Actually, some 100 bodies have already been discovered on orbits which take them so close to the Earth’s orbit that they could hit in the far distant future. This is because the orbits of these bodies change slowly with time.

Although their orbits do not intersect Earth’s orbit at present, they could hit in a few thousand years or more according to scientists.

In the course of searching for Earth-crossing asteroids, they find ones that will hit not in the next year, or even in the next ten years, but might hit in the next hundreds of years.

They believe that the chance that they will find such an object is only 1 in 1,000, even after a complete search. Once such an object is found, then  they have plenty of time to track it, measure its orbit more precisely, and plan a system for deflecting it from its current orbit (hopefully away from the Earth’s).

There is normally no great hurry, and no great panic. It is a project for all the world’s nations to take part in. It could be a globally unifying event. They are normally found long before it actually hits the Earth, it probably would take only a small impulse (rockets, or perhaps mass drivers) to divert it from a threatening path.

There is a much smaller chance that we would find one that could impact in the next 10 years according to scientists. The chance of that happening is 1 in 10,000.

If this were to happen, they would probably still have time to launch a crash program of scientific and technological research, with the goal of characterizing both the structure of the menacing asteroid, and the best means for diverting its orbit.

The least likely scenario is that if they would find one that could hit in the next year. It’s said that the chance is 1 in 100,000. In that case, there would probably be little that could be done to divert it.

Of recent a Russian astronomer said there is an incoming asteroid that is likely to hit the Earth in the year 2029. Asteroid Apophis was discovered in 2004, astronomers made headlines when they said there was a one in 37 chances that the 350-metre-wide rock nicknamed Apophis would collide with Earth in 2029. Further studies ruled out such an impact, but that there remained a one in 250,000 chance it could strike in 2036.

The Russian scientist Perminov said he had heard from another scientist that Apophis is getting closer and may hit the planet. NASA scientists have estimated that if the asteroid hits the Earth, it would release more than 100,000 times the energy released in the nuclear blast over Hiroshima.

Thousands of square miles would be directly affected by the blast but the whole of the Earth would see the effects of the dust released into the atmosphere.

An asteroid can be broken out of its orbit by the other planets’ gravitational pull on the asteroid.

About 250 million years ago, scientists believe an asteroid 4 to 8 miles (6.4 to 12.8 kilometres) across slammed into Earth with the force of more than a million earthquakes, setting off what scientists theorize was the most catastrophic of mass extinctions in Yucatan Peninsula in Mexico.

The impact carved out a crater some 75 miles (121 kilometres) wide and caused huge volcanic eruptions that buried much of the planet in lava. It blasted millions of tons of rock and dust into the sky, blotting out the Sun’s rays. It also brought about changes in sea levels and climatic shifts.

It’s believed these changes killed 90 percent of the marine species and 70 percent of the backboned land animals that lived during that geologic period. Scientists discovered evidence of this devastating extinction by digging deep into Earth’s core.

Trapped with soccer-ball shaped carbon molecules, scientists found a mixture of helium and argon gases similar to that found in certain stars but unlike anything that could form naturally on Earth.

Scientists now believe a flurry of asteroids barraged Earth and the Moon some 4 billion years ago, roughly the same time life was forming on our planet. In a study released at the end of 2000, scientists reported that a rain of asteroids lasting from 20 million to 200 million years melted rocks, blasted out craters, and reshaped the surface of both the Moon and Earth.

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