Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets. Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles. Sixteen asteroids have a diameter of 240 km or greater. They have been found inside Earth's orbit to beyond Saturn's orbit. Most, however, are contained within a main belt that exists between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path and some have even hit the Earth in times past. One of the best preserved examples is Barringer Meteor Crater near Winslow, Arizona.

Asteroids are material left over from the formation of the solar system. One theory suggests that they are the remains of a planet that was destroyed in a massive collision long ago. More likely, asteroids are material that never coalesced into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometres (932 miles) across -- less than half the diameter of our Moon.

Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of Earth. Asteroids that are on a collision course with Earth are called meteoroids. When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor. If the meteoroid does not burn up completely, what's left strikes Earth's surface and is called a meteorite.

Of all the meteorites examined, 92.8 percent are composed of silicate (stone), and 5.7 percent are composed of iron and nickel; the rest are a mixture of the threematerials. Stony meteorites are the hardest to identify since they look very much like terrestrial rocks.

Because asteroids are material from the very early solar system, scientists are interested in their composition. Spacecraft that have flown through the asteroid belt have found that the belt is really quite empty and that asteroids are separated by very large distances. Before 1991 the only information obtained on asteroids was though Earth based observations. Then on October 1991 asteroid 951 Gaspra was visited by the Galileo spacecraft and became the first asteroid to have hi-resolution images taken of it. Again on August 1993 Galileo made a close encounter with asteroid 243 Ida. This was the second asteroid to be visited by spacecraft. Both Gaspra and Ida are classified as S-type asteroids composed of metal-rich silicates.

On June 27, 1997 the spacecraft NEAR made a high-speed close encounter with asteroid 253 Mathilde. This encounter gave scientists the first close-up look of a carbon rich C-type asteroid. This visit was unique because NEAR was not designed for flyby encounters. NEAR is an orbiter destined for asteroid Eros in January of 1999.

Astronomers have studied a number of asteroids through Earth-based observations. Several notable asteroids are Toutatis, Castalia, Geographos and Vesta. Astronomers studied Toutatis, Geographos and Castalia using Earth-based radar observations during close approaches to the Earth. Vesta was observed by the Hubble Space Telescope.

Dangers To Earth

Although the Earth is bombarded by meteorites every day, these are usually very small and burned up in the Earth's atmosphere. But the Earth increases it's weight by 25 tonnes per day, this extra material is composed of space dust and micro meteors. Evidence shows that, on average, a large meteorite - weighing as much as 50,000 tonnes, or about the size of a modern passenger liner - hits the Earth about every 100,000 years, this is what many believe killed the dinosaurs. Craters are found all over the Earth,deep. Even more rarely (every 100 million years or so, but it could happen this decade) a cosmic projectile 5 to 10 miles across strikes, with consequences so terrible that most species (like the dinosaurs 65 million years ago) are threatened with extinction. It is plausible that an even larger object, perhaps 25 miles across or larger will strike the Earth during our Sun's lifetime with the possibility of virtually sterilizing the surface of our planet.

We are lucky in the fact that Jupiter has saved us many times as large meteorites and comets heading for Earth have got caught in Jupiter's gravitational pull and the meteorite has hit Jupiter instead. One of the most famous is the Shoemaker-Levy 9 comet, which impacted Jupiter in 1994, if this object had hit Earth instead of Jupiter it would have stayed intact and the result would have been total devastation to all life on Earth.

Large Asteroids

The population of asteroids of size 1000 meters (1 km) or larger which cross or closely approach Earth's orbit is thought to be about 1,600. We know of many of these, which is why they make the press. They are big, so we see them with telescopes. We know their orbits, and that they probably won't hit us in the next 1000 years or so. It was this sized asteroid which is thought to have caused the dinosaur extinction. A crater has been found named the Chicxulub (pronounced CHEEK shoo loob) Basin centred in Mexico's Yucatan Peninsula. The diameter of the basin is about 190 miles (300 kilometres). Rock samples obtained by drilling into the basin indicate that an asteroid struck the Earth there about 65 million years ago. This was about the time the last dinosaurs became extinct. The impact hurled a lot of debris into the sky. Many scientists believe this debris caused climate changes that the dinosaurs and more than 65% of other life on Earth could not survive.

It is thought that the very massive asteroid Geographos, a cigar shaped asteroid of 5.1 kilometres by 1.8 kilometres (3.2 miles by 1.2 miles) which passed near Earth in 1969 and again in 1994, could collide with Earth in the not too distant future. It's probably an iron or stony iron asteroid. Geographos would cause a global ice age for several years from the dust it would kick up. We can't project Geographos' orbit in the far future with enough precision to determine if it will impact Earth or the Moon, or whether it will have a close encounter which will fling it elsewhere.

