Most asteroids follow fairly regular paths in orbits between Mars and Jupiter. A small fraction, about one in a thousand, have evolved from their regular orbits by slow gravitational effects of the planets, mainly Jupiter, to travel in more elliptical orbits that cross the paths of other planets, including Earth. The first of these discovered was Eros, found in 1898, which crosses the orbit of Mars but not Earth. The first space mission dedicated primarily to visiting an asteroid was the Near Earth Asteroid Rendezvous (NEAR) mission (later renamed NEAR Shoemaker in honor of American astronomer Eugene Shoemaker), which orbited Eros for a year in 2000-2001, before touching down on its surface on February 12, 2001.
Even in 1694, when Edmund Halley discovered that the orbit of the comet that bears his name crosses Earth's orbit, he suggested the possibility of a collision with Earth by a comet, and he rightly suggested that such an event would have a catastrophic effect on Earth and its inhabitants. In 1932, two more asteroids were discovered, named Amor and Apollo, which pass close enough to Earth to suggest the possibility of eventual collision with the planet.
Today scientists refer to asteroids that can come closer than 1.3 astronomical units (AU) to the Sun (0.3 AU to Earth) as near Earth asteroids (NEAs), or collectively along with comets that come that close, near Earth objects (NEOs). By January 2002, 1,682 NEAs had been discovered, 572 of which were estimated to be 1 kilometer (0.6 mile) or larger in diameter. Scientists estimate that the total number of NEAs larger than 1 kilometer in diameter is about 1,000, so somewhat more than half of them had been found by January 2002. The largest asteroid in an orbit actually crossing Earth's orbit is around 10 kilometers (6 miles) in diameter. Scientists do not believe that there are any undiscovered objects larger than 4 or 5 kilometers (2.5 or 3 miles) in diameter.
The Frequency and Energy of Impacts
Given that these cosmic bullets are flying around Earth, the expected frequency of impacts on Earth can be estimated. Any one NEA has a likelihood of hitting Earth in about 500 million years. Since there are about 1,000 NEAs larger than 1 kilometer, one impact about every 500,000 years can be expected. The energy of such an impact can also be estimated. A piece of rock 1 kilometer in diameter, traveling at 20 kilometers per second (12.4 miles per second) on impact, should release an energy equivalent to almost 100,000 megatons of TNT, or about the total energy of all the nuclear weapons in the world. Such a blast should make a crater nearly 20 kilometers (12.4 miles) in diameter.
Evidence abounds of past collisions, on Earth as well as on the surfaces of almost all other solid-surface bodies in the solar system. Impact craters up to hundreds of kilometers across are clearly visible on the face of the Moon and have been found and counted on Mercury, Venus, Mars, planetary satellites, and even the asteroids themselves.
In 1980, the father and son team of Louis and Walter Alvarez, along with two other colleagues, offered a revolutionary explanation for the extinction of the dinosaurs, as well as most other species inhabiting Earth at that time (65 million years ago). They found the rare element iridium in the thin clay layer that caps the rocks of the Cretaceous era. The element was present in concentrations far too high for a terrestrial explanation, but just about right for the debris left from a cosmic impact by an asteroid or comet about 10 kilometers (6 miles) in diameter. This hypothesis, which was first met with widespread skepticism, has gained strength with many subsequent supporting discoveries, including the identification of the "smoking gun"—the remains of the crater at the tip of the Yucatan Peninsula in Mexico. Known as the Chicxulub Crater, it is buried in sediments and invisible from the surface, except for a ring of sinkholes outlining the original rim, approximately 200 kilometers (125 miles) in diameter. Impact cratering is now recognized as an important geological process, which can even affect the evolution of life on Earth.
Potential Effects of a Collision
The world received a "wake-up call" in July 1994 when the pieces of the comet Shoemaker-Levy 9 slammed into the planet Jupiter, leaving giant dark spots in the clouds, easily visible from Earth through a small telescope. Some of the spots were as large as the entire Earth. Based on these observations and computer models of the expected effects of a cosmic impact on Earth, it is estimated that an asteroid 1 to 2 kilometers (0.6 to 1.2 miles) in diameter would form an impact crater more than 20 kilometers (12.4 miles) in diameter. In addition, it would throw enough dust into the upper atmosphere to block out the Sun for about a year, producing a global "im-pact winter."
Such a climatic catastrophe could lead to global crop failures and the starvation of perhaps a quarter of the world's population. The individual numbers boggle the mind: more than a billion deaths, but only once in 500,000 years. Yet the quotient is quite understandable: an average of some thousands of deaths per year, or in the same range as the death toll from commercial airline accidents, floods, earthquakes, volcanic eruptions, and other such disasters that are taken very seriously. Because the frequency of occurrence is so low—indeed there has never been a catastrophic asteroid impact in recorded history—humans have paid less attention to this risk than to the others mentioned. But the consequences are as terrible as the intervals are long, so the importance is about the same as the other natural hazards, with one significant difference. This hazard alone (with the possible exception of a very massive volcanic eruption) has the potential to end human civilization globally.
Preparations for and Responses to Potential Collisions
What can, or should, be done? As a first step, it makes sense to simply look and find all the asteroids and Earth-approaching comets out there and see if one has our name on it. Beginning in the late 1990s, several governments and agencies embarked on what has been loosely called the Spaceguard Survey.* The goal of this survey is to find at least 90 percent of all NEAs larger than 1 kilometer (0.6 mile) in diameter, the lower size limit for objects that could cause a global catastrophe. By the year 2001, the project was about half complete, and it is likely to be finished by 2010. With continued effort, ever-smaller asteroids can be found and cataloged, providing assurance that nothing is coming Earth's way in the foreseeable future (i.e., about the next fifty years).
But if we find that something is coming, what can we do? With many years warning, only a small push of a few centimeters per second would divert an asteroid from a collision course to a near miss. Even without knowing quite how to do it, it is easy to estimate that the energy needed is within the range available from nuclear weapons, and the rocket technology to deliver a bomb to an asteroid is available. Whether such a system should be developed in advance of any specific threat is a more difficult question and one that will need to be carefully addressed by both scientists and policy-makers.
see also Asteroids (volume 2); Comets (volume 2); Impacts (volume 4).
Lewis, John S. Rain of Iron and Ice: The Very Real Threat of Comet and Asteroid Bombardment. Reading, MA: Addison-Wesley Publishing Co., 1996.
Steel, Duncan. Target Earth. Pleasantville, NY: Reader's Digest Association, 2000.
Asteroid and Comet Impact Hazards. Ames Space Research Division, National Aeronautics and Space Administration. <http://impact.arc.nasa.gov/index.html>.
Tumbling Stone. The Spaceguard Foundation and NEO Dynamic Site. <http://spaceguard.ias.rm.cnr.it/tumblingstone/>.
*The term "Spaceguard Survey" is borrowed from Arthur C. Clarke's science fiction novel, Rendezvous with Rama (1973), detailing the story of a huge and mysterious object that appears in the solar system.