis rests on the fact that space debris of varied sizes collides with the Earth. To be sure, astronomers have shown that, in orbiting the Sun, the Earth meets other bodies in the Solar System whose orbits it happens to cross; in short, the Earth is in a 'cosmic shooting gallery' (Chapman 2004). These bodies range in size from dust-like particles, which continuously rain into the atmosphere, to pieces of rock and ice the size of mountains, which strike the Earth on rare occasions. Three broad groups of extraterrestrial objects collide with the Earth - asteroids, meteoroids, and comets. Because it is not always easy to establish the origin of these objects, some authorities use the blanket term planetes-imal for them, though some astronomers confine the term planetesimal to objects over about 10 km diameter, which includes Theia, a huge planetesimal whose supposed collision with the Earth might have created the Moon. Somewhat confusingly, a planetesimal is also an object formed by the coalescence of particles in solar nebulae, of which asteroids and comets are leftover remnants. By convention, objects in and inside Jupiter's orbit are 'asteroids' and those farther out are 'comets', even though comets typically contain more volatiles than the more rocky or metallic asteroids. When considering collisions with the Earth, asteroids, meteoroids, and comets are all bolides (from the Greek bolis, a missile).
Asteroids (meaning 'star-like' objects), when viewed through a telescope, appear as a point of light. Their chief reservoirs are a large torus in the main Asteroid Belt, a region of space lying between the orbits of Mars and Jupiter, and two groups of 'Trojans' that average 60° ahead of and behind Jupiter in its orbit. The largest asteroid in the Asteroid Belt is Ceres, with a diameter of 913 km. At December 2002, the estimated population of Earth-crossing asteroids with diameters 1 km or more was about 2,400; the discovered population was 322.
Asteroids that venture within the inner Solar System fall into three groups: Amor asteroids, Aten asteroids, and Apollo asteroids (Table 3.1). They each have different collision probabilities and impact velocities. Earth-crossing asteroids probably come from the Asteroid Belt. They may also come from comets in the Jupiter-family and Halley-family short-period system, which may themselves be the fragments of a single progenitor giant comet up to 180 km in diameter (Bailey et al. 1994). The largest near-Earth asteroids discovered so far are the Earth-crossing Amors 1627 Ivar and 1580 Betulia, both with diameters of about 8 km, and the Apollo asteroids 1866 Sisyphus, with a diameter of about 10 km, 3200 Phaeton, with a diameter of about 6.9 km, and 2212 Hephaistos, with a diameter of about 5 km. The smallest detected Earth-crossing asteroid so far is probably 1993 KA2, with a diameter of 4-8 m, which passed within 0.001 AU (less than half the distance to the Moon) in May 1993.
A meteoroid is a natural solid object moving in interplanetary space that is smaller than about 100 m, but larger than a molecule. Meteoroids are probably fragments of asteroids too small to observe with a telescope. A meteorite is a natural object of extraterrestrial origin that survives the brief journey through the atmosphere without its being fully vaporized. It is a small asteroid or a meteoroid that has struck the Earth's surface. The annual accretion rate of small meteoroids, as found by examining hypervelocity impact craters on the space-facing end of the Long Duration Exposure Facility Satellite, is about 40,000 ± 20 tonnes (Love and Brownlee 1993). The smallest fragments are micrometeoroids or cosmic dust grains.
Between 1975 and 1994, infrared sensors in satellites detected 136 meteoroid impacts world-wide (Tagliaferri et al. 1994). The sensors detect the heat emitted from the fireballs produced as meteoroids detonate in the atmosphere. The flux rate of meteoroids suggests that at least one 20 kt airburst should occur every year. Such explosions are dangerous as they resemble nuclear explosions. On 1 October 1990, sensors on United States Department of Defense satellites picked up an explosion with over 1 kt of TNT energy equivalence occurring 30 km over the central Pacific Ocean. It took several months to decide that a 100-t stony asteroid striking the atmosphere had caused it (Tagliaferri et al. 1994). The biggest and most well documented encounter between a meteoroid and the Earth was the Tunguska event (p. 34).
Comets (meaning 'long-haired' stars) are diffuse, unstable bodies of gas and solid particles that orbit the Sun. They have a dusty atmosphere, or coma, commonly with tails of plasma
Table 3.1 Earth-crossing asteroids with diameters of 1 km or more (December 2002).
Near-Earth object class
Aten Apollo Amor Total
1GG 8GG 1,5GG 2,4GG
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