Apophis will pass exceptionally close to the Earth in 2029, only 40,000 kilometers away, which is where we have most of our communications satellites. Terrestrial gravity might cause this asteroid to leave its trajectory and collide with the Earth in 2036.

The consequences of such an impact would be much more dramatic than the fallout of the Tunguska event, the meteoroid-caused explosion near the Tunguska River in Siberia, just under a hundred years ago. Yet, the scale of the potential damage would be local rather than global, with Apophis measuring only 350 meters in diameter. A global disaster can only be caused by an asteroid as large as one kilometer across or more.

As far as collisions with space rocks are concerned, the Earth has, in fact, been more fortunate than other planets. Its close neighbors Mars and Mercury have their surfaces riddled with craters.

The Earth's atmosphere and Jupiter bearing the brunt of some of the heaviest impacts, along with other factors, contribute to our planet's being the solar system's only one capable of sustaining life. But it has been hit by large celestial bodies many times before, so there are no guarantees it will remain habitable forever.

The diameter of a relatively new crater formed just 50,000 years ago in what is now Arizona, in the United States, is about 1.5 kilometers.

The Canadian province of Quebec has two craters presumably left by a meteoroid that split apart in midair: one is 32 kilometers across and the other is 22 kilometers.

The 170-kilometer crater on the Yucatan Peninsula, which became the Gulf of Mexico, is the result of an impact believed to have been produced by an asteroid some 65 million years ago. A similar strike today would kill the entire human race, destroying almost all biological links.

The Earth has narrowly escaped being struck by asteroids a countless number of times. The chance of a large asteroid impact is a realistic one. Astronomers say this is a question of "when" rather than "if."

It used to be believed that impacts like the Tunguska event happen every 700 to 900 years. Today scientists contend that such events could occur much more frequently, as often as once every a century. This is because the Earth is now followed by a tail of large celestial bodies. More asteroids have been sighted in the past decade than in the two preceding centuries.

Scientists suggest monitoring all space objects that fly in dangerous proximity to the Earth. This will require 5-meter telescopes, with three or four mounted in each hemisphere. The telescopes will target some particular segment of outer space, and data collected with their help could then form the foundation of a catalogue of asteroids measuring more than a kilometer across.

Installed on board a spacecraft, telescopes with a mirror 20 to 30 centimeters in diameter could monitor asteroids measuring 50-100 meters across at a distance of several million kilometers from the Earth. The main objective of such monitoring will be to follow the trajectory of objects that may have a potential impact on the Earth.

Scientists say that after at least 90% of large asteroids are recorded and constantly observed, it will be possible to warn the humankind about the threat of an impact some 80-100 years ahead.

But it remains unclear how a sighted asteroid can really be stopped from striking the Earth. Would it be a good idea to preempt an impact by hitting the approaching celestial body with a nuclear or hydrogen bomb? And at what distance would such a preemptive strike be safe enough for the planet?

Mathematical modeling has enabled scientists to calculate the lower distance limit for detonating a nuclear warhead in outer space. An asteroid moves with an average speed of 25 kilometers per second, so it should be no closer to the Earth than 464,000 kilometers when a bomb targeting it goes off.

Detractors argue that using such a method would be extremely unwise and fraught with grave implications for the Earth.

Boris Shustov, president of the Astronomy Institute at the Russian Academy of Sciences, is one of the opponents. He heads an academic task force set up in Russia to study threats posed by asteroids and comets to the Earth.

In Shustov's view, the best solution would be to make celestial bodies veer off a potentially dangerous course rather than destroying them. He says modifying an asteroid's velocity by just a few centimeters per second would be enough to divert it.

One way of doing so will be to attach a spacecraft with a solar sail shaped like a concave mirror to an asteroid. The sail will focus solar radiation on a small area of the asteroid's surface. Owing to the asteroid's weak gravity, warmed-up substance will quickly evaporate off the surface and the resultant jet thrust will make the rock shift its trajectory.

Another technique involves attaching rocket engines onto the surface of an asteroid itself to activate them when the need arises.

If an attempt to move a large asteroid off course fails, we could then try to provoke its collision with another, smaller asteroid. Such processes happen naturally all the time, resulting in asteroids shifting their orbits.

Unfortunately, such methods are workable only with well-studied asteroids, whose potential impact on the Earth can be predicted at least several years in advance.

A much bigger danger is posed by as yet undiscovered asteroids and so-called long-period celestial bodies, which approach the Earth once in a thousand years or even less often.

The orbits of asteroids may change gradually overtime. Sharp shifts are also possible, for example, in the event of gravitational perturbation by big-sized planets.

Creating a special asteroid interception service and keeping that service on permanent alert will require major investment. Some of its elements should be tried out while implementing current space programs. This way, we could ensure an immediate mobilization of all available means to counter a specific asteroid impact threat as soon as it arises.

Yury Zaitsev is an expert with the Russian Institute for Space Research.

The opinions expressed in this article are the author's and do not necessarily represent those of RIA Novosti.

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