770,000 years ago an asteroid or comet, estimated to be 3-10 km in diameter, struck somewhere in Southeast Asia. While the crater has yet to be located, and may be buried offshore, the impact and explosion ejected millions of tons of molten rock across as much as 1/10th of the Earth's surface. These pieces of impact melted rock, black fragments of glass called tektites are found from China all the way down to southern Australia, and as far west as Madagascar. They are the main piece of evidence that southeast Asia was devastated by a huge impact.

An asteroid hit Tunguska, Siberia on June 30, 1908. It was a tiny asteroid, only about 30 to 60 meters across (i.e., difficult and unlikely to be detected by even the most modern ground-based telescope in existence today, given their necessarily selective partial coverage of the sky) and 10,000 to 100,000 tons in mass. Fortunately, the asteroid was just grazing the Earth and did not come straight down, causing a long streak in the sky seen over many territories, and was packed with volatiles rather than nickel-iron metal. It exploded in the air about 5 kilometres (3 miles, or 15,000 feet) above remote Tunguska. However, the energy released was equivalent to a nuclear bomb. In fact, the explosion was greater than the Hiroshima or Nagasaki nuclear bombs. The forest was flattened, out to about 30 kilometres (18 miles) from the centre. Below the explosion, trees were incinerated, though some remained standing -- just like at Hiroshima and Nagasaki. Trees were scorched on one side out to 14 kilometres (9 miles) from ground zero. If the same object had exploded over New York City, the scorched area would have reached nearly to Newark, New Jersey. Trees would have been felled beyond Newark. Deafening bangs' might have beenheard in Pittsburgh, Washington, D.C, and Montreal. We can only imagine the devastation this would have caused if this had landed.

The exact damage inflicted by an asteroid or comet depends upon a number of factors -- size, speed, composition of object, and whether it hits land or ocean. For a land impact, it can be said in general that an object of roughly 75 meters diameter can destroy a city, a 160 meter object can destroy a large urban area, a 350 meter object can destroy a small state, and a 700 meter object can destroy a small country. Anything larger than this has the potential of creating a "nuclear winter".

For an ocean impact, the destruction is much greater -- smaller objects can cause far more widespread damage. The effects of an ocean impact are felt much further away than the effects of an airburst due to the more effective propagation of water waves, and the fact that human populations and assets are largely concentrated in coastal cities which historically became established due to water transport (i.e., shipping and trade) and businesses near ports.

Smaller Asteroids

The population of smaller asteroids probably exceeds 100,000 crossing Earth's orbit. But we know of precious few of these small asteroids because they are difficult to detect using today's telescopes until they are very close to us (and possibly just about to hit us!). The Meteor Crater in Arizona, measuring about a kilometre in diameter, was caused by a nickel-iron rock only about 30 meters across. A small asteroid (under 1km) does not have the potential to cause a global catastrophe but the damage caused to the local area would certainly be catastrophic, and result in the loss of many lives and possibly complete cities.

Conclusion

So, is the Earth targeted for an impact?

Not that we know of! None of the asteroids discovered so far is on a collision course with Earth. However, we can't speak for those that are not yet discovered. If the asteroid is a global killer (above 1km) we would possibly have some warning, maybe a few months or even a year, but if the asteroid is smaller the chances are we would have no idea it was coming until it entered Earth's atmosphere. Perhaps a larger threat is posed by comets. Our solar system's natural orbit within the Milky Way takes Earth through a densely populated area every million years or so. As our solar system passes through this area, scientists believe that one of the planets could "knock" one of the many comets in the area, off course. This comet would follow the Sun's magnetic field, taking around 1000 years to catch up with the Sun and normally disintegrate in the Sun's atmosphere. But, sometimes the comet just slingshots around the Sun and heads back out the other side, possibly on a direct collision course with Earth....and yes, it has been 1000 years since we last passed through this area.

TheShoemaker-Levy-9 comet segment "G" piece which hit Jupiter on 18 July 1994 caused a energy pulse 5,000 times that of the Tunguska event in 1908. The Tunguska event was equivalent to 12 mega tons of TNT explosion and the nucleus "G" piece of the comet S-L-9 which hit Jupiter created a 60,000 mega tons explosion. The exact size of the "G" piece, although not the largest, is said to be about 1 km (.6 of a mile). Had the S-L-9 remained whole which is the most likely occurrence as it approaches a smaller body like the Earth the impacts energy pulse would have been more that 5 times 60,000 mega tons or approximately a 306,000 mega tons energy pulse. At the height of the cold war there was about 150,000 mega tons of nuclear weapons which all the models said would have caused a nuclear winter upon use. It is highly probable that a nuclear winter would follow from any comet the size of Shoemaker Levy 9 comet. Although, of the impacts on Earth, only perhaps 5% to 10% can be contributed to comets they are likely to be the ones which cause the most destruction to the environment and cause mass extinction’s.

Comets are also the hardest to detect in time to prevent. Many new comets are of small size and are only seen a few months before their closest approach to Earth. A doomsday comet is coming some day for sure and it may be impossible to divert away from fatal impact to Earth.