INTRODUCTION

However, is there any real chance that bodies known as microplanets (that is, comets, asteroids and meteoroids) may actually collide with the Earth? Can their orbits be set (because of perturbations) into a collision route? If so, how to avert these killer asteroids so as to save mankind from extinction? What do astronomers (and scientists in general) know about killer asteroids?

The Solar System started as a huge cloud of neutral hydrogen gas and dust suspended in or near a spiral arm of our Milky Way Galaxy. As soon as the gas became compressed by its own gravity our proto-sun began its evolution. Primordial materials gradually rearranged in the shape of a proto-planetary disk (a proplyd) out of which the planets would be born. Leftover dust particles, due to electrical charges, began coalescing one with each other, thus beginning a process known as accretion. Particles then became kilometer-sized planetesimals, then, in turn, these became moon-like protoplanets.

It is estimated by some researchers that our early solar system could have contained as much as 1,000 million planetesimals (Freedman, Kaufmann, 2002). Certainly, such a huge number of orbiting bodies must have produced frequent, sometimes violent collisions. A recently published work says, "A quick look at our own moon will suffice to show that we live in a dangerous neighborhood" (Altschuler, 2001).

Even though the process of accretion has essentially come to a halt (it actually lasted only through the first stages of our Solar System) some accretion episodes may, and do still occur (Beatty, Collins-Petersen, Chaikin, 1999). The Solar System is still crowded with large numbers of microplanets, which many regard as actual relics from the early stages of our System. In addition to the asteroid belt located between the orbits of mars and Jupiter, we have know the existence of a Kuiper Belt, extending from just outside the orbit of Neptune, some 30 astronomical units (au) from the sun, to about 500 au.

There is also much evidence pointing to the existence of a gigantic reservoir of comets, called the Oort Cloud, extending from just outside the Kuiper Belt to about 50,000 au (almost one light year). It is estimated that the Oort cloud may contain up to thousand of millions of comets. Occasionally, some of these objects may be perturbed in their orbits (either by a nearby field star, an interstellar molecular cloud, or even by one another) and thrown up into the inner Solar System, posing a collision risk for planets (such our own) orbiting along this area.

The same actually happens with some asteroids located in the main asteroid belt between Mars and Jupiter. Areas exist within the asteroid belt, called Kirkwood Gaps, where orbits enter upon 2-1 and 3-1 resonance with Jupiter (that is, orbital paths equivalent to 1/2 or 1/3 of Jupiter's path). Due to Jupiter's gravity, asteroids located along these areas will be are exposed to very strong perturbations, and may even end up thrown out of their orbits, possibly towards the inner Solar System. That is why Kirkwood Gaps regions are called gaps—they are virtually devoid of asteroids.

These stray objects posing collision risks to the Earth are known as Near-Earth Objects. "Near-Earth Objects (NEOs) are generally defined as those objects whose close approaches to the sun are 1.3 AU or less. As a result, Near-Earth Objects are those comets and asteroids that can come within about 28 million miles of the Earth's orbit." (NASA)

But what about actual collisions recorded in our own planet Earth?

Approximately 150 impact craters have been positively identified on Earth, and the number continues to grow. These are mostly located in North America, Eastern Europe and Australia, since these areas have not been subjected to large scale geological changes, as most other areas have. But also, because impact research has been more complete on those (Beatty, Collins-Petersen, Chaikin, 1999).

The following are some examples of actual collisions of NEOs on planet Earth:

Manicouagan (Quebec, Canada)—
214 million years ago, a 5 kilometer object hit the Earth carving out an enormous crater. It is thought that this collision could have triggered the mass extinction occurring during the late Triassic. The original 100 kilometer crater rim has totally eroded, leaving in its place a 70 kilometer ring-like crater, now filled entirely by a lake [see figure 1].

Figure 1. Manicouagan Impact Crater Source: NASA/GSFC/LaRC/JPL/MISR Team Comments: This is a satellite picture of the Manicouagan Crater in Quebec, Canada, obtained by the EOS mission. A peculiar ring-shaped lake is clearly seen on this natural-color image acquired on June 1, 2001.

Chicxulub (Yucatán, Mexico)—
64.98 million years ago, a 10 kilometer object (possibly an asteroid) hit the Yucatán shoreline, liberating an energy equivalent of 100 million megaton nuclear bomb (Altschuler, 2001). This killer asteroid blew up thousands of millions of tons of dust into the atmosphere, causing a global winter which may have lasted anywhere from six months to a few years. Temperatures plunged down to, or below the freezing point, triggering the late Cretaceous extinction that may have killed more than half of the Earth's animal and plant species, including the dinosaurs. A 175 kilometer crater remains [see figure 2], buried under a kilometer of sediment (The Planetary Society, 2000)

Figure 2. Yucatán Impact Structure Source: NASA/JPL/NIMA Comments: This is a topography map of the Yucatán Peninsula in Mexico generated by the SRTM mission. The crater itself is now buried, but its rim remains visible as an arc-like indentation on the northwestern side of the peninsula.

Tunguska (Eastern Siberia, Russia)—
On June 30, 1908, a 50 meter object (possibly a stony meteor) exploded with a force of about 10 megatons, that is 700 times as powerful as the Hiroshima atomic bomb. This object disintegrated perhaps a second before hitting the Earth, but it started out a tremendous fire, as well a powerful shock wave that totally destroyed a forest area of about 2,000 square kilometers. The blast was certainly heard at about 1,000 kilometers away, and probably much farther than that. When the first expedition arrived in the area nineteen years later, headed by Leonid Kulik, a scene of total devastation was seen. Thousands of trees were laid parallel upon the ground, at an angle opposite to the meteor blast.

An interesting case of a recent near-collision is that of Kusaie (Micronesia) on February 1, 1994. On that day, an extremely bright fireball with an estimated diameter around 10 meters exploded in the atmosphere, near the said island. Events like these are thought to occur approximately every 10 years, but due to the small size of the meteor, they normally disintegrate before hitting the ground (Altschuler, 2001).

But, one may still ask, just what is the real likehood of one being killed by an asteroid impact?

Considering these facts, some governments are taking concrete steps to learn more about NEOs, with the hope of avoiding a possible future collision. The United States established in 1998 a new NASA department, the Near-Earth Object Program Office, with the purpose of detecting, tracking and studying potentially hazardous asteroids and comets that could ominously approach the Earth. The British National Space Centre has recently been active in protecting near-Earth environment. The Australian government has also, in the past, funded NEO research through the Anglo-Australian Near-Earth Asteroid Survey (or AANEAS, later renamed Spaceguard Australia), a very successful program lasting from 1990 to 1996.

These programs have led to precise calculations of asteroid orbits in the hope of predicting some future collision. Not only that, but comprehensive searches have now begun, with the purpose of identifying the thousands of hazardous, stray asteroids that still remain unknown. "Approach reports" are also routinely posted in the Internet. During the month of April, 2003, for example, NASA has predicted that ten known objects will have "close approaches" with the Earth. These bodies are mostly in the 50-500 meter range, and will be passing the Earth from distances ranging from 12.4 to 52.7 lunar distances.

But how does all this measure up? It is estimated that an asteroid smaller than about 100 meters in diameter would only cause regional damage, while a 100-1,000 meter body could bring about widespread regional destruction. A meteor larger than 1 kilometer is considered a true killer asteroid, and may bring about global devastation on a scale similar to that of Chicxulub. This amounts basically to a global winter, equivalent perhaps in its destructive effect to a world-wide nuclear conflict.

The destructive effect of a colliding body is determined solely by its mass and its impact speed. This is described by the following relation: E = ˝M × V² where E is liberated energy, M is the asteroid's total mass, and V its impact velocity. Even though V is given more weight in the equation, this value tends to be rather similar for most killer asteroids, so we tend to place more emphasis on weight (or perhaps size, this of course being a function of weight and density) On the question on how to avert these objects, it has been suggested (Altschuler, 2001) that a nuclear missile may be exploded over the asteroid, thus perturbing the object's trajectory in the hope of preventing an impact. The actual impact of such a missile may be rather insignificant over a large object (say, 8 or 10 kilometer asteroid) so, the best chance would be to intercept the object months or years ahead of its predicted collision, so that the effects of such small perturbation may be considerably magnified through time.